Solar cells versus heat engines

Two basic methods of conversion will be addressed in this series of posts : photovoltaic (PV) and solar dynamic (SD).

Solar Dynamic (SD) uses mirrors to concentrate light on to a heat engine of some type. Various thermal cycles are possible, such as Stirling, Brayton or Rankin. We will look at some actual implementations of  Stirling-type heat engines built for the 2005 and 2010 NASA/Spaceward Space Elevator Games (or “Power Beaming”) Competitions.  The use of solar dynamic technologies can be shown to reduce mass per watt of energy, but have never flown to date in actual spacecraft.  This concept involves focusing photons from sunlight (or laser light energy) on to a heat exchanger to drive a thermal (heat) engine.  So far the power to weight ratio for SD has not been absolutely demonstrated to be superior to PV, but more work is needed to develop SD technology.

Photovoltaic (PV) conversion uses semiconductor cells to directly convert photons into electrical power. Today, PV Gallium arsenide (GaAs)-based solar cells are typically favored over silicon in satellites and spacecraft, because they have a higher output per unit weight. The most efficient solar cells currently in production are multi-junction cells. These use a combination of several layers of both gallium arsenide and silicon to capture the largest spectrum of light energy possible. Multi-junction cells are capable of  almost 30% efficiency under the proper conditions and this blog series will show a real implementation that reached conversion numbers near that with the express goal of getting the maximum output per unit weight.

Another approach is to use both technologies –  lightweight solar concentrators,  which can be much lighter per unit area than the solar cells, and the PV array together to increase the amount of sunlight or laser power that would otherwise get into the PV cells. This blog will also demonstrate that Photovoltaic (PV) cells the performance / efficiency drops off as the temperature increases and that the temperatures that (especially those of a 8 KW Industrial laser can produce at distances of 1 KM or less) are significant.  For GaAs type cells of the Thermal PV (TPV) variety this problem is lessened and they can accept high concentrations of light and consequent rise in temperature and give good output levels if adequate cooling is engineered.

This is used in some spacecraft, e.g. Deep Space-1, lightweight Fresnel lenses (Stretched Lens Array) concentrate the sunlight going into heavy GaAs PV cells. In 2005 Rigid-Panel Stretched Lens Arrays were producing 7 kW per wing. Solar arrays producing 300 W/kg and 300 W/m² from the sun’s 1366 W/m² power near the Earth are now available. Entech Inc. hopes to develop 100 kW panels by 2010 and 1 MW panels by 2015.

Reference: Kepler Space University Course 809 (Certificate Course) 

Renewable vs. Nonrenewable Energy: The SBSP Imperative?


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