Silicon Solar Cell

Reproduced from "Rural Energy" 1989/2 FAO, Bangkok (summarised)

Fig 1 Structure & operation of solar cell

Module and Array
Several solarcells when connected in series and encapsulated within framed glasses form a module as shown in Figure 2.
The purpose of fanning modules and arrays is to increase the capability of providing higher current/voltage output for a single cell.

FIGURE

Current-voltage Curve

A current-voltage curve for a single cell is shown in Figure 3. The curve cuts the voltage axis at open-circuit voltage and this is about 0.45 volt. It cuts the current axis at short-circuit current the value of which depends on a few factors, namely, the incident light intensity, and the cell-area. At the bend or "knee" of the curve is the maximum-power point.

Fig 3 - Characteristic of a Solar Cell

Broadly photovoltaic solar systems can be divided into the following:

A simple system

This consists of an array and load with no control or a minimum of control. A minimal of control is desired in most cases to smooth or optimise the fluctuating output resulting from the solar radiation fluctuation. An example of this type is the Balik Palau protovoltaic system which operates two DC pumps in parallel. This simple control consists of two main parts, namely a cut-in or cutout of the second pump when the operating voltage is above or below 35 volt and to cut-out the pump operation when the water level falls to just above the pump inlet.

A DC/AC System

The electricity obtained by conversion of solar energy is a direct current supply and is changed into an alternating current by an inverter. Generally, for continuous operation, a battery back-up sub-system is incorporated. A control unit for such an operation is essential and may be complex depending upon the application type. A schematic of a DC/AC photovoltaic systems is shown in Figure 4.

FIGURE

A Tracking System

Concentration of sunlight increases its intensity and thereby increases the output for a given application requirement. Also, it reduces the number of photovoltaic modules and thereby cuts the cost of solar cells to a certain extent. One needs to consider the trade-off in the cost of cells and the automatic control sub-system which can be quite sophisticated. Low concentration using booster mirrors is applicable in climatic regions where the ratio of diffusion to direct solar radiation is high. Whereas Fresnel lenses allow capability of much higher concentration, this operates only during direct beam radiation. The solar cells need to be specifically designed to optimise the input/output.

(Source: Chuah, D.G.S., "A General Introduction of Photovoltaic Solar System" A paper presented at US Asean Conference on Comparitive Technology Water Pumping June 14-17, 1988, Penang, Malaysia).