Seminar on Sustainable Building for Military Infrastructure 13th- 15th March 2002

Improved Heat Pump Technology by Senior Engineer Helge Underland, Norwegian Defence Estates Agency

1 Background Main air station Bodø was established during the World War II, an in the early fifties it was enlarged. Water-born heating systems based on oil-fired boilers were installed in each building; in some places two or tree buildings where connected. This was a common way to construct heating systems in the Norwegian defence, in addition to electric heating. During the eighties an energy-plan was developed. We started to plan and build district-heating systems consisting of central heating plants with different energy sources (oil, electricity, heat pump or bio fuel). In our central heating plants, we used to have oil-fired boilers for 100% of the design effect, and electric boilers for 60%. Later we started to install heat pumps or bio fuel boilers witch cover approx. 40 to 60 %. These give us the possibility to use the cheapest kind of energy at any time. If a heat pump or bio fuel boiler is installed, this will be the main load, and oil-fired boilers cover the peak load and will be the back up.

2 Bodø, climatic conditions Bodø is located far north in Norway at approx. 66.5°N. Design outdoor temperature is –13.5°C, and the mean temperature is 4.6°C. The heating season is 274 days. Under these conditions, it is suitable to install a heat pump, because of its long running time per year.

3 Heat pump principles A heat pump can be compared to a refrigerator, working in the opposite direction, where heat is transported from the inside to the outside; from low temperature to a higher temperature. The four main components of a heat pump are connected to a closed circuit. The evaporator where a liquid is boiling and evaporating under low pressure (low temperature energy is added). The vapour is compressed to a higher pressure and higher temperature in the compressor. The hot vapour enters the condenser where the vapour is condensed and the heat is transferred to a heating system. Finally, the high-pressure working fluid is expanded through the expansion valve, which regulates between high and low pressure. In this way we can increase the temperature in a liquid by adding high quality energy (electricity) in small amounts, and low quality energy in large amounts. In general, we add approx. 1 part electricity and 3 parts low quality energy. We are surrounded by inexhaustible energy sources (air, water, and ground).

Mailing address Visiting address Telephone no Fax no No of encl. Oslo mil/Akershus Akershus festning +47 23 09 36 05 +47 23 39 31 76 0 N-0015 OSLO Oslo E-mail address Internet Error! AutoText entry [email protected] www.forsvarsbygg.no not defined. 2 of 2

4 Main air station Bodø, NH3 (ammonia) heat pump In 1990 rehabilitation of the heating systems started, and the decision to build a district heating system with a heat pump as the main source was taken. Approx. 40 buildings where connected. The design effect in each building varied from 72 kW to 940 kW. The total design effect amounts to 5400 kW. Because of differences in time of use of the different buildings (offices, workshops, living quarters, etc), the practical design effect was calculated to 3800 kW, and the energy consumption for heating was calculated to 11 GWh.

4.1 The working fluid In 1990, it was known that CFC based working fluids would be forbidden, so we looked for other working fluids. The new refrigerants were not sufficiently tested so the old well-known NH3 (ammonia) (R717) was devoted. This working fluid has been used in refrigerating plants, but not in a heat pump. NH3 has very good thermodynamic properties. It does not harm the environment in any way; there is no harm to the ozone layer or any global warming effects. This ought to open for an extended use. In a certain mixture with air, it is explosive, and it is toxic.

Compressors used in refrigeration plants (NH3) are built for a pressure of 25 bar (approx. 50°C condensation temperature) but a higher temperature was desirable for the district heating system. (The design temperature of the heating systems in the buildings was 80°C). Therefor we needed a compressor of approx. 40 bar.

4.2 The central heating plant The central heating plant consists of 2 heat pumps of 2 MW, one electrical boiler and two oil- fired boilers at 3.8 MW

4.2.1 Seawater The central heating plant is located close to the beach. Seawater is collected at 170 m depth where the temperature is constantly 7°C around the year. The seawater drains into a 7 m deep basin, where two submerged pumps are located. Their total capacity is 180 m3/h.

4.2.2 The heat pump The heat pump consists of two separate aggregates with shell-and-tube condensers and evaporators. Each has a two-stage piston compressor with interstage receivers. The compressors consist of a low-pressure compressor with 16 cylinders and a high-pressure compressor with six cylinders. The two-stage compressor is preferred because it needs less power than a one-stage compressor and therefore gives better performance. Related to NH3, the pressure and the temperature, it is necessary to use a two-stage compressor to prevent the oil to decompose.

4.2.3 The process The ammonia is heated with seawater, and boils in the evaporator at 4 bar and -0.7°C. The low- pressure compressor compresses the vapour to 14 bar, 100°C. The hot vapour enters the interstage receiver. The high-pressure compressor compresses the vapour from 14 bar, 38.7°C to 30.7 bar, 108°C. The hot vapour enters the condenser where it is cooled to 74°C, and the condensation heat is transferred to the district heating system. The fluid enters the interstage receiver through the expansion valve, and from the inter-stage receiver to the evaporator through the second expansion valve. The two aggregates are connected in serial and the water in the district heating system enter the first ”master” at 60°C and leave at 64°C and enter the second ”slave” at 64°C and leave at 68°C. The heat factor is 3.4 at the master and 3.2 at the slave. 3 of 3

4.2.4 Technical data Compressor Low-pressure compressor High-pressure compressor

Manufacturer SABROE SABROE

Model SMC 116 S HPC 106 S

Rotational speed rpm 1475 1475

Max shaft power 155 kW 124 kW

Piston displacement 905 m3/h 330 m3/h

5 Economy This project was decided carried out as a prototype demonstration plant, and received economic support from NVE (The Norwegian Water Resources and Energy Directorate). The plant got 1.2 mill NOK (150 000 Euro) for instrumentation for follow up in the running-in and test period.

Max output 2000W Delivered quantity of energy 8 GWh per year Used quantity of energy to run the 2.5 GWh per year heat pump COP 3.4 Total cost for the district heating 38 mill NOK , 4,75 mill € system and the heat pump Extra cost for the heat pump 8 mil NOK, 1 mill € Payback for the heat pump 6 years

6 Environmental effect The heat pump installation has a positive influence on the environment by reductions in the air pollution. The reductions are: CO2 3 048 700 kg SO2 4 760 kg NOx 2 830 kg S 2 300 kg

The heat pump has a surplus capacity as is, and we therefore now enlarge the district heating system by connecting more buildings. 7 Other heat pump installations We have installed some other large heat pumps. Those use R134a as working fluid. As heat sources we use ambient air, seawater and ground water. In general, we are very satisfied with our large heat pump installations.

We have considered using CO2 as working fluid in one of our large heat pumps, since we are contributing the research work on CO2 as a working fluid. 4 of 4

In Oslo (Akershus Castle administration area) we now evaluate the heating- and distributing system. The design effect for this is 6 MW. In this project, we consider the use of a heat pump system with NH3 or CO2 as the working fluid.

8 Conclusions Heat pump technology is a sustainable way of heating. Reduction in the use of primary energy with very high quality is the key. To obtain the best performance it is important that we have a continuous heat source. It is important to balance the distribution systems, and design them with as low operating temperature as possible. Thus we can reduce the input energy to the compressor and increase the Coefficient of Performance (COP). In the long term only those technologies are sustainable that can address the dual challenge of protecting the ozone layer and containing adverse climate effects. NH3 and CO2 are working fluids for the future.