PV Solar Collectors

January 2011

Photovoltaics or PV is the direct conversion of the sun’s energy into electricity (not to be confused with thermal plate solar collectors which heat up the domestic hot water or central heating water). This is usually achieved by a complicated process of coating glass with a silicon material which responds to sunlight by creating a DCDirect Current (the sort of electricity that comes out of batteries rather than Alternating Current which is in the mains) current of electricity from two wires connected to the glass coating. By connecting multiple panels together the voltage can be raised to a level where it can be put through an inverter and used like mains electricity in the house or sold into the national grid.

The conditions under which the use of PV makes the most sense are -

• In an extremely sunny climate and where the orientation is correct and shading is minimal.
• If the location of the building is so remote that there is no connection to mains electricity. One of the companies specializing in design and supply of off grid systems is Energy Development Cooperative Limited. (and their shop)
• If it is possible to use the solar panels as the roof and/or wall covering and thereby save costs on other materials. This has been done successfully on several new industrial buildings and houses in the UK

Further information on the Solarserver site

Feed in tariffs

There are now cashback incentives for installing PV collectors and this has made them into a good financial investment

The ESTEnergy Savings Trust have an on-line calculator which calculates costs and savings depending on your house’s roof slope, its orientation, the size of the installation and when it is installed.

Some local authorities have taken a lead by subsidising solar panel installation – see the BBC item on free panel installation

Value for money

The money spent on PVs would normally be better spent on saving energy in the first place through better insulation or the use of more efficient appliances (and when that has been done then on other environmental lifestyle aspects such as transport, food etc). However it is important to keep an eye on market developments. Both Germany and the US are much cheaper for PV than is the UK and things could change quickly here. A recent BBC news item suggests that costs might reach grid parity for most of Europe by 2020.

Given that money is usually a limited commodity it makes sense to calculate where that money is best spent. PV should not be incorporated at the expense of insulation or air tightness so if there is a conflict then go for these two first. There is also the option to make the house PV ready so that when prices drop PV can be added later. Being PV ready involves several design factors.

• Having an area (probably of roof) which is built so that
  • it is facing in the right direction
  • is at the correct pitch
  • has a covering which can be modified to take the supports for PV
  • has some easy means to run cables indoors
  • is, ideally easily accessible
• Having an area set aside where the control gear can be housed

The hope over the last 30 years or so has been that through mass production the rather high price of these PV panels would tumble to the point where everyone would have them on their roofs. Due to the inherent difficulties of manufacturing the panels this has never happened although some people are predicting that it could begin to happen soon. It now looks like costs are dropping at the rate of about 5%/year.

The efficacy of a solar panel is measured in kWpkW peak. This is the output of a PV solar collector in bright sunshine (rather than a cloudy day). This is the measure of power in kWkilowatt - a measure of how fast energy is flowing. e.g. electricity might flow through an electric kettle at the rate of 2 kW when the panel is at its peak output, i.e. bright sunlight striking it directly and cool surrounding air (since if they get too hot the output drops off somewhat)

The price has almost halved over the past decade. At present the cost is about £2.80/watt (peak)  for the panels (including VAT) plus nearly the same again to mount them, control them and tie them into the grid. So to generate a kilowatt of electricity  would cost say £4800.00 in investment. However, in the US state of California, where a certain amount of solar installation is mandatory it looks like a 2kW solar array fitted to a new house roof will soon be costing about £4000 (ie £2000/kW).

To get an idea of payback time you then have to factor in how many hours of sunlight you get per day and this varies with location and time of year. It tends to work out that you get about 800 kWhkilowatt hour. This is a unit to measure an amount of energy. If you run your 30 kW gas boiler for 2 hours a day you use 60 kWh per day /kWp / year.

A long term study in Berlin showed that living roofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living RoofsA roof with a covering of soil or growing medium and plants. They tend to be divided into turf roofs with a 150mm layer of soil and sedum roofs with a thinner layer (about 40mm). see Living Roofs improved the performance of roof mounted PVPhoto Voltaic. referring to the generation of electricity from sunlight collectors by keeping the air cooler around them. They are less efficient when hot.

Green Building magazine has some very informative articles on the subject by Jerry Clark who has fitted two sets of collectors to his own house and goes into considerable detail on their output and costings. The Spring ’09 article (before the current feedin tariffs were introduced) is here. The Autumn ’09 and Autumn ’10 are not on line. However his calculations based on using the new feedin tariffs show a payback time of just under 8.5 years and thereafter a very good return on investment.

The BREBuilding Research Establishment. ’s Green Book Live web site contains lists of microgeneration companies specialising in PVs.