No Fossils in MY house...part 3 - Solar Photo-Voltaic cells

Solar power can either be used to heat something up, or in the case of Photo-Voltaic or PV cells, generate electricity directly.  These are devices, usually made of silicon, that respond to sunlight by producing electricity.  They are rated in kWp, which is kilowatts peak - the amount of electrical power they produce in direct sunlight.

Putting PV cells on the roof to generate electricity sounds like quite a neat idea.  We could easily angle them to face south.  In Edinburgh the average incident solar energy that falls is about 100 watts per square metre.  PV cell efficiency is a bit of a challenge however.  There is a theoretical limit to the efficiency of solar cells due to something called the band-gap problem of about 50%, however the best today are about 20% and typical ones are about 10% efficient.

The other slight problem is that last time I looked, the sun doesn't shine at night, so that suggests that one needs to charge batteries for overnight power.  The other slight problem is that in the winter when the daylight hours are shortest is when demand for electricity tends to be highest.  The battery problem can be overcome in a sense if one is "grid connected".  The theory is that one generates excess electricity during the day and sells it to the grid - effectively using it as a storage system - and then buys it back during the night or during periods of insufficient generation.  Of course, this assumes that someone somewhere has spare capacity when you need it during the night - not an unreasonable assumption.

As well as the PV cells, one needs an inverter to convert the DC from the cells/storage battery in to AC.

On the plus side, solar cells should be relatively cheap at the moment due to overcapacity in manufacturing.  This article describes the problem.  That might explain why Maplin had a sale on a while ago for solar products.

As well as the cells and a set of batteries to charge, you also need an inverter to turn the electricity from DC to AC.  I haven't investigated this completely - but I am assuming that devices are available that will allow one to become grid connected - basically as well as generating AC, it needs to match the phase and frequency of your generator to your local supply.  The Distribution Network Operators have different policies when it comes to connecting one's generation system to the grid.  Again I haven't investigated the details of this yet either.

A quick survey suggests that a 120W cell will work out at about 600 pounds, plus batteries, charge controller and inverter.  I'm guessing we would need 3 or 4 such devices to allow for not having full sunlight all the time to achieve 120 watts - let's say 4 for luck.  I would need to do some more figuring to determine how many watts we should aim to generate and store - but let's assume we want to generate our full 7500 kWh per year.

Assuming an average10 hours daylight per day - which is probably a bit optimistic, and 4 cells generating 120W (let's call it 100 for the sake of round numbers) for 10 hours would generate 1 kWh per day.  That means we'd need 20 such arrays (hmmm...my roof will be a bit full with that lot on it,) - or maybe 80 cells at the cost of about 600 pounds each.  That's not far off fifty thousand quid!  The cost of the other bits disappears into the noise!  Payback time?  Given that one of the cells is about the same as our annual electricity bill, it's going to be about 80 years.

This doesn't look like a very good plan either.  Maybe solar cells will get a lot cheaper and maybe my estimates of the number needed are a bit awry - but the payback period still looks to be pretty long.