Thermal energy storage
Employing widely different technologies, it allows surplus thermal energy to be stored for hours, days, or months.
Storage media include water or ice-slush tanks, masses of native earth or bedrock accessed with heat exchangers by means of boreholes, deep aquifers contained between impermeable strata; shallow, lined pits filled with gravel and water and insulated at the top, as well as eutectic solutions and phase-change materials.
Yet another system is known as a packed-bed (or pebble-bed) storage unit, in which some fluid, usually air, flows through a bed of loosely packed material (usually rock, pebbles or ceramic brick) to add or extract heat.
Energy producer Helen Oy estimates an 11.6 GWh capacity and 120 MW thermal output for its 260,000 m3 water cistern under Mustikkamaa (fully charged or discharged in 4 days at capacity), operating from 2021 to offset days of peak production/demand;[22] while the 300,000 m3 rock caverns 50 m under sea level in Kruunuvuorenranta (near Laajasalo) were designated in 2018 to store heat in summer from warm seawater and release it in winter for district heating.
[23] In 2024, it was announced that the municipal energy supplier of Vantaa had commissioned an underground heat storage facility of over 1,100,000 cubic metres (39,000,000 cu ft) in size and 90GWh in capacity to be built, expected to be operational in 2028.
[24] Solid or molten silicon offers much higher storage temperatures than salts with consequent greater capacity and efficiency.
Using oils as sensible heat storage materials is an effective approach for storing thermal energy, particularly in medium- to high-temperature applications.
An oil that is initially more expensive may prove to be more cost-effective in the long run if it offers higher thermal stability, thereby reducing the frequency of replacement.
Thus in the example below, an insulated cube of about 2.8 m3 would appear to provide sufficient storage for a single house to meet 50% of heating demand.
[32] “Brick toaster” is a recently (August 2022) announced innovative heat reservoir operating at up to 1,500 °C (2,732 °F) that its maker, Titan Cement/Rondo claims should be able cut global CO2 output by 15% over 15 years.
[33] Because latent heat storage (LHS) is associated with a phase transition, the general term for the associated media is Phase-Change Material (PCM).
[9] Miscibility gap alloys [35] rely on the phase change of a metallic material (see: latent heat) to store thermal energy.
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.
The photochemical decomposition of nitrosyl chloride can also be used and, since it needs photons to occur, works especially well when paired with solar energy.
The low cost ($200/ton) and high cycle rate (2,000×) of synthetic zeolites such as Linde 13X with water adsorbate has garnered much academic and commercial interest recently for use for thermal energy storage (TES), specifically of low-grade solar and waste heat.
Typically, hot dry air from flat plate solar collectors is made to flow through a bed of zeolite such that any water adsorbate present is driven off.
Storage can be diurnal, weekly, monthly, or even seasonal depending on the volume of the zeolite and the area of the solar thermal panels.
Because of the low temperature, and because the energy is stored as latent heat of adsorption, thus eliminating the insulation requirements of a molten salt storage system, costs are significantly lower.
[44] In 2013 the Dutch technology developer TNO presented the results of the MERITS project to store heat in a salt container.
The DSPEC generates hydrogen fuel by making use of the acquired solar energy to split water molecules into its elements.
A possible solution to overcome this anti-correlation between the energy density and the red shifting is to couple one chromophore unit to several photo switches.
The basic principles involved in a thermal battery occur at the atomic level of matter, with energy being added to or taken from either a solid mass or a liquid volume which causes the substance's temperature to change.
Early examples of thermal batteries include stone and mud cook stoves, rocks placed in fires, and kilns.
While stoves and kilns are ovens, they are also thermal storage systems that depend on heat being retained for an extended period of time.
Hence a thermal battery that uses a phase change can be made lighter, or more energy can be put into it without raising the internal temperature unacceptably.
It can take in waste heat from alternate sources such as computer server rooms or compost heaps and store it for later distribution.
The picture above depicts what is known as a "horizontal" GHEX where trenching is used to place an amount of pipe in a closed loop in the ground.
They are non-rechargeable electrical batteries using a low-melting eutectic mixture of ionic metal salts (sodium, potassium and lithium chlorides, bromides, etc.)
These contain iron powder moist with oxygen-free salt water which rapidly corrodes over a period of hours, releasing heat, when exposed to air.
Storage heaters are commonplace in European homes with time-of-use metering (traditionally using cheaper electricity at nighttime).
