‘An idea for storing renewable energy’
was first posted on ccq.org.uk, 25th March 2012
Question “Is it a good idea or barmy?”
1. There are times when renewable energy cannot be used. Sources tell me that 20% or more of wind energy is grounded (i.e. thrown away) because sometimes the generated energy cannot be used. Other renewable sources (solar, wave power) are also intermittent. Biomass is an exception.
2. Climate change is worse that the official view. There will be greater awareness of the severity of climate change this year or next. This will increase efforts to extract carbon dioxide from the atmosphere
3. Carbon dioxide can be extracted from the atmosphere by using biomass in power plants which have carbon capture and storage. A reasonable carbon price will give the right market signals to make this way of extracting carbon dioxide from the atmosphere economically viable.
4. Biomass is difficult to burn in conventional power stations needing extra capital expenditure because it is bulky, has high moisture content and rots.
5. Torrefaction is a mild form of pyrolysis at temperatures typically ranging between 200 to 320 °C, where biomass is “roasted”. Torrefaction leads to a dry product with no biological activity (no rotting). The volumetric energy density (GJ/m3) of biomass increases by approximately 70%. This leads to savings in the transport, storage and handling of biomass between source and end-user location.
6 Torrefied biomass is easier and cheaper to cofire with coal. “The coal-like characteristics of torrefied biomass make it possible to blend it with coal in much higher proportions than are achievable with untreated biomass, such as wood pellets. Wood pellets are typically co-fired with coal at a rate of approximately 5 to 10%, whereas torrefied biomass is expected to be achieve co-firing rates in the range of 50 to 80%.”
7. Torrefaction generates various gases and oils as well as the torrified biomass. Typically the non-condensable gases (e.g. CO and CH4) are recycled and burnt providing process energy for the torrefaction.
8. This proposal is for a farm scale torrefaction scheme which uses the surplus energy from other renewable sources such as wind or solar as the process energy for torrefaction (or cheap energy when the grid has too much capacity). The resulting gasses are stored to generate energy when it is required (See Dynamic Demand ). Ideally the torrefied biomass can then be used for cofiring in a plant with carbon capture. It would achieve the aim of storing some renewable energy in the non-condensable gases from torrefaction.
9. Smart grid technology and pricing may be necessary to make such schemes economic.
10. An pilot scheme should consider a farm growing 100 dry tonnes of willow per year.