BioSorB – Biomass heating system with a sorption heat pump for condensing boiler efficiency
Heat transformation: heat pumps & refrigeration units
As part of the project, a system for utilising the condensation heat from biomass boilers is being developed by integrating an absorption heat pump. The development is aimed at boilers in the medium nominal output range (50 kW to 500 kW), which are used to supply local heating networks in small municipalities, districts or neighbourhood associations. Local heating networks are generally operated at return temperatures of around 50 °C. This means that it is not possible to increase energy efficiency by utilising the condensation effect with conventional system technology, as direct heat transfer from the flue gas of the combustion plant to the return flow of the heating network cannot achieve a temperature below the flue gas’s dew point due to the high return temperature. The use of an absorption heat pump enables the heat contained in the flue gas to be utilised almost entirely and transfers it to the district heating network.
Project description
At the low-temperature level, heat from the cooling and condensation of the boiler flue gas is transferred to the evaporator (V). At the high-temperature level, the hot boiler water in the generator (G) provides the thermal drive for the sorption cycle. At the medium-temperature level, useful heat is released via the condenser (K) and absorber (A).
The utilisation of condensation heat in biomass combustion systems enables a significant increase in efficiency. The resulting fuel savings can make a decisive contribution to achieving energy and climate policy objectives. In larger heat supply systems in particular – served via local and district heating networks – the utilisation of condensation heat is only possible to a limited extent due to network return temperatures exceeding 50 °C. The figure below shows an expected efficiency increase of 19 per cent (at a fuel moisture content of 30 per cent) when integrating a heat pump that cools the flue gas down to 25 °C. By comparison, with direct heat transfer from the boiler flue gas to the heating network return flow, the flue gas can, at best, be cooled to around 65 °C, which results in an efficiency increase of only 5 per cent.
The system design envisages the heat pump as an auxiliary unit that can be connected to existing biomass-fired systems. Integrating the heat pump enables the energy efficiency and cost-effectiveness of the biomass-fired systems to be optimised, and this can, in principle, be achieved with biomass boilers from various manufacturers.
The heat pump unit for condensing boiler applications can also be used in renewable heat supply systems in commercial and industrial applications and, particularly when providing high-temperature heat for industrial processes through the cascaded use of heat pumps, enables improved fuel utilisation. By integrating it into the flue gas system of a biomass or hydrogen-fired boiler, additional heat can be recovered, for example for heating buildings and feeding into district heating networks.
You can find more information on the project page.
Project Management
Dept 05 – Utilities and Building Services Engineering
CENERGIE – Research Center for Energy-Efficient Buildings and Districts
Technical University of Munich
Trane Air Conditioning and Refrigeration Engineering Office Ltd
BMH-Contracting GmbH
Jan 2025 – Dec 2027
The German Government’s 8th Energy Research Programme, section on ‘Energy supply from bioenergy – the use of biogenic residues and waste materials for energy purposes’