Potential for using water reservoirs as heat sources in heat pump systems

There are many water reservoirs intended for agricultural applications or flood control in western Japan. These reservoirs may offer energy savings because their water is usually at a higher temperature in winter and lower temperature in summer than that of ambient air. The present work assesses whether thermal energy stored in these reservoirs can be used as heat sources for heat pump systems. Water temperatures were measured over 1 year at six different reservoir depths. During summer there was thermal stratification, in which water temperatures were widely distributed between 10 °C and 30 °C. During winter, active natural convection provided a stable vertical temperature distribution. A one-dimensional fluid dynamics model enabled reproduction of the measured water temperatures. Then, based on an index for cooling in summer (Pc) and another for heating in winter (Ph), the potential of using 16 reservoirs as heat sources was evaluated. These indices are related to differences between ambient air temperatures and water temperatures calculated near reservoir bottoms. Winter weather conditions were found to statistically affect both Pc and Ph. Weather conditions have little influence on water temperatures near reservoir bottoms in summer. Cold winters produce large values of Pc.