Numerical model and experimental validation of heat storage with phase change materials

This paper describes the numeric model developed to simulate heat transfer in phase change materials (PCM) plunged in water tank storage. This model, based on the enthalpy approach, takes into account the conduction and the convection into PCM as well as at the interface between PCM and water of the storage. Furthermore, hysterisis and subcooling are also included. This model has been implemented in an existing TRNSYS type of water tank storage. It allows the simulation of a water storage tank filled with PCM modules made of different materials and different shapes such as cylinders, plates or spheres bed. Comparisons between measurements and simulations has been undertake to evaluate the potential of this model.     

Application of latent heat thermal energy storage in buildings: State-of-the-art and outlook

Latent heat thermal energy storage (LHTES) is becoming more and more attractive for space heating and cooling of buildings. The application of LHTES in buildings has the following advantages: (1) the ability to narrow the gap between the peak and off-peak loads of electricity demand; (2) the ability to save operative fees by shifting the electrical consumption from peak periods to off-peak periods since the cost of electricity at night is 1/3–1/5 of that during the day; (3) the ability to utilize solar energy continuously, storing solar energy during the day, and releasing it at night, particularly for space heating in winter by reducing diurnal temperature fluctuation thus improving the degree of thermal comfort; (4) the ability to store the natural cooling by ventilation at night in summer and to release it to decrease the room temperature during the day, thus reducing the cooling load of air conditioning. This paper investigates previous work on thermal energy storage by incorporating phase change materials (PCMs) in the building envelope. The basic principle, candidate PCMs and their thermophysical properties, incorporation methods, thermal analyses of the use of PCMs in walls, floor, ceiling and window etc. and heat transfer enhancement are discussed. We show that with suitable PCMs and a suitable incorporation method with building material, LHTES can be economically efficient for heating and cooling buildings. However, several problems need to be tackled before LHTES can reliably and practically be applied. We conclude with some suggestions for future work.     

Performance of phase change material boards under natural convection

The high thermal storage capacity of phase change material (PCM) can reduce energy consumption in buildings through energy storage and release when combined with renewable energy sources, night cooling, etc. PCM boards can be used to absorb heat gains during daytime and release heat at night. In this paper, the thermal performance of an environmental chamber fitted with phase change material boards has been investigated. During a full-cycle experiment, i.e. charging–releasing cycle, the PCM boards on a wall can reduce the interior wall surface temperature during the charging process, whereas the PCM wall surface temperature is higher than that of the other walls during the heat releasing process. It is found that the heat flux density of the PCM wall in the melting zone is almost twice as large as that of ordinary wall. Also, the heat-insulation performance of a PCM wall is better than that of an ordinary wall during the charging process, while during the heat discharging process, the PCM wall releases more heat energy. The convective heat transfer coefficient of PCM wall surface calculated using equations for a normal wall material produces an underestimation of this coefficient. The high convective heat transfer coefficient for a PCM wall is due to the increased energy exchange between the wall and indoor air.     

Experimental tile with phase change materials (PCM) for building use

The use of phase change materials (PCM) and their possible architectural integration is a path in the search for optimizing energy efficiency in construction. As part of this path, a pavement has been designed which, in combination with the PCM, serves as a passive thermal conditioning system (new patent n°. ES2333092 A1) [1]. The prototype has been tested experimentally and the results proved that it is a viable constructive solution improving the energy performance of sunny locals.     

A comprehensive review of solar cooker with sensible and latent heat storage materials

Solar energy is available freely and does not create any pollution to the environment. Hence, researchers from all around the world are always working on solar energy applications with enhancement technologies. The solar cooker is one of the largest solar energy application, which is used to cook the food. However, because the solar energy is available only during the daytime, it is not widely used for industry as well as household application. To store the heat during the day, energy storage materials are used. Hence, this study presents a review of sensible and latent heat storage materials used to store the heat during daytime and use for other than daytime hours.     

Monitoring results of an interior sun protection system with integrated latent heat storage

An interior sun protection system consisting of vertical slats filled with phase change material (PCM) was monitored from winter 2008 until summer 2010. While conventional interior sun protection systems often heat up to temperatures of 40 °C or more, the monitoring results show that the surface temperature on the interior side of the PCM-filled slats hardly ever exceeded the PCM melting temperature of 28 °C even in case of long-term intense solar radiation. As long as the PCM is not fully melted, the latent heat storage effect reduces the solar heat gain coefficient (g-value) of the sun protection system to 0.25 for a totally closed blind, and 0.30 for slats set at 45° (the g-values of the same system without PCM are 0.35 and 0.41, respectively). This reduced the maximum air temperature in the offices by up to 2 K in contrast to a reference room with a comparable conventional blind. The sun protection system with PCM therefore considerably improves thermal comfort. In order to discharge the PCM, the stored heat must be dissipated during the night. In climates with sufficiently low outside air temperatures, this is best achieved using a ventilation system in combination with tilted windows.     

Realization, test and modelling of honeycomb wallboards containing a Phase Change Material

The present paper deals with the thermal study of honeycomb panels for short-term heat storage. Using honeycomb panels filled with Phase Change Materials (PCMs) allowed us to fulfil two criteria: enhancement of thermal conductivity and containment to avoid possible leaks. Paraffin whose thermal properties have been measured has been chosen as PCM. The response of the PCM panel to temperature variations was studied with a specific test bench. Temperature and flux measurements clearly showed a significant thermal inertia increase compared to samples filled with air and water. Modelling and numerical simulation have been carried out and validated with the experimental results.     

Thermal analysis of a building brick containing phase change material

This paper presents the thermal analysis of a building brick containing phase change material (PCM) to be used in hot climates. The objective of using the PCM is to utilize its high latent heat of fusion to reduce the heat gain by absorbing the heat in the bricks through the melting process before it reaches the indoor space. The considered model consists of bricks with cylindrical holes filled with PCM. The problem is solved in a two-dimensional space using the finite element method. The thermal effectiveness of the proposed brick-PCM system is evaluated by comparing the heat flux at the indoor surface to a wall without the PCM during typical working hours. A paramedic study is conducted to assess the effect of different design parameters, such as the PCM's quantity, type, and location in the brick. The results indicate that the heat gain is significantly reduced when the PCM is incorporated into the brick, and increasing the quantity of the PCM has a positive effect. PCM cylinders located at the centerline of the bricks shows the best performance.     

空气源热泵相变蓄能除霜方式的实验研究

以目前广泛使用的空气源热泵分体式房间空调器为研究对象,建立了空气源热泵相变蓄热逆循环除霜系统的动态实验台,对系统运行中的结霜过程进行描述,并对空气源热泵与蓄热器之间不同连接方式的除霜情况进行比较分析,总结得出了性能较高的蓄能除霜方式。     

相变储能建筑材料的制备和应用研究

建筑能耗在社会能耗中占有很大比例,把相变材料和传统的建筑材料复合制成的相变建筑节能复合材料是一种较为有效的节能材料,能在建筑节能中发挥良好的作用,把这种材料应用到建筑中,可以改变房屋建筑的室内居住环境,达到减少建筑能耗的目的。     

Correlation between the local climate and the free-cooling potential of latent heat storage

The natural cooling of energy-efficient buildings using latent heat thermal energy storage (LHTES) that is integrated into the building services makes possible energy savings and improved thermal comfort. In this article, studies of the free-cooling potential for different climatic locations are presented. Six cities from around Europe with a wide range of climatic conditions were selected. The size of the LHTES was optimized on the basis of the calculated cooling degree-hours. First, we analysed the influence of the width of the phase change temperature range and determined the optimal melting temperature of the phase change material (PCM). Then, the optimal LHTES was selected, based on the ratio of the mass of the PCM and the volume flow rate of air ventilating the building. We found that the optimum PCM has a melting temperature that is approximately equal to the average ambient air temperature in the hottest month, and that the free-cooling potential is proportional to the average daily amplitude of the ambient air's temperature swings. For all the analysed climatic conditions the PCM with a wider phase change temperature range (12 K) was found to be the most efficient. The optimal size of the LHTES for the free cooling of buildings is between 1 and 1.5 kg of PCM per m³/h of fresh ventilation air.     

Thermal performance of latent heat storage for free cooling of buildings in a dry and hot climate: An experimental study

Experimental studies on the Phase Change Material (PCM) storage unit for building ventilation in dry and hot climates were conducted to determine its thermal performance. The PCM unit stored the night time coolness and used it for cooling the hot ambient air during day time. The influence of air flow rate and the inlet air temperature on cold accumulation in PCM during charging process and cold extraction from the PCM during discharging process were analyzed. The air temperatures used for charging of PCM were 20^oC, 22^oC and 24^oC, while during the discharging process, it was at 36^oC, 38^oC and 40^oC. The air flow rates considered for charging of PCM were 4 and 5 m³/hr/kg of PCM. Experimental observations showed that solidification of PCM was more sensitive to the charging air temperature compared to the air flow rate. When the charging air temperature was reduced from 22^oC to 20^oC, ∼33% less time was needed to completely solidify the PCM. Moreover, when the charging temperature was increased from 22^oC to 24^oC, ∼52% more time was required by the PCM to complete the solidification process. Changing the air flow rate from 4 m³/hr to 5 m³/hr reduced the solidification time period up to ∼16%.     

Experimental and modelling study of twin cells with latent heat storage walls

An experimental and numerical simulation study of the application of phase change materials (PCMs) in building components is presented for thermal management of a passive solar test-room. The experimental study was conducted in an outdoor test cell constituted of two small rooms separated with a wall containing PCM. A specific wall made of hollow glass bricks filled with PCM was studied. Three PCMs were tested: fatty acid, paraffin, and salt hydrate whose melting temperatures are 21 °C, 25 °C and 27.5 °C respectively.Indoor and outdoor temperatures were measured with thermocouples. Ten fluxmeters located at the centre of each wall allowed us to measure the heat fluxes across the walls. Tests were carried out in real climatic conditions.A one-dimensional numerical model has been developed to simulate the transient heat transfer process in the walls. Reasonable agreement between the simulation and the experimental results was observed.     

Temperature reduction due to the application of phase change materials

Overheating is a major problem in many modern buildings due to the utilization of lightweight constructions with low heat storing capacity. A possible answer to this problem is the emplacement of phase change materials (PCM), thereby increasing the thermal mass of a building. These materials change their state of aggregation within a defined temperature range. Useful PCM for buildings show a phase transition from solid to liquid and vice versa. The thermal mass of the materials is increased by the latent heat. A modified gypsum plaster and a salt mixture were chosen as two materials for the study of their impact on room temperature reduction. For realistic investigations, test rooms were erected where measurements were carried out under different conditions such as temporary air change, alternate internal heat gains or clouding. The experimental data was finally reproduced by dint of a mathematical model.     

Heat transfer enhancement of the latent heat storage system using different encapsulating materials with and without fins

This study is aimed at analysing the heat transfer characteristics of different encapsulating materials and the fins on the overall heat transfer in the latent heat storage system (LHSS). Experimentation is done with different phase change materials (PCMs) (d-mannitol, d-sorbitol and paraffin wax) using different encapsulating materials, i.e. copper, aluminium and brass, with and without fins. During the charging process, there is an average 15% heat loss of copper encapsulations without using fins as compared with using fins. The heat losses in aluminium and brass encapsulations are 10% and 15%, respectively. In the discharging process, 23% of heat extraction loss is seen in copper encapsulations, 5% and 18% in aluminium and brass encapsulations, respectively. The results showed that the usage of fins is an effective technique to increase the heat stored and recovered in a LHSS. The most cost-effective encapsulation is aluminium balls with fins as it has the lowest cost/kJ as compared with other two.     

相变储能材料在地源热泵建筑中应用效果的模拟研究

地源热泵系统在建筑环境中,利用介质传输的方法,起到与环境的热交换作用。在冬季,地源热泵传输的地热能在满足使用的同时,部分能够存储在相变材料中。在夜间,关闭使用地源热泵的情况下,相变材料利用自身的结晶放热性能,为建筑物室内持续加热并保持一个较为舒适的温度,实现节约电能及提高热效率的目的。地源热泵使用雨水收集系统中净化的中水作为传输介质,能够满足日常小区住户对热水需求的同时实现节能的目的。     

Positioning of an isothermal heat storage layer in a building wall exposed to the external environment

The paper presents an idea for the enhancement of a building's thermal insulation component by adding into the structure a thin layer of a phase-change material (PCM). The proposed solution can be dedicated, for example, to an external partition of lightweight construction. The capacity for latent heat storage allowed it to stabilize the temperature inside the insulation layer in a daily cycle and to minimize the temperature difference between the internal and external surfaces. In this paper, different positions of a PCM layer in the thermal insulation of an external wall were analysed. The main goal of this work was to find the best position of the PCM layer, taking into account the indoor temperature and the meteorological conditions. The authors propose two parameters: the amount of stored energy and the dynamism of the charging/discharging processes. Taking into account large temperature fluctuations on the external surface during summer and winter, it was concluded that, for this specific case, the most appropriate solution would be an external position of the PCM with a melting temperature of 20°C.     

高温相变蓄热电暖器散热功率调节性能试验研究

对笔者研制的高温相变蓄热电暖器风道、散热肋片的强化散热性能及风门对散热量的调节特性进行了试验研究,结果表明风道、散热肋片可显著强化散热,不同风门开度下散热功率差别明显。     

Low temperature heat storage with phase change materials

A group of promising phase change heat-storage materials was selected through study of the literature, laboratory tests of freeze—melt behaviour and determination of thermophysical properties. Means were developed of encapsulating these materials in metal or plastic containers. Four of these phase-change materials, suitably encapsulated, were tested in a sub-kale thermal storage unit of about 20MJ capacity, using air as the heat-transfer fluid. In most cases, measured thermal-storage capacity exceeded 90% of the theoretical value. After considering a number of heating and cooling schemes employing phase-change heat storage, we selected a forced hot air, central storage design, using CaCl₂.6H₂O encapsulated in plastic pipes. A home was designed, using a two-storey conservatory for solar collection with vertical glazing and movable insulation.