Progress often occurs at the interface between established disciplines. That is why the entree of a journal by The Optical Society of America into the energy field, long the province of engineering, is welcome. This fusion of the science of light (optics) with the science of heat (thermodynamics), highlighted by Shatz et al., is where much of the excitement is today. During a seminar at the Raman Institute in Bangalore, Professor Radhakrishnan famously asked “how come geometrical optics knows the second law of thermodynamics?” This provocative question from Raman’s son serves to frame this inaugural issue on solar concentration. A few observations suffice to establish the connection. As is well-known, the solar spectrum fits a black body at 5670 K. It follows from the Stefan Boltzmann law the solar surface flux is Φs ~ 58.6 W/mm2 while the measured flux at top of the earth’s atmosphere is 1.35 mW/mm2 . That the ratio, ~ 44,000, coincides with 1/sin2 Θs where Θs is the angular subtense of the sun is not a coincidence but rather illustrates a deep connection between the two subjects (the sine law of concentration). Many of the papers in this issue exploit this connection in various ways to develop novel solar concentrating devices; both high concentration (V. D. Rumyantsev, Benitez et al., J. M. Gordon, Araki et al.) and low concentration (R. Winston and W. Zhang explores the limits of stationary concentrators). Taken together, they provide a broad perspective of the state- of- the- art of solar concentrator technology.
The sun is a very bright source indeed; the “brightness” (or radiance) B= Φs /π (B = power/etendue, where etendue in optics has the role of “phase space” in thermodynamics). The conservation of brightness is basic to solar energy concentration. In fact, photovoltaic conversion of solar flux to work (electricity) is made possible by the high temperature of the solar surface and even the limiting performance of a hypothetical sequence of multi-junction cells - which is about 85% - is bounded by Carnot efficiency considerations. Remarkably (see S. Kurtz and J. Geisz) we are nearly half-way there with three junctions. The conservation of brightness can be modified by including processes where the spectrum is down-shifted in a luminescent medium (hv→hv’). Thermodynamics provides the framework since entropy depends linearly on energy (heat) deposited in the medium but logarithmically on brightness. It follows that brightness can increase exponentially with (hv−hv’)/kT, where k is Boltzmann’s constant. Since the energy shift is ~ 0.5 electron volts while kT is ~ 0.025 electron volts at room temperature, this effect can, in principle be large and is not subject to the sine law of concentration. New results in this area of “luminescent concentrators” are presented in two papers by Mulder et al. Additionally, A. Steinfeld and A. W. Weimer reviews how concentrated solar energy can be used to produce clean fuels, which may be an efficient way to store the energy derived from the sun, while X. Jiang’s paper on the vacuum tube solar thermal collector provides a window into one of the most prevalent collector technologies on the planet,
In assembling this inaugural issue, every effort was made to include a broad variety of technologies and invite the most distinguished scientists and engineers in their respective subject areas. To the extent the content is useful both for present and future workers in solar energy, the credit belongs to them. There is one area that is conspicuously absent (not for want of trying by the editor) which is the solar tower. The solar tower is based on the premise that it is more cost-effective to transport light than heat. This concept originated in the former Soviet Union in the middle of the last century and later developed in United States and Europe. Today’s solar towers have come a long way since the demonstrations of the 1980’s and 90’s (Solar One and Two in the Mojave Desert), which were not particularly successful. The figure shows a contemporary20 MW (electric) solar tower outside Seville, Spain (See Fig. 1). The combination of form and functions is stunning and suggests a promising technology path for solar thermal concentration on a large scale.