1. Photovoltaics (PV)
This is the process of turning sunlight into electricity. The theory is that a photon of sunlight can knock an electron off the surface of a semi-conductor (usually Silicon doped with a one atom thick layer of something like Boron). This electron then goes round the circuit made by connecting wires to the surface and back of a crystal of silicon.

This process has been known for a long time but was made use of and developed for powering spacecraft. There are several types of PV collector including: single crystal; amorphous; and thin film. For practical use they should be selected according to cost per watt. However, there is also the consideration of how long a life they have. The single crystal type is believed to be long lived. Some of the early ones are still in use after thirty years. Thin film can be cheap to make but may have only a short life and so needs to be replaced. In practice PV panels are available from stockists. They are manufactured in advanced factories (not suitable for home construction).

 The electricity produced is Direct Current.

2. Uses of the electricity.
PV produces electricity only when the sun shines - during the hours of daylight. (Some power can be produced on cloudy days - light cloud does not turn the power off, though heavy cloud does reduce output to a trickle.)

How can we use the power at night? There are two usual methods.

1). Batteries
In places not connected to the grid this is the only solution. During the day the PV panels charge batteries and the power needed for lighting and other night time activities is drawn from the batteries. The batteries must be of the type used for forklift trucks, invalid chairs and so on - traction batteries. Starting batteries from cars are not suitable (believe me, I have tried them and found they break down quickly if used for energy storage).

2) Grid connection.
During the day power generated by the PV panels is sold to the power company's grid. At night power is drawn off the grid as usual. The best type of grid intertie system is one in which the electricity meter runs backward while power exported is greater than power being used. Effectively the power company is buying the solar power at retail rates. Some companies insist on buying at wholesale price and thus require a more complicated system with two meters. Usually the retail rate will not be paid if the difference is large and so the user will try to maintain the meter at zero, with electricity sold and electricity bought being about the same so the bill will show nothing to pay.

3) Using power as it is generated
On my system the main daytime use is to power the circulating pump of my solar water heating system. This has the advantage that on cloudy days when there is less being generated there is also less demand as the water pump doesn't operate. And of course the water pump doesn't operate at all at night. Thus I have enough power to work the pump and also enough left over to charge the batteries for night time lighting. If I had a larger amount of PV panels I might try to run a fridge off solar power, at least during the day. The Fridge is the main user of electricity in the average house.

(In this case it would be wise to get one of the super-efficient types available for users of solar power as described in Factor 4)

It would also be possible to switch the fridge to mains power automatically when there is insufficient solar power, and at night.

 3. Inverters
The PV system produces Direct current. Usually this will be at a nominal voltage of 12 volts. (Nominal, because in full sunlight it may be as much as 20 volts). This voltage will charge a 12 volt battery.

(Other voltages are possible by connecting panels in series. The higher the voltage the thinner the connecting cables can be. Batteries also can be connected in series to raise the voltage. Thus two 12 volt PV panels and batteries can make a 24 volt system.

To use this energy we can use ordinary mains appliances. The 12 volt Direct Current (DC) power is put through an Inverter which gives out mains voltage and frequency - 220 volts AC in Europe and most of the world). If we are using a grid intertie system the inverter transforms the PV output into a form suitable for feeding into the grid. (This must be done by qualified persons)

 4. Water heating
Solar power can be used to heat water. This is a separate type of process from Photovoltaics. In theory it is simpler. The heat that accompanies sunlight is used to raise the temperature of water. If the solar heat collector is separated from the atmosphere by a transparent layer it acts like a greenhouse.

As I write on 15 July 2002 the temperature in my hot water tank is 55 degrees Celsius - too hot to touch. However the summer of 2002 has so far been very cloudy and there have been many days when I have had to use the gas boiler.

The collector on the roof gets hot. The fluid in it is pumped down to a coil in the hot water cylinder which acts a heat exchanger. The fluid in the collector and heating coil must contain anti-feeeze and is in fact water with an antifreeze additive.

 5. Money
Will the user of solar power make money?

No.

Solar power is more expensive to use than conventional power. At present costs solar water heating cannot cost less than using gas. Of course, once the apparatus is installed there will be days when no gas is used for hot water. But if the money were invested in the building society the interest received would be more than the amount saved.

So, if someone asks whether he will save money, or see a return on his investment, he must be told that that is not what this is about.

In global terms solar energy is cheaper than fossil fuels.

This is because burning fossil fuels has many costs that are not paid by the user. The costs of changing climate are not borne by the motorist or user of coal-fired electricity. There are also health costs. Someone who installs a solar system cannot at present receive the savings from carbon not emitted, or asthma not needed to be treated. The government's grant for Photovoltaics is a first step in recognising this fact. I hope that in the future a tax on carbon emissions will raise money to recognise the savings from solar energy, but at present this money is not available.

Installing solar energy apparatus is a step in the campaign to make governments and business realise the need to make the carbon emitter pay.

And there are some people who want to feel they are part of the solution instead of adding to the problem.

Users of solar power do take steps to save power. They eliminate the phantom loads mains power users don't bother about. A computer printer 'always on' drains power. It has been calculated that if everyone switched off his phantom loads - like the tv being on for the remote control - several power stations could be closed down. Because PV generation costs more, the user will make sure he doesn't install more than needed for the benefits. He will use the most efficient fridge even if it costs more (Lovins, Lovins and Weizsaecker Factor 4 argue a more efficient fridge costs no more to make and so should not have a higher price but there it is).

However, after having written the above, I notice that the latest issue of Home Power magazine contains a calculation that shows a solar electric system to make a positive return, given the long life of the components. The writer, Jeremy Smithson, compares expenditure on a new vehicle of the kind so popular these days, an SUV.

A PV system has a low rate of depreciation, whereas a car has a high rate. After 20 years the car is worthless, but the PV system is still generating.

As the writer suggests: "If you can afford a car, you can afford a PV system".

6. Costs A solar electric system needs the following components 1) Photovoltaic panels 2) Connecting cables 3) An Inverter 4) Possibly batteries. 5) Possibly a connection to the grid 6) Possibly new appliances See Wind and Sun, for a price list.

Dorset Area Renewable Energy Society Climate change action Energy Saving Trust

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