IEEE Senior Mrmbtkr. Monji G . Jaboori. IEEE Student Member. Elect r ica 1 and Computer Engineering Department. Col lege of Engineering and Petroleum.
IEEETransactionson Energy Conversion,Vol. 4, No. 3, September 1989
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Optimal Solar Array Configuration and DC Motor Field Parameters for Maximum Annual Output Mechanical Energy Mohamed M o s t n f a S n i e d I E E E Senior Mrmbtkr
Monji G . Jaboori IEEE Student Member
Elect rica 1 and Computer Engineering Department Col lege of Engineering and Petroleum Kuwdi t University P . 0 . Hox 5969 1 3 O f i O Safat KUWAIT Abstract: This paper presents a procedure for the proper design of systems comprising photovoltaic solar generators directly supplying DC motors. The method is based on maximizing the total annual g r o s s mechanical energy delivered to a mechanical load with a given torque-speed characteristic. The seasonal variations of both the insolation level as well a s the ambient temperature are taken into account. From the required rated output power and the voltage-current characteristic of a single solar cell, the total number of cells can be directly determined. The two unknowns affecting the objective function (the total annual gross energy) are the number of panels in series, o r equivalently, the number of strings in parallel, as well as the motor field constant (for separatelyexcited motors), or the armature-field mutual inductance (for series motors). Using an optimization technique, it is then possible to determine the array configuration, and accordingly the optimal motor voltage and current ratings, as well as the motor field parameter that yield the maximum annual gross mechanical output energy for given annual radiation and ambient temperature variation curves. The proposed technique is applied to two practical case studies. and the resulting optimal design parameters are compared with those available in the literature. A comparison is also made between the optimal separately-excited and series motor design alternatives regarding the annual gross mechanical output energy. KEYWORDS: Photovoltaic Systems, DC Motors, Design Guidelines, Matching, Optimal Design.
Depending on the end use as well as the application. there are many procedures f o r utilizing the solar radiation. They extend from the direct heating of water for domestic applications, to the most sophisticated solar arrays needed for space satellites. This paper deals with the utilization of solar energy, after its conversion in a photovoltaic solar array, to supply 'direct current motors used to operate pumping loads. This represents one of the promising applications suitable for farm irrigation and other similar situations. The proper matching of the solar array to the 1)C motor is one of the crucial issues in such applications. Perfect matching represents the ultimate objective in terms of getting the maximum utilizable output mechanical energy. There a r e several factors affecting the operating points of this array/motor system, and accordingly,enhance o r offset the matching condition. Most important are: 1.
The strongly nonlinear and radiation-dependent voltage current characteristic of the solar cell.
2.
The array configuration, i.e. the number of cells in series and strings in parallel.
3.
The type of mechanical load, as expressed by its torque-speed characteristic .
4.
The type of DC motor used, i.e. separatelyexcited, series or shunt motor.
5.
'The c e l l thermal conditions, particularly the change in the ambient temperature leading to drifts in the cell characteristic and hence the system operating point.
6.
Although there is an optimal operating point reyarding the electrical power delivered by the array, it is impossible to work at these radiation and temperature-dependent points all the time.
7.
The above mentioned optimal operating points in terms of maximum electrical power do not coincide with the optimal points regarding the mechanical output power. This is mainly due to the copper losses in the armature circuit.
INTRODUCTION Extensive research and development activities are currently being conducted all over the world in the area of utilizing renewable energy resources. Despite the recent drop in the o i l pricrs. rompartxi to those prevailed just after the 1973 embargo, there i s R I I awareness of the necessity o f ndtrptjng these available supplies. not only because o f the economical burden and political insecurity o f the oil supply. but a l s o due to the fact that oil should be kept primarily € o r the use as an indespensible raw material in some key petrochemical industries. One of the most promisill,? renewable resources is the solar energy.
A paper recomiended and approved 89 'dM 02r)-9 EC by the IEEE Energy Development and Power Generation Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1989 'dinter Yeeting, New York, New York, January 29 - February 3, 1989. Yanuscript submitted June 20, 1988; nade available for printing December 21, 1988.
One of the possible procedures to achieve good matching over a wide range of insolation levels and ambient temperature fluctuations is t o introduce a power control or conditioning unit such as DC transformers between the solar array and the DC motor. However, the increase in the output energy will be, generally,
