The Aarhus University Hospital Skejby in Denmark has been replacing out of date R22 chillers with new hydrocarbon chillers. Most recently hydrocarbon heat pumps supplied by Johnson Controls have been installed.
The first hydrocarbon chillers were already installed in 2003 and as the hospital has been gradually expanded additional hydrocarbon chillers have been installed.
The larger chiller system:
- Uses 9 air-cooled propane (R-290) based chillers each with a cooling capacity of 250 kW, coefficient of performance (COP) of 4.5 and a total of about 210 kg R290
- Uses a free-cooler with a capacity of 300 kW
- The chillers deliver a 35% propylene glycol/ water solution at 9°C and return at 15°C
On top of the 9 chillers there are also some special chillers around on the site working at different temperature levels, bringing the total number of hydrocarbon chillers on site to 15.
Moreover the latest installation made in the context of the gradual expansion encompassed two heat pumps that were put into operation in November 2010, with:
- Hydrocarbon refrigerant isobutane (R600a) of a total of about 80 kg
- A total heating capacity of about 450 kW and a cooling capacity of about 325 kW
- The units are built as two independent circuits on one frame
Experience with the R600a heat pumps so far and lessons learnt
The primary function of the heat pumps is to deliver cold water. The engaged capacity is therefore given by the cooling requirement. On the warm side, the flow is controlled to keep up the condensing temperature and the pressure in the local district heating system.
The values that were assumped at the beginning of the project are consistent with what has been measured over the last 1½ year since the commissioning of the system. In general, the experience with the heat pumps so far has been very good as they have not only met but also exceeded expectations.
An additional benefit for the University Hospital from installing the heat pumps is security of energy supply: the entire system is connected to a local generator set which ensures stable supply and independence from the local grid in case of a power blackout.
One important lesson learned was that compared to refrigeration systems, heat pump insulation needs more than normal consideration. The heat loss can be quite substantial and Johnson Controls has estimated it to be about 15 to 20%.
Combining heating and cooling
The new system combines the cooling system with the heating system, an approach that is still not widely used in most places in the world, despite the obvious intrinsic energy savings. However, due to increasing energy prices, customers have become more receptive to using new ways of thinking to save on their energy bills.
As a result, the new system offers better performance meaning energy savings, which also translates to CO2emissions savings.
With the hospital normally obliged to take the required heat from the district-heating network, the project involved talks with the local district heating company. In the winter period, the supply temperature of water is 80°C and in the summer it is supplied at 70°C. The return is 40°C. This requires the heat pumps to deliver water at 80°C and at the same time cool water. During winter, there is also a cooling load, which the heat pumps will be able in part to deliver while the rest is managed by the free coolers. In the summer period, there is a base load for heating of tap water and room heating 400 to 500 kW provided by the heat pumps.
Using hermetic design with no shaft seals or semi-welded heat exchangers, Johnson Controls has eliminated most flammability concerns of hydrocarbon refrigerant. For the units installed outdoor with a secondary loop an efficient micro bubble separator is included as a result of which no gas will ever reach inside and the possibility of getting a flammable concentration is not present. Moreover, in case of a leak, all power to the unit is cut out and can only be restarted manually from the panel (remote reset not possible).
Regulatory context: complying with international and national provisions
The project has been a success and the University Hospital has got a new system that complies with both international obligation under the Montreal Protocol that banned the use of HCFCs in new installations as of the year 2000, as well as Danish regulations which ban the use of HFC charges over 10 kg. At the same time, the hospital has avoided the high cost associated with HFC refrigerants, which is intensified by their heavy taxation introduced in Denmark in 2003.
Moreover, a number of R22 chillers have been taken out of service and 2,000 kg R22 (1,700 GWP) has been recycled. This is equivalent to 3,400,000 kg’s of CO2 being removed.