Seasonal Thermal Energy Storage Research Articles

Heat Transfer Analysis of Large Scale Seasonal Thermal Energy Storage for Underground Mine Ventilation

Abstract Deep underground mining is highly energy intensive due to the need to overcome high pressure rise required by ventilation fans, high cooling load in summer due to rise in rock temperature and the auto-compression effect, and heating requirement in winter. Rising energy costs have led the mining industry to look for alternatives in energy-efficient systems to reduce the operating costs as well as to reduce the carbon footprint. This paper addresses the challenge by utilizing naturally available renewable energy source from seasonal cycle for heating and cooling of underground mines: heat in the summer is stored in the rock-pit to be used for heating in winter, and the “cold” energy in winter is captured and is stored in the rock-pit for cooling during summer. A three-dimensional unsteady local thermal non-equilibrium (LTNE) model is developed to evaluate thermal storage and heat transfer between ventilation air and rock-pit (large broken rock mass). The results suggest that the seasonal thermal energy storage (Se-TES) of rock-pit is able to assist thermal management in underground mine and to reduce energy consumption for winter heating and summer cooling. The ventilation air temperature is about 15 to 20 ˚C higher/lower as compared to ambient temperature in winter/summer, respectively. Clearly, this shows potential application of large scale Se-TES in mining industry.

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.

SUSTAINABLE ENERGY SOURCES FOR THE FUTURE: A NEW GEOTHERMAL HEAT PUMP SYSTEM FOR THE BUILT ENVIRONMENT

In the recent attempts to stimulate alternative energy sources for heating and cooling of buildings, emphasise has been put on utilisation of the ambient energy from ground source heat pump systems (GSHPs) and other renewable energy sources. Exploitation of renewable energy sources and particularly ground heat in buildings can significantly contribute towards reducing dependency on fossil fuels. Geothermal heat pumps (GSHPs), or direct expansion (DX) ground source heat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater or surface water as a heat source when operating in heating mode or as a heat sink when operating in a cooling mode. It is receiving increasing interest because of its potential to reduce primary energy consumption and thus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that it utilises the lower temperature of the ground (approximately <32°C), which remains relatively stable throughout the year, to provide space heating, cooling and domestic hot water inside the building area. The main goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternative energy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energy from ground source and other renewable energy sources. The purpose of this study, however, is to examine the means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendly technology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPs applications as an optimum means of heating and cooling, and to present typical applications and recent advances of the DX GSHPs. The study highlighted the potential energy saving that could be achieved through the use of ground energy sources. It also focuses on the optimisation and improvement of the operation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the direct expansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonal thermal energy storage from solar thermal collectors, is extendable to more comprehensive applications.

Economic top–down evaluation of the costs of energy storages—A simple economic truth in two equations

Energy storage is a key technology for increasing energy efficiency and a more extensive integration of renewable energies. Besides technical and physical limits, cost uncertainty is a major barrier to the development and utilization of energy storage systems. In this work, an economic top–down approach has been worked out following the assumption that the maximum acceptable costs of energy supplied by a storage should not exceed the cost of energy from the market. Thereby, maximum acceptable storage capacity costs are calculated from the interest rate assigned to the capital costs, the intended payback period of the user class (e.g., industry or building), the costs of energy from the market, and the annual number of storage cycles. The main findings of the top–down evaluation are: first, for a fixed cycle period, the maximum acceptable storage capacity costs depend on the user’s economic environment. Second, seasonal thermal energy storage with up to 2 cycles per year requires storage costs below 3€·kWhcap−1 in the building sector and below 0.4€·kWhcap−1 in the industry sector, respectively. Third, short-term storages with for example 300 cycles per year allow 300 times higher storage costs. Therefore, if the annual number of storage cycles is sufficiently high, several energy storage technologies are economically viable. In this case, systems should be compared with regard to technical or physical attributes.

Development of a numerical model for the reaction zone design of an aqueous sodium hydroxide seasonal thermal energy storage

This paper describes a thermochemical seasonal storage with emphasis on the development of a reaction zone for an absorption/desorption unit. The heat and mass exchange is modelled and the design of a suitable reaction zone is explained. A tube bundle concept is presented for the heat and mass exchangers and the most demanding working conditions they should fulfil are modelled and discussed. To estimate the performance of such a reaction zone and to design it, numerical models were developed and are described in this paper. Several parameters influencing these models were tested such as the sensitivity of the models to the correlation used to calculate the heat and mass exchanges, the tube diameter and the tube pitch influence. The final contribution of the tube bundle modelling is to size and design the heat and mass exchanger constituting the reaction zone. This work will be used as a basis for the reaction zone construction of an aqueous sodium hydroxide seasonal thermal energy storage prototype.

Low Cost and Marketable Operational Experiences for a Solar Heating System with Seasonal Thermal Energy Storage (SHSSTES) in Hebei (China)

Abstract This paper introduces a seasonal thermal energy storage project which is the largest in Asia. In the project gross collector area is 11546m2 and capacity of the storage tank is 20381 tons. Several pilot solar heating systems with seasonal thermal energy storage (SHSSTES) built in Germany and North Europe since 1996 have been proven to provide stable performance and reliable high solar fraction. However due to construction cost and non-standard industrial manufacture. SHSSTES are not yet commercial prospects. In this project due to innovation in the solar collector system, water circulating system and tank design, the cost has fallen dramatically. Each part of the system has minimized cost without any compromise on performance. Also this project is a typical model of BOT (build-operate-transfer), with maximum benefit possible. This project is most likely to be commercially viable with payback period less than 6 years for this design. The influence of each part in the system is discussed through analysis of the data.

A Review on Borehole Seasonal Solar Thermal Energy Storage

Because of the intermittence and unreliability of solar radiation, a seasonal thermal energy storage system is needed to maximize the potential utilization of solar energy. Borehole seasonal solar thermal energy storage is one of the most common energy storage methods and some applications have been conducted. This paper reviews the studies on borehole seasonal solar thermal energy storage. Analytical and numerical models of underground regenerator and system simulations are summarized here. Large application projects used for center solar heating in European counties and some small scale experimental studies are included. Problems and barriers of the development of borehole heat storage in China are also given.

Data Measurement and Analysis of a Solar Heating System with Seasonal Storage

The technologies for seasonal thermal energy storage are of great significance in balancing the energy utilization and in enabling the renewable sources, especially solar energy, to be more widely used. The solar heating system with seasonal storage discussed in this paper is located in LuoYang City (Approx. E112’, N34.5’), in HeNan Province of China. The working data of the whole system, including temperatures of different parts, water flow, thermal energy, and solar irradiation, etc. are measured continuously and recorded using the technology of internet remote data transmission. The efficiency of the solar heating system is considered to be good when operating in general weather conditions, for example, 45% on sunny days. The average heat loss factor of the heat storage tank is only 0.50W/(K·m3),the ratio between surface and volume is about1.08 1/m.

Seasonal Thermal Energy Storage in Shallow Geothermal Systems: Thermal Equilibrium Stage

This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. The variation of temperature was obtained after 3 heating and cooling cycles for the different types of loads with maximum thermal load of qmax = 15 W.m−1 with variable angular frequency (8 months of heating and 4 months of cooling).and constant angular frequency (6 months of heating and 6 months of cooling) to estimate the necessary number of cycles to reach the thermal equilibrium stage. The results show that we approach a thermal equilibrium stage where the same variation of temperature can be observed in soils after several heating and cooling cycles. Based on these simulations, the necessary number of cycles can be related to the total applied energy on the system and the minimum number of cycles is for a system with the total applied energy of 1.9qmax .

Net energy analysis of a solar combi system with Seasonal Thermal Energy Store

EU targets require nearly zero energy buildings (NZEB) by 2020. However few monitored examples exist of how NZEB has been achieved in practise in individual residential buildings. This paper provides an example of how a low-energy building (built in 2006), has achieved nearly zero energy heating through the addition of a solar domestic hot water and space heating system (“combi system”) with a Seasonal Thermal Energy Store (STES). The paper also presents a cumulative life cycle energy and cumulative life cycle carbon analysis for the installation based on the recorded DHW and space heating demand in addition to energy payback periods and net energy ratios. In addition, the carbon and energy analysis is carried out for four other heating system scenarios including hybrid solar thermal/PV systems in order to obtain the optimal system from a carbon efficiency perspective.

Sustainable Energy Sources for the Future: A New Geothermal Heat Pump System for the Built Environment

In the recent attempts to stimulate alternative energy sources for heating and cooling of buildings,emphasise has been put on utilisation of the ambient energy from ground source heat pumpsystems (GSHPs) and other renewable energy sources. Exploitation of renewable energy sourcesand particularly ground heat in buildings can significantly contribute towards reducingdependency on fossil fuels. Geothermal heat pumps (GSHPs), or direct expansion (DX) ground sourceheat pumps, are a highly efficient renewable energy technology, which uses the earth, groundwater orsurface water as a heat source when operating in heating mode or as a heat sink when operating in a coolingmode. It is receiving increasing interest because of its potential to reduce primary energy consumption andthus reduce emissions of the greenhouse gases (GHGs). The main concept of this technology is that itutilises the lower temperature of the ground (approximately <32°C), which remains relatively stablethroughout the year, to provide space heating, cooling and domestic hot water inside the building area. Themain goal of this study is to stimulate the uptake of the GSHPs. Recent attempts to stimulate alternativeenergy sources for heating and cooling of buildings has emphasised the utilisation of the ambient energyfrom ground source and other renewable energy sources. The purpose of this study, however, is to examinethe means of reduction of energy consumption in buildings, identify GSHPs as an environmental friendlytechnology able to provide efficient utilisation of energy in the buildings sector, promote using GSHPsapplications as an optimum means of heating and cooling, and to present typical applications and recentadvances of the DX GSHPs. The study highlighted the potential energy saving that could be achievedthrough the use of ground energy sources. It also focuses on the optimisation and improvement of theoperation conditions of the heat cycle and performance of the DX GSHP. It is concluded that the directexpansion of the GSHP, combined with the ground heat exchanger in foundation piles and the seasonalthermal energy storage from solar thermal collectors, is extendable to more comprehensive applications

Identification of the objective function for optimization of a seasonal thermal energy storage system

Abstract The article shows the proposed solution of the objective function for the seasonal thermal energy storage system. In order to develop this function the technological and economic assumptions were used. In order to select the optimal system configuration mathematical models of the main elements of the system were built. Using these models, and based on the selected design point, the simulation of the entire system for randomly generated outside temperatures was made. The proposed methodology and obtained relationships can be readily used for control purposes, constituting model predicted control (MPC).

Achieving sustainable work of the heat pump with the support of an underground water tank and solar collectors

This paper deals with the methodology of sizing a sustainable heating–cooling system, which consists of a heat pump, thermo-solar collectors and underground water tank, as a seasonal thermal energy storage. The methodology is based on successive and coupled energy balancing of all system elements for time intervals of 1h, during one representative calendar year. The starting assumption for the calculation contains the size of the underground storage tank, its burial depth, the size of building that is heated or cooled, and the temperature of air in the building. To make the calculation, it is necessary to know the so-called dynamic boundary conditions during a calendar year, comprising the following: hourly outdoor air temperature, hourly insolation (received solar radiation energy) and hourly temperature of the ground surface. The calculation result is the size of the thermal–solar collectors surface, which will ensure the sustainability of the system, i.e. it will ensure that after one year of operation this system is restored to the original conditions, without disturbing natural environment in which it is used.For the purpose of presenting the methodology and checking the influence of individual parameters of the system's operation, a numerical simulation was performed and required surfaces of thermo-solar collectors were determined for one of the system models. Analyses of the influence of the underground tank burial depth, tank size and the initial water temperature in the tank on the required size of thermal–solar collectors were also performed.

Hybrid ground-source heat pump system with active air source regeneration

Ground-source heat pump systems (GSHP) offer great advantages over traditional heating and cooling installations. However, their applications are limited due to the high initial costs of borehole drilling. One way to avoid these costs is by reducing the size of the borefield, e.g. by combining the system with other renewable energy sources or by using active regeneration to increase the system efficiency. In this paper a hybrid ground-source heat pump system (HGSHP) is analyzed. The borefield is split into a warm part and a cold part, which allows for seasonal thermal-energy storage. Additionally, supplementary drycoolers capture heat during summer and cold during winter. The relationship between the underground storage size and temperature and the drycooler capacity is described, using an office building in Flanders (Belgium) as reference case. Results show that with a HGSHP system a significant borefield size reduction can be achieved without compromising system performance; i.e. for the reference case a reduction of 47% was achieved in the cost-optimal configuration. It is also shown that the cooling seasonal performance factor decreases significantly with underground storage capacity. In addition, the HGSHP can be used to maintain or restore thermal balance in the geothermal source when heating and cooling loads do not match.

Performance investigation of a solar heating system with underground seasonal energy storage for greenhouse application

This study reports the performance of a demonstrated 2304 m2 solar-heated greenhouse equipped with a seasonal thermal energy storage system in Shanghai, east China. This energy storage system utilises 4970 m3 of underground soil to store the heat captured by a 500 m2 solar collector in non-heating seasons through U-tube heat exchangers. During heating seasons, thermal energy is delivered by the heat exchange tubes placed on the plants shelves and the bare soil. The system can operate without a heat pump, which can save electricity consumption and further enhance the solar fraction. It was found that in the first operation year, 331.9 GJ was charged, and 208.9 GJ was later extracted for greenhouse space heating. No auxiliary heating equipment was installed so that solar energy covered all the heating loads directly or indirectly. It was demonstrated that this system was capable of maintaining an interior air temperature that was 13 °C higher than the ambient value when the latter temperature was −2 °C at night. The ECOP (electrical coefficient of performance) of the first operation year was approximately 8.7, indicating a better performance than the common heat pump heating system.

Energy Efficient Integration of Heat Pumps into Solar District Heating Systems with Seasonal Thermal Energy Storage

Solar district heating (SDH) with seasonal thermal energy storage (STES) is a technology to provide heat for space heating and domestic hot water preparation with a high fraction of renewable energy. In order to improve the efficiency of such systems heat pumps can be integrated. By preliminary studies it was discovered, that the integration of a heat pump does not always lead to improvements from an overall energy perspective, although the operation of the heat pump increases the efficiency of other components of the system e. g. the STES or the solar collectors. Thus the integration of heat pumps in SDH systems was investigated in detail.Usually, the heat pumps are integrated in such a way, that the STES is used as low temperature heat source. No other heat sources from the ambience are used and only that amount of energy consumed by the heat pump is additionally fed into the system. In the case of an electric driven heat pump, this is highly questionable concerning economic and CO2-emission aspects. Despite that fact the operation of the heat pump influences positively the performance of other components in the system e. g. the STES and makes them more efficient. If the primary energy consumption of the heat pump is lower than the energetic benefits of all other components, the integration makes sense from an energetic point of view.A detailed assessment has been carried out to evaluate the most promising system configurations for the integration of a heat pump. Based on this approach a system concept was developed in which the integration of the heat pump is energetically further improved compared to realised systems. By means of transient system simulations this concept was optimised with regard to the primary energy consumption. A parameter study of this new concept has been performed to identify the most sensitive parameters of the system.The main result and conclusion are that higher solar fractions and also higher primary energy savings can be achieved by SDH systems using heat pumps compared systems without heat pumps.

Reaction Zone Development for an Aqueous Sodium Hydroxide Seasonal Thermal Energy Storage

This paper focuses on the development of a reaction zone dedicated to an absorption/desorption seasonal thermal energy storage. The modelling of the tube bundle constituting the reaction zone is described as well as the boundary conditions in worst working conditions and some modelling results are presented for the desorber/absorber. In parallel to this sizing work, investigations were lead on the tube bundle optimisation by studying the wetting and fluid distribution. A specific developed experimental set up based on imaging enabled to quantify the influence of tube texturing and to improve the manifold design. This work will lead to the reaction zone construction for an aqueous sodium hydroxide seasonal thermal energy storage prototype.

STES—Typical Scenarios for Heat Accumulator Cooperation

The article presents an analysis of real operational data of a Seasonal Thermal Energy Storage system connected to a local heating grid. 8 years of operation time was analyzed to determine typical scenarios for cooperation with the heating grid and solar collectors. The research shows that three main working modes of the accumulator may be particularized: charging, discharging and transient modes, and that the maximum speed of charge/discharge is on average below 0.3°C/day.

Solar District Heating for the Built Environment-Technology and Future Trends within the European Project EINSTEIN

This paper shows the current German state of technology of solar district heating with seasonal thermal energy storage for the built environment. In order to point out the necessity for further development and implementation activities of this technology for future applications, the main contents of the European project EINSTEIN such as adequate system concepts or technical requirements for retrofitting applications are presented.

Seasonal Thermal Energy Storage - A Size Selection

The paper presents a theoretical investigation of using a Seasonal Thermal Energy Storage facility (STES) to cover the heat demand of a complex of four buildings. The STES is placed in the ground and connected to both the local district heating network and solar panels. A number of scenarios were investigated to find an adequate size of the STES (tank size and solar panel area.) The results obtained show that the use of a STES could reduce heat consumption by 22100% depending on the architecture solution chosen.