Stuttgart / Wuppertal, July 2016.
Two years ago, the Institut für Thermodynamic und Thermal Engineering received an order from HAPPICH GmbH, the leading international supplier for interior fittings for commercial vehicles, to develop a climate-neutral process for the stationary air-conditioning of truck cabins.
Dr. Kerskes: You and your employees have now designed and started a first a prototype. Why is the stationary air conditioning of truck cabins an issue for research at all?
Dr. Kerskes: The starting point is that the conventional stationary cooling of a truck driver's cab today is still based more or less climate-damaging refrigerants (CFCs or today C0², among others) and is often still operated by the stationary combustion engine. About 4 litres of fuel per hour – mostly diesel – are consumed here. Not only in warm, tropical countries, but also in temperate climate zones such as in Central and Northern Europe in summer time, a gigantic but avoidable fuel consumption takes place here. For this reason alone, it is clear that our project to develop a climate-neutral stand cooling system for truck cabins is important. This enormous relief potential was also seen by the NRW Ministry of Science, which approved the application for research funding from our client, HAPPICH GmbH.
How did HAPPICH GmbH come to commission you as the Institute for Thermodynamics and Thermal Engineering with the development?
HAPPICH GmbH approached us with the idea and recognised us as a suitable partner for the development of an innovative, resource-saving technology for the stationary cooling of truck cabins. Our institute has developed a high level of expertise over decades. Thus, we were able to convince the developers at HAPPICH that we are not only prototyping a functioning stationary cooling system for truck cabins, but that we are also working on the basis of theoretical and practical research to develop a usable basis for the intended series production readiness for equipping future truck fleets.
What are the specifications? What requirements should the new stationary air-conditioning system for truck cabins meet?
Cold generation should be independent of engine operation, especially when the engine is at a standstill, and should occur without additional energy input, e.g. only by using ambient and exhaust heat. The concept that has been aspired to and now prototypically implemented is a very cheap and simple procedure that makes use of a simple physical process. In addition, the process is based on harmless and unproblematic substances: Water and zeolites. And by dispensing with additional energy, this stationary climate control system is completely emission-free and climate-neutral. Since no moving parts are required, this system is also extremely low-maintenance. This auxiliary cooling system can even be integrated into existing air conditioning systems or retrofitted as an independent system.
This should be very attractive for truck manufacturers. What is the idea behind it? Water and zeolites play an important role in the new climate-neutral cooling of truck cabins. How does it work?
When the engine is switched off, we cool the truck cab by adsorption cooling, which we obtain by evaporating water in a closed vacuum system at low temperature. We take the necessary energy from the warm air in the truck cab. Water is drawn capillary into an evaporator, creating the required evaporative cooling, which is fed directly into the truck cab for cooling.
In parallel, we store water vapour, which is known to take up 1000 times the volume of liquid water, in zeolites in an extremely space-saving way. These originally volcanic minerals (aluminosilicates) in the form of highly porous globules have a fantastic adsorption capacity: only 1 gram spread out results in a surface area of 1,000 square metres! During the cooling phase, the zeolite gradually absorbs the water vapour until the entire quantity of water has been temporarily stored.
When the truck is back in operation, the exhaust heat from the combustion engine is used to heat the zeolite, which is saturated with water, and to dehydrate it completely (desorption). The water in the zeolite escapes as steam, liquefies in a condenser and flows back into the water tank – ready for the next cooling cycle.
By the way: Zeolite and water are unused and unchanged and can be used for adsorption or desorption as often as desired.
How efficient is this climate-neutral stationary air conditioning?
The current prototype is designed for an output of 1.3 kW for the first hour and then for approx. 500 watts for a further 7 hours to keep the cooled air at the target temperature – depending on the cooling requirements of the cabin. The cooling capacity is approx. 6 to 8 kWh.
Are you the only one who is researching the air conditioning of truck cabins?
A number of companies and institutions, truck manufacturers themselves as well as researching colleagues are dealing with alternative cooling processes for truck cabins during the stand. The process described above for the production of adsorption refrigeration using water and zeolite is already on the market – with deviations – for example in domestic air conditioning technology. In the prototype we developed for HAPPICH, the adsorption cooling process based on zeolite and water is less new than its use for cooling in truck cabins. HAPPICH has already applied for a patent for this limited area of application, which is also very important in terms of market volume.
Where are the other challenges now? What are the next development steps?
As a university, our primary goal is to generate new knowledge. We deal experimentally and practically with the physical processes. That is why we are working on numerically mapping and mathematically describing all processes and functions down to the last detail. By commissioning the prototype, we now know that the basic principle works as expected. We still have to secure this in long-term operation. The reliable controllability of the system is also a development step on which we are working.
The next challenge is to increase the performance of air conditioning while simultaneously reducing the size of the unit. This is a particularly important step, as it makes it easier to integrate the system into the vehicle periphery. After that, the next long-term test phase will take place under real conditions, not least on a moving truck.