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
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. Siemens builds a
36 MWh thermal storage near Hamburg with 600°C basalt and 1.5 MW electric
output. A similar system is scheduled for Sorø, Denmark, with 41-58% of the stored 18 MWh heat returned for the town's district heating, and 30-41% returned as electricity
Miscibility
gap alloy (MGA) technology
Miscibility
gap alloys rely on the phase change of a metallic
material to store thermal energy Rather
than pumping the liquid metal between tanks as in a molten-salt system, the
metal is encapsulated in another metallic material that it cannot alloy with (immiscible). Depending on the two materials
selected (the phase changing material and the encapsulating material) storage
densities can be between 0.2 and 2 MJ/L. A working fluid, typically water or steam, is used to transfer the heat into and out of the MGA. Thermal conductivity of MGAs is often higher (up to 400 W/m K) than competing technologies which means quicker "charge" and "discharge" of the thermal storage is possible. The technology has not yet been implemented on a large scale.
Electric
thermal storage heaters
Storage heaters are commonplace in European homes with time-of-use metering
(traditionally using cheaper electricity at night time). They consist of high-density
ceramic bricks or feolite blocks heated to a high temperature with electricity,
and may or may not have good insulation and controls to release heat over a
number of hours.
Ice-based technology
Several applications are
being developed where ice is produced during off-peak periods and used for
cooling at later time For example, air conditioning can be provided more
economically by using low-cost electricity at night to freeze water into ice,
then using the cooling
capacity of 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. Historically, ice was
transported from mountains to cities for use as a coolant. One metric ton of water (= one cubic meter) can
store 334 million joules (MJ) or 317,000 BTUs
(93kWh). A relatively small storage facility can hold enough ice to cool a
large building for a day or a week. In addition to using ice in direct 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 time frame by which the source energy elements can contribute heat back into the system
Cryogenic energy storage
This uses liquefaction of air or nitrogen as an energy store. 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.
Hot silicon
technology
Solid or molten silicon offers much higher storage
temperatures than salts with consequent greater capacity and efficiency. It is
being researched as a possible more energy efficient storage technology.
Silicon is able to store more than 1MWh of energy per cubic metre at
1400 °C
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 which was being
developed by the now bankrupt UK Company Isentropic operates as follows. It comprises two insulated containers
filled with crushed rock or gravel; 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. 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 off-peak 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 500°C (900 °F). 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.
Another proposed system uses turbomachinery and is capable of operating at much higher power levels. Use of Phase Change Material (PCMs) as heat storage material would enhance the performance further.
Endothermic/exothermic
chemical reactions
One example of an
experimental storage system based on chemical reaction energy is the salt
hydrate technology. The system uses the reaction energy created when salts are
hydrated or dehydrated. It works by storing heat in a container containing 50% sodium hydroxide (NaOH) solution. Heat (e.g. from
using a solar collector) is stored by evaporating the water in an endothermic
reaction. When water is added again, heat is released in an exothermic reaction
at 50 °C (120 °F). Current systems operate at 60% efficiency. The
system is especially advantageous for seasonal
thermal energy storage,
because the dried salt can be stored at room temperature for prolonged times,
without energy loss. The containers with the dehydrated salt can even be
transported to a different location. The system has a higher energy density than heat stored in water and the
capacity of the system can be designed to store energy from a few months to
years. In 2013 the Dutch technology developer TNO presented the results of the MERITS project to store heat in a salt container. The heat, which can be derived from a solar collector on a rooftop, expels the water contained in the salt. When the water is added again, the heat is released, with almost no energy losses. A container with a few cubic meters of salt could store enough of this thermo chemical energy to heat a house throughout the winter. In a temperate climate like that of the Netherlands, an average low-energy household requires about 6.7 GJ/winter. To store this energy in water (at a temperature difference of 70 °C), 23 m3 insulated water storage would be needed, exceeding the storage abilities of most households. Using salt hydrate technology with a storage density of about 1 GJ/m3, 4–8 m3 could be sufficient
As of 2016, researchers in several countries are conducting experiments to determine the best type of salt or salt mixture. Low pressure within the container seems favorable for the energy transport especially promising is organic salts, so called ionic liquids. Compared to lithium halide based sorbents they are less problematic in terms of limited global resources, and compared to most other halides and sodium hydroxide (NaOH) they are less corrosive and not negatively affected by CO2 contaminations
Head (m)
|
Volume requirement (m3)
|
Approximate dimensions of reservoir
|
1000
|
366
|
100
m2 x 3,66 m
|
500
|
732
|
200
m2 x 3,66 m
|
250
|
1464
|
400
m2 x 3,66 m
|
100
|
3660
|
1000
m2 x 3,66 m
|
50
|
7320
|
1000
m2 x 7,32 m
|
20
|
18
300
|
2500
m2 x 7,32 m
|
Water volume requirements for various
pressure head.
|
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