Energy storage

In order to meet our energy requirements in the future, energy generated from sustainable sources will have to be available in the right place and at the right time to the end-user, within the built-up environment, in industrial production and in our vehicles etc. In order to close the gap between the time and place that the energy is generated, we need new storage systems that are capable of storing heat or electricity for long periods of time.

Electricity storage

Hydrogen is an important candidate for electricity storage as it is a clean substance and due to the fact that water, the raw material from which hydrogen can be released by means of electricity (electrolysis) for example, is available everywhere. The drawback of hydrogen, however, is that it has a much lower energy density in comparison with petrol for example. One litre of liquid hydrogen, stored at an extremely low temperature (-254 °C) contains one quarter of the energy contained within one litre of petrol. If the fuel cells of the future (that convert the hydrogen into electricity for driving the wheels) are going to provide a higher efficiency than the combustion engines of today, we can nonetheless produce interesting systems. Thorough searches are being conducted into new, economical storage methods, in which hydrogen is encased in suitable materials in molecular or atomic form, as a result of which such extreme conditions are not necessary.

A marvellous solution for storage systems during transportation may also lie in a new type of battery. Light and compact battery systems must be developed for this purpose, which are able to deliver large quantities of power in a short period of time, cause less damage to the environment, and which have a longer life-span than the current storage batteries. Lithium-ion batteries are likely candidates at present, but still require a lengthy development process in order to produce battery materials that can be used in high-power applications. Lithium-ion batteries make use of chemical processes in which Li is charged in the crystals of suitable electrode materials. Another development is that of the ‘supercapacitors’, in which electrical is stored in the surface of nanostructured electrodes. If batteries and/or supercapacitors are combined with combustion engines in cars, this would give rise to considerable savings on fuel consumption. Expectations are that the market share of these so-called hybrid cars, which at present is still relatively small, will increase sharply in the coming years.

Heat storage

Heat can be stored easily in a tank with warm water. The stored heat can be used at a later time to heat tap water or a room. Examples of such heat storage systems include the water tank and a boiler or combination boiler. These traditional systems waste a large amount of energy, however. The energy density is also relatively low, so that a large volume is needed for a completely sustainable energy supply.

The first problem can be solved through the use of vacuum insulation around the storage tank, instead of traditional insulation materials such as polyurethane or polypropylene. Vacuum insulation increases the insulation value of the tank three-fold in comparison to traditional insulation materials. This not only improves the insulation of the heat storage, but also reduces the use of insulation materials that are harmful to the environment. Research into vacuum insulation for heat storage is currently still being conducted at laboratory level, but it is expected that this method will be applied successfully on a large scale in a few years from now.

A promising alternative for storing heat in water is thermochemical heat storage. In this process, a thermochemical material (C) is converted into two other components (A and B) with the addition of heat and each of these is stored separately:

C + heat -> A+B

By combining the two components (A and B) at such time that is required, they revert to the original material. During this reaction, the heat that was previously added to the thermochemical material is recovered:

A + B -> C + heat

Thermochemical heat storage is a highly compact form of heat storage, as ten times more heat can be stored per litre of thermochemical material than in water. Research into thermochemical heat storage is still in the early stages, whereby studies are being carried out into new materials and optimum reactor concepts.

Energy storage systems are less visible than solar panels, windmills or power stations, however are extremely essential to sustainable energy supply systems. A great deal of (fundamental) research is still required in a number of different fields before we can establish a smooth link between the generation of energy from sustainable sources and the use of that energy.