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Latent heat storage materials have been tested by several researchers for decades to be used as passive heating and cooling systems in buildings but their implementation into building components is still stacked as is facing specific technical limitations related to difficulties to be charged both in heating and cooling periods. This paper presents a numerical analysis to evaluate the potential of a disruptive system, which is designed to solve the main drawbacks and to convert phase change materials (PCM) passive heating technology into a competitive solution for the building sector. The novel technology moves PCM layer with respect to the insulation layer inside the building component to maximize solar benefits in winter and be able to actively provide space heating. Design variables such as PCM melting point and control schemes were optimized. The results demonstrated that this technology is not only able to limit heat losses towards outdoors but it can provide space heating from stored solar energy when required. The promising numerical results endorse the possibility to build a future experimental prototype to quantify more in detail the benefits of this system. ; This work was partially funded by the Ministerio de Ciencia, Innovación y Universidades de España (RTI2018-093849-B-C31). The authors at the University of Lleida 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. A.D.G. has received funding from Appl. Sci. 2019, 9, 3688 11 of 12 the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia.

A novel concept based on the dynamic use of phase change materials (PCM) in building envelopes is presented in this paper. The concept aims at breaking the main technical barriers that PCM have been dealing with in its application as passive cooling system: (i) solidification process of PCM is limited and (ii) peak cooling load is delayed but mainly discharged indoors. The concept relies on the ability of the system to modify the position of the PCM layer inside the building envelope with respect to the insulation layer. A proof of concept evaluation based on a numerical tool demonstrates the cooling load reduction potential of this technology when implemented in different construction systems. PCM peak melting temperature as well as the daily activations of the system were optimized using a Particle Swarm Optimization (PSO) algorithm. The numerical results indicate that the dynamic system facilitates dramatically the solidification process of PCM, allowing the system to be designed with lower PCM peak melting temperatures. The potential of the system to charge PCM which solidifies at temperatures lower than indoor set point, allows the technology to be used not only as a thermal barrier but as a cooling supplier system. ; The authors at the University of Lleida 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. Dr. Alvaro de Gracia has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia.

The detailed study of latent packed bed thermal energy storage (TES) system has been a great topic of interest in the literature. Experimental measurements have been conducted to analyze the performance of these systems, however, the complex transient nature of latent TES makes necessary the use of numerical models for detailed study and evaluation of key design parameters, which lead to a numerous scientific contributions in the field. Different and diverse numerical models have been developed, which can be mainly divided into single phase models, Schumman's model, concentric dispersion model, and continuous solid phase model. This paper provides an extensive comprehensive revision of the different numerical models, highlighting the key aspects of each one as well as the main findings in the field. Furthermore, the performance of the different methodologies are discussed and compared. The most important empirical correlations used in the different models in order to take into account physics, such as natural convection inside the spheres or effective thermal conductivity of heat transfer fluid, are also given. ; The work partially funded by the Spanish Government (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 (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). Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

It is well known that there is a need to develop technologies to achieve thermal comfort in buildings lowering the cooling and heating demand. Research has shown that thermal energy storage (TES) is a way to do so, but also other purposes can be pursued when using TES in buildings, such as peak shaving or increase of energy efficiency in HVAC systems. This paper reviews TES in buildings using sensible, latent heat and thermochemical energy storage. Sustainable heating and cooling with TES in buildings can be achieved through passive systems in building envelopes, Phase Change Materials (PCM) in active systems, sorption systems, and seasonal storage. ; The work 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 (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). Alvaro de Gracia would like to thank Education Ministry of Chile for Grant PMI ANT1201.

Buildings should be understood as a process that consumes energy in all their phases (design, construction, use and end-of-life) and, more specifically, the building envelope is clearly involved in all of them. For this reason, the International Energy Agency defines in its latest publication the improvement of building envelopes as one of the key points to reduce the energy consumption in buildings. In the present study, two sustainable construction systems based on rammed earth walls are adapted to modern requirements to be thermally tested and compared against three Mediterranean conventional systems under summer conditions. The experimentation was done by performing several experiments in free floating and controlled temperature conditions at real scale in five cubicle-shape buildings with inner dimensions 2.4 × 2.4 × 2.4 m. The purpose of this study is to demonstrate that more sustainable construction systems can be used instead of conventional ones, with higher embodied energy, and achieve similar thermal response. Results show that the reduction of rammed earth wall thickness strongly penalizes its thermal behavior. However, similar thermal response than conventional systems is reached when 6 cm of wooden insulation panels are added in the outer face of the cubicle-shape building. ; The work is partially funded by the Spanish government (ULLE10-4E-1305 and ENE2015-64117-C5-1-R). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA-UdL (2014 SGR 123) and the city hall of Puigverd de Lleida. 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). Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

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 need to achieve energy efficiency standards in new and existing buildings has triggered both research and design practice aimed at reducing their carbon footprint and improving their indoor comfort and functionality conditions. In this view, a dedicated scientific effort has to be spent while dealing with historical architectures needing to preserve their key testimonial knowledge into the society. Therefore, tailored retrofit strategies have been investigated and implemented without compromising their architectural value, especially when new uses are foreseen in those buildings. This review classifies different examples of the use of energy efficiency approaches and the integration of renewable energies in historical buildings, including solar and geothermal energy, and the use of heat pumps and other high-efficiency Heating Ventilation and Air Conditioning systems. ; Prof. Luisa F. Cabeza would like to acknowledge the Spanish Government for the funding PRX17/00221, that allowed her to visit University of Perugia during 6 months. Prof. Cabeza 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. The project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curiegrant agreement No 712949 (TECNIOspring PLUS) and from the Agency for Business Competitiveness of the Government of Catalonia.

A radiant wall heating system embedded in a heavy brickwork envelope and coupled to a ground source heat pump supplied has been experimentally tested under real outdoor conditions. This system was applied to a room sized cubicle built in Puigverd de Lleida (Spain) test-site, where it was studied in system vs. system analysis in comparison to a reference cubicle built with commercial available technologies (insulated alveolar brick wall and air-to-air heat pump). The results showed the potential of the radiant wall, which in continuous operation reached energy savings between 19.97% and 40.72% based on set-point temperature. Most important, the active thermal mass of radiant wall allowed operating in off peak periods. Otherwise, this peak load shifting ability was completely inexistent in the reference cubicle. However, the results show that the radiant cubicle was unsuited to operate in occupancy schedules due to its slow response time. Furthermore, the tests show that optimization of the radiant wall system requires a control strategy that takes in account the dynamics of the system. ; The work partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER), ENE2015-64117-C5-3-R (MINECO/FEDER), TIN2015-71799-C2-2-P and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123) and the city hall of Puigverd de Lleida. This projects has received funding from the European Commission Seventh Framework Programme (FP/2007–2013) under Grant agreement Nº PIRSES-GA-2013-610692 (INNOSTORAGE) and from European Union's Horizon 2020 research and innovation programme under grant agreement Nº 657466 (INPATH-TES) and from EEA-Grants under grant IDI-20140914. Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

An experimental set-up consisting of a house like cubicle exposed to outdoor weather was used to validate a numerical model of a radiant wall. The 2D transient finite volume model used as inputs the indoor temperature, outdoor temperature, global solar radiation incident on a vertical surface, and temperature and flow of the supply water. The simulation results closely agreed with the temperature profiles and heat fluxes for the three studied orientations (East, South, and West). Furthermore, a parametric study was carried out with the radiant wall model, concluding that pipes spacing between 125 mm and 150 mm and depth between 45 mm and 65 mm minimized the temperature difference on the surface while maximizing the heat flux. Furthermore, a control strategy with shorter activation periods improved the heat transfer efficiency. ; The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER), ENE2015-64117-C5-3-R (MINECO/FEDER), and ULLE10-4E-1305). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123) and the city hall of Puigverd de Lleida. This projects has received funding from the European Commission Seventh Framework Programme (FP/2007–2013) under Grant agreement Nº PIRSES-GA-2013-610692 (INNOSTORAGE) and from European Union's Horizon 2020 research and innovation programme under grant agreement Nº 657466 (INPATH-TES). Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

Buildings account for a significant amount of global energy use and CO2 emissions. Thermally Activated Building Systems (TABS) are a technology with potential for significantly reducing buildings energy use. TABS are heating and cooling systems that are integrated in the building structure. They mainly exchange heat through radiation and are able to store heat in the building thermal mass. TABS high thermal mass and their interaction with the building structure make their energy evaluation and design process difficult. Development of simulation models has been essential to study the design and control of TABS. Control of TABS is challenging due to the slow response time and storage capacity. A lot of research has been conducted to develop control strategies that fully exploit its energy saving potential and that maximize the use of renewable energies. This paper summarizes the main characteristics of TABS and presents the developed simulation models and control strategies. ; The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123). This projects has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement N° PIRSES-GA-2013-610692 (INNOSTORAGE) and from European Union's Horizon 2020 research and innovation programme under grant agreement N° 657466 (INPATH-TES) and from EEA-Grants under grant IDI-20140914. The work was partially funded by the Spanish government grant ENE2015-64117-C5-1-R and grant ENE2015-64117-C5-3-R. Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

A building prototype was built in the experimental set-up located in Puigverd de Lleida (Spain) to study the energy performance of a radiant wall with ventilated facade cooled with a ground coupled heat-exchanger. The installed geothermal heat pump operates only as a ground coupled heat exchanger on cooling mode, thus providing free-cooling. In this case, only the circulation pumps consume power. The summer experimental campaign showed the energy savings potential and the peak load shifting ability of the system. On continuous operation and taking as reference a cubicle equipped with a conventional air-to-air heat pump, the radiant wall cooled with the ground coupled heat-exchanger achieved savings up to 54.17% and 82.08% at set-point temperatures of 24 °C and 26 °C, respectively. The thermal storage capacity of the system was studied in night charging test, when the cubicles were pre-cooled during night-time. During the day, the temperature raise caused by heat loads was small and the system kept the temperature inside comfort range despite it only operated overnight. However, the performance was very sensitive to set-point temperature. Free-cooling was limited by the temperatures in the boreholes, showing that with lower set-points the gradient between supply temperature and room temperature was small, and thus it required a higher water flow to achieve the necessary cooling power. Intermittent operation of the system according to different schedules also affected the radiant walls performance as they interacted with the thermal inertia of the system, which could even have a negative impact on energy use. ; The work partially funded by the Spanish government (ULLE10-4E-1305, ENE2015-64117-C5-1-R(MINECO/FEDER), 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 (2014 SGR 123) and the city hall of Puigverd de Lleida. This projects has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement No. PIRSES-GA-2013-610692 (INNOSTORAGE) and from European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES) and from EEA-Grants under grant IDI-20140914. Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

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).

This paper experimentally evaluates the implementation of heat pipes in latent heat thermal energy storage systems. The well-known performance of heat pipes as a heat transfer technology makes them great candidates to be used as heat exchangers. However, previous studies compared their efficacy against solid metal rods, where heat pipes clearly succeeded. Therefore, the objective of this study is to experimentally evaluate the advantages of using heat pipes instead of a common shell and tubes system, during charging processes. In particular, five latent heat thermal energy storage systems were tested. One based on the shell and tubes, and the remaining four based on heat pipes. The experiments were conducted at constant heat transfer fluid temperature and flow rate, and the results were analysed from the temperature, heat transfer, and visual point of view. The results show that in heat pipes systems the phase change material melts homogeneously through all the storage container. However, the shell and tubes tank performed the charging process in 25 min while the fastest heat pipe one took 40 min for it. On the other hand, in the shell and tubes configuration melt from the heat transfer fluid inlet towards the outlet. Moreover, systems with more heat pipe surface inside the heat transfer fluid collector rovided higher power rates. Comparing the best and the worst heat pipe storage tanks during the first 30 min, the heat transfer rate increased over 40%. But the storing material low conductivity cushioned those high heat transfer rates. ; 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.

Buildings contribute to climate change by consuming a considerable amount of energy to provide thermal comfort for occupants. Cooling energy demands are expected to increase substantially in the world. On this basis, technologies and techniques providing high energy efficiency in buildings such as passive cooling are highly appreciated. Passive cooling by means of phase change materials (PCM) offers high potential to decrease the cooling energy demand and to improve the indoor comfort condition. However, in order to be appropriately characterized and implemented into the building envelope, the PCM use should be numerically analyzed. Whole-building energy simulation tools can enhance the capability of the engineers and designers to analyze the thermal behavior of PCM-enhanced buildings. In this paper, an extensive review has been made, with regard to whole-building energy simulation for passive cooling, addressing the possibilities of applying different PCM-enhanced components into the building envelope and also the feasibility of PCM passive cooling system under different climate conditions. The application of PCM has not always been as energy beneficial as expected, and actually its effectiveness is highly dependent on the climatic condition, on the PCM melting temperature and on the occupants behavior. Therefore, energy simulation of passive PCM systems is found to be a single-objective or multi-objective optimization problem which requires appropriate mathematical models for energy and comfort assessment which should be further investigated. Moreover, further research is required to analyze the influence of natural night ventilation on the cooling performance of PCM. ; The work is partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER) and ENE2015-64117-C5-3-R (MINEDO/FEDER)). The authors would like to thank the Catalan Government for the quality accreditation g iven to their research group GREA (2014 SGR 123). This project has received funding from the European Commission Seventh Framework Program (FP/2007-2013) under Grant agreement Nº PIRSES-GA-2013-610692 (INNOSTORAGE) and from the European Union's Horizon 2020 research and innovation program under grant ag reement No 657466 (INPATH-TES). Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940.

The building envelope has high potential to reduce the energy consumption of buildings according to the International Energy Agency (IEA) because it is involved along all the building process: design, construction, use, and end-of-life. The present study compares the thermal behavior of seven different building prototypes tested under Mediterranean climate: two of them were built with sustainable earth-based construction systems and the other five, with conventional brick construction systems. The tested earth-based construction systems consist of rammed earth walls and wooden green roofs, which have been adapted to contemporary requirements by reducing their thickness. In order to balance the thermal response, wooden insulation panels were placed in one of the earth prototypes. All building prototypes have the same inner dimensions and orientation, and they are fully monitored to register inner temperature and humidity, surface walls temperatures and temperatures inside walls. Furthermore, all building prototypes are equipped with a heat pump and an electricity meter to measure the electrical energy consumed to maintain a certain level of comfort. The experimentation was performed along a whole year by carrying out several experiments in free floating and controlled temperature conditions. This study aims at demonstrating that sustainable construction systems can behave similarly or even better than conventional ones under summer and winter conditions. Results show that thermal behavior is strongly penalized when rammed earth wall thickness is reduced. However, the addition of 6 cm of wooden insulation panels in the outer surface of the building prototype successfully improves the thermal response. ; This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement Nº PIRSES-GA-2013-610692 (INNOSTORAGE), from the European Union's Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES) and the Spanish government (ULLE10-4E-1305, 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 GREA-UdL (2014 SGR 123) and the city hall of Puigverd de Lleida. GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. Álvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014- 19940.