Barton, N. April 17, 2013. ESOI for solar thermal.

http://sunoba.blogspot.com/2013/04/esoi-for-solar-thermal.html

Published information is available to evaluate the ESOI score for the most common solar thermal storage technology – a molten 60-40 mixture of sodium and potassium nitrates, commonly known as solar salt.

Burkhardt, Heath and Turchi [2] made a life cycle assessment of a hypothetical 100 MW parabolic trough concentrating solar plant at Daggett, California. The storage envisaged is 62,000 tons of solar salt, capable of storing 1,988 MWh of thermal energy, which can be converted into an electrical equivalent by multiplying by the thermal-electric efficiency of the plant.

Many individual items were taken into account by Burkhardt et al. to calculate the embodied energy of the storage component of the plant; these included obvious items like steel, concrete, pumps, heat exchangers, insulation and solar salt. However the biggest single item is the energy required to keep the salt molten and stirred for daily operations.

It’s noteworthy that the embodied energy of solar salt is low if it mined (as assumed to be the case in [2]), but high if it produced synthetically. In the latter case, which Burkhardt et al. say applies to slightly more than half of all installations, the manufacturing process involves pre-production of ammonia, for which there is a natural gas requirement.

I have also made an as-yet unpublished estimate for the ESOI score for thermal storage in air-blown pebble beds. This estimate is in the context of a new concept for solar thermal power generation entitled BRRIMS, denoting Brayton-cycle, Re-heated, Recuperated, Integrated, Modular and Storage-equipped. Here what needs to be considered is the embodied energy in hardware such as steel tanks, ducts, concrete footings, insulation and pebbles. Heat exchangers, pumps and fans are not required.

Results of Barnhart & Benson can now be extended as follows, with the new data highlighted. This is a fair comparison (“apples with apples”) between storage technologies since the new figures represent electrical energy that would be produced from the underlying thermal storage.

The simple conclusion from the ESOI metric is that geologic storage is excellent, thermal storage is good, whilst electrochemical storage is poor.

That is not the whole story however. Geological storage is not particularly cheap, and its applicability is limited by the availability of suitable sites. My estimates show that thermal storage is the cheapest option, and I propose to present details of this work at the World Renewable Energy Congress in July.

References
[1] C J Barnhart and S M Benson, “On the importance of reducing the energetic and material demands of electrical energy storage”, Energy Environ. Sci., 6 (2013), 1083.

[2] J J Burkhardt III, G A Heath and C S. Turchi, “Life cycle assessment of a parabolic trough concentrating solar power plant and the impacts of key design alternatives”, Environ. Sci. Technol. 45 (2011), 2457–2464.