Stand-Alone Power Supply (SPS) System Module 3

Renewable Energy Sources

In this section we will look at photovoltaics.

Solar Photovoltaics (PV)

PV systems should not be confused with the more common solar water heater technology. Solar water heaters consist of a hot box, which traps the heat from the Sun in exactly the same way as your car gets hot in the Sun. This heat is used to heat water. Alternatively in photovoltaic technology, sunlight is converted to electricity through what is known as a photovoltaic (or solar) cell. Light creates a difference in the electrical properties in the layers of silicon resulting in electricity.

Figure 1 A schematic of the layers of a typical PV cell.

Terminology

Many solar (PV) cells are combined into a module. These are the smallest units that you can buy for a system. Many modules are combined into an array. A single square or circular unit of silicon forms a solar cell. Many of these cells are combined into a panel which is also known as a module. Modules are the smallest units which you can buy and they are usually sold according to the rated peak Watt (i.e. 100 W, 250 W).


A PV cell	A PV Module	A PV Array

PV Modules

A PV Module is composed of modules of either:


Amorphous silicon	Mono crystalline silicon	Poly crystalline silicon

The amount of energy you can expect to get from a module is about 5 times its rated power output in summer and about double in winter. i.e. 80W panel produces 400Wh per day (0.4 units of electricity) in Summer and 160Wh in winter (0.16 units in winter). The daily energy output from PV panels will vary depending on the type of module, orientation, location, daily weather and season. On average, in summer, a panel will produce about five times its rated power output in watt hours per day, and in winter about two times that amount. For example, in summer a 50 watt panel will produce an average of 250 watt-hours of energy, and in winter about 100 watt-hours. These figures are indicative only, and professional assistance should be sought for more precise calculations. It is possible to design your system for the best or worst month, depending on the application.

PV Arrays

In the southern hemisphere, the Sun is generally in the northern half of the sky. PV arrays face north to so that they can collect the sunlight. Solar panels can either be fitted to the northern side of a building, such as a house or may be mounted on a free standing structure at ground level, or above ground level. Tilting the array also increases sunlight shining on array. A great way to illustrate this is to use spaghetti to simulate the sunlight and a cup or bangle to illustrate the collecting surface. Where there is a right angle between the cup opening and the spaghetti, the greatest amount of spaghetti will end up in the cup. Where there is less than a right angle, say a 45 degree angle, less spaghetti is able to get in the cup.

Figure 2 Positioning the array at a 90 degree angle maximises the amount of sunlight that falls on the array.

This effect can be seen in a graphical form in Figure 3 below.

Figure 3 A comparison of different array angles throughout the year in Australia.

Arrays which lie flat collect slightly more sunlight in summer, but far less energy in winter (blue line). Arrays are mounted such that they are titled with respect to the horizon. This is because they collect a far more even amount of sunlight throughout the year (black and green lines). They are typically mounted on angles similar to your latitude.

Stationary or Tracking Arrays

Systems which follow the path of the Sun across the sky are known as tracking systems. They collect more energy than stationary arrays, but they add to the system cost. Trackers are used to keep PV panels directly facing the Sun, thereby increasing the output from the panels. Trackers can nearly double the output of an array (see Figure 4). Careful analysis is required to determine whether the increased cost and mechanical complexity of using a tracker is cost effective in particular circumstances. A variety of trackers, which will take about 10 panels, are manufactured in Australia.

Figure 4 A power production comparison between a tracking and non-tracking array.

Maximum Power Point Trackers (MPPT)

The device is used to ensure that array is operating at the most efficient point. This is used with photovoltaic modules to optimise the match between the panels and the battery bank. This technique uses a DC to DC converter with circuitry that measures the incoming power from the module and adjusts the voltage so that the maximum power is being sent to the battery bank independent of the battery bank voltage. As the name suggests, a MPPT is an electronic device (or tracker) designed to maximise the power available to the system from the PV panels. It can perhaps be likened to an electronic “gearbox” that ensures that regardless of the power available from the Sun, the MPPT will try to turn a motor (such as a water pump), even slowly early in the morning, adjusting to “direct drive” at noon, then the same as the Sun levels drop in the afternoon.

This is the end of Module 3.

Click here to Return to the Stand-alone Power Supply (SPS) System Page to attempt the Module 3 Quiz.