Bangor University
Photovoltaics
How does it work?
Photovoltaics (PV) is a method of converting sunlight directly into electricity. Layers of semiconductor material trap the light energy and use it to create free electrons which can then form an electric current. It is a completely clean, renewable energy source: the only input is sunlight, and the only output is electricity (and some heat). Operation is silent as the device is solid-state with no mechanical moving parts.A PV system usually consists of several modules (panels) together with their associated wiring and power electronics equipment. The latter is necessary in order to convert the d.c. (direct current) produced by the panels to a.c. (alternating current) which is compatible with electricity from the grid, and can be used by ordinary appliances. Systems which are connected to the grid can export excess electricity when more is produced than is needed, and import electricity when the system is not producing enough (e.g. at night). This means there is no need for a storage battery. Export/import takes place automatically as required, without any action from the user.
What are the different technologies?
Traditionally, PV modules have been made from layers of monocrystalline or polycrystalline silicon. These two technologies still represent over 90% of installed PV at present, but other technologies are also now in production, and capturing a growing share of the market. These newer technologies include thin-film amorphous or nano-crystalline Si, compound-semiconductor thin-films (CdTe/CIS), hybrids and dye-sensitized (Grätzel) cells. Organic solar cells are under development. Thin-film technologies currently have lower conversion efficiencies than crystalline Si, but they have the potential to be considerably cheaper to manufacture, with lower-temperature processes, less materials usage and less wastage. Therefore the cost per watt of electricity produced is potentially less than for traditional Si, and the gap may be expected to widen as efficiencies are improved via research and development.Adoption Issues
Cost is an important factor in PV adoption. The fuel (sunlight) is free, and little or no maintenance is required, so the running costs are essentially zero, but at present the initial purchase cost is quite high. This can be a barrier to adoption, as almost the full cost is “up-front” capital outlay. Countries which have been highly successful in encouraging PV take-up tend to offer incentives such as low-cost loans, grants, or favourable payment schemes for electricity fed back to the grid (feed-in tariffs). The most notable example is Germany, where a combination of low-cost loans and feed-in tariffs has led to world leadership in adoption (55% of installed PV in 2006)It should be noted that although incentives are important in order to stimulate adoption at present, this will not always be the case. Based on predicted reductions in PV prices and predicted increases in conventional electricity prices, it is expected that PV will be an attractive option in the U.K. on purely financial grounds within the next 10 – 20 years. However, in the meantime it is vital that adoption is supported, as this will underpin cost reduction via targeted research, scale-up and local industry development. Clearly there are many advantages to be gained from developing a strong industry position at an early stage in this rapidly-growing market.
Against this background, the potential outlook for Wales is good. Strengths already exist in many parts of the PV supply chain, including fundamental research in PV materials and power electronics, manufacturing, and system design and installation. The Welsh Assembly Government is committed to supporting renewable energy via its Microgeneration Action Plan, and via its aspiration to achieve zero-carbon new build by 2012 (cf. the UK target of 2016).
The Solar Resource
Apart from cost issues, barriers to PV adoption in the UK can arise from misconceptions about the adequacy of the solar resource. Photovoltaic panels do not require direct sunlight, and will still work even on cloudy days. Some technologies (e.g. amorphous Si, some thin-film) are particularly suitable for making good use of the diffuse or scattered light conditions commonly found in the UK. Clearly latitude does affect energy output to some extent, but nevertheless, the amount of solar energy reaching the Earth’s surface in the UK is still around half of that found in the highest insolation regions of the world. The amount of sunlight received in the UK is very similar to the amount received in Germany, the world’s largest PV market.PV systems are rated in terms of kilowatts-peak (kWp). This means that a 1 kWp system should produce 1 kW of electricity under a set of agreed standard conditions. These are: with 1000 watts per square metre of sunlight energy falling upon it, at a system temperature of 25°C. The sunlight spectrum is affected by passing through the atmosphere, so standard conditions specify “AM 1.5” which corresponds to the sun at an angle of about 45° in the sky.
On a bright sunny day in the UK there may be 1000 watts per square metre of sunlight available, but usually it is somewhat less. However, over a period of a year, the total amount of insolation in any given location is quite predictable, so over this time-scale a well-designed system should closely match its expected output. This output will be in the range 750 – 900 kWh/yr for most of the UK.
PV modules have a long lifetime (25 – 30 years or more), and will take only a few years to generate an amount of electrical energy equal to the amount of energy used in their manufacture. Therefore they represent a significant net benefit in the struggle to reduce carbon emissions and combat climate change.
Further Information
For more information please contact:Anne Stafford: a.stafford@bangor.ac.uk
Tel: 01248 382235