A Fascinating Look at Possible Utility-Scale Thermal Storage

Efficient and economical utility-scale energy storage would provide power grids much greater stability and versatility. Here are excerpts from an intriguing look at future prospects for thermal grid storage from Intelligent Utility:

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.

...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.

Conclusions:

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. _IntelligentUtility

The excellent article by Harry Valentine should be read in full at the link above.

One omission from the fine overview of future thermal storage methods, is the cryogen method being developed at the University of Leeds. Such cryogenic energy storage methods extend the temperature range considerably on the low end, with concomitant potential for greater efficiencies.