Energy supply in the future scenarios
Shell International4' has published a projection for different energy sources for the years 1990 up to 2100 (Figure 2.1). Assuming the "Sustainable Growth" scenario, energy consumption will increase by 7 times (at most) during this period. Applying the "Dematerialization" scenario (= much lower consumption driven by sustained economic use), the amount of energy will increase by a factor of 3 (at least). Both scenarios can be explained and are driven by the assumptions of an increase in population from about 6 bn to around 10 bn plus a continuous fast path taken by emerging markets to accelerate their economic growth.
Further, by 2020 the technologies around renewable resources are expected to have reached the potential for full economic use. Shell foresees a fast growth for these future alternatives and has projected that by 2050 the regenerative energy resources will provide 50% of the total energy consumption worldwide. According to Shell, the main source will be solar energy and heat.
Similarly, the WEC (World Energy Council) in 1995 has put forward a scenario in which the primary energy consumption will increase 4.2-fold by 2100 (referring back to 1990), and in its "Ecological" scenario of 1995 it still talks about a 2.4-fold increase.5)
The IPCC (International Panel on Climate Change) expects a 3 times higher energy consumption by 2100 (referring back to 1990), providing a high demand. With sustained economic use of energy, calculations suggest that almost 30% of the total global primary energy consumption in 2050 will be covered by regenerative energy sources. In 2075 the percentage will be up to 50%, and it is expected to continuously increase up to 2100. According to the IPCC report, biomass is going to play the most important role, projected to deliver 50 000 TWh in 2050, 75 000 TWh in 2075, and 89 000 TWh in 2100, in line with the calorific value derived from the combustion of more than 16 bnMg of wood.6)
Many other institutions have developed their own scenarios and done their own projections, as shown in Table 2.1 .
The economic potential of using hydroelectric power to provide energy is already almost fully exploited. All other renewable resources, however, still have huge potential and can still be widely expanded.
Biogas from Waste and Renewable Resources. An Introduction. Dieter Deublein and Angelika Steinhauser
Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31841-4
- Figure 2.1 Projection of the energy supply up to year 2100 (acc. to Shell International).
Table 2.1 Perspectives for energy sources as a percentage of the total energy consumption in Germany.
Federal Ministry for the Federal Ministry of Greenpeace9)
Environment in Germany7) Economy and
Technology in Germany8
Today Projection in In 2020 In 2010
year 2080
Total final energy Static at around Static Declining to ca. Independent consumption 2600 TWha-1 2115TWha-1 oftheenergy consumption
Elect. Heat Elect. Heat Elect. Heat
Natural gas Nuclear energy Hard coal Brown coal Petroleum Renewable energy Hydro-electricity Wind
Photovoltaics Solarthermal heat Geothermal heat Biomass
0.30
1.59
68.0
18.5
30.2
Note: Technical final energy potential = technically usable electrical energy in the system.
Biomass is rich in carbon but is not yet a fossil material. All plants and animals in the ecological system belong to biomass. Furthermore, nutrients, excrement, and bio waste from households and industry is biomass. Turf is a material intermediate between biomass and fossil fuel.
There are several processes to transform biomass into solid, liquid, or gaseous secondary energy carriers (Figure 2.2): these include combustion, thermo-chemical transformation via carbonization, liquefaction or gasification, physico-chemical transformation by compression, extraction, transesterification, and biochemical transformation by fermentation with alcohol or aerobic and anaerobic decomposition.
Today in Germany, 65% of the heat and electricity generated with processes based on biomass are provided by combusting firewood and forest residual wood, followed by the use of industrial residual wood and matured forest. About 14% of the energy comes from the use of liquid or gaseous biological energy carriers. When considering heat only, it is even higher, as shown in Table 2.2.
Thermochemical processing or combustion are the most effective ways to maximize the generation of energy. Combustion is only efficient, however, if the water
Figure 2.2 Applied technologies to transform biomass11) into secondary energy sources.
Uegetabedl thand. arrniarii
Figure 2.2 Applied technologies to transform biomass11) into secondary energy sources.
Table 2.2 Heat generated from biomass.
|
Energy carrier |
Percentage |
Generated heat |
|
Organic residues. By-products, waste (biogas, sewage |
9.4 |
6.25-6.53TWha-1 |
|
sludge gas, landfill gas) | ||
|
Bio fuels |
1.3 |
0.8-0.9TWha-1 |
|
Biogenous solid fuel (firewood, forest residual wood) |
89.3 |
45.8TWha-1 |
|
Industrial residual wood |
11.9TWha-1 | |
|
Matured forest without recovered paper |
3.3TWha-1 | |
|
Other wood-like biomass |
0.3TWha-1 | |
|
Straw |
0.78TWha-1 |
content of the biomass is below 60% to prevent most of combustive energy from going into the evaporation of water. In the worst case, all this energy will have to be generated from the flue gas. The only chance to regain this usable energy will then be to condense the evaporated water in a condensing boiler.12' However, this is only possible if the biomass is free from corrosive materials. From an economic point of view, the temperature of the flue gas is important. Furthermore, the composition of the combustion residue needs to be carefully evaluated for possible use.
If the biomass has very high water contents (e.g., liquid manure, freshly harvested plants', it is best to select and accept a process which provides only about 70% of the energy resulting from the combustion of dry material. As an advantage, the residues can be easily returned to nature, especially since no materials enriched with minerals and thus plant-incompatible ash are generated.
If biomass is to be used to serve as source win liquid fuel, it is best to produce ethanol and/or methanol via alcoholic fermentation. This process is more efficient than anaerobic fermentation referring to the hectare yield.
Overall, the energy balance is particularly favorable for biomass when considering the energy yield from the biomass [output] to the assigned primary energy [input], including all efficiencies as to be seen in Table 2.3.
With the output/input ratio of 28.8 MJ/MJ, biomass appears to be a very efficient source of biogas.
One of the leading countries in developing biogas plants is Germany, where hence a lot of efficiency data have been yet generated. In the following abstracts, these data are presented to show the potential of this technology and to highlight important factors that should be considered before planning a biogas plant. It is
|
Energy source |
Energy balance Output/ |
Remarks |
|
Input [MJ/MJ] | ||
|
Rape oil |
5.7 |
Energy recovery of the colza cake and |
|
green waste included | ||
|
Ethanol |
2.7 |
From wheat |
|
Ethanol |
1.6 |
From sugar beet |
|
Ethanol |
5.0 |
From sorghum |
|
Energy recovery of the bagasse included | ||
|
Electricity and heat |
8.5 |
Combustion of the whole plant |
|
From cereals | ||
|
Electricity and heat |
19.7 |
Combustion of miscanthus plants (not |
|
dried' | ||
|
Electricity and heat |
14.2 |
Combustion of energy plants |
|
Electricity and heat |
20.4 |
Combustion of residual straw |
|
Electricity and heat |
19.0 |
Combustion of forest residual wood |
|
Biogas |
28.8 |
From excrement (CHP cycle) |
Physically
Lowered through e.g. efficiency of transformation
Compared to other energy sources Generally accepted
Figure 2.3 Evaluation of the scope - from the theoretical to the deducible potential.
important to differentiate and carefully evaluate the theoretical, technical, economical, and realizable potential (Figure 2.3).
The theoretical potential comprises all the energy that should theoretically be physically generated within a defined time period and a defined space.
The technical potential is part of the energy of the theoretical potential. It is that specific part which can be provided within the given structural and ecological boundaries and by respecting any legal restrictions.
It may not always make sense to fully exhaust the technical potential, especially if there is no profitable return.
However, the economic potential may not be realizable without any administrative support from certain institutions.
The total yield from biomass results from the maximum area available for cultivation and the energetic yield from the biomass cultivated on this specific area.
Amount of space
The amount of space in Table 2.4 is defined as the land area plus the surface area of the water, because algae or water plants in general are biomass and may have potential in the future.
The right hand columns in the table show the amount of space that is available for cultivation of biomass and may have potential.
In theory all the amount of space AD, including the surface of the water, can be used to produce biomass.
Technically, biomass can be cultivated on all areas except the settlement area, mining lands or badlands. This is an amount of space of ADtechn = 0,88 ■ AD of the total surface of Germany.
As soon as the micro algae production is developed, then technically an even larger surface, means 95% of the entire available space, could be exploited.
Economically, the cultivation of energy plants competes with the cultivation of other agricultural products. The market will probably equilibrate itself. But overall
Physically
Lowered through e.g. efficiency of transformation
Compared to other energy sources Generally accepted
|
Theoretical potential | |
|
Technical potential | |
|
Economical p. | |
|
Deducible | |
|
Potential | |
|
Utilization of the amount of area today |
Total area |
Theoretically usable |
Technically usable |
Economically usable | |
|
Total |
35703 099 |
35 703099 |
31503 678 |
20117031 | |
|
Settlements and areas used |
4393 895 |
= |
374052 |
300812 | |
|
for transport and traffic | |||||
|
The above areas include: |
Buildings and open space |
2308079 |
= |
0 |
0 |
|
including residences, trade, | |||||
|
industry | |||||
|
Areas for winning substances |
73 240 |
= |
= |
0 | |
|
out of the soil without | |||||
|
mining land | |||||
|
Area for recreation incl. parks |
265 853 |
= |
= |
= | |
|
Area for cemeteries |
34960 |
= |
= |
= | |
|
Areas for traffic (roads, streets) |
1711764 |
= |
0 |
0 | |
|
Area for agriculture including |
19102791 |
= |
= |
9551395 | |
|
moor and heathland | |||||
|
Forest area |
10531415 |
= |
= |
= | |
|
Surface of the water including |
808462 |
= |
= |
0 | |
|
Mining land |
179 578 |
= |
0 |
0 | |
|
Other areas |
686957 |
= |
= |
0 | |
|
These include: |
Badlands |
266 593 |
= |
= |
0 |
|
100% |
88% |
56% |
= means same number as in the left column.
= means same number as in the left column.
about 50% of the agricultural area is considered to be available for profitable production of biomass. Some other surfaces will never be agriculturally usable in a profitably way. So the total area for profitable agricultural use for biomass is estimated to be ADtechn = 0.56 ■ AD.
Potential yield from biomass
Theoretical potential161
Biogas results from the microbial degradation of biomass, formed by photosynthesis by solar power ES.
Carbon dioxide + Water + Solar energy ^ Sugar (Glucose) + Oxygen
Metabolic processes in the plants, transform the following compounds into secondary products.
Carbohydrates: Starch, inulin, cellulose, sugar, pectin Fat: Fat, fatty acids, oil, phosphatides, waxes, carotene
Protein: Protein, nucleoproteid, phosphoproteid
Others: Vitamins, enzymes, resins, toxins, essential oils.
During the metabolism of the sugar, the plant releases energy, when necessary, to the environment, so that the possible energy yield from plants may vary greatly.
Multiplying the proportion of the main plant components (see Table 2.5) by the entire vegetation, an averaged elementary composition of plants dry matter results:
C38H60O26
With the help of an approximate equation from Buswell (1930), the theoretical maximum yield of methane can be estimated taking the elementary composition as a base:
Table 2.5 Main components of plants without nitrogen N and sulfur S.
Carbohydrate C6H12O6
Fat C16H32O2
Protein C6H10O2
14 | 2 Energy supply in the future - scenarios where x = 0.125 (4c + h -2o -3n + 2s) y = 0.250 (4c - h - 2o + 3n + 2s)
or, simplified
The hectare yield of methane can hence be calculated from the hectare yield of the dry matter. This again depends on the planting, which should be as productive as possible.
The maximum theoretical possible yield is estimatedat ERmax = 30 MgDM/(ha ■ a) when applying two harvests per year and cultivating C4 plants with an average elementary composition of ME = 932kg/kmol. Based on the simplified equation from Buswell the yield of CH4 is EM = 20kmolCH4/kg biomass and the energy yield P4leor is calculated by the formula:
Ptheor '
"Em to give 144.200kWh/(ha ■ a). If one multiplies the hectare yield by the entire surface of Germany (35 703 099 hectares), the following equation
Ptheor Ptheor ' AD
results in a primary energy quantity from biomass of 5.148 TWh/a. Theoretically the entire amount of primary energy supply in Germany could be covered by biomass alone.
Assuming that the yield of the available cultivable area on earth is proportionally the same as in Germany, an area of 7420Mioha, half of the available area of 14900Mioha on earth, would theoretically be enough to cover the total world primary energy consumption of 107 000TWha-1.
If a precondition is that the maximum yield should be guaranteed on a long-term basis, this could be facilitated by
Accurate and targeted addition of fertilizer
Water and fertilizer can be added very accurately by using hoses which are directly led to the roots. The accuracy depends on the characteristics of the local soil, but the overall yield per hectare of conventional agriculture could perhaps be doubled, particularly, when some missing nutrients are supplied with the water.
Multiple harvests per year
Yields of 25-30 Mg DM/ha.a can be obtained if the field crops shown in Table 2.6 are cultivated immediately after each other during one year.17),18)
Table 2.6 Crop rotation (GPS
2.2 Potential yield from biomass | 15 = Mixture of winter wheat and peas).
1st Planting
2nd Planting
3rd Planting
Wheat Winter rye Winter barley Triticale, Winter oat Winter rape Beets
Winter peas Incarnat clover Winter sweet pea
Maize (mass-producing species)
Sunflower
Sorghum
Sudan grass
Hemp
Mustard
Phacelia
Radish
Sweet pea
Peas
Today the most frequently cultivated crop rotation consists of the following three crops:
1. The domestic cold-compatible C3 plants: winter rape or winter rye
2. The southern C4 plants: corn (mass-producing species),19) as main crop during summer
3. The cold-resistant C3 plants: GPS.20)
In order to generate energy, all the plants are harvested as soon as they finish their growth without leaving them time to fully develop. The costs of cultivation are 61-84 US$/Mg for the cultivation of winter wheat, winter barley, and triticale a crossing of wheat and rye in Germany.21)
Overall the cultivation of energy plants has just started. Besides maize, some other C4 plants like sorghum, sugar cane, or Chinese reed seem to be efficient when used as biomass.22) Their yield, though, still needs to be improved. Also, certain C3 plants such as grain, grasses, hemp, rape, beet, sunflower, or winter peas seem to have good potential as energy sources with a yield still to be increased, too. In future this broader range of energy plants will allow interesting new combinations and an increased level of flexibility in deciding on the crop rotation system.
2.2.1.1 C3 plants (energy plants)
The enzyme most important for the production of energy is RuBisCo (Rubilose 1.5-diphosphate carboxylation-oxygenase). It is the most frequently produced enzyme of all organisms and can be found in the chloroplasts of the plants in the form of proteins. Their level in the proteins amounts to 15%.
RuBisCo catalyzes photosynthesis and photorespiration. It binds oxygen as well as CO 2 and acts as oxygenase. For photorespiration to occur, the chloroplasts,
- 12 3-Phosphoglycerate
6 ATP
6 C02 + 6 Ribulose-biphosphate H
Ribulose-
Reduction phosphat
6 ATP
Regeneration of ribulose
12 Glyceraldehyd-phosphate H-C=0
Starch
Figure 2.4 Calvin cycle.23'
mitochondria, and glyoxisomes, cell components around the mitochondria, need to be involved.
The ratio of photosynthesis to photorespiration is defined by the ratio of CO 2 and O2 in the air. With a higher concentration of CO2, the output of the photosynthesis increases.
In moderate zones, e.g., in Central Europe, photorespiration in plants plays a subordinate role. Predominantly C3 plants occur, which use the light-independent reaction, the Calvin cycle (Figure 2.4), to bind CO22 They are called C3 plants, because the first stable product in the Calvin cycle after the CO 2 fixing 3PGS (Phosphoglycerate) has 3 C-atoms. Also the molecule which is reduced from 3PGS with NADPH+H+ to 3PGA (Phosphoglycerin aldehyde) in the following phase of the Calvin cycle contains 3 C-atoms.
The leaf structure of C3 plants is layer-like. In warm summer weather the transpiration and the evaporation at the surface of the sheets increases. In order to minimize the water loss, the plants close their pores. CO2 cannot be absorbed by the pores any longer. Thus the photosynthesis is stopped and the biomass yield is limited.
In addition, the biomass yield depends on the soil as well as the entire climatic conditions: in some regions of the world the yield can be up to five times higher than in Germany. It is not possible, however, to obtain the theoretically projected yields just by cultivating C3 plants (Table 2.7).
Other typical representatives of C3 plants are onions, wheat, bean, tobacco.
Most C3 plants are well adapted to the moderate climatic zones but not to arid, saline areas with hot and dry air. Under such climatic conditions the ratio of photosynthesis to photorespiration increases from 2 : 1 and negatively impacts the yield.
|
Plant |
Yield [Mg DM |
Water content |
Advice for plantation |
|
(fruit + haulm)/ |
[%] related to | ||
|
ha.a]24) |
the total mass | ||
|
Trees (stored) |
1-2 |
15 - 20 |
Cut every 150 years |
|
Fast - growing wood |
15 |
30 - 60 |
Cut every 6 years |
|
(poplar, willow) | |||
|
Eucalyptus |
15-40 |
High |
- |
|
Rape (whole plant) |
4.2-6.9 |
12 - 34 |
Crop rotation every 4 years |
|
Sunflower (mature |
2.5 |
15 |
Crop rotation every 5 years |
|
plant) | |||
|
Hemp |
3-4 |
65 - 75 |
Crop rotation yearly |
|
Sugar beet |
7.2-18.2 |
74 - 82 |
Crop rotation every 4 years |
|
Potato |
5.8-12.5 |
75 - 80 |
Crop rotation every 4 - 5 years |
|
Jerusalem artichoke |
12-27 |
72 - 81 |
Crop rotation yearly |
|
Straw and grain |
4-15 |
14 - 16 |
Crop rotation yearly |
|
Bastyard Grass25' |
13.7 |
65 - 80 |
For 5 cuts per year |
|
Meadow |
7.7 |
65 - 80 |
For 5 cuts per year |
C3-plants • C4 or CAM-plants
|
Prefixation | ||
|
(spatially or timely) | ||
|
by | ||
|
PEP-Carbolylase | ||
|
co2 | ||
Fixation by RuBisCO
Figure 2.5 Different ways for the CO2-Fixation. 2.2.1.2 C4 plants and CAM plants
There is a large group of 1700 variants of C4 plants and/or CAM plants which are all well adapted to hot and dry climates and do grow in arid, saline areas. This is possible since the CO2 fixing occurs in C4 plants spatially separated from where the Calvin cycle occurs. In CAM plants the CO2 fixing happens at a different time of the day from that when the Calvin cycle occurs (Figure 2.5). Such plants can utilize even the smallest CO2 concentrations.
The separation of the CO2 fixing occurs with the help of the enzyme PEP carboxylase (PEP = phosphoenolpyruvate), which possesses a substantially higher affinity
24) 1km2 = 100ha = 10000a = 1000000m2 25) Cp. WEB 90
to CO 2 than RuBisCo. The first product of the photosynthesis which is stable is oxalacetate (see Figure 2.7), a C4 product. This characterizes the so-called C4 plant.
Compared to C3 plants, the leaves of C4 plants are anatomically different. The spatial separation of the CO2 fixation takes place in cells at a distance from each other, the bundle sheath cells and the mesophyll cells, both containing chloro-plasts but different types: the mesophyll cells contain normal chloroplasts while the budle sheath consist of chloroplasts with grana. The vascular bundles to transport the cell liquid are covered by a layer of thick bundle sheath cells which are surrounded by mesophyll cells.
An intensive mass transfer is continuously happening between the bundle sheath cells and the mesophyll cells. This starts with the formation of oxalacetate (Figure 2.6), a result of the enzymatic reaction of PEP-carboxylase binding CO2 to PEP = (phosphoenolpyruvate). Oxalacetate is then enzymatically transformed into malate and transferred to the chloroplasts of the bundle sheath cells. In the bundle sheath cells it degrades into pyruvate and CO 2 while forming NADPH+H+ as a by - product. CO2 is introduced into the Calvin cycle while pyruvate is transported back into the mesophyll cells.
CAM plants actually belong to the group of C4 plants. The name "CAM plants" is derived from the Crassulaceae Acid Metabolism (acid metabolism of the Crassula-ceae), since the metabolism was first observed in the plant species "Crassulaceae".
Because of the high water loss, these plants open their stomata only at night to take up CO2 which is stored in form of malate. During the day, CO 2 is released and transformed in the Calvin cycle forming ATP as a by-product.
Like C3 plants, the CAM plants have layer-like structured leaves.
Some species of CAM plants are cactuses, pineapple, agave, Kalanchoe, Opuntia, Bryophyllum, and the domestic Sedum spec. or Kalanchoe (Crassulaceae).
C4 and/or CAM plants show the following advantages, compared to C3 plants:
• C4 and/or CAM plants can generate biomass twice as fast if conditions are favorable (see Table 2.8).
• The upper leaves of C4 and/or CAM plants are perpendicularly directed to the sun, so that the low- hanging leaves still get sufficient light even under unfavorable light conditions.
- Figure 2.7 Mass transfer in C4 plants (C4-dicarbon acid path).27'
|
Plants |
Max. yield |
Water content |
Advice for plantation |
|
(approximate) |
(depends on harvest time) | ||
|
[MgDM/ha.a] |
%] |
[Years] | |
|
(fruit+haulm) |
Miscanthus 25-30 15-45 Harvesting 20-25 from the
3rd year on
Sorghum spec. 17 17-60 Harvesting 20-25 from the
3rd year on
Sorghum spec. 5-32 70-80 One year plant
Maize 3 028) 15 One year plant
• C4 and/or CAM plants need only half the water.
• C4 and/or CAM plants adapt to dry and warm locations.
• C4 and/or CAM plants do not need pesticides but only some fertilizers in the first year.
- Figure 2.8 Sorghum (above right), Micro-algae chlorella in glas tuber (below). Energy maize abt. 5 m heigh (above left).
• C4 and/or CAM plants once planted grow again after biomass has been harvested.
2.2.1.3 Micro-algae
By cultivating micro-algae, even the surface of water as well as the area of rooftops can be exploited in a profitable way.
A yield of 15-17 Mg biomass per year seems theoretically be achievable by planting micro-algae and cultivating them in well-lit bioreactors.29
2.3 Technical potential | 21 Figure 2.9 Reactor for micro-algae growth.
Most of the micro-algae naturally grow much better when the light is somewhat diffused rather than in direct clear light. Sun may even limit the growth. In order to control the light in the latest bioreactors, the micro-algae are cultivated in airlift reactors in which a circular flow is caused by changing the direction of the gas bubbles (Figure 2.9). The light reflects at the outer wall of the reactor.
The circular flow is set in such a way that the algae are located mainly in the outer area of the incidence of light. The algae absorb just enough light to keep the Calvin cycle alive for a maximum yield of biomass.
In reactors erected in the sea, the sea water can actually be used to help maintain a moderate temperature inside the reactor. The micro-algae may also serve to clean the water, especially in cases where the reactor is located close to a river mouth and the water is led through the reactor.
Micro-algae can not only be used to produce biogas but also to provide lipids, fatty acids, vitamins, e.g., vitamin E, beta-carotene, or even pigments like phyco-cyanin or carotenoids. Antioxidants like tocopherols or omega fatty acids may also be extracted, which are very interesting from a pharmaceutical point of view.
In 2000 the first farm for micro-algae was inaugurated close to Wolfsburg, in the middle of Germany. Within a fully closed system of bio-reactors (about 6000 m3 in total) chlorella algae are converted into about 150-200 Mg animal food produced annually.30'
Technical potential31'
Technically it should soon be feasible to achieve a yield of ER = 1/2 er^ =15 Mg/ ha.a of biomass.32' This final effective outcome may be lower than the theoretical potential, since certain losses have to be taken into consideration because
• Quite often the biomass that is used to generate the energy is just a leftover after having been consumed as food. Some other parts of biomass had been used to construct houses.
. . . Overall most of the quantity of biomass effectively used has served other purposes before being taken for energy supply, so that part of the energy has already been wasted.
• Technically the transformation from primary energy to effective energy goes along with quite immense losses of around 20-70%.
• Energy plants need to be cultivated in a sustainable way to ensure the continuous energy supply over time. It is important to ensure that the soil is not getting leached.
The real technical potential hence after the equations
results in 72.100 kWh/(ha ■ a) or 2.265 TWh/a when multiplied by the area ofland that is technically available.
The technically realistic yield of energy provided by biomass should provide about 50% of the total energy consumption in Europe.
Humans themselves would be part of a closed CO- cycle (Figure 2.10). Excrement and/or waste are directed into a separator to separate solids and water. The water flows into a constructed wetland and is purified there to drinking water. The concentrated solid is converted into energy by being processed in an anaerobic reactor with a generator attached to it. The fermented residue is composted and used as a fertilizer for food plants. The constructed wetland may be run with water hyacinth and/or common duckweed which can be returned to the cycle.
Water hyacinths are fast- growing plants which should be cut quite frequently. In that way they are well suited to be utilized as an effective renewable source to provide biomass for energy supply.
Economic potential
Prices for crude oil and energy are rising globally. This trend anticipates that any technical feasibility will be profitable sooner or later. The economic potential hence equals the technical potential
2-Mb giving 72.100kWh/(ha - a). When multiplied by the area which is economically available
Pecon Pecon ' °.56 * AD, this results in 1.441 TWh/a, about 35% of the total primary energy supply of Germany.
Realizable potential
There is a huge gap between the technical and profitable potential and the realizable potential. A lot of what is technically feasible is rejected for various reasons, mainly special interests, e.g., landscape protection or job safety. A lot can be explained rationally but a lot is just based on emotion.
Today, almost 20 Mio ha of the agricultural area is cultivated only to produce food without considering the possibility of using it for energy supply. Just about 5% of the agricultural area (about 1.2 Mio ha) is disused. About 30% of this specific area is planted with energy-affording plants. In the next few years it may well be possible that the agricultural area used to produce biomass for energy supply will increase to about 2-2.6 Mio ha, even if we bear in mind that the forest area certainly cannot be simply transformed into an area of cultivable land for energy-affording plants. Such areas are expected only to deliver about 5 Mg/ha.a of dry biomass material.
In the same way, parks will remain on a long-term basis and may only provide a very small, almost negligible amount of biomass.
In the future, additional yield of biomass can only be achieved by exploiting those areas that are agriculturally used today. From a technology point of view, however, this is the only area that can be used to cultivate biomass or to provide output for the production of bio-diesel fuel. Today, about 70% of the non-food rape is consumed by the bio-diesel fuel industry. So just about 0.6 Mio ha are left and realizable for the cultivation of energy plants. The target of 4 Mio ha or about 20% of the total agicultural area in Germany available seems unrealistic and overestimated.33)
Further realizable potential may be provided by waste and sewage, which are already partly exploited in biogas plants. The fermentation of waste materials has to be seen in competition with being fed to animals, combusted, or composted (Table 2.9).34)
The potential to provide biogas is the most important data point and is specific to the kind of organic material used. For example, in Germany the materials shown in Figure 2.11 will be available by taking 2/3 of the entire volume of excrement (liquid manure) of the German agricultural livestock into consideration:
Table 2.9 Possibilities to exploit bio waste (- = not suited; 0 = partially suited; + = well suited).
Feeding Combustion Composting Fermentation
Sewage sludge - 0 0 0
Sewage from industry, biologically + - 0 + contaminated
Waste from slaughterhouse - - 0 +
|
9000 | |||||
|
D 405 | |||||
|
3700 | |||||
|
5750 | |||||
|
□ 435 | |||||
|
□ 580 | |||||
|
=1 |
935 | ||||
|
1250 | |||||
0 2000 4000 6000 3000 10000 12000
0 2000 4000 6000 3000 10000 12000
Yield of biogas in Mio mJ/a Figure 2.11 Yield of biogas from different sources35 in Mio m3/a.
From these sources, the energy potential as to be seen in Table 2.10 can be derived:
190MioMg ofexcrement may deliver the same yield as 500 000 ha ofland to cultivate energy plants. The power generated by two power nuclear plants may be provided by fully exploiting agriculture and forestry.36)
Another source of energy is fermented plants of created wetlands, which deliver a much lower but still appreciable amount of biomass, or the wastewater from the paper industry.
Even human urine may be exploited (Figure 2.12). Around 40 Mio Mga-1 ofurine (500L/person.a)37) could be made available by investing in changing the complete system of sewage disposal; so-called "gray water" from private households, e.g., from washing dishes, laundry, or bathing, should be separated from brown water (containing excrement) and urine. Even the toilet flushing would need to be omitted to avoid too much dilution. A potential solution may look as shown in Figure 1.16.
The importance of input from landfills will decrease in the next few years. Legal regulations and restrictions have become much stricter, which may finally render this source unprofitable.
Table 2.10 Potential of energy from biogas from different sources.
Sources for biogas production Energy potential [TWh a 1
Landfill 6 Communal and industrial sewage water
Organic wastes from households and markets 18 Organic wastes from industry Excrement (190 Mio Mga-1)
Byproducts of agriculture and food production 47 Material from landscape conservation
Plantations of energy plants (area ca. 2.5 Mio ha) (15Mg/ha.a) 141
Wood (10Mioha forest area) (5Mg/ha.a) 187
Urine 4
Nutrition in sewage water 5
Total 408
Figure 2.12 Separation of human excrements.38'
Overall, the biggest output of energy from renewable resources, however, will be provided by using bio waste from the food industry. Pomace, for example, from wine making, consists of grape pods, stones, and stems, which can be used for energy recovery. Today pomace serves as a base for the production of alcohol and/or as animal feed, as does the waste from breweries, sugar refineries, and fruit processing plants. All this bio waste can be a source of profit by fermentation. Even the wastewater from dairies or waste from slaughterhouses will be fermented in the future. The potential is huge. Annually 0.9 Mio sheep and horses, 3.8 Mio cows, 0.4 Mio calves, and 43 Mio pigs are slaughtered in Germany.
The total yield of realizable biogas sources in Germany should be around 408TWha-1, which is about 10% oftoday's primary energy supply and about 48.5% of today' s primary energy consumption of natural gases (natural gas, mine gas, sewage gas) - about 840TWha-1.
When all biogas is used to generate electrical power, the potential yield of power from the biogas amounts to about 143 TWha-1, assuming an efficiency of35% for the power generators. Biogas may hence contribute to 10-12% of the total power supply.391
In some literature, lower yields of biogas of around maximum 74 TWha-140) only are estimated.
In Western Europe, France and Germany are leading just looking at the potential yield of energy resulting from cultivating energy-affording plants and exploiting agricultural by- products. France has the highest potential with 178 TWha-1 (Figure 3.1 ).
This means that so called "passive houses", with a very low energy requirement for heating (primary energy consumption of 120 kWh/m2 - a), can be heated by using biogas, e.g., by generating power by a fuel cell. The required amount of energy will be supplyable by animals. One animal unit (abbreviation used: GVE) 500 kg in weight produces 550 m3 biogas per year or, depending on the energy content of the biogas, ca. 3500 kWhth. One cow can hence supply a small apartment of about 30-60m2 living area with enough heat. Or a passive house of about 400-800 m2, needing ca. 40000kWhtha-1, can be heated with energy plants growing on one ha of field.
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