Solar Concentrator Dynamic Power System

Solar energy can be used in systems other than photovoltaic cells. For example, the sun's energy is collected in the form of heat using a concentrator. The heat, in turn, is used to generate steam and drive a rotating turbo-generator or a reciprocating alternator: either way uses a thermodynamic energy converter.

Years in service

FIGURE 3.8 Degradation of solar array output power versus service years.

Years in service

FIGURE 3.8 Degradation of solar array output power versus service years.

The dynamic power system was a primary candidate for early space station design, having an estimated power requirement of 300 kW. The system configuration is shown in Figure 3.9. A parabolic concentrator focuses the sun's heat on to a receiver, which boils a fluid. The fluid can be a suitable liquid or even a liquid metal, such as potassium chloride. High-pressure steam produced in the receiver drives a steam turbine based on the Rankine cycle. The fluid can also be a gas, such as a mixture of helium and

FIGURE 3.9 Solar concentrator-dynamic system. (Source: NASA Glenn Research Center.)

xenon, having a molecular weight around 40. The heated gas drives a turbine working on the Brayton cycle. The gas-based system, however, minimizes erosion and the problem of sloshing when transporting a liquid. In either case, the high-pressure high-temperature fluid drives the turbine, which in turn drives an electrical generator. The energy conversion efficiency is about twice that of the photovoltaic system. This minimizes the deployed collector area and the aerodynamic drag in the low Earth orbit. An indirect advantage is that the energy storage is interwoven in the system at no extra cost. It primarily resides in the form of latent heat with the phase change at high temperature around 1000 K.

The usable energy extracted during a thermal cycle depends on the working temperatures. The maximum thermodynamic conversion efficiency that can be theoretically achieved with the hot side temperature, Thot, and the cold side temperature, Tcold, is given by the Carnot cycle efficiency, which is

_ Thot Tcold io

Thot where the temperatures are in degrees absolute. The higher the hot side working temperature and lower the cold side exhaust temperature, the higher the efficiency of converting the captured solar energy into electricity. The hot side temperature, Thot, however, is limited by properties of the working medium. The cold side temperature, Tcold, is largely determined by the cooling method and the environment available to dissipate the exhaust heat.

The dynamic power system incorporates the thermal energy storage for hours with no degradation in performance, or for longer duration with some degradation. This feature makes the technology capable of producing high-value electricity for meeting peak demands. Moreover, compared to the solar-photovoltaic system, the solar-thermal system is economical, as it eliminates the costly PV cells and battery. The solar concentrator-dynamic system with a turbo-alternator also offers significant advantage in efficiency and weight, and hence the overall cost over solar PV technology. The efficiency advantage comes from the higher efficiency of the engine (about 30%) as compared to silicon solar cells (about 15%), and higher efficiency of thermal energy storage of the receiver (about 90%) as compared to the battery efficiency (about 75%).

The concept is sufficiently developed for use in the future, particularly in high-power LEO missions. It may also find applications in high-power defense spacecraft where large solar arrays can make the mission nonmaneuverable and vulnerable to enemy detection and attacks. The higher efficiency requiring less solar collection area results in reduced drag and less concern regarding station dynamics, approach corridors, and experimental viewing angles. The reduced drag is particularly important because it allows lower flight altitudes within given constraints of dragmakeup fuel and orbit decay time. At high power approaching the 100-kW range for space-based radar (SBR), the solar array collector area becomes prohibitive. The solar dynamic system can be extremely cost effective over a wide range of power between a few kilowatts and hundreds of kilowatts. It was considered for the dynamic isotopes power system (DIPS) in the 5 to 10 kW power range, and the space station in the 200 to 500 kW power range.

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