The novel use of phase change materials in refrigeration plant. Part 1: Experimental investigation

Phase change materials (PCMs) have found considerable applications in large air conditioning schemes. PCMs were primarily used to introduce capacitance within a system, which enabled them to take into account the fluctuations in the daily cooling load. However, this technology has been developed recently and integrated into the refrigeration system directly to save energy and for better control. This has been achieved through lowering the temperature of the sub-cooled refrigerant and stabilizing the superheat. A prototype demonstrating this novel use of PCMs in refrigeration systems has been developed and tested. Results show that by lowering the temperature of the sub-cooled refrigerant, up to 8% energy savings can be achieved in the UK climate conditions. By using the PCM heat exchanger as a pre-condenser in a refrigeration system, the system’s COP can be improved by 6%. The benefit achieved by reducing the superheat is somewhat offset by its extra pressure drop, although simulation work has showed elsewhere [F. Wang, The passive use of phase change materials in refrigeration system, Ph.D. thesis, London South Bank University, 2003] that the COP can be improved by 4% using a low pressure drop PCM heat exchanger in any climate zone.     

Experimental assessment of heat storage properties and heat transfer characteristics of a phase change material slurry for air conditioning applications

A new microencapsulated phase change material slurry based on microencapsulated Rubitherm RT6 at high concentration (45% w/w) was tested. Some heat storage properties and heat transfer characteristics have been experimentally investigated in order to assess its suitability for the integration into a low temperature heat storage system for solar air conditioning applications. DSC tests were conducted to evaluate the cold storage capacity and phase change temperature range. A phase change interval of approximately 3 °C and a hysteresis behaviour of the enthalpy were identified. An experimental set-up was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the phase change material slurry. First, tests were carried out using water in order to obtain natural convection heat transfer correlations. Then a comparison was conducted with the results obtained for the phase change material slurry. It was found that the values of the heat transfer coefficient for the phase change material slurry were higher than for water, under identical temperature conditions inside the phase change interval.     

Development of a ventilation system utilizing thermal energy storage for granules containing phase change material

We have developed an experimental ventilation system that features direct heat exchange between ventilation air and granules containing a phase change material (PCM). Measurement of outlet air temperature when the inlet air temperature was periodically varied to simulate changes of outdoor ambient air temperature showed that the outlet air temperature was stabilized and remained within the phase change temperature range. This effect is expected to be useful in practical ventilation systems. The potential of such systems for reducing ventilation load was examined through computer simulation for eight representative cities of Japan. This revealed how different temperature conditions would affect required heat storage capacity.     

一种建筑节能技术——水源热泵空调系统

水源热泵是一种高效节能、经济环保、安全稳定、冷暖两用、运行灵活的新型中央空调系统,它利用地表水(江、河、湖水)、地下水、工业废水及生活废水,又可用取之不尽的海水等,借助热泵系统,既能制冷、又能制热,是一种高效建筑节能技术。     

The application of a validated numerical model to predict the energy conservation potential of using phase change materials in the fabric of a building

Phase change materials absorb a large amount of energy as latent heat at a constant phase transition temperature and are thus used for passive heat storage and temperature control, example applications include electronics, and storage of perishable items. Encapsulated phase change materials incorporated into gypsum wallboards of buildings may be suitable for absorbing solar energy directly, reducing temperature rise. Improvement in human comfort is to be expected due to a decrease in the frequency and magnitude of internal air temperature swings, maintaining the temperature closer to the desired set temperature passively for a long period of time.This paper summarises the results of a detailed theoretical investigation and analysis of thermal energy storage and temperature control achieved using passive building construction elements incorporating phase change materials. The predictions detail the effects of using various quantities of different PCM materials with phase change temperatures of 28 and 43 °C incorporated into a selection of wall constructions for selected ambient conditions of temperature and insolation. From this parametric study, optimum arrangements are proposed.     

Thermal management of metal hydride hydrogen storage reservoir using phase change materials

Metal hydride hydrogen storage reservoir should be carefully designed to achieve acceptable performance due to significant thermal effect on the system during hydriding/dehydriding. Phase change materials can be applied to metal hydride hydrogen storage system in order to improve the system performance. A transient two-dimensional axisymmetric numerical model for the metal hydride reservoir packed with LaNi₅ has been developed on Comsol platform, which was validated by comparing the simulation results with the experiment data from other work. Then, the performances of metal hydride hydrogen storage reservoir using phase change materials were predicted. The effects of some parameters, such as the thermal conductivity, the mass and the latent heat of fusion of the phase change materials, on the metal hydride hydrogen storage reservoir were discussed. The results shown that it was good way to improve the efficiency of the system by increasing the thermal conductivity of phase change materials and selecting a relatively larger latent heat of fusion. Due to the relatively lower thermal conductivity of phase change materials, different metal foams were composited with the phase change materials in order to improve the heat transfer from the metal hydride bed to the phase change materials and the hydrogen storage efficiency. The effect of aluminium foam on the metal hydride reservoir was studied and validated. The phase change materials composited with copper foam shown better performance than that composited with aluminium foam.     

The novel use of phase change materials in refrigeration plant. Part 3: PCM for control and energy savings

Phase change materials (PCMs) have been recognized as energy storage tanks since the 1980s. The ‘tank’ has been introduced into the refrigeration system to enable its capacitance to take account of fluctuations in the daily cooling load. However, this part of the paper will present a novel control purpose of using PCM in refrigeration systems. The novel application of PCMs in refrigeration systems at different positions in the refrigeration cycle circuit with a shell and tube structure has been investigated extensively by the novel mathematical model presented in part 2 of the paper. The results show that for energy savings, PCMs at different positions give coefficient of performances (COPs) up to 8% through lowering the sub-cooling. PCMs also improve the system COPs up to 4% and 7% for the thermostatic expansion valve (TEV) and orifice systems, respectively, by minimizing the superheat. Further benefits such as system stabilization were also observed in this investigation.     

联合国环境规划署亚太资源中心办公楼热回收空调系统应用研究

对联合国环境规划署亚太资源中心办公楼采用的新型热回收新风机加空调盘管的集中空调系统进行分析,详细阐述了热回收新风机的特点、优势及节能潜力。对办公建筑的空调系统改造具有一定的指导意义。     

Thermal performance of packed bed thermal energy storage units using multiple granular phase change composites

The present article reports on the utilization of multiple granular phase change composites (GPCC) with different ranges of phase change temperatures in a packed bed thermal energy storage system. Small particle diameter of GPCC allows simple mixing of two or three ranges of GPCCs in a packed bed for enhancement of storage unit performance. Experiments have been carried out to characterize the phase changing characteristics of two GPCCs chosen for this purpose. Packed bed column experiments have been carried out to provide basic understanding of the heat transfer process in the composite bed consisting of a mixture of GPCCs at different values of mixing ratio. A mathematical model has been developed for the analysis of charging and discharging process dynamics. Once validated, the model has been used to perform a parametric study to investigate the overall bed performance at different values of mixing ratio and Reynolds number. An optimization of the value of mixing ratio has been obtained based on the overall charging and discharging times as well as the exergy efficiency. It has been demonstrated that, as compared to the use of single GPCC, careful choice of the mixing ratio of GPCCs in a composite bed can result in a significant enhancement of the overall storage unit performance. As compared to the use of multiple sequential layers of GPCCs, using units composed of a mixture of GPCCs with an optimized mixing ratio results in a remarkable improvement of the unit performance without limitations on the charging and discharging directions during practical applications.     

Thermal analysis of a natural circulation solar air heater with phase change material energy storage

The transient thermal analysis of a natural convection solar air heater is presented. The heater consists of a single-glazed flat plate solar collector integrated with a paraffin type phase change material (PCM) energy storage subsystem and a rectangular enclosure which serves as the working chamber. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The underside of the absorber plate, together with the vertical sides of the PCM module container, serve as air heating vanes. Air flow through the system is by natural convection. Energy balance equations are developed for each major component of the heater and linked with heat and mass balance equations for the heated air flowing through the system. The airflow rate is determined by balancing the buoyancy head resulting from thermally induced density differences and the friction head due to various flow resistances. The predicted performance of the system is compared with experimental data under daytime no-load conditions over the ambient temperature range of 19–41 °C and daily global irradiation of 4.9–19.9 MJ m^–2. Predicted temperatures at specific locations on the absorber plate, heat exchanger plate, glazing, and heated air agree closely with experimental data to within 10, 6, 8, and 10 °C, respectively. Maximum predicted cumulative useful and overall efficiencies of the system are within the ranges 2.5–13 and 7.5–18%, respectively. Correlations of the predicted efficiencies are presented.     

A novel thermoelectric refrigeration system employing heat pipes and a phase change material: an experimental investigation

This paper presents results of tests carried out to investigate the potential application of heat pipes and phase change materials for thermoelectric refrigeration. The work involved the design and construction of a thermoelectric refrigeration prototype. The performance of the thermoelectric refrigeration system was investigated for two different configurations. The first configuration employed a conventional heat sink system (bonded fin heat sink) on the cold side of the thermoelectric cells. The other configuration used an encapsulated phase change material in place of the conventional heat sink unit. Both configurations used heat pipe embedded fins as the heat sink on the hot side. Replacement of the conventional heat sink system with an encapsulated phase change material was found to improve the performance of the thermoelectric refrigeration system. In addition, it provided a storage capability that would be particularly useful for handling peak loads and overcoming losses during door openings and power-off periods. Results showed that the heat sink units employing heat pipe embedded fins were well suited to this application. Results also showed the importance of using a heat pipe system between the cold junction of the thermoelectric cells and the cold heat sink in order to prevent reverse heat flow in the event of power failure.     

High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets

Using exfoliated graphite nanoplatelets (xGnP), paraffin/xGnP composite phase change materials (PCMs) were prepared by the stirring of xGnP in liquid paraffin for high electric conductivity, thermal conductivity and latent heat storage. xGnP of 1, 2, 3, 5 and 7 wt% was added to pure paraffin at 75 °C. Scanning electron microscopy (SEM) morphology showed uniform dispersion of xGnP in the paraffin wax. Good dispersion of xGnP in paraffin/xGnP composite PCMs led to high electric conductivity. The percolation threshold of paraffin/xGnP composite PCMs was between 1 and 2 wt% in resistivity measurement. The thermal conductivity of paraffin/xGnP composite PCMs was increased as xGnP loading contents. Also, reproducibility of paraffin/xGnP composite PCMs as continuous PCMs was manifested in results of electric and thermal conductivity. Paraffin/xGnP composite PCMs showed two peaks in the heating curve by differential scanning calorimeter (DSC) measurement. The first phase change peak at around 35 °C is lower and corresponds to the solid-solid phase transition of the paraffin, and the second peak is high at around 55 °C, corresponding to the solid-liquid phase change. The latent heat of paraffin/xGnP composite PCMs did not decrease as loading xGnP contents to paraffin. xGnP can be considered as an effective heat-diffusion promoter to improve thermal conductivity of PCMs without reducing its latent heat storage capacity in paraffin wax.     

A review of intercalation composite phase change material: Preparation, structure and properties

This paper reviews preparation, structure and properties of the intercalation composite phase change material (PCM). The layered structure of clay and graphite is utilized to prepare the intercalation composite PCM. It is concluded that the preparation methods include liquid phase intercalation and melting intercalation. The thermal conductivity and flame retardancy of organic PCM are improved by intercalating organic PCM into montmorillonite (MMT) or graphite. The phase change properties of the intercalation composite PCM can be measured by differential scanning calorimetry (DSC) and T-history method.     

Thermodynamics and kinetics analysis of hydrogen absorption in large-scale metal hydride reactor coupled to phase change material-metal foam-based latent heat storage system

Phase change materials (PCMs) have recently been coupled with metal hydride storage tanks (MHSTs) to store adsorption heat and subsequently deliver it for hydrogen desorption through melting and solidification cycles. This method might reduce process costs by eliminating the use of HTF (i.e. heat transfer fluid). However, thermodynamics and kinetic data are scarce for large-scale MH-PCM applications, particularly when PCM is loaded in metal foams (MFs) to promote heat transfer. The current work aimed to develop a 2D model for simulating H₂-absorption in a LaNi₅-metal bed integrating a PCM-MF unit in a large-scale tube-and-shell heat exchanger. The constructed model (via Fluent 15.0 CFD-platform) was first-validated using referenced experimental data. The resulting heat transfer was analyzed for different MFs [aluminum, copper, nickel and titanium] of different porosities (0.1–1.0). Excellent outcomes were retrieved. By trapping the H₂-absorption heat, the MF-PCM unit improved the LaNi₅ hydriding. The LaNi₅ charging was achieved after ～500 s, independently of the MF type and porosity. The PCM melting rate depends on tube position, porosity and the MF type. It increased with MFs incorporation (order of enhancement: Cu > Al > Ni > Ti) and MF-porosity decrease (from ε = 100% to 10%). Besides, the PCM tube above the H₂-feeding pipe melts more quickly than the other tubes, presumably to the gravitational-force effect. Longer times (i.e. 9 000 s to >16 000 s, depending on tube position) were recorded for complete melting of the PCM when ε_MF = 100%; however, when ε_MF is less than 80%, the required time for total melting was tremendously reduced to less than 500 s. Nevertheless, the MFs porosity could not be decreased considerably to avoid a huge loss of material storage (PCM), thereby diminishing the thermal storage performance of the PCM matrix.     

某度假村水源热泵空调系统的设计方案选择与分析

通过对宁波银凤度假村空调系统的比较，分析了分体式水源热泵空调系统的优势并提出了对今后设计、施工中应注意的几点意见。     

Synthesis and thermal energy storage characteristics of polystyrene-graft-palmitic acid copolymers as solid-solid phase change materials

A series of polystyrene graft palmitic acid (PA) copolymers as novel polymeric solid-solid phase change materials (PCMs) were synthesized. In solid-solid PCMs, polystyrene is the skeleton and PA is a functional side chain that stores and releases heat during its phase transition process. The heat storage of copolymers is due to phase transition between crystalline and amorphous states of the soft segment PA in copolymer and the hard segment polystyrene restricted the free movement of molecular chains of the soft segments even above the phase transition temperature. The copolymers always remain in the solid state during the phase transition processing and therefore they are described as form-stable PCM. Fourier transform infrared spectroscopy (FT-IR) and polarization optical microscopy (POM) analyses were performed to investigate the chemical structures and crystalline morphology. Thermal energy storage properties, thermal reliability and thermal stability of the PCMs were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) methods. Thermal conductivities of the PCMs were also measured using thermal property analyzer. The analysis results indicated that the PA chains were successfully grafted onto the polystyrene backbone and the copolymers showed typical solid-solid phase transition properties. Moreover, thermal cycling test showed that the copolymers have good thermal reliability and chemical stability although they were subjected to 5000 heating/cooling cycling. The synthesized polystyrene-graft-PA copolymers as novel solid-solid PCMs have considerable potential for such as underfloor heating, thermo-regulated fibers and heating and cooling of agricultural greenhouses. Especially, the polystyrene-graft-PA copolymer including 75% PA is the most attractive PCM due to its highest latent heat storage capacity in the synthesized copolymer PCMs.     

Thermal management of lithium-ion batteries using phase change materials

良好的热管理系统是电池安全及高效使用的保证,电池的热管理需要确保电池温度在安全温度范围以及电池组内最大温差不超过5 ℃.传统的热管理方式,如空气冷却,不仅需要额外的动力输入,而且越来越不能满足高能量密度的新型锂离子电池的热管理需要.使用相变材料的电池热管理系统,利用相变材料的相变潜热吸收电池产生的热量,在不使用外界功耗的条件下,可以长时间保持电池的温度在适宜的范围内.通过与膨胀石墨,金属泡沫复合,相变材料的热导率可以大大提高,电池组内体系温度均匀性可以满足工作要求.而且,相变材料的形状不固定,可以使用在任意形状的电池上.被动式热管理是应用于电池热管理系统中最具前景的技术.     

Second law analysis of a diesel engine waste heat recovery with a combined sensible and latent heat storage system

The exhaust gas from an internal combustion engine carries away about 30% of the heat of combustion. The energy available in the exit stream of many energy conversion devices goes as waste. The major technical constraint that prevents successful implementation of waste heat recovery is due to intermittent and time mismatched demand for and availability of energy. The present work deals with the use of exergy as an efficient tool to measure the quantity and quality of energy extracted from a diesel engine and stored in a combined sensible and latent heat storage system. This analysis is utilized to identify the sources of losses in useful energy within the components of the system considered, and provides a more realistic and meaningful assessment than the conventional energy analysis. The energy and exergy balance for the overall system is quantified and illustrated using energy and exergy flow diagrams. In order to study the discharge process in a thermal storage system, an illustrative example with two different cases is considered and analyzed, to quantify the destruction of exergy associated with the discharging process. The need for promoting exergy analysis through policy decision in the context of energy and environment crisis is also emphasized.     

空调排风热回收在我国应用节能潜力分析

介绍空调排风热回收系统的工作原理,对不同地区的室外温度、焓值的逐时变化参数进行分析.在气象数据基础上动态分析我国不同典型气候区域城市热回收的适应性及热回收方式的选择,并对热回收节能潜力进行分析,进而得出热回收系统在我国不同气候区域的适应范围,研究表明排风热回收装置在我国节能效果明显.     

Numerical heat transfer studies of the fatty acids for different heat exchanger materials on the performance of a latent heat storage system

Theoretical investigations of fatty acids as a phase change material (PCM) for energy storage system have been conducted in this study. The selected fatty acids were capric acid, lauric acid, myristic acid, palmitic acid and stearic acid. For the two-dimensional simulation model based on the enthalpy approach, calculations have been made for the melt fraction with conduction only. Glass, stainless steel, tin, aluminium mixed, aluminium and copper were used as heat exchanger materials in the numerical calculations. Theoretical results show that capric acid was found good compatibility with latent heat storage system. The large value of thermal conductivity of heat exchanger materials did not make significant contribution on the melt fraction.