Biomass driven absorption-heat pump plant for firing and cooling
According to the Kyoto-Protokoll Austria has to reduce its CO2 emissions until 2010/2012 by 13% (based on 1990). The main energy consumer is the building sector, but it is possible to decrease the energy consumption significantly by means of heat pumps and free ambient energy from air, ground and groundwater. With heat pumps cooling is also possible, which becomes more and more important due to the changing architecture and rising comfort needs. In the case of electrically driven heat pumps electricity production is the sector, where CO2-emissions arise. In the case of absorption heat pumps the driving power is heat, and if this heat comes from biomass, heat pumps become almost CO2-neutral.
Absorption heat pumps with "natural" refrigerants - like ammonia/water - are environmentally friendly alternatives to common compression refrigeration units with fluorinated hydrocarbons, whose further use will be restricted.
The goal of the project BioAWP was to develop a small scale high-efficient biomass-driven absorption heat pump for residential heating and cooling which uses the ground as heat source or heat sink and a low-temperature heating or cooling system with integrated hot water production.
For the driving heat of the absorption heat pump a biomass boiler has been modified and tested for operation at supply temperatures up to 180°C. By the use of thermo oil as heat carrier the boiler can be operated at atmospheric pressure and the necessary modifications of the off-the-shelf product are limited.
One goal which has been traced during the development of the absorption heat pump was to reduce the first cost of the system by the use of off-the-shelf components. At the beginning of the project the "Generator Absorber heateXchanger" (GAX) process has been focused on, due to higher efficiency compared to e.g. the single stage process. However, the investigations showed that the use of plate heat exchangers within the GAX process is restricted. Therefore a single stage process has been realised, whereby for all heat exchangers standard plate heat exchangers have been used. The prototype of the absorption heat pump was tested, optimized and connected to the pellet boiler. The results of the tests showed the correct function of the system at both steady state and intermittent operating conditions as well as a satisfying efficiency.
For the use of the ground as heat source or heat sink a CO2-probe has been investigated. For the operation of the CO2-probe as a heat sink - i.e. for cooling mode - the CO2 has to be in liquid state and the circulation must be forced by special pumps. The simulation and experiments showed that the necessary mass flow rate of CO2 for the heat rejection and the associated power demand of the circulation pumps are relatively high. Presently the realization of this technology seems not to be competitive. As an alternative to the CO2-probe a system with a horizontal collector and gas-liquid phase change in the cooling and heating mode has been suggested.
The concept of the project BioAWP for heating, cooling and hot-water preparation has been further investigated for different applications and compared with alternative technologies.
It can be concluded that the BioAWP concept at heating operation is approximately competitive to alternative technologies, but for cooling-only operation the biomass demand of the BioAWP concept is relatively high, thus the competitiveness to compression heat pump is difficult. From an ecological point of view the BioAWP concept offers low CO2-emmisions and has advantages in heating and cooling operation compared to compression heat pumps.
Dr. René Rieberer
TU Graz, Institut für Wärmetechnik
Project or cooperation partner
- Heliotherm Wärmepumpentechnik GmbH
- KWB - Kraft und Wärme aus Biomasse GmbH
- M-TEC Mittermayr GmbH