CSP Technologies

The Ivanpah Solar Electric Generating System in California uses 173,500 mirrors to concentrate sunlight onto three towers, heating a fluid to over 500 degrees C. Unlike solar PV, concentrated solar power (CSP) generates electricity through a heat engine, the same thermodynamic pathway as a coal or gas plant.

CSP systems use mirrors or lenses to focus direct sunlight onto a receiver, heating a working fluid that drives a steam turbine via the Rankine cycle. Three main designs exist. Parabolic trough: curved mirrors focus light onto a receiver tube running along their focal line, heating oil to 400 degrees C. Most commercially proven. Power tower: a field of flat mirrors (heliostats) tracks the sun and reflects light to a central receiver atop a tower, reaching 500-600 degrees C. Higher temperatures enable higher Carnot efficiency. Linear Fresnel: flat mirrors approximate a parabolic shape at lower cost but lower optical efficiency.

Compare temperatures and efficiency. Trough: ~400 degrees C, 15-20% solar-to-electric. Tower: ~560 degrees C, 20-25%. Higher temperature means a higher Carnot ceiling.

Add thermal storage. CSP's key advantage over PV is dispatchability: molten salt can store heat for 6-12 hours, enabling generation after sunset. The Gemasolar plant in Spain has run continuously for 24 hours on stored heat, sustaining round-the-clock output for 36 consecutive days in 2013.

Why can CSP store energy cheaply as heat while PV requires expensive batteries?

Storage is upstream of the turbine. CSP collects heat first, then converts to electricity. Storing heat in molten salt is far cheaper per kWh than storing electricity in batteries. PV converts to electricity first, so storage must happen after conversion.

CSP requires strong direct sunlight (DNI above 5 kWh/m2/day), limiting it to deserts and sun-belt regions.

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