Risk and Economic Analysis on Two Tracker Architectures€¦ · Risk and Economic Analysis on Two Tracker Architectures Report Number: R1-ART161220 Revision Number: FINAL v1.1

TÜV Rheinland PTL LLC.

Test Report

Risk and Economic Analysis on Two Tracker Architectures

Report Number: R1-ART161220

Revision Number: FINAL v1.1

Client: Array Technologies

3901 Midway Place NE Albuquerque, NM 87109

USA

Release Date: September 8th, 2017

TÜV Rheinland PTL 1107 W. Fairmont Dr., Building A Tempe, Arizona 85282 USA

Contact: Mark Skidmore

Mobile: +1.479.468.0293

E-Mail: [email protected]

Report Number: R1-ART161220 Revision Number: FINAL v1.1

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Table of Contents Page

1. TÜV Rheinland ................................................................................................................ 7

2. Introduction ..................................................................................................................... 8 2.1 Contributors ............................................................................................................. 9 2.2 Analysis Procedure ............................................................................................... 10

3. Revision History............................................................................................................ 11

4. Signatures...................................................................................................................... 11

5. Executive Summary ...................................................................................................... 12 5.1 Failure Modes and Effects Analysis Summary ..................................................... 12 5.2 Economic Lifetime Cost Analysis .......................................................................... 17 5.3 Final Comments .................................................................................................... 21

6. Description of Tracking Architecture 1 ...................................................................... 22 6.1 General Description ............................................................................................... 22 6.2 Foundation Design ................................................................................................ 23 6.3 Bearings ................................................................................................................. 27 6.4 Torque Tube .......................................................................................................... 29 6.5 Control ................................................................................................................... 30 6.6 Actuating Method................................................................................................... 33 6.7 Design Philosophy (Key Perceived Compromises to Optimize Cost) .................. 37

7. Description of Tracking Architecture 2 ...................................................................... 41 7.1 General Description ............................................................................................... 41 7.2 Foundation Design ................................................................................................ 43 7.3 Bearings ................................................................................................................. 46 7.4 Round Torque Tube .............................................................................................. 47 7.5 Control ................................................................................................................... 48 7.6 Actuating Method................................................................................................... 57 7.7 Design Philosophy (Key Perceived Compromises to Optimize Cost) .................. 59

8. Summary of Perceived Failure Modes and High Risk Components ...................... 62 8.1 Structural Failures ................................................................................................. 62 8.2 Controller Failures ................................................................................................. 64


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8.3 Bearing Failures .................................................................................................... 68 8.4 Motorized Drive System ........................................................................................ 68 8.5 Damping System ................................................................................................... 69 8.6 Mechanical Drive System ...................................................................................... 71 8.7 Storm Event Failure ............................................................................................... 72

9. Comparative Failure Mode Effects Analysis (FMEA) ............................................... 74 9.1 Evaluation Methodology ........................................................................................ 75 9.2 Summary of CPN................................................................................................... 81

10. Lifetime Cost Analysis (LCOE, NPV) .......................................................................... 83 10.1 Energy Estimation ................................................................................................. 83 10.2 LCOE Equations and Assumptions ....................................................................... 90 10.3 NPV Equations and Assumptions ......................................................................... 92 10.4 NPV and LCOE Results ........................................................................................ 94 10.5 Closing Remarks ................................................................................................... 97

END OF REPORT .................................................................................................................... 97


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Table of Figures Page

Figure 1: Flow Chart of Analysis Steps 10

Figure 2: Pre-Tax NPV and Undiscounted Cash Flows 19

Figure 3: Pre-Tax LCOE 19

Figure 4: After-Tax Equity NPV 20

Figure 5: Side View of Fully Assembled Architecture 1 22

Figure 6: Gear Box Bracket 23

Figure 7: Bushing Housing Bracket 23

Figure 8: Foundation Column Height Minimums 24

Figure 9: Array on Column Foundation (pier view) 25

Figure 10: Array on Column Foundation (motor view) 26

Figure 11: Fully Assembled Bearing Kit 28

Figure 12: Torque Tube (in field) 29

Figure 13: Controller Unit with PLC and Emergency Stop 32

Figure 14: Assembled Drive Column 34

Figure 15: Worm Gear Transmission 34

Figure 16: Dampener on End of Row 35

Figure 17: Drive Line 36

Figure 18: Drive Line Quick Disconnect 36

Figure 19: Foundation Column Design Forces from Solar Assembly from Doc 1 38

Figure 20: Analysis Diagram of Torque Tube Moments and Torques 39

Figure 21: Integrated Stop on Foundation Post 40

Figure 22: Fully Assembled Architecture 2 41

Figure 23: Module Mounting on Architecture 2 42

Figure 24: Motor Piers and Brackets 44

Figure 25: End of Row Piers and Brackets 45

Figure 26: Bushing Housing 46

Figure 27: Torque Tube 47

Figure 28: Self-Powered Controller 49

Figure 29: Controller Box with Mounting Bracket 50

Figure 30: Controller Box Side View 50

Figure 31: Network Control Unit Door 52

Figure 32: Network Control Unit 52


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Figure 33: Repeater 54

Figure 34: NCU network architecture 55

Figure 35: Solar Tracker Network Architecture 56

Figure 36: Slew Drive Assembly 57

Figure 37: Active Safety Stow 61

Figure 38: Architecture 2 Chain of Control 65

Figure 39: Rusting and corrosion within the steel enclosure 66

Figure 40: Circuit Board Beginning to Delaminate 66

Figure 41: Circuit Board Beginning to Delaminate and Track 67

Figure 42: Gas Charged Damper 69

Figure 43: Failed Gas Charged Damper 70

Figure 44: FMEA methodology for evaluating tracking architectures 75

Figure 45: CPN Summary for Architecture 1 82

Figure 46: CPN Summary for Architecture 2 82

Figure 47: Determination of Average Energy Output at Failure 84

Figure 48: Determination of Average Energy Loss Due to Failure 85

Figure 49: Basic System Configuration Used to Determine Per Unit Values 86

Figure 50: Detailed Loss Values for Thermal Parameter Tab 87

Figure 51: Detailed Loss Assumptions for Ohmic Loss Tab 87

Figure 52: Detailed Loss Assumptions for Module Quality Tab 88

Figure 53: Detailed Loss Assumptions for Soiling Loss Tab 88

Figure 54: Shade Scene Example Created for Tracker Failure at Fixed Tilt 89

Figure 55: Shade Factor Table Calculated from Shade Scene Example 89

Figure 56: LCOE Calculation for Architecture 1 94

Figure 57: LCOE Calculation for Architecture 2 95

Figure 58: Summary of Margin Differential at 1% PPA Escalator 96

Figure 59: Summary of NPV Calculations 96


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DISCLAIMER

The data presented in this report was collected specifically for this project and

was gathered from documentation submitted by the Client (Array Technologies,

Inc.), from documentation submitted by 3rd party PV site owners and from direct

observations made in the field by TUV Rheinland PTL personnel.


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1. TÜV Rheinland

Founded in 1872, TÜV Rheinland is a global leader in independent engineering services, ensuring quality and safety for people, the environment, and technology in nearly all aspects of life.

Due to our extensive experience with solar power plants, we support our clients in all project phases and ensure plant safety and reliability. We also support our clients in achieving and maintaining profitability for large-scale PV systems and solar thermal power plants – from choosing the right site to full-scale operations.

We provide a comprehensive range of services, including owner representation, site as-sessment and inspection, supervision of construction, plant monitoring with yield check and yield evaluation, component inspections, and warranty evaluations.


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2. Introduction

Array Technologies, Inc. commissioned TÜV Rheinland PTL (TÜV) as an independent party

to perform a three-phase review of two tracker architectures.

Phase 1 of the review is centered on gathering facts relating to the two architectures and

technically describing them, taking care to identify key differences and critical components.

Phase 2 of the analysis will use the information and observations collected in Phase 1 to

further refine the list of key components, define missing values, and to perform a Failure

Mode Effect Analysis (FMEA) on critical components.

Phase 3 will build on Phase 1 and Phase 2 by creating a model of operational costs for the

expected life cycle of the solar plant. The end goal of this three-phase exploration is to un-

derstand how the tracker‘s architecture affects the lifetime operational costs of the solar fa-

cility.


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2.1 Contributors

TUV-Industrial Group Europe Contributors 2.1.1

• Joerg Folchert:

o FMEA analysis

TUV-PTL Contributors 2.1.2

• Mark Skidmore:

o Professional engineer, project management, systems constructions, design,

performance modelling, and report summary

• Samantha Doshi:

o Performance modelling, report summary, report editing

• Kamerine Kroner:

o Rack and tracker testing expert

Outside Contributors 2.1.3

• James Lane SE:

o Structural engineering consultant with tracker design expertise


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2.2 Analysis Procedure

The following is a brief summary of the procedure of TÜV Rheinland PTL in the assessment

of tracker design for Array Technologies.

Figure 1: Flow Chart of Analysis Steps

Summary of Architecture 1 & 2 Foundation Design, Bearings, Control, Actuating Method, Design Philosophy, Key Perceived Compromises to Optimize Cost

Summary of Failure Modes and High Risk Components Summation of site inspections, anlaysis of suseptability to failures, etc.

Comparative Failure Modes and Effects Analysis (FMEA) and Cost Priority Number (CPN) Analysis Potential failures of each architecture system with associated severity, detectability, probability of occurance, and cost.

Cost of Failure Analysis Expected failures over lifetime of site, part costs, repair time, labor rate, number of technicians required, production loss, etc.

NPV and LCOE Calculations PVsyst production modeling (Module, Inverter, Wire Loss, Soiling, Transformer, Shading, Grid Availability, Weather File), fixed and variable O&M estimations, cost of failure, discout rate, financing, replacement of inverters, EBOS, etc.


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3. Revision History

Revision Number Date [dd/mm/yyyy] Name Reason for Update

FINAL v1.0 09/08/2017 Samantha Doshi and Mark Skidmore Finalization

FINAL v1.1 09/11/2017 Samantha Doshi Updated formatting in Executive Summary.

4. Signatures

Compiled by: Reviewed by:

Name / Date / Signature Name / Date / Signature


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5. Executive Summary

Array Technologies, Inc. commissioned TÜV Rheinland PTL (TÜV) as an independent party

to perform an economic and risk analysis of two tracker architectures. Architecture 1 is a sys-

tem that is driven by a single motor linked by a rotating driveline to multiple tracker rows. Ar-

chitecture 2 is a system where each row operates as a self-contained unit with a dedicated

photovoltaic (PV) panel, battery, motor, and other tracker system components. TUV’s analy-

sis includes descriptions of the technical characteristics of each system, followed by a failure

modes and effects analysis (FMEA) to assess risk associated with component failures, and

concluding with a levelized cost of energy (LCOE) / Net Present Value (NPV) analysis to

assess the economic impact of the two technologies on developers, owners, financiers, and

insurers of utility scale solar power plants.

5.1 Failure Modes and Effects Analysis Summary

Structural Failure 5.1.1

As a technical advisor, TÜV Rheinland is frequently called upon to evaluate solar projects. In

regards to tracking systems, TÜV has found that torsional harmonics (“galloping”) is playing

an increasing role in tracker-based failures. Galloping is of particular concern because of the

potential for consequential damage to solar modules and other ancillary components due to

the severe harmonic oscillations and twisting imparted on the system by the phenomenon.

Galloping has been reported on tracker systems at wind speeds below design limits.

Architecture 1 handles high wind and structural loadings in a unique manner. High torsional

loads initiate a “release mechanism” at each tracker row. The individual row then rotates in a

controlled fashion to a mechanical stop located at the maximum angular travel (52 degrees)

position. The mechanical stops are located at each foundation pier where they transmit the

high structural loads to ground. The torsional forces placed on the system are, therefore, not

transmitted further than the foundation post intervals which are spaced 6.5m apart along a

typical 80m row length.

An independent structural engineer found that Architecture 1 has additional structural capaci-

ty when compared to Architecture 2 (for additional details, see §5.4.7). In addition, Architec-

ture 1’s mechanical load-relief system was found to reduce the likelihood of torsional gallop-

ing by preventing torsion buildup and distributing maximum loads.


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In single axis trackers such as Architecture 2, all torsional structural loads are transmitted

along the length of the torque tube to the center structure where they are then transmitted to

ground. Torsional galloping typically begins at the extremities of the tracker row where the

opportunity for torsional deflection is at its highest. Since Architecture 2 requires the loads to

be transmitted for approximately 42.5m down the length of the torque tube, the opportunity

for torsional galloping is increased.

Architecture 2 utilizes an active stow algorithm to avoid an increase in static loads. The ac-

tive safety stow position is 30 degrees in the prevailing wind direction with the leading edge

of the module down. The 30 degree stow position, however, may increase the likelihood of

torsional galloping. The assertion that 30 degree stow tilt angle imposes the least likelihood

of aerodynamic loads, least likelihood of induced vibration, and least likelihood of torsional

galloping should be evaluated empirically and with computational analysis (e.g. CFD with

dynamic structural modelling). Because Architecture 2 relies on lower structural loads driven

by stow assumptions, the Architecture carries a lesser structural capacity and is less resilient

to loading above design wind loads. The FMEA analysis conducted as part of this report ana-

lyzes the probability and impact of control or drive system failures that prevent the Architec-

ture 2 tracker from reaching stow position, and therefore becoming more susceptible to cata-

strophic structural failure.

While it would be impossible to do a side-by-side comparison in an identical wind event, the

additional structural capacity of Architecture 1 will provide some level of insurance against

structural damage.


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Control Systems & Failures 5.1.2

Both Architecture 1 and Architecture 2 are electronically controlled, and are susceptible to

outside induced failure modes such as lightning and infant mortality of components.

Architecture 1 utilizes commercially rated and readily available components such as a DC

Power Supply and Programmable Logic Controller (PLC). Under normal operation and prop-

er installation, the Architecture 1 control system should prove reliable with failure primarily a

function of infant mortality of the electrical components. These components have lifecycle

data available from the manufacturer.

Architecture 2 utilizes a custom designed printed circuit board (PCB) that appears to be pro-

prietary. The unit provided for this analysis showed signs of significant corrosion within the

critical control elements. Therefore, Architecture 2 appears to be more susceptible to envi-

ronmental induced failure caused by normal operation. Furthermore, should the manufactur-

er of Architecture 2 experience a bankruptcy, or for some other reason be unable to service

warranty claims, their Control System would be challenging to replace. A replacement cus-

tom control system would need to be designed, tested, and programed that could assume all

of the required functions.


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Batteries 5.1.3

Architecture 2 employs lithium-ion (Li Ion) battery packs at each row, providing power to the

row’s motor and control system. Since each row is a self-contained tracking system, a func-

tioning battery is required for normal and safety operations.

The lifespan of Li Ion batteries is determined by multiple factors including maximum number

of discharge cycles, ambient temperatures (ambient temperatures above 40°C/104°F may

reduce charge capacity 35% / year), depth of discharge, and level of charge. Charging of Li

Ion batteries at temperatures below 0°C/32°F or above 50°C/122°F may physically damage

batteries and shorten life expectancy. Given the broad range of operating conditions in harsh

solar power plant environments, the expected battery lifespan can be estimated to be any-

where from two (2) to ten (10) years. For Architecture 2, the manufacturer’s given lifespan

for the Li Ion battery pack is ten (10) years. During TUV’s inspection of a site employing the

Architecture 2 tracker design, ~20 batteries had been pulled from the trackers and decom-

missioned. The site was less than 10 years old. Therefore, for this evaluation the battery life

expectancy has been reduced to 7 years123.

While the Li Ion batteries utilized at each row are commercially available, multiple replace-

ments of each battery must be planned for the lifetime of the tracker system. The Li Ion row

batteries must also be kept in functioning condition at all times, as catastrophic damage risk

is increased if rows are unable to stow due to non-functioning batteries.

1 Addressing the Impact of Temperature Extremes on Large Format Li-Ion Batteries for Vehicle Applications,

https://www.nrel.gov/docs/fy13osti/58145.pdf

2 Effect of Temperature on the Aging rate of Li Ion Battery Operating above Room Temperature, https://www.nature.com/articles/srep12967

3 How to Prolong Lithium-based Batteries, http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries


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Damping Systems 5.1.4

Architecture 1 utilizes automotive style dampers. The failure mechanism of this type of

damper is typically through a change in the viscosity of the hydraulic fluid rather than a loss

of hydraulic fluid. Ultimately this means the dampening effectiveness is reduced, but not lost.

Architecture 2 uses gas charged struts. These damping devices place a high damping ratio

in a small package making them very length-efficient when compared to a traditional damper.

During a site visit of a project that used Architecture 2, one damper out of the twenty-five (25)

inspected had failed due to seal failure. When a gas charged damper fails, the hydraulic oil

is quickly evacuated by the pressurized gas. This failure mode can be seen by the spray of

oil evident on the pile, ground, and damper itself. When gas charged dampers fail, they lose

almost all damping action and are effectively useless.


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5.2 Economic Lifetime Cost Analysis

Two economic analysis methodologies were used to evaluate each tracking architecture:

� Levelized cost of energy (LCOE), and

� Net present value (NPV)

LCOE and NPV are related time value of money economic methodologies. NPV expresses

the project economic summary as a single number at today’s point in time. LCOE expresses

the same result (as NPV) but as a levelized, annual value over the project duration.

LCOE is a metric that allows for the comparison of the combination of capital costs, O&M,

performance and fuel costs. As this is intended to be a comparison of identical systems, this

model does not include financing issues, discount issues, future replacement of inverters,

electrical-balance-of-systems (EBOS), and it omits degradation costs in general. As LCOE

factors in lifetime energy production, the energy output is degraded overtime so energy deg-

radation is included in the calculation. Net present value (NPV), on the other hand, is the

difference between the present value of cash inflows and the present value of cash outflows.

This analysis does include financing, discount rates, replacement of inverters, EBOS, etc.

NPV is used to analyze the profitability of a projected investment.


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Scheduled Maintenance 5.2.1

This category includes manufacturer recommended inspections, vegetation management,

module washing, and other system maintenance requirements.

Scheduled Tracker Maintenance:

Architecture 1 has lower overall scheduled maintenance costs as the system has no sched-

uled maintenance requirements outside of system inspections. Architecture 2 has higher

scheduled maintenance costs due to the required re-lubrication of the slew gear.

Vegetation Management / Mowing:

For systems employing Architecture 1 without the quick disconnect drivelines, a 25% in-

crease in mowing cost is assumed (per mow). For this analysis, quick disconnect drivelines

are assumed. Furthermore, Architecture 1 has a module lower edge to grade clearance of

approximately 18” while Architecture 2 has a clearance of 11“ (a ~40% difference). Therefore,

Architecture 2 will need to be mowed more frequently and is reflected in the analysis.

Module washing:

Module washing costs are consistent between each architecture.

Unscheduled Tracker Maintenance 5.2.2

Unscheduled maintenance costs were calculated using a Cost of Failure (CoF) methodology,

which estimates the expected market cost of a failure. Cost of failure calculations are unique

to each architecture as they are a function of cost of the part being replaced, labor, and pro-

duction losses. Cost of failure was then used to linearly extrapolate yearly unscheduled costs

based on the expected replacements within the 30-year life. Expected replacements are a

function of the life expectancy of each unit and number of units needed to equip a 100MW

site.

Annual Energy Production (AEP) 5.2.3

Annual energy production for a 100MW site was modeled for both systems using the model-

ing software PVsyst. The main difference between the two models was the ROM (range of

motion). Architecture 1 has a ROM of ±52° while Architecture 2 as a ROM of ±60°. Architec-

ture 1 does have the option of ROM of ±62°, but that was not included within this analysis.


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Economic Analysis Conclusions 5.2.4

The Pre-Tax financial results for the two architectures modeled as 100 MWdc systems are

detailed in the chart below.

Pre-tax basis:

Architecture 1 is economically advantaged due primarily to its $517,914 per year O&M

cost advantage:

1. The pre-tax NPV of a project using Architecture 1 at COD (Commercial

Operations Date) is increased by ~$4 million, or 4 cents/Wdc.

2. The LCOE for Architecture 1 is $1.89/MWhr (6.7%) lower than Architecture 2.

3. The undiscounted O&M savings over the life of the project are $15.5 million, or

~15% of the assumed project CAPEX.

Figure 2: Pre-Tax NPV and Undiscounted Cash Flows

Figure 3: Pre-Tax LCOE

Pre-Tax Analysis OutputsPre-Tax NPV Output A1 A2System Value: NPV @ Discount Rate w/ ITC ($) $38,440,293 $34,453,481Increase in System Value: NPV Delta ($) $3,986,811Increase in System Value: NPV Delta (%) 11%

Pre-Tax Undiscounted Cash Flows A1 A2Undiscounted System Cash Flow w/ ITC ($) $285,227,507 $269,933,998Increase in Undiscounted Cash Flow ($) $15,293,509Increase in Undiscounted Cash Flow (%) 6%

Undiscounted O&M Costs (Fixed & Variable) -$34,270,037 -$49,807,458Decrease in O&M Costs (Fixed & Variable) $15,537,421Decrease in O&M Costs (Fixed & Variable) -37%

Pre-Tax Analysis Outputs

LCOE for A1 over 30 year life [$/MWhr]LCOE for A2 over 30 year life [$/MWhr]

Difference ($)Percent Difference (%)

LCOE (Levelized Cost of Energy) Output$27.51$29.40($1.89)-6.7%


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After-tax basis:

On an After-tax Basis, the financial results for the two architectures are similar but modi-

fied for taxes. Tax assumptions include Federal rate at 35%, State rate at 9%, Investment

Tax Credit (ITC) rate at 30%, and MACR’s accelerated depreciation in a 5 year term.

After-tax major conclusions:

• The Equity NPV of the project at COD is $2.3 million (4.5%) higher with

Architecture 1.

Figure 4: After-Tax Equity NPV

The project economic analysis results demonstrate the extent to which Architecture 1 pro-

vides a lower operating cost than Architecture 2. Architecture 1 achieves its lower cost profile

primarily due to its lower parts failure rate, utilization of robust control components, and a

robust structural design. Architecture 2’s expected lifetime service events are 732 times

greater than the service events expected for Architecture 1.

In Architecture 2, the high component failure rate are driven by the duplication of all drive and control systems at each tracker row and the life of those components. The batteries, tracker controls, communications, sensors, and motors drive increased unscheduled maintenance costs for Architecture 2 substantially above those for Architecture 1. During the 30-year anal-ysis period for a 100MW site, Architecture 2’s total maintenance costs over the project life-time were ~$15.5 million (37%) greater than that for Architecture 1. The results shown above quantify the economic impacts of this report’s analysis of the tech-nical and operational approaches of each architecture. To a project owner, the present value costs (at COD) of Architecture 1 are 4 cents/Wdc lower than Architecture 2.

After-Tax Analysis Outputs

Equity NPV for A1 [$]Equity NPV for A2 [$]

Difference ($)Percent Difference (%)

NPV (Net Present Value) Output$53,141,237$50,783,039

4.5%$2,358,199


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5.3 Final Comments

It is important to note that severe weather events are omitted from this analysis. Architecture

1 has clear advantages in areas subject to extreme weather such as microbursts or areas

with high sustained winds. The economic results presented have not been adjusted for the

higher risks events that may affect Architecture 2 more severely due to its lower structural

capacity.

Analytical details supporting all results and conclusions noted in this executive summary are

included in the report body.

Please do not hesitate to contact TÜV Rheinland with questions about tracker performance,

risks, and economic impacts on your solar power plant projects.


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6. Description of Tracking Architecture 1

6.1 General Description

Tracker Architecture 1 is a centrally driven architecture, meaning that one prime motor is deployed per multiple tracker rows. The power from the one motor is transmitted through a centrally located drive shaft that transmits energy through small transmissions located cen-trally throughout the tracker. Architecture 1 is a single-axis tracker that tracks the sun’s movement on a rotating North and South axis. The modules are mounted without a southern elevation and all movement is based on East to West movement. At the end of the opera-tional day, the tracker will reset to a horizontal or user selectable position in preparation for the next day of operation. This flat night stow positon prevents early morning shading and allows the system to activate as early as possible.

Figure 5: Side View of Fully Assembled Architecture 1


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6.2 Foundation Design

Tracker Architecture 1 appears to be compatible with various foundation types, however the

foundation system must have a vertical element providing clearance and attachment of the

tracker system. The Architecture 1 system employs a bolted attachment suitable for bolting

to an “I”,”, “C”, “Sigma”, and other common foundation column/pile shapes. Other foundation

column designs such as screw piles, spread concrete footings, and steel substructures can

be used, but vertical-beam shapes are the most common and typically the least expensive

configurations.

There are two column lengths utilized by this system. The shorter column is located where

the drive motor and gear rack assembly is installed. Each row of tiltable solar panels will

have at least one (1) of these columns. The longer columns are support columns. They sup-

port the long horizontal pipe element that the solar panels are attached to, including the bear-

ing, bushing, and bearing housing. As the horizontal octagonal tube rotates, it rotates the

solar panels around the axis of the horizontal tube. The support columns are spaced at ap-

proximately twenty-five (25) feet (7.62m) on center along the line or row of rotating solar

panels. Official prints for the I-beam brackets show the various configurations needed to

support potential varying sizes and shapes the support columns available in the market. The

attachment points are all bolted and have elongated holes to allow adjustment in the field.

Figure 6: Gear Box Bracket Figure 7: Bushing Housing Bracket

The gear box attachment location requires ten (10”) inches (25.4cm) of clearance above

grade. The bushing housing attachment requires forty-two (42) inches (106 cm) of clearance


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below minimum to grade or other. These dimensional requirements provide a typical clear-

ance between the lower module edge and ground of. The clearance of module lower edge to

the ground impacts the frequency of mowing required, and provides the allowance for snow

buildup before it impacts the tracker movement. This minimum elevation requirement allows

for a steel “A” frame support on concrete spread footing, or a concrete pier of reasonable

diameter. However, the system as drawn identifies driven H-Piles.

Figure 8: Foundation Column Height Minimums


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Figure 9: Array on Column Foundation (pier view)


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Figure 10: Array on Column Foundation (motor view)

Architecture 1 system transfers the tension, compression, shear, and moment stresses as-

sociated with the site design forces to the piers. Site design forces will vary depending on

height of installation, associated snow, wind, and seismic forces. The closer the system is to

the finish grade, the lower the pier forces. Typically, the majority of installations will be con-

trolled by wind forces. However, soil conditions will play a major role in foundation design

requirements on both internal and external rows.


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6.3 Bearings

The bearings for Architecture 1 support an octagon tube (or torque tube) that acts as the central drive mechanism to rotate the row of solar panels mounted to the tube. The bearings are manufactured from a self-lubricating plastic that have a high molecular weight (HMW). HMW polymers are a popular material for tracker system bearings as well as structural plas-tics in a variety of industries, particularly the automotive industry. The bearings are injection molded and have been physically lightened through the use of internal ribbing to provide ad-equate support while reducing required material. The HMW polymer used in Architecture 1 has been fortified with carbon black to protect the polymer from ultraviolet (UV) damage.

The bearing to torque tube contact surface area is increased with the octagonal shaped torque tube. The octagon shape is also highly effective in distributing and withstanding the torsional loads imposed. The contact area between the bearing and the bearing housing is reduced on the upper side of the housing. A cutout is provided to accommodate an aluminum bushing that keeps the bearing in place but also transfers load onto the bottom side of the bearing. The lower half of the bearing fully contacts the bearing surface, thus distributing the gravitational, snow and wind load of the torque tube and solar modules over the largest pos-sible surface area.

The bearing is a critical component for system operation but it is likely a highly reliable com-ponent of the system. Testing documents provided for Architecture 1 demonstrate the bear-ings are resilient and durable. In failure scenarios bearing housings often fail before the bearings. These tracker bearings have been employed in fielded tracker systems of Archi-tecture 1 for over 10 years and have exhibited excellent durability without a single failure according to the manufacturer.


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Figure 11: Fully Assembled Bearing Kit


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6.4 Torque Tube

Architecture 1 incorporates an octagonal shaped torque tube. The octagonal shape is a

highly optimized structural tube shape for resisting torque and beam loads while providing

the means for shape fit splices to multiple torque tube sections and mechanical connections

to the drive gear, bearing stops and module clamps. Shape fit connections are less prone to

fatigue failure since the connection stresses are not localized and thereby lower in magni-

tude.

Figure 12: Torque Tube (in field)


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6.5 Control

The control system for Architecture 1 is an assembly of widely available components. The central control module is a Programmable Logic unit (PLC), a proven industrial tool for con-trol systems. The tracker controller receives input from the site controller that informs the PLC of the time, date, and geographical location. The control module then incorporates these variables into a tracking algorithm that adjusts the tracker‘s East/West position at regu-lar intervals throughout the day. The controller operates solely in a closed-loop capacity based on defined variables, but not on real-time measurements of system output. The con-troller uses the NREL open-source algorithm4 for controlling the tracking behavior. At the end of the day, the tracker employs a back tracking algorithm that adjusts the trackers posi-tion based on a geometrical calculation. The back tracking algorithm is designed to prevent the tracker rows from shading themselves at the beginning and end of the day. Separate settings for morning and evening backtracking are available to optimize backtracking on E/W tilted site geography.

The control assemblies are assembled in the tracker manufacturer’s facility and pre-programed for the project. The tracker manufacturer is approved to assemble the entire tracking system, including the control panels. The tracker rows may be in any position dur-ing the installation of the drivelines and the linked group of rows will synchronize during a calibration or after one full day of operation. The drive motor uses a rotary encoder, so once a known position is defined, the tracker control system will recognize what position the track-er is in.

The control panels utilize dynamic number insertion (DIN) rails, to which key control compo-nents are attached. The DIN rails stops and terminal blocks are utilized to secure compo-nents and to make critical connections. Wire used within the enclosure is a typical machine tool or appliance wiring with designations identified in the NEC and individually listed per the applicable component standard.

A detailed bill of materials was acquired for each controller utilized by this system and each component was researched. As Architecture 1 utilizes off the shelf components, most critical components have operational life data available or calculated life expectancies.

4 Reda, I.; Andreas, A. (2003). Solar Position Algorithm for Solar Radiation Applications. 55 pp.; NREL Report No. TP-560-

34302, Revised January 2008.

http://www.nrel.gov/docs/fy08osti/34302.pdf

http://www.nrel.gov/docs/fy08osti/34302.pdf


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One crucial component that is utilized in multiple control boxes is the PLC. This device has a stated mechanical switching life of ten million (10,000,000) cycles. However, at the rated current this switching life is reduced to one hundred thousand (100,000) cycles. The operat-ing current of Architecture 1 is well below the rated current of the device at only 2mA, which is 1000 times less than the rated capacity. If a tracker moves every fifteen (15) minutes throughout the operational day, which averages ten (10) hours per day, and the loading of the PLC is 2mA then the expected life of the PLC is well beyond that of the solar module.

Another critical component is the direct current (DC) power supply. The supply used has an MTBF of 3.5 million hours. The DC power supply is essentially a robust solid state device that is likely capable of functioning for the plant‘s lifecycle if left unaffected by outside influ-ences. The Power Supply can be damaged by power surges. The DC power supply is the primary connected element to the grid power for the control system and is the likely first ele-ment to be damaged by power surges and voltage spikes and may act as a sacrificial protec-tion to the PLC control surge. The Power Supply supplies control voltage to the system, so if the power supply fails the control system fails and the tracker will remain in the position it was in at the time of the power supply failure.


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Figure 13: Controller Unit with PLC and Emergency Stop


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6.6 Actuating Method

The tracker in Architecture 1 is actuated by a motor which drives worm gearboxes at each row, linked through a series of rotating drive shafts. The drive motor has a gear reduction head to increase delivered torque to the system. Each worm gearbox has an input and out-put shaft that receives the rotational input on one side and passes the action through to the next gearbox via the linked drivelines. The worm is rotated by the drive shaft and in turn ro-tates a worm gear connected to an output shaft. A spur gear is connected to the output shaft and the spur gear drives a rack gear which is connected directly to the torque tube. The rack gear is assembled from two metal plates that are riveted together. The rivets act as the bearing surface and are made of stainless steel. Through mechanical advantage, the drive mechanism is able to drive multiple rows, up to thirty-two (32), achieving tracking of approxi-mately one (1) MW per drive motor.

The worm gearbox allows for high torque transmission but operates at very slow speeds, which reduces heat accumulation from friction. The slow speed of operation helps minimize component wear. The worm gear transmissions also prevent the tracker rows from reversing the direction of power, which helps the tracker remain at the prescribed tilt while the motor is not in operation. While the gears, racks and worms are highly important components of sys-tem operation, they are also industrial components typically meant for higher duty cycles so failure is unlikely. A compression spring is incorporated at the ends of every row to offset the overhung weight of the system to minimize rotational deflections and minimize the drive forc-es within each row. The spring counterbalance further relieves the stresses on the drive sys-tem.


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Figure 14: Assembled Drive Column

Figure 15: Worm Gear Transmission


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Figure 16: Dampener on End of Row


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Figure 17: Drive Line

Figure 18: Drive Line Quick Disconnect


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6.7 Design Philosophy (Key Perceived Compromises to Optimize Cost)

Bearing 6.7.1

The bearing is an injection molded piece that optimizes its cost by removing unneeded mate-rial within the bearing assembly. Removing material lightens the bearing, reduces the placement of HMW plastic, and shortens cooling times for the part. Thinning the material also improves part tolerance and manufacturing efficiency. Based on accelerated life testing which was provided to TUV, these cost compromises are seemingly for optimal manufactur-ing and have no impact on part reliability under operating conditions.

Module Support Rails 6.7.2

Module support rails have been eliminated and a custom clamp secures the module with less hardware. One bolt clamps the module in two locations, secures the clamp around the oc-tagonal torque tube and through a UL approved grounding element electrically grounds the module frame to the tracker structure in two places. Only one clamp assembly is needed on each side of the module. The custom clamp together with the module frame becomes the primary structural member.

Reduction of Electrical Infrastructure 6.7.3

Architecture 1 reduces and optimizes the number of electrical motors and control elements required to actuate the tracker. The reduction of components is an optimization method that enables the use of hardened electronic and electrical components that undergo strict ac-ceptance criteria. While this optimization method increases the number of mechanical ele-ments, these elements are easily maintained and often visually manifest failure in advance of critical failures. The components used and the structure of the system are all robustly de-signed and are assembled from commonly available parts.

Safety Factor 6.7.4

Assessment of the safety factors within Architecture 1 were based on the following doc-

uments:

• Document 1: DTHZ v3 JKM315-76 100-C 0 PSF Ground Force Analysis dated

10/27/2016,

• Document 2: Installation Guide v3 Standard Design 90052-000, and


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• Document 3: Summary of DuraTrack® HZ Solar Tracker DTHZ v3 JKM315-76

Projects for TUV XXX project, Document # 10XXX-902 Fresno, CA.

Document 1 provides a rationale for the establishment of foundation design loads, referring

to a wind tunnel study performed to establish the wind force coefficients. The resulting coeffi-

cients from the wind study are much less than values in the current code. These resulting

coefficients are allowed in the building code.

The analysis within Document 1 uses an ultimate stress design (ult) wind load of 100 mph

(161 kph), Risk Category I. This 100 mph/161 kph value is in accordance with ASCE 7-10,

American Society of Civil Engineer Standard 10 – Minimum Design Loads for Buildings and

Other Structures. One statement in that document reads as follows: “The controlling case

from each designation in the field shall be used for all column designs.” These wind forces

are provided to the structural engineer of record for each site install. The resulting loads on a

typical column style foundation for both interior and exterior columns are shown below.

Figure 19: Foundation Column Design Forces from Solar Assembly from Doc 1

The current official installation guide provided to clients states that a ninety (90) unit solar

module row can resist 130 mph/209 kph 3-sec gust wind for exposure “C” per the 2012 Inter-

national Building Code (IBC). This value appears to be an ultimate wind speed. The majority

of the United States (US) for common buildings classified as Risk Category II utilizes 90

mph/145 kph (asd) and 115 mph/185 kph (ult) with both being recorded as 3-sec gust. For

solar arrays designed as Risk Category I, the majority of the US would be 105 mph/169 kph

(ult).


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Both Document 1 and Document 3 appear to use a load factor of 1.0 in the establishment of

base wind pressure, using wind speeds as cited in ASCE-10 for California. Elements being

designed based on the resulting wind reactions obtained will have at least a safety factor of

around 1.7 for beams and columns. For connections designed per current industry stand-

ards, the safety factor is 3. These safety factors are for the elements based on the design

wind load that were obtained from a wind tunnel study.

Figure 20: Analysis Diagram of Torque Tube Moments and Torques

Document 3 shows simple sample calculations for the determination of moments and torques

on a single torque tube and a series of columns supporting a length of torque tube. ATI uses

wind tunnel analysis to determine the worst case span load that could be applied to each

span and assumes these are being applied simultaneously for all component design. Since it

would be very unlikely that all solar panels would receive maximum wind at the same time, a

more detailed analysis with skip loading should be performed to ensure the most conserva-

tive possible moments are being used. Skip loading loads two adjacent bays, skips a bay,

and then loads the next two adjacent bays with the pattern continued all the way down the

row. Each row follows this pattern, except the position of the initially loaded bay is moved

over one spot. These multiple load combinations will envelope the maximum moments in the

torque tube, though normal LRFD safety factors should already more than account for these

potential variations.


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Self-protection 6.7.5

Architecture 1 self-protects from high static and dynamic torsional loads through a passive

mechanical torsion limiter on every row. The system is designed such that maximum torsion-

al loads will be shared through a stop device that is installed at every foundation post in eve-

ry row. In cases of extreme torsional loads, the torque limiter within Architecture 1’s gearbox

will slip, preventing further torsional loads from being imposed on the torque tube. Unlike the

industry norm that actively stows modules in the horizontal position during a wind event, Ar-

chitecture 1 uses the strong winds to push the array into torque a higher angle, dynamically

stable position until it reaches the limitations of the stop that is integrated into each founda-

tion post. This position is the most stable because it minimizes torsional vibrations on the

system comprised of the torque tube and solar modules, and is usually at the end of rota-

tional travel position. Because each foundation post has an integrated stop the torsional and

hinge moment loads in a wind event are transmitted to ground through the nearest founda-

tion post. Architecture 1’s approach of directing extreme system loads to the nearest founda-

tion, as opposed to the industry norm to carry those loads through long lengths of torque

tube to a center structure, is an inherently more robust approach to the handling of extreme

loads.

Figure 21: Integrated Stop on Foundation Post


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7. Description of Tracking Architecture 2

7.1 General Description

Tracker Architecture 2 is an individual row driven architecture, meaning that each row is moved with a dedicated motor. For every row of up to ninety (90) modules there is one 24V DC motor. Each row operates as a self-contained unit with a dedicated PV panel and battery to provide power to the controller and motor. Architecture 2, like Architecture 1, is a single-axis tracker that tracks the sun’s movement on a rotating North/South axis. The modules are mounted without a southern elevation and all movement is based on East to West rotation. At the end of the operational day, the tracker will reset to an easterly position in preparation for the next day of operation.

Figure 22: Fully Assembled Architecture 2


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Figure 23: Module Mounting on Architecture 2


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7.2 Foundation Design

Architecture 2 appears to be compatible with various foundation types. However, the founda-

tion system must have a vertical element providing clearance and attachment of the tracker

system. The Architecture 2 system employs a bolted attachment suitable for bolting to an “I”,

“C”, “Sigma”, and other common foundation column/pile shapes. Other foundation column

designs such as screw piles, spread concrete footings, and steel substructures can be used,

but vertical-beam shapes are the most common and typically the least expensive configura-

tions.

There is one column height utilized by Architecture 2. The columns support the long round

pipe element that the solar panels are attached to, including the bearing and bearing hous-

ing. This pipe rotates, giving the solar panels the tilting capability. The support columns are

spaced at approximately twenty-five (25) feet (7.62 m) on center along the line of rotation.

Official prints for the I-beam brackets show the various configurations needed to support po-

tential varying sizes of “I” columns available in the market. Bearing and Drive attachment

points are all pinned and feature elongated holes to allow adjustment in the field (see Figure

18 for more details).

The gear box and bearings have a minimum height of 3’ 7” (109 cm) and provide a solar

module lower edge to grade clearance of approximately 11” (28 cm). The smaller clearance

between the lower module edge and ground for Architecture 2 increases the frequency of

mowing required, and increases the risk of module damage due to snow buildup. Attach-

ment to the Architecture 2 system must be such that it transfers the tension, compression,

shear, and moment stresses associated with the site design forces. Site design forces will

vary depending on height of installation, associated snow, wind, and seismic forces. The

closer the system is to the finish grade, the lower the attachment forces. Typically, the ma-

jority of installations will be controlled by wind forces. However, soil conditions will play a

major role in foundation design requirements on both internal and external rows.


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Figure 24: Motor Piers and Brackets


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Figure 25: End of Row Piers and Brackets


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7.3 Bearings

The bearing surface on Architecture 2 is a hardened stainless steel shaft that interfaces with a welded steel housing. The fitment between these devices is loose and does not contain lubricants of any kind. A gap of approximately 1/8“/“/3 mm can be seen throughout the rota-tion cycle. It is not clear if the welded steel assembly is honed to provide a smooth surface or if it is left in its native state after manufacturing. Mild scoring from the expansion and con-traction of the tracker was evident on the hardened steel shaft.

The hardened stainless steel bearing shaft is supported by a welded assembly which is mounted to the foundation post. The shaft is connected to the torque tube by two cast piec-es that are strapped to the cylindrical steel torque tube using U-bolts. Other than clamping friction and geometry, there is no mechanical means to prevent the torque tube from rotating within the U-bolt connection.

Figure 26: Bushing Housing


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7.4 Round Torque Tube

Architecture 2 utilizes a round torque tube. Round is a highly-optimized shape for resisting torsion and beam load forces, however round torque tube in this design requires pin-joints for all structural connections. Connections to the round torque tube in Architecture 2, including the connections to the drive gear and the torque tube splice connections are secured using pin-type blind fasteners. Pinned connections, especially when used for connecting thin round tubes in torsion create high stress points in thin wall tubing at the area where the pins pierce the tube. High stress concentrations when exposed to repeated torsion and beam loading are prone to fatigue failure over time. This effect may be exacerbated once loosened by repeated loading. Once these connections become loose, the forces may increase from the wind and the connection may fail rapidly. Further fatigue analysis of this connection needs to be conducted to determine its integrity over a 30-year design life.

Figure 27: Torque Tube


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7.5 Control

The Architecture 2 tracker controller system incorporates three control units, networked to-

gether using ZigBee wireless mesh communications to provide individual platform tracking

and system-level control and monitoring capabilities. A ZigBee device is installed on each

row and is used to create personal area networks with small, low-powered digital radios. The

three control units are the Self-Powered Controller, Network Control Unit, and Repeat-

er/Weather station. Each Network controller can control 100 Self-Powered Controllers within

500 feet of the controller. If additional distance is required, a repeater station is needed to

allow communication.

Self-Powered Controller 7.5.1

Each tracker row is equipped with a Self-Powered Controller (SPC) that is mounted directly to the torque tube and connects to a single tracker motor. The controller has several key functions, the first of which is a charge controller. A small twenty-four (24) volt photovoltaic (PV) module supplies voltage to the board, which controls the charge of a 14.4 volt, 45Wh lithium-ion battery. The battery can power the tracker for up to three (3) days, eliminating the need for alternating current (AC) power to be wired to the tracker motor or controller. The battery is composed of eight (8) cylindrical lithium ion batteries that have been soldered to-gether into an assembly. Power from the dedicated solar module to the battery is passed through a circuit board that is dedicated to managing the health of the battery. The battery control unit (BCU) monitors battery temperature and charge state. The status of the battery is then sent to the Self Power Controller (SPC) which makes charging decisions based on the status from the BCU. The status of the BCU will not operate without a functional battery. All circuit boards are custom built for this application and feature tight spacing of components and conformal coating. The enclosure that houses the SPC and battery is made of steel, is gasketed to prevent water intrusion, and contains no weep holes.

The second function of the control board is Tracker Row Control. The SPC operates auton-

omously using an onboard clock and inclinometer to adjust the tracker platform position. All

SPCs communicate with the Network Control Unit over a ZigBee wireless mesh network for

monitoring and control functions. When commissioned, the control board receives site specif-

ic details that are used as inputs for a sun tracking algorithm that optimizes the trackers posi-

tion. While the tracker controller does receive updates from a central controller, each tracker

row can function without constant communication from the central controller. Lack of com-


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munication with the central controller would, however, not allow the tracker rows to receive a

stow command in the case of a high wind event.

Figure 28: Self-Powered Controller


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Figure 29: Controller Box with Mounting Bracket

Figure 30: Controller Box Side View


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In the fielded SPC unit inspected, there was evidence of corrosion on both the enclosure and

circuit board. Failed controller units were available on site and one was provided for inspec-

tion. A significant amount of corrosion was present within the SPC enclosure, and the battery

of the failed unit was not charging. Upon testing, the battery control unit (BCU) was found to

be inoperable. The conformal coating applied over the printed circuit board was bubbling in

several locations. In some areas, conductive paths seem to have been established due to

delamination and arcing. It appears that the sealed enclosure is collecting condensate which

then causes the interior of the enclosure and printed circuit boards to corrode and fail prema-

turely.

Network Control Unit 7.5.2

A Network Control Unit (NCU) provides communication, control, and monitoring functions for

up to one hundred (100) SPCs via a ZigBee wireless mesh network. Each NCU is accessible

via a Modbus interface for supervisory control and data acquisition (SCADA) integration for

streamlined data collection. In areas with Ethernet access, the information is sent to the

cloud for storage and is accessible via a web-based interface. For remote areas the infor-

mation can be stored locally on an SD card within the NCU.

NCUs are typically located at inverter pads with other electrical equipment to easily connect

to the required 120 or 240-volt AC power. A battery backup is included with each NCU unit,

and as with the SPC, the backup battery when new, can supply up to three (3) days of pow-

er. Operator generated commands are available via local switches on the NCU for manual

system control. Stowing commands for wind or snow events are generated and issued auto-

matically by the NCU based on data from the Weather Stations.


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Figure 31: Network Control Unit Door

Figure 32: Network Control Unit


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Repeater/Weather Station 7.5.3

Depending on the site, one or more Weather Stations must be installed to measure real-time

wind speed and snow depth (where applicable) using Nova Lynx anemometers or snow and

flood sensors, and report this data to the NCUs over the ZigBee wireless mesh network. Like

the SPC, the Weather Station receives operating power from a dedicated PV module with a

backup battery that can power the station for up to three days. When conditions warrant, and

the ZigBee systems are functional, the NCUs send a stowing command to its associated

SPCs within seconds.


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Figure 33: Repeater


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Network Architecture 7.5.4

An NCU network consists of up to any combination of one-hundred (100) SPCs, weather

station, or network repeaters (used to extend wireless network range).

Figure 34: NCU network architecture

The NCU communicates to all these devices over a ZigBee network. The NCU serves as a

coordinator for the Network, while the SPCs, weather station, and network repeater are

nodes in the network.


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Figure 35: Solar Tracker Network Architecture The status querying rate for the SPC and weather station are configured through the NCU,

and can go up to every five (5) minutes. SPC and weather station status are saved on an SD

card within the NCU. The SD card within the NCU can save up to one month of SPC and

weather station data.


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7.6 Actuating Method

Architecture 2 utilizes a slew ring and dedicated 24VDC motor to drive the tracker. There is

no power transmission between tracker rows, meaning every row in the solar facility can the-

oretically operate independently. The manufacturer markets this feature as potentially reduc-

ing maintenance costs. The ring gear is attached to the torque tube through a collar and

held in place by bobtail pins. The torque tube is offset from the rotational axis of the slew

ring which, if allowed to free wheel, would cause the assembly to center itself in the stow

position. The slew ring prevents the system from freewheeling, allowing the tracker to stay in

a fixed position when no load is applied to the motor. The tracker center of gravity offset

reduces the mechanical energy required to reach the stow position, but increases the energy

required to move to the tracking extremes.

Figure 36: Slew Drive Assembly


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Architecture 2, like most others, employs dampers to reduce vibrations. Architecture 2 utiliz-

es gas charged oil dampers similar to those used in the automotive industry to support

hoods. While inspecting the project site, it was apparent that dust collects on the polished

shaft of the damper, but is cleaned by a seal during movement. In the inspected site, one

seal had seemingly failed resulting in an oil leak. Gas charged dampers provide significant

dampening force in a compact package, but when they reach a failure point the high pres-

sure causes the damping fluid to spray out and lose all damping force.


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7.7 Design Philosophy (Key Perceived Compromises to Optimize Cost)

Architecture 2’s design philosophy entails simplifying the cabling and/or mechanical infra-structure required to drive the tracking system. To achieve this simplification, the design eliminates the wired AC infrastructure to power the drive systems, and also eliminates the usual wire-based communication method within the tracker field. This reduction in AC infra-structure is achieved at the cost of dramatically increasing the quantity of drive motors re-quired. Instead of one motor to rotate two thousand five hundred and twenty (2520) modules, one motor rotates only ninety (90) modules – a 32x difference). Similarly, the quantity of controllers increases at a 128x to 168x rate; one controller is needed for every four to six drive motors. To avoid distributing energy to power the tracker, a self-sufficient supply is located at every controller alongside batteries to provide the deep current draws required to start the tracking motion.

Safety Factor 7.7.1

Based on the reviewed design documents, Architecture 2 employs a wind climate assess-

ment for each site in order to identify suitable design wind speeds. However, standard wind

design speeds are roughly 100 – 130mph (161 – 209kph) with three (3) second gust per

ASCE 7-10, and configurable for higher wind speeds. The documents indicate that the de-

sign winds at the site will come from thunderstorms, and the following was recommended:

U300 of 90 mph/145 kph for directional wind speeds,

Kd = 1.0 when calculating the velocity pressure q

Sample calculations for a project were reviewed for Architecture 2. The analysis calculates

wind loading on the deflected shape of the module. This analysis appears flawed as the col-

umn at the motor would not have zero (0) deflection. Loading applied in the tables did not

explain how the “X” and “Y” forces were calculated.

The wind study provided wind speed to design to, which also gave various loads for a variety

of solar panel tilt angles. There was no apparent difference taken into account between exte-

rior and interior rows, as was the case for the Architecture 1 design criteria. The study shows

that higher loads are gradually being applied as the distance from the gear box col-

umn/center increases. This approach appears to treating the outermost solar panels on each

row somewhat like exterior columns. But the loading analysis results are flawed, as the outer

most column reactions would not equal the interior reactions. It is unclear whether the torque


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forces were applied to the tube in these load combinations and analysis. The loadings used

in an early part of the report do not match what was utilized for later reaction results.

The site-specific wind study reviewed also has issues. It is suspected the Architecture 2 ven-

dor used a wind tunnel model for their system. The site study may have been a low-cost op-

tion to reduce loads without the cost of doing tunnel modeling on their actual system. The

use of a site study vs. wind tunnel testing may indicate a lack of funding and is not prefera-

ble. The wind study is a very vital piece to ensure that appropriate design loading is accu-

rately being applied to the system and can be used to reduce foundation costs.


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Self-protection 7.7.2

Architecture 2 actively protects itself by using site wind and snow (as appropriate) sensors to monitor ambient weather conditions. Should ambient wind speeds exceed a predetermined set point the tracker will move to a stow position. The active safety stow is 30 degrees into the prevailing wind direction with the leading edge of the module down. In areas with higher snow loads, the active stow is 60 degrees in the prevailing wind direction.

Figure 37: Active Safety Stow


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8. Summary of Perceived Failure Modes and High Risk Components

8.1 Structural Failures

Research has been done exploring the phenomenon of torsional galloping, as was most fa-

mously demonstrated by the Tacoma Narrows Bridge failure in in Washington State, USA

[https://www.youtube.com/watch?v=nFzu6CNtqec]. In single axis trackers where all loads are

transmitted through the torque tube to the center structure, torsional galloping begins at the

extremities of the tracker row where the opportunity for torsional deflection is at its highest.

As the wind passes over the module, pressures are created from vortices generated on the

back (rear) side of the module edge that induce a non-uniform torsional force across the

module edge. This vortices force is similar to what an airplane wing experiences. However,

unlike an airfoil which is designed to minimize turbulence, a tracker’s wing magnifies turbu-

lence as the tracker row deflects. The magnification effect is influenced by the natural fre-

quency of the mechanical system – which is ultimately a function of stiffness and weight that

can be further boiled down to mounting method and attachment point as these points define

the deflection areas. Furthermore, unstable torsional deflection can take on a harmon-

ic/cyclical nature, causing the tracker to twist in two directions. This twisting force looks simi-

lar to a propeller, except the deflection alternates shape. The longer the wind event is ap-

plied, the more the torsional galloping is amplified, which increases the likelihood of failure.

An informative and helpful report on torsional harmonic instability (galloping) was released by

CPP Wind and is available at the below link:

• http://www.cppwind.com/wp-content/uploads/2014/01/Torsional-Instability-of-Single-

Axis-Solar-Tracking-Systems-Rohr-Bourke-Banks-2015.pdf

To minimize cost, mounting manufacturers are constantly striving to lighten the structural

material. Moving to a stow position has long been utilized in the solar industry, along with

scaled wind tunnel testing, to justify reduced loading in wind events. While Architecture 1

relies on a passive mechanical system to prevent excessive torsion buildup and distribute

maximum loads, Architecture 2 uses an active stow algorithm to avoid increased static loads

in high wind events. In recent months, TUV has been called out to multiple sites where mod-

ules mounted on single-axis trackers had been violently removed in a wind event. These

sites included push-pull tracking systems. Visible structural damage has also been found,

http://www.cppwind.com/wp-content/uploads/2014/01/Torsional-Instability-of-Single-Axis-Solar-Tracking-Systems-Rohr-Bourke-Banks-2015.pdf

http://www.cppwind.com/wp-content/uploads/2014/01/Torsional-Instability-of-Single-Axis-Solar-Tracking-Systems-Rohr-Bourke-Banks-2015.pdf


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evidenced by bearing housing damage and flange movement on the extremities of the array.

While it is difficult to directly pinpoint what happened on individual sites, it seems likely that

torsional galloping played a role in the recent failures observed by TUV.

While both Architecture 1 and 2 utilize wind tunnel testing to economize their designs, Archi-

tecture 2 uses load assumptions that are more aggressive than Architecture 1. The resultant

lightening of the structure results in material savings, but also makes the structure more flex-

ible. As such, the structure is more susceptible to damage from shifting resonant frequen-

cies and torsional galloping.


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8.2 Controller Failures

Both Architecture 1 and Architecture 2 are electronically controlled, and as such are suscep-tible to damage from faults. Architecture 1 utilizes commercially rated and available compo-nents. Key elements of the Architecture 1 control system, such as the DC Power Supply and the PLC, have lifecycle data available from the manufacturer. Under normal operation and proper installation, the Architecture 1 control system should prove reliable with failure pri-marily a function of infant mortality of the electrical components. Typical industrial controllers such as the Siemens SPC are designed, manufactured and architected to be reliable. Indus-trial SPC’s meet standards such as IEC 61508 that require these devices to have inherent functional reliability of a specific class. However, if critical components fail, those compo-nents or their equivalents are commercially available with the exception of the control soft-ware. Should the company behind Architecture 1 experience a bankruptcy or for some other reason be unable to service warranty claims, their system would be relatively easy to replace failed components and maintain operation without the manufacturer’s support.

The control system for Architecture 2 is a custom designed printed circuit board that appears to be proprietary. The unit provided for analysis showed signs of significant corrosion within the critical control elements. The conformal coating on the circuit boards started to delami-nate in some areas and as a result conductive paths had formed between key components. The gasketed steel enclosure appears to be trapping moisture within itself, causing conden-sation on the interior of the enclosure. In reaction, the interior was rusting and paint was flaking from corrosion. While both Architecture 1 and Architecture 2 are susceptible to out-side induced failure and infant mortality of components, Architecture 2 seems much more susceptible to environmental induced failure caused by normal operation. Should the manu-facturer of Architecture 2 experience a bankruptcy or for some other reason be unable to service warranty claims, their system would be challenging to replace without manufacturer support. A customer control system would need to be designed, tested, and programed that could assume all the functions included on the printed circuit board. This potential redesign effort would add both time and significant initial expense to the end customer.

Furthermore, the chain of control within Architecture 2 is designed such that if one link failed, operation of a row or block would cease. In the case of an extreme weather event, if the an-emometer or ZigBee Network failed to release a signal for active stow, the system and mod-ules could become damaged.

.


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Figure 38: Architecture 2 Chain of Control

Weather Station

Network Control Unit

ZigBee Network

Row Controller Electronics

Row Battery

Row Motor


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Figure 39: Rusting and corrosion within the steel enclosure

Figure 40: Circuit Board Beginning to Delaminate


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Figure 41: Circuit Board Beginning to Delaminate and Track


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8.3 Bearing Failures

Both Architecture 1 and Architecture 2 rely on bearings to support the rotational movement of the tracker. Architecture 1 utilizes a high-density polymer to support rotational movement, while Architecture 2 utilizes a metal-on-metal bearing surface. For both these systems, bear-ings are critical failure points and as such are made of durable materials. Along with allowing the shaft to rotate, the bearings for both systems are designed to allow axial expansion and contraction.

8.4 Motorized Drive System

Both Architecture 1 and Architecture 2 rely on electric motors to actuate the rotational movement of the trackers. Architecture 1 utilizes a three phase AC induction motor, which is a common industrial component. Much like the control system Architecture 1 uses, the mo-tor would be relatively easy to find and replace. AC induction motors are simple machines and are proven to be highly reliable.

Architecture 2 utilizes a small DC motor for every tracker row. DC motors are typically more complicated than AC motors as an alternating field must be created from a constant field. In DC motors that utilize brushes, a commutator is required that mechanically alternates the DC field as the motor rotates. Brushed DC motors are subject to failure from either physical wearing of the brushes or failure of the commutator mechanism. It is not known if the motor used on Architecture 2 is brushless or not, but as there is no visible access point to maintain or replace brushes, it is assumed to be a brushless DC motor. Brushless DC motors utilize permanent magnets and electronic commutation in order to alternate the otherwise constant field. While these motors remove the mechanical wear points, the commutation is done with onboard electronics. These onboard electronics are essentially acting like an inverter, so the control of brushless DC motors utilizes semiconductor components that can be sensitive to moisture intrusion and heat. However, brushless DC motors are more efficient than their AC counterparts. This means that the motors are typically smaller and employ permanent mag-nets that allow less energy to be dedicated in creating an initial magnetic field.


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8.5 Damping System

Architecture 1 utilizes automotive style dampers designed for a serviceable life damping au-tomotive suspension vibration. The Architecture 1 shocks are not gas charged, and achieve their damping action by pushing a piston with metered holes through a viscous fluid. The shaft of the damping element is protected from sunlight and shielded from dust by a metal shroud. The protective shroud allows the damper to retract while not exposing the polished shaft or shaft seal. A traditional hydraulic damper such as this is a highly reliable part. In the automotive application, it can last a decade or more without replacement. When the compo-nent does fail, the failure is typically through a change in the viscosity of the hydraulic fluid rather than a loss of hydraulic fluid. Ultimately this means the dampening effectiveness is reduced, but not lost.

Architecture 2 utilizes gas charged struts. These damping devices place a high damping ratio in a small package making them very length-efficient when compared to a traditional damper. Gas charged dampers such as those used on Architecture 2 are also utilized in au-tomotive applications, but are typically limited to hood and hatch supports. While these de-vices typically have a long service life on an automobile, they are largely protected from sun-light and road grime. In the application of Architecture 2, the dampers are exposed directly to dust and UV light throughout each operational day. On a deployed system, a build-up of dust on the polished shaft could be clearly seen on all dampers inspected. The shaft seal was performing properly on most dampers as evidenced by a clean shaft where travel had just occurred, followed by a build-up of oily dust where shaft travel had not occurred.

Figure 42: Gas Charged Damper


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One damper out of the twenty-five (25) inspected had failed due to seal failure. When a gas charged damper fails, the hydraulic oil is quickly evacuated by the pressurized gas. This failure mode can be seen by the spray of oil evident on the pile, ground, and damper itself. When gas-charged dampers fail, they lose almost all damping action and are effectively use-less.

Figure 43: Failed Gas Charged Damper


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8.6 Mechanical Drive System

Both Architecture 1 and Architecture 2 utilize gear reduction to rotate the torque tube. The mechanical elements of both systems utilize robust components that minimize required maintenance. While Architecture 1 relies on multiple moving parts to transmit the drive mo-tors energy, very few components in the system require lubrication. However, one critical component to tracker operation on Architecture 1 is a gear box that receives rotational movement from a drive shaft. Architecture 1 gearboxes require no scheduled maintenance over their lifetime. The gearboxes are lubed for life with the internal components immersed in synthetic oil. They also incorporate high quality Viton shaft seals to minimize leaks. Ar-chitecture 2 relies on less mechanical transmission of energy as the motors are coincidental-ly located at the slew ring of each row. The slew ring requires grease lubrication and re-quires re-greasing on a regular maintenance schedule. The mechanical design of the key elements is sufficiently robust so outright mechanical failure is unlikely.

A more likely failure method would be induced through water intrusion into the sealed gear housings of either Architecture 1 or 2. If moisture collects in the gear box the polished metal gear surfaces can flash rust. If the rust occurs between the mating surfaces of the gears or bearings a corrosive weld can be created. In Architecture 1 this could result in a drive shaft failure or, more likely, a gearbox output failure. A drive shaft failure, while possible, is unlike-ly since the shafts are designed to transmit the load of thirty-two (32) rows. However, a drive shaft failure in Architecture 1 could affect multiple rows while a gearbox output failure would affect a single row. A corrosive weld in Architecture 2 could lead to a motor failure from over-loading, affecting a single row. Water intrusion into a sealed assembly is possible when the system seal fails. The larger the seal, the more challenging it is to preserve the integrity. The gear box deployed in Architecture 1 minimizes sealing surface area and effectively min-imizes exposure to elements that might damage the seal such as dirt, UV light, and other physical damage. Architecture 1 also employs covers over every seal on the linked gearbox to prevent direct exposure to any elements. Architecture 2’s slew ring requires a much larger seal with a circumference of approximately thirty-one (31”) inches. The likeliness of water intrusions increases as surface area increases.


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8.7 Storm Event Failure

In recent visits to existing solar plants, the existence of storm damage within PV arrays has

become very familiar to TUV Rheinland. Dramatic structural damage has been explored on

sites with recorded wind speeds that are not significantly higher that design levels, though

the duration of wind speed is much longer than assumed. While TUV has not witnessed a

failure occur in real time, significant analysis of failures has been done including ground level

testing on sites that have completely ceased operation and lost thousands of modules due to

extraction from racking systems by wind. Modules were not the only elements affected by

these wind events, however, as key elements of the tracking structures have also been ob-

served to fail. In some cases the structures have been so badly damaged that portions need

to be completely re-built. In other cases, the mechanical components and/or assemblies

simply deflected enough to collide with the solar modules causing visible and uniquely identi-

fiable results. While it is extremely difficult to understand with certainty the cause of these

failures without witnessing them, the apparent causes seem tied to wind induced harmonics

that cycle and stress the tracker eventually leading to failure. It is important to note that nei-

ther Architecture 1 or Architecture 2 have been observed by TUV in a failure scenario, but

the conditions of failure have the potential to touch any solar site and as such are worth dis-

cussing.

As structural failures are quite real, but as yet difficult to predict, it is worthwhile to discuss

the structural capacity of the two architectures and how they might behave in an unforeseen

event. This discussion will not be reflected in the Cost Priority or Levelized Cost analysis

(LCOE), however, as predicting a failure on an individual site is not currently possible. While

not accounted for in the LCOE calculation, a structural failure can significantly affect the op-

erating costs as well as impact the revenue generated by the system. On a recent project

that TUV visited consisting of a push-pull tracking architecture, the site experienced more

than 6 months of ceased operation which means a complete loss of revenue genera-

tion. The replacement of solar modules numbered in the thousands and the damage to the

structure was extensive. While this example is the most dramatic TUV has encountered, it

demonstrates how serious structural failures can be and how much they can impact the

plants financial performance.

Architecture 1 utilizes an octagonal torque tube supported by Aluminum bearing caps, and

steel H-Piles. Architecture 2 utilizes a round torque tube, steel bearing caps, and steel H-

piles. While the materials are quite similar, a detailed review by an independent structural


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engineer highlighted that Architecture 1 has additional structural capacity when compared to

Architecture 2. While both architectures have been optimized for manufacturing as well as

cost factors, the passive mechanical system employed in Architecture 1 requires additional

structural capacity to support the expected loading during a wind event. This same stow

mechanism also reduces the likelihood of torsional galloping by preventing excessive torsion

buildup and distributing maximum loads. Architecture 2, on the other hand, uses an active

stow algorithm to avoid an increase in static loads by stowing the modules in a horizontal

position. This position, however, increases the likelihood of torsional galloping. Furthermore,

the lower structural capacity makes Architecture 2 less resilient to loading above design wind

loads. While it would be impossible to do a side by side comparison in an identical wind

event, the additional structural capacity of architecture 1 as well as it’s stow methodology will

provide some level of insurance against structural damage. To know how much this added

capacity is worth is difficult to quantify though, which is again why it is simply discussed and

omitted from financial analysis.


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9. Comparative Failure Mode Effects Analysis (FMEA)

The failure analysis is a combination of hazard and operability study (HAZOP) and failure

mode, effects and criticality analysis (FMECA), where the HAZOP helps to identify the poten-

tial failures and FMECA to evaluate the failure consequence and failure control measures.

A hazard and operability study (HAZOP) is a structured and systematic examination of a

complex planned or existing process or operation in order to identify and evaluate problems

that may represent risks to personnel or equipment.

Failure mode, effects, and criticality analysis (FMECA) is an extension of failure mode and

effect analysis (FMEA). FMEA is a bottom-up, inductive analytical method which may be per-

formed at either the functional or piece-part level.


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9.1 Evaluation Methodology

An FMEA study uses three evaluation categories of varying caliber: probability of Occurrence

(O), Cost (C) to repair failure, and Detectability (D). Within each category, levels of severity

are defined and assigned a numerical value. These numerical values are used to evaluate

each individual failure, and then as a whole, used to calculate the cost-priority-number

(CPN)5. The CPN is expressed in a monetary unit and can be used to compare multiple de-

signs.

Figure 44: FMEA methodology for evaluating tracking architectures

5 Shafiee, M.; Dinmohammadi, F. An FMEA-Based Risk Assessment Approach for Wind Turbine Systems: A

Comparative Study of Onshore and Offshore, Energies 2014. Available online: http://www.mdpi.com/1996-1073/7/2/619/htm

FME

A M

etho

dolo

gy Probability of

Occurance (O)

Detectability (D)

Cost (C) of failure

Corrective replacement

Spare parts

Transportation

Manpower

Production Loss


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Equations, Definitions, and Evaluation Criteria 9.1.1

The basis for this evaluation is to compare the Expected 30-Year CPN of Architecture 1 and

Architecture 2. The cost-priority-number (CPN6) is defined as,

1) 𝐶𝐶𝐶 = 𝑂𝐶𝐶𝐶,𝑖 ∗ 𝐶𝑖 ∗ 𝐷𝐶𝐶𝐶,𝑖

And the Expected 30 Year CPN is defined as,

2) 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝐶𝐶𝐶 = 𝐶𝐶𝐶 ∗ 𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝑟𝐿𝑒𝑟𝑟𝑒𝐿𝐿𝐿𝑟𝐿 𝑜𝐿 100𝑀𝑀 𝑠𝑠𝑠𝐿𝐿𝐿 𝐿𝑟 30 𝑠𝐿𝑟𝑟𝑠 3) 𝑇𝑜𝐿𝑟𝑟 𝑅𝐿𝑠𝑅 𝐶𝐶𝐶 = 𝐶𝐶𝐶 ∗ (𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝑟𝐿𝑒𝑟𝑟𝑒𝐿𝐿𝐿𝑟𝐿 𝑜𝐿 100𝑀𝑀 𝑠𝑠𝑠𝐿𝐿𝐿 𝐿𝑟 30 𝑠𝐿𝑟𝑟𝑠 +

𝑟𝑛𝐿𝑛𝐿𝑟 𝑜𝐿 𝑛𝑟𝐿𝐿𝑠 𝑟𝐿𝐿𝑒𝐿𝑒 𝐿𝑜 𝐿𝑒𝑛𝐿𝑒 𝑟 100𝑀𝑀 𝑠𝑠𝑠𝐿𝐿𝐿)

The expected replacement of 100MW system in 30 years is a function of the life expectancy

of each major unit within the tracker system and the number of units needed to populate the

entire field. Because this analysis is based on a 30-year duration, any component rated for a

higher life will only be evaluated at the 30-year mark. The major units and assumptions of life

expectancy are defined below.

• Architecture 1

o Tracker Control (30.5 units, 15 years)

o Site Control (1 units, 15 years)

o Communication and Sensors (1 units, 15 years)

o Motor Drive (122 units, 30 years)

o Transmission, worm gear (3400 units, 30 years)

o Module Row

� Torque Tube (27200 units, 30 years)

� Mechanical End Stop & Bearings (34000 units, 30 years)

� Vibration Dampeners (6800 units, 30 years)

o Steel Structure (3400 units, 30 years)

• Architecture 2

o Tracker Control (3400 units, 15 years)

6 Shafiee, M.; Dinmohammadi, F. “An FMEA-Based Risk Assessment Approach for Wind Turbine Systems: A

Comparative Study of Onshore and Offshore”. Energies. 2014. available online: http://www.mdpi.com/1996-1073/7/2/619/htm


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� Battery (3400 units, 7 years)

o Site Control (8 units, 15 years)

o Communication and Sensors (3400 units, 15 years)

� ZigBee wireless system

o Motor Drive (3400 units, 15 years)

o Transmission, slew drive (3400 units, 30 years)

o Module Row

� Torque Tube (34000 units, 30 years)

� Mechanical End Stop (34000 units, 30 years)

� Bearings (34000 units, 30 years)

� Vibration Dampeners (6800 units, 30 years)

o Steel Structure (3400 units, 30 years)

*Note: Architecture 2’s literature gives the life expectancy of the lithium ion battery as 10

years. However, the life expectancy of a lithium ion battery is mainly dependent upon the

battery’s temperature, state of charge and charge protocol. With a listed maximum controller

operating temperature of 50°C, there is a higher probability of increase life expectancy deg-

radation of the battery. During TUV’s inspection of a site employing the Architecture 2 tracker

design, ~20 batteries had been pulled from the trackers and decommissioned. The site was

less than 10 years old. Therefore, for this evaluation the battery life expectancy has been

reduced to 7 years789.

7 Addressing the Impact of Temperature Extremes on Large Format Li-Ion Batteries for Vehicle Applications,

https://www.nrel.gov/docs/fy13osti/58145.pdf

8 Effect of Temperature on the Aging rate of Li Ion Battery Operating above Room Temperature, https://www.nature.com/articles/srep12967

9 How to Prolong Lithium-based Batteries, http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries


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Probability of Occurrence (O)

The probability of occurrence (Oi) is a positive number between 1 and 5 obtained by rating

each individual sub-unit using the below criteria.

Probability of occurrence (O):

• 5: High (once per day)

• 4: Occasional (once a week)

• 3: Low (once a year)

• 2: Very low (once every 10 years)

• 1: Rare (once every 1000 years)

After all sub-units of the major units are defined, the ratings are summed and divided by the

worst case scenario to obtain the CPN occurrence ratio (OCPN,i). OCPN,i is a positive number

between 0 and 1 defined as,

4) 𝑂𝐶𝐶𝐶,𝑖 = 𝑂𝑖𝑤𝑤𝑤𝑤𝑤 𝑐𝑐𝑤𝑐 𝑤𝑐𝑐𝑠𝑐𝑤𝑖𝑤


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Detectability (D) of Failure:

The detectability of failure (Di) is a positive number between 1 and 5 obtained by rating each

individual sub-unit using the below criteria.

Detectability (D) of failure:

• 5: Cannot detect or detection with very low probability, product is not inspected

(0% - 19% -> no DC)

• 4: Remote or low chance of detection, inspection cycle = quality assurance or due

dilligence inspections (20% - 59% -> no DC)

• 3: Low detection probability, inspection cycle = maintenance crew scheduled

inspection (60% - 89% -> DC = Low)

• 2: Moderate detection probability, inspection cycle = post-event inspection (90%-

99% -> DC = Middle)

• 1: Almost certain detection, automatic daily inspection procedures with asssigned

preventative maintenance (>99% DC = High)

After all sub-units of the major units are defined, the ratings are summed and divided by the

worst case scenario to obtain the CPN detectability ratio (DCPN,i). DCPN,i is a positive number

between 0 and 1 defined as,

5) 𝐷𝐶𝐶𝐶,𝑖 = 𝐷𝑖𝑤𝑤𝑤𝑤𝑤 𝑐𝑐𝑤𝑐 𝑤𝑐𝑐𝑠𝑐𝑤𝑖𝑤


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Cost (C) of Failure:

The cost of failure is a positive value including all costs associated with a failure due to

repair, replacement, logistics, transporation, manpower, and production loss. It is defined as,

6) 𝐶𝑖 = 𝐶𝑅,𝑖 + 𝐶𝑆 + 𝐶𝑇 + 𝐶𝑀,𝑖 + 𝐶𝐶,𝑖

a) 𝐶𝑅,𝑖 = 𝑒𝑜𝑠𝐿 𝑜𝐿 𝑛𝑟𝐿𝐿 𝑤ℎ𝐿𝑒ℎ 𝑟𝐿𝐿𝑠𝑒 𝐿𝑜 𝑛𝐿 𝑟𝐿𝑒𝑟𝑟𝑒𝐿𝑒 𝑜𝑟 𝑟𝐿𝑒𝑟𝐿𝑟𝐿𝑒,

b) 𝐶𝑆 = 𝑒𝑜𝑠𝐿 𝑜𝐿 𝐿𝑒𝑛𝐿𝑒𝑒𝐿𝑟𝑒 𝐿𝑟𝐿𝑟𝐿𝐿𝑟𝑟𝑟𝑒𝐿 𝑒𝑟𝐿𝑤,𝑜𝑟𝑒𝐿𝑟𝐿𝑟𝑒 𝑠𝑒𝑟𝑟𝐿 𝑒𝑟𝑟𝐿𝑠, 𝐿𝐿𝑒., c) 𝐶𝑇 = 𝑒𝑜𝑠𝐿 𝑜𝐿 𝐿𝑟𝑟𝑟𝑠𝑒𝑜𝑟𝐿𝐿𝑟𝑒 𝐿𝑟𝐿𝑟𝐿𝐿𝑟𝑟𝑟𝑒𝐿 𝑒𝑟𝐿𝑤 = 2𝑒𝑒𝑇 ,

d) 𝐶𝑀,𝑖 = 𝐿𝑟𝑟𝑒𝑜𝑤𝐿𝑟 𝑒𝑜𝑠𝐿 𝐿𝑜𝑟 𝐿𝑟𝑠𝑒𝐿𝑒𝐿𝐿𝑜𝑟 𝑟𝑟𝑒 𝐿𝑟𝐿𝑟𝐿𝐿𝑟𝑟𝑟𝑒𝐿 = 𝐿𝑖𝑒𝐿�2𝑒𝐿𝑇 + 𝐿0 + 𝐿𝑖𝑅�, e) 𝐶𝑖𝐶 = 𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝑒𝑜𝑠𝐿 𝑜𝐿 𝑒𝑟𝑜𝑒𝑛𝑒𝐿𝐿𝑜𝑟 𝑟𝑜𝑠𝑠 𝑒𝑛𝐿 𝐿𝑜 𝐿𝑟𝐿𝑟𝑛𝑟𝐿

Where,

i) 𝑒 = 𝑟𝑎𝐿𝑟𝑟𝑒𝐿 𝑒𝐿𝑠𝐿𝑟𝑟𝑒𝐿 𝑛𝐿𝐿𝑤𝐿𝐿𝑟 𝐿ℎ𝐿 𝑟𝐿𝑒𝑟𝐿𝑟 𝑠ℎ𝑜𝑒 𝑟𝑟𝑒 𝑠𝐿𝐿𝐿 𝐿𝑟 𝐿𝐿𝑟𝐿𝑠 ii) 𝑒𝑇 = 𝐿ℎ𝐿 𝐿𝑟𝑟𝑟𝑠𝑒𝑜𝑟𝐿𝑟𝐿𝐿𝑜𝑟 𝑒𝑜𝑠𝐿 𝑒𝐿𝑟 𝑛𝑟𝐿𝐿 𝑒𝐿𝑠𝐿𝑟𝑟𝑒𝐿 𝐿𝑟 $/𝐿𝐿𝑟𝐿

iii) 𝐿𝑖 = 𝑟𝑛𝐿𝑛𝐿𝑟 𝑜𝐿 𝐿𝐿𝑒ℎ𝑟𝐿𝑒𝐿𝑟𝑟 𝑟𝐿𝑒𝑛𝐿𝑟𝐿𝑒 𝐿𝑜 𝑟𝐿𝑒𝑟𝐿𝑟 𝑜𝑟 𝑟𝐿𝑒𝑟𝑟𝑒𝐿 𝐿𝑟𝐿𝑟𝑛𝑟𝐿

iv) 𝑒𝐿 = 𝑒𝑟𝐿𝑟𝑠 𝑟𝑟𝐿𝐿 𝑜𝐿 𝐿𝑟𝑟𝑒𝑜𝑤𝐿𝑟 𝐿𝑜𝑟 𝐿𝐿𝑒ℎ𝐿 𝑤𝑜𝑟𝑅𝐿𝑟𝑒 ℎ𝑜𝑛𝑟𝑠 𝑒𝐿𝑟 𝑒𝑟𝑠 𝐿𝑟 $/𝑒𝑟𝑠

v) 𝐿𝑇 = 𝐿𝑟𝑟𝑟𝑠𝑒𝑜𝑟𝐿𝑟𝐿𝐿𝑜𝑟 𝐿𝐿𝐿𝐿 𝑒𝐿𝑟 𝑛𝑟𝐿𝐿 𝑒𝐿𝑠𝐿𝑟𝑟𝑒𝐿, ( 𝑑𝑐𝑑𝑚𝑖𝑚𝑐𝑤)

vi) 𝐿0 = 𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝐿𝐿𝐿𝐿 𝑟𝐿𝑒𝑛𝐿𝑟𝐿𝑒 𝐿𝑜 𝑠𝐿𝐿𝑛𝑒 𝐿ℎ𝐿 𝐿𝑟𝐿𝑟𝐿𝐿𝑟𝑟𝑟𝑒𝐿 𝑟𝑒𝐿𝐿𝑜𝑟𝑠 𝐿𝑟 𝑒𝑟𝑠𝑠 vii) 𝐿𝑖𝑅 = 𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝐿𝐿𝐿𝐿 𝑟𝐿𝑒𝑛𝐿𝑟𝐿𝑒 𝐿𝑜 𝑟𝐿𝑒𝑟𝐿𝑟 𝑜𝑟 𝑟𝐿𝑒𝑟𝑟𝑒𝐿 𝐿ℎ𝐿 𝐿𝑟𝐿𝑟𝑛𝑟𝐿 𝐿𝑟 𝑒𝑟𝑠𝑠 viii) 𝐿0 = 𝐿𝑒𝑒𝐿𝑒𝐿𝐿𝑒 𝐿𝐿𝐿𝐿 𝑟𝐿𝑒𝑛𝐿𝑟𝐿𝑒 𝐿𝑜 𝑠𝐿𝐿𝑛𝑒 𝐿ℎ𝐿 𝐿𝑟𝐿𝑟𝐿𝐿𝑟𝑟𝑟𝑒𝐿 𝑟𝑒𝐿𝐿𝑜𝑟𝑠 𝐿𝑟 𝑒𝑟𝑠𝑠

For this analysis, only CR,I , CT, and CM,I and CiP will be used to develop the total cost of fail-

ure (Ci) for each unit. CS, the cost of equipping a maintenance crew and ordering spare parts,

is not included.

Since CPN methodology is rooted on a wind turbine study, cost of production for a solar site

is calculated differently. Therefore, cost of production for this assessment is estimated based

on PVsyst modeling of average energy lost during a failure and the production time loss as-

sociated with repair.


– Page 81 of 97 –

9.2 Summary of CPN

The cost priority number builds on the Failure Mode effect analysis in an effort to quantify the

component risk in the system. The CPN takes this analysis one step farther by calculating a

monetary value per major component assembly. The monetary value expressed in the CPN

should not be perceived as a direct replacement cost, but rather viewed as a scalar that iden-

tifies the relative importance of components within a system. Components with a higher CPN

have a higher impact on the operation of the system should they fail, and items with a lower

CPN have a lesser impact.

Just as an FMEA has been done for both architectures, a CPN analysis was done for both

architectures to gage the relative importance to one another. While it would be impossible to

perform a true side-by-side comparison due to the quantity of design elements and compo-

nents that differ, the FMEA and CPN analysis as utilized provides a viable method for tech-

nical comparisons.

Lifetime CPN is a function of the expected replacements within the system’s 30-year lifetime.

This, in turn, is based on the assumed life expectancy of each unit within the tracker architec-

ture. For Architecture 1, the tracker controller has the highest risk of contributing to unsched-

uled maintenance. For Architecture 2, it is the battery pack and motor drive.

Total Risk CPN looks at not only the expected replacements for the system’s lifetime, but

also the total number of each component within the site. This number provides an insight to

the amount of risk each architecture may have if unforeseen faults in design or manufactur-

ing occurred. For both architectures, torque tubes have the highest risk, followed closely by

bearings and mechanical end stops.


– Page 82 of 97 –

Tracker Control Site Control

Communica-tion

& Sensors Motor Drive Transmission

(Gear Box) Torque Tube Bearings & Mechanical End Stop

Vibration Dampeners

Steel Structure

CPN $541 $336 $124 $2,667 $701 $1,707 $701 $347 $1,272

Life Expectancy 15 15 15 30 30 30 30 30 30

Units/100MW 30.5 1 1 122 3400 27200 34000 6800 3400

Expected Replacements 30.5 1 1 0 0 0 0 0 0

Lifetime CPN $16,507 $336 $124 $0 $0 $0 $0 $0 $0

Total Risk CPN $33,014 $671 $248 $325,313 $2,383,712 $46,443,746 $23,822,027 $2,359,916 $4,323,729

Figure 45: CPN Summary for Architecture 1

Tracker Control Battery Site

Control Communication

& Sensors Motor Drive Transmission (Gear Box) Torque Tube Mechanical

End Stop Bearings Vibration Dampeners

Steel Structure

CPN $119 $58 $543 $88 $182 $1,498 $1,562 $392 $436 $209 $3,509

Life Expectancy 15 7 15 15 15 30 30 30 30 30 30

Units/100MW 3400 3400 8 3400 3400 3400 34000 34000 34000 6800 3400

Expected Replacements 3400 13600 8 3400 3400 0 0 0 0 0 0

Lifetime CPN $403,994 $787,976 $4,348 $298,019 $620,231 $0 $0 $0 $0 $0 $0

Total Risk CPN $807,988 $984,970 $8,696 $596,037 $1,240,462 $5,092,923 $53,100,849 $13,317,516 $14,830,012 $1,418,693 $11,929,720

Figure 46: CPN Summary for Architecture 2


– Page 83 of 97 –

10. Lifetime Cost Analysis (LCOE, NPV)

10.1 Energy Estimation

PVsyst was used for system output estimates. A standard list of loss assumptions were de-termined and then used to model system performance of Architecture 1 and Architecture 2. The only difference between the performance models was the range of motion the tracker possesses. Architecture 1 is capable of +/- 52 degrees where Architecture 2 is capable of +/- 60 degrees. Architecture 1 does have a +/- 62 option, but was not included in this anlaysis. The approach of the simulation was not to model an entire 100MW plant, but to model a characteristic piece of a plant that can be derived into a per-unit-number; a per-unit-number in KWh per KW allows for easy extrapolation. As the focus of this analysis is the difference in contributions to performance from the two tracker designs, only the range of motion was changed while all other assumptions were left equal. As the PVsyst models are only different at the extremes of motion, generic models were run for 0 degree tilt and +/- 30 degree tilt. Each technology has a unique energy simulation at the extreme of their range of motion. Each system was also modeled in proper operation as the baseline operational output of each system. Below is a summary of the performance scenarios simulated through PVsyst:

Energy Models Created for Analysis:

1) Architecture 1 proper operation

2) Architecture 1 failure at max tilt, 52 degrees east and west.

3) Generic failure at moderate tilt, 30 degrees east and west

4) Generic failure at no tilt, 0 degrees

5) Architecture 2 failure at max tilt, 60 degrees east and west

6) Architecture 2 propper operation

In order to determine a typical energy output of a failed tracker controller, the failure values at 0 degrees, +/-30 degrees, and +/- tracker extreme were averaged to create an average en-ergy output at a fixed tilt failure. This average was then compared to the baseline operation-al value at proper operation and a delta value was determined for energy lost per unit of fail-ure downtime.


– Page 84 of 97 –

Figure 47: Determination of Average Energy Output at Failure

Scenario for Energy Modeling Total Energy Produced (Whrs)

Total Energy Produced (kWhrs)

Yearly Production Hours

Proper Operation 166721987.87 166721.99 2450.35Failure Extreme West (52 degrees) 139465805.04 139465.81 2049.76Failure Extreme East (52 degrees) 124941857.12 124941.86 1836.30Failure West (30 degrees) 157976779.33 157976.78 2321.82Failure East (30 degrees) 157976779.33 157976.78 2321.82Failure Flat (zero degrees) 166841795.56 166841.80 2452.11

Avg. Failure Energy 149440603.28 149440.60 2196.36Δ(avg failure energy, proper op) 17281384.60 17281.38 253.99

Scenario for Energy Modeling Total Energy Produced (Whrs)

Total Energy Produced (kWhrs)

Yearly Production Hours

Proper Operation 166841795.56 166841.80 2452.11Failure Extreme West (60 degrees) 131209415.50 131209.42 1928.42Failure Extreme East (60 degrees) 113295408.37 113295.41 1665.13Failure West (30 degrees) 157976779.33 157976.78 2321.82Failure East (30 degrees) 157976779.33 157976.78 2321.82Failure Flat (zero degrees) 166841795.56 166841.80 2452.11

Avg. Failure Energy 145460035.62 145460.04 2137.86Δ(avg failure energy, proper op) 21381759.94 21381.76 314.25

Arch

itect

ure

2Ar

chite

ctur

e 1


– Page 85 of 97 –

Figure 48: Determination of Average Energy Loss Due to Failure

203190.88 Average KWh/yr due to Failure556.69 Average Kwh/day due to Failure$33.40 Average Energy Cost per day



Deficit per affected block (A1)

Deficit per affected row (A2)

Deficit per affected row (A1)


– Page 86 of 97 –

Figure 49: Basic System Configuration Used to Determine Per Unit Values


– Page 87 of 97 –

Figure 50: Detailed Loss Values for Thermal Parameter Tab

Figure 51: Detailed Loss Assumptions for Ohmic Loss Tab


– Page 88 of 97 –

Figure 52: Detailed Loss Assumptions for Module Quality Tab

Figure 53: Detailed Loss Assumptions for Soiling Loss Tab


– Page 89 of 97 –

Figure 54: Shade Scene Example Created for Tracker Failure at Fixed Tilt

Figure 55: Shade Factor Table Calculated from Shade Scene Example


– Page 90 of 97 –

10.2 LCOE Equations and Assumptions

Simple levelized cost of energy (sLCOE) is a metric that allows for the comparison of the

combination of capital costs, O&M, performance and fuel costs. As this is intended to be a

comparison of identical systems, this equation does not include financing issues, discount

issues, future replacement of inverters, electrical-balance-of-systems (EBOS), and it omits

degradation costs in general. As LCOE factors in lifetime energy production, the energy out-

put is degraded overtime so energy degradation is included in the calculation.

𝑠𝐿𝐶𝑂𝑠 = (𝑂𝐶𝐶∗𝐶𝑅𝐶+𝑓𝑖𝑓𝑐𝑑 𝑂&𝑀 𝑐𝑤𝑤𝑤+𝐶𝐹𝑐𝑚 𝐶𝑤𝑤𝑤+𝑉𝑐𝑤𝑖𝑐𝑉𝑚𝑐 𝑂&𝑀 𝑐𝑤𝑤𝑤)𝐶𝑤𝑐𝑑𝑖𝑐𝑤𝑐𝑑 𝐿𝑖𝑓𝑐𝑤𝑖𝑚𝑐 𝐸𝑠𝑐𝑤𝐸𝑑−𝐶𝑤𝑐𝑑𝑖𝑐𝑤𝑐𝑑 𝐸𝑠𝑐𝑤𝐸𝑑 𝐿𝑤𝑤𝑤 𝑑𝐹𝑐 𝑤𝑤 𝑆𝑑𝑤𝑤𝑐𝑚 𝐶𝑐𝑖𝑚𝐹𝑤𝑐𝑤 ) 10

Where

• OCC = Overnight Capital Cost, initial cost of a generation technology per kilowatt of

capacity, if it could be conducted overnight (aka the cost of building a power plant

overnight). OCC is a a function of o Soft Cost is an expense item that is not considered direct construction cost

(aka architectural, engineering, financng, and legal fees, etc.) o Labor Cost is the cost of labor to build the power plant o Module Cost is the cost of module needed to build the power plant o Inverter Cost for power conversion equipment o EBOS, electrical balance of system costs o Engineering / Dilligence o Tracker Cost

• CRF = Capital Recovery Factor, a ratio of a constant annuity to the present value of

receiving that annulty for a given length of time. Using an interest rate i, the capital

recovery factor is:

o 𝐶𝑅𝐶 = 𝑖(1+𝑖)𝑛(1+𝑖)𝑛−1

� n is the number of annuitie recieved, 1 � i is interest rate, 10%

10 National Renewable Energy Laboratory, Energy Analysis, Simple Levelized Cost of Energy (LCOE) Calculator

Documentation, http://www.nrel.gov/analysis/tech_lcoe_documentation.html


– Page 91 of 97 –

• Fixed O&M cost per year includes11

o Mowing ($143,000), assumes 1 mow per year for Architecture 1 and 2 mows

per year for Architecture 2.

� For systems employing Architecture 1 without the quick disconnect

drivelines, a 25% increase in mowing cost is assumed (per mow).

� Architecture 1 has a module lower edge to grade clearance of

approximately 18” while Architecture 2 has a clearance of 11“ (a ~40%

difference). Therefore, Architecture 2 will need to be mowed more

frequently and costs are assumed to be +40% more annually.

o Module washing ($300,000), assumes 1 wash per year

o Scheduled Maintenance, Architecture 1 ($400,000)

� Inspection of system

o Scheduled Maintenance, Architecture 2 ($500,000)

� Inspection and slew gear maintenance (lubrication)

• Fuel Cost

o $0.06 per kwh consumed on site for parasitic load from tracking operation

o Tracker energy consumption based on 0.03% of annual energy output for

Architecture 1

o Load from Architecture 2 is considered negligable and omitted from analysis

• Variable O&M Cost = determined by expected repairs over lifetime of project

11 Enbar, N., Weng, D., & Klise, G. (2016). Budgeting for Solar PV Plant Operations & Maintenance: Practices

and Pricing. Sandia Laboratories , 11.


– Page 92 of 97 –

10.3 NPV Equations and Assumptions

Net present value (NPV) is the difference between the present value of cash inflows and the

present value of cash outflows. This analysis includes financing, discount rates, replacement

of inverters, EBOS, etc. NPV is used to analyze the profitability of a projected investment.

Any NPV greater than $0 is a value-added project.

𝐶𝐶𝑁 = ∑ 𝐶𝑖(1+𝑤𝑐𝑤𝑐)𝑖

𝑠𝑖−1 − 𝐶0

Where

• Ci = net cash inflow during the period i

• C0 = total initial investment costs

• n = the number of cash flows

• Rate = rate of discount over the length of one period

To calculate NPV, several assumptions need to be made to determine the net after-tax cash

flow. All of the input variables except for variable / fixed O&M costs and annual energy pro-

duction were kept consistent between the two architectures for a better side-by-side compar-

ison. The following assumptions were made:

System Design

• System Cost for Architecture 1 and 2 is assumed to be cost-competitive at $1.00 /

Watt per DC at a total system size of 100MW. For further details, please see section

LCOE Equations and Assumptions: Fixed O&M costs per year includes.

Performance Inputs

• Annual energy produced was modeled using PVsyst. For further details, please see

section Lifetime Cost Analysis (NPV, LCOE): Energy Estimation.

Tax Assumptions

• Federal Tax Rate = 35%

• State Tax Rate = 9%

• Investment Tax Credit Rate = 30%

• MACRS (Modified Accelerated Cost Recovery System) term = 5 years


– Page 93 of 97 –

Financing

• Percentage financed with equity = 20%

• Debt Interest Rate = 6.9% at a term of 30 years

• Discount Rate = 10%

• Weighted average cost of capital = 8.25%

Other

• Fixed O&M Costs of $8.43/kW at an escalation rate of 2% per year for Architecture 1 and $10.00/kW at an escalation rate of 2% per year for Architecture 2

o Includes system inspection, system maintenance, module washing, vege-tation mowing, etc.

• Variable O&M Cost has been calculated and is a function expected failures within a 30 year life and a calculated cost per failure.

• Inverter replacement cost of $0.08/W, every 15 years

• Insurance cost of $10/kW at an escalation rate of 2% per year

• Land lease of $400,000 per year at an escalation rate of 1% per year

• Wholesale price of electricity at $0.06 / kW at an escalation rate of 1% per year


– Page 94 of 97 –

10.4 NPV and LCOE Results

The goal of the NPV and LCOE analysis was to determine the influence the tracker architec-

ture exerts on the levelized cost of energy over a 30 year period. In order to minimize the

effect of other variables skewing the result, all attributes of the system other than those relat-

ed specifically to the tracker were held constant for Architecture 1 and 2. For Architecture 1,

the LCOE assumptions are stated below.

Figure 56: LCOE Calculation for Architecture 1

sLCOE $0.0275 $/kWhr

System Size DC (MW) 100.00 MWOvernight Capital Cost 142,000,000.00$ $

Soft Cost 0.25$ $/WLabor Cost 0.15$ $/W

Module Cost 0.45$ $/WInverter Cost 0.10$ $/W

EBOS 0.34$ $/WEngineering/ Due Diligence 0.03$ $/W

Tracker Cost Arch 1 0.10$ $/WDiscount Rate (%) 10% %Term (yrs) 30 yearsCapital Recovery Factor 110% %Interest Rate 10% %Number of annuities 1.00 --Periods 30 --Fixed O&M Cost 25,290,000.00$ $

Mowing 143,000.00$ Washing 300,000.00$

Schedule Maintenance 400,000.00$ Capacity Factor 28% %Cost of Electricity 0.06$ $/WFuel Cost 132,318.39$ $Variable O&M Cost $ 61,359.49$ $Variable O&M Cost $/kwh 0.0003$ $/kwhLifetime Energy Estimate with Failures (Year 1) 245,034,049.39 kwhLifetime Energy Estimate with Failures (Year 1) 245,034.05 MWh

Architecture 1

Architecture 1


– Page 95 of 97 –

For Architecture 2, the LCOE assumptions are stated as follows.

Figure 57: LCOE Calculation for Architecture 2

sLCOE $0.0294 $/kWhr

System Size DC (MW) 100.00 MWOvernight Capital Cost 142,000,000.00$ $

Soft Cost 0.25$ $/WLabor Cost 0.15$ $/W

Module Cost 0.45$ $/WInverter Cost 0.10$ $/W

EBOS 0.34$ $/WEngineering/ Due Diligence 0.03$ $/W

Tracker Cost Arch 2 0.10$ $/WDiscount Rate (%) 10% %Term (yrs) 30 yearsCapital Recovery Factor 110% %Interest Rate 10% %Number of annuities 1.00 --Periods 30 --Fixed O&M Cost 30,006,000.00$ $

Mowing 200,200.00$ Washing 300,000.00$

Schedule Maintenance 500,000.00$ Capacity Factor 28% %Cost of Electricity 0.06$ $/WFuel Cost 132,389.02$ $Variable O&M Cost $ 7,961,443.32$ $Variable O&M Cost $/kwh 0.03$ $/kwhLifetime Energy Estimate with Failures (Year 1) 245,164,850.52 KWhLifetime Energy Estimate with Failures (Year 1) 245,164.85 MWh

Architecture 2

Architecture 2


– Page 96 of 97 –

The figure below summarizes the modeled savings over a 30 year period between Architec-

ture 1 and Architecture 2. Architecture 2 offers savings in the following areas: parasitic ener-

gy to power tracker, mowing efficiency, and initial purchase price. While offering the afore-

mentioned savings, the lifetime operation costs are still higher with Architecture 2 due to the

volume of components on site, the additional time to inspect and service those components,

and the increased variable maintenance component from higher predicted failure rates.

LCOE savings over 30 years with Architecture 1 vs Architecture 2 $12,412,895

Percent difference in savings over 30 years with Architecture 1 vs Architecture 2 6.44%

Number of people required to perform Variable O&M for Archi-tecture 1 over site lifetime 0.003

Number of people required to perform Variable O&M for Archi-tecture 2 over site lifetime 1.195

Figure 58: Summary of Margin Differential at 1% PPA Escalator

NPV at Year 30, Architecture 1 $53,141,237

NPV at Year 30, Architecture 2 $50,783,039

Percent difference between NPV of A1 over A2 4.5%

Figure 59: Summary of NPV Calculations


– Page 97 of 97 –

10.5 Closing Remarks

While both systems offer a competitive lifetime cost of operation, Architecture 1 offers a low-er operating cost than Architecture 2. Architecture 1 accomplishes this primarily due to its utilization of robust control components and a robust structural design. It is important to note that severe weather events are omitted from this analysis. Architecture 1 has clear ad-vantages in areas subject to extreme weather such as microbursts or areas with high sus-tained winds. It is also important to note that Architecture 1 uses a control system that is based on commercially available components. Should either company experience a bank-ruptcy or other business failure, Architecture 1 offers a platform in which spare parts can be sourced from the general industry where Architecture 2 would require a control system re-design in order to restore operation.

END OF REPORT

Documents

Risk and Economic Analysis on Two Tracker Architectures€¦ · Risk and Economic Analysis on Two Tracker Architectures Report Number: R1-ART161220 Revision Number: FINAL v1.1