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Sustainable Energy: Energy From Wood
Energy used directly for heating requires no conversion. However, if we derive our electrical energy from a biological fuel like wood, two conversion processes are involved. Each of these loses some of the energy it receives, delivering only a fraction of its input energy in the new form. These fractions are known as conversion efficiencies.
I shall represent these conversion efficiencies as follows:
ke = efficiency of thermo-mechanical process (eg. in a heat engine)
ka = efficiency of electro-mechanical process (eg. in an alternator)
Two further 'distribution' efficiencies must be taken into account:
kb = efficiency of electrical storage process (eg. as in a battery)
we = efficiency of the electrical distribution (ie. in the wiring)
- The total thermal energy E required from the fuel for both heating and electricity is therefore given by:
- E = eh + ee * ke * ka * kb * we (megajoules per year)
- eh = thermal energy for space heating and hot water
- ee = electrical energy for lighting and electrical devices
Please email me if you have access to rigorous research figures which disagree significantly with the default efficiency percentages above.
Fuel Value of Wood
A given amount of fuel, when burned, releases a corresponding amount of heat. The amount of fuel is expressed in terms of mass, which is measured in kilograms. The corresponding amount of energy released is measured in megajoules. The amount of thermal energy released when a kilogram of fuel is burned is called its thermal yield.
- Therefore, let
- ew = thermal yield of oven-dry wood in Mj/kg.
Unfortunately, it is not economical or practical to dry fuel wood in an oven before using it. We must let the wood dry at first by natural seasoning and then by being left to stand in stacks after cutting. This leaves some residual moisture in the wood. When we burn the wood, some of the heat produced goes to supply the latent heat of evaporation for this bound moisture. This reduces the useful heat delivered when the wood is burned.
- We must therefore include a factor:
- kw = relative yield of wood dried by seasoning and stacking
- Therefore the energy yielded by burning normally dried wood:
- em = kw * ew megajoules per kilogram
- The amount of wood W required to supply our domestic energy needs in kilograms per year is therefore:
- W = E / em
- W = E / (ew * kw)
To produce the wood for our domestic energy needs, we must establish and maintain a plantation which yields a gain of W kilograms of wood per year.
- If we let:
- wy = rate of wood mass production in kg/m² per year
- then the required area of the plantation in m²:
- A = W / wy m²
Please email me if you have access to rigorous research data which disagrees significantly with the default figures shown above.
Part of the total wood yield must be in the form of cut logs for burning in a fire or stove. The remainder must be converted to combustible dust for a heat engine. The logs can be cut from a tree's trunk. The engine fuel dust can be made from its branches, twigs and saw dust. These fractions are computed automatically below from the given data.
If wood off-cuts are reduced to dust using a bacterial method, the energy required to do this is minimal and self-acquiring.
The next step is to convert wood off-cuts and dust into electricity. To do this we need a heat engine, an alternator and a back-up battery.
© January 2001 - Robert John Morton