ESR Cycles
An Electrolysis-Storage-Reconversion cycle (ESR cycle) consists of three main processes: (1) electrolysis of a chemical compound to produce chemical energy in the form of a combustible fuel which can be stored indefinitely, (2) combustion of the fuel in a heat engine to produce mechanical energy, and (3) utilization of the mechanical energy to perform a desired task. The most common ESR cycles involve the electrolysis of water to produce hydrogen. Hydrogen can be stored as a compressed gas or it can be converted to a secondary fuel such as ammonia, which, in its liquid state, has a much greater energy density than compressed hydrogen. In any case, a fuel’s stored chemical energy can be released when needed by burning it in a heat engine, thereby producing mechanical energy which can be used to power a transportation vehicle or drive an electric generator. In this way, ESR cycles provide long-term storage of electrical energy, and that is especially useful when the electrical energy is derived from a variable resource such as solar radiation.
As recently as 5 or 10 years ago, ESR cycles were not considered to be cost-effective options for long-term storage of solar electricity, primarily because of inefficiencies in the processes comprising various proposed cycles. Commercially available water electrolysis units had maximum efficiencies (with respect to hydrogen production) in the range of 55 to 65 per cent. Best options for heat engines included gas turbine engines with efficiencies in the range of 35 to 40 percent and reciprocating diesel engines with efficiencies of to 30 to 35 per cent. If all other energy losses were ignored, ESR cycles utilizing these electrolysis/fuel/heat engine technologies resulted in end-to-end efficiencies (electrical to fuel to electrical) in the range of only 20 to 30 per cent. These low end-to-end efficiencies, when coupled with the relatively high cost of solar electricity, meant that solar electricity stored and regenerated by ESR cycles was not cost-competitive with electricity generated directly from fossil or nuclear fuels. But a lot has changed in the past few years.
First, the cost of solar-generated electricity has dropped dramatically because of increases in solar cell efficiency and decreases in the fabrication costs of solar cell arrays. In the near future, the cost of solar electricity will decrease even further as advanced cogeneration units like the SEA solar energy conversion system are deployed on a broad scale.
Second, modular water electrolysis units with efficiencies in excess of 82 per cent are now commercially available and are being brought into operational service in large numbers.
Third, more efficient heat engines are now commercially available. There has been steady progress in this area. Two-stroke diesel engines with efficiencies approaching 50 per cent have been developed and are currently in operation in a variety of marine applications (www.wartsila.com). Combined-cycle power plants (Brayton cycle gas turbines integrated with Rankin cycle steam turbines) have been operated at efficiencies exceeding 50 per cent (ge.com/power/gas/power-plants).
More recently, an even more important advance has occurred. SEA has designed a high-performance two-stroke reciprocating piston engine which will have an efficiency near 70 per cent (US Patent pending 15,702,597; International Application PCT/US18/49338). In addition to its efficiency, the SEA engine design offers many other advantages, including modularity, mechanical simplicity, and the ability to respond quickly to load changes, all of which are important when dealing with the unavoidable variations of the solar resource.
When SEA high-efficiency engines are integrated with Thyssenkrupp electrolyzers, ESR cycles with end-to-end efficiencies of nearly 60 per cent can be achieved. If Thyssenkrupp electrolyzers are powered by low-cost electricity from SEA solar energy conversion systems, solar electricity can be efficiently stored on a seasonal basis, and the cost of the regenerated electricity will be considerably less than the cost of electricity generated from fossil fuels. The SEA engine design closes the technology loop for ESR cycles. Low-cost seasonal storage of solar electricity will soon be a reality.