| Amount of waste heat is released by industrial yearly, which not only reduces theenergy efficiency in production, but also burdens the energy demand of our country.To solve this issue, the waste heat recovery and utilization technology have beenstudied and used in industries recently. However, it is found the temperature of mostwaste heat recovered and utilized is mainly above230°C. That below230°C is stillunderutilized. On the other hand, with the improvement of people’s living standard,the heating demand for building is increasing. Limited by the network of districtheating (DH), some users without DH usually choose coal-fired, oil-fired or gas-firedheating systems. They do not only reduce the energy efficiency and economy ofsystems but also release more pollution. Hence, the mobilized waster heat utilization(MWHU) system is studied in this paper from above. The following researches havebeen carried out:The erythritol was selected as the material for recovering waste heat below230°Caccording to the melting temperature, latent heat, safety, environmental protection,economy and other factors. The material was tested by differential scanningcalorimetry (DSC) to obtain accurate parameters. In addition, the super-cooling testwas also conducted, which can provide references for analyzing super-cooling in thefollowing experiment.A lab-scale MWHU system was set up with the selected material according to theprinciple of MWHU system. The charging and discharging experiments with indirectcontact MWHU container were carried out. The temperature variation and theprocesses of melting and solidification were analyzed. The performance of thecontainer was studied with two indexes: thermal efficiency and heating intensity.Due to the limited measuring points in experiment, simulation of charging anddischarging processes was conducted to further understand the melting andsolidification. The models were built based on the assumptions and simplifications ofexperiment. They were verified by comparing the calculated results with those ofexperiment. Afterwards, they were used to optimize the container. Three options, i.e.improving the material thermal conductivity, adjusting the diameter and arrangementof tubes and adding straight fins, were studied. Results showed that the appropriatescheme was with the thermal conductivity of3.7W/m·K, the diameter of DN22withthe related arrangement of heat transfer tubes and the fins area of0.468m2. The heat capacity of optimized container was as much as90%of the original one, however,with cutting74%of charging time and67%of discharging time.In addition, in order to strengthen the heat transfer in container, the direct contactMWHU container, within which the thermal oil and material contact directly, wasdesigned. The charging and discharging experiments were carried out. To know if theoperating parameters would affect the processes and provide some references for fullscale system, the melting and solidification rates of material under different thermaloil flow rates were investigated. Since the thermal oil flow was weakened bysolidified material in the initial stage of charging process, the option using electricalbars to form quick flow channels was discussed. Furthermore, the thermal efficiencyand the heating intensity were employed to evaluate the performance of direct contactMWHU container.Finally, according to the experimental system, the cost and benefit of full scaleMWHU system were estimated. The economy of system was analyzed with threeeconomic indexes (net present value, payback period and internal rate of return). Thesensitivity analysis was studied with considering three uncertainty factors such ascharging time, distance between waste heat and users and the waste heat price.Results showed that the descending order of three uncertainty factors is: charging time,distance between waste heat and users and the waste heat price. |
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