The development of the high-pressure steam engine by James Watt at the University of Glasgow in Scotland during the early 1800's led to the discovery of thermal energy storage. When the water level was at the specified level inside a drum-type boiler, the high-pressure saturated water would briefly remain at high temperature inside the drum after the flame under the boiler had been extinguished. That water would produce flash steam still drive a steam piston engine for a considerable amount of time.

Pressure Vessels:

By the late 1800's, insulated pressure vessels appeared that could store saturated water at high-pressure and high-temperature for extended durations. The world's first submarine craft were powered by stored thermal energy, as were some ferryboats. Fireless steam ferryboats usually operated from a terminal that was near a source of high-pressure industrial steam and could rapidly recharge after each return crossing of a narrow channel. Railways that owned shunting yards located near a source of industrial steam operated fireless steam locomotives for shunting duties.

Unlike batteries that quickly expired after repeated deep-cycle discharges and recharges, steam-based stored thermal energy technologies could easily withstand many hundreds of repeated deep-cycle discharges and recharges with zero deterioration in performance. Fireless steam locomotives, ferryboats and submarines now grace museums of technology. However, steam-based stored thermal technology has found application in several industries as well as becoming part of some small-scale, low-cost solar thermal energy applications.

Heat-of-Fusion Storage:

During the era of the fireless steam locomotive, several locomotive builders experimented with heat-of-fusion thermal energy storage concepts and also with heat-capacity thermal storage to generate steam. Some of these early efforts began in Denmark around 1924 and involved molten caustic soda, except the thermal storage technology proved problematic. The solar thermal power industry has undertaken further research into heat-of-fusion thermal energy storage technology.

They seem to have developed a successful thermal energy storage system, based on a mixture of naturally occurring salts such as sodium nitrate (60%) and potassium nitrate (40%) that melts at 565-deg C. This mixture can raise superheated steam that drives turbine overnight, receiving heat during daylight hours. There seems to be scope to apply this storage technology to the nuclear industry, especially small-scale nuclear being developed by Toshiba and by Nu-Scale that involve power outputs of 10MW to 100MW. The stored thermal energy system would recharge overnight and deliver power to the market during the AM and PM peak periods.

There are numerous alternative industrial applications for thermal energy storage technology. Several businesses operate industrial furnaces and stoves during the weekday peak periods when power prices are high. They have use for heat-of-fusion technology that may use lower-cost off-peak electric power to recharge thermal energy storage systems. In large cities, numerous restaurants and diners that offer a breakfast are packed with customers during the AM peak period. They may also use stored thermal energy to prepare for their lunchtime rush of customers.

Small-Scale Generation and Storage:

During the overnight off peak periods, the small-scale nuclear power generation may operate at constant maximum output. During daylight hours, all the steam drives turbines that drive generators to produce electric power. During the off-peak period, the steam is directed to the thermal energy storage system that may supply the market with power during the peak periods. At the present day, there is much research involving high-temperature nuclear reactors that operate at temperatures in excess of 1000-deg C and that may be cooled by helium, carbon dioxide or even compressed air.

The aluminum industry is able to provide a high-temperature thermal energy storage mixture of naturally occurring bauxite (Al2O3) and cryolite (AlNa3F6) that may melt within the temperature range of high-temperature nuclear reactors in the 10MW to 100MW power output range. There are also naturally occurring b-metallic aluminum compounds such sodium aluminate (Na-O-Al=O) and potassium aluminate (K-O-Al=O) that occur in parts of the USA that may also be used as a basis for a low-cost thermal energy storage mixture. When in the molten state, such a mixture could sustain the operation of an externally heated, closed-cycle gas turbine engine that circulated compressed air or compressed carbon dioxide.

Heat Capacity Thermal Storage:

While some ancient cultures use pre-heated rocks for cooking, there are modern applications for such thermal storage technology. Based on mass or on volume, the heat storage capacity of most compounds is generally too low to be of interest to the power industry. Like compressed air storage, heat-capacity thermal storage technology requires massive volume and massive mass to be of interest to the power industry. New research from the USA and from the UK involves steam-based, heat capacity thermal storage on a massive scale of mass and volume that also involves relatively low cost. It has its origins in fireless steam power of an earlier era, with some assistance from the natural world.

Large-Scale Thermal Energy Storage:

A research group headed by Dr Charles Forsberg at MIT has explored the possibility of large-scale, underground or geothermal thermal energy storage in porous rock located deep underground. The MIT research aims to use the economy of scale as a means by which to reduce overall costs. In many geographic regions, geothermal energy wells eventually cool and lose the ability to raise steam. These wells become candidates for larger-scale geothermal energy storage, including seasonal energy storage.

The research at MIT has indicated that the sheer volumetric scale of a geothermal energy storage system compensates for thermal energy losses due to conduction through adjacent rock. Such a system could theoretically sustain the operation of a 1,000MW thermal power plant for up to 12 months. There are power markets around the world that peak during a single season every years, either to provide electrical energy for air conditioners or to provide energy to heat homes and buildings.

A competing team at Isentropic Energy in the UK seeks to store thermal energy in two by nearby geothermal wells composed of porous rock, one being the hot reservoir at 500-deg C and the other being the cold reservoir near the freezing temperature of water. Insulated pipes may carry steam from the heated reservoir while similar insulated pipes would carry cooled water from the heat sink, with the thermal power station being located near the midpoint between the two underground reservoirs.

Challenges of Geothermal Storage:

Large-scale geothermal energy storage could prevail at locations where an abundance of water deep underground in permeable and porous rock and gravel is present. Saturated water at high temperature and pressure would involve low density. It would require substantial difference between the main reservoir and ground level to remain in the saturated state, example: a vertical elevation of 4000-ft below ground surface for saturated water with a density of 40-lb/cu.ft would maintain a pressure of up to 1100-psia and temperature of 290-deg C or 556-deg F.

Geothermal installations produce contaminated steam that produces mineral deposits that form inside piping systems, resulting in steam pipes that need to be cleaned frequently. Special turbines need to be able to operate using steam that may contain small particles. Depending on the temperature and pressure of the saturated water deep underground, special separators could swirl out impurities and droplets of water to allow dry steam to expand in the turbines.

Exhausted natural gas wells that occurred in permeable rock in arid regions may be free from water, allowing for circulation of superheated compressed carbon dioxide that would heat the porous rock to over 500-deg C or 1000-deg F during the recharge cycle. When power is needed, the circulation of compressed carbon dioxide would transfer heat from the underground rock to a boiler that would raise superheated steam. The steam would activate turbines that would drive electrical generating equipment.

The circulation of cooled carbon dioxide or atmospheric air could help remove heat from porous rock, perhaps cooling the rock strata to below the freezing point of water as the heat pump transfers heat to the surface. The temperature difference between a hot well and a cold well could sustain the operation of a heat engine that drives electrical machinery for several hours, weeks or even months as could be possible with extensive hot and cold geothermal reservoirs.

Applications for Geothermal Storage:

Large-scale geothermal storage would provide a thermal reservoir for clean-coal and nuclear power stations. During a future emergency, steam may rapidly be re-directed from the turbines to geothermal storage, with a residual amount of steam going through the turbines to maintain temperature or to allow for gentle cooling over an extended duration. While China leads the world in building clean-coal and nuclear power stations, several Middle Eastern countries plan to introduce nuclear power while South Africa has 2-clean coal power stations of almost 5,000MW each slated to open soon.

There is much porous rock deep underground across the Middle East, where the oil and gas industry has used massive volumes of seawater to displace the oil and natural gas. Exhausted wells that are filled with seawater may form the basis of either a geothermal heat sink, or a high-temperature geothermal reservoir. While South Africa is known to have hot springs, it is unknown as to whether the new and planned thermal power stations will be located anywhere near occurrences of underground porous rock. A peak seasonal demand for electric power occurs in all 3-markets that also have an urgent need to develop new generating capacity along with energy storage capacity.

Emergencies:

Events such as solar storms and ice storms can rapidly incapacitate long-distance transmission lines. During such times, the close proximity between thermal storage capacity and thermal power station will allow for a rapid re-direction of thermal energy from generation, into storage. A minimal amount of steam may flow through turbines to allow for a gentle cool-down, greatly reducing the onset of thermal stresses that may result in costly repairs.

Conclusions:

Large-scale seasonal geothermal energy storage promises to lower the cost per MW of grid-scale energy storage and improve the productivity and cost effectiveness of thermal power stations. Much of the storage medium already exists in the earth, or is in the process of being developed as a water-displacement byproduct of the oil and gas industry. Even small-scale thermal storage technology offers the prospect of low cost per kW (or MW) using naturally occurring compounds. Both large and small-scale thermal storage compounds and technologies offer the prospect of greatly extended service lives that may perhaps be measured over a period of centuries.

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