Research On The Operation Characteristics And Application Of Cold Storage Condensation Heat Recovery System Using Phase Change Materials

Large amounts of condensation heat emitted by cold storage refrigeration systems and discharged directly into the atmosphere can not only lead to increased energy consumption but also result in environmental pollution and global warming. Consequently, there is growing interest in utilizing low-grade waste heat from cold storage to provide heat for domestic hot-water preparation and industrial hot-water supply in order to reduce global energy consumption and protect the environment. Thus far, the major studies in this field continue to focus on economic analysis and cold storage refrigeration system design. Moreover, the application of phase change materials(PCMs) as a prospective technique for residual or stored energy recovery is expected to play a significant role in enhancing condensation heat recovery from cold storage refrigeration systems in the future.Therefore, the present paper focuses on the preparation of a newly developed phase change material(PCM), which is suitable to condensation heat from cold storage refrigeration systems, the design of a PCM heat recovery unit with the function of heat storage and heat transfer, and the design of a cold storage condensation heat recovery system using the PCM heat recovery unit. Several tests were performed to demonstrate the effectiveness of condensation heat recovery from the cold storage refrigeration system using the PCM.Firstly, a novel and suitable phase-change composite material is developed for cold storage condensation heat recovery, which has a relatively large latent heat, high thermal conductivity, and an appropriate phase-change temperature(i.e. 80°C). With carnauba wax(CW) as the PCM and expanded graphite(EG) as the additive, a composite was developed with an optimal mass ratio of CW:EG=10:1. Furthermore, the thermal and physical properties and the interior structure of the composite were then investigated using a scanning electron microscope(SEM), thermal constants analyser(Hot Disk), differential scanning calorimeter(DSC), and Fourier transform infrared spectrometer(FT-IR). Experiments on the melting and solidification processes and accelerated thermal cycling were also conducted.Then, heat storage and release experiments were conducted to examine the thermal properties of the cylindroids CW/EG heat recovery unit. Is was found that there were three phases such as sensible heat, latent heat, sensible heat in the heat storage and release processes, the time of the final phase was more than the other two phases. Owing to the high conductive of EG, the whole heat storage and release rates of the cylindroids CW/EG heat recovery unit were enhanced, and the heat release time is more than the heat storage time. Moreover, based on the above experiments, the present paper design a PCM heat recovery unit with the function of heat storage and heat transfer.Finally, a cold storage condensation heat recovery system that uses PCMs has been designed and analysed. According to the principle of energy cascade recycling, the three different operation modes could be effectively switched to recycle condensation heat only by the switch control of valves. The experiments showed that the system could work well in both the refrigeration mode and the refrigeration and heat storage mode, the required time to equilibrium of the latter is more than that of the former. In the heat release mode, the temperature of hot-water prepared by the system could achieve around 26°C, and the temperature increase speed and the obtained volume amount of hot-water was different when the flow rate of inlet water was changed. However, the built-in heat transfer pipes were mainly fixed in the center of the PCM heat recovery unit, which would contributed to the temperature distribution non-uniformity and a low heat recovery efficiency. Therefore, to enhance the temperature distribution uniformity and heat recovery efficiency, it is extremely necessary to design a cold storage condensation heat recovery system according to the real operation parameter such as operating load and thermal demand side load.