WINTER PRODUCTION OF GREEN ASPARAGUS BY USING SURPLUS HEAT FROM MACHINERY ROOM AND USED HOT WATER FROM HOTEL'S SPA

Obtaining of heat source is a serious problem in greenhouse production during winter because of expensive oil cost. Japan is rich in hot springs and hot water from the hot spring is used for spa in hotels. Mostly used hot water from the spa is drained into the river. However, the temperature of the used hot water is approximately 30°C, and has a potential to warm the enclosed places. In addition, surplus heat is emerged from the machinery room which controls the environment in the spa. The experiment was carried out in Yubari city, Hokkaido, 470 m above sea level and about 2 m of snow covers in winter. Small plastic house with 4 cultural beds (0.9 m x 6.3 m x 0.6 m, W x L x H,) was built close to the area of the spa. More than 30°C surplus heat air in the machinery room next to the spa was vacuumed into the plastic house. The plastic house was covered with only one plastic sheet (0.15 mm of thickness) and kept the temperature above 0°C even if -17°C in outside. The used hot water from the spa flowed to storage tank (1,000 L) set up in underground between the spa area and the plastic house. In the storage tank, anti-freeze solution was warmed by heat exchange with the used hot water from the spa. It was passed into the pipe installed in the bottom and side of the culture bed to regulate the soil temperature at 20°C. Average temperature 10-15°C was shown in the soil of culture bed in mid winter, 2011. One year old asparagus root stocks were planted densely in the culture bed on Dec. 30, 2010, and emerged spears from the rootstocks were harvested from Jan. 24. Similar yield per rootstock was recorded compared with the spear production in the room controlled at 20°C. Asparagus root stocks were planted 4 times, every 2 weeks, and spear harvest continued till March 8 for hotel use. The present system using the surplus heat and the used hot water made it possible to produce asparagus spears in a small energy compared to conventional electric or oil heater.