The World's Largest “Natural” District Heat Pump

Drammen is a town 40 km south west of Oslo (Norwegian Capital). During the last decade it has gone through a major transformation from being a rundown industrial town to a newly developed town centre with new hospital, housing, ice rink, hotels and shopping centre.

Image 1. External view of Drammen city

Drammen Fjernvarme KS was established in 1999, and is owned by Energiselskapet Buskerud (Buskerud Energy Company) and Fortum Holding. The same year Drammen Municipality decided to make connection to the district heating system mandatory in the concession area. This means that every new building larger than 1000 m²has to be built with a water-based heating system connected to the district heating system. Today the area that receives district heating has been expanded, and includes most of central Drammen.

These new developments have all been connected to a district heating network. The first district heating plant in Drammen was installed in 2002 using 8 MW biomass boilers.

Knowing that the European Commission has designated heat pumps a renewable technology for heating and cooling, Drammen decided to use heat pumps and had several additional goals in mind for this capacity increase project:

·       The supply water temperature from the heat pump  would be 90°C

·       The highest coefficient of performance (COP) possible the ratio of heat extracted compared to energy consumed.

·       A technology solution with low annual operating and maintenance costs.

·       A system using a non-ozone depleting refrigerant with zero global warming impact.

With the second phase of the district heating network extension being a 13 MW of heat pump duty (for the base load) and additional 2 x 30 MW gas fired boiler (backup for the peak duties) have been installed. The maximum network peak heat demand is 45 MW duty.

Installed system

The ammonia heat pump that has been installed on site consists of 3 x 2 stage Vilter single screw compressor systems in series each with a heating duty of approximately 4.5 MW.

Image 2. Internal view of the Drammen installation building: machine room

The installation design and these heat pumps have been manufactured by Star Refrigeration Ltd, located in United Kingdom.

The supply temperature of the district heating water varies across the year depending on the heat demand. In the summer time where there is a very small demand (less than 2 MW) the supply water temperature is 75ºC, when the ambient temperature falls and there is an increase in heating demand, the supply water temperature increases up to 120ºC at peak load. The return water temperature from the district heating loop is very steady at 60ºC to 65ºC all year around. When the gas boilers are being utilised they are working on a constant flow with temperature difference of 10ºC between inlet and outlet. The water is then being mixed with the district heating water to achieve the desired outgoing temperature. To optimise the performance of the heat pump it was important to have variable flow system where the water is taken directly from the district heating return line as every degree subcooling is important and any degree overheating is wasted energy.

The heat source for the heat pump is sea water. Norway has a famous rocky coastline. The thermodynamic beauty of this landscape is that the water gets very deep just off the coast. When taking in the water at 40 m depth there is a constant water temperature of 8°C to 9°C most of the year. At this depth the water temperature is not affected by changes in the air temperature from +30°C in the summer to -20°C in the winter. The water intake pipe runs 800 m into Oslo Fjord and the return pipes are 600 m long to ensure that the 4°C outlet water is not mixed with the inlet water. The seawater pumps are situated on land but below sea level.

The seawater is cooled directly in spray chillers, where ammonia is sprayed across titanium pipes with the seawater inside. 


The installed heat pumps cover 85% of the Drammen district heating heat demand. The heat supplied is modulated from 2 MW during summer to 13.2 MW in winter with a peak up to 15 MW using gas boilers. Even with such a large temperature difference between the sea watrer as heat source and the district heating water loop, the heat pump installation can deliver a pretty constant COP all over the year of 3.05 for the water at 90°C.

The table below summarises the summer and winter majors operating conditions




Heating Capacity

2 MW

13.2 MW

Heat source

Sea water cooling from 8°C to 4°C

Heat sink-

Water loop temperature

60 °C to 80°C

60°C to 90°C





On average, the annual energy supply is 67 GWh. This year end 2014, the total amount 200 GWh will be reached. This is equivalent carbon dioxide equivalent of for a car driving 188 million kilometers; except Drammen has saved it.

These installation based on those 67 GWh per year at the following current gas price in Norway are approximately 50€ per MWh and the electricity price is approximately 30 € per MWh.

By using the ammonia heat pump, the total cost of electricity would be around 659,000 € per year vs. 3,350,000 € per year for a gas boiler installation. There is an estimated saving of 2,690,000 € per year.

Image 3. External view of the Drammen installation building

The global warming benefit of the ammonia heat pump is also significant. With a yearly equivalent CO2 emission practically zero (hydroelectricity), this compares  to burning gas which would give a CO2 emission of 15 thousand tons per year at the given usage profile.

Low carbon heat without combustion or transport emissions.


The main challenge for heat pump in general is to convert the heat source to the right heat level offering the best return on investment possible.

This type of installation shows that high temperature and also high heating capacity heat pump can be achieved using a natural refrigerant like Ammonia with the right compression technology. 

This specific heat source (sea water at 8°C) proves that the range of heat source for heat pump can be widened (sea water, river, waste process water, heat recovery,…) and can provide high COP allowing optimised return on investment.

Further improvements can be made by harnessing the cold return water for data centres. A truly thermally integrated solution.


-        Emerson Climate Technologies GmbH + Star Refrigeration Ltd. internal data

-        Star Refrigeration ltd. Case Study n°64: Neatpump

-        Hoffman, & Pearson, D. 2011. Ammonia heat pumps for district heating in Norway 7 – a case study. Presented at Institute of Refrigeration, 7 April, London.