Losses and Gains of Eco Exergy by Human Activities Included Pollution

When contaminants, for instance heavy metals, are widely dispersed, eco-exergy is lost. When leaded gasoline was used to obtain a higher octane number, on the order of 400 0001 of lead were dispersed annually around the globe. Lead was even found in the ice pack of Greenland! A typical concentration in lead ore is about 5% or 0.05 kgkg_1ore, while a typical concentration in the environment after the dispersion is 1 mgkg"1 soil. If we presume 300 K, the annual eco-exergy lost can be found by eqn [7] as

where 207 is the atomic weight of lead. The consumption oflead has decreased due to shifts to other additives in the gasoline in most countries. This loss of eco-exergy by the use of lead as additive to gasoline is therefore today only estimated to be around 40 000 GJ yr"1.

'The loss of exergy due to dispersion of resources in general' can be calculated parallel to the application of eqn [7] as shown in eqn [21]. In addition to lead (only the dispersion of lead as gasoline additive is considered in this context), the loss of exergy by dispersion of other non-renewable resources is shown in Table 2.

Table 2 Loss of eco-exergy due to dispersion of nonrenewable mineral resources

Element GJyr 1

Chromium 32 000

Nickel 15 000

Zinc 80000

Copper 18000

Mercury (included fossil fuel) 27 000

Lead (today) 40 000

Calculations based upon principles shown in eqn [21].

The 'loss of eco-exergy due to the consumption of fossil fuel' is found by addition of the chemical free energy (the work capacity) of the fossil fuel and the loss of eco-exergy due to the dispersion of the gases resulting from the chemical processes. The exergy loss due to dispersion of the components of fossil fuel is found by the following calculations: if we consider 1 g of coal that contains 1% of sulfur and 99% of carbon (coal contains also ash, but let us not consider it in our calculations), the exergy loss due to the dispersion can be determined by the following calculations:

+ 0.99(8.314 x 0.300/12)ln0.99/4 x 10-4) = 1617J « 1.6 kJ

where 50 x 10"9 and 4 x 10"4 represents concentrations (expressed as ratios, i.e., no units) of sulfur dioxide-S and carbon dioxide-C in a typical town atmosphere. The chemical exergy content of 1 g coal is about 32 kJ. The loss of exergy by dispersion is therefore only 5% of the loss directly of chemical exergy by burning coal. As all our calculations will have a higher uncertainty than 5%, and the quality of coal may vary more than 5%, it seems acceptable not to include the dispersion exergy loss by use of fossil fuel or as alternative to multiply all exergy losses due to our consumption of fossil fuel by a factor of 1.05 to compensate approximately for the exergy loss due to the dispersion of the formed gases in the atmosphere.

The deterioration of ecosystems. By the use of eqn [20] it is possible to find the eco-exergy of an ecosystem or rather of the ecosystem corresponding to our model of the ecosystem. Consequently, the loss of eco-exergy due to deterioration and pollution of ecosystems can be found by calculation of the eco-exergy before and after deterioration. The difference will directly yield the loss.

The use of renewable resources. The formation of renewable resources are found separately by multiplication of the annual consumption of the various resources by the exergy content of each renewable resource. If, for instance, the annual fishery in the North Sea has the last many years been in the order of 100 000 t, which implies that the eco-exergy of the North Sea has been reduced 1011 x 499 x 18.7 kJ — 9.3 x 1017 J, then 499 is the /3-value for fish (Table 1). Sustainability requires that the growth of fish biomass compensate for this loss of eco-exergy. It has, unfortunately, for a couple of decades in many marine ecosystems, including the North Sea, not been the case due to over fishing.

Dispersion of waste. This to a certain extent can be calculated parallel to eqn [21]. This is often named the external costs of our activities including the industrial and agricultural activities, but it is actually as the other four points just a question about the loss of eco-exergy. It is not surprising that the cost of treating waste is increasing as the environmental agencies require a more and more complete elimination of these exergy losses. Or expressed differently: we are coming closer and closer to the carrying capacity of the Earth for man-made production. In this context it should not be forgotten that also the treatment of waste costs eco-exergy.

Consumption of nonrenewable fuel, including fossil fuel and nuclear fuel. The annual loss of eco-exergy is found by multiplication of the exergy content and the annual consumption.

We calculated above the loss of eco-exergy by dispersion of waste due to consumption of nonrenewable resources. This is, however, not the entire story, as energy is required in producing various materials from ore and raw source materials. The energy requirement when the material is produced from scrape is included. As it can be seen, the energy requirement is less when scrap ifused for the production. Reuse and recycling gives therefore double benefits: we save to draw on the limited resources and we save energy. Notice, particularly for aluminum, that the energy requirement by the use of scrap instead of ore is considerable. As energy consumption explains one of our major losses of eco-exergy, the latter benefit is of great importance.

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