Phase Change Cold Storage And Its Application In Solar Energyair Conditioning Systems

In the circumstance that the energy and electric power are in short supply in China,cold storage plays an important role in conserving available energy, improving energyutilization efficiency and successful load shifting. Solar energy is available only duringdaytime, and hence, its application usually requires an efficient thermal storage so thatthe excess energy during sunshine hours may be stored for later use during the night.Moreover, solar cooling systems are usually intermittent and susceptible to the weather.Therefore, applying cold storage methods to solar cooling air-conditioning systems isfavorable to utilize renewable energy and enhance system stability.In this thesis, phase change materials, cold storage spherical capsule, cold storagetank and solar air conditioning system integrated with cold storage are investigatedexperimentally and theoretically. The main work is as follows:1. Combined with the current domestic and overseas researches of the cold storageair conditioning system, some commonly used phase change materials, structures ofcold storage tank and several typical systems are summarized.2. According to the working conditions of the cold storage solar air conditioningsystem, suitable phase change materials are developed using capric acid and lauric acidas the phase change material base fluid. The eutectic point of the binary system isdetermined. Methyl salicylate, ethyl benzoate, caproic acid, oleic acid, tetradecane andhexadecane are used as organic additives and are added at different mole fractions intothe phase change material base fluid in order to modify the phase transition temperature,phase change properties and heat transfer characteristics for cold storage application.3. Cooling curve methods, differential scanning calorimetric apparatus, Hot Diskthermal analyzer are employed to test the self-developed phase change materials. Theproper mole fractions of each additive which can match the target of the refrigerationunit well are obtained. Phase change materials are selected and screened according totheir thermal physical properties. By contrast, the phase change material, C-L/O0.08,which employs oleic acid as the additive has a phase change temperature of14.97℃ and a heat of fusion of114.1kJ/kg. It shows the least supercooling degree and the moststeady phase transition. Moreover, its thermal conductivity and specific heat capacityboth manifest an obvious advantage. Therefore, C-L/O0.08is considered as the mostsuitable phase change material among all the candidate materials for cold storageapplication in solar air conditioning systems.4. Based on the self-developed phase change material C-L/O0.08, physical coldstorage spherical capsules are fabricated. A single cold storage spherical capsulecontains materials with the quality of126.4g. The external diameter of the capsule is70mm and the wall thickness is2mm. The spherical shell is made from polyethylene.By experiments, the temperature distribution and temperature variations within thespherical capsule during charging and discharging processes are tested. Amathematicalmodel of the steady-state charging and discharging working conditions of the coldstorage spherical capsule is set up. Influencing factors on the charging and dischargingperformance such as the operating conditions and the structural parameters of thecapsule are analyzed and discussed. It is found that the temperature of the heat transferfluid, the diameter, wall thickness and shell material of the capsule all have significanteffects on the charging/discharging capacity, charging/discharging rate and charging/discharging duration of the cold storage spherical capsule.5. A spherical capsule cold storage tank is fabricated, and a phase change coldstorage solar air conditioning experimental system is set up. During the daytime, thecold storage tank charges under unsteady-state working conditions of chilled waterfrom a solar adsorption chiller. During the night, the tank discharges and suppliescooling to users through the indoor cooling radiation terminal. The variations of theinlet and outlet water temperature of the cold storage tank and the central temperatureof the spherical capsules are determined by experiments. The variations of the charging/discharging capacity and charging/discharging rate are analyzed. Meanwhile, basedon the experimental data of the chilled water temperature of a solar absorption chillerand a solar adsorption chiller, unsteady-state working conditions of the cold storage insolar air conditioning systems are simulated. Thus, the temperature distribution andtemperature variations within the capsule are investigated. Through above work, someuseful conclusions are obtained for optimizing the operation and control of the solar airconditioning system with phase change cold storage.