Back to knowledge

Heat captured during phase change

Thermal energy storage (TES) systems can  store 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 (HTF) 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, the most important design criteria are the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system. 


Integration of active TES systems in a building [10] 


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. 


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; the materials used in latent TES are called phase change materials (PCMs). The last technology includes sorption and chemical energy storage  and it is usually known as thermochemical TES. 

Integration of PCM in a TABS system: PCM in concrete slab with exchange with air for free-cooling in summer and free-heating in winter [15] 


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]


This factsheet focusses on the use of PCM for heat recovery and free-cooling and free-heating. In this context, core activation, thermally-activated building systems (TABS), suspended ceilings, and ventilation systems are used [10]. In free-cooling, coldness from the night is captured and stored to be used when cooling is needed in the building [12]. Free-heating is a similar concept, when heating from waste heat recovery or available solar energy are captured and stored to be used at a later time, in the heating system. PCMs help in increasing the amount of coolness or heat that can be stored; moreover, PCMs help storing the thermal energy at the desired temperature. These technologies may use ventilation systems to improve their performance [13]. Heat recovery can be done from any waste energy source available in the building. An example is the use of PCMs to recover the rejected heat from air-conditioning equipment [14].  

Phase change materials integration in a ceiling (https://www.building.co.uk/news/phase-change-materials/3111760.article) 


Even if these technologies cannot provide thermal comfort by themselves, and some supplementary equipment might be needed, they allow removing/downsizing the mechanical ventilation systems, while providing better thermal conditions, circulating fresh air, reducing energy costs, and improving demand side management. 




TES-based free-cooling and free-heating are mature technologies (TRL=9), heat recovery is at a less advanced stage (TRL=7-8). PCM materials and their technologies are still under development, although some products can be found on the market (TRL=5-8). 

Comments ()




Under license CC BY-NC-SA
This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format for noncommercial purposes only, and only so long as attribution is given to the creator. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.