Concentrated Solar Power

Introduction

Fig. 1: View from the ground of one of the towers of the Ivanpah Solar Electric Generating System located near the border of California and Nevada in the Mojave Desert. The glare in the image is a result of the ground-based mirrors concentrating sunlight towards the top of the tower. (Source: Wikimedia Commons)

Concentrated solar power (CSP) is a means of concentrating energy (heat) from the sun which can then be used for a variety of purposes, chiefly among them powering the electric grid. This is as opposed to photovoltaic solar farms which generates electricity directly from solar rays. This distinction is important because in the case of CSP the capture/collection of energy is decoupled from generating electricity which provides a variety of benefits in terms of versatility and storage. As of July 2020, CSP is responsible for 7 GW of power globally while the United States produces about 1.7 GW where the majority of this power is from parabolic trough mirrors/lenses. [1] Recent technology uses a tall central collection tower where solar rays can be concentrated to achieve higher temperatures as shown in Fig. 1. Other promising technology includes dishes that can actively track the sun as it passes across the sky and funnels sunlight directly to a Stirling engine to generate electricity. [2] The result of this is even greater efficiency; dish Stirling systems have a solar efficiency above 30%, which is measured by the amount of incident solar rays converted into electricity on the grid. [3] Unfortunately, this is the least popular of all methods of CSP by manufacturer, with only 4 out of 42 solar heat for industrial processes suppliers surveyed by Solar Payback produce concentrated dishes (for reference, 18 of the 42 produce parabolic troughs and 10 produce flate plate systems); since this technology is the only system likely to use Stirling engines, they are correspondingly not popular in industry and reflect a very small market share. [5]

Achieving Higher Temperatures

Existing deployed CSP technology is typically the parabolic trough mirror system concentrating into a long tube filled with a liquid used to transfer heat from the mirrors to an engine or storage unit. [1] This ubiquity is due to the fact that this technology was developed in the 80's, and has become more mature to the point of commercialization. The limitations of this existing technology is that it cannot achieve a very high temperature. The reason this is important has to do with the ideal efficiency equation for an engine (given below) used to convert the heat energy from the CSP to useful energy.

Fig. 2: Ideal Stirling engine efficiency with cold reservoir of 370°K as a function of the hot reservoir temperature, per Table 1. [4] (Source: J. Barnett)

The efficiency is proportional to the difference between the hot and cold reservoir divided by the temperature of the hot reservoir. The practical implication of this equation is that if the cold reservoir can be made colder - or the hot reservoir hotter - the efficiency can be increased. This is why cutting-edge CSP technologies given in Table 1 are important as they offer much hotter temperatures for the hot reservoir for a Stirling or (more frequently) a steam engine, providing greater efficiency.

Referring to Fig. 2, we see that the parabolic dish and Heliostat field offer much greater maximum theoretical efficiency than the parabolic trough configuration. It should be noted that in real life performance, such numbers are not achieved, which is limited by - among other reasons - the properties of the material used in construction. For instance, the parabolic disk peak efficiency a little over 30%, higher than any other CSP. [3] Further, if it were to be made of steel, it would be limited to a temperature a little under 870°K when it begins to lose its strength. Using this new peak temperature yields merely an efficiency of 57%. For some context, a typical steam engine can achieve a thermal efficiency of over 70%, although this is using natural gas instead of solar. [6]

Storage Efficiency

There is a severe mismatch between peak solar activity and peak energy consumption, requiring a form of storage to utilize this energy when it is needed the most. With existing technology, it is far more efficient to store thermal energy as opposed to chemical in the form of battery storage that PV requires. For now, CSP research and development will continue to focus on commercial viability with the support of government agencies to provide an effective means of renewable energy generation.

© Joshua Barnett. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] "Energy Storage Grand Challenge - Draft Roadmap," U.S. Department of Energy, July 2020.

[2] T. Mancini, et. al, "Dish-Stirling Systems: An Overview of Development and Status," J. Solar Energy Eng. 125, 135 (2003)

[3] J. Coventry et al., "Dish systems for CSP," Solar Energy 152, 140-170, (2017).

[4] M. U. H. Joardder, et al., "Solar Pyrolysis: Converting Waste Into Asset Using Solar Energy," in Clean Energy for Sustainable Development, ed. by A. K. Azad, A. K. Rasul, and S. C. Sharma (Academic Press, 2016).

[5] "Solar Heat for Industry," Solar Payback, March 2017.

[6] "Combined Heat and Power Technology," U.S. Office of Energy Efficiency and Renewable Energy, DOE/EE-1334, July 2016.