Design of low temperature heating system in solar greenhouse using capillary tube mat exchanger

Abstract: Solanaceous fruit vegetables are able to be planted in a solar greenhouse without a heating system during winter in northern China. But low temperatures under 8℃ often happen at night in a solar greenhouse. So a heating system in a solar greenhouse is a crucial technology for agricultural fresh produce such as vegetables in winter in order to improve the inner environment. A new heating technique using a capillary tube mat exchanger was introduced to a solar greenhouse. Thermal characteristics and property of flow resistance were tested under 0.0306 kg/s, 0.0222 kg/s and 0.0139kg/s water flow rates and 20~50℃ excess temperature in a laboratory. The diameter and wall thickness of the capillary tube was 4 mm and 1 mm. The length, width, and distance between neighboring capillary tubes were 2800 mm, 600 mm, and 20 mm. Function curves of heat flux and local resistance under different water flux were obtained. The influence of water flux could be ignored, but set position had a great influence on the results. The heat fluxes were 258~323 W/m2 and 307~381 W/m2 when sample capillary tube mat exchangers were set with vertical position and horizontal position under an excess temperature of 25~30℃. In order to test the feasibility of the capillary tube mat exchangers in a solar greenhouse, a low water temperature heating system using capillary tube mat exchangers was designed for a solar greenhouse with a 518.5m2 covered area seated in Beijing Changping district. 120 m2 capillary tube mat exchangers were installed along the back wall and side wail in the solar greenhouse. A biomass boiler supplied the 60℃ heat water. Low 40℃ water was supplied to the capillary tube mat exchangers using a water mixing valve. Contrast tests were conducted between two same structure solar greenhouses in winter during the year 2012 through 2013, during which one solar greenhouse with a heating system and the other without were monitored. Soil temperature at the 15cm depth and canopy temperature were measured every 10 minutes during the experiment in the two greenhouses. Average night canopy temperature and soil temperature at 15cm depth under ridge cultivation could be kept at 10℃ and 15℃ above while using 125 kg of straw biomass fuels every night. The average night canopy temperature, lowest temperature, and soil temperature at 15 cm depth under ridge cultivation during the experiment were 9.0℃, 6.5℃, and 5.7℃ higher than compared to a solar greenhouse.