EVALUATION OF THE HEAT LOSS FROM PARTIALLY BURIED,BERMED THERMAL ENERGY STORAGE TANKS

A seasonal heat storage model is developed to describe the steady-state thermal behaviour of the soil regime surrounding large, cylindrical, partially buried heat storage tanks having soil bermed against the above grade portion of the tank wall. The increase in tank heat loss rate occurring when an in-ground tank is repositioned at a higher level and bermed is determined for several berm configurations, using a numerical approach. The results are presented in both graphical and analytical forms, for a range of berm heights, soil conductivities and insulation thicknesses. The results appear to provide convenient and accurate design aids for predicting the heat loss characteristics of bermed seasonal heat storage tanks, and as well to be applicable with little error to most non-circular tanks normally encountered. The analytical aid, which is more convenient and as accurate as the graphical aid, appears to be particularly useful for activities involving numerical simulation, economic analysis, and optimization.     

The use of berms in thermal energy storage systems: energy-economic analysis

The results are reported of an energy-economic analysis of the use of berms in thermal energy storage (TES) systems. The analysis compares the initial cost savings derived from using a bermed tank instead of an in-ground tank, with the additional costs associated with the greater heat losses for the bermed tank over the life of the installation. The main factors considered include: (1) the increased excavation associated with an in-ground tank, (2) the increased wall structural support required for an in-ground tank, (3) the haulage and disposal of excavated soil for an in-ground tank, compared with the haulage and disposal or acquisition of soil for a bermed tank, (4) the forming of soil into a berm, and (5) the increased heat loss associated with a bermed tank. In evaluating the last factor, the findings of previous studies are used by the author into the effects of berms on TES heat losses. As space for the berm is assumed available, the cost associated with any land additionally required for the berm is neglected. The results indicate that tanks having berms are, in most practical instances, economically superior to other tank configurations.     

Optimal sizing of a heat pump/thermal storage system based on the linear programming method

An optimal planning method is proposed for a heat pump/thermal storage system that utilizes time-of-use pricing of the electrical utility. Equipment capacities are determined so as to minimize the annual total cost in consideration of system's operational strategies for energy demand requirements. This optimal planning problem is solved by the linear programming method. Through a numerical study on a heat pump/thermal storage system for a commercial building, the effect of thermal storage tank is investigated on the long-term economics of the system. A parametric study is also performed with respect to the initial capital unit cost of thermal storage tank. The relation is clarified among the optimal capacities of thermal storage tank and other pieces of equipment. It is ascertained that this optimal planning method is a useful tool for evaluating the economic properties of heat pump/thermal storage systems.     

Analytical approach to ground heat losses for high temperature thermal storage systems

A new approach to estimate the heat loss from thermal energy storage tank foundations is presented. Results are presented through analytical correlations based on numerical solutions for the steady‐state heat conduction problem for thermal energy slab‐on‐grade tanks with uniform insulation. Model results were verified with other well‐established benchmark problems with similar boundary conditions and validated with experimental data with excellent agreement. In addition to the TES foundation heat loss, new correlations for the maximum temperature and for the radial evolution of the temperature underneath the insulation layer are also provided, giving important information related to the tank foundation design. The correlated variables are of primordial importance in the tank foundation design because, due to the typical high operating storage temperatures, an inappropriate tank foundation insulation would lead not only to a not desired loss of energy but also to an inadmissible increase of the temperatures underneath the insulation layer, affecting the structural stability of the tank. The proposed correlations provide a quick method for the estimation of total tank foundation heat losses and soil maximum temperature reached underneath the insulation layer, saving time, and cost on the engineering tank foundation design process. Finally, a comprehensive parametric analysis of the variables of interest is made and a set of cases covering a wide range of tank sizes, insulation levels, depths to water table, and storage temperatures are solved.     

热电联产机组储热罐最佳容量的确定方法

为解决“风热冲突”下储热罐的容量选择问题,以热电联产机组整个采暖期为研究对象,引入特征日概念,对配置储热罐后的热电机组建立了逐小时的运行模型。分别以机组深度调峰空间的增量、全年总收益和10年净现值为目标函数,寻找储热罐容量的最优值。结果表明,热负荷越高储热罐的最佳容量也越大,同时机组配置储热罐后所能获得的深度调峰空间也越大;不考虑初投资时,以全年总收益为目标的储热罐最优容量约为820 MW;在考虑初投资后,以10年净现值为目标的储热罐最优容量约为430 MW,容量几乎减半。     

Optimal insulation of solar heating system pipes and tanks

A compact and time-effective insulation design procedure for solar heating system piping and water-filled thermal storage tanks was developed. Recognizing the particular sensitivity of solar systems to cost, the economic aspect of the problem was treated by a comprehensive present-value life-cycle cost analysis. In the development of the method, a numerical sensitivity analysis was performed to determine the relative effects of all relevant independent variables (within their pertinent ranges) on piping and tank heat transfer coefficient values. For the acceptable error limits of ± 14% for pipes and ± 19% for tanks, it was found that one may assume that only the nominal pipe diameter (or tank diameter), the thermal conductivity of the insulation, and the insulation's thickness have an effect on the overall heat transfer coefficient. Based on this result, design graphs and tables are presented which can be used to determine the optimal insulation thickness and type, total annual heat losses, present-value annual costs of insulation and lost heat, and overall insulation R-values. The use of the method is illustrated by calculating all the above quantities for all piping and storage tanks for the University of Pennsylvania SolaRow House. The present method provided insulation thicknesses slightly greater than those obtained by the ETI technique.A major conclusion of the study is that the cost of insulation in solar systems is not insignificant (e.g., ~15% in SolaRow), and that heat losses through insulation could amount to an important percentage of the useful solar energy collected (e.g., 24% in SolaRow). This re-emphasizes the need for a careful design of insulation in solar systems.     

分布式太阳能集中供暖系统用户与集中蓄热装置容量匹配优化设计

以蓄热系统各部分蓄热容积为决策变量,蓄热系统全寿命周期费用总现值为优化目标建立蓄热系统优化模型,并通过Hooke-Jeeves优化算法进行求解。以西宁市某小区为案例进行蓄热系统优化,结果表明：用户总蓄热容量与集中蓄热容量的最优配比为3∶2。分布式各蓄热水箱最优容积相比传统太阳能系统蓄热容积计算结果减少40%以上。随着蓄热水箱容积的增大,系统辅助热源供热量先降低后升高,太阳能产热量先升高后趋于定值,系统初投资呈增加趋势,运行投资与蓄热系统寿命周期费用总现值均呈先降低后升高的趋势。最后以寿命周期费用节约百分比为评价指标分析影响因素的影响程度,敏感性大小依次为锅炉供热单价>折现率>寿命周期。     

Design and performance of hot-oil storage tanks

Because the ratio of surface area to capacity decreases with increasing volume for a particular shaped hot-oil storage tank, there is a trend towards larger tanks in order to incur only relatively small heat losses per unit capacity. The present investigation suggests, for conditions encountered in the UK, that the aspect ratio (i.e. height-to-radius) for least heat losses from bare or fully insulated hot-oil tanks should be approximately 0·4. If the cylindrical walls of the tank are to be insulated, the roof remaining bare, then the optimal aspect ratio should be appropriately greater. However, when designing a tank, the running cost is only one of several considerations—although an increasingly important one as fuel costs inflate—the capital investment in the tank and site also radically affect the choice. The economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank, is considerably in excess of the optimal aspect ratio corresponding to minimum rate of energy loss. This maximum energy thrift or minimum financial expenditure dichotomy is now serious when neither sufficient, cheap fuel nor adequate industrial investment is available.     

Underground storage of heat in solar heating systems

The thermal interaction between underground heat storage and the surrounding ground is studied. The storage medium may be water in a tank, rocks, the ground itself, etc. For the case of hemispherical geometry, analytic steady-state solutions and numerical solutions to select time-dependent situations are presented and discussed.The quasi-steady-state average daily net heat loss into the surrounding ground from uninsulated underground heat-storage facilities is estimated to be only a few per cent of the heat storage capacity for typical home-size units in low-conductivity ground in the absence of ground-water flow. The economics of added insulation under these circumstances appears to be critically site and system specific.The time required for the average heat-loss rate of a new system to approach the steady-state value is on the order of a year.The ground surrounding a typical heat-storage facility contributes very little to the system's storage capacity either on a daily or a seasonal time scale.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

A simplified approach to economic analysis of solar heating and hot water systems and conservation measures

Using a new manipulation of standard relationships, the economic optimization and analysis of solar heating and hot water systems has been simplified. A payback ratio is defined as the reciprocal of the capital recovery factor and used to calculate the minimum or marginal annual thermal return per unit area acceptable for the system. Selecting the point at which the slope of a plot of solar fraction versus area (expressed as area per unit annual demand) equals the marginal annual thermal return per unit area optimizes the system. The ratio of coordinates at that point is the overall annual thermal return per unit collector area. This may be easily transformed into the return on investment, net worth, and payback time of the system. A new equivalent life-cycle payback time is defined that is derived from life-cycle analysis.The analysis is then extended to optimization of conservation measures. A closed formula for optimal insulation thickness is developed and illustrated.     

Thermal load levelling of heat flux through an insulated thermal storage water wall

This communication presents an investigation of the thickness distribution of a given total thickness of the insulation inside and outside a thermal storage water wall for acheiving the maximum load levelling of the heat flux entering through the wall. Analysis is based on the solution of the heat conduction equation for the temperature distribution in the insulated wall subjected to periodic solar radiation and atmospheric air on one side and in contact with room air at constant temperature (corresponding to air-conditioned rooms) on the other side. an explicit solution for a temperature distribution satisfying the apporpriate boundary conditions at the surface has been derived to obtaing a periodic heat flux through the storage water wall. It is found that for a given total thickness (cost) of insulation the thicknesses of outside and inside insulation must be equal for best load levelling. Moreover, more load levelling is achieved when the whole of the insulation is outside rather than inside the thermal storage water wall.     

结构参数对全玻璃真空管太阳热水器夜间热损失的影响研究

文章建立了三维非稳态的全玻璃真空管太阳热水器的数值模型,分析了夜间散热时,该热水器内的流动和传热特征,并且在夜间同一工况下,模拟研究了贮热水箱保温材料的导热系数、保温厚度,以及真空管涂层的发射率对贮热水箱温度、真空管温度和该热水器夜间热损失的影响。分析结果表明:随着散热过程的持续进行,全玻璃真空管太阳热水器内温度分层情况越来越明显,内部流体的流速越来越小,真空管内静滞区域自下往上逐渐扩大;当贮热水箱保温材料的导热系数由0.035 W/(m·℃)减小至0.020 W/(m·℃)时,该热水器的夜间热损失减少了8.5%;当贮热水箱保温厚度由50 mm增加至60 mm时,该热水器的夜间热损失减少了5.0%;当真空管涂层的发射率由0.06减小至0.05时,该热水器的夜间热损失减少了4.0%。     

Heat-transfer considerations for large liquefied-natural-gas storage tanks

Natural gas is used worldwide as a practical energy source. In order to have a concentrated form of energy, natural gas is liquefied and stored under a pressure slightly above atmospheric and at a corresponding temperature just above its normal boiling point (112 K). This investigation presents a general steady-state study of the heat transfers into such a storage tank. Two mathematical models are proposed in order to help improve the thermal design process for such a tank. It is concluded that the aspect ratio (i.e. height-to-radius) of such a tank for the minimum rate of heat gains should be approximately unity. This value differs from the economically most favourable aspect ratio, i.e. that leading to the least total financial expenditure over the lifetime of the tank. However, the two models allow a complete simulation of the thermal costs. As the unit cost of fuel rises relative to other costs (e.g. for the construction of the tank), the overall most economic tank design approaches that of the optimal thermal design of tank as deduced in this investigation.     

A novel rainwater–ground source heat pump – Measurement and simulation

This paper presents the results of experimental measurement and numerical simulation of the performance of a heat pump system designed to make use of rainwater and ground as heat sources/sinks. The system was tested under laboratory conditions. A refrigerant was circulated through a closed loop heat exchanger to transfer heat between the heat pump and rainwater in a storage tank and another heat exchanger made of solid bars or heat pipes to transfer heat between the stored rainwater and surrounding soil. Physical and thermal properties of soil such as water content, density, specific heat, thermal diffusivity and thermal conductivity were determined. Numerical simulations were also carried out for a rainwater storage tank installed under ground for domestic application of the heat pump with different operating modes, heating loads and the sizes and types of heat exchanger.     

Heat transfer characteristics of in-ground heat exchangers

Most in-ground heat storage installations use a system of horizontal or vertical plastic pipes to carry heat exchanger fluid. In designing these systems it is generally assumed that the thermal effects of the plastic pipe can be neglected. This paper reports a laboratory study of the pattern of heat flow around fluid-carrying plastic pipe buried in clay soil. Heat flow measurements as well as estimated contact resistances are presented for a number of configurations. In addition, numerical model computations are given for steady-state, transient and cyclic behaviour of several configurations, and it is shown that substantially reduced heat flows are obtained when plastic pipe is used.     

Optimization of a thermal storage unit combined with a biomass boiler for heating buildings

The performance of a boiler with a built-in thermal storage unit is presented. The thermal storage unit is an insulated water tank that absorbs surplus heat from the boiler. The stored heat in the thermal storage unit makes it possible to heat even when the boiler is not operating, thus increasing the heating efficiency. A system with three components is described. The model of the system and the mathematical model were made using the TRNSYS program package and a test reference year (TRY). The degree of efficiency, which optimizes the thermal storage volume and the heating power of the boiler, was determined. The thermal storage must also ensure that the heat is stored at the highest possible exergy level, and complete mixing of the water is a condition for optimizing the thermal storage. The matching of the boiler’s heating capacity with the thermal storage unit ensures a supply of heat even when the boiler is not operating.     

太阳辐射填料床储热特性研究

针对给定太阳日辐射曲线,研究集成蓄热单元的太阳光热系统的整体能量的动态转化特性及关键参数影响规律。结果表明：填料床总储热量与传热流体进口流速呈非线性变化,当传热流体进口流速 u_f =0.006 m/s时,填料床总储热量最大;在给定填料总容量和u_f =0.006 m/s的条件下,填料床高径比为5的填料床具有更高的储热能力;在该计算条件下,u_f =0.006 m/s、填料床高径比为5及填料量相对值为1时,太阳光热能实现最大程度上的转化和储存。     

A cost-optimal solar thermal system for apartment buildings with district heating in a cold climate

Finding the global optimal combination of the main components for a solar thermal energy system is an important topic in utilising solar radiation in a cost-effective way. However, selecting an optimal solar thermal system in a cold climate condition is a challenging task due to the dependency on the heat demand and the limited availability of solar radiation. This research presents several sets of optimum combinations of a solar thermal collector and a hot water storage tank regarding energy efficiency and the life cycle cost. Since domestic hot water consumption forms the significant part of the heat demand in new energy efficient apartment buildings, the applied consumption information were extracted precisely according to measured data. The solar thermal system with cost-optimal component sizes was able to save district heat energy consumption up 24% to 34% and made 4 €/m^2 to 23 €/m^2 in financial profit.     

Performance and economics of annual storage solar heating systems

Annual storage is used with active solar heating systems to permit storage of summer-time solar heat for winter use. This paper presents the results of a comprehensive computer simulation study of the performance of active solar heating systems with long-term hot water storage. A unique feature of this study is the investigation of systems used to supply backup heat to passive solar and energy-conserving buildings, as well as to meet standard heating and hot water loads.

Findings show that system performance increases linearly as storage volume increases, up to the point where the storage tank is large enough to store all heat collected in summer. This point, the point of “unconstrained operation”, is the likely economic optimum. In contrast to diurnal storage systems, systems with annual storage show only slightly diminishing returns as system size increases. Annual storage systems providing nearly 100% solar space heat may cost the same or less per unit heat delivered as a 50 per cent diurnal solar system. Also in contrast to diurnal systems, annual storage systems perform efficiently in meeting the load of a passive or energy-efficient building. A breakeven cost 4¢–10¢/kWh is estimated for optimal 100 per cent solar heating in the U.S.A.