Wednesday, September 23, 2009

Heat Storage at Drake Landing Solar Community

The Drake Landing Solar Community (DLSC) is a master planned neighbourhood in Canada that has successfully integrated energy efficient technologies with a renewable, unlimited energy source - the sun.

The first of its kind in North America, DLSC is heated by a district system designed to store abundant solar energy underground during the summer months and distribute the energy to each home for space heating needs during winter months.

The system is unprecedented in the World, fulfilling most of each home’s space heating requirements from solar energy and resulting in less dependency on fossil fuels.

A typical Canadian home's energy requirement can be broken down into 60% for space heating, 20% for domestic hot water heating and 20% for appliances, lights, and other. Estimates for homes in the Drake Landing Solar Community indicate that, in a typical year, over 90% of the energy used for space heating will come from solar energy. Even in an unusually cold winter and spring, 80% of the required heat is expected to come from the sun.

Seasonal Heat Storage

The Drake Landing Solar Community use a borehole thermal energy storage (BTES) system for storing large quantities of solar heat collected in summer for use later in winter. It is basically a large, underground heat exchanger.

A BTES consists of an array of boreholes resembling standard drilled wells. After drilling, a plastic pipe with a “U” bend at the bottom is inserted down the borehole. To provide good thermal contact with the surrounding soil, the borehole is then filled with a high thermal conductivity grouting material.

The BTES in the Drake Landing Solar Community (DLSC) consists of 144 boreholes, each stretching to a depth of 37 meters and planned in a grid with 2.25 meters between them. The BTES field covers 35 metres in diameter. At the surface, the U-pipes are joined together in groups of six that radiate from the center to the outer edge, and then connect back to the Energy Centre building. The entire BTES field is then covered in a layer of insulation and then soil – with a landscaped park built on top.

When solar heated water is available to be stored, it is pumped into the centre of the BTES field and through the U-pipe series. Heat is transferred to the surrounding soil and rock, and the water gradually cools as it reaches the outer edge and returns to the Energy Centre.

Conversely, when the homes require heat, cooler water is pumped into the edges of the BTES field and as the water flows to the centre it picks up heat. The heated water passes to the short-term storage tank in the Energy Centre and is then circulated to the homes through the district heating loop. All pumps and control valves are housed in the neighbouring Energy Centre building.

Even with sunny Alberta weather, it will take approximately three years to fully charge the BTES field. In the first years of operation, the field will operate at relatively low temperatures, and the recoverable energy will be largely depleted before the end of the heating season. However, after a few years of operation, the core temperature of the BTES field will approach 80°C by the end of summer, with sufficient heat for almost an entire heating season.

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