Solar energy conversion

Two basic methods of converting photons to electricity have been studied, solar dynamic (SD) and photovoltaic (PV).

SD uses a heat engine to drive a piston or a turbine which connects to a generator or dynamo. Two heat cycles for solar dynamic are thought to be reasonable for this: the Brayton cycle or the Stirling cycle. Terrestrial solar dynamic systems typically use a large reflector to focus sunlight to a high concentration to achieve a high temperature so the heat engine can operate at high thermodynamic efficiencies; an SPS implementation will be similar. A major advantage of space solar is the efficiency with which huge mirrors can be supported and pointed in zero gravity and vacuum conditions of space. They can be constructed with very thin aluminum or other metal sheets and very light frames, easily constructed from materials available in space (eg, on the Moon's surface).

PV uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert sunlight photons into voltage via a quantum mechanical mechanism. These are commonly known as “solar cells”, and will likely be rather different from the glass panel protected solar cell panels familiar to many and in current terrestrial use. They will, for reasons of weight, probably be built in a membrane form not suitable to terrestrial use which is subject to considerable gravitational loading.

It is also possible to use Concentrating Photovoltaic (CPV) systems, which like SD are a form of existing terrestrial Concentrating Solar Energy approaches which convert concentrated light into electricity by PV, thus avoiding thermodynamic constraints which apply to heat engines. On Earth, they also use tracking systems, mirrors, and lenses to achieve high concentration ratios and are able to reach efficiencies above 40% Concentrating Photovoltaic Technology. Because their PV area is rather smaller than for conventional PV, the majority of the deployed collecting area in CPV systems is mirrors, as with SD systems; so they share the advantages of building and pointing large (simple) mirror arrays in space as opposed to (complex) PV panels.