Performance of underground heat storage system in a double-film-covered greenhouse

Performance of underground heat storage system in a double-film-covered greenhouse 1 first-author †E-mail:wywzju@zju.edu.cnlxfcjl@163.com †E-mail:wywzju@zju.edu.cnlxfcjl@163.com wywzju@zju.edu.cn 2 †E-mail:wywzju@zju.edu.cnlxfcjl@163.com †E-mail:wywzju@zju.edu.cnlxfcjl@163.com wywzju@zju.edu.cn Department of Biosystem Engineering, Zhejiang University, Hangzhou 310029, China Department of Biosystem Engineering, Zhejiang University, Hangzhou 310029, China School of Mechanical and Electrical Engineering, China Jiliang University, Hangzhou 310018, China School of Mechanical and Electrical Engineering, China Jiliang University, Hangzhou 310018, China An underground heat storage system in a double-film-covered greenhouse and an adjacent greenhouse without the heat storage system were designed on the basis of plant physiology to reduce the energy consumption in greenhouses. The results indicated that the floor temperature was respectively 5.2 °C, 4.6 °C and 2.0 °C higher than that of the soil in the adjacent reference greenhouse after heat storage in a clear, cloudy and overcast sky in winter. Results showed that the temperature and humidity were feasible for plant growth in the heat saving greenhouse. Greenhouse Underground heat storage system Performance of heat storage Energy saving INTRODUCTION 1 Temperature is one of the most important greenhouse climate variables influencing the growth, crop production and product quality of plants. It is a challenge to improve the economic potential by controlling ambient air and soil temperature in greenhouses. However, protected horticulture is an industry with high energy consumption. The UN reported that energy consumption for heating greenhouses amounted to 35% of the production cost (Pan et al., 1999 1999 ). In Europe the energy used for heating and cooling of greenhouse represents approximately 1.5% of the total energy consumption (Santamouris et al., 1995 1995 ; 1996 1996 ). It is obvious that the cost for heating greenhouses in winter is one of the primary components of the production cost limiting the development and economic potential of protected agriculture. The underground heat storage system in greenhouses can store much solar energy underground and retrieve it from the floor when needed. This was applied for conditioning greenhouses and livestock buildings with favorable effects on energy saving (Baxter, 1992 1992 ). However, the storage unit in the existing underground heat storage systems in greenhouses was insulated on the sides and the top but not on the base, so the efficiency of the heat storage system was very low. The poor performance was mainly due to the large heat loss from the uninsulated base of the storage unit (Kurata and Takakura, 1991a 1991a ; 1991b 1991b ). In order to reduce energy consumption and to increase the utilization of solar energy, an underground heat storage system in double-film-covered greenhouse was designed. Previous experimental results showed that this heat storage system had a performed well in heat exchange and storage (Wang, 2004 2004 ). COMPONENTS 1 The system consists of an outer greenhouse, an inner greenhouse, a floor, a ventilator, inlet pipes and outlet pipes etc. as shown in Fig. 1 1 . 1: Ventilator (diameter: 560 mm); 2: Inlet pipe (30 cm×40 cm); 3: Heat storage layer (thickness: 112 mm); 4: Heat exchanging pipe (diameter: 76 mm); 5: Insulated layer (thickness: 10 cm); 6: Basement layer (thickness: 10 cm); 7: Outlet pipe (30 cm×40 cm); 8: Inner greenhouse; 9: Outer greenhouse A Hua-dong type greenhouse was used as a protecting outer cover in the experiment for accumulating heat during the day. This greenhouse was a single arc structure width ridge top height of 4.5 m, a shoulder height of 2.5 m, width of 6 m, length of 24 m from east to west, and covered width a 0.08 mm thick polyethylene film. The inner greenhouse with ridge top height of 2 m and width of 6 m increases heat conservation. The floor was composed of a heat storage layer, an insulated layer, a base layer and heat exchanging pipes. The heat storage layer and the basement layer were built up with concrete. A mixture of concrete and cinder was chosen to build the insulate layer. Heat exchanging pipes were set up in the heat storage layer. They had diameter of 76 mm, total length of 18 m and the distance from the center of the pipes to the floor surface was 15 cm. Forty-five pipes were distributed equally over the greenhouse width so their center distance was 133 mm. A ventilator with a flow of 9000 m 3 /h and a drive power of 250 W was installed in the air entrance (Wang et al., 2003 2003 ; 2004 2004 ; 2005a 2005a ; 2005b 2005b ). PRINCIPLE OF HEAT STORAGE 1 The daytime temperature inside the greenhouse was often very high and even exceeded the limiting values for plant growth due to solar heat accumulation while the temperature of the floor remains lower because the heat is transferred very slowly between the ambient air and the floor. In the system, the ventilator drives the hot air of the greenhouse in the heat exchanging pipes that act as air-to-floor heat exchangers. The excess heat in the air is transferred to the floor by convection during the day and thus decreases the air temperature. The excess heat is stored in the floor simultaneously which increases the floor temperature and the cooler air is returned to the greenhouse. During the night the floor acts as a heat source and the heat transfer from the floor surface to the air is achieved by conduction resulting in increased air temperature. EXPERIMENTAL METHODS 1 No plants were grown in the greenhouse in the experiment. During clear and cloudy days, the covering film of the inner greenhouse was opened at 9:00 a.m. and closed at 15:00 p.m. The ventilator started to operate at 10:00 a.m. and stopped at 15:00 p.m. In days with overcast sky, the covering film of the inner greenhouse was opened from 10:00 a.m. to 14:30 p.m. during which the ventilator was running for heat storage. The ventilator never turned at night. Parameters including the ambient air temperature, the humidity and the floor temperature inside the greenhouse with heat storage and the air temperature and soil temperature in the adjacent reference greenhouse as well as the outside air temperature were measured. ZDR-20 temperature sensors and humidity data recorders were installed along the longitudinal center of the inner greenhouse at distance of 1 m, 5 m, 9 m, 13 m and 17 m from the air entrance respectively at a height of 50 cm above the floor (range: temperature −40 °C~100 °C, humidity 0.1%~100% RH; resolution: temperature 0.1 °C, humidity 0.1% RH). At the same time the air temperatures outside and in the adjacent reference greenhouse were also recorded by ZDR-20 sensors. The temperature of only a half floor was measured because the floor was symmetrical and the air flux in the heat exchanging pipes was distributed equally according to Wang et al.( 2005b 2005b ). Fig. 2 2 shows the location of the fifteen thermocouples TDW2001 in the floor at depth of 10 cm from the reaching points. The thermocouples were placed between the heat exchange pipes. The floor temperature was recorded at 30-min intervals and so was the soil temperature of the adjacent reference greenhouse. RESULTS 1 The measurements conducted at Zhejiang University, started on January 15 and ended on February 8, 2004. The results from the recorded data showed similarities between the clear, cloudy and overcast sky weather conditions. Figs. 3a and 3b 3a and 3b show that the maximum temperature inside the reference greenhouse exceeded 28 °C to 25 °C while the outside temperature was only 10 °C and 12.7 °C in clear and cloudy days respectively. During heat storage, the temperature of the floor increased from 8.3 °C at 10:00 a.m. to 20.3 °C at 3:00 p.m. during the clear day on 2004-01-28 and from 12.9 °C at 10:00 a.m. to 22.1 °C at 3:00 p.m. during the day (2004-01-30) with cloudy sky. Thus an increase of 12 °C to 9.2 °C over 5 h in a clear and cloudy day was realized. The floor temperature was respectively 2.5 °C to 7.5 °C and 3.5 °C to 5.1 °C higher than that of the reference greenhouse. The average temperature difference was 5.2 °C and 4.6 °C respectively. During a day with overcast sky (Fig. 3c 3c ), the floor temperature increased from 13.1 °C at 10:00 a.m. to 18.4 °C at 2:30 p.m. or 5.3 °C over 4.5 h and 1.6 °C to 3.1 °C higher than that of the adjacent reference greenhouse or an average temperature difference of 2.0 °C. It was obvious that the heat storage system increased the floor temperature significantly. This temperature increase will benefit the substrate of the seedling containers on the warmer floor and so improve plant growth. As shown in Fig. 3 3 , the air temperature inside the heat storage greenhouse in the afternoon was lower than that of the adjacent reference greenhouse because the heat was transferred to the floor. These cooler temperatures are favorable for leaf transpiration and CO 2 -uptake. CONCLUSIONS 1 1. The underground heat storage system in double-film-covered greenhouses is a useful approach for energy saving in greenhouses. At the same time, the solar heat will be stored for overnight use. 2. The heat reduction in the afternoon could benefit the physiological growth conditions (transpiration, CO 2 -uptake) of plants in the heat storage greenhouse. 3. During days with clear, cloudy and overcast sky in winter, the floor temperature was respectively increased by 5.2 °C, 4.6 °C and 2 °C compared to the soil in the reference greenhouse. This higher floor temperature will benefit the nursery bed on it and also seedling development. 4. Further research will be undertaken on the relation between the flow of the ventilator and the heat exchange of the system. References DO Baxter . Energy exchanger and related temperatures of an earth-tube heat exchanger in the heating mode . Transactions of the ASAE , 1992 . 35 ( 1 ): 275 - 285 . . Energy exchanger and related temperatures of an earth-tube heat exchanger in the heating mode K Kurata , T Takakura . Underground storage of solar energy for greenhouse heating. I. Analysis of seasonal storage system by scale and numerical models . Transactions of the ASAE , 1991 . 34 ( 5 ): 563 - 569 . . Underground storage of solar energy for greenhouse heating. I. Analysis of seasonal storage system by scale and numerical models K Kurata , T Takakura . Underground storage of solar energy for greenhouse heating. II. Comparison of seasonal and daily storage system . Transactions of the ASAE , 1991 . 34 ( 5 ): 2181 - 2186 . . Underground storage of solar energy for greenhouse heating. II. Comparison of seasonal and daily storage system Q Pan , ZD Huang , CW Ma . Study on the energy conservation of huabei-type multispan plastic greenhouse and its practice . Transactions of the CSAE , 1999 . 15 ( 2 ): 155 - 159 . (in Chinese) . Study on the energy conservation of huabei-type multispan plastic greenhouse and its practice M Santamouris , G Mihaladakou , CA Balaras , A Argriou , D Asimakopoulos , M Vallindras . Use of buried for energy conservation in cooling of agricultural greenhouses . Solar Energy , 1995 . 55 ( 2 ): 111 - 124 . DOI: 10.1016/0038-092X(95)00028-P http://doi.org/10.1016/0038-092X(95)00028-P . Use of buried for energy conservation in cooling of agricultural greenhouses M Santamouris , G Mihaladakou , CA Balaras , DN Asimakopoulos . Energy conservation in greenhouse with buried pipes . Energy , 1996 . 21 ( 5 ): 353 - 360 . DOI: 10.1016/0360-5442(95)00121-2 http://doi.org/10.1016/0360-5442(95)00121-2 . Energy conservation in greenhouse with buried pipes YW Wang . Study on the Underground Heat Storage and Floor Broad Irrigation System in Greenhouse , 2004 . : Hangzhou Zhejiang University , (in Chinese) . Study on the Underground Heat Storage and Floor Broad Irrigation System in Greenhouse YW Wang , XW Miao , SR Cui , XF Liang . Heat exchanging characteristics of underground heat storage system in greenhouses . Transactions of the ASAE , 2003 . 19 ( 6 ): 248 - 251 . (in Chinese) . Heat exchanging characteristics of underground heat storage system in greenhouses YW Wang , XW Miao , SR Cui . Research on design of the underground heat storage system in greenhouse . Journal of Agricultural Mechanization Research , 2004 . ( 1 ): 137 - 139 . (in Chinese) . Research on design of the underground heat storage system in greenhouse YW Wang , XW Miao , SR Cui , XF Liang . Study on the heat storing and heating performances of an underground heat storage system in greenhouse . Transactions of the CSAM , 2005 . 36 ( 1 ): 72 - 75 . (in Chinese) . Study on the heat storing and heating performances of an underground heat storage system in greenhouse YW Wang , XF Liang , XW Miao , SR Cui . Study on the distribution of airflow velocity within heat exchange pipes and heat saving performance of the underground heat storage system in greenhouse . Journal of Zhejiang University (Agriculture & Life Sciences) , 2005 . 31 ( 3 ): 311 - 314 . (in Chinese) . Study on the distribution of airflow velocity within heat exchange pipes and heat saving performance of the underground heat storage system in greenhouse 10.1631/jzus.2006.B0279 Project (No. 20050912) supported by the Education Department of Zhejiang Province, China 2005-08-31 2005-11-13 2006-03-13 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 04 07