Increase Performance of Absorption Cooling Devices

Possible applications of this technology are transport cooling, vehicle air conditioning and solar cooling of buildings.

THOKA – Thermally driven High Performance Cooling

The goal of the Fraunhofer internal project THOKA was the development of adsorption cooling devices with enhanced performance characteristics based on cheap energy resources like the sun of the excess heat of processes. This project is a joint effort of the Fraunhofer institutes ISE, IFAM-DD, IFAM-HB, IVV and ITWM. The ITWM supports the development effort by optimization techniques utilizing virtual models of the corresponding energy conversion processes.

The main goal of the project is the Improvement of the power density of adsorption devices by the development of a new generation of the adsorber technology. The key characteristics are 100 W per liter of the volume and simultaneously an improved coefficient of performance (above 0.6). Possible applications of this technology are transport cooling, vehicle air conditioning and solar cooling of buildings.

Logo THOKA
Schematische Darstellung des THOKA Adsorberdesigns.
© ITWM

Diagram of the THOKA thermal absorber.

The key parameters for the obtainable power density are:

  • a realizable load amplitude determined by the sorption material
  • short cycles due to a fast material transport through the pore system
  • the fast recovery of the emitted sorption heat
  • a high vaporization enthalpy of the cooling agent.

 

Sorption material: ceolith with a high absorption rate

The basic design principle of the adsorber cell is shown on the left. On a heat transmitter sheet, a zeolith coated porous microstructure (pore size 10-100 microns) is soldered, see 4. On the opposite side, heat exchanger channels are attached to the sheet (diameters of a few centimeters).  This forms one stack element of the complete device. Each stack element is connected by valves with the evaporator on the cooling side and with a condenser on the back cooling side. By dynamically optimizing the valves, back heating with favorable energy consumption could be achieved. Key performance characteristics of such a device are the load amplitude in the zeolith, the cycling frequency between loading and unloading of the composite, the performance of the heat exchange moderated by the channels and the evaporation enthalpy of the working liquid.

The ITWM supports the construction and process design of the device by using optimization techniques utilizing virtual simulation models on all scales.

THOKA Thermisch angetriebene Hochleistungskälte
© ITWM

Bottom-Up approach is used for modeling.

The process modeling follows a bottom-up approach. The adsorbing zeolite coating is modeled by the phenomenological description of Dubinin with parameters fitted to experimental data. This model forms a source term for the heat conduction in the porous medium. This micro model of the composite structure is then homogenized using a mathematical two scale procedure in order to fit to the scales of the heat exchanger.

 

Simulation of the heat exchanger and heat conduction within the microstructure

The working liquid, water in our case, in the channels of the heat exchanger is modeled by the Navier Stokes and energy balance equations. To this end, we developed structure generators for the channels following bionic principles. The composite model and the heat exchanger model are then coupled dynamically in order to simulate the processes in one stack element. A further coarse graining to a network flow model, who has variables associated with the branching points of the bionic structure only, allows for simulations of the stack including heat recovery mechanisms.