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Storing energy

Many forms of renewable energy, such as wind or wave power, produce energy at times when the energy is not required. For example, a windmill can produce a lot of electricity after midnight but there is only a small demand for electricity at this time. One way to store electricity is in a battery, but the energy density of batteries is too low for many applications.



A lead battery only stores about 0.1 Megajoule of energy for each kilogramme in weight.

More efficient ways to store electricity need to be investigated.

A chemical compound, such as dimethylether (DME, CH3-O-CH3) stores about 31 Megajoules of energy for each kilogramme in weight, which is a 300 times higher energy density than a lead battery.

Electricity produced during the night could be used to produce dimethylether by combining methane and carbon dioxide. A similar process can be designed to convert biogas which is produced by the anaerobic digestion of biomass.

Biomass, and many forms of organic waste, can be converted into biogas, which is a mixture of methane (CH4) and carbon dioxide (CO2). This is achieved by the process of anaerobic digestion in which in the absence of air or oxygen, microorganisms break down the organic material to give biogas.

Biogas can be converted into dimethylether using catalysts, through the following simple chemical steps:

CH4  +  CO2  =  2H2  +  2CO

2H2  +  CO  =  CH3OH (methanol)

2CH3OH  =  CH3-O-CH3  +  H2O  

DME is a gas at room temperature but is easily compressed into a liquid and so it is easy to store. It can then be transported to be used as a substitute for diesel fuel in an engine or it can be used to generate electricity when the demand for electricity increases during daylight hours.

A diesel engine can run on dimethylether without any modification of the engine.