Solar cells and modules. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Solar cells and modules. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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The method of obtaining electricity from sunlight has long been known. The phenomenon of the photoelectric effect was observed for the first time by E. Becquerel in 1839. But this phenomenon was not paid attention until 1873, when W. Smith discovered a similar effect when a selenium plate was irradiated with light. Already in the twentieth century. the silicon solar cell was invented, which by the 50s had reached a high degree of perfection.

The simplest design of a solar cell based on single-crystal silicon is shown in Fig.1. At a shallow depth from the surface of the p-layer silicon wafer, a pn junction with a thin metal contact is formed.

A solid metal contact is applied to the back side of the plate. When a solar cell is illuminated, the absorbed photons generate non-equilibrium electron-hole pairs. The electrons generated in the p-layer near the pn-junction approach the pn-junction and are carried out to the n-region by the electric field. Similarly, excess holes created in the n-layer are partially transferred to the p-layer. As a result, the n-layer acquires an additional negative charge, and the p-layer acquires a positive one.

The initial contact potential difference between the p- and n-layers of the semiconductor decreases, and a voltage appears in the external circuit. The negative pole of the source corresponds to the n-layer, and the positive pole to the p-layer.

Unlike other current sources, the characteristics of solar cells depend on the amount of light falling on the surface. For example, an incoming cloud can reduce output power by more than 50%. In addition, solar cells have a spread in parameters, so they need to be sorted by output current.

By loading the element, it is possible to plot the dependence of the output power on the voltage (Fig. 2). Peak power corresponds to a voltage of 0,47 V.

Thus, in order to compare solar cells with each other under the same conditions, it is necessary to load them so that the output voltage is 0,47 V. Matched cells must be soldered in series to obtain a higher voltage or in parallel to obtain a higher current. You can also use a series-parallel connection.

An important point is the temperature regime. When the element is heated one degree above 25°C, it loses 0,002 V. For comparison, Fig. 3 shows a family of curves of the current-voltage characteristic for temperatures of 25 and 60°C. On a bright sunny day, the elements heat up to 60-70°C. This is the main reason for the drop in the efficiency of solar cells (usually it is 10-16%). An element measuring 100x100 mm can therefore generate 1...1,6 watts.

The connection of solar cells in parallel and serial chains is called a solar module. When used to charge batteries with a nominal voltage of 12 V, as a rule, 36 solar cells are required, which gives 16 ... in the charger.

All photovoltaic systems can be divided into two types: autonomous and connected to the electrical network (the latter give excess electrical energy to the network). The autonomous system consists of a set of solar modules placed on a support structure, a battery (battery), a battery charge-discharge controller, and connecting cables. If the consumer needs to have an alternating voltage, then an inverter converter of direct voltage to alternating voltage is added to this kit.

The calculation of the FES includes: determining the nominal power of the modules, their number, connection schemes, choosing the type and capacity of the battery, the power of the inverter and the charge-discharge controller.

The power of consumers is indicated in the product data sheets. The inverter power must be selected based on the total power of consumers multiplied by 1,25. It should be borne in mind that some consumers at the time of start-up consume power many times more than the nominal one (electric motors).

The nominal range of inverters is 150, 300, 500, 800, 1500, 2500, 5000 W. For powers above 1 kW, the station voltage is chosen at least 48 V.

Determination of battery capacity. Battery capacity is selected from a standard range of capacities. And the calculated capacity is obtained by dividing the total power of consumers by the product of the battery voltage and the depth of discharge of the battery. For example, if the total power of consumers is 1000 W, incl. per day, and the depth of discharge of a 12 V battery is 50%, then the calculated capacity will be 1000 / (12x0,5) = 167 A.h. This calculation is made for the condition when all days are sunny.

Determination of the total power and the number of solar modules. The average value of solar radiation for different latitudes and different months of the year are given in meteorological tables. For example, for a latitude of 50°, the value of solar radiation in July is 180 kWh/m2 when the site is oriented to the south at an angle of 40° to the horizon. This means that on average the sun shines in July for 180 hours (6 hours per day) with an intensity of 1000 W/m2.

The Pw module will generate the following amount of energy during the selected period:

W = k Pw E/1000,

where E is the value of insolation for the selected period; k is a coefficient equal to 0,5 in summer and 0,7 in winter (it takes into account the correction for the heating of elements in the sun). For example, with a module power of 1000 W and E = 180 kWh/m2, it will generate 90 kWh of electricity in July.

Based on these data, you can calculate the total power of the modules and, dividing it by the power of one module, calculate the number of modules.

See other articles Section Alternative energy sources.

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