A new type of heat pump helps to use heat more efficiently

Half of all energy in Switzerland is consumed for the heating and cooling of buildings, the production of hot water or the driving of industrial processes. If we exclusively look at electricity consumption, 40 % of this is used for heating and cooling rooms or materials. And this energy still primarily comes from fossil sources and nuclear energy. “If thermal energy was used more efficiently, this would reduce both CO₂ emissions and our dependence on nuclear energy”, says Andreas Häberle, a Professor for Renewable Energies and Environmental Technology at the University of Applied Sciences Rapperswil.

Among other things, more efficient use means making more systematic use of waste heat. At present, valuable heat energy is still lost in many places. This could be collected using so-called adsorption heat pumps. These could, for example, collect waste heat from factories or computer centres or increase energy from solar thermal systems. Until now, however, the use of such systems has failed due to their high investment costs. For this reason, Häberle and his research team have further developed the design of adsorption heat pumps in a sub-project of the joint project “Heat utilisation with solid sorption technology”. They equipped the interior of the system with newly developed, optimised components and in doing so improved the heat flow within the system. This makes the newly designed heat pump more efficient and economical than previous systems.

Heat exchanger as a cost factor

Like the compression heat pumps already widely used today, adsorption heat pumps can also extract heat from their surroundings and increase it. To this end, however, they need a higher initial temperature of 50 to 60 degrees Celsius. They do, however, have a decisive advantage over conventional heat pumps: they require almost no electricity and instead primarily use heat as a drive source.

The process works as follows: the environmental heat is firstly used to evaporate a refrigerant – this is usually simply water in adsorption heat pumps. The water vapour is fed into an adsorption heat exchanger and then adsorbed and compressed inside by a sorption material. This further heats the adsorbed steam and the sorption material. The system now needs some additional thermal energy at a higher temperature to release the hotter steam once more. In technical jargon, this is referred to as desorbing. The heat gained can finally be fed into a heating circuit. However, just like conventional heat pumps, adsorption heat pumps can also work in the opposite direction for cooling.

The most expensive part of such a system is the adsorption heat exchanger. The researchers have therefore newly developed the components of the heat exchanger and initially built and characterised them on a small scale under laboratory conditions. To this end, they constructed a measuring chamber which they were able to place under vacuum. In this measuring chamber, they tested the new heat exchanger element with adsorption-desorption cycles of different lengths and at different drive temperatures. Using a scale installed in the chamber, they determined how much refrigerant was absorbed by the sorption material inside the heat exchanger in each case. For example, the researchers investigated the effects of a drop in pressure and also determined the performance of the heat exchanger during heating and cooling.

In performing their tests and optimisations, the researchers had one specific application in mind: the cooling of a computer centre. Here, they based their activities on the work of another sub-project in which a total of four promising scenarios for the future application of adsorption heat pumps had been designed and evaluated.

The system becomes larger

After the environmental technicians had optimally adjusted the heat exchanger element for computer centre cooling in the laboratory tests, they increased the scale: they constructed somewhat more efficient elements from aluminium – with a length of 80 centimetres instead of 50 as before – and combined six such elements to create a heat exchanger.

When designing the elements, the team made sure that they could be adapted for different sorption materials. This is because the sorption material inside the heat exchangers is also decisive for the efficiency of the process – a separate sub-project therefore aimed to find a new, high-performance material for the new heat pump. It should be possible to integrate this into the new heat exchangers at a later time.

Häberle’s team also developed a new evaporator and condenser. The result was an optimised adsorption heat pump with a cooling capacity of 10 kilowatts. For the sake of comparison, a 160- to 200-square-metre room could be air-conditioned with this output.

The environmental engineers initially tested the new heat pump with a widespread and inexpensive sorption material made from silica gel beads before later doing so with a more efficient but much more expensive organic-metallic material made of aluminium fumarate.

In future, however, the system is to function with the carbon sorption material developed specifically as part of the associated sub-project – and will do so more efficiently and at a lower cost than all previous adsorption heat pumps. According to environmental engineer Häberle, there should thus be nothing more standing in the way of the optimisation and increased introduction of the systems.