How do solar panels work?
Electricity is produced from sunlight through a process called a photovoltaic system. Photovoltaic panels are typically made up of between 60 or 72 cells, each made of semi-conducting materials, which convert sunlight into electricity.
Solar cells are made of thin layers of silicon. When sunlight strikes a PV cell electrons within are knocked loose. The absorption of a photon (light particle) leads to a negative electron being shunted away from the silicon atom, leaving a positive ‘hole’.
The free electron and the positive hole together are neutral. Therefore, to be able to generate electricity, the electron and the hole need to be separated from each other. This is done by giving layers within the cell opposite charges, so that the freed electrons cannot return to the positively charged holes and are instead attracted to the next layer.
When the electrical contacts on the front and rear of the cells are connected through an external circuit, the freed electrons can only return to the positively charged holes by flowing through this circuit, thus causing electricity to flow. Greater intensities of light (i.e. more photons) give a greater flow of electricity.
What happens if the sun isn’t shining?
Even if the sky is overcast, PV panels still produce electricity. This is because PV panels not only use direct sunlight, but also diffuse solar radiation, which is light that has been scattered by dust and water particles in the atmosphere. Obviously the amount of useful electricity generated is proportional to the intensity of light energy that falls onto the panels, so less electricity is produced in overcast conditions, however, you can still have an electricity supply even without the sun!
Solar PV Efficiency
The efficiency of a PV panel is defined as its output power divided by the power of solar radiation that it is receiving (the solar radiation is referred to as insolation). Although the power output is directly affected by the level of insolation, it does not greatly affect the efficiency. In other words, reducing the insolation by 10% will lead to a drop in output power of approximately 10% so the ratio between input and output stays much the same.
A common source of confusion concerns the difference between power and energy. Power is an instantaneous measurement expressed in Watts (abbreviated to W) or kilowatts (kW) where 1000W equals 1kW. Energy is a measurement of power over time usually expressed in kilowatt hours (abbreviated to kWh). For example a 100 Watt light bulb burning for 10 hours uses 100W x 10hrs = 1000 Watt hours or 1 kWh.
Types of solar cell
Thin film solar cells promise to make solar power more cost-effective in the future but currently they are only used on commercial properties due to their comparatively low efficiently of 9%, however they are exceptional in low light conditions. Most solar panels currently sold in the UK are Polycrystalline, made from multicrystalline silicon and have efficiencies of 11% to 14%. Monocrystalline silicon panels are slightly more expensive and efficient, around 15%. Hybrid panels consist of two types of cells, Monocrystalline and Amorphous (thin film). This makes these panels exceptional in all conditions and the most efficient MCS accredited panel at 19%. Efficiencies over 40% have been achieved using exotic materials and processes but the costs of such cells are prohibitively expensive.
Each PV installation needs at least one inverter to manage the DC (direct current) output from the panels and to convert this into AC (alternating current) suitable for feeding into the national electricity grid. The percentage peak efficiencies quoted for quality inverters around 96, and in practice the energy loss is typically around 6%. Losses in the DC and AC wiring and other connections will amount to a further 2% or 3%. All losses are factored in to the SAP prediction.
Using a single inverter, the maximum total output of all the panels is usually at least 2% less than the sum of the maximum outputs of the individual panels because the panels behave slightly differently.
Dust and dirt will also reduce the output by a few percent and snow of course can have a considerable effect in winter. In heavily polluted arid areas overseas losses of up to 25% have been reported, but frequent showers in the UK keep our panels relatively clean, especially if they are installed at a reasonable tilt.
Manufacturers measure the output of solar panels at 25 degrees centigrade under “full sun” insolation of 1000 Watts per square metre. However, in practice full sunlight will heat the panels to considerably higher temperatures. This is significant because the power output for most crystalline silicon solar cells falls approximately 0.5% per degree rise in temperature. Tests have shown that solar panels reach about 45 degrees centigrade under 800 Watts per square metre of insolation, so power outputs can be at least 10% less than expected when the sun is shining strongly in summer. (http://www.tuvptl.com/doc/abstract-NOCT.pdf) It is therefore advisable to have an air gap between solar panels and a roof or wall to aid air circulation. Consideration should also be given to leaving gaps between panels on larger installations to aid the dissipation of warm air. Hybrid panels lose the least efficiency with increasing temperatures.
Tilt and orientation of the panels
As most of the PV energy will be generated in the summer when the sun moves round in a wide arc, the direction of solar panels is not too critical (providing they don’t face northwards).
Some people recommend that the tilt of the panel above the horizontal should be equal to the latitude where the panel is sited: this would be 52 degrees in the UK Midlands which lies around 52 degrees north of the equator. Others say panels should be tilted 20 degrees less than the latitude. NASA’s satellite data suggest the optimum tilt in the Midlands is 40 degrees whereas the EU gives a figure of approximately 36 degrees for the Midlands. In practice, most householders will want to fix their panels to their roof and fortunately most domestic roofs have a slope of around 40 degrees - which is just fine. As regards the compass direction of panels, turning a moderately tilted panel 30 degrees away from due south reduces the output by no more than 5%.
Where panels are sited on the ground or on a large flat roof it is possible to mount them in devices which direct them toward the sun throughout the day, rotating them either vertically, horizontally or both. This can increase the output by around 30% compared with panels fixed in the optimum direction, but there are obviously cost, maintenance and planning implications.
A solar cell typically produces about half a volt when it is illuminated. Each solar panel generally contains 60 or 72 cells that are joined in series so their voltages add up to give a useful output voltage. In addition, PV systems contain a number of solar panels connected in series to produce an even greater voltage.
A problem arises if just one of the cells is shaded because this results in an increase in resistance in that cell. The current from the other illuminated cells produces heat in this resistive cell which could irreparably damage it. To prevent this from happening, a bypass diode is placed in parallel with groups of cells. A diode is a simple electrical component that acts as a one-way valve, and if shading occurs within a group of cells it allows the current from other groups in the series to bypass the shaded group without causing damage.
For a solar panel containing 3 bypass diodes, if one or more cells in a group is shaded sufficiently in comparison with the other cells then the output of the panel will be reduced by approximately one third. This can happen if just 1% of the panel’s area is shaded. If a long narrow shadow falls across the panel this could lead to all the bypass diodes operating and no energy being produced by the panel. Furthermore, if a shadow affects a number of panels then the overall output voltage can fall below the minimum operating input voltage of the inverter and the whole system will shut down. This can be avoided by using a micro-inverter for each panel, or minimised by installing Hybrid panels as these deal with shading much better than other panels.
The amount of solar radiation received annually does not vary greatly across the UK. In the Midlands approximately 1100 kilowatt hours are received a year on a one square metre of panel facing south and tilted at the optimum (fixed) angle. The figure is roughly 10% less for central Scotland and 10% more for south-west England.
PV panels contain no moving parts and have proved to be very reliable in the past. The majority of panels being installed now can be expected to be still working in 25 years. Their efficiency does drop as they age. For example, a study by the National Renewable Energy Laboratory in the USA found that a sample of commercially available solar cells lost an average of 0.71% of their output annually. The 28,672 solar panels owned by the Sacremento Municipal Utility lost 30% of their output over 25 years. However, the long-term performance of panels made 25 years ago may not be a good guide to the latest generation of panels. As well as being tested to be water and frost proof, PV panels are designed to work in much sunnier and hotter climates than in the UK. It is therefore reasonable to hope that the annual rates of deterioration will be less in the UK’s relatively mild climate.
However, Panasonic (formally Sanyo) have run a performance analysis on one of their older 'H series' module systems. Ten years after installation in 1995, the entire system had only lost 3% efficiency; manufacturers will never guarantee this level of performance but choosing the right panel will maximise total profit / future potential. This system was installed in Japan which has a considerably higher average annual solar irradiation figure. They have also been tested for degradation by TUV Rheinland, this test resulted in only a loss of 8% over the term of the sequential test. This is tremendous achievement and will make this panel stand out over it's competitors.