Central electric thermal storage (ETS) feasibility for residential applications: Part 1. Numerical and experimental study

Detailed models have been developed and integrated into a TRNSYS calculation tool to evaluate the optimal storage capacity of central electric thermal storage (ETS) units for residential applications. The tool uses hourly weather data to establish the energy use profile of residential buildings under different heating system configurations and control strategies. The reliability of the system for each configuration is determined as the percentage of time the heating system meets the heating requirement of the building. The level of reliability is used to evaluate the appropriate capacity of the system. This study investigates the theoretical development and modelling of a multi‐zone building equipped with a central ETS in TRNSYS. The TRNSYS simulation results are then compared and validated against experimental data obtained during the project. Furthermore, the effect of various parameters such as system configuration, charging capacity, control strategies, heat losses from the building, and electric utility time‐of‐use schedules are analysed. Copyright © 2001 John Wiley & Sons, Ltd.     

German central solar heating plants with seasonal heat storage

Central solar heating plants contribute to the reduction of CO₂-emissions and global warming. The combination of central solar heating plants with seasonal heat storage enables high solar fractions of 50% and more. Several pilot central solar heating plants with seasonal heat storage (CSHPSS) built in Germany since 1996 have proven the appropriate operation of these systems and confirmed the high solar fractions.Four different types of seasonal thermal energy stores have been developed, tested and monitored under realistic operation conditions: Hot-water thermal energy store (e.g. in Friedrichshafen), gravel-water thermal energy store (e.g. in Steinfurt-Borghorst), borehole thermal energy store (in Neckarsulm) and aquifer thermal energy store (in Rostock). In this paper, measured heat balances of several German CSHPSS are presented. The different types of thermal energy stores and the affiliated central solar heating plants and district heating systems are described. Their operational characteristics are compared using measured data gained from an extensive monitoring program. Thus long-term operational experiences such as the influence of net return temperatures are shown.     

Thermal management and desorption modeling of a cryo-adsorbent hydrogen storage system

Adsorbent based hydrogen storage systems using activated carbon or metal organic framework (MOF) materials are among the most promising material based hydrogen storage systems. In this effort, a complete 3-D discharge model to simulate extraction of hydrogen from the storage bed for fuel cell operation is developed. The desorption process is endothermic and the heating requirement is met by a heating element which includes a helical coil and a central longitudinal rod placed inside the bed. Since thermal conductivity of adsorbent bed plays a crucial role in determining the temperature distribution, simulations were performed to find an optimum conductivity range that can facilitate maximum extraction of hydrogen from the bed along with a fairly uniform bed temperature distribution. Finally, storage system designs including the adsorbent bed and heating system for two adsorbents – an activated carbon (AX-21) and a MOF material (MOF-5) – are presented.     

Thermal modelling of a few novel solar collector-cum-storage units for passive heating

Analytical models have been put forward to predict the thermal performance of passive heating systems, which have previously been suggested. The systems consist of a water vessel for heat storage and a structure positioned on its outside wall, which act as a solar collector and a thermal insulation for the storage, respectively. Four different variations of structures have been considered and numerical calculations performed corresponding to the physical parameters of an earlier reported experimental study. The analysis is able to predict the experimental results fairly satisfactorily.     

Study on key parameters design and economic evaluation of the electric heating and solid sensible heat thermal storage device

电制热固体储热系统对可再生能源消纳、能源清洁化利用具有重要意义。电制热固体储热装置的关键参数设计以及经济性分析是提高经济效益的重要手段。因此,本文提出了电制热固体储热装置投资运行费用计算方法。通过对比不同供暖方式所需费用分析了电制热固体储热装置的经济性。同时研究了谷电利用系数对电制热固体储热装置经济性的影响。最后,采用案例分析验证本文所提经济性评估方法的合理性与正确性。本文的研究内容为用户对电制热固体储热装置的选择提供参考。     

A design procedure for solar air heating systems

A natural extension of the design procedure for liquid-based solar space and water heating systems is a similar analysis for solar heating systems using air as the heat transfer fluid. In this paper, a solar air heating system incorporating a flat-plate air heater and packed bed thermal storage is described and a simulation model for the system is developed. The results of many simulations of the air heating system are used to establish the relationship between system performance and the system design and meteorological variables. The results are presented in analytic and graphical form, referred to as an f-chart for solar air heating systems. The results of simulations in several widely different climates suggest that the information presented in the f-chart is location independent. Methods of estimating the performance of air heating systems having a collector air capacitance rate and a storage capacity other than those used to generate the f-chart are included. A comparison of the performance of air and liquid based systems is afforded by a comparison of their respective f-charts. The air system is shown to perform better at high load fractions supplied by solar energy than a liquid-based system with the same collector thermal performance parameters.     

An applied methodology for assessment of the sustainability of biomass district heating systems

In order to maximise the share of biomass in the energy supplying system, the designers should adopt the appropriate changes to the traditional systems and become more familiar with the design details of the biomass heating systems. The aim of this study is to present the development of methodology and its associated implementation in software that is useful for the design of biomass thermal conversion systems linked with district heating (DH) systems, taking into consideration the types of building structures and urban settlement layout around the plant. The methodology is based on a completely parametric logic, providing an impact assessment of variations in one or more technical and/or economic parameters and thus, facilitating a quick conclusion on the viability of this particular energy system. The essential energy parameters are presented and discussed for the design of biomass power and heat production system which are in connection with DH network, as well as for its environmental and economic evaluation (i.e. selectivity and viability of the relevant investment). Emphasis has been placed upon the technical parameters of biomass logistics, energy system's design, the economic details of the selected technology (integrated cogeneration combined cycle or direct combustion boiler), the DH network and peripheral equipment (thermal substations) and the greenhouse gas emissions. The purpose of this implementation is the assessment of the pertinent investment financial viability taking into account the available biomass feedstock, the economical and market conditions, and the capital/operating costs. As long as biomass resources (forest wood and cultivation products) are available and close to the settlement, disposal and transportation costs of biomass, remain low assuring the sustainability of such energy systems.     

Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications

The melting temperatures of the palmitic acid (PA) (61.2 °C) and the myristic acid (MA) (52.2 °C) are high for solar thermal energy storage under the climate conditions of the some regions in Turkey. However, their melting temperatures can be adjusted to a suitable value by addition of MA to PA. In this study, the thermal analysis by differential scanning calorimetry technique shows that the mixture of MA and PA with 58.0 wt% MA forms a eutectic mixture with melting temperature of 42.6 °C and the latent heat of fusion of 169.7 J g⁻¹. The melting temperature and latent heat of fusion of MA–PA eutectic mixture make it a suitable material for energy storage in passive solar space and greenhouse heating systems under the climate conditions of some residential and agricultural regions in Turkey. The present study also deals with the experimental establishment of thermal characteristics of the eutectic mixture as a phase change material in a vertical concentric two pipes––energy storage system. Reynolds and Stefan numbers were selected as experimental parameters and used for the evaluation of the results. The experimental results prove that the MA–PA eutectic mixture has attractive thermal characteristics during the melting and the solidification processes and it is a potential latent heat storage material for heating applications with respect to the climate requirements in Turkey.     

A design procedure for wind powered heating systems

This paper presents a generalized design procedure for the month-by-month prediction of performance of wind powered residential heating systems. In its initial form, the technique is restricted to the simulation of residential heating systems using conventional horizontal axis wind turbines that dissipate their output into water based thermal storage systems (via electrical resistance heaters or fluid dissipation devices). The procedure using a graphical formulation (Wf-chart) is designed to give the monthly heating energy fraction supplied by a wind heating system with a minimum of site, wind turbine, and system input parameters. Such information is useful for simple performance studies or as input for economic assesment of wind heating systems. In addition example results of the calculational procedure for a typical wind heating system are presented.     

工业余热跨季节储热系统优化及经济性分析

以工业余热跨季节储热用于区域供热系统为研究对象,在一个已有的1 MW工业余热系统的基础上,搭建工业余热跨季节储热系统设计方案,通过系统模拟对系统储热、取热过程进行分析。分析结果表明:工业余热跨季节储热适合长周期、大规模蓄热;储热体体积和循环流量应根据系统热源和取热装置特性进行合理匹配,在合理匹配范围内可参考系统经济性确定系统最优配置。最后,通过费用年值法分析优化后的系统经济性,分析表明工业余热跨季节储热用于区域供热成本仅略高于燃煤供热,相比燃气供热具有非常可观的经济性。     

Experimental results of a sensible heat storage system for residential and light commercial application

This paper presents the performance results for a sensible heat storage system. The system under study operates as an air source heat pump which stores the compressor heat of rejection as domestic hot water or hot water in a storage tank that can be used as a heat source for providing building heating. Although measurements were made to quantify space cooling, space heating, and domestic water heating, this paper emphasizes the space heating performance of the unit. The heat storage system was tested for different indoor and outdoor conditions to determine parameters such as heating charge rate, compressor power, and coefficient of performance (COP). The thermal storage tank was able to store a full charge of heat. The rate of increase of storage tank temperature increased with outdoor temperature. The heating rate during a charge test, best shown by the normalized rate plots, increased with evaporating temperature due to the increasing mass flow rate and refrigerant density. At higher indoor temperature during the discharge tests, the rate of decrease of storage tank temperature was slower. Also, the discharge heating rate decreased with time since the thermal storage tank temperature decreased as less thermal energy became available for use. Copyright © 1999 John Wiley & Sons, Ltd.     

SOLAR INDUSTRIAL PROCESS HEAT POTENTIAL IN ALEXANDRIA,EGYPT

The present paper investigates the economic viability of the solar domestic hot water systems for industrial processes in Alexandria, Egypt. Also, the optimum parameters of such systems in Alexandria, Egypt have been determined.

The industries covered in the present study are soft drinks, and vegetable oil. The weather data for Alexandria, Egypt (latitude 31.2°) was used. The data file of Alexandria contains monthly average values of daily radiation on a horizontal surface, clearness index, ambient temperature, wind speed, and relative humidity.

The economic calculations for this study are based on life-cycle savings calculations. The f-chart design method is used to estimate the long-term annual thermal performance of solar domestic hot water systems.

The main results obtained can be summarized in the following: 1) solar domestic hot water systems are preferable alternatives to the conventional heating systems currently used in food industries in Egypt. 2) significant life-cycle savings can be achieved with solar heating systems.     

Experimental and theoretical investigation of a solar heating system with heat pump

In this study, the performance of a solar heating system with a heat pump was investigated both experimentally and theoretically. The experimental results were obtained from November to April during the heating season. The experimentally obtained results are used to calculate the heat pump coefficient of performance (COP), seasonal heating performance, the fraction of annual load met by free energy, storage and collector efficiencies and total energy consumption of the systems during the heating season. The average seasonal heating performance values are 4.0 and 3.0 for series and parallel heat pump systems, respectively. A mathematical model was also developed for the analysis of the solar heating system. The model consists of dynamic and heat transfer relations concerning the fundamental components in the system such as solar collector, latent heat thermal energy storage tank, compressor, condenser, evaporator and meteorological data. Some model parameters of the system such as COP, theoretical collector numbers (N_c), collector efficiency, heating capacity, compressor power, and temperatures (T₁, T₂, T₃, T_T) in the storage tank were calculated by using the experimental results. It is concluded that the theoretical model agreed well with the experimental results.     

Cost optimization of the design of CHCP (combined heat,cooling and power) systems under legal constraints

Combined heat, cooling and power (CHCP) systems are interesting for the supply of different energy services in urban districts and in large buildings. CHCP systems utilize a fuel's energy to a greater extent, because the cogenerated heat can be used for heating in winter as well as for cooling in summer with an absorption refrigerator. The use of thermal energy storage (TES) provides the additional advantage of covering variable thermal demands while the production system operates continuously at nominal conditions. Thus, energy supply systems integrating the technologies of cogeneration, absorption refrigeration and thermal storage can provide substantial benefits from economic, energetic and environmental viewpoints. In this paper an optimization model is developed, using mixed integer linear programming (MILP), to determine the preliminary design of CHCP systems with thermal storage. The objective function to be minimized is the total annual cost. Taking into account the legal constraints imposed on cogeneration systems in Spain, the optimization model is applied to design a system providing energy services for a set of buildings constituted of 5000 apartments located in the city of Zaragoza (Spain). The effect of legal constraints in the design and operation of CHCP systems is highlighted in this study.     

Optimization of the heat exchanger in a flat plate indirect heating integrated collector storage solar water heating system

Due to the environmental impact of energy usage, consumers need to be encouraged to use renewable energy sources such as solar energy. The indirect heating flat plate integrated collector storage solar water heating system is one of the compact systems for domestic water heating. It incorporates the collection of a solar energy component and a hot water storage component in one unit. The objectives of this study were to investigate the effect of different parameters on the thermal performance of this system with the aim of reducing both the initial and the running costs. The outlet service water temperature was used as a measure of performance, because it is an indicator of the energy acquired from the solar radiation. The continuity, momentum and energy equations of the fluids involved in the system were numerically solved in a steady state condition, using FLUENT software. Three-D CFD models were developed and validated using previous experimental results. A standard k–ω turbulent model was used in the optimization of the heat exchanger because it produced good agreement with the experimental results. The surface-to-surface radiation model was included. The effect of single and double row heat exchangers with different lengths was investigated. Circular and elliptic cross-section pipes were also examined. Mass flow rates of 500 and 650 L/h were chosen. The results showed that the single row HX of 10.8 m length for both the elliptical and type B tube gave high service water outlet temperature (acceptable for heat exchanger design) and with low pumping power. This resulted in an increase in the thermal efficiency and a significant reduction in both the initial and the operating costs of the system.     

Thermal Modelling of a Natural Convection Solar Water Heating System with a Thermal Trap Collector

A mathematical model has been developed to study the performance of a solar water heating system with a thermal trap flat plate collector and in which the flow of water between the collector and the storage tank is maintained by natural convection. An expression has also been developed for the mass flow rate in terms of known parameters. The model yields exact expressions for the temperature of water in the storage tank as a function of time in terms of collector's parameters and the solar insolation. Numerical calculations have been performed to compare the performance of the hot water heating system with a thermal trap collector with the one with an ordinary flat plate collector.     

小高层学生公寓太阳能中央热水系统方案分析

针对小高层学生公寓太阳能中央热水系统储热水箱、管网和辅助加热系统,叙述了适合小高层学生公寓的最优太阳能中央热水系统设计方案及运行控制系统的边界条件。     

Characteristics of medium deep borehole thermal energy storage

Seasonal energy storage is an important component to cope with the challenges resulting from fluctuating renewable energy sources and the corresponding mismatch of energy demand and supply. The storage of heat via medium deep borehole heat exchangers is a new approach in the field of Borehole Thermal Energy Storage. In contrast to conventional borehole storages, fewer, but deeper borehole heat exchangers tap into the subsurface, which serves as the storage medium. As a result, the thermal impact on shallow aquifers is strongly reduced mitigating negative effects on the drinking water quality. Furthermore, less surface area is required. However, there are no operational experiences, as the concept has not been put into practice so far. In this study, more than 250 different numerical storage models are compared. The influence of the characteristic design parameters on the storage system's behaviour and performance is analysed by variation of parameters like borefield layout, fluid inlet temperatures and properties of the reservoir rocks. The results indicate that especially larger systems have a high potential for efficient seasonal heat storage. Several GWh of thermal energy can be stored during summertime and extracted during the heating period with a high recovery rate of up to 83%. Medium deep borehole heat exchanger arrays are suitable thermal storages for fluctuating renewable energy sources and waste heat from industrial processes. Copyright © 2016 John Wiley & Sons, Ltd.     

The use of CHP and absorption cooling in cold storage

In recent years, it has become standard practice to consider combined heat and power (CHP) systems early in the design stage of commercial buildings. With new initiatives from the UK government on reduced energy use, energy efficient systems such as CHP have been considered for a wider application particularly within industrial building design. The viability of CHP in a typical cold storage application is described in this paper. The electrical energy and heating requirements are defined and used to assess the annual energy consumption of a traditional cold storage design using a thermal model. The analysis is then used to consider the economics of different CHP configurations, which includes an integrated CHP and absorption system used to provide chilled glycol for the cold storage facilities. The additional capital cost of each configuration is shown and this is used to calculate the payback period. The results show that an attractive payback period of approximately four years for a combined CHP and absorption system may be achieved.     

Central solar heating plants with seasonal storage in Germany

In the house building sector, central solar heating plants presently offer the most cost-favourable application of all possibilities of solar-thermal systems. By the integration of seasonal heat storage, more than 50% of the annual heating demand for space heating and domestic hot water can be supplied by solar energy. Since 1995, eight central solar heating plants with seasonal heat storage have been built in Germany within the governmental R&D-programme ‘Solarthermie-2000’. This report describes the technology of central solar heating plants and gives advice about planning and costs. The pilot and demonstration plants for seasonal heat storage already built in Germany are described in detail to give an idea about possible system design and applications of central solar heating plants.