Thermal energy storage

District heating accumulation tower from Theiss near Krems an der Donau in Lower Austria with a thermal capacity of 2 GWh

Thermal energy storage (TES) is achieved with greatly differing technologies that collectively accommodate a wide range of needs. It allows excess thermal energy to be collected for later use, hours, days or many months later, at individual building, multiuser building, district, town or even regional scale depending on the specific technology. As examples: energy demand can be balanced between day time and night time; summer heat from solar collectors can be stored interseasonally for use in winter; and cold obtained from winter air can be provided for summer air conditioning. Storage mediums include: water or ice-slush tanks ranging from small to massive, masses of native earth or bedrock accessed with heat exchangers in clusters of small-diameter boreholes (sometimes quite deep); deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and top-insulated; and eutectic, phase-change materials.

Other sources of thermal energy for storage include heat or cold produced with heat pumps from off-peak, lower cost electric power, a practice called peak shaving; heat from combined heat and power (CHP) power plants; heat produced by renewable electrical energy that exceeds grid demand and waste heat from industrial processes. Heat storage, both seasonal and short term, is considered an important means for cheaply balancing high shares of variable renewable electricity production and integration of electricity and heating sectors in energy systems almost or completely fed by renewable energy.

Solar energy storage

Main articles: Solar hot water storage tank and Seasonal thermal energy storage

Most practical active solar heating systems provide storage for from a few hours to a day's worth of energy collected. There are a growing number of facilities that use seasonal thermal energy storage (STES), enabling solar energy to be stored in summer (primarily) for space heating use during winter. The Drake Landing Solar Community in Alberta, Canada has now achieved a year-round 97% solar heating fraction, a world record and possible only by incorporating STES.

Molten salt is a means of storing heat at a high temperature. This is a current commercial technology used in conjunction with concentrated solar power for later use in electricity generation, to allow solar power to provide electricity on a more continuous basis. These molten salts (Potassium nitrate, Calcium nitrate, Sodium nitrate, Lithium nitrate, etc.) have the property to absorb and store the heat energy that is released to the water, to transfer energy when needed. To improve the salt properties it must be mixed in a eutectic mixture.

The use of both latent heat and sensible heat are also possible with high temperature solar thermal input. Various eutectic mixtures of metals, such as Aluminium and Silicon (AlSi12) offer a high melting point suited to efficient steam generation, while high alumina cement-based materials offer good thermal storage capabilities

Economics

High peak loads drive the capital expenditures of the electricity generation industry. The industry meets these peak loads with low-efficiency peaking power plants, usually gas turbines, which have lower capital costs and, since the recent drop in natural gas prices, have low fuel costs as well. A kilowatt-hour of electricity consumed at night can be produced at much lower marginal cost. Utilities have begun to pass these lower costs to consumers, in the form of Time of Use (TOU) rates, or Real Time Pricing (RTP) Rates.

Heat storage in tanks or rock caverns

See also: steam accumulator

A steam accumulator consists of an insulated steel pressure tank containing hot water and steam under pressure. As a heat storage device, it is used to mediate heat production by a variable or steady source from a variable demand for heat. Steam accumulators may take on a significance for energy storage in solar thermal energy projects.

See also: Hot water storage tank

Large stores are widely used in Scandinavia to store heat for several days, to decouple heat and power production and to help meet peak demands. Interseasonal storage in caverns has been investigated and appear to be economical.

Heat storage in hot rocks, concrete, pebbles etc

Water has one of the highest thermal capacities Heat capacity - 4.2 J/(cm³·K) whereas concrete has about one third of that. On the other hand, concrete can be heated to much higher temperatures – 1200 °C by e.g. electrical heating and therefore has a much higher overall volumetric capacity. Thus in the example below, an insulated cube of about 2.8 m would appear to provide sufficient storage for a single house to meet 50% of heating demand. This could, in principle, be used to store surplus wind or PV heat due to the ability of electrical heating to reach high temperatures. At the neighborhood level, the Wiggenhausen-Süd solar development at Friedrichshafen has received international attention. This features a 12,000 m³ (420,000 cu ft) reinforced concrete thermal store linked to 4,300 m² (46,000 sq ft) of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water.[12]

Electric thermal storage heaters

Main article: Storage heater

These are commonplace in European homes and consist of high-density ceramic bricks heated to a high temperature with electricity, and well insulated to release heat over a number of hours.

Ice-based technology

Main article: Ice storage air conditioning

Air conditioning can be provided more economically by using cheaper electricity at night to freeze water into ice, then using the cool of the ice in the afternoon to reduce the electricity needed to handle air conditioning demands. Thermal energy storage using ice makes use of the large heat of fusion of water. One metric ton of water, one cubic meter, can store 334 million joules (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a "ton" of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. This is the heat flow one would expect in a 3,000-square-foot (280 m²) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC capacity = 12,000 BTU/hour (~3.5 kW). Either way, an agreeably small storage facility can hold enough ice to cool a large building for a day or a week, whether that ice is produced by anhydrous ammonia chillers or hauled in by horse-drawn carts.

As such there are developing and developed applications where ice is produced during off-peak periods and used for cooling at later time.

In addition to using ice in cooling applications it is also being used in heat pump based heating systems. In these applications the phase change energy provides a very significant layer of thermal capacity that is near the bottom range of temperature that water source heat pumps can operate in. This allows the system to ride out the heaviest heating load conditions and extends the timeframe by which the source energy elements can contribute heat back into the system.

Cryogenic energy storage

Main article: Cryogenic energy storage

This uses liquification of air or nitrogen as an energy store.

A pilot cryogenic energy system that uses liquid air as the energy store, and low-grade waste heat to drive the thermal re-expansion of the air, has been operating at a power station in Slough, UK since 2010.

Molten salt technology

Molten salt can be employed as a thermal energy storage method to retain thermal energy collected by a solar tower or solar trough of a concentrated solar power plant, so that it can be used to generate electricity in bad weather or at night. It was demonstrated in the Solar Two project from 1995-1999. The system is predicted to have an annual efficiency of 99%, a reference to the energy retained by storing heat before turning it into electricity, versus converting heat directly into electricity. The molten salt mixtures vary. The most extended mixture contains sodium nitrate, potassium nitrate and calcium nitrate. It is non-flammable and nontoxic, and has already been used in the chemical and metals industries as a heat-transport fluid, so experience with such systems exists in non-solar applications.

The salt melts at 131 °C (268 °F). It is kept liquid at 288 °C (550 °F) in an insulated "cold" storage tank. The liquid salt is pumped through panels in a solar collector where the focused sun heats it to 566 °C (1,051 °F). It is then sent to a hot storage tank. This is so well insulated that the thermal energy can be usefully stored for up to a week.

When electricity is needed, the hot salt is pumped to a conventional steam-generator to produce superheated steam for a turbine/generator as used in any conventional coal, oil or nuclear power plant. A 100-megawatt turbine would need a tank of about 9.1 metres (30 ft) tall and 24 metres (79 ft) in diameter to drive it for four hours by this design.

Several parabolic trough power plants in Spain and solar power tower developer SolarReserve use this thermal energy storage concept. The Solana Generating Station in the U.S. has 6 hours of storage by molten salt.

Pumped-heat electricity storage

In pumped-heat electricity storage (PHES), a reversible heat-pump system is used to store energy as a temperature difference between two heat stores.

Isentropic

One system currently being developed by the UK based company Isentropic operates as follows. It comprises two insulated containers; a hot vessel storing thermal energy at high temperature and high pressure, and a cold vessel storing thermal energy at low temperature and low pressure. Both vessels are filled with crushed rock or gravel which acts as the heat storage medium. The vessels are connected at top and bottom by pipes and the whole system is filled with the inert gas argon.

During the charging cycle the system uses surplus electricity to work as a heat pump. Argon at ambient temperature and pressure from the top of the cold store is compressed adiabatically to a pressure of 12 bar, heating it to around . The compressed gas is transferred to the top of the hot vessel where it percolates down through the gravel, transferring its heat to the rock and cooling to ambient temperature. The cooled, but still pressurized, gas emerging at the bottom of the vessel is then expanded (again adiabatically) back down to 1 bar, which lowers its temperature to -150 °C. The cold gas is then passed up through the cold vessel where it cools the rock while being warmed back to its initial condition.

The energy is recovered as electricity by reversing the cycle. The hot gas from the hot vessel is expanded to drive a generator and then supplied to the cold store. The cooled gas retrieved from the bottom of the cold store is compressed which heats the gas to ambient temperature. The gas is then transferred to the bottom of the hot vessel to be reheated.

The compression and expansion processes are provided by a specially designed reciprocating machine using sliding valves. Surplus heat generated by inefficiencies in the process is shed to the environment through heat exchangers during the discharging cycle.

The developer claims that a round trip efficiency of 72-80% is achievable. This compares to >80% achievable with pumped hydro energy storage.

Other systems

Another proposed system uses turbomachinery and is capable of operating at much higher power levels.

Research

Storing energy in molecular bonds is being investigated. Energy densities equivalent to lithium-ion batteries have been achieved.

See also

• District heating
• Eutectic system
• Fireless locomotive
• Heat capacity
• Ice storage air conditioning
• Liquid nitrogen economy
• List of energy storage projects
• Phase change material
• Pumpable ice technology
• Steam accumulator
• Storage heater
• US DOE International Energy Storage Database

External links

• ASHRAE white paper on the economies of load shifting
• MSN article on Ice Storage Air Conditioning
• ICE TES Thermal Energy Storage - IDE-Tech
• http://thermalbatterysystems.com/featured-systems/laramie-wyoming-thermal-battery-system-example/#.U8Whdo1dXx8
• "Prepared for the Thermal Energy Storage Systems Collaborative of the California Energy Commission" Report titled "Source Energy and Environmental Impacts of Thermal Energy Storage." Tabors Caramanis & Assoc energy.ca.gov

Further reading

• Hyman, Lucas B. Sustainable Thermal Storage Systems: Planning, Design, and Operations. New York: McGraw-Hill, 2011. Print.
• Henrik Lund, Renewable Energy Systems: A Smart Energy Systems Approach to the Choice and Modeling of 100 % Renewable Solutions, Academic Press 2014, ISBN 978-0-124-10423-5.