Renewable Energy Vol. 2, No. 6, pp. 543-549, 1992 096(~1481/92 $5.00+.00 Printed in Great Britain. Pergamon Press Ltd

MAXIMUM POWER POINT TRACKING: A COST SAVING NECESSITY IN SOLAR ENERGY SYSTEMS

J. H. R. ENSLIN

Department of Electrical and Electronic Engineering, University of Stellenbosch, Stellenbosch 7600, South Africa

(Received 2 January 1991 ; accepted 30 March 1991 )

Abstraet--A well engineered renewable remote energy system, utilizing the principal of Maximum Power Point Tracking (MPPT) can improve cost effectiveness, has a higher reliability and can improve the quality of life in remote areas. A high-efficient power electronic converter, for converting the output voltage of a solar panel, or wind generator, to the required DC battery bus voltage has been realized. The converter is controlled to track the maximum power point of the input source under varying input and output parameters. Maximum power point tracking for relative small systems is achieved by maximization of the output current in a battery charging regulator, using an optimized hill-climbing, inexpensive microprocessor based algorithm. Through practical field measurements it is shown that a minimum input source saving of between 15 and 25% on 3 5 kWh/day systems can easily be achieved. A total cost saving of at least 1~ 15% on the capital cost of these systems are achieveable for relative small rating Remote Area Power Supply (RAPS) systems. The advantages at large temperature variations and high power rated systems are much higher. Other advantages include optimal sizing and system monitor and control.

INTRODUCTION

Energy, together with the other production factors, forms the primary input to the development and pros- perity of any community, and more specifically to Southern Africa. Energy is a prerequisite for human existence at large, while electric energy, at a moderate cost, is necessary for the development of any devel- oping community. Whilst Southern Africa is blessed with an abundance of insulation, more than 3000 hours sunlight per year, the solar energy is however spread over a large area with a peak power density of less than 900 W / m 2 at midday on a horizontal surface. This implies that to use this large resource of renew- able energy in an economic way, a large area and power converters with ultra-high efficiencies, are necessary [1, 2, 3]. Several solar-electric power con- verter topologies have been introduced, which include heat engines, thermal systems, photovoltaic cells, chemical systems, biomass, wind systems, hydro sys- tems and others [1, 7].

For the generation of electricity in remote areas at a moderate price, sizing of the power supply is of the utmost importance. Photovoltaic systems and some other renewable energy systems, are therefore excel- lent choices in remote areas for low to medium power levels, because of the easy scaling of the input power source [1, 7]. The technologies associated with photo- voltaic (PV) power systems have not yet benefitted from the low costs of large quanti ty production and

therefore the price of an energy unit generated from a PV system is an order of magnitude higher than conventional energy supplied to city areas, by means of the grid supply [2, 3].

In Southern Africa the power supply grid is not extended to remote areas and thus PV systems can be compared favourably to conventional energy systems by means of grid extension on capital, development and energy costs [2, 3]. When energy systems are com- pared in developing areas, photovoltaic and some other renewable energy systems can even be more cost effective than extended grid or diesel generator systems if Maximum Power Point Tracking is included in the total energy system [2, 3, 5]. Hybrid energy topologies can also make PV systems more cost effective in remote areas [2, 8].

Renewable energy systems in stand-alone appli- cations have, however, a disadvantage in the sense that practical systems have an extremely low total efficiency. This is the result of the cascaded product of several efficiencies, as the energy is converted for example from the sun, through the PV array, the regulators, the battery, cabling and through an inverter to supply the AC load [4].

This paper will address the advantages of using dedicated M P P T systems in a single regulating con- verter or even in a compound converter using a single converter system to perform several tasks including battery charging, regulating and inversion [3, 4].

543

544 J. H. R. ENSLIN

INPUT • + +

SOLAR TRACKING MPPT

WIND FOLOWING LIFETIME

CONCENTRATION OPIMIZATION

[~CONTROL i l ~ OUTPUT TO SYSTEM

~ COMPLEX

LOAD

AC/DC

BATTERY I N V E R T E R NONLINEAR

HYDRO CONVERTER REACTIVE

Fig. 1. Structure of a typical renewable energy system.

STRUCTURE OF A RENEWABLE ENERGY

SYSTEM

A typical stand-alone renewable energy system comprises an input section, an energy storage section and an output section, as shown in Fig. 1. The energy from the sun Ei, is converted to electric energy by means of a photovoltaic array (q ~ 18%), a wind generator (turbine efficiency ~35%), or a thermal plant (r /~ 18%) at an efficiency q~. Solar tracking, solar concentration or wind direction controllers can enhance the converted energy by 50% [1, 3].

The electric energy Er, can be passed through a Maximum Power Point Tracker (MPPT), and/or bat- tery lifetime optimization regulator with efficiency qr-,~ 95%. Some of this energy normally passes directly to the load Em, while the other portion is used to charge the battery or other energy storage system E~ with efficiency r/b ~ 80%. The load receives its energy from the storage E , and from the energy system directly Era, through an inverter or converter with efficiency t/c ~ 70%. The load is in general complex, AC, DC and nonlinear. Normally a controller is

necessary to control the energy flow through the power system in an optimal way. The total efficiency of the power system is thus the cascaded product of the individual efficiencies, shown in eq. (1)

,7~" ~," ,7c" [Em + ~b "E~] ( l ) q' - (E, + Era)

For a typical RAPS PV installation this efficiency can be as low as 6-8%. With an effective load and power control algorithm, solar tracking, MPPT and a high efficiency inverter, this total system efficiency can increase to 15% without an excessive increase in capital expenditure. Another method to express this result is in the decrease in the cost of the input power source section by nearly 50%, which is the most costly portion of the system, as shown in Fig. 2. This analysis shows that in a PV system the PV arrays contribute to nearly 60% of the total cost, with the battery stor- age the second major contributor at 30%.

Another possibility to increase the total system efficiency is to combine the different functions of MPPT, battery charging, regulation and inversion of

PV SYSTEM COST DISTRIBUTION 15 kWh/day

BATTERIES

INVERTER O T H L , _ _ . ,

ARRAY (57.0%)

Fig. 2. Cost distribution within a typical PV installation (15 kWh/day) [2, 3].

Maximum Power Point Tracking : a cost saving necessity in solar energy systems

+ s

Rs3

RL

Fig. 3. Schematic diagram of 1.5 kW MPPT regulator.

545

the power into a single converter system. This will in effect increase the total system efficiency q~ dramati- cally, higher than 20%, which compare favourably with other power conversion efficiencies from i.e. coal generation and diesel systems [4].

MAXIMUM POWER POINT TRACKING (MPPT)

Schematic diagram of M P P T converter Since the output characteristics of a PV array, a

wind or hydro turbine or even a diesel generator, show peak power points with solar insolation, cell temperature and battery voltage [1, 2, 4, 5, 10], wind speeds [1, 6], flow rate [1] and torque [8] as parameters, adaptive control algorithms are to be used to utilize the input power source to its fullest capability. Several higher power MPPT systems have been introduced with reasonable reliabilities and efficiencies [2]. A small 500 W-1.5 kW MPPT system was developed with an efficiency between 94-98% [2, 3, 5]. The schematic diagram of the MPPT converter is shown in Fig. 3.

The converter is based on a Buck-Boost converter to optimize in a single design the input and output voltage variations. The system is fully digital con- trolled with an inexpensive microprocessor with a hill- climbing adaptive algorithm [6, 9] to perform MPPT under any input or output voltage variation. The out- put current charging the battery and supplying the load, is maximized under any operating condition. The dynamics of the basic unit is also adequate to perform MPPT in wind turbines [5, 6] and even micro hydro generators. All the current measurements are made with ultra-low resistive current shunts. Other higher order control functions, i.e. battery regulation,

load shedding and system monitor are also performed with the same microprocessor.

Control algorithm of the M P P T converter The MPPT converter is controlled by means of

a 8051 based microprocessor with a PWM output, switching the FET switch S directly. A well known [6, 9] adaptive hill-climbing algorithm is used to stay on the maximum power point of the input source. Figure 4 shows a simplified flow diagram of this algorithm.

BEGIN

L f

[MEASURE[ Us. Is. I L

1 PO : PN DE AY

Fig. 4. Simplified flow diagram of MPPT control algorithm.

546

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~'400

30o 0 m

[-- 2 0 0

100 0

O 22

J. H. R. ENSUN

OPERATION OF MPPT

Pmut3

P, wu~2

T ~ A R T

3'2 PANEL VOLTAGE [V]

Fig. 5. Operation of the 500 W 1.5 kW MPPT converter.

Since the converter is controlled as a current source, input or output voltages are not part of the opti- mization algorithm. The output current is maximized under any variation of input and output parameters, i.e. insolation, cell temperature, battery voltage and load current.

The operation of the MPPT converter is illustrated in Fig. 5. The system seeks the maximum power point of the solar panel from open-circuit voltage to the maximum power point. The time of power point seek, shown in Fig. 5, is within 1.5 seconds, which shows that the dynamics of the system are adequate for any PV or wind energy system.

Industrialized M P P T converter Figure 6 shows a photograph of the industrialized

MPPT converter. The converter also performs the tasks of battery regulation, load shedding, system con- trol and monitor [5]. The optional display unit can display open and short circuit data of the solar panel, cell temperature, state of charge of the battery based on battery open-circuit voltage, power throughput and energy delivered over a 24 hour cycle.

FIELD EVALUATION OF MPPT CONVERTER

The MPPT converter was evaluated through field tests. The output power delivered from the MPPT converter was compared to the output of a standard series linear battery regulator, normally used with smaller remote area power supplies (RAPS).

The experimental evaluation configuration is shown in Fig. 7, and comprises two separate strings of PV panels, a linear series regulator (LR), and the MPPT converter (MPPT), a single 48 V battery bank, Fig. 6. Photograph of industrialized MPPT convertor.

Maximum Power Point Tracking : a cost saving necessity in solar energy systems 547

M P P T EVALUATION

SOLAR i I

ARRAY~

@t

) .,) m ~SOLAR

T ARRAY

| Fig. 7. Setup for comparative measurements between the MPPT and LR converters.

a typical household load curve was implemented. A data acquisition system was used to measure the input voltages and currents, cell temperatures, output cur- rents and battery voltage. The load curve of a typical 3--5 kWh/day remote household was subjected to the evaluation system. Experiments showing the advan- tage of different system topologies and configurations were also measured but is beyond the scope of this paper. The output power from the series linear regu- lator (LR) was compared with the output of the MPPT converter (MPPT). The average daily increase of the energy delivered from the MPPT compared to the linear regulator varied from 16% to 43% with the lowest increase on a clear day, plotted in Fig. 8. The evaluation tests were performed during a time interval of 3 months through mid-summer in Pretoria, South Africa with an average increase over the measured interval of 23%. As shown in Fig. 8, the cell tem- peratures were moderate which resulted in the mini- mal increase in the output. Current field tests are

under way in the Northern Transvaal and Karoo, where cell temperatures of 70°C are encountered.

Even for the moderate cell temperatures of 27- 43°C, the output from the MPPT converter shows an increase of 16% over the output, from the series regulator in Fig. 8 on this particular day. The efficiency of the MPPT converter is high even with relative low output powers (~/MPPT > 94% for Pout > 100 W). The measured efficiency of the linear regulator (LR) was higher than 98%.

Through the charge and discharge cycle of the bat- tery, the battery voltage varies from 46 V to 51 V for the specific load curve. The indicated maximum power point of the 58 W at 25'~C, Solarex semicrystalline panels are given as 17.8 V at 25':C, and 15.5 V at 50°C with a 0 .38%/C decrease in maximum power. These data imply an ideal voltage match for four panels in series as 62 V, and three panels in series as 46.5 V at 5ff'C. Both these configurations are not ideal for the 46-51 V battery voltage variation. With the MPPT

EVALUATION OF MPPT CONVERTER 4X1 (LR) and 2X2 (MPPT) Panels (15.5 V)

160-

.~ PIr t o~ 120- v ~ t I

12L

I 13-

06.00 8.00 10.00 :12.013 ' :14.00 ' :t6.00 18.00 TIME OF DAY (HOURS)

Fig. 8. Output results of comparative tests between the MPPT and LR converters.

548 J. H. R. ENSLIN

Table 1. Summary of cost analysis for LR and MPPT

Equipment LR-system MPPT system

16 panels 12 panels (a) Solar array (60 W, 15.5 V @ $710) $11 360 $8520 (b) Battery (30 kWh) $6700 $6700 (c) Regulator/MPPT $230 $1100

Total $18290 $16320

Cost analysis : 3 kWh/day to the load, 48 V battery bank system.

converter the input panel topology is however irrel- evant to the output battery voltage. To show the prin- ciple, the MPPT converter uses only two panels in series charging the 48 V battery at the maximum power point of the panels. The ideal topology will however result in as high as possible input voltage, typically 100 V, to minimize the cable losses and costs.

COST EVALUATION OF MPPT CONVERTER

The cost breakdown of a typical PV system rated for 3 kWh/day in a dc-load, is shown in Table l, using the South African price structure converted into U.S.S. With an average M P P T efficiency of 95% and an average battery throughput efficiency of 85%, 3.7 kWh/day energy is required from the solar array. Using a load autonomy from battery of 3 days, and a battery charge reset within 14 days, inclusive of full load, will result in a 30 k w h tubular cell battery bank with a maximum of 40% discharge capacity. This will result in a total of 4.6 kWh/day energy required from the solar panel, which imply a peak rating of 0.9 kW of 5.5 sunshine hours per day for a typical series regulator. Using the above-mentioned M P P T con- verter with an average output enhancement of 20%, results in a 0.72 kW peak power solar array. The price of the industrialized M P P T converter is $1100.

Thus using the basic capital components used in such a system, results in an 11% decrease in capital cost under the assumption of an average output increase of 16%. Higher savings can be achieved when larger temperature variations are experienced.

A secondary, but not less profitable, advantage in using a M P P T converter is the ability to control the power flow, and thus the energy storage lifetime can be optimized with the same controller. The PV con- figuration can furthermore be chosen independently from the battery voltage and configuration. Other functions as for example battery regulation, load con- trol, metering etc., can easily be integrated within the same control system [5, 7]. The industrialized POW- E R M A X T M unit includes all the above-mentioned as standard features.

SUMMARY AND CONCLUSIONS

Renewable energy systems can be utilized in remote areas and compare cost competitive with other power supply systems. Fur thermore to implement renewable energy systems cost effectively, without losing reliability, it is necessary to use innovative techniques and appropriate high technology.

This paper described an industrialized MPPT regu- lator, described some evaluation results and some sim- ple cost analysis were performed. It can finally be concluded that MPPT techniques, even for smaller RAPS, can be implemented cost effectively and in some cases are a necessity to size RAPS accurately.

Acknowledgments--The author acknowledges the teamwork from Mr S. J. B. Hartman for the industrialization of the converter and Mr P. D, van den Heever for the field measure- ments both from the Department of Electrical Engineering, University of Pretoria and the financial support from the National Energy Council in Pretoria, with gratitude.

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Maximum Power Point Tracking : a cost saving necessity in solar energy systems 549

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