Environmental Thermodynamics - relationship between energy, emergy and biomass
By A Murray, Consulting food process engineer
Electricity+Control, April 2014 (pages 50 - 52)
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We measure the quantity of food that we consume in units of energy, kilocalories or kilojoules. Our bodies convert energy from the food we eat into human biomass. Part of the study of environmental thermodynamics is concerned with tracing the path of this energy through the trophic layers of the food chain and its conversion into biomass.
Energy enters the biosphere, that part of the earth and its atmosphere which supports the life of animals and other living organisms, from three sources. The largest portion, approximately 99,97%, is in the form of radiant energy from the sun. The balance is accounted for by heat coming from the interior of the earth and tidal energy coming from the movement of the moon round the earth.
Approximately half the radiant energy from the sun is in the visible range and half in the infrared, producing heat.
In practice, the only production of biomass is through the action of photosynthesis. Terrestrial biomass is produced through photosynthesis in green plants and aquatic biomass is produced by the photosynthesis in phytoplankton. Effectively there is no other way in which large quantities of biomass is produced. The production by green plants is approximately 0,5% efficient in terms of the light energy falling on them.
In order to make a comparison between different kinds of energy in the universal hierarchy a new term has been defined: emergy. Emergy is the available energy of one kind that is used up directly and indirectly in transformations to make a product or service. The name is derived from the term embodied energy. Emergy (Em) is measured in emcalories or emjoules.
- The only production of biomass is through the action of photosynthesis.
- Every trophic level of the food chain produces greenhouse gases through decay of its waste and through respiration.
- The waste or decay of biomass is a critical component of both biomass energy production and energy management.