With the “rocketing” (to use the language of the tabloids) prices of petrol and diesel, Prince Charles telling us that he’s running his car on wine, and British Gas raising their gas prices by 35%, it’s got to be time to look at alternative fuels.
So…what are our options for the future? There are some marvellous technologies out there that we can use for homes, buildings and cars. And so…here’s a new series of occasional posts covering some of the basics of these technologies, starting with:
Using the light of the sun to excite electrons in materials, and generate usable electricity, PV is something that we are all familiar with. In fact, chances are that you have at least one PV cell in your house somewhere, on a calculator if no where else.
A quick note on the language of PV
Photovoltaic is greek, combining the words for light and electrical, PV therefore does pretty much what it says on the tin. There is often some confusion between references to PV cells, PV modules and PV panels, essentially a module is made up of lots of cells and a panel contains the modules, the connections and often a frame of some sort.
When talking about the size of a PV system, people usually refer to it in terms of kilowatt peak or kWp. This refers to the amount of instantaneous electricity that the panel can generate in optimum test conditions. It can also be useful to discuss the output in terms of kilowatt hours per annum (kWh), i.e. the power generated by the panel over the course of the year.
How much Carbon Dioxide does PV displace?
The amount of CO2 that you avoid by using PV, depends on the source of the electricity that it is replacing. For example, if your current supply of electricity comes from a coal fired power station, you will be avoiding more CO2 than if your electricity comes from a gas fired power station. Of course, you can’t tell where your grid electricity is coming from, and so a typical figure is generally used, based on the generation mix to the grid. The figure quoted in the Building Regulations is that displacing 1 kWh of grid electricity saves 0.568 kg of CO2. This figure is highly debated, but that’s a topic for another time.
But we live in the UK, we don’t have any sunshine!
PV won’t generate as much electricity in the UK as it will somewhere with year round sunshine and clear skies, but it will still generate a significant amount. PV benefits most from direct sunshine, but will also generate from reflected light on cloudy days. For maximum output, you do want your panels to be facing towards the sun as much as possible. For big fancy systems it’s possible to install a tracker, so the panels change their position throughout the day or year to follow the course of the sun, for smaller systems it’s best to position the panels at one point to get ensure the most solar gain over the course of the day. In the northern hemisphere this means panels that face south, and in the UK you are looking for the panels to be on an incline of about 30 degrees from the horizontal.
What types of PV are there?
PV cells can be created in a number of different ways, and from different materials, which results in different efficiencies and costs.
Mono-crystalline cells are formed by slicing a large cylinder of material to produce a single wafer. The efficiencies of the cell are high, but because the shape of the cells makes it difficult to completely cover a given area, the efficiencies of the panels are less good.
Polycrystalline cells are more fragmented, with a lower efficiency, but are easier (and cheaper) to create, and can cover a larger area of the module.
Other available options include amorphous silicon cells and hybrid modules, which combine monocrystalline and amorphous silicon.
PV is available as large panels; tiles that can be incorporated into a roof without altering the roof line; glass laminates that can be used as architectural features, generating electricity while still allowing light into a building; thin film flexible laminates that can be attached to almost anything…the list goes on.
So, why haven’t we all got PV then?
The easy answer? PV isn’t cheap. But as electricity becomes more expensive and PV production becomes cheaper, it will become more viable for more and more people to choose PV for homes and offices.
P.S. If you notice any factual anomalies in this, or think I’ve missed something really crucial out, please let me know.