Both NewScientist and SciAm report that a native North American perennial grass, switch grass Panicum virgatum can produce 540% of the energy used to grow it. It has been naturally growing at field margins in U. S. for years, yet recently become a new focus in bio-fuel development. Without doubt its efficiency in producing energy beats the criticism that bio-fuel growing is not energy-efficient. It even has nearly permanent underground root system that offset 94 percent of the greenhouse gases emitted to cultivate it and from derived ethanol. Neat isn't it?
However I think environment aware farmers will face certain dilemma when they decide whether to plant it or not. By definition it can be grown on marginal land without intensive care. But it would take many households' yield to form a quantity that is worth further process. The ethanol yield on a field is 2,000 to 3,500 litres/ha, which about equals to 1,300 to 2,250 litres of petrol (using data from here and here). For an average truck that carries the grass from a farm to refinery, the fuel supports it to run for 860 to 1,500 kilometres (using 2005 data from here). There was not an estimate of farm-refinery data that I can find, but this report claims that local food travels on average 56 miles to reach institutional market, convert this to kilometres and times 2, the travel from a refinery to a farm and back along takes about 180 kilometres. This part was not calculated in the model used to calculate net energy yields. Considering that the bio-fuel production is very distributed, it is necessary to incorporate this into the analysis. If the switchgrass is really grown on marginal land, it is very possible that the collector will need to travel more miles before obtaining adequate amount of raw materials for production.
If the grass is grown in field scale, not on marginal land, problems for other bio-fuel plants also exist: investment of land and water, fertilizers and so on. Like all technology, it is dangerous if people are too obsessed and invest their land and money blindly on the switchgrass. With a similar unit area ethanol yield to corn, it still will take considerable land to make the production practical and profitable. The discovery itself is remarkable. But it will not be the single solution to energy problem in the US, nor should it become an excuse to put more greenhouse gases into the atmosphere.
Powered by ScribeFire.
没有评论:
发表评论