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Investigation of New Heat Exchanger Design Performance for Solar Thermal Chemical Heat Pump

The emergence of Thermally Driven Cooling system has received more attention recently due to its ability to utilize low grade heat from engine, incinerator or simple flat plate solar collector which are considered as renewable energy sources. ClimateWell AB located in Stockholm has been developing this cooling system based on its patented chemical heat pump technology.

The heat pump with its tube shape is put under the absorber as in simple flat plate solar collector making it possible be directly attached on the roof without any additional solar collector. A high performance heat exchanger is needed by its reactor to absorb the energy efficiently during the desorption process as well as to recover heat during the absorption process.

Current heat exchanger design has direct contact with the tube’s surface, yet air gaps between the tube and heat exchanger result in a lower amount of heat transferred and non-uniform heat distribution across this surface. Moreover, a special treatment which cannot be done by machinery has to be performed in attaching the tube with this heat exchanger. It becomes a problem during mass production since a lot of man power is needed. A new heat exchanger design was proposed to overcome those limitations.

This design has annulus which is filled with thermal fluid inside. This fluid will make perfect contact to the heat pump tube’s surface and eliminate the air gap. Furthermore, the need of man power in its production can be minimized. Even though perfect contact can be achieved, the fluid in this new design will increase thermal resistance in the radial direction. Therefore, an investigation has to be conducted to evaluate the performance of this new heat exchanger design based on heat transfer coefficient under steady state condition.

The performance investigation also included the influence of various thermal fluids which will be used for this new heat exchanger. The work performed by doing simulation in COMSOL continued with validation of the result with experiment in laboratory. New heat exchanger design efficiency was only 50% while the current one was 82% during the desorption process.

In this process, the fluid’s thermal conductivity was the most influencing fluid property. During absorption process, two heat recovery methods are used. First is by flowing the fluid inside the annulus and second is by using additional heat recovery pipe that is attached outside the heat exchanger surface. The first method gave the highest UA value around 34 W/K while the second one gave almost the same value as the current design which is around 11 W/K. In the first method, the thermal fluid’s viscosity strongly influenced its UA value while the second method is greatly influenced by fluid’s heat conductivity.
Source: KTH
Author: Cordova, Cordova

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