In a coarse grain global analysis the
average total power used by humans is given, and compared with
total solar insolation on land. The theoretically possible, and
the actual overall efficiency of the conversion of solar energy
by technical and biological means is determined. The resulting
limitations of biomass energy for replacing fossil fuels are considered.
Other problems of energy farming are analyzed. Conclusions are
drawn, and future energy policies are recommended.
There is a world wide trend to switch from fossil
fuels to biomass energy. While it may be useful to use biomass
waste and energy farming in some locations, the large scale use
of biomass to replace of fossil fuels is problematic and needs
careful analysis. The first question is to see what the energy
needs of humankind are.
Average Total Power Consumption
Humankind's total primary energy consumption is some 470 EJ/a
, which translates into an average total power of
some 15 TW. With a world population of 6.5 billion people ,
the average total power use is at present 2300 W per person.
Total energy use of countries can be derived
from the same sources, or from . Canada for example
uses 14.3 EJ/a, which translates into average power of 0.46 TW,
or 14 kW/person. By comparison, Niger's total energy use is 0.017
EJ/a. which translates into an average power consumption of 43
The power consumption by sector is approximately
33% of total power each for industry and commerce, households,
and transportation; in per capita terms, the average world citizen
consumes 800 W for each sector: production/trade, residential,
Electricity is practical in many applications,
and hence an essential part of total power in each sector. The
average electric power used is according to the US Energy Information
Administration : global average 300 W/person, in
Canada 2000 W/person, and in Niger 2 W/person.
The composition of the world's primary energy
can be found on a University of Michigan website .
It is in approximate numbers:
|Biomass and wastes
|Solar wind geothermal
Fossil fuels supply at present the bulk of world
energy; as their availability is limited, and as their use contributes
to global warming, they need to be replaced. Nuclear energy has
problems of its own, and should also be replaced by more benign
technology based on solar energy.
Insolation: the Physical Base of Green Energy
The solar constant at the Earth's orbit is 1370
W/m2 perpendicular to the solar rays. 30 % is reflected back into
space. Thus, the Earth receives 960 W/m2 of its cross section
(1.27 *1014 m2), which is a total insolation available at the
Earth's surface of 1.22*1017 W, or 19 MW/person for the present
world population; solar energy received at the Earth's surface
is some 10 000 times more than humans are presently using from
Distributed over the surface of the sphere,
which is 4 times the cross section, yields a day and night global
average of 240 W/m2 on the surface. Equatorial regions get some
400 W/m2, while the inhabited regions in higher latitudes will
receive around 200 W/m2 on a horizontal surface .
Using the global average insolation, 10 m2/person of horizontal
surface receive the amount of energy presently used by humans
on a global average.
Technical Solar Energy Conversion
The collection area required to satisfy human
energy needs depends on the efficiency of the collection method.
Solar cells reach efficiencies greater than 20% ,
producing on average some 50 W/m2 of electrical power. Electrical
energy can supply both, the electricity proper, and transportation.
Therefore, in order to supply 300 W/person electrical power consumption
and 800 W/person in transportation needs, some 22 m2/person of
solar cell collectors are required.
The global average need for thermal power is
1200 W/person; this is determined by subtracting electrical power
and the power for transport from the total power. The achievable
solar thermal efficiency is above 60% , which delivers
on average 145 W/m2 of thermal power. Therefore, the direct use
of solar thermal power requires a collector surface of approximately
In total, technical collection of all of humankind's
present energy needs requires solar collector area of some 30
m2/person on buildings or on dry land. By contrast, biomass conversion
of solar energy is less efficient, and requires water, fertilizers,
and biologically productive land.
Biomass Energy Generation in Theory and Practice
The central part of the solar spectrum is photosynthetically
active radiation. Only 45% of solar radiation energy is carried
by this part of the spectrum. A further reduction of biological
solar energy conversion efficiency is due to the fact that some
of the qualified photons absorbed by the plant fail to perform
photosynthesis; the quantum efficiency is given as 25%, which
reduces the conversion efficiency to 11%. In addition, some of
the solar radiation is reflected, and photosynthesis requires
respiration which requires energy. Thus, a realistic expectation
for the efficiency by which solar radiation energy can be converted
into biomass energy is 3% to 6% . This theoretical
efficiency is 10 times lower than the technical conversion efficiency.
Hence some 300 m2/person of biologically productive land is required
to supply the total present energy needs of humankind. In addition,
transpiration of water is required for this photosynthesis to
take place. Water needs for transpiration depend on conditions;
the University of Prince Edward Island website states that between
250 g to 700 g water are needed for the photosynthesis of 1 g
of dry biomass .
In practice, the efficiency of biomass conversion
is much less than the theory predicts. An energy crop data base
developed by the Oak Ridge National Laboratory 
offers realistic yields of unirrigated switchgrass and hybrid
poplar plantations. The data for Barbor, Alabama may serve as
an example. The median annual yield for switch grass, planted
on former cropland, is 8.6 dry tons/acre; for hybrid poplar it
is 4.1dry tons/acre. In SI units this represents an average dry
matter production rate per square metre of 61µg/s, and 29
µg/s respectively. Using a heating value of 15 kJ/g, the
biomass power generation rate is 0.92 W/m2 for switchgrass, and
0.44 W/m2 for hybrid poplar. These values represent the energy
harvested. The net overall efficiency is further reduced by the
energy requirements to plant, harvest, dry, transport, process
the crop into a suitable transportation fuel, and by the thermodynamic
efficiency in electricity generation. In the end, the realistic
overall power of biological conversion of solar energy to satisfy
present human needs is less than 0.5 W/m2. Therefore, replacing
the 2080 W/person presently derived from fossil fuels and nuclear
energy with biomass energy requires more than 4000 m2/person of
biologically productive land.
Global limits to Food and Energy Crops
A study of net primary productivity and energy
fixation for the world done by Lieth  confirms
the low efficiency of biological conversion of solar energy; only
tropical rainforests and wetlands generate biomass energy at a
rate of 1 W/m2; other forms of vegetation have lower yields.
According to Lieth 1.4*1013 m2 of land world
wide is cultivated or used for permanent crops; this amounts to
2150 m2/person. The land used world wide for agriculture produces
biomass energy at a rate 0.36 W/m2, or 774 W/person. Systematic
utilization of agricultural waste and byproducts of the food system
can contribute a few hundred watts per person to the total power
consumption. However, to supply the remaining present energy needs
from biomass is physically not feasible, as it requires additional
4000 m2/person of biologically productive land, which is not available
on Planet Earth.
There are other reasons that prevent the large
scale use of biomass for oil replacement. Energy farming is in
direct competition with food production for land, for water, and
for fertilizer. It is no secret that humankind is already struggling
to eliminate hunger; therefore, to take land, water, and fertilizers
away from food production is, in a global perspective, not an
option. For example, to run one SUV on ethanol would require an
amount of grain sufficient to feed 26 people, according to Lester
Furthermore, energy farming, like agriculture,
is an enemy of biodiversity. Any land taken away from wilderness
destroys habitat and contributes to the mass extinction of species.
However, this will inevitably happen with increasing use of biomass
fuels. Indonesia is planning to cut down rainforests in order
to supply more palm oil . Brazil threatens the
Amazon rain forest by exporting ethanol from sugarcane, and soya
based diesel fuel .
The problems of large scale global use of biomass
can be visualized by comparing it with food energy. A person needs
some 100 W of food energy -- some 2000 Cal/day. Feeding the present
world energy system with biomass power of 2300 W/person is equivalent
to feeding an additional 23 'energy slaves' for each person; it
is quite obvious that a healthy World ecosystem cannot spare sufficient
biomass production capacity to feed the equivalent of 156 Billion
The replacement of fossil fuels and nuclear
energy in the present world energy system by direct technical
conversion of solar energy requires some 30 m2/person of solar
collectors, and is technically feasible. Due to the lower efficiency
of biological collection of solar energy the land area needed
for bulk replacement of fossil and nuclear energy is 4000 m2/person;
this is not feasible due to several reasons. There is a global
shortage of biologically productive land, water, and fertilizer;
furthermore, energy farming is in direct competition with food
production, and contributes to further reduction of biodiversity
in the Earth's ecosystem.
Policy Recommendations to Governments Worldwide