ABSTRACT
Solar energy can be used in systems other than the photovoltaic. A dynamic energy conversion system is an example, where the sun’s energy is collected in the form of heat using a concentrator. The heat, in turn, is used to produce steam and drive a rotating turbo-generator or a reciprocating alternator to generate electricity. Such a system was a primary candidate for the space station design in the 1980s with an estimated power requirement of 300 kW. The system configuration is shown in Figure 21.1. A parabolic concentrator focuses the sun’s heat on to a receiver, which boils a fluid. The fluid can be a suitable liquid metal, such as potassium chloride. A highpressure stream of liquid metal produced in the receiver would drive a turbine based on the Rankine cycle. The fluid can also be a gas, such as a mixture of helium or xenon having the molecular weight around 40. The heated compressed gas would drive a turbine working on the Brayton cycle. A gas-based system minimizes erosion and sloshing problems in transporting the liquid metal. A functional schematic of dynamic power system for space is shown in Figure 21.2. In either a liquid metal or a gas-based system, the high-pressure high-temperature fluid drives the turbine, which in turn drives an electrical generator. Waste heat transferred to the liquid coolant is dissipated via radiator panels to space. The energy conversion efficiency is
much higher than the photovoltaic system. This minimizes the deployed collector area and the aerodynamic drag in low Earth orbit. The usable energy extracted during a thermodynamic 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
carnot ¼ Thot Tcold
Thot ð21:1Þ
where the temperatures are on the absolute scale. 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, however, is limited by properties of the working medium. The cold side temperature is largely determined by the cooling method and the environment available to dissipate the exhaust heat. An indirect but major advantage of this system is that the energy storage
is interwoven in the system at no extra cost. It resides in the latent heat of phase change at high temperature — around 1000 K. The systems can store thermal energy for hours with no degradation of electrical performance, or longer with some degradation. This feature makes the technology capable of meeting peak power demands with no added mass or cost of separate energy storage. It eliminates the battery requirement altogether. Today’s space power systems using well-proven PV technology provide
up to a few tens of kilowatts with system specific power of 10 W/kg and life up to 15 years. Although the solar dynamic technology is not yet proven in space flights, it offers potential advantages in efficiency, weight, scalability, and the overall cost in high power spacecraft. The cost advantage comes
extremely cost effective in a few kilowatts to hundreds of kilowatts power range. The concept is sufficiently developed for use in the near future, particularly in high-power LEO missions. It may also find applications in high-power defense spacecraft where a large solar array can make the mission nonmaneuverable and vulnerable to detection and attack by enemy. It has been considered for a 300-kW space station and for a dynamic isotopes power system (DIPS) for space defense. The efficiency advantage in the dynamic system comes from the higher
efficiency of the engine (20 to 40%) as compared to silicon solar cells (15 to 20%), and higher efficiency of thermal energy storage of the receiver (85 to 90%) as compared to the battery efficiency (70 to 75%). The greatly improved overall system efficiency as compared to the PV system translates into less solar collection area. This 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 drag-makeup fuel and orbit decay time. At power levels near 100 kW, such as for space-based radar, the PV solar array collector area becomes prohibitive. It is in this power range that solar dynamic power system is expected to find advantageous applications. It offers the following additional advantages:
Low solar collection area High voltage a.c. power generation Highly scalable and mass producible Potentially long life components Inherent radiation tolerance
Recent prototype testing of a 2 kW non-optimized solar dynamic systems reported by Mason1 demonstrated a conversion efficiency of almost 30% using 1990s component technologies. Significant improvements in efficiency can be realized for large systems in ratings above 100 kW using newer technology components and optimized design parameters. The dynamic system uses an electromechanical energy converter in
which an armature coil moves in a rotary or linear motion under an alternating magnetic field of north and south poles. The generated voltage in the armature is therefore alternating at a certain frequency. The terminal voltage output equals that generated in the armature less the internal voltage drop. The mechanical power source driving the armature is typically a steam or gas turbine or a reciprocating engine. The electromechanical energy conversion efficiency is in the 85 to 98% range depending on the system rating and configuration. The machine size and mass primarily depend on the power rating, and secondarily on the adopted thermal design to remove the internal power losses. A smaller
loss and greater thermal system mass. A rough rule of thumb in trading weight and loss over a narrow range using the same materials and design configuration is to assume that the product of the machine mass and the power loss remains constant.
