• Energy Research

The performance of a conventional Ground-Source Refrigeration and Air Conditioning (GSRAC) system with a borehole heat exchanger (BHE) can be enhanced by addressing the soil thermal imbalance issue that affects these systems. This study proposes a novel concept for seasonal cold energy storage using a Thermal Diode Tank (TDT). The TDT consists of an insulated water tank fitted with an array of heat pipes. By integrating the TDT into a conventional GSRAC system, “cold” energy can be passively collected from ambient air during winter, injected into the BHE, and stored in the soil. The stored “cold” energy can then be retrieved in the summer, facilitating cross-seasonal cold energy storage (CS). Thus, a conventional GSRAC system can be transformed into a GSRAC system with cross-seasonal cold energy storage capability, i.e., GSRAC + CS system. The validated BHE model previously developed by the authors is used to predict the performance improvements achieved using the GSRAC + CS system. The results indicate that the Annual Net Cold Energy Storage Efficiency (ANESE) increased from 5.7% to 10.7% over a ten year period. The average Borehole Performance Improvement (BPI) due to the addition of cold storage capability is 11% over the same timeframe. This study also discusses the impacts of varying design and operational parameters on ANESE and BPI. The results demonstrate that GSRAC + CS systems not only mitigate the soil thermal imbalance issue faced by conventional GSRAC systems, but also require less BHE depth to achieve equivalent performance.

The performance of a conventional Ground-Source Refrigeration and Air Conditioning (GSRAC) system with a borehole heat exchanger (BHE) can be enhanced by addressing the soil thermal imbalance issue that affects these systems. This study proposes a novel concept for seasonal cold energy storage using a Thermal Diode Tank (TDT). The TDT consists of an insulated water tank fitted with an array of heat pipes. By integrating the TDT into a conventional GSRAC system, “cold” energy can be passively collected from ambient air during winter, injected into the BHE, and stored in the soil. The stored “cold” energy can then be retrieved in the summer, facilitating cross-seasonal cold energy storage (CS). Thus, a conventional GSRAC system can be transformed into a GSRAC system with cross-seasonal cold energy storage capability, i.e., GSRAC + CS system. The validated BHE model previously developed by the authors is used to predict the performance improvements achieved using the GSRAC + CS system. The results indicate that the Annual Net Cold Energy Storage Efficiency (ANESE) increased from 5.7% to 10.7% over a ten year period. The average Borehole Performance Improvement (BPI) due to the addition of cold storage capability is 11% over the same timeframe. This study also discusses the impacts of varying design and operational parameters on ANESE and BPI. The results demonstrate that GSRAC + CS systems not only mitigate the soil thermal imbalance issue faced by conventional GSRAC systems, but also require less BHE depth to achieve equivalent performance.

This study regards the evaluation of the performance of a thermally stratified tank as an intermediate combi-storage tank for a solar-driven residential thermal system coupled to a seasonal energy storage system. In such applications, the efficient operation of this intermediate tank is crucial to the enhanced exploitation of the harvested solar energy and the minimization of heat losses. In this perspective, the development of a dedicated model in TRNSYS software and its validation with experimental results are investigated. With respect to the simulation model’s discretization, it was found that beyond 60 nodes, the benefits to the model’s accuracy are almost negligible. Comparing the experimental data with the simulation’s results, the predicted temperature profile converges accurately to the measured values under steady-state conditions (threshold stabilization period of 1000 s after charging/discharging has occurred). However, the response of the model deviates considerably under transient conditions due to the lack of detailed inertia modeling of both the tank and the rest of the system components. Conclusively, the developed 1D simulation model is adequate for on- and off-design models where transient phenomena are of reduced importance, whereas for dynamic and semi-dynamic simulations, more detailed models are needed.

This study regards the evaluation of the performance of a thermally stratified tank as an intermediate combi-storage tank for a solar-driven residential thermal system coupled to a seasonal energy storage system. In such applications, the efficient operation of this intermediate tank is crucial to the enhanced exploitation of the harvested solar energy and the minimization of heat losses. In this perspective, the development of a dedicated model in TRNSYS software and its validation with experimental results are investigated. With respect to the simulation model’s discretization, it was found that beyond 60 nodes, the benefits to the model’s accuracy are almost negligible. Comparing the experimental data with the simulation’s results, the predicted temperature profile converges accurately to the measured values under steady-state conditions (threshold stabilization period of 1000 s after charging/discharging has occurred). However, the response of the model deviates considerably under transient conditions due to the lack of detailed inertia modeling of both the tank and the rest of the system components. Conclusively, the developed 1D simulation model is adequate for on- and off-design models where transient phenomena are of reduced importance, whereas for dynamic and semi-dynamic simulations, more detailed models are needed.

Seasonal Thermal Energy Storage (STES) is widely researched for having benefits in that it utilizes excess energy which would be wasted otherwise. The purpose of this study is to analyze energy efficiency of seasonal solar thermal energy system as heating system for greenhouses and compare it with conventional variable air volume (VAV) heating system. Greenhouse was chosen as a simulation model because it requires constant and stable heating through winter season to extend growing season and also because greenhouse can provide enough area to install solar collectors and heat storage tanks. The proposed seasonal solar thermal energy storage system consists of solar thermal collector, fully mixed heat storage tank, and VAV system. Energy simulation was conducted in two steps: heat storing in summer season, and heating in winter season. For greenhouses with area sizing 1600 m2, solar thermal collector of 1250 m2 and heat storage tank of 2000 m3 were designed. TRNSYS 17 and engineering equation solver (EES) were implemented for simulation and calculation of the systems thermal data. Simulation results showed the tank water temperature rising up to optimal temperature (95 oC) before heating season, and STES heating contributed to 55% of total heating load. Consequently, 30% of total heating cost was cut down showing energy efficiency of seasonal solar thermal energy storage system.

Seasonal Thermal Energy Storage (STES) is widely researched for having benefits in that it utilizes excess energy which would be wasted otherwise. The purpose of this study is to analyze energy efficiency of seasonal solar thermal energy system as heating system for greenhouses and compare it with conventional variable air volume (VAV) heating system. Greenhouse was chosen as a simulation model because it requires constant and stable heating through winter season to extend growing season and also because greenhouse can provide enough area to install solar collectors and heat storage tanks. The proposed seasonal solar thermal energy storage system consists of solar thermal collector, fully mixed heat storage tank, and VAV system. Energy simulation was conducted in two steps: heat storing in summer season, and heating in winter season. For greenhouses with area sizing 1600 m2, solar thermal collector of 1250 m2 and heat storage tank of 2000 m3 were designed. TRNSYS 17 and engineering equation solver (EES) were implemented for simulation and calculation of the systems thermal data. Simulation results showed the tank water temperature rising up to optimal temperature (95 oC) before heating season, and STES heating contributed to 55% of total heating load. Consequently, 30% of total heating cost was cut down showing energy efficiency of seasonal solar thermal energy storage system.

Sweden is only utilizing half of the available excess heat. To utilize more of the excess heat a seasonal thermal energy storage could be implemented to store excessed heat from the summer when the demand is lower to the winter when the demand is higher. This can be achieved by an integration of a seasonal thermal energy storage to the district heating system. A seasonal thermal energy storage may also reduce the need of the system’s peak load, which often is economically costly and adversely affect the environment. The purpose of the paper is to investigate the possibility for Skövde Värmeverk to implement a seasonal thermal storage. The paper is performed by a literature collection and calculations are made by software programs. The result shows that it is technically possible to implement a pit thermal energy storage and a borhole thermal energy storage, but no outcome shows a profitability within 20 years. A pit thermal energy storage can replace the system’s peak load up to 79 percent and a borhole thermal energy storage up to 2,8 percent. The most suitable case for Skövde Värmeverk is to install a pit thermal energy storage with a storage capacity of 4 GWh.

Sweden is only utilizing half of the available excess heat. To utilize more of the excess heat a seasonal thermal energy storage could be implemented to store excessed heat from the summer when the demand is lower to the winter when the demand is higher. This can be achieved by an integration of a seasonal thermal energy storage to the district heating system. A seasonal thermal energy storage may also reduce the need of the system’s peak load, which often is economically costly and adversely affect the environment. The purpose of the paper is to investigate the possibility for Skövde Värmeverk to implement a seasonal thermal storage. The paper is performed by a literature collection and calculations are made by software programs. The result shows that it is technically possible to implement a pit thermal energy storage and a borhole thermal energy storage, but no outcome shows a profitability within 20 years. A pit thermal energy storage can replace the system’s peak load up to 79 percent and a borhole thermal energy storage up to 2,8 percent. The most suitable case for Skövde Värmeverk is to install a pit thermal energy storage with a storage capacity of 4 GWh.

Sensible Heat Storage is the most common method of thermal energy storage, particularly in the form of hot water tanks. Essentially, sensible heat storage systems work by charging them with heat from a higher temperature source to raise the temperature of the thermal store, and by extracting heat to discharge them. On a larger scale, these sensible heat stores should be designed to store heat long term over seasons, which allow the thermal storage systems to be charged using solar thermal systems to then supply heat over colder periods and can be applied in an array of buildings, including individual dwellings and larger buildings. These seasonal storage systems consist of: Tank Thermal Energy Storage (TTES), Pit Thermal Energy Storage (PTES), Borehole Thermal Energy Storage (BTES) and Aquifer Thermal Energy Storage (ATES). The aim of this report is to provide useful information about the different construction techniques for the mentioned systems in addition to FP7 Einstein Project, where a big information research has already been done, analysing the main characteristics that interfere in the various proceedings. In addition, a general study for the three different CHESS-SETUP pilots is done regarding the availability and constraints of every case to introduce the different technologies. Finally, in order to ensure the correct operation of the installations, some guidance of the different types of maintenance is done as well as maintenance plans for the different elements of the system.