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Water-cooled MV motors drive 'largest' heat pump system

11 June, 2013

A Scottish company has supplied the heat pump system for the world’s largest natural heat pump district heating installation, which is being commissioned in Norway. The installation includes six ammonia compressors driven by large water-cooled MV motors whose waste heat is being recovered to maximise efficiency.

The district heating system extracts energy from seawater to deliver more than 13MW of heat to Drammen, a community of around 60,000 people living on the Drammen Fjord, near Oslo. Hot water is pumped to several thousand homes and businesses in the city through a network of underground pipes.

The heat pump system, supplied by Star Refrigeration of Glasgow, uses heat from the seawater to raise the temperature of an ammonia refrigerant via a heat exchanger. The now-gaseous ammonia is then compressed by the motor-driven compressors, raising its temperature to around 90°C. This heat is transferred to the hot water supply via another heat exchanger.

The ammonia compressors are powered by six ABB 11kV motors – three rated at 1.25MW, and three at 660kW. The motors, which are usually air-cooled, have been fitted with air-to-water single-tube coolers. To optimise the cooling of the motor while delivering the best heat output, the cooling water contains 30% glycol.

Three of the ammonia compressors and their 11kV motors

Water enters the coolers at 30°C. Every hour, about 3.1m3 of water is fed into the 1.25MW motors, and leaves them at 46°C. The 660kW motors have an inflow of 2.8m3 per hour and they raise its temperature from 30°C to 42.2°C.

The motors have to handle frequent starts and stops to accommodate the varying heating demands during the day and year.

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