The reduction in fossil fuel use for electricity generation, heating/cooling and trans- port may involve a large number of measures, some of which are as follows:
Increased engine efficiency.
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Increased power generation efficiency.
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Local electricity generation and distribution.
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Better home insulation.
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Fewer car and lorry journeys.
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Greater use of public transport.
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Greater use of biofuels.
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Alternative power systems.
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Changes in house design.
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Reduction in long-distance transport of material which can be sourced locally,
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and reduction in air miles.
Energy efficiency measures such as insulation, building design, light bulbs, stand-by default on televisions and other consumer electronics have been estimated to make a major contribution to reduction in energy use. The EU Emissions Trading Scheme and the Climate Change Levy should also encourage cost-effective energy saving, estimated at reducing carbon emissions by 6–9 Mt. Transport uses over 30% of the total energy, therefore continued increases in engine efficiency should give significant savings.
Distributed energy
Electricity is mainly generated in large power stations (2000 MW and above) and 75% of home heating comes from gas supplied through a nationwide network. While centralized systems deliver economies of scale, safety and reliability, the transfer of
electricity to remote users loses 20.3 Mtoe, which is 8.7% of the total energy gener- ated (Table 1.4). However, new and existing technologies, especially advances in gas turbines, have achieved maximum efficiency in small power plants of up to 10 MW (Poullikkas, 2005). This makes it possible to generate energy close to where it is used, which is known as ‘distributed generation’. Distributed generation has been defined as ‘a small scale power generation technology that provides electric power at a site closer to customers than central station generation and is usually interconnected to the transmission or distribution system’ (Edinger and Kaul, 2000).
Distributed energy includes:
All plants connected to a distribution network rather than transmission network.
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Small-scale plants that supply electricity to a building, industrial site or community.
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Microgeneration, small installations such as solar panels, wind turbines, biomass
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burners supplying one building or small community.
Combined heat and power plants (CHPs), including large, community- or build-
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ing sized and micro-CHP, replacing domestic boilers in homes.
Non-gas sources of heat such as biomass, wood, thermal, solar or heat pumps for
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households and small communities.
These smaller systems can be more flexible and reduce the distribution losses incurred with a centralized system. At present less than 10% of electricity comes from micro- generation and CHP plants but these are increasing. The advantages of the distrib- uted generation include:
These plants can be more reliable.
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They are flexible in their energy source. These can handle renewable sources of
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power.
They avoid transporting fuel long distances.
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Less power is lost in distribution.
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They enable the introduction of alternative power systems which can be intermit-
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tent such as wind power and photovoltaics.
These distributed systems could fundamentally change the way energy is supplied, and reduce transmission losses and fuel imports.
Alternative drive systems
Although not directly involving biofuels, the development of alternative drive systems that do not use fossil fuels for transport is important in the reduction in fossil fuel use and greenhouse gas emissions. There are a number of systems being tested including fuel cells, electric cars and hybrid systems.
Fuel cells
Fuel cells have had a long development, including use in the NASA Apollo pro- gramme in 1960, and since 1990 an experimental transportation system has been introduced. A fuel cell consists of two electrodes – the anode and cathode – divided by an electrolyte (Fig. 3.7). Hydrogen is run into the anode, where a platinum-coated
proton-exchange membrane splits the hydrogen into hydrogen ions (protons) and electrons. The protons pass through the electrolyte to the cathode where they combine with oxygen, forming water. The electrons produce an external current which can be used to run an electric motor.
Fuel cells are classified by their operating temperature which is also determined by the electrolyte (Stambouli and Traversa, 2002). Table 3.5 gives some of the char- acteristics of fuel cells. Fuel cells can be combined in stacks, connected in series to produce the desired voltage. The number of fuel cells in a stack determines the voltage and the surface of each cell determines the current. Proton exchange and solid oxide fuel cells are the most advanced and have been fitted into experimental cars.
Two recent developments in fuel cell technology are the direct carbon fuel cell and the microbial fuel cell. In the direct carbon fuel cell, fine particles of carbon (10–1000 nm) are mixed with molten lithium, sodium, or potassium carbonate at 700–800°C (Cooper, 2006). The molten salt is introduced into the anode compart- ment and air to the cathode (Fig. 3.8). Electrons are carried from the carbonate to the cathode. Oxygen passes through a membrane which reacts with carbon, releasing electrons, forming carbon dioxide.
The microbial fuel cell derives energy from organic compounds metabolized by microorganisms. Figure 3.9 shows the layout of a microbial fuel cell. Microbes in the anode chamber oxidize substrates added to the chamber, generating electrons and protons as found in the chemical fuel cell. Carbon dioxide is formed but as organic substrates are used, the carbon dioxide released is only that fixed during photosynthesis.
Hydrogen (H2)
Water (H2O) Oxygen
(O2)
Anode Cathode
Electrolyte Proton exchange membrane H+
E– E–
Fig. 3.7. Outline of a fuel cell.
The reactions are as follows when using acetate as a substrate:
CH3COOH+ 2H2O® 2CO2+ 7H++ 8e- (3.1) At the cathode the protons react with oxygen:
O2+ 4e-+ 4H+® 2H2O (3.2)
To extract electrons to the anode, mediators have to be added to the anode chamber.
These mediators move across the microbial cell membrane where they are reduced Table 3.5. Characteristics of fuel cells.
Operating
Type Electrolyte Temperature (°C) Fuel
Proton-exchange Polymer 50–200 Hydrogen membrane (PEMFC)
Phosphoric acid (PAFC) Phosphoric 160–210 Hydrogen or hydrogen
acid from methane
Molten carbonate Molten salt, 630–650 Hydrogen, carbon (MCFC) nitrate, sulfate monoxide, natural
carbonate gas, propane Solid oxide (SOFC) Zirconia 600–1000 Natural gas, propane,
hydrogen
Solid polymer (SPFC) Polystyrene 90 Hydrogen
Alkaline (AFC) Potassium 50–200 Hydrogen, hydrazine
hydroxide, KOH
Direct methanol (DMFC) Polymer 60–100 Methanol
Oxygen (O2)
Anode Cathode
Air
Proton exchange membrane E–
E–
E– Carbon C
dioxide (CO2)
Carbon (C)
Fig. 3.8. Direct carbon fuel cell. (Redrawn from Cooper, 2006.)
and pass out of the cell, releasing the electrons to the anode. Mediators are dyes and metallorganics such as neutral red, methylene blue, thionine, Meldola’s blue and 2-hydroxy-1,4-naphthoquinone. However, the instability of the mediators limits their use, but recently a group of bacteria, the anodophiles, have been isolated. These bac- teria (including Shewanella putrefaciens, Geobacteraceae sulfurreducens, Geobacter metallireducens and Rhodoferax ferrireducens) attach themselves and transfer elec- trons directly to the anode (Du et al., 2007). Some microbial fuel cells have been inoculated with bacteria mixtures such as sewage sludge and sediments which have the advantage of a wider substrate range. The amount of electricity provided by the microbial fuel cells is still very low, but they can be stacked and used to produce hydrogen, for wastewater treatment and as biosensors.
Alternative biological fuel cells have the microbial cells replaced with enzymes.
This has the advantage of having a higher volumetric catalytic capacity, and it avoids toxic oxidation products. One of the fuels tested in an enzyme-based fuel cell is gly- cerol, one of the by-products of biodiesel production. Glycerol is a non-toxic, non- volatile, high-energy density substrate (6.3 kWh/l) for a cell containing the enzymes alcohol dehydrogenase and aldehyde dehydrogenase (Arechederra et al., 2007).
Battery electric vehicles
The electric vehicle is ideal for use in cities as it emits no fumes and can be integrated into city-wide traffic systems where travelling distances are short. The electricity needed to charge the battery can be generated from renewable sources, which leads to a
Organic substrate
Carbon dioxide (CO2)
Bacteria Anode Cathode
Proton exchange membrane
Water (H2O) Oxygen (O2)
E– E–
Fig. 3.9. Biological fuel cell.
considerable reduction in carbon dioxide emissions. However, for long journeys battery technology and electricity storage is the key point. New battery systems such as Li-ion, nickel hydride and high temperature are under development (Van Mierlo et al., 2006).
Recently Mercedes has introduced a battery-powered Smart car. It is powered by a high temperature (260–330°C) sodium–nickel–chloride battery with an output of 15.5kWh.
The battery can be recharged from the domestic supply and takes about 8 h and gives the car a range of 50 miles. Considering the developments in battery technology driven by the mobile phone and computer industries it is likely that the range will be extended.
Hybrid vehicles
For long-range travel, hybrid vehicles appear to offer the best option. The hybrid is a combination of a battery-powered electric engine plus an internal combustion engine or fuel cell. The internal combustion engine can be used to charge the battery when in use. There are a number of options for hybrid drive trains and a number of cars available from a range of manufacturers which combine electric motors with a small petrol engine, notably the Toyota Prius.