Study On Heat Storage And Release Performance Of Binary Phase Change Composite Heat Storage Unit

In recent years,due to the energy crisis of fossil fuels and emerging ecological problems,the development of clean,renewable and sustainable energy and technologies has become increasingly important.The final consumption of energy usually depends on the form of direct thermal energy consumption.A large amount of greenhouse gas emissions lead to global warming and the sustainable development of human society is seriously threatened.More and more attention has been paid to green,low carbon and environmental protection.The research,development and utilization of renewable energy are extremely urgent.At present,phase change materials are widely used in solar energy,building energy conservation,industrial waste heat,waste heat exchangers,building energy conservation and air conditioning energy conservation.However,there are few power saving applications at the user end.How to apply power "peak shaving and valley filling" to the daily life of residents can save expenses and protect the environment.The more mature technology in electric heat storage is mainly magnesium brick phase change electric heater,which mainly uses the low valley electricity in winter to store heat and release the stored heat during the day.However,its effect in practical application,service life and price are difficult to meet the needs of residents.The direction of electric heat storage still needs further in-depth research on material selection and recycling performance.In order to find a suitable phase change material,this paper selects sodium pyrophosphate decahydrate,barium hydroxide octahydrate and sodium nitrate hexahydrate as phase change materials based on suitable phase change temperature and high latent heat of phase change.However,sodium pyrophosphate decahydrate has low latent heat of phase change,barium hydroxide octahydrate is highly toxic and sodium nitrate hexahydrate is unstable.According to Schroeder's formula,magnesium sulfate heptahydrate and aluminum ammonium sulfate dodecahydrate are selected as basic materials of binary phase change materials.The thermophysical properties of nine mixtures were analyzed,and the results showed that when the ratio of magnesium sulfate heptahydrate to aluminum ammonium sulfate dodecahydrate was 5:5,the phase transition temperature was 77.1?,and the latent heat of phase transition was 222.1 kJ/kg.The binary eutectic with this ratio has better thermal properties and stability.Inorganic phase change materials have low supercooling and thermal conductivity.In order to reduce the influence of supercooling degree of hydrated salt on the cyclic stability of phase change materials,three inorganic substances of calcium hydroxide,borax and disodium hydrogen phosphate dodecahydrate are selected as nucleating agents of magnesium sulfate heptahydrate-aluminum sulfate dodecahydrate according to hydrated salt(5:5).After many heat storage and release experiments,it was found that the latent heat loss of the composite phase change material added with 2% disodium hydrogen phosphate dodecahydrate was the smallest,and the latent heat loss of the phase change was only 3.9%.Porous expanded graphite was selected to shape the composite phase change material to increase its thermal conductivity.when the amount of expanded graphite was 4%,the thermal conductivity of the composite phase change material was 1.583 w/(m k),which significantly improved the thermal conductivity.FLUENT is used to model the heat storage unit and verify the accuracy of the model.The electric heating model is designed,and the thermal performance of the prepared composite phase change material in the heat storage unit is simulated.When the indoor environment is 18?,the material in the heat storage unit can completely melt within 100 min,and the influence of ambient temperature on the heat storage and release of the heat storage unit.Furthermore,the heating effect of the electric heating system of the phase change heat storage unit is simulated.According to the time-of-use electricity price in Beijing,heat storage is carried out during the low electricity price(20:00-8:00 the next day)and heat release is carried out during the daytime(8:00-20:00).The simulation results show that when the outdoor temperature is-5?,the indoor temperature is maintained at about 20?,the indoor temperature distribution is relatively uniform,and the PMV-PPD value in the heating room is between-0.5 and+0.5,with high indoor comfort.