What kind of energy diagnosis ?

How should the energy expenditures due to the farm productions be taken into account, how should the global energy efficiency be evaluated and how can it be optimized ?

 1. The ’farm system’ and the « bilan PLANETE »

The most common method used for energy and greenhouse gas effect assessment in France is the PLANETE balance , from the Solagro association. This “energy balance on the farm” takes into account :
 INPUTS : the consumption of direct energy (fuels) and of indirect energy (energy that was needed to produce fuels or synthetic fertilizers and to make them available on the farm)
 OUTPUTS : products’ intrinsic energy, often based upon the lower heating value (LHV : heat obtained from combustion of the products)

The global energy efficiency is expressed as the energy output over energy input ratio. But this results in an artifact consisting in giving priority to selling products outside of the farm instead of self consuming them, which goes against common sense.

Thus, a farmer whose rape production is self consumed to make pure vegetable oil, which he’ll be using as a source of fuel, will provoke his energy balance to get worse as :

 the energy output decrease (reduction of rape sales) is higher than…

 the input energy decrease (reduction of fuel purchases) _ _

Indeed, quite often the LHV of a kilogram of product (OUTPUT energy) is much higher than the energy expenditures required to produce that kilogram. (INPUT energy).

 2. The life cycle analysis

The PLANETE method originates from the life cycle analysis (LCA) method, which takes into account the environmental impacts caused by the production of a good/service from the cradle to the grave, that is to say from extraction of raw material to destruction or recycling of the product (see picture above for a car !). However, the PLANETE method differs from the Life cycle analysis in that it prides itself on a full balance of the “farm system”, including productions as a whole without making distinction between them.

On the contrary, the LCA expresses fossil energy consumption for each type of production, for example for every kilogram of cereal, meat or milk produced on the farm. Therefore in the example given above, using pure vegetable oil would result in a decrease in this kind of energy consumption in the farm’s product balance (through the substitution of diesel by vegetable oil). Therefore the production of one liter of milk would enable the reduction of fossil energy consumption by 10% to produce one kilogram of cereals.

Of course, the difficulty would be to diversify the farm’s energy consumption between these different products, and you still have to fix, independently from the energy balance, production goals.

 3. Should the energy balance really be optimized ?

The reflexion carried out within the Grignon Positive Energy project suggests :

 either to include the INTRINSIC energy of the input products in the global energy efficiency calculation so as to avoid the artifact mentioned previously

 or to keep an indicator such as the LCA (Mega joules consumed/meat, milk or grains sold), the difficulty is then to be able to set with a certain degree of flexibility the production of each.

In any case, the optimization mustn’t rely only on the energy criterion-rather, this criterion should be regarded as a result (maybe restrictive) of the different change scenarios of the farm system targeting production practices considered as interesting at first sight. (see article about good practices )