The Performance of a High Solar Fraction Seasonal Storage District Heating System – Five Years of Operation

The Drake Landing Solar Community in Okotoks, Alberta, Canada utilizes a solar thermal system with borehole seasonal storage to supply space heating to 52 detached energy-efficient homes through a district heating network. Systems of similar size and configuration have been constructed in Europe, however, this is the first system of this type designed to supply more than 90% of the space heating with solar energy and the first operating in such a cold climate (5200 degree C-days). Solar heat captured in 2293 m2 of flat-plate collectors, mounted on the roofs of detached garages, is stored in soil underground and later when needed for space heating, is extracted and distributed through a district system to each home in the subdivision. Independent solar domestic hot water systems installed on every house are designed to supply more than 50% of the water heating load. Annual greenhouse gas emission reductions from energy efficiency improvements and solar energy supply exceed 5 tonnes per house. The seasonal storage utilizes approximately 34,000 m3 of earth and a grid of 144 boreholes with single u-tube heat exchangers. The system is configured to maintain the centre of the field at the highest temperature to maximize heating capacity and the outer edges at the lowest temperature to minimize losses. A short -term thermal storage consisting of 240 m3 of water is used to interconnect the collection, distribution and seasonal heat storage subsystems. The system has undergone detailed monitoring since it was brought into service in July 2007 to characterize its performance and to improve the TRNSYS model employed in its design. A solar fraction of 97% in its fifth year of operation, convincingly confirms the design target, a solar fraction of more than 90% in year five, has been met. This paper describes the system and its operation, presents 5 years of measured performance and compares those results against the TRNSYS predicted performance for the same period.