On the use of polymer heat exchangers in closed adsorption chillers.
Closed adsorption chiller systems (ACS) show a high potential for the future cooling and air conditioning of buildings due to the resource-saving use of district heat, industrial waste heat and solar heat. However, low cooling efficiencies and power densities are reasons why closed adsorption chillers with low to medium cooling capacity have so far a low market share compared to electric refrigeration chillers. Ways to increase the coefficient of performance (COP) of ACS are the use of suitable adsorption working pairs and the minimization of the energy consumption during the refrigeration cycle. The present study investigates the possibilities of increasing the COP by reducing the heat capacity of adsorber beds with the help of polymer heat exchangers, without neglecting the specific cooling power (SCP).
The experimental investigations carried out on simple model systems demonstrate that the heat capacity of adsorber heat exchangers can be reduced by up to 60 % through the substitution of stainless steel or aluminum with thermoplastic materials. Depending on the specific adsorber bed design and its integration into the refrigeration system, COP-increases of 10 to 30 % can be achieved. Due to their low volumetric heat capacity and their chemical resistance against hot water polyolefins are among other thermoplastics suitable as construction materials for adsorber heat exchangers.
The limited heat transport in unfilled thermoplastics leads to an adsorbate sorptions rate slowed by the factor of two to three and therefore to a reduced cooling capacity of the ACS. In order to increase the heat transfer in polymer heat exchangers metallic or ceramic fillers with a high thermal conductivity can be incorporated in the thermoplastic matrix. In the case of plate and tube heat exchangers with wall thicknesses of 2 to 3 mm and a loose grain configuration (monolayer), a thermal conductivity of 2 W/mK is sufficient for an appropriate heat transport from the adsorbent to the heat transfer fluid during adsorption and desorption. A surface enlargement of polymer heat exchangers by means of additional fins should generally be avoided. Thermal conductivities of 20 W/mK and more are necessary to achieve a typical fin efficiency of 80 to 90%.
A significant increase in the thermal conductivity of thermoplastic parts can be reached by the addition of anisometric particles of expanded natural graphite with an optimized particle size of about 200 µm. In contrast to delaminated graphite and other filler platelets with the same particle size, due to its specific treatment expanded graphite shows a significantly increased aspect ratio. This leads to a higher number of particles and to a closer filler network in the bulk material.
In addition to an increased thermal conductivity, for a successful implementation of polymer heat exchangers in ACS it is important to adapt their structural design to the specific material properties and to manufacturing conditions. Thin-walled plate and foil heat exchangers promote the heat transport and ensure the maximization of the heat exchanger surface in the given installation space of the adsorber.