Experimental Study On Improving Thermal Environment In Solar Greenhouse With Active-passive Phase Change Thermal Storage Wall

The solar greenhouse,an important part of facilities agriculture,is a significant facility for the production of anti season vegetable growth in the north.It has played an active role in ensuring the long-term,safe and stable supply of vegetables for urban and rural residents,and made great contributions to the continuous supply of vegetables in winter in China.Lanzhou region is cold in winter,and there is a large temperature difference between the inside and outside of the greenhouse,however,the heat storage/release performance of the wall has a great influence on the indoor thermal environment,but most of local traditional solar greenhouses absorb solar energy using passive method so that heat storage capacity is limited.It cannot fundamentally satisfy the indoor thermal environment of crops growth in greenhouses.In order to improve the thermal environment in greenhouses,enhance the heat storage/release performance of the wall and make full use of solar energy.This paper focuses on the construction method,the methodology evaluating thermal performance and the storage/release performance of the active heat storage system of the wall.The studies are as follows:Firstly,the thermal performance of existing walls,including soil wall,brick wall,and phase-change material wall,is analyzed by using evaluation indexes,such as indoor air temperature,temperature distribution and heat storage /release capacity of the wall.The results show that the existing typical walls have a temperature stability zone,and the heat storage capacity is limited.By increasing the thickness of the wall,the heat storage capacity is increased,but the land utilization rate is lower.In addition,due to the influence of outdoor environment,the inside thermal environment for crop growth cannot be meet.To solve this problem,a novel active-passive wall with phase change material in solar greenhouse is designed.The combination of active and passive methods improves the indoor thermal environment in solar greenhouse.Secondly,an experimental setup is built in Lanzhou to compare the active-passive solar greenhouse and tradition greenhouse.Using indoor air temperature and soil temperature inside the solar greenhouse,surface temperature and internal temperature inside the wall as the thermal performance evaluation index,combined with test data,the temperature improvement effect of active-passive greenhouse relative to tradition greenhouse under different weather conditions is analyzed.The temperature of the inlet and outlet of the tank and the heat storage coil with PCM in active-passive greenhouse is also tested and analyzed.Thirdly,the result shows that,in sunny days,during the turning on and off the insulation blanket,compared to the tradition greenhouse,the active-passive solar greenhouse’s indoor air temperature increases 4.5℃ and 2.4℃;the average temperature on inner surface of the wall increases 4.6℃ and 2.5℃;the soil temperature on surface increases 1.9℃ and 1.6 ℃,the soil temperature at 20 cm depth increases 1.8℃ and 1.5℃,respectively;in the whole day,the total heat storage of the active-passive solar greenhouse increases 42.1%and the suitable time for crops growth increases 6h.The average temperature of active-passive solar greenhouse is 4.0°C higher than that of tradition greenhouse at 120 mm from the inner surface of the wall.The average temperature of active-passive solar greenhouse is 2.4°C higher than that of tradition greenhouse in the height direction at 320 mm from the inner surface of the wall.In cloudy days,during the turning on and off the insulation blanket,compared to the tradition greenhouse,the active-passive solar greenhouse’s indoor air temperature increases 2.2℃ and 1.2℃;the average temperature on inner surface of the wall increases 2.1℃ and 1.3℃;the soil temperature on surface increases 1.1℃ and 0.9 ℃;the soil temperature at 20 cm depth increases 1.0℃ and 0.8℃,respectively;in the whole day,the total heat storage of the active-passive solar greenhouse increases 28.7%and the suitable time for crops growth increases 4h.The average temperature of active-passive solar greenhouse is 2.6°C higher than that of tradition greenhouse at 120 mm from the inner surface of the wall.Fourthly,from turning on the insulation blanket until the beginning of active heating,the average temperature of tank E is 0.5℃ higher than that of tank B;the average temperature of heat storage pipes inlet is 8.1℃ higher than that of outlet.From the beginning of active heating until turning on the insulation blanket in next day,the average temperature of tank B is 0.6℃ higher than that of tank E;the average temperature of heat storage pipes inlet is 3.3℃ higher than that of outlet;the result shows the heat storage/release capability of PCM is well.The solar greenhouse with active and passive phase change wall proposed in this paper can make full use of solar energy,increase the internal temperature of the wall,greatly improve the thermal environment inside the greenhouse,ensure the suitable temperature of crops growth.It provides a reference for modernize development for the solar greenhouse in Lanzhou.