Energy and Exergy Efficiency of a Packed-bed Heat Storage Unit for Greenhouse Heating

Abstract

In this research, solar energy was stored daily using the volcanic material with the sensible heat technique for heating the tunnel greenhouse of 120 m 2. The external heat collection unit consisted of 27 m 2 of south-facing solar air heaters mounted at a 55° tilt angle. The dimensions of the packed-bed heat storage unit were 6 m by 2 m by 0·6 m deep. The packed-bed heat storage unit was built under the soil at the centre of the tunnel greenhouse. The heat storage unit volume per square metre of ground surface of the tunnel greenhouse was 0·06 m 3, while the storage volume per square metre of the heat collection unit was about 0·27 m 3. The heat storage unit was filled with 6480 kg of volcanic material equivalent to 54 kg of heat storage material per square metre of the greenhouse ground surface. Energy and exergy analyses were applied in order to evaluate the system efficiency. During the charging periods, the average daily rates of thermal energy and exergy stored in the heat storage unit were 1242 and 36·33 W, respectively. It was found that the net energy and exergy efficiencies in the charging periods were 39·7 and 2·03%, respectively. During the discharging periods, the average daily rates of the thermal energy and exergy recovered from the heat storage unit were 601·3 and 20·9 W, respectively. The results showed that 18·9% of the total heating requirement of the tunnel greenhouse was obtained from the heat storage unit.