Font Size: a A A

Design And Optimization Of Active Heat Storage-release Associated With Heat Pump Heating System

Posted on:2015-02-24Degree:MasterType:Thesis
Country:ChinaCandidate:W T SunFull Text:PDF
GTID:2253330431463539Subject:Agricultural Biological Environmental and Energy Engineering
Abstract/Summary:PDF Full Text Request
The Chinese solar greenhouse has a unique greenhouse structure which regards solar energy asmain energy source and has characteristics such as high efficiency, energy saving and low cost. Duringcold winter night, air temperature inside a solar greenhouse is low for crop growth which would affectcrop yield and quality, due to the heat-transfer characteristics and heat capacity limit of the north wall.In recent years, around how to promote the heat storage capacity of the solar greenhouse, the thought ofactive heat storage-release came forward. Solar energy is a kind of clean renewable energy but hasintermittent and unstable performance when used for greenhouse heating, meanwhile the heat collectingefficiency of the solar thermal collector decreases with an increase in operating temperature. Thus,active heat storage-release system (AHSRS) is difficult to ensure appropriate temperature for solargreenhouse in frigid region or encounters weather conditions with weak solar radiation. As an efficientmeans of raising low-grade energy, the heat pump has been more and more applied to greenhouseheating which can reduce operating temperature of the AHSRS when used in combination. In order topromote heating performance and stability of the AHSRS and improve air temperature inside a solargreenhouse at night, based on the thought of active heat storage-release, an active heat storage-releaseassociated with heat pump heating system (AHSRHPS) applicable to solar greenhouse heating wasdesigned in the present study. During the day, the solar energy reaching the north wall surface wasabsorbed by the circulating water and stored in reservoirs when AHSRS was running. Running the heatpump unit to promote low-grade heat energy and reduce circulating water temperature whichcontributes to increasing the heat collecting efficiency of the AHSRS and maximum water temperatureof the reservoir. When air temperature inside the solar greenhouse was low at night, the heat energy wasreleased through the AHSRS. Tests for the AHSRHPS were carried on from5Dec.2012to5Feb.2013.The results showed that when sunny and cloudy day in winter, the air temperature inside theexperimental greenhouse with the AHSRHPS was higher than that in comparative greenhouse both inthe day and at night and the air temperature difference ranged from5.26to6.64℃. The coefficient ofperformance of the heat pump unit reached4.38~5.17. The heat source temperature of the heat pumpunit was ideal because of the sufficient heat supplied by the AHSRS and the outlet water temperature ofthe evaporator became the dominant factor influencing COPHpof the heat pump unit, meanwhile, theCOPHpvalue decreased with an increase in outlet water temperature of the evaporator. Under thespecific thermal environment of the solar greenhouse, with running the heat pump unit for1.5~3hourper day, the heat collecting efficiency of the AHSRS increased to72.32%~83.62%and the heatcollecting power was156.26~258.05W/m2. The COPSysof the overall system reached5.59and theenergy-saving effect was obvious. Made from cheap materials, the active heat storage-release devicesare much cheaper than traditional solar energy collectors. Compared with ground and water source heatpump units, the AHSRHPS doesn’t need fan coil units or other heat dissipation facilities, meanwhile,deep wells or buried pipes used as heat sources are never needed either. The high performance and low cost make AHSRHPS have a good application prospect.Active heat storage-release associated with heat pump heating system, which has remarkableheating and energy-saving effect, takes the same principle with indirect-expansion solar heat pump,while the technical parameters and processes remain to be further optimized. The system in this studyhad been designed and constructed in the experimental glass greenhouse at Institute of Environment andSustainable Development in Agriculture, Chinese Academy of Agricultural Sciences. The mainobjective was to investigate performance evaluation and thermoeconomic analysis of AHSRHPS forgreenhouse heating in winter; to obtain the exergy loss of the system and components, define thespecific locations and primary causes that exergy losses occur, find the methods and technical routesused to reduce exergy loss, by exergy analysis based on the second law of thermodynamics; and then tooptimize the system further. The heat collecting efficiency of the system ranged from89.0%to100.5%during the test and was much higher than the common solar water heating systems. Increasing the heatconvection area between active heat storage-release device and heated indoor air contributed topromoting the heat collecting efficiency. Coil heat exchanger of heat pump equipment integrated withheat storage water tank avoided power consumption of the circulating water pumps at heat source andload sides, meanwhile the water temperature of heat source side had a relatively high temperature, sothe coefficient of performance of heat pump equipment had reached from5.48to6.08which was muchhigher than traditional water and ground source heat pumps. However, the discharge pressure andtemperature had a tendency of increasing, which resulted in a reduction on COPHp, as the watertemperature at load side increased. Over-high temperature requirements went against for the systemoperation reliability and economy. The exergy loss and efficiency of the overall system was obtained tobe9.77×104kJ and48.7%per day. The component which had the largest exergy loss and the lowestexergy efficiency was active heat storage-release device, followed by heat pump equipment, circulatingwater pump and heat storage water tank, and the exergy loss ratio, in order, were78.7%,8.3%,7.7%,5.3%, the exergy efficiency in order, were25.6%,38.3%,75.0%,88.2%, respectively. Among them, theexergy loss of the active heat storage-release device was mostly caused by the heat transfer betweensolar radiation and circulating water, improving production process could help to decrease the exergyloss to some extent. The exergy losses of the heat pump equipment were mainly caused by heatexchange losses of the heat exchangers and power consumption of compressor, controlling to get theproper evaporating and condensing temperature was the emphasis of this optimization. The primarycause of circulating water pump exergy loss was mechanical friction, pump selection was the key to thisoptimization. The exergy loss of heat storage water tank was mainly caused by the heat loss duringnighttime, and heat-retaining capacity should be enhanced. In the view of the overall system, thecomponents that needed technique optimization most were active heat storage-release device and heatpump equipment, the exergy loss mainly caused by heat exchange with finite temperature difference,decreasing the temperature difference of heat transfer, reducing the quantity of heat transfer process andimproving production technology were the emphasis of optimization. In addition, enhancement of the greenhouse insulation could promote the exergy efficiency of the system during heat release period atnight. Economy, reliability and thermodynamic property should be considered synthetically to select thebest balance during the optimization of the system and its components. Greenhouse warming is the mostimportant part in greenhouse production in winter, which has various heating methods and unevenperformance, and rational use of energy and power savings are the imperatives. This study will providea new thinking for performance evaluation and optimization of systems for greenhouse heating.
Keywords/Search Tags:greenhouses, heat storage, heat release, heat pump, thermodynamics
PDF Full Text Request
Related items