Alternative and sustainable heat production for drinking water needs in a subarctic climate (Nunavik, Canada): Borehole thermal energy storage to reduce fossil fuel dependency in off-grid communities

摘要

The development of renewable energy technologies in the Arctic faces technical barriers mainly related to extremely cold temperature. Moreover, storage issues to bridge the gap between supply and demand are more compelling than in temperate climates. Can underground thermal energy storage be efficiently used in such a cold environment to offer a viable seasonal storage alternative? This working hypothesis was tested by designing and simulating for the first time a borehole thermal energy storage facility in a subarctic climate. A system comprising a 1000 m(2) gross solar area and one hundred 30-m-deep borehole heat exchangers was simulated in TRNSYS to cover part of the heating demand of a pumping station that supplies drinking water in Kuujjuaq (Northern Quebec, Canada). The Nunavik capital is characterized by more than 8000 heating degree days below 18 degrees C and average spring-summer solar radiation of 4.6 kWh m(-2) d(-1). Despite the presence of discontinuous scattered permafrost in the area, the study site is free of frozen ground, likely due to a talik that developed around a nearby lake. A number of scenarios reveal that solar fraction of 45 to 50% and heat recovery of more than 60% can be achieved by the 3rd year of operation, resulting in annual savings of 7000 l of regular diesel consumption. A 50-years life-cycle cost analysis demonstrates that a specific incentive program can guarantee similar net present cost and levelized cost of energy compared to the current diesel-dependent situation, or better if electricity comes from renewable source. An additional 10% loss of thermal energy occurs when groundwater advection is a factor. FEFLOW simulations demonstrate that square-shaped storage together with a newly-proposed borehole connection design can reduce advection heat loss by 60% and improve the overall performance of the system. This work validates the technical viability of underground thermal energy storage in subarctic climates and indicates it could help reduce fossil fuel consumption in remote arctic regions across the world. Moreover, the novel type of borehole connection designed for this study can be useful in seasonal storage systems facing low heat recovery due to groundwater flow, regardless of climate.