Performance of positive pressure fan-pad cooling system and cooling load model for Chinese solar greenhouse

Weituo, S., Bo, Z., Fan, X., Chao, S., Lu, C. ORCID: 0000-0002-0064-4725 and Wenzhong, G., 2019. Performance of positive pressure fan-pad cooling system and cooling load model for Chinese solar greenhouse. Transactions of the Chinese Society of Agricultural Engineering, 35 (16), pp. 214-224. ISSN 1002-6819

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Abstract

Year-round and efficient production for crop products of high yield, quality and cleanliness is the development trend of the Chinese solar greenhouse (CSG). However, this is limited by unfavorable climate conditions inside the CSG, such as high air temperature in warm seasons. The fan-pad cooling system, normally adopting negative pressure ventilation, has been widely used for greenhouse cultivation. But it generates a large air temperature gradient in greenhouse, limits the greenhouse dimensions. Above deficiencies are more serious in the CSG. Because CSG always has a long distance between the sidewalls, fans and gaskets are installed separately on the sidewalls. In order to overcome the limitations of negative fan-pad cooling system and improve ability of the CSG in coping with high temperature, a positive pressure fan-pad cooling system (PPFPCS) was designed in this study. By using this system, the cold and humid air enters the CSG from bottom of south roof, and then hot air leaves the CSG through roof vents. Performance of the PPFPCS was tested in a CSG without crops in Beijing area during summer. Results showed that in typical summer hot days, the PPFPCS cooperating with external shading net could decrease mean air temperature of the CSG experimental area to 30.7-33.4 ℃, which was lower than that in the CSG contrast area using natural ventilation combination with external shading net by 5.4-11.1 ℃. Air temperature of the CSG experimental area was also lower than that outside the CSG with a temperature difference of 2.4-5.4 ℃. Nevertheless, both natural and mechanical ventilations were tested to have limited cooling capacity to meet climate requirement for CSG cultivation. The PPFPCS could also decrease the CSG air temperature at night, but had a poorer performance in comparison with daytime cooling due to the smaller vapor pressure deficit (VPD). The contrast area of CSG encountered an extreme low air humidity state with mean VPD of 3.4-6.1 kPa. PPFPCS could effectively alleviate low humidity stress: the average relative humidity in CSG experimental area was between 49.8% and 62.3%, which was 13.6% - 21.2% higher than that in CSG control area and 13.6%-24.6% higher than that in outdoor area. Wind velocity inside the CSG experimental area ranged from 0.35 to 1 m/s, which indicated a relative uniform air flow distribution. Cooling efficiency of the PPFPCS was about 91%, which was over 10 percentage points higher than that of the traditional negative pressure fan-pad cooling system. Low temperature of the PPFPCS circling water contributed to the high cooling efficiency. Average water consumption rate of the PPFPCS used for CSG cooling was 0.035-0.079 g/(m2·s) during the test. It had a positive linear correlation with VPD of outdoor air, that is drier outdoor air anticipates larger water consumption and better cooling performance. Both cooling load model of the CSG and selection method for fan-pad cooling system were derived. Cooling load model is the basis for capacity calculation of cooling equipment to be installed. Cooling load of the CSG in summer was 299.1 W/m2. Contribution ratios of convective heat transfer between north wall and indoor air, convective heat transfer between greenhouse floor with indoor air, hot air infiltration, as well as heat transfer between indoor and outdoor air though south roof, north roof and side walls were 11.0%, 73.3%, 1.3% and 14.4%, respectively. The maximum specific ventilation rate of the PPFPCS used for CSG cooling was recommended to be 0.067 m/s. This study can provide technical support for the application of PPFPCS in CSG cultivation and provide theoretical basis for the climate control of CSG production in summer.

摘 要:负压湿帘风机降温被广泛应用于温室生产中,但存在降温均匀性差、限制温室长度及对温室密闭性要求高等不
足。为克服负压湿帘风机降温的局限性,提高日光温室降温能力,该研究设计了日光温室正压湿帘冷风降温系统,其气
流组织方式为湿冷空气从南屋面底部进入日光温室,热空气由顶开窗排出室外。在北京地区无作物的日光温室对系统夏
季降温增湿效果及性能进行试验,试验结果表明:在典型夏季高温白天,正压湿帘冷风降温系统配合遮阳网可将日光温
室试验区内平均气温控制在 30.7~33.4 ℃,比采用自然通风配合遮阳网的对照区低 5.4~11.1 ℃,比室外低 2.4~5.4 ℃,
降温效果良好;夜间系统对温室降温幅度减小。该系统可有效缓解低湿胁迫,日光温室试验区空气平均相对湿度为49.8%~
62.3%,比对照区及室外分别高 13.6%~21.2%和 13.6%~24.6%。室内风速 0.35~1 m/s,气流分布差异性较小。试验条件
下,正压湿帘冷风降温系统的平均降温效率为91%,比传统的负压湿帘风机高10个百分点以上;实际平均耗水量为0.035~
0.079 g/(m2
·s),且耗水量与室外空气水蒸气饱和压差(VPD,vapor pressure deficit)呈正相关(P<0.01,r=0.64)。同时,
研究构建了日光温室冷负荷计算模型及湿帘冷风降温设备合理选型方法,其中冷负荷模型是降温设备选型的基础,普遍
适用于各种日光温室降温方法的研究。计算得到日光温室夏季降温冷负荷为 299.1 W/m2,应安装的正压湿帘冷风降温系
统最大比通风量为 0.067 m/s。该研究为日光温室正压湿帘冷风降温方法的工程应用提供了技术参考,为日光温室安全越
夏生产环境控制提供了理论基础。
关键词:温室;温度;模型;日光温室;正压通风;湿帘风机;降温;冷负荷

Item Type: Journal article
Description: In Chinese with English abstract.
Publication Title: Transactions of the Chinese Society of Agricultural Engineering
Creators: Weituo, S., Bo, Z., Fan, X., Chao, S., Lu, C. and Wenzhong, G.
Publisher: Chinese Society of Agricultural Engineering, Beijing
Date: 16 August 2019
Volume: 35
Number: 16
ISSN: 1002-6819
Identifiers:
NumberType
10.11975/j.issn.1002-6819.2019.16.024DOI
1367601Other
Divisions: Schools > School of Animal, Rural and Environmental Sciences
Record created by: Linda Sullivan
Date Added: 25 Sep 2020 09:28
Last Modified: 25 Sep 2020 09:28
URI: http://irep.ntu.ac.uk/id/eprint/40922

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