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Buildings are responsible for one-third of the world's energy consumption, of which 60% is due to heating and cooling. To accomplish the low-carbon energy goal in the building sector, thermal energy storage offers a number of benefits by reducing energy consumption and promoting the use of renewable energy sources. This manuscript reviews recent advances in the development of thermal energy storage materials for building applications oriented towards zero energy buildings. Volumetric heat capacity of sensible, latent and thermochemical energy storage materials developed for low-to-moderate temperature applications are reviewed and assessed with a special focus on their technical characteristics and development stage. This encompasses most recent publications, international programmes and projects, and commercially available solutions. Physical, thermodynamic, kinetic and chemical properties are addressed, as well as costs. Advantages, drawbacks and challenges of the diverse alternatives are discussed. The analysis shows that solutions with the highest potential for competitive energy efficiency measures are based on latent and sensible energy storage systems, which present a volumetric thermal energy storage density up to 430 and 250 MJ/m3 respectively. Their applications in free-cooling ventilation systems, solar energy storage solutions for short and long-term storage periods, and demand-side management strategies towards the road to zero energy buildings are highlighted as promising, leading to a reduction of energy consumption of more than 30%. On the other hand, thermochemical energy storage does not yet show clear advantages for building applications, despite the potentially high energy density (up to 1510 MJ/m3) and heat availability for long-term storage periods. Currently, there is no available material for thermochemical energy storage that satisfies all the requirements for building operations. Besides, thermochemical solutions require different tanks and heat exchangers that should be carefully addressed for small-scale applications. Additional research efforts are needed to optimise operation conditions, efficiency, costs and system designs. ; European Union SOE1/P3/P0429EU ; Ministerio de Economía y Competitivdad CTQ2014-52763 -C2 -2-R

This work investigates commercially available granular phase change materials (PCMs) with different transition temperatures for the use of thermal-energy storage systems in fluidized beds. The hydrodynamic characteristics of granular PCMs were tested in cylindrical-3D and planar-2D fluidized beds. The density, particle size distribution and angle of repose were measured for various PCM materials. Further attrition studies were conducted with changes in particle surface from abrasion, which were characterized using a Scanning Electron Microscope (SEM). The results indicate that some materials with smaller particle size and thinner supporting structure can lose the paraffin during the fluidization process, when paraffin is in a liquid state. As a consequence, the particles agglomerate, and the bed defluidizes. For all of the tested materials, only GR50 (with a transition temperature of 50 °C) properly fluidizes when the paraffin is in the liquid state and has shown to endure >75 h of continuous operation and 15 melting-solidification cycles in a fluidized bed. Additional differential scanning calorimetry (DSC) measurements of the cycled particles did not show a decrease in energy storage capacity of the granular PCM, which corroborates that there is no loss of material after >75 h of fluidization. ; The work is partially funded by the Spanish government (ENE2010-15403, ENE2011-22722 and ENE2015-64117-C5-1). The authors would like to thank the Catalan Government for the quality accreditation to their research groups GREA (2014 SGR 123). The study that led to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n°n°PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 657466 (INPATH-TES).

Thermal energy storage (TES) allows the existing mismatch between supply and demand in energy systems to be overcome. Considering temperatures above 150 C, there are major potential benefits for applications, such as process heat and electricity production, where TES coupled with concentrating solar power (CSP) plants can increase the penetration of renewable energies. To this end, this paper performs a critical analysis of the literature on the current and most promising concrete energy storage technologies, identifying five challenges that must be overcome for the successful exploitation of this technology. With these five challenges in mind, this paper proposes an approach that uses a new modular design of concrete-based TES. A preliminary study of the feasibility of the proposed system was performed using computational fluid dynamics (CFD) techniques, showing promising results. ; Funding: This work is part of PCI2020-120695-2 project funded by Ministerio de Ciencia e Innovación— Agencia Estatal de Investigación (MCIN/AEI/10.13039/501100011033) and by the European Union "NextGenerationEU/PRTR". This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31—MCIU/AEI/FEDER, UE) and by the Ministerio de Ciencia, Innovación y Universidades—Agencia Estatal de Investigación (AEI) (RED2018-102431-T). Acknowledgments: The authors at University of Lleida would like to thank the Catalan government for quality accreditation (2017 SGR 1537). GREiA is a certified TECNIO agent in the category of technology developers from the government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.

In the current study a novel geopolymer created from waste materials (namely fly ash and black slag) has been characterised as a potential high temperature thermal energy storage material option. Several geopolymer samples have been fabricated and characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Additionally, the density and heat capacity have been determined. Lastly, the cost of the material and packed bed system was estimated showing that the geopolymer-based systems are potentially 35% cheaper than the traditional 2-tank molten salt systems. ; This research was performed as part of the Australian Solar Thermal Research Institute (ASTRI), a project supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA). The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement Nº PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Aran Solé would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI-2015-25741. The authors would also like to acknowldege the Diopma group from the University of Barcelona thanks the funding by the Spanish Government (RTI2018-093849-B-C32), and the Catalan Government for the quality accreditation (2017 SGR 118).

The final goal of this study is to implement and to test a thermal energy storage (TES) system using different phase change materials (PCM) for solar cooling and refrigeration applications. A high temperature pilot plant able to test different types of TES systems and materials was designed and built at the University of Lleida (Spain). This pilot plant is composed mainly by three parts: heating system, cooling system, and different storage tanks. The pilot plant uses synthetic thermal oil as heat transfer fluid (HTF) and has a working temperature range from 100 ºC to 400 ºC. Two different PCM were selected after a deep study of the requirements of a real solar cooling plant and the available materials in the market, finally d-mannitol with phase change temperature of 167 ºC and hydroquinone which has a melting temperature of 172.2 ºC were used. ; The work is partially funded by the Spanish government (ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2009 SGR 534). Antoni Gil would like to thank the Col·legid'Enginyers Industrials de Catalunya for his research appointment. Eduard Oró would like to thank the University of Lleida for his research fellowship.

AbstractThe multitube design in the shell-and-tube type latent heat thermal energy storage (LHTES) system has received intensive attention due to its promising benefits in enhancing heat storage efficiency. In this paper, single and multi-tube shell LHTES systems were experimentally investigated. First, this study experimentally compared the thermal characteristics between a multiple-tube heat exchanger (MTHX) and a single-tube heat exchanger (STHX). The STHX's geometrical parameters coincided with a virtual cylindrical domain in the MTHX, being similar to the single-tube model formulated by simplifying the numerical solution to investigate the MTHX. The experimental data was then used to validate the simplified numerical model commonly used in the literature that converted the multi-tube problem to a single-tube model by formulating a virtual cylindrical domain for each tube in the MTHX system. The results showed that there was a noticeable difference in the thermal characteristics between the actual STHX and the virtual cylindrical STHX domain in the MTHX system. The comparison indicated that the simplified numerical model could not accurately reflect the thermal performance of the MTHX system. An experimental study or three-dimensional numerical modelling was required for the thermal analysis of the multi-tube problems. Second, the effect of tube number in the MTHX was experimentally investigated. It was found that an increase in tube number boosted both charging and discharging rates without inhibiting the natural convection. The five-tube configuration decreased the total charging and discharging duration by 50% compared to the two-tube one. Finally, the effect of heat transfer fluid (HTF) operating parameters on the system performance was evaluated on the five-tube MTHX system. The results revealed that the adoption of higher HTF temperature considerably improved the charging performance. The charging time decreased by up to 41% with the HTF temperature increasing from 70 to 80 °C. Meanwhile, a variation in the HTF flow rate from 5 to 20 L/min showed a more pronounced influence on charging than on discharging due to the different dominant heat transfer mechanisms.

This paper describes H2020 project TESSe2b, which the main objectives are to increase energy efficiency in buildings, enhance green technologies and promote advanced thermal energy storage. The target of TESSe2b is to design, develop, validate and demonstrate a modular and low cost thermal storage technology, based on Phase Change Materials (PCMs), for residential building applications. TESSe2b solution combines thermal solar collectors and geothermal heat pump with latent thermal energy storage batteries for cooling, heating and domestic hot water (DHW) production in single family houses. Thesse2b solution will be demonstrated in three European countries with different climates, Austria, Spain and Cyprus. The project will develop modular thermal storage batteries and an advanced self-learning control system to maximize the performance of the entire solution. TESSe2b project is funded by the European Union's Horizon 2020 Research and Innovation Programme, under the Grant Agreement number 680555.

This systematic review presents and discusses the previous research about hybrid devices which combine latent thermal energy storage (TES) technology and heat pipes. A bibliometric analysis of this issue shows how hybrid systems have globally grown popularity during time, providing details about the main researchers and research centres on this particular field. Then, the identified papers are assessed and categorized in two main sections, the experimental research carried out, and the numerical modelling of hybrid systems. Experimental research is later classified regarding the operating temperature range, and their final application. Numerical studies are also further categorized, accordingly to how heat pipes were modelled in this case. This review points out the lack of experimental studies at high temperatures, especially when many simulations extended their models (validated at low temperatures) to higher temperature designs. The paper provides details about the research performed, so the gap for future investigations can be spotted. ; This work was partially funded by the European Union's Horizon 2020 Research & Innovation Programme under Grant Agreement 723596 with reference name Innova MicroSolar. This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31 - MCIU/AEI/FEDER, UE) and by the Agencia Estatal de Investigación (AEI), of the Ministerio de Ciencia, Innovación y Universidades (RED2018-102431-T). José Miguel Maldonado would like to thank the Spanish Government for his research fellowship (BES-2016-076554). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREiA is a certified TECNIO agent in the category of technology developers from the Government of Catalonia. This work is partially supported by ICREA under the ICREA Academia programme.

The increase of the capacity factor of thermal processes which use renewable energies is closely linked to the implementation of thermal energy storage (TES) systems. Currently, TES systems can be classified depending on the technology for storing thermal: sensible heat, latent heat, and sorption and chemical reactions (usually known as thermochemical energy storage). However, there is no standardized procedure for the evaluation of such technologies, and therefore the development of performance indicators which suit the requisites of the final users becomes an important goal. In the present paper, the authors identified the energy density as an important performance indicator for TES, and evaluated it at both material and system levels. This approach is afterwards applied to prototypes covering the three TES technologies: a two-tank molten salts sensible storage system, a shell-and-tube latent heat storage system, and a magnesium oxide and water chemical storage system. The evaluation of the energy density highlighted the difference of its value at the material value, which presents a theoretical maximum, and the results at system level, which considers all the parts required for operating the TES, and thus presents a significantly lower value. Moreover, the proposed approach captured the effect of the complexity and overall size of the system, showing the relevance of this performance indicator for evaluating technologies for applications in which volume is a limiting parameter. ; The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2017 SGR 1537). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. Jaume Gasia would like to thank the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya for his research fellowship (2018 FI_B2 00100). Aran Solé would like to thank Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva, FJCI2015-25741. The authors would also like to thank the participants of IEA ECES Annex 30 for their critical view and feedback during the development of the methodology.

Molecular solar-thermal energy storage (MOST) systems are based on photoswitches that reversibly convert solar energy into chemical energy. In this context, bicyclooctadienes (BODs) undergo a photoinduced transformation to the corresponding higher energy tetracyclooctanes (TCOs), but the photoswitch system has not until now been evaluated for MOST application, due to the short half-life of the TCO form and limited available synthetic methods. The BOD system degrades at higher temperature via a retro-Diels-Alder reaction, which complicates the synthesis of the compounds. We here report a cross-coupling reaction strategy that enables an efficient synthesis of a series of 4 new BOD compounds. We show that the BODs were able to switch to the corresponding tetracyclooctanes (TCOs) in a reversible way and can be cycled 645 times with only 0.01% degradation. Half-lives of the TCOs were measured, and we illustrate how the half-life could be engineered from seconds to minutes by molecular structure design. A density functional theory (DFT) based modelling framework was developed to access absorption spectra, thermal half-lives, and storage energies which were calculated to be 143-153 kJ mol-1 (0.47-0.51 MJ kg-1), up to 76% higher than for the corresponding norbornadiene. The combined computational and experimental findings provide a reliable way of designing future BOD/TCO systems with tailored properties. ; The project made use of the NMR Uppsala infrastructure, which is funded by the Department of Chemistry-BMC and the Disciplinary Domain of Medicine and Pharmacy. The authors would like to thank the Knut & Alice Wallenberg Foundation, The Swedish Research Council FORMAS, the Swedish Energy Agency, The Swedish Research Council (Vetenskapsrådet) and the European Union's Horizon 2020 Framework Programme under grant agreement number 951801 for financial support. ; With funding from the Spanish government through the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000917-S). ; Peer reviewed

Thermal energy storage (TES) is seen today as a key technology to reduce the existing gap between energy demand and energy supply in many energy systems. There are, currently, three well known methods to store thermal energy and they are: sensible heat storage (SHS), latent heat storage (LHT) and thermochemical heat storage. Every method has its own thermophysical requirements for the mediums of storage, such as thermal stability, high enthalpy of phase change or reaction, high heat capacity and suitable temperature of the thermal phenomenon for a respective application, among others. In this regard, the composition of materials usually needs to be modified in order to improve their performance or to reach a determined requirement. As a consequence, the costs of potential TES materials to be applied in renewable energy systems are too high to compete with traditional systems using fossil fuels. On the other hand, several wastes and by-products from the nonmetallic mining, such as salt hydrates and double salts, are available without any application but accumulating in the mining sites. This is the case for astrakanite (Na2SO4·MgSO4·4H2O) and lithium carnallite (LiCl·MgCl2·7H2O) with no current application, and potassium carnallite (KCl·MgCl2·6H2O) used as a supplementary raw material to obtain KCl. Since the costs of these materials are close to zero, they were characterized as TES materials taking into account the properties required for the three methods of storage. Results showed that astrakanite and potassium carnallite have potential to be applied as thermochemical material at lowmedium temperature (< 300 °C). Also, a dehydrated product obtained from astrakanite showed potential to be applied as phase change material (PCM) at high temperature, from 550 °C to 750 °C. Nevertheless, lithium carnallite did not show potential to be applied as TES material due to it low thermal stability, presenting partial decomposition below 200 °C. ; The work at the University of Antofagasta was supported by CONICYT/FONDAP N° 15110019, FONDECYT N°1170675 and partially, by ERANet-LAC 2nd Joint Call, ERANET-LAC 2015–2016, project ELAC2015/T06-0988. Veronica Mamani Barreneche would like to thank CONICYT for her doctorate scholarship CONICYT N° 21150145. The authors from the University of Lleida and the University of Barcelona would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and DIOPMA (2014 SGR 1543), respectively. The work was partially funded by the Spanish Government (ENE2015-64117-C5-1-R (MINECO/FEDER)). This work has received funding from the European Commission Seventh Framework Programme (FP/2007–2013) under Grant agreement N°PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Dr. Camila Barreneche would like to thank Ministerio de Economia y Competitividad de España for grant Juan de la Cierva, FJCI-2014-22886.

This report summarizes experiences and lessons learned on Underground Thermal Energy Storage (UTES) systems from the participating EU project partners and is supplemented with input from publications on other relevant cases in, and outside, Europe. The report covers important experiences from the first pre-investigation phase and feasibility studies and throughout the construction phase, system integration and operations. Furthermore, different national legislative, political and public issues are described. The HEATSTORE project and this report comprise four different UTES systems: - High Temperature Aquifer Thermal Energy Storage (HT-ATES) - Borehole Thermal Energy Storage (BTES) - Pit Thermal Energy Storage (PTES) - Mine Thermal Energy Storage (MTES) The ideas behind MTES is state of the art and the HEATSTORE demonstration site in Bochum, Germany, will be the first of its kind. Experiences and lessons learned regarding this specific system is therefore not existing, see chapter 5. In Appendix I and II, specific report contributions from partners are included. These contributions have served as an important basis for this deliverable.