Category Nonimaging Optics in Solar Energy

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...

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Applications in the Lunar Environment

In the lunar environment, solar energy is almost half again as intense as typical terrestrial levels (1350 W/m2 versus about 900 W/m2) and nearly constant during the 2-week-long “lunar day.” Of course, accommodation of the lunar night will preclude long-term, continuous solar-driven produc­tion and, lacking some kind of long-term thermal storage, will require some kind of two-phased monthly cycle. However, the energy requirements for processing lunar materials into cement are such that abundant long-term average production can easily be maintained. In particular, 1 kg of cement requires an input of about 1000 kcal...

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Using Highly Concentrated Sunlight in Space

The techniques of nonimaging optics are particularly valuable in space or lunar environments in which the use of solar thermal energy has obvious advantages. Earlier preliminary studies have ex­plored this concept for the production of cement from lunar regolith and for solar thermal propulsion in space. For example, extremely high temperatures, in the range of 1700-1900 °C, are necessary for the production of cement from lunar minerals. Such temperatures will in turn require very high levels of solar flux concentration. Energy budgets for the support of permanent manned operations on the lunar surface are expected to be limited. For high-temperature thermal (i. e., >300 °C) end uses, direct solar energy has obvious advantages over most other practical power sources...

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Some exotic applications for ultrahigh solar fluxes

From the earliest development of these nonimaging devices, when it became apparent that the thermodynamic limit on concentration could be approached, it was appealing to consider how one might practically develop very high levels of concentrated solar flux, in principle approaching even those found on the surface of the sun. However, pursuit of such objectives was deferred while the lower-concentration applications were developed.

Eventually, however, there occurred very rapid progress from the first ultrahigh flux measure­ments conducted on the roof of the high-energy physics building of the University of Chicago in 1988 to the experimental investigation of potential laser pumping and materials processing experi­ments carried out at the National Renewable Energy Laboratory High Flux Solar...

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It must be emphasized that this is a preliminary study. The assumptions on which it is based are very idealized. For instance, the treatment of the incident angular distribution as a pillbox and the requirement that the secondary and target be sized so as to achieve 100% intercept are somewhat extreme. In practice, the optical errors are more likely to be better represented by some form of Gaussian distribution and the trade-off between concentration and intercept may yield relationships that differ in some details from those found here...

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Summary and Conclusions

A preliminary analysis of various performance trade-offs involved in designing a two-stage central receiver plant secondary to achieve ultrahigh concentrations using a nonimaging CPC type has been carried out. The approach was based on simple geometry and the optical characteristics of CPCs.

We find that the highest possible concentrations can only be achieved with an axially sym­metric circular field surrounding a central tower with a CPC looking vertically downward. In this configuration, the achievable concentration increases asymptotically toward the ideal limit as the CPC acceptance angle is reduced and the tower height increases...

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Limits to Central Receiver Concentration

The approach is based on simple geometry and the optical characteristics of CPCs. The goals are (1) to fill the field of view of the CPC as much as possible and (2) to identify those design factors





FIGURE 9.1: (a) The simplest geometry for a two-stage central receiver is a central tower (height H) surrounded by a circular heliostat field. The secondary is a simple CPC with acceptance angle в. The field radius is R = H*tanec. (b) Alternative geometry for a two-stage central receiver has a CPC whose optical axis is tilted at an angle g (toward the north in the northern hemisphere) to accommodate a he­liostat field that has lower obliquity corrections when tracking the sun...

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Ultrahigh Concentrations


9.1.1 Introduction

In this chapter, a preliminary analysis that carries out various performance trade-offs involved in the design of a two-stage central receiver plant that could achieve ultrahigh concentrations with a nonimaging CPC type secondary is presented. It should be noted that the study was carried out in the context of using a solar central receiver plant to generate hydrogen from the direct thermo­chemical splitting of water. However, the analysis is independent of that context and applies to any and all central receiver (or the so-called power-tower) configurations...

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