Properties Investigation On Resorption Energy Storage Cycle Driven By Low-grade Heat Source

Sorption/resorption thermal energy storage is broadly perceived as one prospective way of thermal energy storage owing to its advantages of high energy density,negligible heat loss,flexible working modes and long-term storage capability.Based on resorption energy storage theory,four energy storage cycles were constructed in this paper,namely resorption refrigeration mode,direct heating mode,heat and cold cogeneration mode and temperature upgrade mode.Working principle and characteristics of each mode were summaried and compared.MnCl₂-CaCl₂-NH₃ was selected as the resorption working pair based on literature review,theoretical calculation and cost factors to experimentally investigate the four aforementioned energy storage cycles.The cross-seasonal heat storage performance was theoretically anlyzed.The resorption energy storage system is mainly consisted of high temperature salt bed（HTS bed）,middle temperature salt bed（MTS bed）,heating oil tank,cooling tower,water thermostat and data collecting system.4.8 kg MnCl₂ and 3.9 kg CaCl₂impregnated in expanded natural graphite treated with sulfuric acid are filled in HTS bed and MTS bed respectively.The experimental and analytical results can provide some support for the design and construction of this type of chemical energy storage system.Main conclusions are yielded as follows:（1）Resorption refrigeration mode:160^oC heat source temperature is the inflection temperature of coefficient of performance for refrigeration（COP）,and a maximum cooling power of 2.98 kW is achieved.When the heating temperature is higher than160^oC,sensible heat load of the system increases which means keeping heating HTS bed will reduce the overall refrigeration efficiency.For refrigeration temperature of15^oC,COP of the system ranges from 0.284 to 0.396 while specific cooling power（SCP）ranges from 100.3 to 338.8 W/kg.SCP increases with the increment of heat source temperature.（2）Direct heating mode:The maximum temperature increment and discharging time increase with the decease of discharging temperature.The outlet temperature increment is about 5^oC and lasts for 25 minutes when inlet temperature is 70^oC.In contrast,the outlet temperature increment can reach to 15^oC and lasts for 40 minutes when the inlet temperature is 50^oC.For heat charging temperature higher than 145^oC,heat discharging temperature increment and time lasted are relatively closed.The highest energy storage density is about 1836 kJ/kg when the heat charging and discharging temperature are 155^oC and 55^oC,respectively.With different heat charging and discharging temperature,mass energy density of heat storage increases with the increase of heat charging temperature and the decrease of discharging temperature respectively.The mass density ranges from 702 to 1836 kJ/kg.For the volume energy density of heat storage shares the same trends as the mass energy density with different heat charging and discharging temperature,which ranges from 144 to 304 kWh/m³.The highest ratio of sensible heat to latent heat is about 1.145 when the discharging temperature is 55^oC.The energy efficiency decreases from 97%to 73%when the discharging temperature increases from 55^oC to 75^oC.The ratio of sensible heat to latent heat and heat storage efficiency change little when the charging temperature is higher than 145^oC.For the charging temperature lower than 145^oC,ratio of sensible heat to latent heat decreases sharply.The lowest ratio and heat storage efficiency are about 0.54 and 58%respectively at 135^oC charging temperature and 55^oC discharging temperature.（3）Heat and cold cogeneration mode:The energy storage density and largest outlet temperature increment increase with the decrease of discharging temperature.Under the discharging and cooling temperature of 30^oC and 15^oC respectively,the maximum outlet temperature increment is 20^oC at the charging temperature of 160^oC.The energy storage density ranging from 912 to 1706 kJ/kg also achieves its peak value under that working condition.The ratio of sensible heat to latent heat ranges from 0.72to 0.90 while the energy storage efficiency covers the span of 67%to 79%and energy storage efficiency changes little when charging temperature is higher than 150^oC.With a cooling temperature of 15^oC,the average cooling power rises from 0.59 kW to 1.07kW during discharging phase and the refrigeration time is 20 minutes.Corresponding COP and SCP range from 0.05 to 0.07 and 15.00 to 27.33 W/kg,respectively.Taking total cycle time 90 minutes into consideration,the average cooling power falls to as low as 238 W.Both the energy and the exergy efficiency of the resorption energy storage system increase with the augment of charging temperature.Energy efficiency increases from 72%to 87%while exergy efficiency rises slowly from 29%to 35%.（4）Temperature upgrade mode:This working mode can effectively improve the quality of low-grade heat source thus expand its application.The highest energy storage density is 614 kJ/kg;corresponding charging temperature,discharging temperature and MTS temperature are 135^oC,140^oC and 70^oC,respectively.Energy storage density ranges from 535 kJ/kg to 614 kJ/kg under different charging and discharing temperature.For MTS bed of 70^oC,energy storage density increases with the increase of charging temperature.The highest ratio of latent heat to sensible heat is 0.418;corresponding charging temperature,discharging temperature and MTS temperature are135^oC,140^oC and 70^oC,respectively.The highest energy storage efficiency is 28.6%;corresponding charging temperature,discharging temperature and MTS temperature are125^oC,130^oC and 70^oC,respectively.（5）Cross-seasonal heat storage:For different mass ratio of metal to sorbents,theoretical energy storage density of the resorption system ranges from 727.2 to 750.7kJ/kg.Both energy storage density and energy storage efficiency increase with the decrease of mass ratio of metal to sorbents.Reducing the mass ratio of metal to sorbents can slightly improve the energy storage density,but can significantly improve the energy storage efficiency of the system.For charging temperature 89^oC and discharging temperature 57^oC,energy storage density of cross-seasonal heat storage mode increases sharply with the increase of the reaction conversion coefficient.Energy storage efficiency has the similar trend,but the increment is limited.Energy storage density ranges from 424.5 to 744.7 kJ/kg while energy storage efficiency covers the span of 77.2%to 85.6%.When mass ratio of metal to sorbents,charging temperature and discharging temperature are 3.5,89^oC and 57^oC respectively,both energy storage density and efficiency increase with the increase of heat transfer efficiency,and the value of each parameter is 558.5⁷44.7 kJ/kg and 48.1%⁸5.6%,respectively.