Dedicated agricultural production for energy
Short rotation forestry
Short rotation forestry refers to the growing of trees (usually willow or poplar) in extremely dense stands, harvested at 3-4 years intervals and regenerated from the stools, which are expected to survive 5 rotations at least. As a rotation crop, SRC is harvested at specific intervals, to provide a regular and constantly renewable supply of fuel. The development of SRC for renewable energy production is a new sector with potential for considerable expansion, offering benefits for growers, developers, consumers, local communities and the environment.
A typical plot might be from 10,000 to 20,000 cuttings per hectare. Planting in twin rows allows harvesting of two rows at a time, usually using direct cut and chip methods (see Harvesting and Transport chapter for details). There would usually be about 1.5 metres between double rows of cuttings so that the side walls of the tires of the tractor are not damaged by the cut crop. 90 cm spacing between rows of cuttings and 75 cm spacing between cuttings in a double row will allow for tractor access. Research is continuing into the optimum spacing between varieties. It is one of the factors, together with better pest management, which may lead to increased productivity. Converting existing arable land to SRC will reduce the amount of agricultural chemicals required as SRC is a low input crop: once established it requires a very much lower input of chemicals than conventional arable crops.
Typical data on short rotation coppice in Europe
|Part of Europe||Scandinavia, British Islands||Central Europe||Mediterranean Europe|
|Crop density stools/ha||18-25,000||10-15,000||8-12,000|
|Av. butt diameter at harvest (mm)||15-30||20-50||20-40|
|Av height at harvest (m)||3.5-5.0||2.5-7.5||2.0-5.0|
|Growing stock at harvest (fresh tons/ha)||30-60||20-45||15-40|
|Moisture content (% weight)||50-55||50-55||40-45|
The system used to harvest, store and transport the SRC crop depends on the scale of the operation, the specification of the end user and a host of local factors such as access and road size. SRC is usually harvested after two to five years’ growth. The average harvesting of SRC, using a chipper or mechanical harvester, is around three hectares a day, depending on the type of harvester used and the size and layout of the plantation. With this in mind, it might be possible to have three different age-classes of SRC in one 10-hectare field.
There are two main systems for harvesting SRC:
Direct cut systems are based on principles used for other agricultural crops, where the whole crop is cut and chipped or billeted in one operation. This system is most likely to be operated by contractors or grower co-operatives because of the initial high investment in machinery. Less expensive tractor mounted versions are also available and may be feasible for smaller operations. In this system, the chipped material is blown into a trailer alongside as the machine moves through the crop. The chips can then be stored on the farm or, if being used elsewhere, be transported to an intermediate store and then on to the end-use plant.
Stick harvesting systems involve a number of operations before the chips are available for use. Sticks are cut with one pass of either a self-propelled or trailed machine, which are less expensive than cut and chip harvesters. The sticks are then laid on the headland. From here they are loaded onto a tractor with suitable attachments and transported to the farm storage area, where they are stacked and stored. The advantage of this system is that there is less investment in machinery, and if necessary the wood can be stored outside, without decomposing. The sticks can then either be chipped and transported as chips, or can be transported as sticks and processed at the end-use plant.
For more information