DESCRIPCIÓN DEL PROCESO DE DESHIDRATACIÓN DEL DIGESTO

Two different sources of energy come from the solid earth. The first one is geother- mal energy, which is considered as a renewable resource, but it has found less ap- plicability on a global scale. The second one is the non-renewable nuclear energy, coming from the mining of radioactive minerals found on earth, mainly from ura- nium isotopes. The latter, although socially and politically controversial constitutes

nowadays a key source of energy for many countries. Next, both sources of energy will be explained in detail.

4.4.1

The Geothermal energy

The temperature of the earth’s interior increases with depth. The geothermal gra- dient varies in different parts of the world from 15 to 75oC/km. The geothermal gradient creates obviously a heat flow leading to a heat loss escaping the crust. The amount of heat that escapes through the earth’s surface is due to the superposition of four components[168]:

Q= Q_C+ Q_L+ Q_B+ Q_T (4.1)

where Q_Bis the heat input at the base of the lithosphere1due to mantle convection,

Q_T is a long-term transient due to cooling after a major tectonic or magmatic pertur- bation, Q_L is the radiogenic heat production in the mantle part of the lithosphere, and Q_C is the radiogenic heat production of the crust.

The radiogenic heat production is due to the decay of the radioactive elements

238_U, 235_U, 232_{U and} 40_{K either in the crust or in the upper mantle. For geologi-}

cal provinces older than _{∼100 million years, QL}, Q_B and Q_T are lumped together into a single parameter called mantle heat flow Q_M. There are different ways to estimate the bulk crustal heat flow of the earth. Some estimates[251], [7], [104]

are obtained by redistributing the heat producing elements in the bulk silicate earth between the continental crust and various reservoirs in the mantle. They require assumptions regarding the structure of the convecting mantle, the composition and the homogeneity of the reservoirs. Other estimates are based on measurements ei- ther from representative rock types and their proportions in crustal columns derived from geophysical profiles[129], [60], [404], [32] or on large-scale production data

sets[81], [307] [106].

Jaupart[168] suggested to estimate the bulk crustal heat production directly from

the heat flow data and local studies of crustal structure and estimating the mantle heat flow Q_M with different ways. He obtained the values of heat production for three age groups: Archean, Proterozoic, and Phanerozoic (see table4.2). The aver- age of heat production was estimated to be between 0,79 and 0,95µW m−3and the crustal heat flow component ranges from 32 to 38 mW m−2, considering an average crustal thickness of 40 km. According to these numbers, the continental crust con- tributes to 5,8 to 6,9 TW to the total energy budget of the earth2. Active provinces and continental margins now represent 30% of the total volume of the crust; 50% error on their heat production would lead to a 15% error in the global budget. These

1_{The lithosphere is the rigid strong outer layer of the earth, consisting of the crust and upper}

mantle, approximately 100 km thick.

2

Table 4.2. Estimates of bulk continental crust heat production from heat flow data

[168].

Age group Range of heat pro- ductionµW m−3

Range of crustal heat flow, mW m−2

Fraction of total con- tinental surface, %

Archean 0,56-0,73 23-30 9

Proterozoic 0,73-0,90 30-37 56

Phanerozoic 0,95-1,10 37-43 35

Total continents 0,79-0,95 32-38

numbers differ from the values given by Skinner[317], in which the the flow is es-

timated to be 63 mW m−2 or 32,3 TW across the entire earth’s surface (not only the crust). It seems though that the numbers given by Jaupart are more updated and in consonance with the order of magnitude of the geothermal studies mentioned be- fore. Extrapolating Jaupart’s values to the entire surface of the earth, would lead to an average geothermal energy contribution3of 17,9 TW.

Geothermal energy constitutes a renewable source of energy. However, its reserves represent only a tiny fraction of all geothermal heat. Besides, like tidal energy, geothermal energy can be important locally but will be minor on a global scale. Ac- cording to the Renewables Global Status Report[208], the 2005 worldwide geother-

mal capacity was 28 GWth for direct thermal use and 9,3 GW for electricity produc- tion. The Geothermal Energy Association [109] reports that geothermal resources

using today’s technology have the potential to support between 35.448 and 72.392 MW of electrical generation capacity. Using enhanced technology currently under development (permeability enhancement, drilling improvements), the geothermal resources could support between 65.576 and 138.131 MW of electrical generation capacity. Assuming a 90% availability factor, which is well within the range experi- enced by geothermal power plants, this electric capacity could produce as much as 1, 09_{× 10}9 MWh of electricity annually (124 GW) (Table4.8). Nevertheless, these values need to be taken with precaution, until the USGS submits its geothermal energy report updating these numbers.

4.4.2

Nuclear energy

Nuclear energy derives from the huge binding force of the nucleus of elements. The- oretically, there are two kinds of processes that can release nuclear energy: fusion and fission.

Fusion consists in binding light elements, such as hydrogen and lithium, and thereby forming heavier elements. This is the process that goes on in the sun. Fusion has not yet been achieved in the laboratory under conditions such that the energy produced

3

exceeds the energy used. Nevertheless, many scientists believe that it might be the solution of future energy supply. Hermann [138] estimated the exergy reservoir

for the fusion cycle between deuterium (coming from the ocean) and tritium (bred from an isotope of lithium) as around 74 Ttoe. Furthermore, if deuterium, the isotope of 1 in every 5000 hydrogen atoms, is fused with another deuterium nucleus at higher temperatures, the resulting resource contained in the ocean is on the order of magnitude of 10 million YJ.

Fission nuclear energy is produced during controlled transformation of suitable ra- dioactive isotopes, when neutrons are fired into the nucleus, making the atoms unstable and subject to spontaneous disintegration. Uranium is the crucial fission energy raw material due to the fact that as mined it contains 0,71% of 235U (the only naturally occurring fissionable atom). Thorium, beryllium, lithium and zirco- nium are other low-demand raw materials with potential or specific uses in nuclear power production [71]. When235U undergoes fission, it releases heat and forms new elements and ejects some neutrons from its nucleus. These neutrons are then used to induce more 235U to fission. According to Skinner[317], once separated

235_Ufrom238_{U (an energy intensive process), the disintegration of a single atom re-}

leases 3, 2_×10−11J; because one gram of235Ucontains 2, 56_×1021atoms, fission of a gram of uranium produces 8, 19_{× 10}10J (equivalent to the energy released when 2,7 metric tons of coal are burned). Eq. 4.2shows a representative fission process of235U. 1 0n+ 235 92 U → 137 37 Cs+ 95 37Rb+ 3n + 3, 2 × 10−11J (4.2)

Estimated uranium resources in the continental crust amounted in year 1986 to 3.457 kton[317] (see table4.3), representing an exergy reservoir of 2, 8_×1014GJ or 6.741 Gtoe. More recent estimations of uranium sources indicate that these amount to about 13 Mt according to Grubler [128] and 14,8 Mt according to the OECD

[247], which represent an exergy reservoir of around 23.800 and 27.100 Gtoe, res-

pectively. With current state of technology, which makes use of only 0,7% of the natural fuel in a “once-through” fuel cycle, the reserves would last only a few hun- dred years (174 Gtoe). With fast spectrum reactors operated in a “closed” fuel cycle by reprocessing the spent fuel and extracting the un-utilized uranium and pluto- nium produced, the reserves of natural uranium may exceed 5.200 Gtoe (Table4.8). However, if advanced breeder reactors could be designed in the future to efficiently utilize recycled or depleted uranium and all actinides, then the reserves of natural uranium may be extended to several thousand years at current consumption levels [249].

Additionally, Hermann [138], estimated the exergy reserves of thorium as around

7.500 Gtoe and of seawater uranium as around 8.350 Ttoe.

At the end of year of 2006, 6% of the world’s primary energy consumption was derived from nuclear power plants (see figure4.4), and amounted to 635,5 Mtoe. In

Table 4.3. Estimated uranium resources in ores rich enough to be mined for use

in 235U power plants [317], together with estimated rates of production for 2005

according to the BGS[139]. Data reported as ktons of metal content. No distinctions

are drawn between reserves and resources, and no data for resources are reported by the former URSS countries.

Country Reasonably assured

resources, kton

Production rate in 2005, kton

Australia 1357 9,516

USA 758 1,034

Rep. Of South Africa 332 0,674

Canada 199 11,627 Niger 136 3,093 Namibia 113 3,08 France 47 - Other 516 12,876 Total 3457 42

France, more than half of all the electrical power comes from nuclear plants and in other European countries and Japan, the fraction is high too. Nuclear power capacity forecasts out to 2030 vary between 279 - 740 GWe when proposed new plants and the decommissioning of old plants are both considered [163]. Nuclear energy has

the advantage against fossil fuels that it does not emit greenhouse gases and its reserves are greater (see table 4.3). Some renowned scientists such as Lovelock [200] claim that: “there is no alternative but nuclear fission energy until fusion

energy and sensible forms of renewable energy arrive as a truly long-term provider”. However, other problems are associated with nuclear energy. The isotopes used in power plants are the same used in atomic weapons, so a political problem exists. The possibility of a power plant failing in some unexpected way creates a safety problem as it happened in the Chernobyl disaster in 1986. Finally, the problem of safe burial of dangerous radioactive waste matter must be faced, since some of the waste matter will retain dangerous levels of radioactivity for thousand years.