Solar Salt Latent Heat Thermal Storage for a Small Solar Organic Rankine Cycle Plant

The design of the latent heat thermal storage system (LHTESS) was developed with a thermal capacity of about 100 kW h as a part of small solar plant based on the organic Rankine cycle (ORC). The phase change material (PCM) used is solar salt with the melting/solidification temperature of about 220 °C. Thermophysical properties of the PCM were measured, including its phase transition temperature, heat of fusion, specific heat, and thermal conductivity. The design of the thermal storage was finalized by means of the 3D computational fluid dynamics analysis. The thermal storage system is modular, and the thermal energy is delivered with the use of thermal oil, heated by Fresnel mirrors. The heat is transferred into and from the PCM in the casing using bidirectional heat pipes, filled with water. A set of metallic screens are installed in the box with the pitch of 8–10 mm to enhance the heat transfer from heat pipes to the PCM and vice-versa during the charging and discharging processes, which take about 4 h. This work presents a numerical study on the use of metallic fins without thermal bonding as a heat transfer enhancement method for the solar salt LHTESS. The results show that the absence of the thermal bonding between fins and heat pipes (there was a gap of 0.5 mm between them) did not result in a significant reduction of charging or discharging periods. As expected, aluminum fins provide better performance in comparison with steel ones due to the difference in the material conductivity. The main advantage observed for the case of using aluminum fins was the lower temperature gradient across the LHTESS

Black box modelling of a latent heat thermal energy storage system coupled with heat pipes

This paper presents black box models to represent a LHTESS (Latent Heat Thermal Energy Storage System) coupled with heat pipes, aimed at increasing the storage performance and at decreasing the time of charging/discharging. The presented storage system is part of a micro solar CHP plant and the developed model is intended to be used in the simulation tool of the overall system, thus it has to be accurate but also fast computing. Black box data driven models are considered, trained by means of numerical data obtained from a white box detailed model of the LHTESS and heat pipes system. A year round simulation of the system during its normal operation within the micro solar CHP plant is used as dataset. Then the black box models are trained and finally validated on these data. Results show the need for a black box model that can take into account the different seasonal performance of the LHTESS. In this analysis the best fit was achieved by means of Random Forest models with an accuracy higher than 90%.This study is a part of the Innova MicroSolar Project, funded in the framework of the European Union’s Horizon 2020 Research and Innovation Programme (grant agreement No 723596). 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. 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

Development of a small solar thermal power plant for heat and power supply to domestic and small business buildings

The small solar thermal power plant is being developed with funding from EU Horizon 2020 Program. The plant is configured around a 2-kWel Organic Rankine Cycle turbine and solar field, made of Fresnel mirrors. The solar field is used to heat thermal oil to the temperature of about 240°C. This thermal energy is used to run the Organic Rankine Cycle turbine and the heat rejected in its condenser (about 18-kWth) is utilised for hot water production and living space heating. The plant is equipped with a latent heat thermal storage to extend its operation by about 4 hours during the evening building occupancy period. The phase change material used is Solar salt with the melting/solidification point at about 220°C. The total mass of the PCM is about 3,800kg and the thermal storage capacity is about 100kWh. The operation of the plant is monitored by a central controller unit. The main components of the plant are being manufactured and laboratory tested with the aim to assemble the plant at the demonstration site, located in Catalonia, Spain. At the first stage of investigations the ORC turbine will be directly integrated with the solar field to evaluate their joint performance. During the second stage of tests, the Latent Heat Thermal Storage will be incorporated into the plant and its performance during the charging and discharging processes will be investigated. It is planned that the continuous field tests of the whole plant will be performed during the 2018-2019 period

Black box modelling of a latent heat thermal energy storage system coupled with heat pipes

This paper presents black box models to represent a LHTESS (Latent Heat Thermal Energy Storage System) coupled with heat pipes, aimed at increasing the storage performance and at decreasing the time of charging/discharging. The presented storage system is part of a micro solar CHP plant and the developed model is intended to be used in the simulation tool of the overall system, thus it has to be accurate but also fast computing. Black box data driven models are considered, trained by means of numerical data obtained from a white box detailed model of the LHTESS and heat pipes system. A year round simulation of the system during its normal operation within the micro solar CHP plant is used as dataset. Then the black box models are trained and finally validated on these data. Results show the need for a black box model that can take into account the different seasonal performance of the LHTESS. In this analysis the best fit was achieved by means of Random Forest models with an accuracy higher than 90%

High-temperature phase change materials for thermal energy storage

The development of energy saving technologies is very actual issue of present day. One of perspective directions in developing these technologies is the thermal energy storage in various industry branches. The review considers the modern state of art in investigations and developments of high-temperature phase change materials perspective for storage thermal and a solar energy in the range of temperatures from 120 to 1000 °C. The considerable quantity of mixes and compositions on the basis of fluorides, chlorides, hydroxides, nitrates, carbonates, vanadates, molybdates and other salts, and also metal alloys is given. Thermophysical properties of potential heat storage salt compositions and metal alloys are presented. Compatibility of heat storage materials (HSM) and constructional materials have found its reflection in the present work. Data on long-term characteristics of some HSMs in the course of repeated cycles of fusion and solidification are analyzed. Article considers also other problems which should be solved for creation of commercial high-temperature heat storage devices with use of phase change materials.Heat storage materials (HSMs) Phase change materials (PCMs) Latent heat storage Thermal energy storage (TES) Salt mixes and compositions Metal alloys Thermophysical properties Corrosion

Passive thermal control in residential buildings using phase change materials

This paper analyzes the state of the art in R & D on integration of phase change materials into building structures for their passive thermal control. Such perspective phase change materials as paraffins, fatty acids and their blends, as well as fatty acid esters, are considered for passive thermal control of buildings. Gypsum wallboards, concretes, porous and other materials used for building structures forming are discussed. Various technologies of the PCM integration into building structures are described. The characteristics of three laboratory small models of buildings, which were subject of investigations, are presented in this paper. The results of comparative tests on fifteen full size buildings containing elements with PCMs are summarized. Experiments conducted by a number of researchers on passive solar buildings demonstrated that the application of phase change heat storage materials decreases the variation in the air temperature in the rooms; shifts the peak of energy consumption for heating and cooling of lightweight buildings by several hours and decreases energy consumption for maintaining the comfort temperature levels in buildings. Recommendations for further research activities in this field are proposed at the end of this review article

Salt hydrates as latent heat storage materials:Thermophysical properties and costs

Thermal energy storage is considered as one the most perspective technologies for increasing the efficiency of energy conversion processes and effective utilization of available sources of heat. Advantages and technical attractiveness of the thermal energy storing have resulted in continuously increasing numbers of research activities, especially in the last four decades. Among various applications of thermal energy storage, the heat or cold accumulation in the temperature range from −50 °C to 120 °C has a greater market potential and this can be carried out using a wide range of phase change latent heat materials. Among these materials the salt hydrates deserve a special attention and currently a large number of phase change compositions based on salt hydrates are produced commercially and available on the market. However, reliable data on thermophysical properties as well as their thermal stability over their lifetime is required to build effective storage systems. Currently this data is insufficient and is scattered across numerous sources that are often difficult to access for potential consumers. This paper summarises practically all available original experimental data on the phase change diagram of salt–water systems, melting temperatures, heat of fusion, specific heat, density, thermal conductivity, and thermal diffusivity in solid and liquid states and viscosity in the liquid state for 18 salt hydrates. The above information is provided for major market products on the basis of the salt hydrates for latent heat storage. The wholesale prices for pure salt, salt hydrates, and salt hydrate heat storage compositions are also additionally discussed