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Thermochemical thermal energy storage (TES)

Thermal energy storage (TES) systems can store heat or cold to be used later 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 main requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the HTF and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility of 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).  

 

There are three technologies of TES systems, each one with different performance, which will drive for which technology each one is more appropriate. Moreover, each technology is in a different maturity status. Sensible TES is when the energy is stored 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 material (PCM). The last technology includes sorption and chemical energy storage and is usually known as thermochemical TES. 

 

Sorption + PCM + electrical storage system developed within the H2020-funded project HYBUILD 

 

Several review can be found in the literature on TES for building applications, such as 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 about the use of TES in building applications [9–11]. Moreover, TES systems also have an important role in district heating and cooling systems [12]

Zeolite used as sorption material 

  

This factsheet describes thermochemical TES. Thermochemical TES uses reversible chemical reactions or physical/chemical sorption, with potential to yield higher energy densities and minor thermal losses. This technology can be divided into sorption TES and chemical-reaction TES (which are commonly used of 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 are more compact but with lower energy storage efficiency [13]. These systems can be applied to cooling, heating, and dehumidification, and are much used in long-term storage (seasonal storage) [14,15].  

 

MATURITY:  

This technology is still in the research status, with TRL considered to be between 3 and 5 [16,17]

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