In the context of energy supply, chemistry and process engineering play a dual role: on the one hand, they are solution providers and develop materials for batteries and for thermal or chemical storage systems. On the other hand, they themselves use energy storage in order to optimise process heat management. Reason enough to choose ’Innovative energy carriers and storage’ as one of the focus topics of Achema 2012.
The Author: Dr. Kathrin Rübberdt Head of PR and Biotechnology, Dechema
No sooner had Germany announced an energy turnaround, than this triggered an intense public discussion on energy storage. So far, the focus has been mainly on electricity storage for electromobility, but with the increasing importance of renewable energies, securing grid stability has moved to the top of the agenda. The U.S. Energy Storage Association (electricitystorage.org) has compiled an overview that gives a first impression of the broad variety of available technologies and of the factors that are relevant to choosing the right solution.
From the range of technologies available, Li-ion batteries are the solution of choice for electric vehicles; researchers are working intensively on their further development. Grid electric storage systems are currently being broadly discussed, the emphasis being on mechanical (pump storage systems, compressed air storage plants) and electrical storage, for example redox flow systems.
Thermal storage as an alternative is basically still eclipsed by electrical storage. In some solar-thermal power plants, it is used to balance the electricity production between day and night. But there are many more instances where energy is released in the form of heat – and which depend on yet more heat. According to Fraunhofer ISI, for example, German industry accounts for about 30 % of the total power demand. In many industries, but especially in the chemical industry, energy mainly appears as heat. What could be more obvious than to dispense with conversion into other forms of energy, which is bound to cause losses, and to focus on thermal storage instead?
Taking conditions into account
Thermal storage systems differ; a whole panoply of basic technologies with various parameters, depending on specific requirements, is on offer. Essential decision factors include storage capacity, charging and discharging time, storage periods, efficiency factors, lifetime, the number of charging cycles, and costs. Moreover, questions of system integration with regard to transmission media, charging and discharging conditions like temperature and pressure and, not least, space requirements, are also important.
In principle, thermal storage systems can be classified into four categories. Sensible heat stores absorb heat by changing their own temperature: heat is transmitted in one step to a suitable carrier, water being the simplest example; in technical applications, usually more sophisticated tailor-made media are used, such as heat carrier oils, molten nitrates in solar-thermal power plants, ionic liquids or ceramic solids. The German Aerospace Center (DLR) is currently working on solid storage materials that are at the same time cheap and usable with diverse heat carriers – thermal oil, steam or ionic liquids – so that they can be adapted to different applications. The actual heat storage takes place in the solid; the carrier – for example steam – is released directly from the process. Unlike salts, these solids can be employed over a much larger temperature range.
Phase change materials (PCMs), by contrast, are latent heat stores; they absorb heat and change their condition of aggregation without changing their temperature. Latent heat storage systems based on paraffin are already used in buildings. Their major drawback is their low thermal conductivity that limits charging and discharging time and temperature. One solution is microcapsules that significantly increase the surface, or – for industrial applications – macro-encapsulation whereby steel or graphite fins with good thermal conductivity are arranged in parallel in the storage material thus ensuring efficient heat transfer.
Adsorption storage a beacon of hope?
At the interface between heat storage and chemical storage, adsorption storage is currently a beacon of hope on the horizon. When gas molecules are adsorbed on a surface, heat is released; when heat is applied, the molecules can be removed from the surface. To date, some technical applications based on zeolites have been implemented. However, many projects targeting the development of new, functional materials with optimised structures and properties are well underway.
Chemical storage takes the concept one step further. It includes not only hydrogen or methane generation, but also a whole list of reversible reactions, the best known being hydrate formation of salts. The advantages of durability and transportability are counteracted by technical difficulties caused, among other factors, by the volume and, therefore, the pressure change in the course of the reaction. The latest developments are based on composite materials based on salt and zeolites or bentonite. They combine the benefits of both systems, namely high storage density, as in salts, with the transportability and lower agglomeration tendency of zeolites or passive carrier materials. What is more, the higher surface of the composites leads to faster charging and discharging.
Opening up new horizons
At first glance, the technical implementation of heat storage may seem simple enough. But the large number of adjustable parameters requires on the one hand exact setting, on the other hand it opens up a variety of opportu-nities for customising heat storage systems to specific requirements. Apart from the choice of basic technology – latent storage, adsorption or chemical storage – the design of the appa-ratus and the system leaves scope for adjustments: heat transmission in one or more steps, using pressure differences in multi-step storage processes, optimizing capacities and charging/discharging time by varying the bed and the flow or adopting moving bed reactors, researchers and developers can give free rein to their imagination. Accordingly, there is still a tremendous need for R&D in this area; but it is worth the effort, as in many cases new hori-zons will open up beyond the often already tapped potential for energy saving.
The special show ’Innovative energy carriers and storage’ is located in Hall 9.2. Topics are: Concepts for efficient energy storage and transport; chemical energy storage, solar chemical processes; photovoltaic materials and applications; innovative batteries and supercapacitors; advances in fuel cell technology; concepts for e-mobility; hydrogen economy. More information at achema.de.