Thermal energy storage (TES) systems can stock heat or cold to be used at a later time under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The key requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the heat transfer fluid and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility after a number of cycles, low thermal losses during the storage period, and easy control. Moreover, one design criteria could be the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system.
Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU. The applications considered were seasonal solar thermal systems (L=25,287 MWth; E=46,150 GWhth; RCO2=12,517,676 tons), district and central heating systems (L=1,453,863 MWth; E=2,326,182 GWhth; RCO2=630,957,558 tons), solar short-term systems (L=416,180 MWth; E=319,269 GWhth; RCO2=86,599,153 tons), and passive cold systems (L=9,944 MWth; E=18,148 GWhth; Ee=6,481 GWhe; RCO2=3,085,135 tons). The subscript “th” stands for “thermal” and the subscript “e” stands for electric.
Three TES technologies can be identified depending on whether sensible heat, latent heat, or thermochemical concepts are used. Each technology comes with a different performance that dictates the best-fitting application. Moreover, each technology is in a different maturity status. Sensible TES is realized when the energy is stored by increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material (i.e., water turns into ice). The materials used in latent TES are therefore called phase change materials (PCMs). Finally, sorption and chemical energy storage is usually known as thermochemical TES.
Several reviews can be found in the literature on TES for building applications, i.e., PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5], adsorption for cooling in buildings [6], TES in hybrid systems [7], TES for seasonal storage [8], or more general building applications [9–11]. Moreover, TES systems also have an important role in district heating and cooling systems [12].
This factsheet is devoted to seasonal TES. Seasonal TES are normally used to store thermal energy produced in an array of solar collectors during the summer months for its use in winter, but other energy sources can also be used [9]. The available technologies are sensible TES (with large water tanks, in underground TES systems –aquifer (ATES) and borehole (BTES)) and thermochemical TES [9]. The main characteristics are:
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Water tank TES: the tank can be built almost independently from the geological conditions and should be placed to avoid groundwater as much as possible. It is usually 5 to15 m deep and features a heat storage capacity of 60-80 kWh/m3. The tank is usually made of concrete, stainless steel, or fibre reinforced polymer, with a coating layer inside the tank surface and an insulation layer outside.
Water tank from Jenni Energietechnik being implemented in a building for water seasonal TES [9]
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Pit TES: the tank can be built almost independently from the geological conditions and should be placed to avoid groundwater as much as possible. It is usually 5 to15 m deep and features a heat storage capacity of 30-50 kWh/m3 when a mixture of water and gravel is used as storage material.
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BTES (borehole TES): It is suitable for soils with rock or saturated water with no or only very low natural groundwater flow. It is usually 30 to100 m deep and features a heat storage capacity of 15-30 kWh/m3. The heat is directly stored in the water-saturated soil, and it is injected in it with U-pipes.
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ATES (aquifer TES): It uses aquifers with high porosity, ground water, and high hydraulic conductivity, as well as small flow rate. Typically, it exhibits a heat storage capacity of 30-40 kWh/m3.
ATES systems for seasonal storage [16]
Thermochemical energy storage can be divided into sorption TES and chemical-reaction TES (which are commonly used at high temperatures, therefore with limited applications to buildings). Sorption TES can be further divided into solid-adsorption TES or liquid-adsorption TES. In a sorption system, a liquid sorbate (usually water) interacts with a solid or liquid sorbent (i.e., zeolites, silica gels, activated carbons, salts, salt composites). Adsorption systems ae more compact but exhibit lower energy storage efficiency [13]. These systems can be used for cooling, heating, and dehumidification, and are typically adopted for long-term storage (seasonal storage) [14,15].
MATURITY:
Sensible TES systems for seasonal storage are quite mature (UTES TRL=9, water tank TES=8-9, pit seasonal storage TRL=7), while thermochemical TES is going through conceptualisation and lab validation (TRL=2-4).
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