Experiment to explain the magic of thermal cooling

In the following the driving force of thermally driven machines (i.e. the “magical heat pumping”) is explained briefly by an experiment.

On the left hand side in Figure 1 one can see the experimental set-up at start conditions (steady state), i.e. the ambient temperature is at approximately 26,5 °C. The set-up consists of two vacuum tight volumes containing water (left-hand) and dried zeolite (right-hand). These two volumes are separated from each other through a manual valve.

Opening the valve several things occur at the same time. Due to the dry and therefore highly hydrophilic zeolite (high affinity to adsorb water) a mass transfer of water vapour (cf. blue arrow) from left (water reservoir) to the right (zeolites) takes place.

This so called adsorption process is accompanied by heat dissipation (exothermal process) heating the zeolite up to approximately 50°C. Furthermore the water vapour pressure in the whole system is reduced, as water molecules are adsorbed at the zeolite’s surface. The zeolite acts as a vacuum pump, sucking continuously more and more water vapour from the water reservoir. This, in turn, causes continuous evaporation of water on the left side. As the process is quite fast, the necessary heat to evaporate water (evaporation enthalpy for the phase change from liquid to vapour) is taken from the water reservoir itself, hence the temperature of the remaining water decreases accordingly. Considering this entire process energy - more precisely heat - is pumped from the left to the right volume. Thus the temperature on the left hand side decreases while the temperature on the right hand side increases. The way of heat and mass transport is displayed through a blue arrow within the heat image and the temperature scale shows the impact on the temperature distribution. As can be seen in the cut-out picture, the heat necessary for the evaporation is huge compared to the heat capacity of the water itself (latent vs. sensible heat), therefore the water reservoir freezes. The adsorption process either stops if there is no more liquid water available or more significant if the zeolite is fully hydrated (no more water can be attached to the inner volumes and surface).

In practice heat exchangers are inserted into the volumes in order to supply heat for the evaporation (i.e. cooling of the heat exchanger’s fluid circuit) and to remove the heat at the corresponding volume (e.g. heat rejection at cooling tower, or useful heat for heating purposes). To realise continuously cooling or heating – as it is intended for practical applications – the adsorption material has to be regenerated. Therefore heat is required in order to remove the water molecules from the surface of the zeolite (desorption process). This is realised through the use of solar, waste heat or “gas heat” as it is the case for a gas driven heat pump.

Annotation: The physic, i.e. the driving force, behind thermally driven absorption machines is similar (adsorption versus absorption affinity). Here, solid material is replaced through a liquid mixture “only” which causes significant consequences in constructing machines and its system management.