A preliminary model for phase change thermal energy storage in a shell and tube heat exchanger

A preliminary model for estimating possible thermal energy storage in a phase change shell and tube heat exchanger is presented. Effect of various parameters such as thermal and physical properties of PCM and convective fluid, heat exchanger dimensions and heat transfer fluid flow rates both in laminar and turbulent regime on energy storage times are discussed. The model is illustrated for specific cases.     

Heat storage in direct-contact heat exchanger with phase change material

This paper describes the development and performance of a direct-contact heat exchanger using erythritol (melting point: 391 K) as a phase change material (PCM) and a heat transfer oil (HTO) for accelerating heat storage. A vertical cylinder with 200-mm inner diameter and 1000-mm height was used as the heat storage unit (HSU). A nozzle facing vertically downward was placed at the bottom of the HSU. We examined the effects of flowrate and inlet temperature of the HTO using three characteristic parameters of heat storage – difference between inlet and outlet HTO temperatures, temperature effectiveness, and heat storage rate. The temperature history of latent heat storage (LHS) showed three stages: sensible heat of solid PCM, latent heat of PCM, and sensible heat of liquid PCM. Further, the operating mechanism of the DCHEX was proposed to explain the results. The average heat storage rate during LHS was proportional to the increase in flowrate and inlet temperature of HTO. Thus, latent heat can be rapidly stored under large HTO flowrate and high inlet temperature in the DCHEX.     

Performance studies of a solar energy storing heat exchanger

The design, construction, and performance of a solar energy storing heat exchanger is presented as a step toward a solar cooking concept. The solid-solid transition of pentacrythritol is the principal mechanism for energy storage. The methods for describing the system performance are explained and applied to a test system containing a controllable replacement for the solar input power. This first stage of the project will be followed by another in which the heat exchanger is connected to a concentrating array of CPC cylindrical troughs. Although a size appropriate to commercial cooking cussed herein is sized for domestic use and addresses the question of what solar collector area and PCM mass are needed in order to provide adequate energy for several family-size meals with sufficient storage to cook at night and one or two days later.

The performance is described from efficiency measurements and the determination of a figure of merit.     

Numerical study of nanoencapsulated phase change material inside double pipe heat exchanger

Nanoencapsulated phase change material (NPCM) slurry is a dispersion where the phase change material (PCM) is dispersed in fluid. Compared with fluid, these nanofluids have a higher heat capacity during the phase change and a possible enhancement, as a result of this phase change, in the heat transfer phenomenon. To appreciate the merits, in terms of energy, a numerical study has been carried out with fluid based on NPCM inside double pipe heat exchanger. The numerical simulation results have been validated using experimental heat transfer data. The Reynolds and Nusselt numbers have been determined using thermal conductivity and viscosity evaluated in the same conditions as those in numerical model. The results obtained show an improvement of this energetic criterion at low mass flow rate compared with the base fluid. Analysis of the numerical and analytical results reveal that higher inlet flow rate and NPCM concentration results in higher heat transfer rate. In addition, increasing NPCM slurry temperature decreases its performance due to fast melting of PCM inside the tube.     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

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%。     

Hydrogen release from a metal hydride tank with phase change material jacket and coiled-tube heat exchanger

Hydrogen fuel cells are received increasingly wide attention in order to develop green ships and reduce greenhouse gas emissions in the field of waterway transportation. Metal hydrides (MHs) can be used to store hydrogen for green ships due to their high volumetric storage capacity and safety. Various measures should be considered in the design and manufacture process of the MH reactor to strengthen its performance of heat and mass transfer and obtain an acceptable hydrogen storage capacity. In this work, LaNi₅ hydride is used as the hydrogen storage material and packed in the reactor. A basic axisymmetric numerical model for the hydrogen storage system without a heat exchanger has been developed and proved to be effective through the comparison between its simulation results and the published data during dehydriding. A hybrid heat exchanger, which is consisted of a phase change material (PCM) jacket and a coiled-tube, has been applied into the hydrogen storage system to relieve the thermal effect of MH in the dehydriding process on system performance. Effects of the heat transfer coefficient between the circulating heating water in the coil-tube and the MH bed, the temperature of circulating heating water and the pressure at the outlet on the dehydriding performance have been investigated. Based on parametric study, the relationships among the average dehydriding rate, the heat transfer coefficient, the heating water temperature and the outlet pressure have been found and fitted as simple equations. These fitted equations can be considered as a reference, which provides an important method to effectively control the dehydriding rate in order to satisfy the fuel requirement of the power unit and ensure the safe navigation of green ships in the future.     

A new theory for a rotary-kiln heat exchanger

From theoretical considerations, two limits for the length required to achieve a prescribed temperature rise in rotary kilns are established. The “well-mixed” condition represents the lower limit (minimum length), while the “non-mixed” condition represents the upper limit (maximum length). The actual kiln length lies between these two limits. Procedures for the calculation of both limits are presented in the paper.Results from the “well-mixed” theory agree remarkably well with published data for an existing kiln.     

Energy and exergy analysis of a latent heat storage system with phase change material for a solar collector

Analysis of energy and exergy has been performed for a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. CaCl₂·6H₂O was used as PCM in thermal energy storage (TES) system. The designed collector combines in single unit solar energy collection and storage. PCMs are stored in a storage tank, which is located under the collector. A special heat transfer fluid was used to transfer heat from collector to PCM. Exergy analysis, which is based on the second law of thermodynamics, and energy analysis, which is based on the first law, were applied for evaluation of the system efficiency for charging period. The analyses were performed on 3 days in October. It was observed that the average net energy and exergy efficiencies are 45% and 2.2%, respectively.     

Internal surfaces including phase change materials for passive optimal shift of solar heat gain

The thermal inertia of building internal components may be used to shift the irradiation (solar) heat load, which could result in substantial energy saving. In this paper, a numerical model was implemented to determine the thermal performance of internal surfaces including Phase Change Materials (PCM). The model was exploited to compare a typical concrete floor with a floor with PCM. Thermal performance was defined by three different objective functions, each pinpointing different thermal characteristics of the heat load. First, parametric studies were performed to understand the influence of the thickness of a typical concrete floor. Then, the optimization of the melting temperature, thickness and position of a PCM layer included in a floor was performed. These analyses used either simplified or real weather conditions (for Québec City). Results showed that the thickness of the concrete floor could be optimized based on the three criteria retained. Also, the floor performance may be enhanced by the inclusion of a PCM layer. It was shown that the gain of performance brought by the internal surfaces thermal mass strongly depends on the weather conditions considered. This paper provides a fundamental understanding of PCM influence on internal surfaces.     

Analytical model of solar pond with heat exchanger

This paper presents an analytical model of a three zone solar pond with heat exchange pipes laid in its bottom convective zone. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted are derived as a function of geometrical and operational parameters of the system. The transfer of heat from the pond bottom convective zone to the heat exchange fluid is expressed in terms of a heat removal factor, F_R. Analytical results, characteristic of the optimum performance of the pond, are presented and the criteria for the size and heat transfer characteristics of the heat exchanger are investigated. The annual average efficiency of heat extraction exhibits the asymptotic increase with the increase of length per unit pond area of heat exchange pipe.     

Innovative sensible heat transfer medium for a moving bed heat exchanger in solar central receiver power plants

Renewable energies are gaining importance due to the steadily increasing scarcity of fossil fuels, the ongoing climate change and last but not least the risks which accompany the use of nuclear power. In this growing market, solar thermal power plants offer a centralized, potentially load following electricity production. To serve this need, the integration of thermal energy storage systems is essential. The Moving Bed Heat Exchanger MBHX storage concept for CSP systems using sensible heat transfer medium aims at using a low cost solid storage media. This concept requires intermediate bulk cycles to transfer heat between the solar field and the storage material (the bulk). Heat Transfer Fluids (HTF) such as synthetic oils (mobiltherm 603) are typically used. In this work, granular materials such as sand and rocks are studied to present an additional HTF to represent an efficient and cost-effective alternative. Low cost solid particulates can store and transport heat at temperatures over 1000°C. For the purpose of heat recovery, a moving bed heat exchanger (MBHX) is applied and tested. In this study, the dense granular mass is gravity-driven through a heat exchanger. The performance of the MBHX with the utilization of Sand, Basalt, and a Mixture of Sand and Basalt as a granular material was experimentally investigated. It is found that the effectiveness of the MBHX using a mixture of 50% sand and 50% basalt improved by 30% compared to using sand alone.     

Experimental performance evaluation of a novel heat exchanger for a solar hot water storage system

The performance of a novel heat exchanger unit (‘Solasyphon’) developed for a solar hot water storage system was experimentally investigated. The ‘Solasyphon’ is a simple ‘bolt-on’ heat exchange unit that can be integrated externally to a traditional single-coil hot water cylinder (HWC) avoiding the costly replacement of an existing HWC with a twin-coil HWC. The installation cost of a ‘Solasyphon’ is lower compared to a traditional HWC thus offers greater cost effectiveness. A data acquisition system was designed to compare the thermal performance of an integrated ‘Solasyphon’ HWC with a traditional twin-coil HWC under controlled simulated conditions. The analysis was based on experimental data collected under various operating conditions including different primary supply temperatures (solar simulated); primary supply patterns and draw off patterns. The results indicated that the ‘Solasyphon’ delivered solar heated water directly to the top of the HWC producing a stratified supply at a useable temperature. Under variable solar conditions the ‘Solasyphon’ would transfer the heat gained by a solar collector to a HWC more efficiently and quickly than a traditional HWC. The ‘Solasyphon’ system can reduce installation costs by 10–40% and has a lower embodied energy content due to less material replacement.     

A parabolic solar collector heat-pipe heat exchanger reactor assembly for cyclohexane's dehydrogenation: A simuation study

Hydrogen is an optimum large scale fuel for the future, although there remain some problems in transport and long-term storage. One has to develop the use of alternative fuels that are easily transformed into hydrogen and that can be stored in liquid form, and, thus, more safely and economically. A simulation study on the possibility of the utilisation of cyclohexane as a vector for hydrogen storage, itself used as a vector of solar energy storage, has been carried out. The results of this study indicate that a dehydrogenation muti-tube reactor for cyclohexane is feasible. Neverthless, a good heat transfer, which is a limiting process, has to be assured. To heat this reactor a system of a Parabolic Solar Collector heat pipe heat exchanger reactor assembly (PSCHPHERA) is studied.     

Countercurrent water-heating system with a heat exchanger in a solar energy collector loop

The counter current heat exchanger theory with periodic inlet temperatures has been applied to analyze the thermal performance of a water heating system. The theoretical results obtained are in agreement with the experimental data reported in the literature.     

An experimental investigation into the operation of a direct contact heat exchanger for solar exploitation

The use of a direct contact heat exchanger (DCHX), if properly applied, will allow several benefits. Primarily these include the elimination of the cost of a closed heat exchanger and the ability to operate with much lower temperature differences. This paper examines the operation of a liquid–liquid type DCHX in harnessing the solar energy. Heat is delivered to the working fluid (heat transfer fluid) in the collection loop composed of solar collectors and a circulation pump. Two different kinds of working fluid were tested for their thermal characteristics that are immiscible with water. Texatherm 46 and diethyl phthalate (C₆H₄(CO₂C₂H₅)₂) and those that are experimented in the present analysis. Different schemes were used to introduce these fluids into the DCHX as they are either lighter (Texatherm 46) or heavier (diethyl phthalate) than water. A series of outdoor tests were conducted to determine the overall performance of DCHX as well as transient behaviors as the sun's energy is exploited. It is worthwhile to note that no thermal stratification was observed throughout the DCHX when in operation regardless of the working fluid. Stability and thermal performance, however, appear to improve when the working fluid is dispersed from the top of a DCHX. A difference of 8% is measured in the heat exchanger effectiveness, which gives a measure of the heat exchanger's overall ability in heat transfer.     

Experimental determination of optimal size of a heat exchanger for solar furnaces with large aberrations

A method is presented for experimentally determining the optimal area of a heat exchanger to be used in solar furnaces with large aberrations and relatively low coefficients of reflection. This process is intended to be used in domestic solar furnaces with low power requirements. The method first determines the intensity distribution of radiation in the focal plane by measuring it with a photovoltaic cell which has a linearized output and which has been calibrated with a standard radiometer. It is then possible to determine the area which will collect the maximum energy. An example is given where 100 W of net power are obtained by means of a small solar furnace.     

A novel design of a heat exchanger for a metal-hydrogen reactor

A metal-hydride reactor equipped by a spiral heat exchanger is experimentally studied. The inserted exchanger provides significant insights into the problem of minimizing the total storage time by manipulating the operating parameters. Performance studies are carried out by varying the supply pressure, volume of the tank, absorption temperature, and overall heat transfer coefficient.