Prospects for Future Grid-Scale Thermal Energy Storage
During the latter 20th century, various underground geological formations were adapted for seasonal low-grade thermal energy storage, to provide heating and cooling inside buildings. A recent advance in this technology in Alberta, Canada uses solar thermal collectors to achieve seasonal geothermal energy storage at a temperature of up to 80°C (176°F), to provide winter heating for a community. While parabolic troughs may collect solar thermal energy at some locations, purpose built solar thermal salt ponds may achieve the same task at lower cost at other locations.
The combination of a multitude of exhausted oil/gas wells and extensive strata of porous rock saturated with ground water enhances prospects for such storage at multiple locations across the USA and Western Canada. During winter, temperatures may drop to -40°C (-40°F) at several northern locations. The temperature difference between the cold winter air and stored subterranean heat at 80°C can sustain the operation an Organic Rankine-Cycle (ORC) engine. The size and extent of the thermal storage reservoir would determine the output (Mw-hrs) of such an engine driving electrical generating equipment.
Salt caverns that occur near ground surface are unable to hold natural gas or compressed air at high pressure. There may be scope to hydrate the rock salt and use it for seasonal thermal energy storage at temperatures of up to 150°C (300°F). Large subterranean thermal storage reservoirs may provide heat for buildings, energize banks of ORC engines and/or thermal chimney engines that flow air through turbines. Stored thermal energy may also sustain the wintertime operation of multi-level greenhouses that use up to 80% less water than conventional outdoor agriculture.
Caverns that contain massive volumes of saline groundwater close to ground surface near large urban centers may serve as a seasonal heat sink, due to the freezing point of saline water being below that of pure water. During the cold winter months, the atmosphere may serve as the heat sink to cool the saline water. During summer, the cooled saline water may sustain viable district cooling of large buildings and reduce electrical energy demand required for air conditioning.
Grid-Scale Thermal Storage:
The solar thermal industry identified a need for low-cost, grid-scale energy storage to provide power when clouds cast a shadow over solar collectors, as well as after sunset. One leader in the field is the SENER group of Spain who developed molten mixtures of naturally occurring sodium and potassium salts that store thermal energy. The largest planned thermal energy storage installation in the SW USA will be capable of raising steam at some 220°C to power turbines that generate over 200MWe output for almost 8-hours, sufficient to cover the demands of both AM and PM peak periods.
The molten-salt heat-of-fusion thermal storage initiative has potential in the nuclear power industry. Nuclear power stations operate optimally when the reactors and steam lines remain at constant temperature, with steam lines also operating at near constant pressure. To achieve such an objective, owners of nuclear power stations may sell off-peak power at bargain-basement prices or even pay outside utilities to take the excess off-peak nuclear-electric power. Such operation also enhances prospects for cost-competitive and viable energy storage.
At geographic locations where pumped hydraulic or compressed air storage is unavailable, thermal storage may become a potentially attractive option. Steam lines may carry off-peak thermal energy from nuclear reactors to molten salt-based thermal energy storage installations. The useful life expectancy of thermal energy storage technology greatly exceeds that of various chemical battery storage technologies that offer 4,500 to 5,000-deep-cycle recharges and discharges. Over the long-term, thermal energy storage may be cost-competitive against grid-scale chemical battery storage.
High Temperature Storage:
While older generation, heavy-water nuclear reactors operate at temperatures that are comparable to molten-salt heat-of-fusion stored thermal energy installations, modern light-water nuclear power station operate at higher temperature. The reactors are cooled by helium that transfers the heat to boilers at a temperature near the melting point of aluminum. At such temperatures, boilers may raise super-critical steam capable of producing power at over 40% thermal efficiency.
Instead of using molten aluminum for thermal storage, there may be scope to use molten mixtures of naturally occurring metallic oxide ores that melt near the same temperature. The mineral ore cryolite (Na3AlF6) melts at 900°C to 1000°C and may be mixed with bauxite hydrate (Al2O3.H2O) to reduce melting temperature to near that of a helium-cooled nuclear reactor. Other variations of aluminum fluoride contain potassium (NaK2AlF6) or lithium (Li3AlF6) and may used in thermal storage material.
Some naturally occurring bauxite ores such as diaspore and bhoemite contain hydrogen [AlO (OH)] while other variations contain sodium (NaAlO2) or lithium (LiAlO2). There are numerous possible mixtures of bauxites and cryolite ores that can melt at temperatures that are near the operating temperature of newer generation, light water nuclear technology. Alternative thermal energy storage systems may be based on alternative a compound between the heat of decomposition and heat of formation.
When heated, several metallic carbonates such as calcium carbonate (CaCO3) will decompose and release carbon dioxide (CO2), leaving the metallic oxide calcium oxide (CaO). Unglazed calcium oxide may be reacted under pressure with carbon dioxide to produce the metallic carbonate and release massive amounts of heat. The temperature of the heat of formation of some metallic carbonates is sufficiently high to raise super-heated steam and/or super-critical steam. At some locations, it may be possible to storage massive volumes of compressed carbon dioxide in subterranean caverns and the metallic oxides in above ground silos.
Several compounds that are hydrates release water vapor (H2O) when heated. When some dehydrated compounds encounter water and/or steam, there is either a heat of reaction or a heat of formation as a hydrate is formed. The heat of reaction/formation may occur at a sufficiently high temperature to generate steam that may drive turbines and electrical generating machinery. Banks of insulated and pressurized accumulators may hold saturated water to produce the steam needed to sustain the heat of formation operations.
During off-peak hours, special piping systems may transfer heat from the reactors to thermal storage. During peak periods, stored heat would raise steam to drive turbines and electrical machinery to meet market demand for electric power. During off-peak periods, it may be possible to flow minimal amounts of steam through the piping system to maintain constant temperature and pressure in steam lines connected to the thermal storage system. Such operation may reduce thermal stress problems caused by thermal cycling of thermodynamic components.
While chimney engines that operate on low-grade heat generate air currents capable of driving turbines, other developments are underway that involve ultra-high temperature super-heated air and gas. While some of these developments involve ultra-high temperature concentrated solar thermal energy, other related developments involve pebble-bed modular nuclear technology. Researchers in China are testing ultra-high temperature, helium cooled reactors that operate at over 900°C and sustain the re-circulating flow of helium between the reactor and an externally heated Brayton-cycle turbine engine.
Materials such as silicon carbide maintain constant mechanical and thermal properties at temperatures of up to 1400°C. Turbine wheels made of carbon fiber reinforced silicon nitride can operate at the same sustained temperature. Several metallic oxides and mixtures of such oxides that melt at 800°C to 1000°C may provide thermal energy storage. Several other materials that thermally decompose at elevated temperature also produce the heat of reaction/formation near that temperature and may be used as an alternative to heat-of-fusion thermal storage technology.
The naturally occurring ore cryolite (Na3AlF6) that melts at some 900°C may be used as heat-of-fusion thermal energy storage material, as can a mixture of different concentrations of bauxite and cryolite. The heat of reaction of calcium oxide and pressurized carbon dioxide occurs at over 1000°C, sufficient to sustain the operation of an externally heated Brayton-cycle engine. A compact package of the high-density mixture of the oxide and hydroxide of thorium melts at 900°C to 1000°C and may be suitable thermal storage material for micro-nuclear reactors.
There is scope to combine ultra-high-temperature thermal energy storage with compressed air energy storage. Compressed air may be super heated to over 1000°C (1800°F) and drive a multi-stage turbine engine system that include reheat capability and exhaust heat recovery, along with preheating of the incoming compressed air. The super heated compressed air may energize turbines that drive electrical machinery during peak demand periods, while diverting the power normally allocated to driving turbo-compressors to instead drive electrical generating equipment.
Depending on final exhaust temperature, there may be scope to use the exhaust heat to sustain the preheating requirements for a Rankin-cycle engine or to sustain the operation of thermal seawater desalination during peak periods. As with steam-based power systems, there may be scope during off-peak periods to flow a small amount of super heated compressed air through the piping systems, to minimize problems related to cyclic thermal stresses in the thermal components.
Future thermal energy storage would likely cover the temperature range from the sub-freezing point of water to ultra-high temperatures of some 1000°C. Heat-of-fusion technologies offer greatly extended useful service lives and cost-competitive long-term costs. While compact thermal energy storage systems are possible, most such systems would likely be built on a large scale that involve massive volume. Most future research into thermal energy storage may involve high-temperature systems that generate steam and energize air turbine engines.