Rising energy costs, ambitious CO2 targets, and perhaps most of all, public image – compelling reasons why many firms are paying ever more attention to improving their energy conservation. Using waste heat efficiently is a good place to start. Many firms already recycle waste heat, though often not to its full potential. The Steinbeis Transfer Center for Applied Thermodynamics, Power and Combustion Engineering (ATEV) advises clients on how to make maximum use of waste heat.
Heat energy can be stored in many ways. In “sensitive” storage devices, the storage material – which may be water or a solid – is simply heated. Thermochemical storage devices use a highly porous storage material which is dried while being charged with energy. Neither of these methods is particularly suitable for mobile applications – which would require a transportable heatabsorbing storage container. Latent heat storage devices are far more suitable. These store waste heat at a constant temperature, via state change from solid to liquid in the storage material. Upon discharge, heat is released as the material solidifies again. Various storage materials such as paraffins, salts, salt hydrates or fatty acids can be used, depending on the temperature range of the waste heat source.
However, the thermal conductivity of most materials used is relatively low. To attain optimal efficiency and cost-effectiveness when storing and releasing heat, the heat transfer method during charging and discharging of the latent heat storage device must be adapted to the needs of the system. Customizing the device’s dimensions for each application improves the cost-effectiveness of this type of heat transport, even compared to local or district heating. The efficiency depends not only on the distance between the heat source and heat absorber, but also on the speed of charge and discharge. The Steinbeis Transfer Center ATEV in Bayreuth helps firms assess the economic viability of systems, brings customers and heat providers together, and works permanently on the improvement of storage methods.
If no customers can be found within an acceptable distance of the waste heat source, then power can be generated using the Organic Rankine Cycle (ORC). Power produced this way can be used for individual consumption or fed back into the power grid. The ORC process is fundamentally analogous to the classic Rankine cycle. Still used by steam power plants today, the classic process cannot easily be scaled down to efficiently generate energy from waste heat at low temperatures and quantities. So instead of water, the ORC method uses organic liquids which are far more suitable on a smaller scale. Simply selecting the most suitable fluid can result in significant optimization – and the Steinbeis team routinely selects the ideal fluid from a choice of over 1000. Energy efficiency is just one of many criteria considered here – factors such as toxicity, flammability and chemical stability are also key to safe operation and low maintenance. Using a mixture of fluids also increases efficiency, and changing circuit layouts is another area with optimization potential.
Mobile latent heat storage devices and power generation via ORC devices both result in increased energy efficiency. Crucially, these optimization concepts must be individually adapted to each application – they can’t just be applied as a simple, all-in-one package. When it comes to custom optimization, the Steinbeis team in Bayreuth are the ideal goto people.
Professor Dr.-Ing. Dieter Brüggemann
Steinbeis Transfer Center for Applied Thermodynamics, Power and Combustion Engineering (ATEV)
(Bayreuth)
stz311@stw.de