We are planning a‘Biomass’ heating system at Tory Bush, where all our energy will come from wood pellets, the pellets used to be made from compressed surplus sawdust from one of N I’s biggest saw mills. Now because of the increased demand ordinary lumber is being used to make the pellets, no longer the waste-product of the mill and I am sure timber is being specifically grown for pellet production. I see a time, with this switch to Biomass, which includes wood burning stoves, that timber will become a premium here in Ireland and we will have to import it and the question will be, is that any better than importing coal and oil.
As part of my planning research for this switch from Fossil Fuel to Biomass I came across the following perceptive quote,
“The fuel of the future is going to come from fruit, weeds, sawdust and almost anything. There’s enough energy in an acre of potatoes to drive the machinery necessary to cultivate the field for one hundred years.”
Henry Ford 1925.
I just wondered if this was a correct statement so I did the following calculation.
According to Scottish Agriculture it takes about 112 litres of Diesel to grow and harvest an acre of Potatoes, Diesel has an energy content of 32-40 megajoules per Litre so conservatively 3600 megajoules of energy is used per acre.
100 grams of raw potato has 322 kilojoules, which is actually quite low against other staples such as Rice and Wheat, both about 1500 kilojoules per 100gms, but then ‘spuds’ are 80% water.
A kilogram of potatoes would have 3,220 kilojoules, or 3.2 megajoules and a tonne of potatoes would have 3,200 megajoules.
According to Wiki “Potato farms in the United States were the most productive in 2010, with a nationwide average of 44.3 tonnes per hectareUnited Kingdom was a close second.”
This is equivalent to about 18 tonnes per acre which using the energy value of potatoes is 57,600 megajoules of energy produced per acre.
So dividing the energy produced by the acre of potatoes by the energy required to produce them it gives the figure of 16, i.e. the energy in one crop of potatoes gives enough to cultivate the field for 16 years, not quite 100 years.
However when Henry Ford was doing his calculation it was probably based on 3-4 mechanical actions preparing the land, sowing the seed potatoes and then harvesting.
In modern potato farming there can be as many as 15 different actions in the total cultivation of potatoes, three or four of which can be the spraying of Herbicides, Insecticides, Fungicides (the dreaded Blight), fertilisers, stalk shortners, etc. I studied agriculture in the early 80’s and the policy was to spray by the calendar whether the crop required it or not, at least now a bit more attention is paid to the weather and spraying is more proactive than prescriptive.