- Agricultural canals can be used as heat sources for heat pump as they are 15 times highly efficient than underground installation -
NARO has revealed that installing a sheet type heat exchanger in a water flow such as agricultural irrigation canals can exchange heat approximately 15 times more efficiently than installing it underground. Agricultural irrigation canals widely used in rural areas can be effectively used as heat sources for heat pumps by this research result. It also helps to reduce the consumption of energy for cooling and heating of greenhouses and its operational costs.
In agricultural facilities, heat pumps have been utilized for cooling, heating, and dehumidification to reduce energy consumption and operational costs by performing heat exchange from the air and underground. Currently, the method of installing heat exchangers in the air or underground is the mainstream, but in recent years, research is carried out for the installation in water, which has a higher exchange efficiency.
In rural areas, there are irrigation ponds and wells (still water: velocity of about 0 m / min) and irrigation canals (flowing water: at least about 30 m / min for main irrigation canals). By using a sheet type heat exchanger submergible in water, we clarified the differences in heat exchange efficiency of heat pumps with various flow rates and installation methods using a full-scale hydraulic model.
As a result, when the heat exchanger is installed in flowing water, the water around the heat exchanger constantly flows, so in either case of cooling or heating operation of the heat pump, it was found that heat could be exchanged more efficiently than when installed underground or in still water. During cooling operation, the heat exchange efficiency when the heat exchanger is installed in flowing water is: about 2.5 times that of still water installation, about 15 times that of underground installation, and about 25 times that of conventional slinky heat exchangers that were installed underground.
This result showed that agricultural irrigation canals widely distributed in rural areas can be effectively used as heat sources for heat pumps. This result will help to reduce the energy consumed for cooling and heating the greenhouses, reduce operational costs and also expected to be used in heat-consuming facilities adjacent to agricultural irrigation canals, such as detached houses, apartment houses, convenience stores, and commercial facilities.
Fig.1 Sheet type heat exchanger
Fig. 2 Experimental System
Fig. 3 Relationship between flow velocity in canal and heat transfer liquid flow rate
and heat transfer co-efficient (cooling operation)
Table 1 Comparison of heat transfer co-efficient depending on
installation method (cooling operation)
Fig. 4 Case when heat exchanger is integrated with expanded metal
and installed on the side wall of canal
Fig. 5: Case when a blocking plate is installed at the upstream end
Fig. 6: Relationship between the installation interval between the heat exchanger
and the sidewall of canal and the heat transfer co-efficient (during cooling operation)