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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderAnalysis of energy cost of integrated systems
View the document(introduction...)
View the documentEnergy cost and energy requirement
View the documentWhy energy analysis?
View the documentNet energy intensity
View the documentWhat criteria are offered by energy analysis?
View the documentConclusion
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Net energy intensity

Energy analysis of biological systems reveals its strength most clearly when dealing with the net energy of systems aimed at producing energy, i.e., biomass systems (Table 1). Net energy is the output energy of the system minus all of the energy equivalents of inputs drawn from other parts of the economic system expressed as GJ/ha* yr.* The decision on which process to espouse will then depend on the relative value of a GJ of energy in relation to a hectare of land. Today a barrel of oil, delivered on site, costs perhaps US$20. Table 1 presents data on alcohol produced by enzyme hydrolysis of cassava tops. The net energy of such a process is in the order of 2.6 barrels of oil per hectare of land per year. Thus, the process can only be economical if land rent is less than $20 x 2.6 = $52/ha, which is a low price for land. Process 2 in the table can never be economical. Processes 3 and 4 are viable at land rents of less than $104 and $160 per ha, respectively. If land rents already exceed these levels, biomass systems can never succeed unless land is deliverately taken out of the market system.

TABLE 1. Net Energy of Photo-Biological Fuels

Fuel Raw material Process Net




barrels of


Alcohol Cassava tops

+ tubers

1. Enzyme


17. 2.6
Alcohol Eucalyptus 2. Acid


- 180 - 27
Methane Cereal straw 3. Anaerobic


34. 5.2
Pyrolytic oil Eucalyptus 4 Flash


52. 8.

Source: Rawitscher and Mayer 19). Net energy is the energy content of the product minus the GER of ail the inputs to make the product, but not counting solar energy, whose contribution is a function of land area, climate, latitude, and plant type

Net energy is related to cost. Consider a system of h hectares, having a land rent of r $/ha yr and a production system (growing, harvesting, and conversion) having an amortized cost of c $/yr and current inputs of p $/yr, yielding a gross output of Q GJ of biomass energy. If m is the internal recycle of that biomass energy, the apparent energy output of the system is y = Q - m. The true net energy is less, because c + p have an energy requirements given by c x lc + p x lp, where lc and lp are the energy intensities of capital and inputs, respectively (GJ/$).

Net energy/ha x yr,

While costs are

From this it follows that,



That is, cost drops as N increases. Empirical evidence on the few systems for which data are available suggests that biomass becomes economical at a net energy of about 100 GJ/ha x yr at present energy prices.

It is not always realized by proponents of biomass energy how land-intensive it is. Table 2 lists the land area that must be sequestered for one year to provide certain economic services (assuming a net energy of 100 GJ/ha x yr).

TABLE 2. Land Areas Sequestered for One Year for Biomass System of 100 GJ/ha x yr Net Energy to Provide the Services Listed Below

3,000 mile flight in Boeing 707 .15 ha/person
3,000 mile flight in jet .38 ha/person
Trucking 10 tons 1,000 miles .23 ha/ton
Air freighting 10 tons 1,000 miles 2.2 ha/ton
Making copper 1.0 ha/ton
Making nitrogen fertilizer .35 ha/ton
Pumping 1,000 m³ of irrigation .4 ha/1,000 m³
water from 100 m depth .3 acre/acre-foot