Every day, the sun delivers huge amounts of energy to the earth free of charge. We can use this free energy thanks available technology both to generate electricity and to heat water for domestic or commercial use.

Solar photovoltaic cells (PV) that turn this sun light into electricity are a very different technology to the solar thermal collectors that use this light to heat the water. In this article we introduce the two technologies and explain how they work and how they differ.

Solar Water Heating Introduction

Solar water heating systems consist of a panel, usually mounted on the roof, which is designed to absorb heat from daylight. When there is sufficient daylight, water is pumped through the panel to the hot water cylinder and back in a closed continuous loop.

As the heated water passes through the solar coil in the hot water storage cylinder, the heat is transferred, via a heat exchange coil, to the surrounding water in the tank. It is this water in the tank that is then used in the showers, sinks and baths. This is known as an indirect system because the water that is used is heated indirectly by the solar energy.

The existing method for heating water (e.g. gas-fired boiler or electric immersion) acts as a backup for when there is insufficient sunlight. The system works automatically. The customer will always have hot water at the required temperature, whenever they want it. In summer, almost all hot water needs are met by the solar system, the rest of the year it pre-heats the water so the boiler only tops up to the required temperature.

The collector is made up of a metal sheet (copper) called the absorber plate to which a selective surface coating has been applied allowing for very efficient heat absorption (i.e. it gets very hot!). Copper pipes are soldered to the back of the absorber plate. The pipes and the absorber plate are then placed in a well-insulated box and covered by a hardened low-iron glass which has a coating that allows for good light absorption and low light emissivity.

As water is circulated through the solar panel, heat is transferred from the absorber plate to the water, which is then circulated through the system to the hot water cylinder.

Solar Photovoltaic System

Photovoltaic panels are made out of materials called semiconductors. Typically silicon is used, and it is a sustainable resource, making up 12% of the earth's crust (commonly found in sand). The silicon is sliced into wafers or made into thin layers and assembled into the modules.

When light strikes the surface of the silicon, the energy of the light is capable of knocking electrons in the silicon free, so they can move around. Two different impurities are added to the silicon, which sets up an electric field that forces all the free electrons to move in one direction - producing an electric current. The brighter the sunshine, the greater the electric current produced.

A thin silicon cell, four inches across, can produce about one watt of direct current electrical power in full sunlight. The direct current is converted into alternating current to operate household appliances. One PV cell alone may not produce much power, but a number of photovoltaic cells are grouped together into arrays, large panels or sheets that collectively form a solar collector system.

PV solar parts have no moving parts, do not make noise or produce any wastes or emissions. Solar electric systems can be connected to local utility grids in urban areas to reduce costs or they can be off-grid, stand alone systems. Some electricity providers offer consumers credit for excess power produced by the system.