Denali Commission - acep.uaf.eduacep.uaf.edu/media/56065/ACEP-Final-Project-Report-1-.pdfDenali Commission Emerging Energy Technology Grant An Investigation of Organic Rankine

Denali CommissionEmerging Energy Technology Grant

An Investigation of Organic Rankine Cycle Technology in AlaskaA Project by the Tanana Chiefs Conference and the Univeristy of Alaska Fairbanks

University of Alaska Fairbanks PO Box 755910 Fairbanks, AK 99775-5910 (907) 474-5402 www.uaf.edu/acep

About the Author

The Alaska Center for Energy and Power (ACEP) is an applied energy research group housed under the Institute of Northern Engineering at the University of Alaska Fairbanks. ACEP is serving as the program manager of the EETG program on behalf of the Denali Commission.

A key deliverable for each EETG project is a lessons learned report by ACEP. As the projects deal with emerging energy technology, provid-ing lessons learned and recommendations is critical for understand-ing the future of the technology in Alaska, and the next steps needed in developing energy solutions for Alaska.

ACEP’s technical knowledge and objective academic management of the projects, specifically for data collection, analysis, and report-ing, are vital components to the intent of the solicitation.

Emerging Energy Technology Grant

Emerging energy technology is a critical phase in the development process of energy technology, linking research and development to the commercialization of energy solutions. Although the Arctic possesses bountiful energy resources, the Arctic also faces unique conditions in terms of climate, environment, population density, energy costs, logistics, and the isolated nature of electrical generation and transmission systems. These conditions, challenging under the best of circumstances, making the Arctic an ideal test bed for energy technology. Emerging energy tech-nology provides a unique opportunity to meet Arctic energy needs, develop energy resources, and create global expertise.

In 2009 the Denali Commission, an independent federal agency in Alaska, released a public solicitation entitled the Emerging Energy Tech-nology Grant (EETG). The EETG targeted (1) research, development, or demonstration projects designed to (a) test new energy technologies or methods of conserving energy or (b) improve an existing energy technology; and (2) applied research projects that employ energy tech-nology with a reasonable expectation that the technology will be commercially viable in Alaska in not more than five years.

The following are the 9 projects funded under this solicitation:

Alaska SeaLife Center, Seawater Heat Pump Demonstration Project

Cordova Electric Cooperative, Psychrophiles for Generating Heating Gas

Kotzebue Electric Association, Feasibility of Solar Hot Water Systems

ORPC Alaska, Nenana Hydrokinetic Turbine

Sealaska Corporation, Commercial Scale Wood Pellet Boiler

Kotzebue Electric Association, Flow Battery Energy Storage Systems

Tanana Chiefs Conference, Organic Rankine Cycle Heat Recovery System

University of Alaska, Fairbanks, High Penetration Hybrid Power System

Kotzebue Electric Association, Wales Diesel-Off High Penetration Wind System

For further information, please visit the EETG program website:

http://energy-alaska.wikidot.com/emerging-energy-technology-grant

Tanana Chiefs Conference

The Tanana Chiefs Conference (TCC) formed in 1962 to represent 32 communities in Interior Alaska; this number has since grown to the 42 communities that are currently represented. TCC represents a total population of 86,130 over a total area of 235,000 square miles spanning from central to eastern Alaska. TCC originally submitted the ORC heat recovery project to the Denali Commission for consideration under the EETG solicitation, and is the primary stakeholder in this project.

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An Investigation of Organic Rankine Cycle Technology in Alaska

A Project by the Tanana Chiefs Conference

and the University of Alaska Fairbanks

Recipient:

Tanana Chiefs Conference

EETG Funding: $250,000

Total Project Budget: $304,306

Project Timeline:

January 2010—February 2012

Report OverviewThis report investigates the demonstration of a 50 kW Organic Rankine Cycle (ORC) system designed to generate electricity through the utilization of “waste heat” from diesel electric generators (DEGs). Given the prevalence of DEGs and the high cost of fuel in many Alaska communities, a diverse range of groups expressed interest in the applications of this technology for Alaska. While waste heat is currently recovered for space heating applications in many of these communities, there is need to formally assess the comparative opportunities of recovered heat utilized for electricity production. This report includes an overview of the demonstration project, an analysis of performance and economic data, and a summary of findings relevant to future applications of ORC technology in Alaska.

This demonstration was funded by the Denali Commission Emerging Energy Technology Grant (EETG) program and implemented by the Tanana Chiefs Conference (TCC) and the University of Alaska Fairbanks (UAF). For comprehensive project information, data, and report appendices, please visit the EETG program website at :

http://energy-alaska.wikidot.com/emerging-energy-technology-grant

Project IntroductionThe goal of this project was to investigate the applicabil-ity of a commercial-scale ORC system designed to generate electricity by utilizing waste heat from DEGs in rural Alaska communities. Project tasks included modeling, technology demonstration, and performance and economic assessment.

The technology used in the investigation was a 50 kW ORC system, manufactured by ElectraThermi and known as the Green Machine (GM). The project commenced in January 2010 with initial procurement and modeling activities. System installation commenced in March 2011 and was completed in November 2011. Demonstration activities commenced in December 2011 and were finalized in January 2012. Final project reporting and analysis was completed by the end of February 2012.

The following organizations were involved in this project:

Tanana Chiefs Conference: TCC, the primary stakeholder in this project, is the tribal consortium of the 42 villages within the Interior of Alaska. TCC submitted the ORC heat recovery project to the Denali Commission for consideration under the EETG solicitation.

University of Alaska Fairbanks: UAF is the primary contractor to TCC responsible for fulfilling project activities. The UAF power plant provided the space and utilities required for testing and demonstration. UAF Facility Services assisted with project installation.

Institute of Northern Engineering: UAF faculty and staff led modeling, installation, and data analysis efforts for this effort and are from the Institute of North Engineering (INE), the research department of the UAF College of Engineering and Mines.

ElectraTherm: ElectraTherm is the manufacturer that pro-vided the 50 kW Green Machine. McKinley Services is an ElectraTherm distributor in Alaska and assisted with com-missioning the system.ii

Alaska Center for Energy and Power: The Alaska Center for En-ergy and Power (ACEP), an applied energy research program based at UAF, provided technical support for installation and operation of the ORC unit. In addition, ACEP provided in-dependent project and performance analysis and reporting. This report is the final product of that effort.

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Denali Commission Emerging Technology Grant

Technology Overview

Heat Recovery from Diesel Electric Generators

Alaska is home to many rural communities which are not connected to a larger, centralized electrical grid. The nearly ubiquitous method of generating electrical energy in these communities is the use of DEGs. In a DEG, a diesel engine burns fuel in order to develop mechanical work. The me-chanical work is transmitted through a rotating shaft to an electric generator, which then produces usable electricity. Typical conversion efficiencies of DEGs are on the order of 35%. Heat and frictional losses in the engine account for the remaining 65% of rejected energy, commonly referred to as “waste heat”iii, which is a form of low-grade heativ.

Heat recovery is the capture and utilization of waste heat through heat transfer processes. In a DEG, heat can be recov-ered using either stack heat or jacket water recovery.v The simplest and most common uses of recovered heat in Alaska includes space heating, domestic hot water (DHW) heating, and freeze protection for such things as municipal water supplies.

Stack Heat Recovery: Stack heat recovery directly extracts heat from the hot exhaust port of the diesel engine; the ex-haust is piped through an air-liquid heat exchanger where heat is transferred to a heat transfer fluid (HTF) which can then be used in various applications.

Jacket Water Heat Recovery: Jacket water heat recovery utilizes heat from the internal cooling circuit of the DEG for similar purposes; heated cooling fluid enters a heat ex-changer, transferring heat to a HTF which can then be used in various applications.vi A “marine diesel” is a special class of engine, designed for use in confined spaces such as boat engine rooms, which additionally runs the internal cool-ing circuit through the exhaust manifold, increasing heat recovery.

The amount of heat that can be recovered from a DEG de-pends primarily upon the size of the diesel engine and the resulting exhaust temperature (stack method)vii or outlet temperature of the cooling water (jacket method)viii. Higher temperature recovered heat results in a more efficient con-version to work.

Community Heat Recovery Applications in Alaska

In addition to relying on DEGs for generating electrical energy, many rural Alaska communities depend on heating oil for community heating needs. Using recovered heat for

warming buildings provides these communities with the opportunity to reduce heating costs. For example, the Alaska Village Electric Corporation (AVEC), the primary power pro-vider for 54 villages in rural Alaska, has installed heat recov-ery systems in 49 of their power plants. The recovered heat is used for space heating, typically in schools and community buildings. Given the significant economic value of recovered heat for community heating applications, alternative uses of the recovered heat, such as for electricity generation, are generally not pursued.

As an example of the potential economic value of recovered heat, a 2011 study completed by the Alaska Native Tribal Health Consortium (ANTHC) Division of Environmental Health and Engineering (DEHE) notes that “high energy costs and economic conditions in rural communities challenge the sustainability of water and sewer systems” while “waste-heat recovery efforts are effective at reducing fuel costs in all system types.”ix In 2012, ANTHC completed a heat recovery system in Minto, a village in rural interior Alaska, between the co-located AVEC power plant and the village water treatment plant. Original estimates indicated annual fuel displacement of 5,200 gallons saving $20,800 per year and resulting in a project payback of 9.9 years.x While final fuel numbers from a full year of operation are not yet available, ANTHC estimates that over 5,000 gallons of fuel were saved in 2012; in some months, the water plant did not use any fuel.xi

Community Heat Recovery Considerations in Alaska

Given that a heat recovery system is one component of a larger community-scale energy infrastructure system, there are many factors to consider when planning, designing, con-structing, or analyzing a specific heat recovery system. While it is beyond the scope of this report to discuss all of these factors, several considerations that may inform the findings of this report are noted below.

In Alaska communities, heat recovery using the jacket water method is more common than stack recovery because higher stack temperatures are essential to operation. Removing too much heat from the exhaust reduces the exhaust tempera-ture in the engine stack, creating a higher risk of soot buildup and condensation, which leads to the formation of sulfuric acid and corrosion. Historically, this has led to higher costs for maintenance and operation as well as increased risks, such as plant failure or even fire, which have mitigated potential payback from use of recovered waste heat systems. How-ever, advancements in exhaust heat exchanger design, use of ultra-low-sulfur-diesel (ULSD) fuelxii, and new operational strategies have reduced issues related to corrosion and soot


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problems—thereby potentially in-creasing the opportunity for future utilization.

There are many other factors to con-sider in terms of modern DEG opera-tion, including Tier 4 requirementsxiii, ULSD use, electronic fuel injection and modern engine advancements, and engine sizing and dispatch strat-egies. These factors are important as they impact total system efficiency and the ability to technically and economically apply heat recovery. DEG operational priorities, require-ments, and needs are not always aligned with community heat recovery needs; while there is potentially significant economic value to recovered heat, the affordable and reliable provision of electricity must be prioritized.

Given the role that piping and distance plays in project economics, heat recovery typically must be utilized in close proximity to the heat source. Right-of-way issues are also consequential; the use of utilidors in many rural Alaska communities, as an example, can be logistically constraining. AVEC power plants are typically centrally located within a community with end users in close proximity.

It was mentioned by several organizations interviewed for this report that heat recovery in rural Alaska communities has only recently seen wide-spread adoption and application. Historically, there were significant challenges concerning the operation and maintenance of the systems, understanding and quantifying the value of the recovered heat, and system designs. Technical and policy innovations such as the use of brazed plate heat exchangers and the development of a boil-er-plate recovered heat purchase agreement—which outlines the roles and responsibilities of the provider and end-user and assigns costs to the provision of recovered heat—have been instrumental in the current wide-spread application of systems throughout rural Alaska communities.xiv

Heat Recovery Using the Organic Rankine Cycle

The Rankine cycle is a four-step thermodynamic process commonly used in steam power plants to generate electricity from heat. The four steps are as follows:

1. Pressurization of a working fluid (typically water) using a pump

2. Generation of steam at high pressure, with a boiler/

economizer/super-heater

3. Expansion of steam through a turbine to a lower pres-sure (this is the step where work is generated)

4. Condensation of steam back into water using a cooling source (often a body of water, such as a river or with a cooling tower)

This process is dependent on the ability of the heat source to boil water and generate high-pressure steam, which is not a problem with hot-burning coal or nuclear facilities. However, if the heat source is at a lower temperature than the boiling point of the working fluid, vapor cannot be generated and no work can be extracted. This is the central challenge when generating work from low-temperature heat sources, such as some geothermal heat or waste heat from a DEG.xvi

An ORC system employs the same principles as a normal steam Rankine Cycle system with one key difference: the working fluid. The working fluids used in traditional ORC sys-tems are organic compounds (hence the name of the cycle) which have a much lower boiling point than water and a high stability temperature and high latent heat capacity. There are a number of organic fluids that can be used in ORC units; commonly used fluids include R245fa and R134fa.

The ORC steps are the same as those of the traditional Rankine Cycle. However, there are some differences in terminology:

1. Pressurization of the organic working fluid using a pump

2. Generation of vapor at high pressure using a source of low-grade heat (evaporator)

3. Expansion of vapor through an expander (analogous to a turbine) to generate work

4. Condensation of vapor back into liquid using a cooling source

Figure 1. Rankine Cycle and the Green Machinexv

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One relevant application of ORC technology in Alaska can be found at Chena Hot Springs resort (located 60 miles east of Fairbanks). The resort’s electricity needs are met by two 200 kW United Technology Corporation (UTC) prototype R134 ORC generator and a commercial 280 kW Pratt and Whitney (P&W) ORC generator. The system is heated using low-grade 165°F geothermal heat. Other relevant applications of ORC technology in Alaska include the application of a 280 kW UTC/P&W ORC generator at Cordova Electric Cooperative’s Orca Diesel Generation Plant in Cordovaxvii, a combined heat and power (CHP) biomass/ORC project in North Polexviii, and the use of Ormat ORC units along the TransAlaska pipelinexix.

ORC Opportunities: Utilizing recovered heat to generate electricity and increase system efficiency, in particular when no other end use for waste heat is available, is a tremen-dous opportunity for this technology. One benefit of ORC electricity generation is the absence of net emissions. The heat source for an ORC unit is the exhaust stream or engine jacket water from a DEG. Any emissions associated with the process have already been released regardless of whether or not an ORC unit is present. The advantages of generating electricity from heat recovery, as opposed to simply using the heat, are also worth noting. Electricity is a form of work, which can typically be easily transmitted, used, or convert-ed back into heat. It can also be used year-round whereas heat is primarily demanded during the winter season and is difficult to store.

ORC Challenges: Even though low-grade heat theoretically can be used to generate electricity using ORC technology, it may be too technically and economically challenging to be feasible, in particular for the (relatively) small scale of

application proposed by this project; small-scale commer-cially-available ORC products are either pre-commercial or very limited.xx In rural Alaska communities, for example, the competing economic advantage of utilizing recovered heat for space heating is a significant economic challenge to the application of the technology (although the cost of waste-heat infrastructure may negate cost savings from heating fuel displacement).

Perhaps the largest challenge facing the practical applica-tion of ORC technology is the maximum thermal efficiency of such a process. The second law of thermodynamics outlines restrictions on the maximum thermal efficiency of any de-vice that converts heat into work (generally called a “heat engine”). This limit depends only on two values: the absolute temperature of the heat source and the absolute temperature of the cooling source. The greater the difference between the temperature of heating and cooling sources, the higher the theoretical limit on thermal efficiency. For example, an ORC heat engine that operates between 160°F and 55°F can only be, at most, 17% efficient (gross). Note that a DEG is also a thermodynamic heat engine, though it operates at a much higher temperature.xxi It can, therefore, be made much more efficient than a device operating in an ORC. However, there is no thermodynamic restriction on the fraction of heat that can be recovered and used as heat.

ORC Project SummaryThe primary activity of this project was to investigate the application potential of a commercial-scale ORC system designed to generate electricity through the utilization of waste heat from DEGs in a rural Alaska community setting. Original project tasks included:

1. Evaluation of the performance and maintenance re-quirements of a commercial 50 kW ORC system

2. Development of guidelines for potential ORC system selection, operation, and maintenance

3. Quantitative evaluation of the potential benefits of applying ORC heat recovery technology in rural Alaska communities; specifically the reduction in fuel con-sumption and greenhouse gas emissions

4. A performance and economic comparison of two ORC systems: a 50kW system developed by ElectraTherm and a 250 kW system developed by Pratt & Whitney

Changes to the original proposed project scope were made due to a reduction in available funding from the Denali Com-mission, from $562,497 to $304,306. These changes included:

Any HeatEngineDevice

High TemperatureHeat Source, TH

Useful WorkWMAX

Low TemperatureHeat Sink, TL

WMAX=(1- ) X 100%TL+460°FTH+460°F

Figure 2. Heat Engine Energy Conversion Process


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• A reduction of time for reliability testing, from 1,000 hours to 600 hours

• Elimination of second phase of the project, which con-sisted of field testing of an ORC unit in a village within the TCC region

In addition, changes to the revised scope of work were made due to unforeseen delays outside of the control of the re-searchers. These changes include:

• Delayed evaluation of the 250 kW Pratt & Whitney system, installed at Cordova Electric Cooperative. A supplemental report is to be published at a later date, independent of final project reporting.

• Delayed development of guidelines for potential ORC system selection, operation, and maintenance, given delays in comparative evaluation of the 250 kW Pratt & Whitney system. These guidelines are to be included in the forthcoming supplemental report.

The final milestones and timeline for this project were as follows:

Milestone Description Timeline

ProcurementPurchase of the ORC system, cooling system, heating system, instrumentation, and controls.

01/01/10 – 12/31/11

Installation and Instrumentation

Installation of the ORC unit, electrical harness, heating and cooling loops, instrumentation, and controls.

03/16/11 – 11/11/11

Commissioning and Testing

A 600-hour reliability test and a 50-hour performance test of the ORC system.

12/12/11 – 01/31/12

ReportingIntermediate and final experi-mental reports.

11/01/11 – 02/29/12

Green Machine System Description

The GM is a self-contained ORC power generator designed for use with traditionally low-quality heat sources including stationary internal combustion engines, biomass boilers, co-produced petroleum products, geothermal, and concen-trated solar thermal systems. ElectraTherm has patented a twin-screw expander—used in the GM—which ElectraTherm claims increases the efficiency of this generator.xxii

The capacity of the GM model tested was 50 kW and was purchased at a cost of $119,388.xxiii The GM performance specifications are detailed in Figure 3, above.

Performance Modeling

Before project testing activities commenced, a thermody-namic and heat transfer model of the GM system was created in order to predict performance characteristics as a function of adjustable system parameters such as heat source tem-perature and flow rate.xxvii Modeling was also used to inform test bed configuration, including sensor selection. A diagram of the simplified GM system modeled is shown in Figure 4. There are five primary engineering components in the model: 3 heat exchangers, a variable frequency drive pump, and the expander (turbine).

Test Bed Design and Installation

The GM was installed in the UAF power plant building, which had access to the necessary heating supply and cooling water required for operation. The piping and instrumentation diagram (P&ID) below shows the primary instrumentation, flow control, heat source, and heat sink. The heat source was taken from the power plant steam supply and transferred to a hot-water loop via a heat exchanger. This hot water loop then functioned as the heat source for the GM. The heat sink was municipal water made available from a fire hydrant and

Cooling Water RequirementsFlow Rate [gpm] 220

Temperature [°F] 40 – 110xxvi

Heat Source Input ParametersFlow Rate [gpm] 50 – 200

Temperature [°F] 180 – 300

Working Fluid R245fa (Pentafluoropropane)

Ambient Operations [°F] -20 – 120xxv

Electric Power [kW] 5 – 50xxiv

Figure 3. 50 kW Green Machine Performance Specifications

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connected directly to the GM using a bypass valve to control the temperature of the heat sink; a pump was available on the coolant line to allow for recirculation and temperature control, if needed, while pressure from the fire hydrant pro-vided circulation pressure.

Performance Evaluation

Two tests were scheduled for the GM to obtain data related to its operation and performance. The first, a reliability test, was designed to test the GM under full load and operating capacity to verify its steady-state operation including net power production. The second was a performance test de-signed to test the GM under different input (flow rates and temperatures of heat source and heat sink) conditions to de-termine the performance (e.g., power output and efficiency) of the GM at each of the given input conditions.xxviii

Reliability Testing

A reliability test was conducted with the GM operating at full capacity for 600 hours under constant conditions (Heat source: 160 gpm, 225°F; Cooling source: 160 gpm, 50°F). The primary purpose of this test was to observe the GM under long term operation near the rated screw expander (i.e., gross) output of 50 kW.

At three different times during the reliability test, readings from the temperature gauges and flow meters were taken. The gross output of the GM was 50.1 kW at each interval. The net output was consistently around 46.4 kW at each mea-surement point due to the power requirements of the GM working fluid pump.

During the test on January 3, 2012, an automatic shutdown of the GM occurred repeatedly. Logs indicate the shutdown

occurred seven times. The shutdown occurred because excessive pres-sure was recorded in the high-tem-perature, high-pressure part of the system. The ElectraTherm remote observation facility in Nevada was contacted. They then informed the project team that the expander high-pressure switch was malfunc-tioning. The project team bypassed the switch and the rest of the reli-ability test occurred without incident. It is not clear what potential issues were presented by this modification. However, the GM appeared to oper-ate as expected for the remainder of

the tests. The defective expander high-pressure switch was scheduled to be replaced after conclusion of testing. It was noted by the project team that the frequency of “expander high pressure switch” failure is very rare, and would not be expected during the operational lifetime of the unit.xxix

Based on reliability testing, the project team found the sys-tem suitable for performance testing. In addition, the project team indicated that under normal operating conditions, no foreseen technical problems were expected for long term operation.

Performance Testing

The primary purpose of performance testing was to collect data in order to characterize the GM performance under a range of different heat source and cooling source conditions. This information would provide further data points to inform and refine the preliminary GM system model. The range of selected conditions considered the operational constraints of the test bed heat source and heat sink, and the GM. In addition, the range selection considered the properties of an envisioned heat source and heat sink (i.e., a village-scale DEG and available heat sink). There were four dimensions to the experimental matrix: hot water (HW) temperature, HW flow rate, cold water (CW) temperature, and CW flow rate. The set points investigated in each dimension are listed as follows:

HW Tem-perature [°F]

HW Flow Rate [gpm]

CW Tem-perature [°F]

CW Flow Rate [gpm]

155175195215225

120160200250300

5068

120160200

Figure 4. Model Diagram of the Green Machine


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In each test, the four system variables were adjusted to the desired value and then held at that value for approxi-mately 30 minutes in order to obtain a near steady-state of operation. At that point, performance metrics were recorded including gross power production (kW, neglecting pump re-quirements), pump requirements (kW), rate of heat input (kW), and low temperature heat output (kW). From this data, the net efficiency of the system could be calculated.

Additional relevant information regarding performance test-ing is as follows:

• In calculating the net system efficiency, the project team considered two parasitic loads; the GM pump power consumption and cold water pump power con-sumption. The project team did not consider the hot water pump power consumption assuming the engine jacket water pump will overcome the GM heat ex-changer pressure drop.

• Cold water pump power was estimated from measured hot water pump power consumption at the relevant flow rates (120gpm, 160gpm, and 200gpm).

• When testing at a CW temperature of 68°F, the highest obtainable HW temperature was 215°F. xxx

The most illustrative results of the performance tests are for the extreme high and low values of variables investigated, in particular HW temperature and HW flow rate; a review of these results is as follows.xxxi

Lower Limitations to Heat Energy Utilization

Figure 6 shows the net power and efficiency of the GM at the lowest HW temperature investigated, 155°F, as a function of the HW flow rate. This figure shows the net efficiency in-creasing then decreasing as the flow rate of the heat source increases. The implication of these results is that increas-ing the HW flow rate to extract more energy from a lower temperature heat source was not necessarily advantageous. Although slightly more power was made available, the in-creasing power demands of parasitic loads reduced overall system efficiency.

Upper Limitations to Heat Energy Utilization

Figure 7 shows the gross power production of the GM over the testing range of HW flow rates at the highest heat source temperature investigated, 225°F.

This figure indicates an important performance characteristic

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23

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1819

20

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1314

14

1

2 3 4 5

6

6

8

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9

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4 1112

21

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Green Machine

Condenser

Evaporator/PreheaterHeat Supply Loop

Heat Sink (Hydrant) Loop

Steam Supply Loop

Figure 5. Piping and Instrumentation Diagram for the Green Machine Installation at UAF

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of the GM, namely the satura-tion of energy production at a certain level of heat energy availability. As the HW flow rate is increased, an increasing portion of the available heat energy by-passes the screw expander due to the capacity limitation on maximum GM power output (i.e. the GM can-not use the extra heat energy to generate more than about 50kW power).xxxii In addition, there is a corresponding drop in efficiency as increasing power demands of parasitic loads reduced system effi-ciency.

Of note, the initial point of power saturation and maxi-mum efficiency (in the case of Figure 7, a HW flow rate of 160 gpm) may not necessarily be the operational ceiling. For example, the HW temperature leaving the GM increases, as shown in Figure 8. Because the return temperature is still relatively hot (at 300 gpm, less than 20°F colder than when supplied at 225°F), the resid-ual heat may still be useful

for other thermal purposes. Therefore, although the GM efficiency decreases, it may be more economical to use a high flow rate and then route the returning heat source for use in additional thermal purposes.xxxiii

Effect of CW Flow Rate

The heat sink utilized municipal water made available from a local fire hydrant. For this project, the effect of CW on GM perfor-mance was investigated for two CW temperatures, 50°F

Figure 6. Green Machine Power Production and Efficiency with 155°F HW (CW of 50°F at 120 gpm)

Figure 7. Green Machine Power Production and Efficiency with 225°F HW (CW of 50°F at 200 gpm)

Figure 8. Green Machine Efficiency and HW Temperature Profiles with 225°F HW (CW of 50°F at 200 gpm)


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and 68°F. The selection of 50°F was based on the CW source limitation; the temperature of the hydrant water was nearly constant, nominally 50°F. The second CW temperature of 68°F was obtained using a by-pass line to re-circulate part of the warm water returning from the GM condenser. Base on observation it was found very difficult to control the GM for stable operation for CW temperatures higher than 68°F. The three CW flow rates used in testing were 120, 160, and 200 gpmxxxiv. Of note, these flow rates were only used during testing at a CW temperature of 50°F; variable flow rates were not fully controlled to spe-cific values for CW at 68°F.xxxv

Figure 9 displays the net efficiency of the GM with CW at 50°F for the highest (HW at 225°F and 300 gpm) and lowest (HW at 155°F and 120 gpm) heat availability, the highest and lowest cases for 75 (15x5) experimental permutations with CW at 50°F.

A conclusion of the project team notes that “It was observed that the effect of cold water flow rate on heat input, heat rejection, power output was minimum for a given cold water supply temperature, hot water flow rate and hot water supply temperature.” While this conclusion is verified through test-ing data, results such as those shown in Figure 9 indicate that the CW flow rate has an increased effect on GM performance as heat availability decreases. This follows the thermody-namic principle of a higher temperature difference across a thermal engine normally equating to more efficiency.

There are several important implications of these results for possible heat recovery applications. First, the minimum CW flow rate that can be used before a significant loss in efficiency should be established. Not only should extraneous pumping requirements be avoided as a general practice, but in many locations year-round water sources are not so readily and sufficiently available that they effectively can be wasted. This may become more important when cold water pump power requirements (e.g., open loop cooling and water from deep wells) and cold water consumption (e.g., open loop cooling especially for non-evaporation system) are in-volved. Second, the upper and lower limits on the heat sink temperature before significant loss in efficiency also need

to be established. A heat sink significantly colder than 50°F is possible in many Alaska locations for much of the year, and may be worth utilizing. The upper limit on temperature is also important because it may be possible to utilize even warmer heat sinks, such as the return line on municipal heat-ing loops.

Findings

Project Team Conclusions

The project team found the fuel efficiency of a diesel gen-erator may be improved by about 4% with an ORC system. Based on experimental results, the maximum net efficiency experienced at the heating fluid temperature of 195°F is about 7.4%. If more than 50% of fuel energy is wasted as heat (in jacket water and exhaust), the potential of fuel efficiency improvement is about 3.7%. In addition, the project team de-veloped the following conclusions:xxxvi

1. Application of this 50 kW ORC power unit for waste heat recovery application from stationary diesel gen-sets is expected reliable and feasible in rural Alaska as the maintenance requirement and level of expertise re-quired to operate the power unit is expected minimal.

2. It was observed that the effect of cold water flow rate on heat input, heat rejection, power output was min-imum for a given cold water supply temperature, hot water flow rate and hot water supply temperature.

3. For a given hot water supply temperature with the in-crease of hot water flow rate, the heat input to power unit and system operating power output reached as-ymptotic condition.

Figure 9. Net Green Machine Efficiency for Various CW (50°F) Flow Rates

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4. Performance curves were plotted for heat input to evaporator, heat rejected to cold water, system operat-ing power output, efficiency, payback period and CO2 emission reductions with respect to hot water supply temperature for 50°F and 68°F cold water supply tem-peratures respectively.

The project team, using reliability and performance testing results to refine their preliminary system model, conducted environmental and economic evaluations resulting in the following conclusions:xxxvii

1. For all hot water supply temperatures except for 155°F (or lower), the payback period of less than 6.5 years and 8 years could be achieved for 50°F and 68°F cold water temperatures respectively.

2. For jacket water temperature at 210.2°F, 41.7 kW sys-tem operating power output was achievable with 7.2% efficiency and 2.6 years payback. From our observation of example results, it is possible to generate 45.7 kW system operating power output with 7.4% efficiency and 2.3 years payback using this ORC power unit work-ing on waste heat from stationary diesel engines if the waste heat is from both jacket water and exhaust heat exchanger.

3. Considerable amount of annual emissions and CO2 (GHG) reductions could be obtained if the ORC power unit was operated year round on waste heat from die-sel engines.

4. Considering the 370,000 MW-h of electrical consump-tion of whole Alaska and taking 38% fuel efficiency of diesel engine, nearly 486,800 MW-h of heat energy is present in jacket water and exhaust heat. Using this waste heat, at 7% ORC efficiency, about 34,080 MW-h of electricity can possibly be generated which would increase the diesel engine fuel efficiency to 41.5%, with CO2 reductions of 27,000 short tons/year, fuel savings of 9,214,800 lit/year (2,434,300gal/year), and fuel cost savings of $12,171,500/year.

Overall, the project team found that “An example to evaluate the present ORC system using the field diesel engine data is presented for jacket water heat recovery, and combined jacket water and exhaust heat recovery systems using the developed performance curves. The example shows that the performance data obtained from this experiment can be used to simulate and evaluate the application of this ORC system to Alaska village genset for power output, efficiency, payback period, emissions reductions etc.”

Project Next Steps

Currently, all laboratory testing for the GM has been com-pleted and the project team has commenced field-testing, previously removed from the project scope of work under the Denali Commission funding, with funding of $472,787 provided through the Alaska Energy Authority Renewable Energy Fund.

Field-testing is further investigating performance, opera-tions, and the economics of the GM such as the installation and maintenance costs, fuel savings, and emissions. Tok, a TCC community, is the location of the field test; the powerhouse is owned and operated by Alaska Power and Telephone (AP&T), the utility for Tok. Installation of the GM took place during the summer of 2013. The system was commissioned by McKinley Services and is undergoing demonstration through October 31, 2013 with potential for long-term deployment and data monitoring contingent on initial performance.

Future Project Considerations

The ongoing field demonstration of this project is critical to validate the reliability and performance findings of this project and to further inform the widespread applicability of this technology in rural Alaska communities. In particular, system performance information within the context of a rural community-scale and quality heat source and sink is needed to refine the preliminary model developed through these project activities. Information regarding the installation, op-eration, and maintenance of the system is needed to refine preliminary economic assessments. The following are other considerations for future projects:

Cooling

The GM was tested in an open-loop configuration where cooling water was used once and discarded which demanded significant cold water flow rates, from 160–200 gpm. While the demonstration in Tok has successfully identified an open-loop configuration source (a dedicated well), it is question-able whether many communities of rural Alaska, particularly those in Arctic conditions, can logistically, economically, or sustainably provide such flow rates in an open-loop scenario. A closed-loop configuration of the GM is also possible (e.g. the use of a cooling tower), which could be a suitable op-tion for some applications. In addition, since the laboratory demonstration of the GM, ElectraTherm has developed a re-vised product of similar specifications that incorporates air cooling directly into the overall system. The resulting GM is much larger, approximately the size of a conex, but it is inte-grated and still suitable for transport and delivery to most


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rural Alaska communities. It should be emphasized that the project team’s findings are specific to the original GM unit as tested. Performance may vary for both the closed-loop and air cooled systems.

Economics

The most compelling economic question is whether or not the use of ORC technology can achieve cost parity with alter-native uses of waste heat, such as space heating or domes-tic hot water (DHW) heating. For a case study, consider 100 gallons of diesel fuel being used to power a DEG. It follows that the value of one kW-hour of electricity is about $0.32 using the parameters in the table below. If instead of using this fuel to generate electricity, it is burned for space or DHW heating, the value per kW-hour of heat is only $0.13, assum-ing a heater efficiency of 85%. Assuming waste heat from the DEG could be used for heating, its value would also be $0.13 per kW-hour. It would appear from these numbers that elec-tricity is on the order of two and a half times as valuable, per kilowatt-hour, as heat. When coupled with the seasonality of heat demand, it appears that year-round electricity genera-tion is the more valuable use of diesel fuel.

Economic Assumptions and Calculations

Base amount of fuel considered 100 gallons

Unit cost of fuel $4.50/gallon

Heating value of fuel 39.6 kW-hrs/gallon

Net efficiency of DEG 35%

Electricity generated from fuel 1386 kW-hrs

Theoretical unit cost of electricity $0.32/kW-hr

Efficiency of typical fuel oil heater 85%

Theoretical unit cost of heat $0.13/kW-hr

Option 1 – Waste Heat Recovery

Fraction of fuel emitted as waste heat 65%

Waste heat capture efficiency 85%

Months heat is useable each year 6 months

Energy available for waste heat recovery 2188 kW-hrs

Energy that can be recovered and used 1094 kW-hrs

Value of waste heat recovered and used $142.22

Option 2 – ORC for Electricity Generation

Thermal efficiency of Green Machine 7.5%

Energy generated by ORC 164.1 kW-hrs

Value of electricity generated by ORC $52.51Figure 10. Heat Conversion Process and Energy Balances for Eco-nomic Assessment

35%

10%

55%

Energy Balance for Option 1 - Heat Recovery

Useful Electrical Workfrom DEGUnusable Waste Heat

Useful Heat fromRecovery System

35%

60%

5%

Energy Balance for Option 2 - ORC Process

Useful Electrical Workfrom DEGUnusable Waste Heat

Useful Heat fromRecovery System

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However, the thermal efficiency of the ORC unit tested in this project was about 7.5% at full rated load. In the process of extracting that 7.5% of additional electrical power, the re-maining 92.5% of thermal energy rejected from the DEG is moved from a high temperature (~225°F) to a low tempera-ture (~50°F). While high-temperature waste heat can likely be used for heating purposes, once it is cooled to a lower temperature within the ORC, it has marginal value for any other purpose. The total value of the waste heat recovered and seasonally delivered for use in space or DHW heating applications is about 2.75 times higher than the value of the small amount of electricity generated by the ORC machine. Note that since diesel fuel is being used to generate either electricity or heat, the price of fuel does not affect the cost parity analysis.

Installation of an ORC unit as the primary recovered heat application, under this analysis, would only make economic sense if there was no alternative use for this heat.xxxviii There are a few scenarios where this may be the case, such as a community with a remotely sited powerhouse (e.g., Elim). Another potential scenario is being investigated through the Tok demonstration. AP&T’s power plant currently uses a por-tion of its recovered heat for office and shop space heating. The GM is installed to receive remaining waste heat. Given the estimated payback periods calculated by the project team, this may be a viable economic configuration. There are concerns, however. In winter, more recovered heat is used for space heating, leaving an unknown amount available for the GM (which is secondary to space heating), and coinciding with optimal cooling conditions. In summer, less heat is used for space heating, leaving more available for the GM, but the GM’s cooling needs may be more difficult to meet.

Sizing

Overall, the potential for widespread ORC deployment in rural Alaska communities appears rather limited at this time. Small-scale ORC generators are still an emerging technology; the selection of ORC units for future application is presently made difficult by the lack of commercially available sys-temsxxxix. Many ORC manufacturers advertise small capacity systems but do not have any in production. In terms of appli-cation, the small-scale 50 kW unit tested requires a minimum of 650 kW DEG capacity.xl This may preclude a significant number of rural communities or applications from consider-ing ORC technology. It should be noted that the focus of this report has been specific to community-scale DEGs. There are industrial or commercial applications suitable for ORC use in general, such as those other projects ongoing in Alaska and highlighted previously in this report.

References and Notesi. For more information on ElectraTherm, please visit:

http://www.electratherm.com

ii. McKinley Services is based in Soldotna, Alaska. http://www.mckinleyservice.com

iii. The term “waste heat” is commonly used by many professionals and end-users in Alaska. Inherently this term is not a neutral phrase but rather incorporates several possible pre-conceived implications, such as a valuable commodity, heat, is/was not being captured and applied thus being wasted, or the heat given off by the DEG is/was wasted energy to the process of converting fuel to electricity. The term “recovered heat” is perceived as more technically accurate and neutral in implication, and is thus used by this report.

iv. “Low-grade heat refers to low- and mid- tempera-ture heat with a low energy density that cannot be converted to work efficiently. Although there is no unified specification on the temperature range of low-grade heat, it is understood that a heat source with a temperature below 698oF (370°C) is low-grade. Heat below this temperature cannot be efficiently converted to work using a standard Rankine cycle. The main low-grade heat sources are solar thermal, geothermal, and industrial waste thermal sources” (Modified from source: http://www.eng.usf.edu/~h-chen4/Low-Grade%20Heat%20Sources.htm). For some applications, low-grade heat is considered to be heat which is not resulted from direct combustion of fossil fuels.

v. There are other less common sources of heat recov-ery for DEGs, such as charge air. For the purposes of this paper, jacket water and stack heat recovery are considered.

vi. There are also systems that directly utilize the cooling circuit for heat applications, but are not common in heat recovery systems found in communities of rural Alaska.

vii. Based on “American J. of Engineering and Applied Sciences 2 (1): 212-216, 2009” the maximum combus-tion temperature for a DEG is approximately 2,192°F (1,200°C). From comparative research experience, measured exhaust temperatures ranged from 867°F to 918°F (464°C to 492°C).

viii. A diesel generator set with adequate load (~60-90%) will typically have 195°F jacket water outlet, and


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180°F jacket water inlet. A well-designed cooling sys-tem or heat recovery system should have a 12 - 15°F temperature drop across it, and result in lowering the return water temperature to the engine water inlet. The working fluid in the heat recovery system could be 10°F less than the engine jacket water.

ix. Daniel Reitz, Art Ronimus, and Carl Remley, “Energy Use and Costs for Operating Sanitation Facilities in Rural Alaska: A Survey,” Alaska Native Tribal Health Consortium, Division of Environmental Health and Engineering, October 2011. http://www.anthctoday.org/dehe/cbee.html

x. “Minto, Alaska Heat Recovery Study,” Alaska Energy and Engineering, #ANTHC-09-P52187, December 2010. http://www.anthctoday.org/dehe/cbee/documents/DE-HE-171525-v1-Minto_Heat_Recovery_Analysis.PDF

xi. “Minto Rural Power System Upgrade Project Closeout Summary Report,” Denali Commis-sion, #01296, March 2013. https://www.denali.gov%2Fdcpdb%2Findex .c fm%3FfuseAct ion%-3DAttachments.ShowAttachment%26attachment_id%3D31953%26attachment_path%3D1296%-2520Minto%2520RPSU%2520Project%2520Fi-nal%2520Closeout%2520Summary%2520Report.doc&ei=Mf2LUZGvIqKeiALj3oDgBA&usg=AFQjCNEumT-Fq4VBiazhN0S5VNrRjfyEMSw&bvm=bv.46340616,d.cGE

xii. Ultra-low-sulfur-diesel (ULSD) is a diesel fuel with significantly less sulfur than historic diesel fuel. Its implementation is being mandated by the Environ-mental Protection Agency with progressive roll-outs for various applications in the US from 2006 to De-cember 2014.

xiii. Environmental Protection Agency has put into effect Tier 4 regulations for all new diesel generators. Tier 4 regulations call for a 50-90% reduction in partic-ulate emissions as well as up to a 90% reduction in nitrous based oxides emissions. Any modification to the exhaust system of a Tier 4 DEG will result in the voidance of its Tier 4 status, which is now required for all new diesel generators capable of producing more than 125 kW of power. This may limit heat recovery applications for new DEGs to jacket water heat recov-ery only.

xiv. Alaska Village Electric Cooperative (AVEC) Recovered Heat Utilization Policy. AVEC and AEA have both bro-kered recovered heat sales agreements. AVEC has a general multipurpose agreement that it applies to all

the communities that it provides recovered heat for space heating too. AVEC can charge up to 50% of the avoided fuel cost saved by users by using their recov-ered heat. In the city of Hoonah, AEA helped to write a document for the power plant stating that they could charge up to 50% of the avoided fuel cost as well.

xv. Note that the GM system depicted in this figure uti-lizes an air cooling system as opposed to a plate-and-frame heat exchanger, as tested.

xvi. See note IV.

xvii. ftp://ftp.aidea.org/RENEWABLE%20ENERGY%20FUND/RFAOctober08/22_OrcaPlantEfficiencyUpgrade_Cor-dovaElectricCooperative/C%20-%20AEA%20ORCA%20Application.pdf

xviii. http://energy-alaska.wikidot.com/north-pole-chp

xix. Ormat Energy Converters, ranging from 200 to 3000 kW, have been applied to remote applications in-cluding along oil pipelines to produce power for valve operation and cathodic protection. http://www.ormat.com/research/papers/organic-rankine-cycle-pow-er-plant-waste-heat-recovery

xx. Typical commercially available, industrial scale ORC units range from 200 kW to 22 MW.

xxi. Diesel exhaust gas temperature can range from 600-1100°F while cylinder combustion temperatures can be several thousand deg. F when the engine is oper-ating at rated load.

xxii. Langston RK. Power Compounder, U.S. Patent 7,637,108, Dec. 29, 2009. Oxner, AJ, Langson RK, Gas Pressure Reduction Generator, U.S. Patent applica-tion 2012/0169049. Most existing screw devices for heat energy to power conversion (or vice versa) are designed for compressors. The GM expander has a different screw profile (a new design) for effective conversion of thermal flow energy into power.

xxiii. Originally, the GM was to be a 65 kW system; how-ever, the system delivered for testing was de-rated to 50 kW due to a factory recommendation from known safety concerns with system over-speed and plumb-ing (which were subsequently addressed by ElectraTh-erm). Although the instrumentation components were specified with a 65 kW system in mind, this did not have any effect on actual testing.

xxiv. The GM will by-pass extra heat flow to constrain the maximum output to 50 kW. Technically, the machine can produce down to 5 kW; below this output, the ma-

chine turns off. For practicality, however, the project team recommends that the machine turn off when-ever the system net output is less than pump parasitic power.

xxv. GM notes that extreme operating conditions require the specification of option system equipment.

xxvi. The starting the GM (not regular operation), the min-imum temperature difference between the hot input and cold input is 80°F

xxvii. The heat transfer characteristics of the heat exchang-ers were assumed based on their physical design. The fluid within the GM was modeled as refrigerant 245fa. Other key assumptions include (1) all the ORC heat exchangers i.e. evaporator, pre-heater and condenser, are 100% efficient, (2) the quality of refrigerant out of the evaporator in the ORC system is controlled, (3) the quality of liquid out of pre-heater and condenser are saturated liquid and (4) the isentropic efficiency of screw expander and pump (within the ORC system) are taken to be constant at 78% and 70% respectively.

xxviii. The reliability and performance tests were completed a few weeks after the expected date (late January 2012) due to cold weather conditions affecting the availability of cold water as well as minor delays in instrumentation.

xxix. Assessment based on the professional experience of personnel involved in testing. For more information, Please see the final report on the project website, http://energy-alaska.wikidot.com/emerging-ener-gy-technology-grant

xxx. System testing at a cold water temperature of 68°F was supplemental to testing performed under the De-nali Commission project and occurred after February 2012. The inability to test with HW temperature over 215°F was due to reduced pressure of the test bed steam heat source, resulting from a reduction in load requirements for the UAF power plant as summer was approaching.

xxxi. Of note, these case studies only consider testing at a CW temperature of 55°F. See note xxxv for more information.

xxxii. As mentioned above, the unit tested was a 65 kW machine de-rated to 50 kW. The system was designed to limit power generation to 50 kW by by-passing ad-ditional heat flow. These results simulate the actual driver of peak power production, namely the perfor-mance saturation of the system heat exchanger.

xxxiii. It should be noted that although more heat may be available for secondary use, removing additional heat from the loop will lower the return temperature to the GM, which may impact system performance.

xxxiv. These flow rates were selected based on the minimal and critical pump power requirements (relative to the power generated by GM).

xxxv. Due to effect of the flow rate on the GM performance is observed minimal, variable flow rates were not fully controlled to specific values or CW at 68°F.

xxxvi. Avadhanula, V., Lin, C., and Johnson, T., “Testing a 50kW ORC at Different Heating and Cooling Source Conditions to Map the Performance Characteristics,” SAE Techni-cal Paper 2013-01-1649, 2013, doi:10.4271/2013-01-1649. http://papers.sae.org/2013-01-1649/

xxxvii. Ibid.

xxxviii. A caveat to this may be if the only alternative use for heat is an application that requires low-grade heat, such as preheating for DHW. It is possible that such need could still be met with the incorporation of an ORC unit.

xxxix. The final project report has generated a detailed market survey. Please see the project website, http://energy-alaska.wikidot.com/emerging-energy-technolo-gy-grant, for more information.

xl. The 650 kW figure comes from dividing 50 kW by the ORC efficiency (50 / 0.075 is approximately 650 kW).

Documents

Denali Commission - acep.uaf.eduacep.uaf.edu/media/56065/ACEP-Final-Project-Report-1-.pdfDenali Commission Emerging Energy Technology Grant An Investigation of Organic Rankine