An Integrated Maximum Power Point Tracker for Photovoltaic Panels Wernher Swiegers Department of Electrical Engineering University of Stellenbosch Stellenbosch, 7600 South Africa [email protected] Abstract - This paper proposes a maximum power point tracker (MPPT) for a photovoltaic panel, that is to be integrated with the panel during manufacturing. The MPPT is inexpensive, efficient and has few components that serve to increase the MPPT’s mean time between failures (MTBF). The MPPT uses an inexpensive micro-controller to perform all of its functions. This includes maximum power point tracking, series battery voltage regulation, sensorless short circuit protection of the MPPT’s converter and intelligent shutdown and wakeup at dusk and dawn. The MPPT can source 10 A to a 6 V - 36 V lead-acid storage battery and can be connected in parallel or series with other MPPTs. The MPPT may be easily configured to perform output voltage regulation on passive and water pumping loads. It could also control the actuation of a diesel generator in a hybrid remote area power supply (RAPS). Energy transfer enhancements of up to 26%, compared to solar panels without MPPTs, have been measured. The complete component and materials cost of the MPPT is approximately 2’8% of the cost of photovoltaic panels with a peak power rating of 154 W. The integrated MPPT also consumes no stored energy at night. 1. INTRODUCTION Photovoltaic panels convert incident solar energy into electrical energy. The capital cost of solar panels is high. Solar panels, however, become an economically viable source of energy for areas remote from existing electricity distribution networks [ 11. It is necessary to utilise the solar panels maximally as they typically form the most expensive component of a photovoltaic remote area power supply [2]. To this end, maximum power point trackers (MPPTs) have been introduced. MPPTs are controlled power electronic converters that ensure that a photovoltaic panel or array always deliver maximum available power. It is important that MPPTs be efficient, reliable and inexpensive or no benefits will result from their use. The electrical characteristic of a solar panel is a function of the intensity of solar irradiation and the operating temperature of the solar panel. An increased insolation level and a decreased panel temperature result in higher solar panel power output. Fig. 1 shows a typical set of a solar panel’s temperature related electrical characteristics. It is clear that a panel with characteristics as in Fig. 1 would not deliver maximum power if it were directly connected to a 12 V storage battery. This is typically the case with standard RAPS, using regulators to connect the load through low impedance output cables. 0-7 803-4756-0/98/$10.00 1998 IEEE Johan H.R. Enslin Department of Electrical Engineering University of Stellenbosch Stellenbosch, 7600 South Africa [email protected] EFFECT OF CELL TEMPERATURE ON POWER 60 W Panel 60 50 40 8 3 30 8 20 10 E -d-L-d . Oh 6 8 10 12 14 16 18 20 22 24’ Panel Voltage [VI Fig. 1. Electrical Characteristics of a Photovoltaic Panel, [3] MPPTs remove the mismatch between the load and maximum power operating point of the photovoltaic array [4]. The load clamps the output voltage of the MPPT’s converter (Buck, Buck-Boost, Boost, Chk etc.). By controlling the converter’s duty cycle, the input voltage to the MPPT may thus be controlled to the operating point where the solar panel or array delivers maximum power. The reliability, efficiency and cost of a single higher power MPPT are seen as its main drawbacks. It is thus proposed that small, low power MPPTs be integrated with every solar panel or every second solar panel produced. The solar panel without the MPPT can be connected as a “slave” to the “master” unit with a MPPT. The MPPT thus provides power point tracking for both panels. the The integrated MPPTs are fully modular in nature. Should the consumer’s energy requirements increase, more of the solar panel MPPTs may be connected in parallel to the existing units. Tis holds two advantages: the overall efficiency of the conversion stage is not reduced, as the converter efficiencies are not cascaded. Secondly, the power ratings of the integrated MPPTs are always ‘‘just right”. With a single high power MPPT, the power rating of the converter is either not fully utilised, or, as soon as the photovoltaic array is expanded the rating of the converter might be just not enough. In the current drive by Renewable Energy for South Africa, a division of the Central Energy Fund, to electrify remote area residences [SI with one or two solar panels, the expensive high power MPPTs are oversized and not economically viable. The 40low power integrated MPPTs thus become viable products. 11. DESIGN OF THE MPPT’S HARDWARE It was decided to use a Buck converter as the integrated MPPT’s converter. The Buck converter features low ripple and switching device currents compared to other converter topologies. Its circuit diagram is shown in Fig. 2. From Control1 4 I I A b - - Fig. 2. Circuit Diagram of the MPPT’s Converter As the maximum power point voltage of typical solar panels is designed to be close to the battery voltage used, two or three solar panels have to be connected in series to obtain a high enough input voltage for the Buck converter. Another option is to reconfigure solar panels during manufacturing to be higher voltage sources. More solar cells would be connected in series and less strings in shunt during manufacturing of the panel. Such a high voltage solar panel has been constructed by Africa Solar for this project. It is based on a standard 77 W panel and has an open circuit voltage of 60,02 V @ 25 “C, 1 kW/m2. The Buck converter may always maintain chop down functionality from the higher voltage source provided by this panel. A unique and simple power supply is implemented for the MPPT. The circuit diagram is shown in Fig. 3. To dc Bus Dl I!;, Fig. 3. MPPT’s Power Supply This power supply holds several advantages. As the input voltage to the linear regulator is the difference between the source and battery voltage, the power supply may be
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