Energy balance model of combined photovoltaic solar-thermal system incorporating phase change material

In this paper an energy balance model and simulation results are presented for a generic combined photovoltaic (PV) solar thermal (ST) system that incorporates phase change material (PCM). This is a promising integration of technology because the PV converts visible and ultra-violet parts of the solar spectrum, the ST utilises infra-red parts of the spectrum and waste heat from the PV, and the PCM reduces the temperature of the PV which increases PV efficiency. Parameters that are investigated cover PCM physical and thermal properties and water flow schemes. By including an appropriate PCM in an optimised system, the PV output can be increased by typically 9% with an average water temperature rise of 20 °C. Although any increase in PV performance has an associated decrease in ST performance, a practical and worthwhile compromise can still be achieved. This paper demonstrates that there is considerable scope for experimental realisation of the combined PV/ST/PCM system particularly if this is coupled with the development of PCMs with tuneable melting points and high thermal conductivity.     

Latent heat thermal energy storage using cylindrical capsule: Numerical and experimental investigations

This paper is aimed at analyzing the melting behavior of paraffin wax as a phase change material (PCM) encapsulated in a cylindrical capsule, used in a latent heat thermal energy storage system with a solar water heating collector. The heat for melting of PCM in the capsule is provided by hot water surrounding it. Since it is observed experimentally that the phase change occurs in a range of temperature, the present analysis considers this range instead of constant phase change temperature and the deviation between the results of these two is presented. The numerical analysis has been carried out by using enthalpy method and the results are verified with the experimental data. The experiments have been done by visualization technique without disturbing the actual process of melting. Three distinct stages of melting process have been identified as revealed by visualization studies. Results indicate that the melting process is chiefly governed by the magnitude of the Stefan number, Ste, phase change temperature range and the capsule radius. The analysis shows that the agreement between analytical and experimental results is significantly improved when the results are obtained considering phase change temperature range and the natural convection in the liquid phase instead of considering the process to be conduction dominated only.     

Unsteady thermotechnical model of a closed flat plate solar collector–storage tank system

The main provisions of the developed numerical unsteady model of the flat plate solar collector–storage tank (SC–ST) system are given. The model takes into account all of the main parameters of the SC (receiver parameters, radiative properties of the transparent fence for solar radiation, and thermophysical properties of the thermal insulation of the bottom) and the ST (thermophysical parameters of thermal insulation). An algorithm and a program for calculating the thermal properties of a closed SC–ST system have been developed based on this model in the Delphi language. The results of the calculated estimates of the time variation of water heating temperatures in the ST and the efficiency of the closed SC–ST system as a function of the ST volume are given for the case where the water temperature at the SC inlet is equal to the mean water temperature in the ST. The model and the program also make it possible to determine the time variation of temperatures of SC elements and water in individual sections along the length of the SC as well as the thermal losses of the SC sections. The model makes it possible to estimate the effect of the thermal properties of SC elements and the water velocity on the thermal properties of the SC–ST system. The program can be modified for the case of the open SC–ST system, i.e., where the water temperature at the SC inlet does not depend on the water temperature in the ST.     

Solar water heaters with phase change material thermal energy storage medium: A review

Latent heat thermal energy storage is one of the most efficient ways to store thermal energy for heating water by energy received from sun. This paper summarizes the investigation and analysis of thermal energy storage incorporating with and without PCM for use in solar water heaters. The relative studies are classified on the basis of type of collector and the type of storage used i.e. sensible or latent. A thorough literature investigation into the use of phase change material (PCM) in solar water heating has been considered. It has been demonstrated that for a better thermal performance of solar water heater a phase change material with high latent heat and with large surface area for heat transfer is required.     

Experimentation with a water tank including a PCM module

Storage of heat is seen as a major issue for the development of solar energy for house heating and cooling under all climates. Most of the storage systems available on the market use water as the storage medium. The idea studied here was to add a phase change material (PCM) module at the top of a hot-water storage tank with stratification. An experimental solar pilot plant was constructed to test the PCM behaviour in real conditions. The PCM module geometry adopted was to use several cylinders. A granular PCM–graphite compound was chosen as the PCM for the experiments presented here.     

Alternative storage options for solar thermal systems

The efficient implementation of solar systems in buildings depends on the storage of energy yielded, as it can both increase the solar system's autonomy and make it a feasible solution for zero energy buildings, and make storage vessels more compact, reducing precious space requirements. This is of particular important in places with reduced time of sunshine, where solar systems are less effective, because of the deviation between solar radiation and the demand. The traditional storage options use water, which is practical, safe and low‐cost, especially when the storage requirements are small. However, when larger storage is needed, limits concerning the use of water exist, mainly due to the need for larger installation space and the increased thermal losses. The use of phase change materials (PCM) for thermal energy storage seems an upcoming technology. The main idea is the substitution of water with PCM, which feature larger specific energy storage capacity compared to other conventional materials. In the context of the specific paper, a combined solar thermal system used for the preparation of domestic hot water (DHW) and space heating (Solar Combi System) with two different types of storage is studied, for two Greek cities. The aim is to find out which is the most efficient way of storing energy with respect to the autonomy of the system, for a solar combi system. This is being achieved by determining the comparative autonomy of PCM and water storage system for various climates. It was proven that using PCM is advantageous, as it can extend the autonomy duration of the solar system for 2 to 8 hours, depending on the season and the climatic conditions. However, it was also seen that in solar combi systems used throughout the whole year, PCM are inefficient during summer period.     

Thermal behaviour of integrated solar collector/storage unit with 65 °C phase change material

A transient finite volume model was used to predict collection and retention of heat for rectangular cross section solar collector/storage systems when filled with water and various concentrations of phase change material (PCM) slurries with a 65 °C phase change temperature. It was found that the PCM slurry systems collected heat marginally less effectively than water filled stores. Retention of heat at higher temperatures in the PCM slurry systems may allow higher solar savings fractions to be realised depending on heat demand patterns.     

Experimental investigation of a domestic solar water heater with solar collector coupled phase-change energy storage

Phase change materials (PCMs) have good properties such as high thermal capacity and constant phase change temperature. Their potential use in solar energy storage is promising. Tests of exposure and constant flow rate are performed to investigate the thermal performance of a domestic solar water heater with solar collector coupled phase-change energy storage (DSWHSCPHES). Due to the low thermal conductivity and high viscosity of PCM, heat transfer in the PCM module is repressed. The thermal performance of the DSWHSCPHES under exposure is inferior to that of traditional water-in-glass evacuated tube solar water heaters (TWGETSWH) with an identical collector area. DSWHSCPHES also performs more efficiently with a constant flow rate than under the condition of exposure. Radiation and initial water temperature have impacts on system performance; with the increase of proportion of diffuse to global radiation and/or initial water temperature, system performance deteriorates and vice versa.     

Analytical optimization of interior PCM for energy storage in a lightweight passive solar room

Lightweight envelopes are widely used in modern buildings but they lack sufficient thermal capacity for passive solar utilization. An attractive solution to increase the building thermal capacity is to incorporate phase change material (PCM) into the building envelope. In this paper, a simplified theoretical model is established to optimize an interior PCM for energy storage in a lightweight passive solar room. Analytical equations are presented to calculate the optimal phase change temperature and the total amount of latent heat capacity and to estimate the benefit of the interior PCM for energy storage. Further, as an example, the analytical optimization is applied to the interior PCM panels in a direct-gain room with realistic outdoor climatic conditions of Beijing. The analytical results agree well with the numerical results. The analytical results show that: (1) the optimal phase change temperature depends on the average indoor air temperature and the radiation absorbed by the PCM panels; (2) the interior PCM has little effect on average indoor air temperature; and (3) the amplitude of the indoor air temperature fluctuation depends on the product of surface heat transfer coefficient h_in and area A of the PCM panels in a lightweight passive solar room.     

Integrated solar collector storage system based on a salt-hydrate phase-change material

A new integrated collector storage (ICS) concept for low-temperature solar heating of water is described. The solar energy is stored in a salt-hydrate phase-change material (PCM) held in the collector and is discharged to cold water flowing through a surface heat exchanger located in a layer of stationary heat transfer liquid (SHTL), floating over an immiscible layer of PCM. A theoretical model for the charging process of the proposed integrated collector is presented. The model assumes one-dimensional transient heat conduction in the PCM and SHTL layers and neglects the effect of convection heat transfer in these regions. The model was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The results of parametric studies on the effect of the transition temperature and of the thickness layer of the salt-hydrate PCM on the thermal performance of the charging process are also presented.     

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.     

Experimental study of a compact PCM solar collector

The performance of a compact phase change material (PCM) solar collector based on latent heat storage was investigated. In this collector, the absorber plate–container unit performs the function of both absorbing the solar energy and storing PCM. The solar energy was stored in paraffin wax, which was used as a PCM, and was discharged to cold water flowing in pipes located inside the wax. The collector's effective area was assumed to be 1 m² and its total volume was divided into 5 sectors. The experimental apparatus was designed to simulate one of the collector's sectors, with an apparatus-absorber effective area of 0.2 m². Outdoor experiments were carried out to demonstrate the applicability of using a compact solar collector for water heating. The time-wise temperatures of the PCM were recorded during the processes of charging and discharging. The solar intensity was recorded during the charging process. Experiments were conducted for different water flow rates of 8.3–21.7 kg/h. The effect of the water flow rate on the useful heat gain (Qu) was studied. The heat transfer coefficients were calculated for the charging process. The propagation of the melting and freezing front was also studied during the charging and discharging processes. The experimental results showed that in the charging process, the average heat transfer coefficient increases sharply with increasing the molten layer thickness, as the natural convection grows strong. In the discharge process, the useful heat gain was found to increase as the water mass flow rate increases.     

An analysis of a packed bed latent heat thermal energy storage system using PCM capsules: Numerical investigation

This paper is aimed at analyzing the behavior of a packed bed latent heat thermal energy storage system. The packed bed is composed of spherical capsules filled with paraffin wax as PCM usable with a solar water heating system. The model developed in this study uses the fundamental equations similar to those of Schumann, except that the phase change phenomena of PCM inside the capsules are analyzed by using enthalpy method. The equations are numerically solved, and the results obtained are used for the thermal performance analysis of both charging and discharging processes. The effects of the inlet heat transfer fluid temperature (Stefan number), mass flow rate and phase change temperature range on the thermal performance of the capsules of various radii have been investigated. The results indicate that for the proper modeling of performance of the system the phase change temperature range of the PCM must be accurately known, and should be taken into account.     

Review of the phase change material (PCM) usage for solar domestic water heating systems (SDWHS)

The shortage in energy resources combined with the climb in greenhouse emissions is the main incentive beyond the deployment of solar energy resource in various applications. One of the most successful applications is the utilization of solar energy in the domestic water heating systems (DWHS) because 70% of the consumed energy in the residential segment is utilized for space heating and appliances in cold climates 1 . However, the full deployment of solar energy in domestic water heating is only possible when an energy storage system with acceptable price is available. Recently a new tendency for deploying phase change materials (PCMs) as an energy storage system is introduced in several solar DWHS. These systems are known as integrated PCM in solar DWHS and offer several advantages including high storage capacity, low storage volume, and isothermal operation during the charging and discharging phases. The present study reviews various techniques utilized for integrating the PCM in solar water heating systems and the utilized methods for enhancing the heat transfer characteristics of the PCM through the usage of extended surfaces and high conductive additives. Copyright © 2017 John Wiley & Sons, Ltd.     

Numerical analysis of an integrated storage solar heater

This work aims to evaluate the performance of an integrated phase change material (PCM) solar collector. The dynamic behavior of the system is investigated via a theoretical model based on the first law of thermodynamics and oriented to deliver a maximum outlet water temperature. A parametric study is used to assess the effects of the inlet water temperature, the PCM thicknesses and properties and the mass flow rates on the outlet water temperature and the melt fraction. A comparison with a conventional solar water heater without heat storage is made. Results indicate that charging and discharging processes of PCM offer six stages. It is observed that the complete solidification time is longer than the melting one. The latent heat storage system increases the heating requirements at night. The rise is most enhanced for higher inlet water temperature, melting PCM temperature and PCM thickness and for lower mass flow rate.     

Analysis of implementing phase change materials in open-air swimming pools

Open-air swimming pools in Mediterranean climate regions are heated by direct solar radiation with no auxiliary heating systems. In order to extend the swimming season or improve comfort conditions, solar collectors or pool coverings may be used. In this paper, another approach was followed through the use of phase change materials (PCM). Two methods of introducing the PCM were considered: (1) encapsulated in the sidewalls and bottom of the pool, and (2) use the PCM in an external heat exchanger. Heat is stored when water temperature is sufficiently high, while it is released when water temperature drops below comfort levels. A numerical model was built and validated for predicting the evolution of the water temperature, by taking into account meteorological data at three different locations in north-east of Spain. The simulations showed that using phase change materials provided some improvement of water conditions, especially when used in an external heat exchanger.     

Three—Dimensional Heat Transfer Analysis for A Thermal Energy Storage Canister

IlltroductionSolar dynamic power modules (SDPM) with phasechange material (PCM) is a vital solution to ensureuninterrupted power supply for low-earth orbitapplication. The advantage of SDPM is its longerlifehme and higher efficiency. Longer lifetime results insubstanhal savings in hardware replacement, launch, andon-orbit installation costs. Because of SDPM's higherefficiency, its solar collection area is only about 25percent of that for a PV system. This would allowspacecraft operatin…     

相变储能材料的研究和应用

相变材料的研究与开发是一个新兴的领域。本文介绍了相变材料的功能、选择方法、分类、特性、新发展,并指出了各类相变材料存在的问题以及解决的方法。评述了相变材料在太阳能、工业余热利用、电力调峰、纺织业和建筑节能等领域的应用。     

Investigating the performance of a test phase change material chamber for passive solar applications: experimental and theoretical approach

This paper presents the results of a numerical and experimental study for the performance of a test phase change material (PCM) chamber for passive solar applications. The numerical part of the study was based on a one-dimensional model for the phase change problem. The numerical treatment was based on a finite-difference technique and the results were compared with literature data and field measurements. The experimental test PCM chamber has been designed and developed at the campus of Central Greece University of Applied Sciences at Psachna, Evia. The PCM used in the numerical and experimental tests was GR 27 of Rubitherm. Results show that the PCM does not operate as an insulation material but as a mean of temperature stabilisation in the indoor environment during its phase change period. Regarding the application of the numerical model to simulate the performance of the experimental PCM chamber, results show good qualitative and quantitative agreement.     

管内流体流动管外PCM发生相变的贮能系统热性能研究

建立了分析空调贮能系统中管内流体流动管外PCM发生相变的相变的贮能器热性能的数学模型，并进行了数值计算。其中，把传热流体看作是沿轴向的—维无粘流动，对PCM相变过程的求解用显热容法。计算结果与文献中的计算结果吻合较好。所得结论对该类贮能系统的设计和性能优化有一定指导作用。