Copen­ha­gen airport, Denmark

Indus­trial heat pump improves the effi­ciency of a gas boiler at Copen­ha­gen Airport

Renov­a­tion of old gas boilers expan­ded into a project to care­fully utilise the energy of exhaust gases with an indus­trial heat pump. The result is, that effi­ciency of the gas com­bus­tion is more than one hundred per cent.

In the western Magleby­lille part of Copen­ha­gen Airport (more neatly known as CPH) there is an internal heating network. At the begin­ning of 2018 a decision was made to replace the old and low effi­ciency gas boilers that dated from the 1960s. Now the system con­sists of one three mega­watt (3 MW) and one 2 MW natural gas boilers together with one 600-​kilowatt (kW) indus­trial heat pump.

Flue gases are con­densed with a heat pump

“When we started renov­at­ing the heating system, we wanted to look at the entirety in a new way,” says Klaus Borge, project manager at Copen­ha­gen Airport. “The energy con­sult­ant par­ti­cip­at­ing in the design sug­ges­ted recov­ery of the exhaust gas tem­per­at­ure with a heat pump, so we can improve the effi­ciency of the boiler con­sid­er­ably. At the same time we moved the focus of energy usage from gas to renew­able elec­tri­city, which is our object­ive.”

Accord­ing to Borge many of the facil­it­ies in the area are rel­at­ively old and the tem­per­at­ure of return­ing water from the heating network is quite high. This is because in the old days they tried to build as inex­pens­ively as pos­sible and the heat exchangers are small.

The tem­per­at­ure of supply water in the winter is 85–90 degrees cen­ti­grade (°C) and the return about 60°C. In the summer the supply is 70°C and the return 50–55°C. With such warm return it would not be pos­sible to cool the flue gas very much and the energy in the flue gas would remain mostly wasted. With a heat pump it is pos­sible to cool the gas to a lower tem­per­at­ure while the pump oper­ates at excel­lent effi­ciency.

“The tem­per­at­ure of flue gas from the boiler is about 150°C,” con­tin­ues Borge. “It moves through the con­denser, where the tem­per­at­ure is lowered to some 30–35°C with the heat pump. Thanks to the low tem­per­at­ure the humid­ity of the gas is con­densed into water, which adds to the amount of recovered energy and improves the effi­ciency of the system con­sid­er­ably.”

High tem­per­at­ure and great control

“At the begin­ning of the project we dis­cussed a heat pump from another man­u­fac­turer but there was no pump the right size in their selec­tion,” says Borge. “Then we dis­cussed with Søren Gram, dir­ector of Svedan Indus­tri Køleanlæg A/S, retailer of Oilon. Søren presen­ted us with the heat pumps in the Oilon Chill­Heat P 300 series, that had the perfect spe­cific­a­tions for us.”

“The Oilon heat pump can produce a very high tem­per­at­ure that is easily high enough for the water supply in dis­trict heating. Thanks to its four com­pressors and inverter control the pump can be con­trolled very well at high effi­ciency with partial loads and its envir­on­ment friendly HFO refri­ger­ant is also approved in Denmark. Under the design con­di­tions of this project, the elec­tri­city con­sump­tion of the pump is 155 kW and COP 4.0.”

“Our exper­i­ence of the quality of the Oilon heat pump and the oper­a­tion of the factory as a whole is excel­lent. At the install­a­tion and com­mis­sion­ing stage we got com­pet­ent and high quality support from the factory so we got the system in oper­a­tion with just minor prob­lems. Like­wise the service by Svedan Industry has been strong and good.”

Heat pump util­ises the heat in flue gas

The airport heating centre now has two boilers, one 3 MW natural gas boiler and one 2 MW boiler. The 3 MW boiler is con­nec­ted to a con­denser that cools down the flue gas. The second boiler is in reserve and for the coldest weather. It will start if the tem­per­at­ure has been below -10°C for a week. The spare boiler blows the hot flue gases into the sky.

At present world­wide, heat pumps are rarely used for the recov­ery of heat from flue gases.

The dew point of flue gas from a natural gas boiler is about 55°C. By con­dens­ing the flue gas it is pos­sible to produce about 10% more power at its best. Con­densed water is formed at up to 180 litres per hour. Water con­dens­ing also absorbs impur­it­ies from the flue gas thus clean­ing the flue gas. Neut­ral­ized con­dens­ate is dis­charged into the sewer.

The heating plant pro­duces about 10,000 mega­watt hours (MWh) heat yearly, which equals to the con­sump­tion of about 500 private houses. To produce the heat about one million cubic meters of natural gas and 400 MWh elec­tric power is used. The savings with the heat pump is estim­ated to be about 1,000 MWh per year.

Elec­tri­city and energy effi­ciency instead of natural gas

“There is high pres­sure to reduce our green­house gas emis­sions and show that CPH will be an emis­sion free airport in 2030,” says Klaus Borge. “The Oilon heat pump with its 600 kW heating capa­city has some excess power for cooling our flue gases. Our inten­tion, however, is to furnish the system with glycol cir­cu­la­tion heat exchangers outside, that recover energy from the outside air from spring to autumn and then we can close the gas boiler for the summer period.”

“On the roof of the house next to us there are now 125 kW solar panels and 250 kW more are being installed right now. We have even con­sidered acquir­ing bat­ter­ies so the heat pump could even run on solar energy at night.”

Accord­ing to Borge the price of elec­tri­city for CPH is roughly one Danish crown per kilo­watt hour (1 DKK/kWh). The price of natural gas is about 5 DKK/m3 i.e. 0.50 DKK/kWh. The heat pump however pro­duces four times more heat com­pared to its elec­tri­city con­sump­tion so it costs only half the price to heat with the pump com­pared to gas. Denmark has some gas fields in the North Sea but the present gov­ern­ment wants to close them. On the other hand more and more biogas is being pro­duced in Denmark.

“CPH has been able to reduce its energy con­sump­tion con­sid­er­ably year after year, but there is still a lot to do,” says Borge. “It is clear that we will move to using more renew­able elec­tri­city instead of gas. All excess elec­tri­city is cur­rently used for char­ging elec­tric cars and it takes time to rein­force the grid. The amount of solar energy will increase strongly.”

“We are invest­ing heavily in saving energy and heat pumps have an import­ant role in this. A heat pump will pay itself back in four years so it is also a very prof­it­able invest­ment.”