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 potential of load reduction (L) |
Energy savings (E) |
climate change mitigation (as CO2 emissions reduction – RCO2) |
|
Seasonal solar thermal systems |
25,287 MWth |
46,150 GWhth |
12,517,676 tons |
|
District and central heating systems |
1,453,863 MWth; |
2,326,182 GWhth |
630,957,558 tons |
|
Solar short-term systems |
416,180 MWth |
319,269 GWhth |
86,599,153 tons |
|
Passive cold systems |
9,944 MWth |
18,148 GWhth |
3,085,135 tons |
There are three technologies of TES systems, each one with a different performance, which will drive which technology each one is more appropriate. Moreover, each technology is in a different maturity status. Sensible TES is 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 (e.g., 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.
Several reviews 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 [5,6], adsorption for cooling in buildings [7], TES in hybrid systems [8], TES for seasonal storage [9], or more general about the use of TES in building applications [10–12]. Moreover, TES systems also have an important role in district heating and cooling systems [13].
This factsheet describes latent TES for active systems. Common applications are their inclusion in HVAC systems, thermal load management, peak shaving both of electricity and thermal energy, etc. [14]. Since PCMs absorb and release heat at nearly constant temperatures, they are particularly attractive for building applications [15].
The most common PCM is water/ice but other PCMs include organic (i.e., paraffin, fatty acids, esters) and inorganic (i.e., salt hydrates, salts) [15–17]. Today push towards sustainability has driven research towards the development of bio-based PCMs [18] or the use of wastes as PCM [19]. The optimal PCM to be used depends on the physical, chemical, and technical requirements of the application. The materials use the latent heat between the solid and liquid phase change; therefore, they must be encapsulated, that is included in a container or entrapped in another material such as gypsum, concrete, or polymer. Moreover, different device designs and system configurations can be adopted for using PCMs, depending on the chemical and physical compatibility of the storage material and the heat transfer fluid (HTF) and the application requirements.
Latent TES can be included in heat pump systems, since it balances system operation and allows heat pumps to operate at full capacity throughout the year, increasing the seasonal performance factor (SPF) [20,21]. Another usual application of PCM is its inclusion in (solar) water tanks [6,22]. PCMs are also used in conjunction with photovoltaic (PV) panels to increase their performance by reducing the panel temperature [23]. Ice storage is widely used in Asia, mostly in big buildings to produce cold during the night and use it during the day when the electricity is more expensive [24]. Space heating and space cooling are also common applications where PCM is used [4].
Below you can see two examples of latent TES systems. The first one is a demonstration plant at Politecnico di Torino (Italy), with two tanks with PCM for space heating. The second one is encapsulated PCM in a so-called TubeICE container, to be installed inside tanks for space cooling applications; this product was designed with an EU-funded project, and it is today commercial.
PCM storage system installed in a building at the Politecnico di Torino within the EU-funded project RE-cognition
TubeICE PCM product (www.pcmproducts.net)
MATURITY:
Although there are companies already commercializing PCM and PCM systems, the technology is still under development; it is considered to be in TRL 4-7 [14].
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