Wednesday, April 17, 2013

ESOI for solar thermal

Recently, Barnhart & Benson [1] introduced a new metric to evaluate various technologies for energy storage.  They analysed seven storage technologies based on batteries, flow batteries and geologic storage, but did not consider thermal storage. 

The unanswered billion dollar question is how well do solar thermal storage technologies rate on their metric?

The Barnhart-Benson metric, Energy Stored On Invested (ESOI), is the ratio between the energy a device can store in its entire life and the energy required to build the device.

The larger the ESOI, the better is the storage system.  Larger values of ESOI can be obtained by
  • increasing the number of cycles
  • increasing the round-trip efficiency
  • increasing the depth of discharge
  • decreasing the embodied energy
Barnhart & Benson gave the following ESOI values:
 
Technology
ESOI
compressed air energy storage
240
pumped hydro storage
210
Li-ion battery
10
Sodium-Sulphur battery
6
Vanadium redox battery
3
Zinc-Bromine battery
3
Lead-acid battery
2

The conclusion by Barnhart & Benson was that

“over their entire life, electrochemical storage technologies only store 2-10 times the amount of energy that was required to build them”.

Clearly that news will not be welcomed by proponents of electrochemical storage. You can bet that feverish work is under way in hundreds of research laboratories around the world to boost the ESOI score.

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 t 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. 

Technology
ESOI
compressed air energy storage
240
pumped hydro storage
210
pebble bed thermal, BRRIMS
62
solar salt, parabolic trough [2]
47
Li-ion battery
10
Sodium-Sulphur battery
6
Vanadium redox battery
3
Zinc-Bromine battery
3
Lead-acid battery
2

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), 24572464.

 
 

3 comments:

  1. There are some other important variables.

    Ease of siting. Building a new pump-up could mean years of permitting and a fairly long construction period.

    Transmission. Build a new pump-up/CAES in a somewhat remote area and there could be a significant cost, and more permitting problems, running transmission.

    Storage built into "shipping containers" can be parked around the grid, creating distributed storage and minimizing permitting/transmission costs.

    And containerized storage is modular. Easy to add more or move to a different site in order to maximize the value.

    The "10" for lithium batteries may change, or at least a new category of zinc-air batteries may need to be added. Eos is apparently manufacturing a battery at $160/kWh and may have as many as 10,000 cycles in it.

    That might still be 2x as much as pump-up, but the price might be low enough to get a lot installed. Convenience.

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