Seasonal Heat Storage System Research Articles

Improved heat transfer mathematical model forpyramid shaped seasonal heat storage water pit and the influence of boundary conditions

Seasonal heat storage can make use of urban waste heat for low-carbon heating throughout the year. Previous models for pyramid shaped water pit have limited adaptability and ignore the impact of soil temperature in the early stages of operation on the performance of water pits. This article combines two classic mathematical models to establish a mathematical model that can adapt to various sizes of inverted pyramid shaped water pits. The validation of this model was conducted using experimental data from 60000 m3 water pit of Dronninglund, and the influence of boundary conditions for this module were calculated and discussed. The results showed that the root mean square errors of the simulated top, bottom, and average temperature of the water pit were 3.07 °C, 2.39 °C, and 1.37 °C, with relative deviation ratios of 4.25 %, 2.27 %, and 6.62 %, respectively. Furthermore, it was found that for every 5 °C increase in the set average soil temperature, the storage efficiency of the water pits increases by 0.4 %∼0.5 %. In the case of common top insulation of the water pit, the total heat loss at the top, side walls, and bottom are 66.83 %, 31.8 %, 1.37 %. While considering comprehensive insulation in all directions, the heat loss rate for the top, side walls, and bottom are similar with the former, being 67.84 %, 30.79 %, and 1.37 %, respectively. The proposal and findings of this article provide a basis for simplified optimization planning and design of large-scale seasonal heat storage systems.

The design of a seasonal heat storage system with a geothermal heat pump for a residential building in Tehran

Purpose In this study, a solar water heating system along with a seasonal thermal energy storage and a heat pump is designed for a villa with an area of 192 m2 in Tehran, the capital of Iran. Design/methodology/approach According to the material and the area of the residential space, the required heating of the building was calculated manually and then the thermodynamic analysis of the system and simulation was done in MATLAB software. Finally, regarding the waste of system, an efficient solar heating system, providing all the required energy to heat the building, was obtained. Findings The surface area of the solar collector is equal to 46 m2, the capacity of the tank is about 2,850 m3, insulation thickness stands at 55 cm and the coefficient of performance in required heat pump is accounted to about 9.02. Also, according to the assessments, the maximum level of received energy by the collector in this system occurs at a maximum temperature of 68ºC. Originality/value To the best of the authors’ knowledge, in the present work, for the first time, using mathematical modeling and analyzing of the first and second laws of thermodynamics, as well as using of computational code in MATLAB software environment, the solar-assisted ground source heat pump system is simulated in a residential unit located in Tehran.

Performance analysis of seasonal soil heat storage system based on numerical simulation and experimental investigation

Renewable energy has become very prominent these days because of its sustainable and environment-friendly nature. The soil heat storage system plays an important role in the long-term storage of solar energy to ensure a consistent power supply. The experimental analysis and practical implementation of soil heat storage system are very difficult due to its large size and the simulation-based calculation is also very complex for the entire storage system. In order to reduce the system from the perspective of space and time, this paper has applied a similar function relationship for soil heat storage system and develops the traditional CFD simulation model and similar model. According to the dimensions of similar model, the corresponding experimental setup is constructed and the experimental results of long-term heat storage in the soil are compared with the calculation results of a CFD similar model. The relative error of inlet and outlet temperature difference is measured 10% and the error for soil temperature rise is below 1 °C. The proposed method has reduced the simulation and experimental workload for soil heat storage using similarity theory. Hence, the proposed method has great significance in the research domain of soil heat storage and the application of similarity theory.

Performance and Exergy Analyses of a Solar Assisted Heat Pump with Seasonal Heat Storage and Grey Water Heat Recovery Unit.

The main research objective of this paper was to compare exergy performance of three different heat pump (HP)-based systems and one natural gas (NG)-based system for the production of heating and cooling energy in a single-house dwelling. The study considered systems based on: 1. A NG and auxiliary cooling unit; 2. Solely HP, 3. HP with additional seasonal heat storage (SHS) and a solar thermal collector (STC); 4. HP with SHS, a STC and a grey water (GW) recovery unit. The assessment of exergy efficiencies for each case was based on the transient systems simulation program TRNSYS, which was used for the simulation of energy use for space heating and cooling of the building, sanitary hot water production, and the thermal response of the seasonal heat storage and solar thermal system. The results show that an enormous waste of exergy is observed by the system based on an NG boiler (with annual overall exergy efficiency of 0.11) in comparison to the most efficient systems, based on HP water–water with a seasonal heat storage and solar thermal collector with the efficiency of 0.47. The same system with an added GW unit exhibits lower water temperatures, resulting in the exergy efficiency of 0.43. The other three systems, based on air–, water–, and ground–water HPs, show significantly lower annual source water temperatures (10.9, 11.0, 11.0, respectively) compared to systems with SHS and SHS + GW, with temperatures of 28.8 and 19.3 K, respectively.

Dynamic model of solar heating plant with seasonal thermal energy storage

The article focuses on existing technologies developed to harvest and store solar irradiance as a source of primary energy in district heating systems. In the study particular attention was given to solar collector systems cooperating with seasonal heat storage solutions. Motivating this work was the search for a tool to assess achievable solar fraction ‒ the ratio of usable solar energy to total energy consumption. The outcomes of this work are: (i) a simplified dynamic model facilitating evaluation of the proposed solutions, and (ii) selection of optimal operating parameters of the proposed system.A brief review and discussion is made of the technical and legal limitations on possible investments. The ground rules of cooperation between solar based heat sources with seasonal storage systems and conventional industrial boilers and district heating network have been set out. The work touches on the physical components of the discussed systems. Commercially available solar collectors and heat exchangers are presented and their pros and cons discussed. Some in-depth analysis of seasonal heat storage solutions is provided, in particular on tank and pit thermal energy storage as well as storage solutions that use boreholes or aquifer layers.Examples are given of existing plants characterized by high solar fraction located in the EU region and outside of it. A simplified dynamic model developed in the Aspen Hysys software environment is described and the results discussed. Due to the high complexity of the primary problem, the model has been limited to a solar collector installation, seasonal heat storage system and auxiliary boiler. The results obtained from the model are discussed and future steps are presented in the last part of the article. These recommendations seek to further the development of an efficient way of analysis and commercial assessment of such systems.

Dynamic Simulation and Operating Characteristics of Ground-Coupled Heat Pump with Solar Seasonal Heat Storage System

A hybrid ground-coupled heat pump (GCHP) is an efficient and sustainable technology for space heating and cooling. A demonstration house equipped with GCHP with a solar seasonal heat storage (SSHS) system had been built in Harbin, a severe cold zone of China. A dynamic simulation model was built for the house and GCHP with the SSHS system using TRNSYS. The model used a newly developed vertical ground heat exchanger (VGHE) module which considered coupled heat and moisture transfer (CHMT) in ground with variable soil properties (VSPs) and phase change of soil moisture (PCSM). In the simulation, a large amount of computing is consumed for VSP and PCSM, while the computing amount for moisture transfer is small. The model with the new VGHE module produced better simulated results, compared with the field data. So, CHMT-VSP-PCSM affects the performance of VGHE and system to some extent, especially CHMT. Hourly variation laws of temperatures and energy parameters were analyzed, and different characteristics were showed up at different operating stages in heating and cooling seasons for both long and short terms. The GCHP with the SSHS system can meet the heating and cooling demands of the house in general. In cooling season, adjusting the ratio of the two groups of VGHE for heat storage and cooling will increase the utilization efficiency of VGHE and make the soil temperature more balanced.

Simulation Study of a Novel Solar Thermal Seasonal Heat Storage System Based on Stable Supercooled PCM for Space Heating and Domestic Hot Water Supply of Single Family Houses

A TRNSYS model of a novel PCM heat storage, utilizing stable supercooling of Sodium Acetate Trihydrate (SAT), is presented. To achieve high solar fractions in heat supply of single family houses, the necessary integration of big water volumes is challenging. To evaluate its functionality, a system model of a solar thermal combisystem for space heating and domestic hot water supply for dynamic system simulation was built. The key component is a PCM volume for long term heat storage. While conventional heat storage concepts with SAT release the latent heat a few degrees below the melting temperature, with the concept of stable supercooling latent heat can be stored for long periods of time at ambient temperature. This allows the design of a partly loss-free storage. Solar fractions were evaluated for simulation runs with two building variations. Annual specific space heating demands of 15 and 30 kWh/(m2a) and a domestic hot water demand of a typical single family house were considered. A sensitivity analysis on solar fractions of domestic heat supply was performed by variation of the collector field and the PCM volume. While the increase of the PCM volume from 4.5 m3 to 9 m3 shows moderate effects in all simulation runs, an increase of the collector area has substantial effects on the share of solar heat on the total energy demand of the building.

Decentralized heat supply with seasonal heat storage systems: Comparison of different heating systems

To reach international climate goals, the research about energy consumption have to focus more on existing buildings. In the building sector, the increase of energy efficiency of buildings will not be sufficient to reduce the consumption of fossil resources significantly. Therefore the application of renewable energies have to increase as well. Seasonal heat storage systems can support this process because the storage enables heat supply and heat demand to be decoupled time-wise. Nevertheless, innovative systems, which are using renewable energy for heating, are always in a competition with conventional heating systems, which are using gas or oil. They must be equally efficient in technical and economic aspects. For that reason, this study is focused on the comparison of a heat supply with seasonal heat storage systems in connection with a solar thermal system and a heating system with gas. For this comparison a technical system and an operating model was established. This system was analyzed by his economic parameters with complete finance plans. As a result, it is shown, that sustainable heat supply is not much more expensive than conventional heat supply. In addition, the social acceptance of different stakeholder is affected by these parameters. Therefor expert interviews with investors were done.

Verification and analysis of a dynamic simulation model of ground-coupled heat pump with solar seasonal heat storage system

Ground-coupled heat pump (GCHP) is an efficient and sustainable air-conditioning technology. A demonstration house equipped with ground-coupled heat pump with solar seasonal heat storage (GCHPSSHS) system in severe cold zone has been carried out in Harbin, China. A dynamic simulation model using TRNSYS has been built for the house equipped with GCHPSSHS system. The model used a newly developed vertical ground heat exchanger (VGHE) module which considered coupled heat and moisture transfer in ground with variable soil properties and phase change of soil moisture (CHMT-VSP-PCSM). It was verified that the model with new VGHE module produced better simulation results (relative deviations are less than 2%), comparing with the field data. The average soil temperature and inlet and outlet temperatures of VGHE decrease obviously in the heating season. The GCHPSSHS system can meet the heating and cooling requirements of the building in general.

A DFT based equilibrium study of a chemical mixture Tachyhydrite and their lower hydrates for long term heat storage

Chloride based salt hydrates are promising materials for seasonal heat storage. However, hydrolysis, a side reaction, deteriorates, their cycle stability. To improve the kinetics and durability, we have investigated the optimum operating conditions of a chemical mixture of CaCl2 and MgCl2 hydrates. In this study, we apply a GGA-DFT to gain insight into the various hydrates of CaMg2Cl6. We have obtained the structural properties, atomic charges and vibrational frequencies of CaMg2Cl6 hydrates. The entropic contribution and the enthalpy change are quantified from ground state energy and harmonic frequencies. Subsequently, the change in the Gibbs free energy of thermolysis was obtained under a wide range of temperature and pressure. The equilibrium product concentration of thermolysis can be used to design the seasonal heat storage system under different operating conditions.

Performance comparison of heat exchanger designs for a seasonal heat storage system

Heat exchanger design for low energy buildings with thermal block energy storage is a critical component of the overall system performance. Geometric modifications proposed to improve the effectiveness of a heat exchanger in such a building have been examined through simulations and experiments. Of the considered modifications, one configuration (inserting chimney-sweep brushes) was only considered experimentally due to the difficulty of meshing such a complex geometry. Other configurations were considered numerically as possible future modifications. Comparison of the resulting experimental and numerical results indicate that a 30% increase in heat exchanger effectiveness is achieved through use of the brushes, which exceeds the improvements predicted for the other configurations. These results indicate that no further modification of the heat exchanger is needed.

Thermal investigation of in-series vertical ground heat exchangers for industrial waste heat storage

This paper presents a novel ground heat exchanger configuration with vertical boreholes connected in series to be used for the seasonal thermal storage of industrial high-temperature condensation heat. The transient simulation model of multistage-series circuits is developed and the simplex method is employed to solve the outlet/inlet temperatures and the heat transfer rate fraction of each circuit. A simplified steady-flux model is also proposed to design a large-scale seasonal heat storage system. Finally, the effects of some critical parameters on the thermal performance of the GHE with multistage-series circuits are investigated. The results show that both optimum borehole spacing and the higher thermal properties of the ground can improve the thermal efficiency of the storage system and reduce the capital cost accordingly.

A low cost seasonal solar soil heat storage system for greenhouse heating: Design and pilot study

A low cost Seasonal Solar Soil Heat Storage (SSSHS) system used for greenhouse heating was invented and investigated. With soil heat storage technology, the solar energy stored in soil under greenhouse can be utilized to reduce the energy demand of extreme cold and consecutive overcast weather in winter. Unlike conventional underground heat systems, heat pumps are not needed in this system and so the cost is drastically reduced. After the tests, the system proved that seasonal thermal energy storage (STES) is feasible and can partially solve the solar heat demand and supply imbalance problem between summer and winter. TRNSYS is used to simulate the process and effect of solar energy collection and soil heat storage, and the model is calibrated by operational data in a full season. Energy consumption of the SSSHS system and conventional solar heating system have been compared under the same condition: when the indoor air temperature of the greenhouse is kept above 12°C throughout the year, the energy saving in Shanghai was 27.8kWh/(m2 typical greenhouse area⋅year). In the end, the paper discusses the system optimization, including the optimized solar collector area and depth of buried U-pipes, and the results of a pilot test.

Design and Application of a Seasonal Solar Soil Heat Storage System Applied in Greenhouse Heating

A seasonal solar soil heat storage (SSSHS) system applied in greenhouse heating has been designed and introduced. The system consists of solar collector subsystem, soil heat storage subsystem, greenhouse heating subsystem, hydronic subsystem and control subsystem. By applying soil heat storage, solar energy stored in the soil under the greenhouse can be transferred and utilized in winter to realize the utilization of cross-seasonal energy. TRNSYS is used to simulate the process and effect in the system of the solar energy collection and soil heat storage in Shanghai, and the simulation is calibrated to improve the precision of the TRNSYS model. When the indoor air temperature of the greenhouse is kept at 12°C throughout the year, the energy saving by using the SSSHS system in Shanghai can be 46.2kWh/(m2∙year).

Numerical Analysis of Seasonal Heat Storage Systems of Alternative Geothermal Energy Pile Foundations

AbstractGeothermal energy pile foundations are sustainable, cost-effective alternative energy systems for heating and cooling needs of buildings. This paper presents the thermal modeling of two different configurations of energy pile foundations used for seasonal energy storage. Several two-dimensional (2D) transient analyses were carried out to find the optimum pile configuration and to obtain preliminary information about the heat exchange between the ground and the building. Two scenarios were modeled. In Scenario 1, a combination of long and short piles was simulated, while in Scenario 2 only long piles were considered. Long piles are structural piles and work as a heat exchanger throughout the year, while the nonstructural short piles operate only in the summer months to inject surplus solar energy into the ground. Simulation results show that in Scenario 1 the vertical temperature distribution was uneven, resulted in high temperatures at shallow depths (0–10 m) and near-freezing temperatures at grea...

A DFT based equilibrium study on the hydrolysis and the dehydration reactions of MgCl2 hydrates

Magnesium chloride hydrates are characterized as promising energy storage materials in the built-environment. During the dehydration of these materials, there are chances for the release of harmful HCl gas, which can potentially damage the material as well as the equipment. Hydrolysis reactions in magnesium chloride hydrates are subject of study for industrial applications. However, the information about the possibility of hydrolysis reaction, and its preference over dehydration in energy storage systems is still ambiguous at the operating conditions in a seasonal heat storage system. A density functional theory level study is performed to determine molecular structures, charges, and harmonic frequencies in order to identify the formation of HCl at the operating temperatures in an energy storage system. The preference of hydrolysis over dehydration is quantified by applying thermodynamic equilibrium principles by calculating Gibbs free energies of the hydrated magnesium chloride molecules. The molecular structures of the hydrates (n = 0, 1, 2, 4, and 6) of MgCl2 are investigated to understand the stability and symmetry of these molecules. The structures are found to be noncomplex with almost no meta-stable isomers, which may be related to the faster kinetics observed in the hydration of chlorides compared to sulfates. Also, the frequency spectra of these molecules are calculated, which in turn are used to calculate the changes in Gibbs free energy of dehydration and hydrolysis reactions. From these calculations, it is found that the probability for hydrolysis to occur is larger for lower hydrates. Hydrolysis occurring from the hexa-, tetra-, and di-hydrate is only possible when the temperature is increased too fast to a very high value. In the case of the mono-hydrate, hydrolysis may become favorable at high water vapor pressure and at low HCl pressure.

Study of the reversible water vapour sorption process of MgSO4.7H2O and MgCl2.6H2O under the conditions of seasonal solar heat storage

The characterization of the structural, compositional and thermodynamic properties of MgSO4.7H2O and MgCl2.6H2O has been done using in-situ X-ray Diffraction and thermal analyses (TG/DSC) under practical conditions for seasonal heat storage (Tmax=150°C, p(H2O)=13 mbar). This study showed that these two materials release heat after a dehydration/hydration cycle with energy densities of 0.38 GJ/m3 for MgSO4.7H2O and 0.71 GJ/m3 MgCl2.6H2O. The low heat release found for MgSO4.7H2O is mainly attributed to the amorphization of the material during the dehydration performed at 13 mbar which reduces its sorption capacity during the rehydration. MgCl2.6H2O presents a high energy density which makes this material interesting for seasonal heat storage in domestic applications. This material would be able to fulfil the winter heat demand of a passive house estimated at 6 GJ with a packed bed reactor of 8.5 m3. However, a seasonal heat storage system built with this material should be carefully set with a restricted temperature at 40°C for the hydration reaction to avoid the liquefaction of the material at lower temperature which limits its performances for long term storage.

Characterization of MgSO4 Hydrate for Thermochemical Seasonal Heat Storage

Water vapor sorption in salt hydrates is one of the most promising means for compact, low loss, and long-term storage of solar heat in the built environment. One of the most interesting salt hydrates for compact seasonal heat storage is magnesium sulfate heptahydrate (MgSO4⋅7H2O). This paper describes the characterization of MgSO4⋅7H2O to examine its suitability for application in a seasonal heat storage system for the built environment. Both charging (dehydration) and discharging (hydration) behaviors of the material were studied using thermogravimetric differential scanning calorimetry, X-ray diffraction, particle distribution measurements, and scanning electron microscope. The experimental results show that MgSO4⋅7H2O can be dehydrated at temperatures below 150°C, which can be reached by a medium temperature (vacuum tube) collector. Additionally, the material was able to store 2.2 GJ/m3, almost nine times more energy than can be stored in water as sensible heat. On the other hand, the experimental results indicate that the release of the stored heat is more difficult. The amount of water taken up and the energy released by the material turned out to be strongly dependent on the water vapor pressure, temperature, and the total system pressure. The results of this study indicate that the application of MgSO4⋅7H2O at atmospheric pressure is problematic for a heat storage system where heat is released above 40°C using a water vapor pressure of 1.3 kPa. However, first experiments performed in a closed system at low pressure indicate that a small amount of heat can be released at 50°C and a water vapor pressure of 1.3 kPa. If a heat storage system has to operate at atmospheric pressure, then the application of MgSO4⋅7H2O for seasonal heat storage is possible for space heating operating at 25°C and a water vapor pressure of 2.1 kPa.

Thermoeconomics of seasonal latent heat storage system

SUMMARY A simple thermoeconomic analysis is performed for a seasonal latent heat storage system for heating a greenhouse. The system consists of three units that are a set of 18 packed-bed solar air heaters, a latent heat storage tank with 6000 kg of technical grade paraffin wax as phase-changing material, and a greenhouse of 180 m 2 . The cost rate balance for the output of a unit is used to estimate the specific cost of exergy for a yearly operation. Based on the cost rate of exergy, fixed capital investment, operating cost, and economic data, approximate cash-flow diagrams have been prepared. The systems feasibility depends on the cost rate of exergy, operating cost, internal interest rate, and rate of taxation strongly. A cash-flow diagram based on exergy considerations may enhance the impact of thermoeconomic analysis in feasibility studies of thermal systems. Copyright # 2006 John Wiley & Sons, Ltd.

05/02589 Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating

05/02589 Experimental evaluation of energy and exergy efficiency of a seasonal latent heat storage system for greenhouse heating