The solar energy resource
Solar energy is the source of most of the World's energy resources, from fossil fuels to wind energy, with the notable exceptions of radioactivity and geothermal energy. It thus has the potential to supply all of our daily energy requirements, until we take note of its geographical distribution and its temporal cycle. Despite the mismatch of supply and demand, it remains the most important energy resource available to mankind. Also, the problem of intermittency of (renewable) energy resources, like solar, can be solved/alleviated by improving devices for energy storage and conversion. Direct solar radiation provides ~1 kW m-2 on the Earth's surface when the Sun is directly overhead and the sky is clear. When the Sun is lower in the sky, this is reduced by increased scattering and absorption according to the length of the path through the atmosphere, and the weather conditions. The maximum solar power is thus reduced to ~800 W m-2 on a horizontal plane in summer at the latitude of Edinburgh. Interactive tools are available to enable calculations of the solar power falling on inclined surfaces at any location, and hence to model the power that a solar energy converter may provide. For instance, the European Commission Joint Research Centre provides data for the annual "global" irradiation in kWh m-2 for modelling the annual solar electricity that may be generated by a photovoltaic array at any location.
For example, the JRC calculation of the annual mean global irradiation in Wh m-2 day-1 on an optimally inclined surface (36o) at the latitude of the south coast of England (50o50” North) is 3290 and for the optimally inclined plane (39o) at the latitude of Shetland (60o17” North) is 2520, for Glasgow it is 2760. (Moving North over this range of latitude, there is an increase in the ratio of diffuse irradiation to direct irradiation, from 0.55 to 0.61.)
Solar energy research in SUPA is mainly into photovoltaic cells and not into solar thermal collectors (e.g. water heaters) or photochemistry (e.g. biomass). These pages provide some details of our solar cell research projects that are investigating different semiconducting materials and new techniques to enhance optical absorption in solar cell structures. Projects range from enhancing the performance of highly efficient crystalline silicon cells to studying lower performance polymer cells that are potentially cheaper.
How do solar cells work?
There are three essential features that enable photovoltaic cells to generate electrical energy directly from photons:
- optical absorption within the solar spectrum creating pairs of electrons and holes
- charge pair separation by a built-in electric field, often preceded by carrier diffusion
- charge collection at conducting electrodes on the cell surface
The first practical solar cells used a PN junction in crystalline silicon, which continues to be the most widely implemented type of cell. Silicon cells absorb that part of the solar spectrum lying at higher energy than the bandgap of Si at 1.1 eV (i.e. wavelengths below ~1100 nm) and each photon generates at most one electron-hole pair, with the excess energy given to the Si lattice as heat. These pairs are separated by the electric field in the depletion region of the PN junction, holes moving to the P-side and electrons moving to the N-side. An opaque metal contact on the rear face of the cell, the P-type side, collects holes and a gridded contact (to enable light to pass through) on the front face, N-type, collects electrons. An electrical load connected between these two contacts will receive current and voltage according to the cell and the ambient conditions.
Solar cell characteristics
It will be appreciated that the current from a solar cell depends mainly on the illumination (and on cell area) whereas the voltage depends on the magnitude of the built-in field. The operating conditions may vary between the extremes of a short-circuit load (maximum current) and an open circuit load (maximum voltage), with the highest output power occurring somewhere in between. Power conversion efficiencies are usually quoted for the maximum output at a particular irradiance. Poor quality contacts add series resistance which reduces the short circuit current, and defects within the junction region or around the cell edge reduce the shunt resistance and hence the open circuit voltage. These parasitic effects may be included in an analytical model of cell operation that will predict the combination of current and voltage for various operating conditions. A further engineering parameter that is frequently used is the fill factor which is a measure of how rectangular is the current vs voltage characteristic: it is given by the ratio of maximum power to the product of open circuit voltage and short circuit current.
Finally a few words on the spectral response of solar cells: there is an optimum bandgap for a semiconductor to absorb as much as possible of the illuminating spectrum, which for standard sunlight conditions lies around 1.5 eV. Photons above this energy but close to it, will be weakly absorbed and those with much greater energy will be strongly absorbed. Consequently, the conversion process for longer wavelengths (but those still able to be absorbed) requires a thick absorber. The conversion process for shorter wavelengths requires the built-in electric field to be close to the entry surface of the cell to prevent rapid recombination of the pairs of carriers at the surface. Much solar cell design and engineering is aimed at addressing these shortcomings.
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