Nowadays, more and more office buildings are cooled due to the increased needs in terms of comfort. At the same time, the legal requirements concerning energy efficiency and sustainability in building systems have increased as well. Therefore, the use of air-stable refrigerants is no longer legal for sites exceeding a certain capacity.
Within the scope of this project a novel, reversible air-to-water heat pump system has been developed. The system introduces the use of the natural cooling refrigerant propane combined with a reversible heat distribution system. The demonstration unit is implemented at a Minergie-P-labeled office building. A range of passive and active measures, such as a highly insulated building envelope or an effective night cooling ensure a very low final energy demand of the building system. Additionally, a sharp distinction between heating and cooling mode has been accomplished. The demand for hot water is covered by a separate air-to-water heat pump that is also cooling server room. The propane heat pump is solely used to ensure the air-conditioning and heating of the building.
Due to the use of propane, the heat pump is installed outside of the building to guarantee safety. The entire unit including condenser and evaporator is installed outside. The switching between cooling and heating circuit is enabled by change-over valves within the refrigerant circuit. To allow for partial-load operation the heat pump is equipped with four compressors. This measure enables the modulated power output in the range of 15 to 100% of the total capacity. A very compact design of the heat pump could be achieved. Despite the reversibility, the unit’s hydraulics on both, the hot as well as the cold-water side, are simple and cost-efficiently realized. The problem of lubrication due to the solubility of the propane in the lubricants could be solved at all operating points by means of high suction gas superheating and a highly efficient oil separation and recirculation system.
A detailed screening of the building and the heat pump system has been conducted and allowed for optimization in a subsequent step. The heat demand regarding the room heating and ventilation amounts to 42’131 kWh (adjusted for heating degree days, 13.9 kWh/m2), the cooling demand to 3’156 kWh/m2, respectively. Considering the evaluation period, these values are low. The electricity demand for the ventilation system is with 1.8 kWh/m2 slightly lower than the 2.0 kWh/m2 targeted by SIA.
Within the optimization of the heat pump system, the electrical energy demand of the secondary aggregates could be halved, and the average return flow temperature could be decreased by 1.5 K. The result is a COP in heating mode of approximately 3.0. An increased efficiency during cooling could be achieved by lowering the power of the secondary aggregates as well as improving the regulation to upkeep the temperature levels needed. Thanks to these measurements, the EER of the heat pump during cooling could be increased from 1.9 to 2.8 during the compared summer periods.
The project shows that reversible heat pump sytems using propane as refrigerant are already applicable in today’s building systems. The safety level of an outdoor installed heat pump is achieved with reasonable efforts. Due to the legal requirements of sound protection, the unit is being placed within an machine body.
The achieved COPs during winter are high. This is especially impressive, considering the low heat limit meaning that the heat pump only runs at very low and high outside temperatures. This leads to the system often working at non-efficient operation points.
However, the system as a whole is very efficient. The achieved level of comfort in all zones is excellent.