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Efficient useful heat with CO2 hybrid heat pump

New hybrid heat pump achieves higher efficiency for useful heat supply through a CO2 compression and adsorption cycle coupled via stratified heat storage.

An integrated stratified heat storage is at the heart of the hybrid heat pump. It is used for heat buffering between the compression and adsorption cycles, heat recovery within the adsorption cycle and useful heat storage (between electricity and heat use). (Image: Institute for Applied Thermofluidics / KIT)

In the field of useful heat supply, a trend towards sustainable heating technologies such as heat pumps can be observed. Heat pumps use ambient heat or waste heat through heat transformation.

State of the art

In the new construction sector, classic compression heat pumps are already used as primary heat generators. When outside temperatures drop, a second separate heating device is usually switched on to handle the peak load as a bivalent heating system. Despite their high coefficient of performance, the conversion to heat pumps is not very widespread in the renovation of existing buildings. Reason for this: They work best with low-temperature radiators, such as underfloor heating.


With a new hybrid heat pump that combines a transcritical CO2 compression cycle (CC) and an adsorption cycle (AC), scientists at the Institute for Applied Thermofluidics (IATF) are creating the prerequisites for the use of heat pumps for heating and domestic hot water preparation in existing buildings. In contrast to existing systems, both cycles are coupled via an integrated stratified heat storage in order to buffer heat from the CO2 cycle and recover it within the adsorption cycle. This hybrid operating mode provides very high flexibility: The AC can be operated either as an auxiliary cycle to the CC (current mode) or as an adsorption heat pump using an additional driving heat source (e.g. gas burner). At peak load, both cycles are operated simultaneously.


By using the integrated stratified heat storage, a better adaptability of the adsorption cycle to the heat supply through the CO2 cycle is achieved and thus up to 30 percent higher overall efficiency, without further expensive additional components. This means that higher supply temperatures can be reached for conventional radiators and for domestic hot water preparation. In contrast to the most commonly used fluorinated operating materials, CO2 is a cost-effective and environmentally friendly operating material.

Options for companies

In addition to supplying useful heat, it could also be used for cooling or air conditioning. KIT is looking for industrial partners for the joint further development and implementation of the hybrid heat pump.

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Your contact person for this offer

Dr. Aude PĂ©lisson-Schecker
Innovation Manager Energy
Karlsruhe Institute of Technology (KIT)
Innovation and Relations Management (IRM)
Phone: +49 721 608-25335
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