The comeback kid. Return of CSP to aid round-the-clock green power

Concentrated Solar Power (CSP), which had been almost elbowed out of business by the low-cost solar PV, is now making a comeback. Quantum learns that recently NTPC, the state-owned power generating company, sent out feelers to various companies asking if they would be interested in putting up CSP plants; and the response was overwhelming.

“CSP with thermal storage can provide 24x7 renewable energy at less than ₹3 per kWhr,” says Rajan Varshney, Deputy General Manager at NTPC, in a LinkedIn post.

While producing electricity from CSP is one option, a few others are being explored in the scientific and business worlds. CSP works by concentrating sun’s rays onto a point to generate heat. Why convert the heat to steam and producing electricity? Why not use the heat directly?

NTPC wants CSP companies to produce and sell hot steam, which reduces the load on the boilers to heat up the steam—implying burning lesser coal and smaller carbon footprint. Some others are suggesting that CSP heat can be used to produce hydrogen.

Prof KS Reddy and Saurabh Mohite of the Heat Transfer and Thermal Power Laboratory, Department of Mechanical Engineering, IIT-Madras, have recently published the results of their research into photo-electrolysis of water using heat from CSP.

Their method is simple. Use the heat produced by the CSP plant to heat zinc oxide, splitting it into zinc and oxygen. Put the zinc into water, the metal will grab the oxygen in the water to form zinc oxide again, which leaves hydrogen in the water. The second reaction—hydrolysis of zinc—releases heat, about 104 KJ/mole. This is lesser than the 456 KJ/mole demanded by the first reaction, but there is some additional heat kick-back  from the second reaction. In a paper in Energy Conversion and Management, Reddy and Mohite say that the method yields significantly more hydrogen. “Though CSP is an old system, it is gaining popularity among researchers for hydrogen production due to its high temperature applications in the range of 500–2000°C,” the paper noted.

Further, it doesn’t have to be zinc oxide—any metal oxide will work, though each one would need to be studied for its yield.

CSP technologies come in different forms—Parabolic trough collectors (PTC), linear fresnel reflectors, parabolic dish collectors and solar power towers (SPT). In 2020, the world had 6.5 GW of CSP, most of it was PTC. However, solar power towers are gaining more attention as they are able to achieve high solar concentration factors — over 1,000 suns — and operate at higher temperatures, 1000o C and above. “SPT presents a higher room for improvement and a bigger potential for cost decrease due to a higher solar-to-electric efficiency, higher energy densities, a low-cost solar field, less maintenance and oil-free plants with lower environmental impact,” says Javier Bigorri et al of the National Renewable Energy Center, Navarre, Spain.

Experts now are suggesting combining CSP plants with solar PV and wind, so that the same system is used for storing electricity from multiple sources. The conventional way of storing CSP energy has been molten salts, but more advanced CSP-linked storage technologies are emerging—(i) Sensible Heat Storage including novel molten salts, sensible packed-bed thermocline and liquid metals; (ii) Latent Heat Storage: phase change materials packed-bed thermocline and (iii) Thermochemical Energy Storage: hydrides, hydroxides, carbonates and redox reactions.

In sum, CSP is padding up for its second innings.

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