System requirements

The total energy requirement of a mission (i.e. its integrated power requirement) is the most fundamental parameter for designing the power system. If that energy requirement is sufficiently low, the primary energy source can be practicably provided as chemically stored energy - it is practical to drive a wristwatch with a battery, but a washing machine or stove cannot be practically driven this way. The condition 'practicable' is determined usually by mass and volume constraints.

For higher energy requirements there are two solutions. One option is to extract energy from the environment - providing a power source rather than an energy source. In principle, the energy provision of such a power source is infinite (just run the source for longer to acquire more energy). The practical examples here are solar power (usually by photovoltaic arrays) and possibly wind power for Martian surface systems.

The other option is to use an energy source that has a higher energy density (watt-hours per kilogram) than chemical storage - radioisotope sources. Since reliability concerns or operations costs limit missions to a fraction of the half-life of the most common isotope, 238Pu, radioisotope devices may be considered power sources rather than energy sources.

Most spacecraft in Earth orbit have higher energy requirements than can be practicably provided as stored chemical energy - usually only systems that perform their function within a few hours or days, such as launch vehicle stages and planetary entry probes, can be powered this way.

The choice of power source can be usefully demonstrated on a map (Figure 9.1).

Key design parameters for primary power are specific mass and (for solar power) specific area. Solar array performance for arrays of a few kW at 1 AU is typically 65-80 Wm— 2, with specific mass ranging from around 25kgkW— 1 (flexible) to 50kgkW— 1 (folded). To first order, performance is a factor of 3 poorer on the Martian surface. Higher power per unit area is possible using high-efficiency multi-junction solar cells, although at greater fiscal cost.

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