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Heat pumps can be installed both inside and outside buildings.
© STIEBEL ELTRON
Gas adsorption heat pumps
14.01.2016

In the ADOSO project, an initial working model of a sorption module was developed based on fibre heat exchangers. The fibre-composite adsorbent material used is shown here.
© Fraunhofer IFAM

The ADOSO project is aiming to develop a heat pump that, among other aspects, is more compact than devices currently on the market. The scientists have managed to double the power density of the new device (right) relative to the construction used for conventional finned tube heat exchangers (left).
© SorTech AG

Doubling the power density with metal fibres

Zeolite can adsorb significant amounts of water vapour, thereby releasing latent heat. This property is used among others in gas adsorption heat pumps. Scientists have found a way to increase the heat transfer area, thereby increasing the power density of the heat pump module. An initial sorption module with an innovative zeolite coating for the heat exchanger is now available.

It is difficult to connect compact zeolite layers directly and permanently with a metallic heat exchanger. The two substances have different thermal expansions. This would cause the zeolite layers to flake off after a few heat pump cycles. Stainless steel finned tube heat exchangers have been usually used until now, whereby the spaces between the fins are filled completely with zeolite beads. To ensure that the beads remain in place, the entire heat exchanger is surrounded by a grill. The geometry of the zeolite layers in the adsorber has a decisive influence on the heat transfer and ability to store water, and thus on the power density of the heat pump.

Increasing this was one of the aims of the ADOSO research project. To achieve this, scientists at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM soldered a sintered fibre structure to flat tubes belonging to a heat exchanger. Developers from a manufacturer of adsorption chillers then crystallised thin and compact layers of the zeolite sorbent material directly onto the surface of the new fibre heat exchanger. “Compared with standard fins, the fibrous material provides a fourfold greater surface area. This enables the sorbent mass to be increased significantly without increasing the layer thickness and thus slowing down the mass and heat transfer in the adsorption layer,” explains Ursula Wittstadt, project coordinator at the Fraunhofer Institute for Solar Energy Systems ISE. The power density of the sorption module increased in comparison to the construction used for conventional finned tube heat exchangers from 145 W/l to 300 W/l without reducing the efficiency.

The research project is aiming to develop a heating device based on zeolite with water as the working fluid. The heat pump is designed to be more compact and less expensive than the devices currently on the market with comparable performance. In addition, the new device is intended to achieve an annual performance factor of 1.3. This shall be helped by the innovative adsorption heat exchanger and an optimised operation of the evaporator and condenser. A simulation study is currently being conducted that is testing the operation of the gas heat pump in different European climates.

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Addresses

Development of the device
STIEBEL ELTRON GmbH & Co. KG

Development of the sorption module
SorTech AG

Development metal fibres
Fraunhofer IFAM

Characterisation and simulation
Fraunhofer ISE

Links

This is how a zeolite gas heat pump works

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Heating with gas adsorption heat pumps
BINE-Projektinfo 03/2015