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Energy efficient cooling at Aarhus University Hospital

06 January 2012

A new centralised cooling plant will annually save 800,000 kWh energy and secure optimal operation at Denmark’s second biggest hospital. Stability and efficiency are the key achievements of a new centralised cooling plant at Aarhus University Hospital, Skejby. This new centralised plant, which has replaced many old decentralised cooling units, has a cooling capacity of 2.5MW. The cooling supply to surgery rooms, scanners, server rooms and IT-equipment is now more stable and efficient.

The new cooling plant consists of two large air/liquid heat pumps, nine air-cooled chillers and a number of compressors, with Danfoss VLT drives controlling the speed of the units.

During the months of June, July and August the heat pumps are able to deliver all the heat required by the hospital. Therefore, no extra heat has to be delivered from the public district heating network. Surplus heat from the cooling production during the summer is reused for heating purposes. During the winter the heat pumps deliver a smaller portion of the heat consumption, while the main heat supply is taken care of by the district-heating supplier, AffaldVarme Aarhus.

The two 180kW heat pumps (actually two times two separate heat pump circuits, which are built together into one unit) are designed for temperatures of 35/80degC. They achieve energy savings by re-using the heat instead of exhausting it into the environment.

"The heat pumps´ COP (coefficient of performance) is 3.6 or more, depending on outside temperature and cooling demand, says Svend Madsen, Sales/ Project Engineer from Johnson Controls, which installed the cooling plant and hold the service agreement. "This means that their heating power is 3.6 times the electricity consumed. On the old system, the COP was only between 2 and 3, so the heat pumps are an energy saving solution. Compared with ten decentralised cooling plants running separately, which was the situation before, there are huge energy savings to be achieved by co-ordinating and controlling the different units (heat pumps, chillers, etc).”

The chillers´ COP is 5 or more depending on cooling demand and outside temperature. The 90kW VLT drives combined with the control system, ensure that only the required amount of cooling is delivered. A small 5.5kW VLT drive runs a pump on the cooling system and is regulated according to the speed of the larger VLT drive, depending on cooling demand.

Easier control and service

A new PLC monitoring system measures and analyses the efficiency of each machine in order to secure the right combination of chillers and heat pumps, so that the most energy efficient production is chosen at any time. For instance, the supervisory system can change the production from three chillers at maximum load to four chillers running at 80% load, based on a snapshot on what achieves the biggest energy savings. This way the CO2 emissions are reduced as much as possible.

Svend Madsen explains that the cooling plant also uses 'free cooling' in the wintertime in order to utilise the cold days of the year, so no cooling is produced during this period. “It’s also very important in terms of energy savings that surplus heat from the chillers is reused for heating purposes instead of being exhausted to the environment,” he says.

The highly efficient centralised cooling plant is easier to monitor, control and service than the many old cooling units. “The entire system is computerised and PLC controlled, so the physical monitoring will be brought to a minimum, says project manager Lars Torp Larsen from consulting engineer Alectia. "In the design phase and during the installation process we have co-operated closely with the operations department. They are the ones who will work with the plant on a daily basis, so their input is important for the project."


If, for instance, one cooling machine has a lower performance than the others, the staff will be warned by an alarm an immediate action will be taken to optimise it. A CTS system monitors a number of parameters, which takes care of the efficiency of the IT monitoring system.  Poul Hvid Nielsen, Chief of Technical Department at Aarhus University Hospital, Skejby, is satisfied: “In this way the hospital has got double security for optimal operation. Being a hospital, security of supply is fundamental to us”.

Poul explains that three new buildings have been built and added to the hospital recently, and the new centralised cooling plant is prepared for a further growing demand for cooling. "The cooling system is designed so that, for instance, a cooling machine or a heat pump can cut out on a warm summer day without creating any kind of problems to the cooling supply”, he says.

Environmentally friendly solution

The new cooling plant is not only more efficient but also more environmentally friendly than the old system. Many old units used the Freon 22 as refrigerant - this is forbidden in all new cooling systems in Denmark, and at Skejby Hospital it has been replaced by natural refrigiants. The choice fell on chillers using R290 (propane, with a global warming potential of 3.3) and heat pumps using R600a (iso buthane, with global warming potential of 4). Other options like CO2 had also been evaluated but were found to be less efficient. This way, about 750 tons of CO2 emissions are saved annually. Moreover, the new cooling system saves around 800,000kWh of energy per year and surplus heat from the cooling plant is reused.

The VLT drives at-a-glance:

• Installed on each chiller are: a VLT HVAC Drive FC 102 90kW, IP55 enclosure, EMC filter , a safe stop and graphic display with an integrated service switch, in order to run the compressor (eleven units).

• Installed on each heat pump are: a VLT HVAC Drive FC 102 9 kW, IP55 enclosure, EMC filter, a safe stop and a graphic display with an integrated service switch, in order to run the compressor (two units).

• Each chiller also features: a 5,5kW VLT HVAC Drive FC 102 IP55 with an integrated service switch to run the pumps (14 units).

• Installed on the central pump are: a VLT HVAC Drive FC 102 37kW, IP55, EMC filter, safe stop and graphic display with an integrated service switch and a corresponding 5,5kW drive (two units).

Facts about the cooling plant

• Minimum load on cooling demand is 350 - 450kW annually.

• The heat from previously used air is separated and sent into the heat pumps and reused. This saves electricity and minimises CO2 emissions.

• All in all the cooling and heating systems of the new cooling plant will result in energy savings of 3.5m kWh annually and a reduction in CO2 emissions of 750 tons annually.

The top picture shows the environmental chillers in roof-top installation at Aarhus University Hospital. On each chiller are installed a VLT HVAC Drive FC 102, 90 kW, with IP55 enclosure, and a VLT HVAC Drive FC 102, 5.5kW (also IP55). The lower picture shows the VLT HVAC Drive FC 102, 37kW, with IP55 enclosure, installed on the central pump.

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