A thermal energy storage process for large scale electric applications

A new type of thermal energy storage process for large scale electric applications is presented, based on a high temperature heat pump cycle which transforms electrical energy into thermal energy and stores it inside two large regenerators, followed by a thermal engine cycle which transforms the stored thermal energy back into electrical energy. The storage principle is described, and its thermodynamic cycle is analyzed, leading to the theoretical efficiency of the storage system. A numerical model is developed, and the results show the feasibility of the process, even with sub-optimal parameters. Finally, key factors for improving the process performances are identified.     

Feasibility study for a large thermal storage and heat pumping installation

Calculations are described relating to a large thermal storage/heat pump project based at a quarry in Aberdeen, Scotland.Storage on an annual basis is shown to permit a reduction of nearly 50 per cent in heat pump capacity while overall performance is improved because heat is mostly drawn from the surroundings at summer temperatures. Heat losses from the quarry are low due to its very large size and favourable surface-volume ratio. Direct internal combustion (IC) engine drive allows exhaust heat recovery with reduction of heat pump delivery temperatures.Solar energy collection as an addition to the scheme was found not to be economic.     

A method of testing for rating thermal storage devices based on thermal performance

This paper describes a proposed test method for determining the “effective capacity” and heat loss characteristics of thermal storage devices. The prescribed series of tests should provide useful data for the rating of thermal storage devices based on thermal performance. The apparatuses and major components used in the tests have been prescribed so a liquid or air can be used as the transfer fluid. The series of tests to be conducted consist of one steady-state test to determine the heat loss characteristics and eight transient tests to determine the “effective capacity” for both heat storage and heat removal. During the transient tests, the entering fluid temperature is changed in a step-wise manner and amount of energy either stored or removed over a specified test time is determined. Sample experimental data are given in the paper to demonstrate the concept of the transient tests.     

储热材料的研究进展及其应用

综述了储热材料的研究进展和实际应用.介绍了储热材料的分类以及各类材料的性能、储能机理和优缺点;介绍了一些新型的相变材料,并结合实例探讨了储热材料在太阳能利用、建筑节能等领域的应用;指出了储热材料的研究方向和未来的发展趋势.     

Development of a new Biot number and lag factor correlation for drying applications

The present paper deals with the development of a new Biot number-lag factor (Bi-G) correlation for drying applications. Development of this correlation is based on the experimental data acquired from various sources in the literature. Using the developed correlation, moisture transfer parameters such as moisture diffusivity and moisture transfer coefficient for three regular shaped objects, e.g. slab, cylinder and sphere are calculated and compared with the experimental moisture content variations. The results showed an appreciably high agreement between the measured and predicted moisture content values from the correlation. Hence, the present correlation is considered as a useful tool for practical drying applications and a good contribution to the state-of-art of drying.     

Dimensionless lumped formulation for performance assessment of adsorbed natural gas storage

Adsorbed natural gas (ANG) has been emerging as an attractive alternative to compressed natural gas or liquefied natural gas, on various circumstances. However, in spite of the advantages associated with ANG over other storage modes, there are some issues that need be properly addressed in order to ensure a viable employment of such alternative. One major problem is that the thermal effects associated with the sorption phenomena tend to diminish the storage capacity, thereby resulting in poorer performance. Hence, in order to design commercially viable storage vessels, the heat and mass transfer mechanisms that occur in these devices must be carefully understood and controlled. With the purpose of improving the understanding of mass and energy transport within ANG vessels, dimensionless groups associated with this problem have been developed in this study, resulting in an innovation to the ANG literature. Along with the dimensionless groups, a lumped-capacitance formulation has been also proposed. Although this type of formulation is limited compared to the multi-dimensional formulations present in the literature, its computational solution is remarkably faster. Numerical solution results using the proposed lumped formulation are compared with those of a previous study, suggesting that the simpler model can be applied to larger process times. The process of charging and discharging ANG vessels was then simulated employing the proposed formulation for different combinations of the developed dimensionless groups. In order to properly assess charge and discharge processes, a performance coefficient was employed. The results show that increasing the heat capacity ratio and dimensionless heat transfer coefficient tend to augment the performance coefficient, whereas an increase in the dimensionless heat of sorption worsens performance. The proposed normalization scheme is applicable to both multi-dimensional and spatially-lumped formulations, thereby facilitating the analysis of heat transfer enhancement in these storage vessels.     

An experimental study on the quantitative interpretation of local convective heat transfer for a plate fin and tube heat exchanger using the lumped capacitance method

An experimental study has been performed to investigate the heat transfer characteristics of a plate fin and tube heat exchanger. Existing transient and steady methods are inappropriate for the measurement of heat transfer coefficients of the thin heat transfer model. In this study, the lumped capacitance method based on liquid crystal thermography was adopted. The method is validated through impinging jet and plate flow experiments. The two experiments showed very good agreements with those of the well-known transient method with the thick acryl model. And the lumped capacitance method showed similar results regardless of the thickness of the polycarbonate model if the Bi of the fin is small enough. The method was also applied for the heat transfer coefficient measurements of a fin and tube heat exchanger. Quantitative heat transfer coefficients of the plate fin were successfully obtained.     

Performance comparison of a group of thermal conductivity enhancement methodology in phase change material for thermal storage application

Phase change materials (PCM) are able to store thermal energy when becoming liquids and to release it when freezing. Recently the use of PCM materials for thermal energy storage (TES) at high temperature for Concentrated Solar Power (CSP) technology has been widely studied. One of the main investigated problems is the improvement of their low thermal conductivity. This paper looks at the current state of research in the particular field of thermal conductivity enhancement (TCE) mechanisms of PCM to be used as TES. This work considers a numerical approach to evaluate the performance of a group of TCE solutions composed by particular configurations of two of the principal TCE systems found on the literature: finned pipes and conductive foams. The cases are compared against a single PCM case, used as reference. Three different grades of graphite foams have been studied, presenting a charge time 100 times lower than the reference case for the same capacity. For fins two materials are analyzed: carbon steel and aluminum. The charge times of fin cases are from 3 to 15 times faster, depending on the amount and type of material employed. The internal mechanisms are analyzed to understand the results and locate possible improvement.     

Thermal ignition in packed particulate solids: Criticality under conditions of variable Biot number

The existence of both spatial variations of internal temperature and a temperature step at the surface of reacting, particulate cellulosic materials is established experimentally in small cube-shaped masses. When the temperature excess at the center of the mass is about 25K, the surface may be as high as 5K above that of the surrounding air. The effect is greatest in the smallest sizes.The measured temperature gradient normal to the reactant surface and the magnitude of the surface step are used to evaluate the Biot number. For 50–100 mm cubes of the cellulosic reactant the Biot number is 5.75–14.00. This means that the critical values for Frank-Kamenetskii's dimensionless parameter δ vary between 2.60 and 3.14 for the equivalent sphere in this range of cube sizes.δ_cr = 3.663 at the limit of pure conduction (Bi = ∞).Such variations have to be taken into account if scaling rules are to be applied to predict the safe storage temperature for large stockpiles from small-scale laboratory tests. The present data are used to show the extent to which there is an overestimate of the safe conditions when allowance is not made for the changing Biot number.     

Estimation of storage costs for large hydrogen storage facilities

The paper presents an economic evaluation of alternative hydrogen storage methods (pressurized gas, metal hydride and cryogenic storage) for large stationary (utility-scale) applications. The presentation of cost calculation clarifies the importance and influence of a set of relevant parameters depending on the charging-discharging schedule, the relation of the capacity to power level etc. The results presented define the useful range of application for each storage method, whereas the corresponding cost composition (power related capital costs, capacity-related capital costs, energy costs) is analysed and discussed.     

Modelling of thermal energy storage in industrial energy systems the method development of MIND

Industrial energy efficiency is of vital importance as regards environment and industrial profitability. Optimisation of industrial energy systems may show a way towards improved use of resources in energy supply as well as in production processes.The deregulation of the electricity market in some countries increases flexibility in electricity contracts. Taking advantage of the price structure in these contracts is one of the ways to minimise the energy costs and decrease the influence on the environment. Thermal energy stores are very suitable facilities for achieving these goals, having the capability of moving energy use from one period of time to another and thereby influencing not only energy cost but also costs related to power demand if electric energy use is involved.In this paper, the influence on energy costs, energy and material flows resulting from the use of energy storage is discussed. Energy storage has been modelled by using MIND (Method for analysis of INDustrial energy systems) in the form that has recently been developed by the author. A case study from the pulp and paper industry has been used to verify this.     

Numerical exploration of thermal radiation and Biot number effects on the flow of a non‐Newtonian MHD Williamson fluid over a vertical convective surface

A theoretical and computational study of the magnetohydrodynamic flow and free convection heat transfer in an electroconductive polymer on the external surface of a vertical plate under radial magnetic field is presented. The Biot number effects are considered at the vertical plate surface via modified boundary conditions. The Williamson viscoelastic model is employed which is representative of certain industrial polymers. The nondimensional, transformed boundary layer equations for momentum and energy are solved with the second‐order accurate implicit Keller box finite difference method under appropriate boundary conditions. Validation of the numerical solutions is achieved via benchmarking with earlier published results. The influence of Weissenberg number (ratio of the relaxation time of the fluid and time scale of the flow), magnetic body force parameter, stream‐wise variable, and Prandtl number on thermo fluid characteristics are studied graphically and via tables. A weak elevation in temperature accompanies increasing Weissenberg number, whereas a significant acceleration in the flow is computed near the vertical plate surface with increasing Weissenberg number. Nusselt number is reduced with increasing Weissenberg number. Skin friction and Nusselt number are both reduced with increasing magnetic field effect. The model is relevant to the simulation of magnetic polymer materials processing.     

Simple thermal decomposition reactions for storage of solar thermal energy

Simple thermal decomposition reactions have been investigated for the purpose of solar thermal energy storage. Ten criteria regarding the thermodynamics and kinetics of the reaction and the physical properties of the components of the reaction have been established. One particular reaction, the decomposition of ammonium hydrogen sulfate, has been evaluated in a preliminary manner and appears to satisfy all of the established criteria. The efficiency of storage is high and the decomposition occurs in the vicinity of 500°C. Other compounds such as ammonium halides, alkali and alkaline earth metal hydroxides, carbonates, sulfates and oxides have also been examined.     

Development and application of organic phase change mixtures in thermal storage gypsum wallboard

A latent heat storage material which would be useful in hot climate is proposed. It is a mixture of 93–95 wt% commercial Methyl Palmitate (MeP) with 7-5 wt% commercial Methyl Stearate (MeS), having a melting-freezing interval of approximately 23–26.5°C and a latent heat of transition of at least 180 kJ/kg.Normal wallboard may be impregnated with up to 25 wt% of one of these mixtures. Air passing over such an impregnated wallboard would be cooled down due to the heat absorption taking place between 23–26.5°C when the mixture melts. Recharging the wallboard will occur at 22–23°C during freezing, when the latent stored heat is released to the cool night air.The total storage capacity of such an impregnated wallboard, in a 3.5°C temperature interval, is at least twelve times higher than the storage capacity of wallboard alone over this range.     

电热锅炉蓄热在空调采暖系统中的应用

介绍了电热锅炉蓄热的意义、系统形式及原理，在实例中对系统设计提出了新思路，并介绍了电热锅炉及蓄热槽的选型。     

上海市应用蓄热电锅炉的技术经济性分析

简要介绍了蓄热电锅炉的设计原理、设计方案及其应用前景。并且根据上海某宾馆的锅炉房设计实例，与各种其他的系统，例如燃油系统、燃煤系统等，进行了技术经济性分析，并比较了动态以及静态回收年限，为城市节能提出了一个新的思路。     

Characterization of granular phase changing composites for thermal energy storage using the T‐history method

The present article reports on the characterization of granular phase changing composites using the T‐history method. Further modifications and improvements of the method are employed to handle granular materials undergoing phase change over a temperature range. The accuracy of the T‐history method is shown to be limited by the assumption of temperature‐independent specific heats and the difficulty of determining the limits of solid and liquid phases. The concept of enthalpy and its relationship with temperature has been employed in the analysis to overcome these difficulties. Enthalpy–temperature and apparent heat capacity curves similar to those obtained using DSC have been developed. These characteristic curves are necessary for accurate design, modeling, and optimization of latent heat thermal energy storage systems. Experiments have been also carried out to measure the transient temperature distribution inside a cylindrical packed bed using phase changing granulates. Analysis of temperature variation along the bed shows good agreement with the measured phase change characteristics. Copyright © 2009 John Wiley & Sons, Ltd.     

A method to determine stratification efficiency of thermal energy storage processes independently from storage heat losses

A new method for the calculation of a stratification efficiency of thermal energy storages based on the second law of thermodynamics is presented. The biasing influence of heat losses is studied theoretically and experimentally. Theoretically, it does not make a difference if the stratification efficiency is calculated based on entropy balances or based on exergy balances. In practice, however, exergy balances are less affected by measurement uncertainties, whereas entropy balances can not be recommended if measurement uncertainties are not corrected in a way that the energy balance of the storage process is in agreement with the first law of thermodynamics. A comparison of the stratification efficiencies obtained from experimental results of charging, standby, and discharging processes gives meaningful insights into the different mixing behaviors of a storage tank that is charged and discharged directly, and a tank-in-tank system whose outer tank is charged and the inner tank is discharged thereafter. The new method has a great potential for the comparison of the stratification efficiencies of thermal energy storages and storage components such as stratifying devices.     

Economic implications of thermal energy storage for concentrated solar thermal power

Solar energy is an attractive renewable energy source because the sun's energy is plentiful and carbon-free. However, solar energy is intermittent and not suitable for base load electricity generation without an energy backup system. Concentrated solar power (CSP) is unique among other renewable energy options because it can approach base load generation with molten salt thermal energy storage (TES). This paper describes the development of an engineering economic model that directly compares the performance, cost, and profit of a 110-MW parabolic trough CSP plant operating with a TES system, natural gas-fired backup system, and no backup system. Model results are presented for 0–12 h backup capacities with and without current U.S. subsidies. TES increased the annual capacity factor from around 30% with no backup to up to 55% with 12 h of storage when the solar field area was selected to provide the lowest levelized cost of energy (LCOE). Using TES instead of a natural gas-fired heat transfer fluid heater (NG) increased total plant capital costs but decreased annual operation and maintenance costs. These three effects led to an increase in the LCOE for PT plants with TES and NG backup compared with no backup. LCOE increased with increasing backup capacity for plants with TES and NG backup. For small backup capacities (1–4 h), plants with TES had slightly lower LCOE values than plants with NG backup. For larger backup capacities (5–12 h), plants with TES had slightly higher LCOE values than plants with NG backup. At these costs, current U.S. federal tax incentives were not sufficient to make PT profitable in a market with variable electricity pricing. Current U.S. incentives combined with a fixed electricity price of $200/MWh made PT plants with larger backup capacities more profitable than PT plants with no backup or with smaller backup capacities. In the absence of incentives, a carbon price of $100–$160/tonne CO_2eq would be required for these PT plants to compete with new coal-fired power plants in the U.S. If the long-term goal is to increase renewable base load electricity generation, additional incentives are needed to encourage new CSP plants to use thermal energy storage in the U.S.     

A review on theory and application of chemical heat pump in thermal energy storage

化学热泵是高效,环保的新型能源技术,在余热回收,储热,可再生能源等领域具有广泛的应用前景.本文综述了化学热泵系统的一般理论和在储热技术中的应用,介绍了化学热泵系统技术在反应与工质对选择,传热强化以及工业研究与应用等方面的发展.