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| Fig. 1: A Photovoltaic (PV) power plant at Nellis Air Force Base northeast of Las Vegas. (Source: Wikimedia Commons) |
Photovoltaic (PV) and Concentrated Solar Power (CSP) technologies, as depicted in Figs. 1 and 2, are two of the principle means of converting solar energy into electricity. PV systems use solar panels to convert energy from the sun into direct current (DC) before an inverter converts DC into alternating current (AC), which is then distributed. [1] In contrast, CSP systems utilize an array of mirrors that reflect sunlight to a central tower which concentrates thermal energy to power a steam engine, thereby producing AC electricity. [1] This paper will compare and contrast (i) the energy efficiencies of PV and CSP and (ii) the economic considerations associated with both. In particular we will examine the levelized cost of electricity (LCOE), which is defined as the price at which electricity could be sold in order for the given power system to break even by its expiration.
The efficiency of PV systems in large part depends on the material used in the construction of the given solar cells. [2] Initial PV systems with cruder technologies were only able to convert approximately 1% of sunlight into electricity. [2] Today, that metric is in the range of 17% to 20% for many commercial PV power plants, though research has demonstrated potential efficiencies closer to 50%. [3] One notable efficiency advantage of PV systems is that they are less sensitive to clouds than CSP, enabling them to produce electricity across a spectrum of weather patterns. [4] On the other hand, since electricity storage at the commercial scale is a non-trivial problem, a disadvantage of PV systems is that they are not productive at night.
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| Fig. 2: A Concentrated Solar Power (CSP) plant in Spain known as Gemasolar. (Source: Wikimedia Commons) |
The efficiency of CSP systems rather depends on two chief factors: the thermal efficiency of the steam cycle at hand, and the clarity of the sky. [5] Modern steam cycles currently in use for CSP systems lie in the range of 42% thermal efficiency; nonetheless, combined cycles, which essentially recycle a portion of excess heat and are already used for other power systems, may raise this figure to 60% or more for CSP systems. [5] CSP also requires the sun's irradiation to be focused onto a central tower receiver, and thus, haze and clouds which diffract light can drastically limit the efficiency of the system. [4] This makes CSP practically constrained to arid or semi-arid regions, in contrast to PV. [1] However, when CSP systems are equipped with thermal storage made possible by molten salts heated during sunlight hours, they can continue to generate electricity in the night, which is an advantage over PV. [1] In total, the theoretical maximum efficiency of a CSP plant is approximately 65% with high-tech selective absorbers, though the practical maximum efficiency for most plants is closer to 30%. [6]
Despite the best-case efficiencies of CSP systems outperforming PV systems, the actual investment trends have favored PV; as of 2021, the United States was host to more than 50 PV power plants with capacity greater than or equal to 100 Megawatts (1 million Joules / second) but only 4 CSP plants under the same capacity criterion. [4] PV is currently favored over CSP for a variety of reasons, including relative simplicity, reliability, and projected lower levelized cost of electricity (LCOE). [4]
Specifically, the real LCOE of PV is less than that of CSP. Boretti and Castelletto estimate the LCOE of large-scale (min. 100 MW capacity) United States PV systems at 0.03 US$ / kWh while CSP systems considered under the same criteria are closer to 0.06 US$ / kWh. [4] Notably, Boretti and Castelletto claim that CSP with thermal energy storage is cheaper than PV with battery storage. [4] Future advancements in battery technology may eventually nullify this claim.
Also worth consideration is the fact that CSP systems in principle require less land per unit of electricity than PV systems. [7] That said, CSP systems are not particularly modular and require a certain amount of land in order to be practical, while modern PV systems are available in many relatively small consumer goods, and are thus in a sense a more generalizable technology.
In this paper we saw that the energy efficiency of modern commercial Photovoltaic (PV) power systems is approximately 20%, while Concentrated Solar Power (CSP) systems are closer to 30%. Despite this, PV is still economically favored, as it is cheaper than CSP power by roughly a factor of 2. CSP as equipped with thermal energy storage offers the advantage of providing electricity during the night, which is a shortcoming of PV, but CSP is less reliable and more technologically difficult than PV. In total, the future of solar power may involve a complementary relationship between both systems.
© Nathanael Cadicamo. 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.
[1] J. Hernández-Moro and J. M. Martinez-Duart, "Analytical Model For Solar PV and CSP Electricity Costs: Present LCOE Values and Their Future Evolution," Renew. Sustain. Energy Rev. 20, 119 (2013).
[2] R. Venkateswari and S. Sreejith, "Factors Influencing the Efficiency of Photovoltaic System," Renew. Sustain. Energy Rev. 101, 376 (2019).
[3] "Photovoltaic Energy Factsheet," University of Michigan, Pub. No. CSS07-08, August 2023.
[4] A. Boretti and S. Castelletto, "Cost and Performance of CSP and PV Plants of Capacity Above 100 MW Operating in the United States of America," Renew. Energy Focus 39, 90 (2021).
[5] M. T. Dunham and B. D. Iverson. "High-Efficiency Thermodynamic Power Cycles For Concentrated Solar Power Systems," Renew. Sustain. Energy Rev. 30, 758 (2014).
[6] K. Burlafinger, A. Vetter, and C. J. Brabec, "Maximizing Concentrated Solar Power (CSP) Plant Overall Efficiencies By Using Spectral Selective Absorbers at Optimal Operation Temperatures," Sol. Energy 120, 428 (2015).
[7] A. H. Alami et al., "Concentrating Solar Power (CSP) Technologies: Status and Analysis," Int. J. Thermofluids 18, 100340 (2023).
