Solar Thermal Propulsion in Space

Solar thermal propulsion systems in space will require very high temperatures to generate necessary levels of thrust by the direct solar heating and resulting expansion and expulsion of the propellant material. The generation of such temperatures, in the range 1400-2200 °C, will in turn require very high levels of solar flux concentration. In practice, to attain such levels, it may be useful and perhaps even necessary to incorporate some form of ideal or near ideal nonimaging concentrator. An analy­sis of the benefits associated with such a configuration deployed as a solar concentrator in the space shows that the thermal conversion efficiency at the temperatures required can be about three to five times that of the corresponding conventional design. Operational constraints on configurations that may be suitable for selected solar thermal propulsion applications are reviewed.

image136Although a variety of propellant materials and systems for direct solar propulsion in space are being considered, all will require very high temperatures, ranging from a low of about 1400 °C to more than 2300 °C. The efficiency for conversion of solar radiation to useful heat depends on the difference between the energy collected optically and that lost unavoidably through thermal processes driven by temperature differences between the hot absorber and its surrounding environ­ment. Since these losses are small at low to moderate temperatures, it is comparatively easy to attain respectable efficiencies at temperatures below several hundreds of degrees Celsius. However, as one tries to operate at increasingly higher temperatures, radiation losses, which are proportional to differences in the fourth power of the absolute temperatures, increase very rapidly and can only be overcome by applying increasingly higher concentrations. The thermal efficiency of any concentra­tor collector system results from a balance between optical gain (typically between 75% and 85% due to reflection, absorption, and intercept effects) and thermal losses. Concentration can be used to reduce the latter. The consequences of the concentration limits for efficient solar thermal en­ergy collection are illustrated in Figure 9.4. Here, the geometric concentration required to suppress

Thermodynamic Limit

Accessible Range using Nonimaging Optics

Range Accessible with

Conventional

Concentrators

Efficient Solar Furnace

Подпись: 500 Подпись: 1000 Подпись: 1500 Подпись: 2000 Подпись: 2500 Подпись: 3000 Подпись: 3500

in Lunar Environment

Operating Temperature (deg. C)

FIGURE 9.4: The geometric concentration required to achieve a respectable operating solar thermal conversion efficiency (~70%) in the near-earth space environment is shown as a function of the desired operating temperature. Also shown are the achievable concentration ranges for conventional and non­imaging concentrators.

thermal losses to about 5% of the solar gain so as to achieve a respectable operating solar thermal conversion efficiency (~70%) in the near-earth space environment is shown as a function of the desired operating temperature. Also indicated are the concentration levels attainable using either conventional concentrators or those that incorporate nonimaging designs. These levels are based on the optical quality thought to be achievable with reasonable cost for large state-of-the-art reflecting surfaces and correspond to an effective Gaussian slope error distribution with a standard deviation of 0.5-1.0 millirad. As can be seen, the temperature range for solar thermal propulsion requires concentrations that are unattainable (or only marginally attainable) with conventional designs and are only possible using some form of nonimaging concentrator.

image144

[1]

and thus falls short of the maximum concentration limit by a factor of—————– 1 , ■ j, • The highest

cos2 f sin2 f

concentration for a single stage given by Eq. (5.1) occurs for f = 45°, where cos2f sin2f = 1/4. Thus, even for this best single-stage concentrator, the shortfall with respect to the limit of Eq. (1.2b) is at least a factor of 4. However, by a suitable choice of secondary, one can recover much of this loss in concentration.

Nonimaging secondary designs are quite similar in concept to the single-stage CPCs al­ready discussed in that their shape is developed by application of the so-called edge-ray principle (Welford and Winston, 1978, 1989) in the particular geometry being employed. For most types of conventional nonimaging secondary, the device is characterized by a design acceptance angle, fa, and in three dimensions the corresponding secondary can achieve a geometric concentration factor of

Updated: August 23, 2015 — 7:38 am