Cool Copenhagen – district cooling in Copenhagen

Reusing a power station

District cooling comes to Copenhagen

For the first time in Denmark, large-scale district cooling is now available. In March 2010, the Adelgade district cooling plant in central Copenhagen came into operation, supplying district cooling to customers around the Kongens Nytorv area of the city. The Adelgade plant is on the same site and uses some of the buildings that originally housed Copenhagen’s very first power station that dates back to 1892.

PHOTO: Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

District cooling
Reusing a power station fits well with the concept of district cooling, which aims to utilise local resources that would otherwise have been left unused or even wasted. In Copenhagen, a combination of seawater and excess heat from power and waste incineration plants are utilised.

Centralisation of cooling production increases efficiency, and with 40-50% of cooling demand now being used for year-round server cooling, this provides significant savings.

Seawater and excess heat

Copenhagen Energy’s district cooling system utilises the same basic principles as district heating, i.e. to use the excess heat from the production of electricity at power plants. But where district heating can utilise this heat directly, the district cooling plant transfers the waste heat to district cooling with absorption chillers and compression chillers as stand-by for peak load situations. Seawater from the harbour is used for free cooling when the water temperature is low enough.

Copenhagen Energy
Henrik Bøgeskov is Manager of District Cooling at Copenhagen Energy, a utility company with 1 million domestic and commercial water customers and half a million district heating customers. Annual turnover of EUR 600 million – and the company invests almost EUR 140 million in pipes alone – every year!

PHOTO: Inside the district cooling plant at Adelgade, Copenhagen
The three pillars

The Copenhagen solution combines free cooling, absorption chillers and compression chillers for highest efficiency

The district cooling system in Copenhagen is built on three pillars, which enable Copenhagen Energy to prioritise and optimise the energy efficiency of the system. These are free cooling, absorption chillers and compression chillers.

Free cooling – the first pillar

Could you tell us about your cooling technologies?

Henrik Bøgeskov: If we start with the factor that really makes a difference compared to decentralised electrical cooling, we have free cooling for approximately a third of the year. We have 5-6 months of the year where we can use seawater that is so cold that it can be used for free cooling. The salty seawater is not sent directly out into the distribution network due to the risk of corrosion, it is separated from the treated water in the distribution network by plate type exchangers.

So you do not need to cool it down at all?

Henrik Bøgeskov: No, and this provides large energy savings. We only need to use electricity to pump seawater into the system and to circulate the water in the cooling network. Free cooling is very efficient. So the backbone of a district cooling station is access to cold seawater. The Adelgade plant is only 800 metres away from the harbour where we have a seawater intake.

Absorption chillers – the second pillar

During the summer the seawater is too warm for free cooling. What happens then?

Henrik Bøgeskov: The second pillar is our absorption chillers. This technology is based on steam produced via excess heat from our CHP (Combined Heat and Power plant) power station, which itself primarily runs on biomass, straw, wood pellets and waste refuse. This technology uses excess heat in the form of steam produced during the production of electricity at the CHP plant to deliver cooling effect. This is done in a two-tier process in a closed system. Heat is converted to cooling in the system, where heat from the steam first dries out the water in a solution of lithium bromide. Later in the same process the water is absorbed again, which also requires heat that is again taken from the cooling water.


"We have 5-6 months of the year where we can use seawater that is so cold that it can be used for free cooling."

Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

PHOTO: Production distribution of cooling for the Adelgade plant

So you use absorption chillers during the summer months, when cooling demand is high?

Henrik Bøgeskov: Yes, our peak season is from May to September. At the same time, this is the low season for our CHP plant, as buildings do not require heating in the summer. But the CHP plant is still operational, meaning we can receive their excess heat in form of steam to run our absorption chillers. This technology also has a low CO2 consumption.

Compression chillers – the third pillar

PHOTO: Grundfos pumps are installed in the plant

What is the third pillar and why do you need a third option?

Henrik Bøgeskov: The final pillar is our electrically driven compression chillers, which we mainly use during peak load periods. However, even though these are electrically driven, it can still make sense for us to use them. For example, there are times when plenty of cheap and sustainable electricity is available, both from Copenhagen Energy’s wind turbines, as well as hydroelectric power from Sweden and Norway. Here it can make sense to use compression chillers.  

Synergy between the three pillars
Because Copenhagen Energy’s district cooling solution is built on three pillars, it is always possible to select the most energy efficient and cost-effective option. Combined with the economies of scale offered by a centralised cooling solution, this translates into very low electricity consumption compared to alternative cooling solutions.

The second pillar is our absorption chillers. This technology uses excess heat in the form of steam produced during the production of electricity at the CHP plant to deliver cooling effect.

Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

Are there other areas where district cooling provides synergies?

Henrik Bøgeskov: Yes, there are. A hotel needs a decentralised solution with a rather large cooling capacity to deliver cooling to all rooms in the middle of the summer. However, we do not need to design for such high levels. And why is that? Well firstly, if you supply ten hotels rather than one, it is highly unlikely that they will all be fully booked at the same time.

Also, our customer load is spread throughout the day. While office buildings typically require peak cooling in the morning when people arrive, but not in the evening, hotels require more cooling in the evening when guests are in the rooms and restaurants. So we are able to utilise the different consumption patterns of our different customers, who also include shopping centres, restaurants and museums, to lower our peak load requirement.



Are there customers who require constant levels of cooling?

Henrik Bøgeskov: Yes, here we should distinguish between comfort cooling for offices and hotels etc. and server cooling. While comfort cooling is season dependent, server cooling has a constant level of demand throughout the year. Regardless of how warm it is outside, servers and similar IT equipment needs to be cooled down. This provides us with a baseload of cooling.


"The final pillar is our electrically driven compression chillers, which we mainly use during peak load periods. There are times when plenty of cheap and sustainable electricity is available, both from Copenhagen Energy’s wind turbines, as well as hydroelectric power from Sweden and Norway. Here it can make sense to use compression chillers."

Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

Hot and cold

Water temperature is an important factor

To be able to operate with free cooling, the temperature of the seawater taken from Copenhagen’s harbour has to be lower than the flow temperature in the district cooling system, which is 6 OC. The seawater temperature rises above 6 OC during the spring in April or May and does not fall below that figure again until late autumn or winter.

Are there factors that limit the amount of free cooling?

Henrik Bøgeskov: One factor to bear in mind is the physical limitation we have, as we cannot fetch seawater from 20 metres below the surface in Copenhagen harbour, because the harbour is not so deep. Our intakes are placed 3 metres below the water surface and, of course, the water is warmer there.

How warm can the water get?

Around 30 OC! There is a relatively large temperature variation, from just above freezing in the winter up to 30 OC in the summer. Of course, we have the option of closing our seawater intake all together and using the water being returned from cooling once the temperature of the seawater exceeds the return temperature, which is 6 OC.

The Adelgade plant is fully dimensioned for a maximum cooling capacity of 20 MW, which translates to a cooling capacity of 350,000 m3.

Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

Is 6 OC a high temperature for free cooling?

Henrik Bøgeskov: No, I think we could go up to 9 OC. However, we have looked into this and many of the existing cooling systems that our customers have are designed for 6/16 OC. So while we could use more free cooling with say 9 OC, our customers would not only have to refurbish their cooling system significantly, they would also need new ventilation systems etc. With so many existing customers, we cannot run at a higher temperature.

Sucking up water
The seawater intakes in Copenhagen harbour are designed for a maximum capacity of around 4,000 m3 per hour. At the current capacity they take in around 1,100 m3 per hour. The Adelgade plant is fully dimensioned for a maximum cooling capacity of 20 MW, which translates to a cooling capacity of 350,000 m3. At present it produces a little over half of that capacity. Extra chillers can be installed to reach full capacity.

How many seawater pumps do you use?

Henrik Bøgeskov: We have 4 pumps today and can have a maximum of 5. At present there is still some free capacity with 4 pumps. Again, we can optimise energy consumption by running only one or two pumps depending on the level of consumption and the optimal utilisation of the pump performance curve.

Basically we use a cascade system for the pumps, but we are still in the process of testing the optimal set-up under various conditions. We are determining the optimal method to provide the lowest possible energy consumption.

Cool future

Widening the net

Customers for Copenhagen Energy’s district cooling typically have a decentralised cooling plant that they want to replace, either due to refurbishment plans or because it is at the end of its lifecycle.

Which factors are important in deciding to sign up for district cooling?

Henrik Bøgeskov: The decisive factor for many of them is that we transfer all operation and maintenance to a central cooling station and provide economies of scale. So instead of 20 customers each having their own decentralised cooling system, we have one central system, delivering cold water to the door.

Customer benefits
From a customer viewpoint, district cooling has numerous advantages. It is simple to operate and, with no production, there are savings on investment and maintenance costs. For customers in the inner city, saving space previously used for cooling towers and the like is also important, as this space can be utilised for other purposes. There is also an architectural enhancement of the building, with no condensator units on building facades and rooftops.

District cooling sounds like an easy solution for customers. Are there any requirements they must meet?

Henrik Bøgeskov: They are required to return the used cooling water at a temperature of 16 OC. The customer’s heat exchanger must also be designed to withstand a pressure of 15,2 bar in the distribution pipelines.

Our Kw/h prices are competitive compared to the alternatives available.

Henrik Bøgeskov, Manager of District Cooling, Copenhagen Energy

What about prices, is district cooling competitive?

Henrik Bøgeskov: Our Kw/h prices are competitive compared to the alternatives available. Our prices are individually negotiated with the customer since a large part of the costs are related to the pipe from the cooling plant to the customer. The real challenge is the level of investment required, typically DKK 100 million for a district cooling plant. But even though we have only recently started supplying district cooling, the company managed a good level of turnover and a better than expected profit.

The future

Copenhagen Energy is already at an advanced planning stage for the next district cooling plant. This will be located near the town hall square right in the middle of Copenhagen. At present, it is expected that the new plant will be fully operational in 2015. The two district cooling plants will have a combined capacity of around 53 MW delivered to the customers.

Isn’t the town hall square rather a long way from the harbour?

Henrik Bøgeskov: We will need fairly long seawater intake pipes. We already have a few large customers in this area, such as the Tivoli Congress Centre and Danish Industry’s headquarters, who are supplied from Adelgade via pipes that are more than 2 km long. Of course, two district cooling plants mean two production units that can support and supplement one another.

How many customers are you aiming for?

Henrik Bøgeskov: Currently we have 20 customers and with both plants we would be aiming for around 50 customers. We are concentrating on larger customers in fairly large commercial buildings. That is also the reason for placing our plants right in the middle of the city, as this is where the large customers are.

Eight interesting district cooling sites
Beyond 2015 Copenhagen Energy has earmarked a further six locations in and around the city as offering the potential to become district cooling sites. This would provide a fully expanded district cooling system with a total of eight district cooling plants supplying commercial buildings in central Copenhagen – and supplying them with a local resource – seawater taken from the harbour. If they want, customers can simply take a walk down to the harbour and see the water that is being used to cool their buildings.

The Municipality of Copenhagen has a goal of being 100% CO2 neutral by 2025. How does district cooling fit in?

Henrik Bøgeskov: District cooling is a small piece of the puzzle, with lower CO2 emissions for our customers compared to decentralised systems. Our own production of district cooling will have low CO2 emissions, but we need to use some electricity for distribution pumps and peak loads. What we can clearly say is that there is a very, very low electricity or energy consumption for free cooling and also for cooling using absorption chillers. So we are certainly making a difference.


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