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Using Frigid North Sea Water to Heat a City

Winters in Norway can be bitterly cold, with average air temperatures below freezing throughout much of the season and the waters of the North Sea dipping as low as 43°F. Despite these frosty conditions, when the city of Drammen, a community of 60,000 people located on the Drammen Fjord near the capital city of Oslo, needed hot water at 194°F for a new district heating system serving local residents and businesses, it turned to the frigid North Sea as a renewable energy source.

Installing heat pumps to extract heat from water or air is increasingly popular in Europe, largely because the heat they deliver far exceeds the energy they consume, greatly reducing the reliance on fossil fuels and the need for additional renewable energy sources. In fact, the European Commission has designated heat pumps a renewable technology for heating and cooling.

Drammen had several additional goals in mind for this project: the highest coefficient of performance possible; a technology solution with low annual operating and maintenance costs; and a system that used a non-ozone depleting refrigerant with zero global warming impact. Such requirements seemed unachievable, because most industrial heat pumps in Europe use R-134A.

One of the few refrigerants that could meet all of Drammen’s efficiency and environmental requirements was ammonia (R-717), which is most commonly used by the food and beverage industry for process cooling and refrigeration. Ammonia does not contribute to ozone depletion or global warming, but it also had never been used in a high-temperature industrial heat-pump application of this design. In fact, not long ago the application was deemed impossible by the International Energy Agency’s Heat Pump Centre, which said there were no suitable high-pressure compressors available to make using ammonia a reality for high-temperature industrial heat pumps.

The application
Emerson engineers, collaborating with project partner Star Refrigeration, utilized the Vilter single screw compressor used for industrial refrigeration.

The single screw compressor is a rotary, positive displacement compressor that incorporates a main screw and two gate rotors. Compression of the gas is accomplished by the engagement of the two gate rotors with the helical grooves in the main screw. The drive shaft imparts rotary motion to the main screw which in turn drives the intermeshed gate rotors.

The compressor is comprised of three fundamental components that rotate and complete the work of the compression process. This typically includes a cylindrical main screw with six helical grooves and two planar gate rotors, each with 11 teeth. The rotational axes of the gate rotors are parallel to each other and mutually perpendicular to the axis of the main screw.

It delivers higher performance than twin screw compressors and has fewer moving parts than reciprocating compressors. Besides ultra-low bearing wear, the balanced design decreases vibrations and sound levels.

For this specific application, water is extracted from a fjord of the North Sea, which has an average annual temperature of 45°F. The heat is absorbed through the evaporation of ammonia refrigeration in three two-stage ammonia systems. Each ammonia system includes a 2,100-cfm single screw compressor on the low stage operating at 36°F suction (53 psig) and 135°F discharge (335 psig) and a 790-cfm single screw compressor on the high stage operating at 195°F condensing (735 psig).

The facility is designed to provide 48 million Btuh of heat to the residents and businesses of the city as a hot-water utility at 194°F, with a total power consumption of 6,000 hp, for an average COP of 3.15. The compressor packages were factory assembled, and the heat exchangers were erected and piped on site.

Utilizing this compressor technology and ammonia enabled Drammen to achieve all of its goals: a non-ozone depleting and zero global warming impact system that delivers higher temperatures and efficient performance. In addition, the balanced radial and axial force design of the single screw compressor reduces stress on the unit’s bearings, resulting in very low operating and maintenance costs.

The single screw compressor technology is proving extremely adaptable in meeting the needs of other customers seeking increased performance efficiency.
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