| The Chinese Solar Greenhouse(CSG), is characterized by a lean-to south-facing roof, a removable blanket for night insulation and a solid north wall. CSG stores solar radiation for night heating so it is energy saving at low cost. However, one disadvantage of CSG is low land utilization efficiency which is only 30%-45% because of the needed distance between the separate greenhouses to prevent shadowing. Moreover the small unit volume results in intensive labor and low mechanization. And the capability of greenhouse to store and release solar energy is low which lead to low air temperature and low rhizosphere soil temperature.To solve these problems a new-type large-scale greenhouse was designed. It is tunnel type, wide span with steel frame, and oriented south-north. In this way, the space between greenhouses can be shortened and the land utilization efficiency can be increased up to 87.4%. It also has high night insulation due to both a removable external blanket and an internal thermal screen. An active heat storage-release system(AHS) was added in greenhouse to promote solar energy collecting performance and air temperature inside greenhouse at night. In AHS system simple thermal collectors gather excess solar energy by flowing water between two sheets of black plastic foil, depositing the energy in water storage tanks. When greenhouse heating is needed, the same heating collectors are used for heat releasing. To further improve the thermal performance of the new-type large-scale greenhouse, heat pump(HP) was incorporated in AHS system to form a new design-defined as AHS-HP system. And another direct expansion solar collector assisted with HP heating system was also designed. The results were as followed:1) The large-scale greenhouse(experimental) and CSG(reference) located in Beijing, were compared in terms of indoor air temperature, relative humidity(RH), construction consumption and the heat collecting and releasing by AHS in winter. The average temperature in experimental greenhouse was above 10℃ during the whole night. Nighttime air temperature in large-scale greenhouse was 1.2~3.1℃ higher whether on sunny day or on cloudy, on average, compared with the reference greenhouse. And it was 12.5~19.3℃ higher than the outdoor at night. The RH in large-scale greenhouse was 7%~10% lower than in CSG which can decrease the incidence of plant diseases. Compared with traditional heating method using fossil fuels, the AHS achieved 47% energy savings. The coefficient of performance(COP) of AHS was 3.4~4.2 and the average daily electricity consumption of the AHS is 0.013 kWh/m~2. The construction cost of the large-scale greenhouse is 307.2 yuan/m~2, which is 144.5 yuan/m~2 lower than CSG.2) HP was incorporated in AHS system to form a new design-defined as AHS-HP. The HP was used to regulate the water temperature in three storage tanks that serve as heat reservoir for the AHS system. By regulating the water temperatures, the heat collecting and releasing can be done more efficiently than without HP. During night when the circulating water temperature was too low to heat the greenhouse, HP was running to improve the low grade heat energy and increase the temperature between the air and circling water. Two large-scale greenhouses located in Beijing, one with AHS-HP(experimental greenhouse) and one without(reference greenhouse) were compared in terms of indoor air temperature and electricity consumption in winter. Nighttime air temperature in the experimental greenhouse was increased by 2.0℃~4.5℃, on average, compared with the reference greenhouse. The COP of HP was 4.8~6.3 during the daytime and 3.1~4.9 at night. The COP of the AHS-HP system was 3.2. The average daily electricity consumption of the AHS-HP is 0.04 kWh/m~2.3) In the direct expansion solar collector assisted with heat pump heating system, solar collecting planes were installed in outside as the evaporation side of HP, using alloy and circulating the R22 to collect solar energy. HP was running to collect energy in water tank during daytime. During night when rhizosphere soil temperature was under the set-point, water pump was running to circulate hot water through pipes under the matrixes to heat rhizosphere soil. The experimental greenhouse(using the heating system) and the reference greenhouse(without heating system) were compared in terms of indoor air temperature, rhizosphere soil temperature and electricity consumption in winter. Nighttime rhizosphere soil temperature in experimental greenhouse was above 15℃, and increased by 8.1℃, on average, compared with the reference greenhouse. Nighttime air temperature in the experimental greenhouse was increased by 2.4℃~2.5℃, on average. On the different weather condition the COP of the HP was 2.0 during the daytime. The average daily electricity consumption of the system is 0.06 kWh/m~2.The large-scale greenhouse with active heat storage is lower cost, better heat preservation, which is worthy of popularization and application. Greenhouse warming is the most important part in greenhouse production in winter, AHS-HP heating system and direct expansion solar collector assisted with heat pump heating system are used in greenhouse, improving the heat collecting during the daytime and heat releasing at night. The low cost of solar energy assisted with HP has the potential to be applied in solar greenhouses if it is properly designed. |
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