Electricity from the sun
Electricity can be generated from solar energy in two ways. The first is to capture heat from the sun and use this to power a conventional turbine or generator. The other is to use the photovoltaic effect, which converts light directly into electricity using materials called semiconductors.
Solar Thermal Electric Power Plants
The two main types of solar thermal power plants are Solar Chimneys (where heated air in a tower rises to drive turbines) and Concentrating Solar Power (CSP) plants (which use various types of reflectors to concentrate sunlight into a heat absorber). These are both industrial scale applications which are not suitable for the urban environment.
A solar thermal electric power plant
Photovoltaic Cells
The word photovoltaic is a marriage of the words ‘photo’, which means light, and ‘voltaic’, which refers to the production of electricity. Photovoltaic technology generates electricity from light. Electricity is the existence (either static or flowing) of negatively charged particles called electrons. Certain materials, called semiconductors, can be adapted to release electrons when they are exposed to light. One of the most common of these materials is silicon (an element found in, amongst other things, sand), which is the main material in 98% of solar PV cells made today.
All PV cells have at least two layers of such semiconductors: one that is positively charged and one that is negatively charged. When light shines on the semiconductor, the electric field across the junction between these two layers causes electricity to flow - the greater the intensity of the light, the greater the flow of electricity.
Although the photovoltaic effect was known to the Victorians, it was not until humanity launched into the space race that the unique qualities of solar PV as a power source began to be fully explored. Following this kick-start the technology has raced along a path to commercialisation and the cost of PV generated electricity has plummeted as manufacturing costs have decreased and cell efficiencies have improved.
Types of PV cells
By far the most common material for solar cells is crystalline silicon and these cells can be divided into a number of categories:
Monocrystalline wafers are the most efficient of the PV technologies in good light conditions. However, since they are cut from cylindrical ingots the cells are normally pseudo-square and cannot completely cover a module without a substantial waste of space. This makes them more expensive than other technologies.
Poly or multi crystalline cells are made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells and used to be less efficient but steady developments in PV technology are now delivering comparible performance. They can easily be formed into square shapes that cover a greater percentage of a panel than monocrystalline cells.
These technologies utilise wafer-based manufacturing techniques. In other words, in each of the above approaches, wafers are processed into solar cells and then soldered together to form a module.
Thin film approaches, in contrast, are module-based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Thin film PV is efficient in low light conditions and very sturdy.
Hybrid cell
Hybrid cells are a combination of monocrystalline and thin-film technologies, this has high peak output coupled with excellent performance in poor light conditions.
Usually, solar cells are electrically connected, and combined into “modules”, or solar panels. Solar panels have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add together.
There are four main different types of solar PV. The table below gives an indication of how they compare to each other.
|
Type of Solar PV |
'Thin Film' |
Polycrystalline |
Monocrystalline |
'Hybrid'* |
|
Cell Efficiency at STC** |
8 - 12% |
14 - 15% |
16 - 17% |
18 - 19% |
|
Module Efficiency |
5 - 7% |
12 - 14% |
13 - 15% |
16 - 17% |
|
Area needed per kWp*** |
Kaneka module: 15.5m² Unisolar modules: 16m² |
Sharp modules: 8m² |
Sharp modules: 7m² |
Sanyo modules: |
|
Area needed per kWp |
Solar metal roofing: 23.5m² Glass-glass laminate: 25m² |
Glass-glass laminates: |
C21 tile: 7.8m²
Glass-glass laminates: |
n/a |
|
Annual energy generated per kWp (in UK) |
800 kWh/kWp |
810 kWh/kWp |
830 kWh/kWp |
865 kWh/kWp |
|
Annual energy generated per m² |
50 - 52 kWh/m² |
100 kWh/m² |
107 kWh/m² |
139 - 150 kWh/m² |
|
Annual CO2 savings per kWp |
344 kg/kWp |
323 kg/kWp |
323 kg/kWp |
387 kg/kWp |
|
Annual CO2 savings per m² |
22 kg/m² |
40 kg/m² |
46 kg/m² |
60 - 65 kg/m² |
* ‘Hybrid’ PV combines both monocrystalline and thin-film silicon to produce cells with the best features of both technologies
** Standard Test Conditions are: 25 °C, light intensity of 1000W/m², air mass = 1.5
*** kWp = kilowatt ‘peak’. Solar PV products and arrays are rated by the power they generate at STC
Read on for further information or download our Solar Energy Fact Sheet
