Experimental And Numerical Study On The Performance Of Shell-tube Based PCM Storage By Utilizing Off-peak Power

With the increasing building heat demand and the difference of electricity consumption in peak and valley period,traditional electricity grid appears to be facing unprecedented challenges in recent years.The development of efficient and flexible energy storage technology to shift off-peak surplus power could significantly improve the regulation ability of the electricity grid,which is of great signicance to China's energy security,energy saving and emission reduction.However,the whole system efficiency is relatively low when the electricity energy is conversed into heat,and thermal is difficult to extract due to the low heat conductivity of PCM,which limits the application of heat storage by utilizing off-peak power.In this study,the thermal performance of a latent heat storage equipment by utilizing off-peak power was experimentally and numerically investigated in addition to being analyzed by economics,hoping to provide useful references for the system design and engineering application of the latent thermal storage equipment.The research platform of thermal performance of latent heat storage system was built with the aluminum alloy hexahydrate selected as PCM,die-casting aluminum electrical heating plate as the heat source,shell-tube thermal storage box as the heat transfer structure.The experiment was carried out from three aspects:the heat charge process,heat storage process in some certain ambient circumstance,and heat discharge process.It shows that there are main reasons for the different temperature rise of the PCM in the circumferential and axial direction:the distance of PCM from the electricity heat plate,the initial temperature of PCM and the natural convection effect.The emphasis was put on the heat discharging process,where the effects of three variable operating conditions on the heat release performance of the thermal storage device were discussed.In the conditions of variable HTF flow rate,the effective release time and effective release energy efficiency are inversely proportional to the flow rate.Only the conditions of 2L/min and 3L/min can effectively extract the heat stored in the PCM.While in the condition of changing the number of coils,with the increase of the number of coils,the effective release time and effective energy extract efficiency are gradually increased,bringing obvious emhancemet when the number of coils was increased form 5 to 7.When pumping water from the bottom of every coil to the top side,the effective release time and efficiency of the 2L/min sub-condition are 179.2min and 91.3%respectively,which are respectively increased by 97%and 39.7%compared with the sub-working condition of 3L/min.In the condition of varying coil connecting sequence,the effective heat release efficiency and effectiveness of adopting the method of pumping water form bottom to top side are better than those of reversing water flow direction.Taking into account of the effective heat discharging quantity and heat transfer rate,the sub-conditions of 7 coils,2L/min and 7coils,3L/min are selected as the initial preferred heat transfer condiions.In the numerical simulation section,the numerical results were compared with the experimental results firstly.The termperature field and liquid distribution field of typical working condition were analyzed,and the reults show that the effect of natural convection on the internal heat transfer of PCM should be considered to make the results consistent with the experiment.Secondly,the number of coils and HTF flow rate are further selected.When choosing the number of coils,two factors of effective heat release quantity and heat storage density are taken into account,in the condition of 5L/min,the effective heat extract increased by 11.49 when the number of coils was increased from 7 to 9,but the volume heat storage density of tank decreased by 16.39%.It was found that the effective heat release are close to the effective heat storage when the heat sorage equipment running under the condionts of 2L/min,2.33L/min,2.66L/min.Finally,the sub-conditon of 7 coils,2L/min is selected as the preferred heat discharging condition.In the economic analysis section,according to the experimental and numerical results,the cost and benefit were estimated.The economy of the enlarged system was analyzed with three economic indexes(net present value,payback period and internal rate of return).Considering two main factors such as price difference of electricity bills between peak and valley period,heat extract rate,the sensitivity analysis was studied.The results show that the descending orders of two uncertainty factors are price difference and heat extract rate.When the discount rate is equal to 7%,the heat using rate varies in the range of 70%to 100%,and the price difference is great than 03 yuan/kWh,the project is profitable during the operating period.