Investigation of thermal performance of a ground coupled heat pump system with a cylindrical energy storage tank

An analytical and computational model for a solar assisted heat pump heating system with an underground seasonal cylindrical storage tank is developed. The heating system consists of flat plate solar collectors, an underground cylindrical storage tank, a heat pump and a house to be heated during winter season. Analytical solution of transient field problem outside the storage tank is obtained by the application of complex finite Fourier transform and finite integral transform techniques. Three expressions for the heat pump, space heat requirement during the winter season and available solar energy are coupled with the solution of the transient temperature field problem. The analytical solution presented can be utilized to determine the annual variation of water temperature in the cylindrical store, transient earth temperature field surrounding the store and annual periodic performance of the heating system. A computer simulation program is developed to evaluate the annual periodic water and earth temperatures and system performance parameters based on the analytical solution. Copyright © 2003 John Wiley & Sons, Ltd.     

Natural convection heat exchangers in solar water heating systems: Theory and experiment

Natural convection heat exchangers can be used in solar hot water systems to replace the pump on the tank side of the exchanger. There is currently no experimentally verified way of designing this type of heat exchanger. An experimental apparatus to test natural convection heat exchangers was built and an extensive set of measured data obtained on two different exchangers sized for low-flow stratified tank system. Two theoretical models for the exchanger are presented: a finite-volume primitive variable numerical solution of the fundamental laminar equations of fluid motion and a laminar forced-convection-based solution method. Comparison of the model predictions with the experimental data showed good agreement when the modified Rayleigh number is less than about 400. The poor agreement under other conditions was attributed to turbulence and recirculation neither of which was accounted for in the models.     

Study of Heat Transfer in Ice-storage Tank

IntroductionThe use of ice-storage for aircondihoulng systeinscan shift some or all of the sPace-cooling load to offpeak period. It follows that icestorage systeths havetbree advantaes in Practice. First, lOad eq is usefulto decrase the peak load of power netWork because aircondihonin systems are major factor brining about theelectrical peak load in sununer Second, ice-storagesystems can reduce the cOInPrssor size tO mect peakcooling load. Thir, load shiftin often resuhs inoPeIating cost …     

Thermal performance of a solar-assisted heat pump drying system with thermal energy storage tank and heat recovery unit

Thermal performance parameters for a solar-assisted heat pump (SAHP) drying system with underground thermal energy storage (TES) tank and heat recovery unit (HRU) are investigated in this study. The SAHP drying system is made up of a drying unit, a heat pump, flat plate solar collectors, an underground TES tank, and HRU. An analytical model is developed to obtain the performance parameters of the drying system by using the solution of heat transfer problem around the TES tank and energy expressions for other components of the drying system. These parameters are coefficient of performances for the heat pump (COP) and system (COPs), specific moisture evaporation rate (SMER), temperature of water in the TES tank, and energy fractions for energy charging and extraction from the system. A MATLAB program has been prepared using the expressions for the drying system. The obtained results for COP, COPs, and SMER are 5.55, 5.28, and 9.25, respectively, by using wheat mass flow rate of 100 kg h⁻¹, Carnot efficiency of 40%, collector area of 100 m², and TES tank volume of 300 m³ when the system attains periodic operation duration in fifth year onwards for 10 years of operation. Annual energy saving is 21.4% in comparison with the same system without using HRU for the same input data.     

储冰桶换热管组流动特性研究

分析了储冰桶换热器管组的流动过程,建立实际流动的物理模型,通过理论推导,获得了分析解。为准确计算换热器组流量分布,系统压降以及设计提供了一种理论方法。     

Thermal stratification in a hot water tank established by heat loss from the tank

This paper presents numerical investigations of thermal stratification in a vertical cylindrical hot water tank established by standby heat loss from the tank. The transient fluid flow and heat transfer in the tank during cooling caused by standby heat loss are calculated by means of validated computational fluid dynamics (CFD) models. The measured heat loss coefficient for the different parts of the tank is used as input to the CFD model. Parametric studies are carried out using the validated models to investigate the influence on thermal stratification of the tank by the downward flow and the corresponding upward flow in the central parts of the tank. Tank design parameters such as tank volume, height to diameter ratio and insulation and different initial conditions of the tank are investigated.It is elucidated how thermal stratification in the tank is influenced by the natural convection and how the heat loss from the tank sides will be distributed at different levels of the tank at different thermal conditions. The results show that 20–55% of the side heat loss drops to layers below in the part of the tank without the presence of thermal stratification. A heat loss removal factor is introduced to characterize the effect of the buoyancy driven flow on exchange of heat loss between tank layers by natural convection. Based on results of the parametric studies, a generalized equation for the heat loss removal factor is obtained by regression which takes into account the influences of tank volume, height to diameter ratio, tank insulation and initial conditions of the tank. The equation is validated for a 150–500 l tank insulated with 0–7 cm mineral wool and a tank height to diameter ratio of 1–5. The equation will be implemented in an existing tank optimization and design program for calculation of thermal performance of a hot water tank.     

变质量体系能量转换及储存系统充能过程动态特性研究

给出了变质量体系能量转换及储存技术的基本概念．讨论了在变质量体系能量转换及储存技术充能过程中因质量变化所产生的内部扰动的系统充能过程的动态特性。结果表明，当充能始、终溶液储罐内溴化锂质量分数差为0．11时，蓄能系统中仅用了一个0．6m^3的溶液储罐和一个0．17m^3的水储罐可储存109kWh的制冷潜能，转移电网负荷23．3kWh，蓄能系统运行的COP值可达到4．67。     

真空管热水器倾角的数值模拟分析

利用fluent软件中的太阳载荷模型对真空管家用太阳热水器进行了三维数值模拟计算,分析了30°、45°、60°倾角下真空管热水器内的流场和温度场随加热时间的变化及传热和流动过程。结果表明,真空管热水器在加热期间,在真空管管口的上壁面出现温度最大值,水箱内真空管管口以下水的温度相对于管口以上的温度分层很明显,水箱内真空管管口以上的热水温度几乎相同,说明水箱内管口以上的水进行了充分混合;对于30°和45°倾角的系统,随着加热时间的增加,水箱内管口以下的水温与管口以上的水温从10 K增加到30 K,以45°倾角为例,当考虑了水箱内管口以下的冷水区时,随着加热时间的增加,真空管与水箱内的温差从0.73 K增大到1.13 K,仅考虑水箱内均匀分布的三点时,随着加热时间的增加,真空管与水箱内的温差从0.15 K减小到了0.03 K;随着系统倾角从30°增加到60°,水箱内管口以下的冷水区域在逐渐减小。因此,为了减小水箱底部的冷水区,插入水箱内的真空管应尽可能短。     

新型定形板状相变材料的蓄/放热特性

对填充了新型定形板状相变材料的蓄热槽的蓄／放热特性进行了数值计算分析和实验比较。根据数值计算，对影响蓄热槽蓄／放热特性的主要因素——相变材料的几何尺寸、相变材料的导热系数、流体流速、对流放热系数、相变材料填充率等的影响规律进行了分析研究，计算出了蓄热槽内温度分布随时间的变化；并在实验台上测试了蓄热槽初始温度、流人和流出蓄热槽流体温度、作为蓄热体的相变材料测点温度随时间的变化，计算结果与实验结果具有较好的一致性。     

油井单体储油罐太阳能加温装置的开发研制

根据油井单体储油罐太阳能加温装置的开发研制情况，设计了油井单体储油罐太阳能加温装置，该装置由太阳能集热器、储能器、热交换器、控制器四部分组成，具有节能、安全、高效、环保等特点，可实现对油井单体储油罐自动加温的功能，避免了使用电加热棒浪费能源、不安全等问题。实践证明，使用该装置能够完全满足对油井单体储油罐的加热要求，具有良好的经济效益和社会效益。     

Thermal performance of a solar-aided latent heat store used for space heating by heat pump

In this study, the cylindrical phase change storage tank linked to a solar powered heat pump system is investigated experimentally and theoretically. A simulation model defining the transient behaviour of the phase change unit was used. In the tank, the phase change material (PCM) is inside cylindrical tubes and the heat transfer fluid (HTF) flows parallel to it. The heat transfer problem of the model (treated as two-dimensional) was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The experiments were performed from November to May in the heating seasons of 1992–1993 and 1993–1994 to measure both the mean temperature of water within the tank and the inlet and outlet water temperature of the tank. The experimentally obtained inlet water temperatures are also taken as inlet water temperature of the simulated model. Thus, theoretical temperature and stored heat energy distribution within the tank have been determined. Solar radiation and space heating loads for the heating seasons mentioned above are also presented.     

A design method to determine the optimal distribution and amount of insulation for in-ground heat storage tanks

The seasonal sensible heat storage model developed by Hooper and Attwater [1] is modified to describe the thermal behaviour of the soil regime surrounding cylindrical, in-ground, heat storage tanks with optimally distributed insulation. The model assumes steady-state heat transfer, and the surrounding soil is considered to be homogeneous and isotropic. Changes in soil thermal properties due to moisture migration, whether driven by thermal or hydrostatic gradients, are assumed negligible [2]. The optimal distribution is determined using the method of Lagrange multipliers. It is shown that the marginal cost per unit of energy lost and per unit of tank surface area must be the same at all points on the surface of the tank as the condition for minimum total heat loss with a given total investment in insulation. This condition appears to apply for all axi-symmetric in-ground tank geometries. For a given volume of insulation, the incremental increase in storage efficiency with an optimal redistribution of the insulation is a function of tank geometry. The problem of determining the optimal total investment in insulation for a given marginal cost of fuel is described and a method of solution is outlined.     

Energetic modeling,simulation and experimental of hydrogen desorption in a hydride tank

This paper presents a zero-dimensional (0D) model of hydride tank. The model aims to study the dynamic heat and mass transfers during desorption process in order to investigate the thermal-fluidic behaviors of this hydride tank. This proposed model has been validated experimentally thanks to a tailor-made developed test bench. This test bench allows the hydride characterization at tank scale and also the energetic characterization. The simulation results of the heat exchanges and mass transfer in and between the coupled reaction bed, show good agreement with the experimental ones. It is shown that the heat produced by a Proton Exchange Membrane Fuel Cell (PEMFC) (estimated starting from an electrical model) is enough to heat the metal alloy (FeTi) and therefore release the hydrogen with a sufficient mass flow rate to supply the PEMFC. Furthermore, the obtained results highlight the importance of the developed model for energy management of the coupling of fuel cell and hydride tank system.     

太阳能地面采暖系统蓄热水箱容积分析

通过分析太阳能采暖系统所需蓄热鼍与建筑热负荷、太阳能集热量日变化规律之间的关系,得出太阳能采暖系统所需蓄热水箱容积的理论算式.根据拉萨、银川、西宁、西安等地的太阳辐射强度及建筑热负荷的日变化规律,模拟得出系统所需蓄热量变化规律;并对各种蓄热温差下对应的蓄热水箱容积进行了模拟分析,结果表明:太阳能采暖系统所需蓄热量随太阳集热器的集热量与建筑热负荷之间的差值增大而增加;蓄热水箱容积随蓄热温差增大而减小,当蓄热水温达到80℃时,在各种地面采暖系统取水温度下,单位集热器面积所需蓄热水箱容积趋于相等.     

The Influence of Return Loop Flow Rate on Stratification in a Vertical Hot Water Storage Tank Connected to a Heat Pump Water Heater

A temperature-controlled hot water heat pump was simulated using heating in a vertical, domestic hot water storage tank. The influence of the return loop flow rate on stratification was investigated experimentally. The return loop is the water line that supplies a long line of consumers with hot water, and returns colder water to the middle of the hot water storage tank. The return temperature is a function of the length of the loop, insulation, and ambient conditions. Temperatures were measured as a function of time at different vertical locations on the centerline of the storage tank. The temperature distributions in the tank were compared for different return flow rates. A return flow rate of three tank volumes per day was identified as preferable, although good results were also obtained for less than three tank volumes per day.     

大直径高温熔盐储罐罐顶接管的强度及罐体稳定性分析

随着可再生能源技术的大力发展与应用,对太阳能热发电站中持久储热的钢制储罐的安全性能进行研究具有重要意义.以青海省某塔式太阳能热发电站中直径为25 m的高温熔盐储罐(储热罐)为例,利用ANSYS软件的平面实体轴对称单元建立了储罐有限元模型,着重对储罐顶部与熔盐管道接口处进行了强度和稳定性分析,通过对管道连接部位进行局部精...     

Integral-type solar-assisted heat pump water heater

An integral-type solar-assisted heat pump water heater (ISAHP) is designed and tested in the present study. The storage tank and the Rankine cycle unit are integrated together to make a more compact size. A thermosyphon loop is used to transfer the heat from the condenser to the water storage tank. The highest COP obtained in the tests is 3.83.     

A new type of phase change heat storage tank in solar energy combination system

介绍了一种新型笼屉式相变蓄热水箱,通过实验测试对比分析相变蓄热水箱与普通蓄热水箱对太阳能组合系统的太阳能保证率及系统能效比的影响。实验表明：同等水箱容积,使用相变蓄热水箱时太阳能集热系统的小时集热量为普通蓄热水箱的3.7倍,相变蓄热水箱有利于提高太阳能保证率及系统能效比。在太阳能辐照强度相似的情况下,相变蓄热水箱会使太阳能保证率平均提高72%,使系统能效比平均提高26%。同时相变蓄热水箱可减少夜间水箱上部的热损失,使水箱上部水温降减少50%。     

Coupled thermal simulation of hydrogen storage tank-Dewar flask system

The thermal effects during hydrogen charge and discharge processes are critical for improving hydrogen storage performance. The heat transfer of the hydrogen storage tank couples with its surrounding Dewar flask in thermal simulation. An additional model for studying the natural convection characteristics in the Dewar flask of the hydrogen storage system is developed and integrated into the system model. The Navier–Stokes equation is used to describe the air flow in the Dewar flask. The thermal dispersion is considered to describe the effective thermal conductivity of the storage tank. In the hydrogen storage tank-Dewar flask system, eight monitoring points in hydrogen storage tank and eight points in Dewar flask are selected to monitor temperature variations. The simulated pressures and temperatures in the hydrogen storage tank-Dewar flask system-agree well with experiments. The simulation results show that the model can well describe the natural convective heat transfer characteristics in the Dewar flask and the hydrogen storage behavior in the porous activated carbon tank, and further prove the feasibility and correctness of the coupled thermal simulation of hydrogen storage tank-Dewar flask system. The heat flow rates from the hydrogen tank and the Dewar flask is obtained by integrations along their boundaries to analyze the heat transfer coefficients. The coupled model is useful for optimizing empirical heat transfer coefficients of hydrogen storage tank.     

The effects of infill on hydrogen tank temperature distribution during fast fill

The temperature rise of hydrogen tank during fast fill poses challenge on the safety of hydrogen-powered vehicles. Researchers have been continuously looking for methods to mitigate the challenge of overheating. In this paper, we proposed an innovative solution by introducing porous infill in gas tanks to slow down gas-to-wall heat transfer. The porosity of the infill is no less than 97% to maintain the volume capacity of gas tanks. To evaluate the impact of infill heat capacity, we modelled the filling process with a lumped-parameter model and obtained various time-independent temperature evolution curves. Then, we set up a 2D and a 3D finite volume model and investigated the spatial distribution of temperature rise. Four cases with different infill properties were simulated and compared. At the end of the fast fill, the infill resulted in lower tank wall temperature at the cost of higher gas temperature. The combined effect of internal gas temperature and gas-phase effective thermal conductivity largely determines the final temperature distribution. The presence of infill effectively slowed down convective heat transfer, yet overly resistive porous infill may overly slow down the gas flow and result in thermal stratification. Further studies on infill design can be done to seek more effective solutions.