C08 -- Seasonal Solar Thermal Energy Storage for Balancing Cold Climate Geo-exchange Systems PDF

A closed-loop, vertical borehole ground heat exchanger (GHX) serving a residential water-to-air heat pump system located in Winnipeg, Manitoba, Canada became frozen due to inadequate GHX sizing and/or borehole spacing for the heating dominant load profile. The heat pump entering water temperature (EWT) regularly dropped below the lower limit of the heat pump, leading to excessive use of electrical back up heating. Polypropylene solar thermal collectors are used to thaw and reheat the GHX during the summer and shoulder seasons. A direct digital controller is used to control, monitor, and optimize the hybrid system. Algorithms were developed to (1) maximize the amount of solar energy that is collected from the panels, (2) limit excess pumping energy during times when marginal solar collection occurred, and (3) limit instantaneous and seasonal overheating of the GHX. These algorithms were critical to the success of the experiment. As hypothesised, the solar re-heat system has started to thaw the GHX and has increased the average temperature of the ground surrounding the boreholes. The system design and control algorithms can be used to increase the efficiency and decrease the capital cost of heating dominant ground source heat pump systems in a safe and dependable manner.