Optimization Of Combined Sensible And Latent Heat Storage Unit And Its Application For Heating In Buildings

The pollutant emission from buildings heating in the north of China is an important factor of the serious environmental problem.How to transform heating from conventional fossil energy to clean energy is an urgent and must-be task.With the rapid development of photovoltaics and wind power,it is possible to use the abandoned power of photovoltaics and wind electricity for heating if only the shortages of these renewable electricity such as serious instability,discontinuity and fluctuation be overcome.Therefore,thermal energy storage has become a key technology to for photovoltaic,wind power and abandoned power heating.In addition,with extending the national peak-valley time-of-use price policy to residential use,off-peak electricity heating is gradually accepted and becoming economically feasible.However,this also needs thermal energy storage to achieve real‘peak cutting and valley filling’.In this paper,sensible-latent heat composite storage technology is proposed for small house heating users and a series of studies have been carried out.In order to determine the feasibility of this technology,a sensible-latent heat composite storage water tank（with water and Ba（OH）₂·8H₂O as heat storage medium and coil heat exchanger as heat charging and discharging unit）is designed and its heat charging and discharging characteristics are studied by numerical simulation.The main work is as follows:Two house models,of floor area 100 m~2 and 300 m~2,respectively,were selected in Beijing and their heating load was simulated by Dest software.According to the heating load data,a rectangular sensible-latent heat composite storage tank is designed with a length 600 mm,width 600 mm and height 500 mm.Heat exchanger is a double-layer U-coil tube,and Phase change material（PCM）Ba（OH）₂·8H₂O is encapsulated into cylinders.The ratios of latent heat and sensible heat to the total heat storage are 15%and 85%,respectively.The influences of the coil position,coil tube diameter,coil form,and inlet and outlet positions on the heat transfer performance of the water tank is studied for both heat-charging and heat-discharging processes.The simulation results show that the change of the relevant structural parameters of the coil heat exchanger has a stronger impact on heat-discharging process than on heat-charging process.The best heat transfer performance is acquired when the distance of two-layer coils is 60 mm and placed in the center of the tank.Under the same mass flow,and the smaller the coil diameter,the more beneficial to the heat release/storage process.The U-coil heat exchanger shows better performance for PCM solidification than the spiral coil and has a larger heat removal rate.The position of the inlet and outlet of the coil,that is,the top-in-bottom-out model and the bottom-in-top-out model,gradually shows a certain difference in relevant operation parameters in the latter half of the heat discharging process,but have little influence on the first half of the heat discharging process and the whole heat charging process.Study on heat transfer and optimization of finned coil heat exchanger.The shape of fins is optimized by selecting various types of fins.To this end,three types of fins,including the cross-type,the H-type the circular fin,are selected and compared with smooth tube.The results show that the cross-type fin has the best heat transfer enhancement.The structural parameters of the cross-type fins are further optimized.The influences of fin height,width,thickness and spacing on heat transfer are studied.The results show that all these geometrical parameters have a certain effect on heat transfer process.The best combination of parameters is 1 mm in fin thickness and width,9-10 mm in height,10 mm in width.Study on heat transfer performance and corrosion of PCM encapsulation unit.Optimization was made of the diameter,the placement method,and the addition of inner fins of PCM unit.The results show that the smaller the encapsulation diameter,the better the heat transfer with the same PCM encapsulation.Horizontal placement of PCM units has a better heat transfer performance.The strip fin is the best among the inner fins simulated,but its influence is quite limited.The corrosion simulation results show that copper and aluminum alloys have the best compatibility with the PCM Ba（OH）₂·8H₂O.The influences of the latent heat ratio on the heat transfer performance is studied.By changing the PCM quantity in the water tank,the heat transfer performance was compared for the latent heat ratios of 10%,15%,20%and 30%.The results show that for the heat discharging process,when the latent heat ratio is 10%,15%and 20%,the heat discharging and the PCM solidification rate do not slow down obviously.However,when the proportion of latent heat is 30%,the PCM solidification rate begins to slow down obviously.It was also found that the latent heat ratio has little effect on PCM melting.For the heat charging process,there is no obvious difference in the heat charging and the PCM melting rate for various latent heat ratios.