This review is focused on the study of the requirement of high thermal conductivity of thermal energy storage (TES) materials and the techniques used to enhance it as this is one of the main obstacles to achieve full deployment of TES systems. Numerical and experimental studies involving different thermal conductivity enhancement techniques at high temperature (>150 °C) are reviewed and classified. This article complements Part 1, which reviews the different requirements that TES materials and systems should consider for being used for high temperature purposes and the approaches to satisfy them. The enhancements identified for this temperature range are the addition of extended surfaces like fins or heat pipes and the combination of highly conductive materials with TES material like graphite or metal foam composites and nanomaterials. Moreover the techniques presented are classified and discussed taking into account their research evolution in terms of maturity and publications. ; The work is partially funded by the Spanish Government (ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). The research leading to these results has received funding from the European Union׳s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. PIRSES-GA-2013-610692 (INNOSTORAGE). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861).
Industrial activities have a huge potential for waste heat recovery. In spite of its high potential, industrial waste heat (IWH) is currently underutilized. This may be due, on one hand, to the technical and economic difficulties in applying conventional heat recovery methods and, on the other, the temporary or geographical mismatch between the energy released and its heat demand. Thermal energy storage (TES) is a technology which can solve the existing mismatch by recovering the IWH and storing it for a later use. Moreover, the use of recovered IWH leads to a decrease of CO2 emissions and to economic and energy savings. Depending on the distance between the IWH source and the heat demand, TES systems can be placed on-site or the IWH can be transported by means of mobile TES systems, to an off-site heat demand. Around 50 industry case studies, in which both on-site and off-site recovery systems are considered are here reviewed and discussed taking into account the characteristics of the heat source, the heat, the TES system, and the economic, environmental and energy savings. Besides, the trends and the maturity of the cases reviewed have been considered. On-site TES systems in the basic metals manufacturing are the technology and industrial sector which has focused the most attention among the researchers, respectively. Moreover, water (or steam), erythritol and zeolite are the TES materials used in most industries and space comfort and electricity generation are the most recurrent applications. ; The work is partially funded by the Spanish government (ENE2011-22722, ENE2015-64117-C5-1-R (MINECO/FEDER)). 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). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). 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 (2016FI_B 00047). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861).
High temperature thermal energy storage offers a huge energy saving potential in industrial applications such as solar energy, automotive, heating and cooling, and industrial waste heat recovery. However, certain requirements need to be faced in order to ensure an optimal performance, and to further achieve widespread deployment. In the present review, these requirements are identified for high temperature (>150 °C) thermal energy storage systems and materials (both sensible and latent), and the scientific studies carried out meeting them are reviewed. Currently, there is a lack of data in the literature analysing thermal energy storage from both the systems and materials point of view. In the part 1 of this review more than 25 requirements have been found and classified into chemical, kinetic, physical and thermal (from the material point of view), and environmental, economic and technologic (form both the material and system point of view). The enhancements focused on the thermal conductivity are addressed in the Part 2 of this review due to their research significance and extension. ; The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R, ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). 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). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). 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 (2016FI_B 00047).
Within the framework of HYBUILD, an EU Horizon 2020-funded project, two innovative compact hybrid electrical/thermal storage systems for stand-alone and district connected residential buildings will be developed and tested in three demos located in Spain, France, and Cyprus. One of the innovative systems is aimed to be placed in buildings located in Mediterranean climate regions, where cooling loads are dominant, while the other system is intended for Continental climate regions, where the heating demand is dominant. Each system will include, among others components such as a sorption storage system and domestic hot water tanks, a latent thermal energy storage (LTES) system that will be connected to a heat pump through an innovative heat exchanger made of aluminium and filled with phase change material (PCM). In both cases, the heat pump works with electricity provided by a photovoltaic system that is, at the same time, connected to an electrical storage battery. The aim of using the LTES system is to enhance the use of solar energy, which will be translated into a reduction of the building energy consumption and related costs. This study focuses on the selection of the most suitable PCM to be used in each system. On the one hand, the LTES system of the Mediterranean system will be used to store cold to reduce the cooling demand. Taking into account that, according to the design parameters, the heat pump will require a refrigerant evaporation temperature around 2 ºC, and the building cooling system will require water supply in the range from 7 ºC to 12 ºC, the PCM melting temperature range should be within 0 ºC and 7 ºC. On the other hand, the LTES system of the Continental system will be used to store heat to reduce the domestic hot water (DHW) demand. The LTES will be located at the compressor outlet and will be charged by the hot refrigerant that exits the compressor at temperatures as high as 120 ºC. During the discharge process, the heat stored in the LTES will be supplied to the DHW at a temperature in the range between 50 ºC to 55 ºC. As a consequence, the range for the PCM melting temperature investigated in this case should be between 62 ºC and 68 ºC. Besides the melting temperature, other selection criteria considered include the PCM melting enthalpy and melting range, maximum allowed working temperature, density, thermal conductivity, availability, cost, and compatibility with aluminium. To decide the ideal PCM candidate for each system, a decision matrix was defined and used, by applying a weighted score to the selection criteria items according to their importance. The preliminary results indicate that for the Mediterranean system the best candidate is the commercial savE OM3 PCM, while for the Continental system, another commercial product PureTemp 63 is the most adequate option. ; This work was partially funded by the Ministerio de Economía y Competitividad de España (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 (GREiA 2017 SGR 1537). GREiA 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). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 768824 (HYBUILD)
The supply intermittency of energy sources like solar energy or industrial waste heat should be properly addressed when studying latent heat thermal energy storage (TES) systems, since it might cause an incomplete melting/solidification of phase change materials (PCM). In the present paper, and experimental study was performed to analyse the storage period (also known as stand-by period) in a latent heat TES system working under partial load operating conditions and the effect of its duration on the subsequent discharging process. In the experimental set-up, 99.5 kg of high density polyethylene (HDPE) was used as PCM in a 0.154m3 storage tank based on the shell-and-tube heat exchanger concept. Four different percentages of charge were evaluated: 58%, 73%, 83% (partial charge), and 97% (full charge). Each charging level was followed by three different periods of storage: 25 min, 60 min, and 120 min. The fact of working at different levels of charge caused that in some regions of the TES system the PCM was not completely melted. Thus, at the end of the charging process different levels of thermal homogenisation were observed. However, during the storage period, the PCM temperature showed a tendency to homogenisation, which was influenced by the energy distribution within the PCM, the heat losses, and the duration of the storage period. Focusing on the discharging period, it was observed that the duration of the storage period slightly affected the temperature and heat transfer profiles, causing the main differences of performance during the first 30 min of process. ; This work was partially funded by the Ministerio de Economía y Competitividad de España (ENE2015-64117-C5-1-R (MINECO/FEDER) and ENE2015-64117-C5-3-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).
One of the features that should be considered when designing a thermal energy storage (TES) system is its behaviour when subjected to non-continuous (partial loads) operating conditions. Indeed, the system performance can be sensibly affected by the partial charging and discharging processes. This topic is analysed in the present study by means of a two-dimensional axisymmetric numerical model implemented in COMSOL Multiphysics. A latent heat TES system consisting of a vertical concentric tube heat exchanger is simulated to investigate the effect of different partial load operating conditions on the system behaviour. The effects of different heat transfer distributions and evolutions of the solid-liquid interface, are evaluated to identify the optimal management criteria of the TES systems. The results showed that partial load strategies allow to achieve a substantial reduction in the duration of the TES (up to 50%) process against a small decrease in stored energy (up 30%). The close correlation between the energy and the duration of the TES cycle is also evaluated during the discharge using detailed maps related to the melting fraction. These maps allow for the evaluation of the most efficient load conditions considering both charging and discharging processes to satisfy a specific energy demand. ; Simone Arena and Efisio Casti would like to thank the Department of Mechanical, Chemical and Materials Engineering of the University of Cagliari for their founding research grants. 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. 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 (2017 FI_B1 00092).
This work presents a numerical investigation on latent heat thermal energy storage (LHTES) systems during the phase change process. The numerical analysis, based on the apparent heat capacity formulation, was carried out through a two-dimensional axisymmetric numerical model developed by means of the COMSOL Multiphysics software. A thermal energy storage system based on the configuration of a double tube heat exchanger with finned surfaces was used as an experimental test case and the commercial paraffin RT35 was selected as phase change material (PCM). The influence of the heat transfer by convection, in particular the influence of the term describing the mushy zone in the momentum equation, was investigated during the whole charge and discharge processes. Three different values of the constant Amush, equals to 104, 106 and 108 were selected as well as two different values of the HTF volumetric flow rate were adopted in order to reproduce both laminar and turbulent flow regimes. The results are reported in terms of temperature, melting fraction and phases evolution during the whole melting and solidification processes, and compared to previous experimental tests carried out in the laboratories of the University of Lleida, Spain. A good agreement with the experimental results was obtained showing that the mushy zone constant has a significant influence on the interface shape and motion. The results show that large values of Amush determine an increase of the mushy region reducing the natural convection effects during the charge phase. Thus, the proper evaluation of the mushy zone constant allows providing a deeper understanding of the phase change behaviour, resulting in an important parameter for accurate modelling of LHTES systems. ; The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER)). Jaume Gasia and Prof. Luisa F. Cabeza 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). 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 (2017 FI_B1 00092).
The present paper provides on one hand, a literature review of the different studies available in the scientific literature where the concept of multiple phase change materials (PCM) configuration, also named cascaded or multi-stage, has been presented and on the other hand, an experimental evaluation of the advantages of using the multiple PCM configuration instead of the single PCM configuration in thermal energy storage (TES) systems at pilot plant to fill the gap of experimental and high scales studies on this concept in the literature. Two PCM with melting temperatures in a temperature range of 150 e200 C were selected due to their high value of heat of fusion and compared: d-mannitol and hydroquinone. Three configurations were evaluated: (1) single PCM with hydroquinone, (2) single PCM with dmannitol and (3) multiple PCM with hydroquinone and d-mannitol. A discussion regarding the results on the specific energy stored and effectiveness as well as the evolution of the PCM and heat transfer fluid (HTF) through the time and at different and representative locations of the facility is presented. Results showed that the multiple PCMs configuration introduced an effectiveness enhancement of 19.36% if compared with single PCM configuration as well as a higher uniformity on the HTF temperature difference between the inlet and outlet. ; The work is partially funded by the Spanish Government (ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n PIRSES-GA-2013-610692 (INNOSTORAGE). Laia Miro would like to thank the Spanish Government for her research fellowship (BES-2012-051861).
In this paper, the influence of the addition of fins and the use of two different heat transfer fluids (water and a commercial silicone) have been experimentally tested and compared in four latent heat thermal energy storage systems, based on the shell-and-tube heat exchanger concept, using paraffin RT58 as phase change material. Three European institutions were involved under the framework of the MERITS project. A common approach (temperature and power profiles), and five different key performance indicators have been defined and used for the comparison: energy charged, average power, 5-min peak power, peak power to energy ratio, and time. For the same heat transfer fluid, results showed that finned designs (4.7-9.4 times more heat transfer surface) showed an improvement of up to 40%. On the contrary, for the same design, water (which has a specific heat 3 times higher and a thermal conductivity 4.9 times higher than silicone Syltherm 800), yielded results up to 44% higher. ; The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007–2013) under grant agreement No ENER/FP7/295983 (MERITS) and 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). The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER) and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to the research group GREA (2014 SGR 123). 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 (2017FI_B 00092).
This paper experimentally studies and compares the addition of fins and the addition of metal wool in a latent heat thermal energy storage (TES) system as heat transfer enhancement techniques. Despite the well-known suitability of fins as enhancement technique, their implementation cost in the TES system is one of its main drawbacks. Therefore, the objective of this study is to evaluate the potential of adding a cheap and commercially available metallic wool in order to overcome the abovementioned drawback. In particular, four different latent heat TES systems based on the shell-and-tube heat exchanger concept were designed using n-octadecane as phase change material (PCM). One of them was used as a reference, while in the remaining configurations the heat transfer surface was increased by means of seventeen rectangular fins and by means of metallic wool arbitrarily distributed within the PCM and compacted in a finned shape. Charging and discharging processes with constant heat transfer fluid temperature and flow rate were evaluated from the temperature and heat transfer points of view. Results were focused on the metal wool because is a cheap and handmade solution which can be implemented in an already made heat exchanger. The addition of metal wool showed an enhancement, during the charge, higher than 10% when it was arbitrarily distributed, while compacting the metal wool in a finned shape showed practically no improvement. During the discharge, both metal wool configurations allowed minimal improvements. ; 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). José Miguel Maldonado would like to thank the Spanish Government for his research fellowship (BES-2016-076554). Francesco Galati would like to thank the Erasmus+ Programme: Traineeship Bet for Jobs for his research fellowship.
The selection of a proper heat transfer fluid (HTF) is a key factor to increase the efficiency of concentrated solar power plants and therefore, to reduce their internal associated CAPEX (capital expenditures of developing and constructing a plant, excluding any grid-connection charges) and OPEX (operating expenditures from the first year of a project's operation). This paper presents a comparative study of two commercial HTF which are widely used in different industries and CSP plants: thermal oil Therminol VP-1 and silicone fluid Syltherm 800. First, the authors theoretically studied the properties of both HTF based on the data given by the manufactures. Afterwards, the authors experimentally perform the comparison in a two-tank molten salt thermal energy storage pilot plant built at the University of Lleida (Spain). The study is focused on the plate heat exchanger of the facility during several charging processes with a counter flow arrangement. Results from both studies showed that, for the same working conditions, Therminol VP-1 is the best candidate for the above-mentioned purposes due to its higher heat transfer, lower thermal losses and lower power consumption associated to the HTF pump. However, it presents problems a low crystallization point, which should also be considered. ; The research leading to these results has received funding from Spanish goverment (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005) and from Abengoa Solar NT. The work is partially funded by the Spanish government (ENE2008-06687-C02-01/CON, ENE2011-22722, ULLE10-4E-1305 and 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 GREA (2014 SGR 123). This project has also received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement NºPIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). 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 (2017FI_B1 00092). The authors would like to thank Dr. Eduard Oró from Catalonia Institute for Energy Research (Spain) and Dr. Antoni Gil from Massachusetts Institute of Technology (USA) for their help during the initial stages of the experimentation.
In some processes, latent heat thermal energy storage (TES) systems might work under partial load operating conditions (the available thermal energy source is discontinuous or insufficient to completely charge the phase change material (PCM)). Therefore, there is a need to study how these conditions affect the discharge process to design a control strategy that can benefit the user of these systems. The aim of this paper is to show and perform at laboratory scale the selection of a PCM, with a phase change temperature between 120 and 200 ºC, which will be further used in an experimental facility. Beyond the typical PCM properties, sixteen PCMs are studied here from the cycling and thermal stability point of view, as well as from the health hazard point of view. After 100 melting and freezing cycles, seven candidates out of the sixteen present a suitable cycling stability behaviour and five of them show a maximum thermal-stable temperature higher than 200 ºC. Two final candidates for the partial loads approach are found in this temperature range, named high density polyethylene (HDPE) and adipic acid. ; 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 GREA and DIOPMA (2014 SGR 123, 2014 SGR 1543). GREA and DIOPMA are certified agents 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). 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 (2017 FI_B1 00092). Camila Barreneche and Aran Solé would like to thank the Ministerio de Economía y Competitividad de España for Grant Juan de la Cierva FJCI-2014-22886 and FJCI-2015-25741, respectively.
Despite the fact that there are some commercial concentrated solar power plants worldwide, there is currently a lack of experimental reports about the operational characteristics of this type of plants. Therefore, a two-tank molten salts thermal energy storage (TES) pilot plant at the University of Lleida (Spain) was used to analyze charging and discharging processes under real conditions. In this facility, 1000 kg of molten salts are used as TES material and Therminol VP-1 is used as heat transfer fluid (HTF). This facility is equipped with measurement equipment which allows an exhaustive analysis of the processes. In this study, the fact of varying the flow arrangement in the heat exchanger (parallel and counter flow arrangements) and the temperature difference between the molten salts and the HTF have been studied and discussed in terms of temperature profiles, energy and power stored/released from/to both HTF and molten salts, efficiencies and effectiveness. The best working conditions found were counter flow arrangement with a temperature grading of about 65 °C. ; The research leading to these results has received funding from Spanish government (Fondo tecnológico IDI-20090393, ConSOLida CENIT 2008-1005) and from Abengoa Solar NT. The work is partially funded by the Spanish government (ENE2008-06687-C02-01/CON, ENE2011-28269-C03-02, ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123). This project has also 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). Laia Miró would like to thank the Spanish Government for her research fellowship (BES-2012-051861). 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 (2016FI_B 00047).
In the present paper, an experimental study is carried out to evaluate the effect of the dynamic melting concept in a cylindrical shell-and-tube heat exchanger using water as the phase change material (PCM) and a potassium formate/water solution as the heat transfer fluid (HTF). The dynamic melting concept is a new heat transfer enhancement technique which consists of recirculating the liquid PCM during the melting process with a pump and thus increasing the overall heat transfer coefficient as a result of the dominance of the forced convection. The HTF flow rate was kept constant at 1 l/min and four different PCM flow rates of 0, 0.5, 1 and 2 l/min were tested. Results from the experimental analysis showed enhancements up to 65.3% on the melting period, up to 56.4% on the effectiveness, and 66% on the heat transfer rates when the PCM flow rate was twice the HTF flow rate. From these experiments, it can be concluded that dynamic melting is an effective technique for enhancing heat transfer during melting of PCM. ; The authors acknowledge the South Australian Department of State Development who have funded this research through the Premier's Research Industry Fund - International Research Grant Program (IRGP 33). This project 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). 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 (2016FI_B 00047). The work is partially funded by the Spanish Government (ENE2015-64117-C5-1-R). The authors at the University of Lleida would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123).
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.
