Energy for the future

If demand continues to rise at the present rate it is expected that most of the world's fossil fuel resources will run out around the middle of this century. There are still those who put their faith in the commercial application of nuclear fusion perhaps relying on the fact that an energy vacuum will radically alter the definition of 'commercial'. However, this goal still seems as elusive as ever. On a global scale there is no real optimism about the capacity of current renewable technology to meet the energy needs of the next century especially the exploding economies of the Far East. At the same time there is considerable anxiety about allowing the proliferation of nuclear technology.

More than ever before there is hope that someone will find the 'Rosetta Stone' that will redefine physics and lead to limitless cheap, clean energy. For example, on the threshold between science fiction and reality is the greatest potential energy source of all, namely the exploitation of antimatter. Physicists from Germany, Italy and Switzerland have managed to combine single antiprotons and positrons to create antiatoms of antihydrogen. Antimatter is destroyed when it comes into contact with normal matter, releasing massive amounts of energy (New Scientist, 'Antiworld flashes into view', 6 January 1996). It's a case of 'watch this space, but don't stand too close'.

The predicted rise in oil consumption considered in Chapter 2 is nothing compared with the anticipated demand for electricity. One of the leading think-tanks in this sphere is the Electric Power Research Institute (EPRI) at Palo Alto in California. Its chief executive Kurt Yeager points out that 2 billion people are without electricity. By 2050 this will have risen to 5 billion unless there are fundamental changes in the way we produce and distribute electricity. This in part will be driven by the digital revolution; as computers get faster they will require reliability of power in the order of 99.9999999 per cent. At present the reliability is 99.9 per cent which amounts to stoppages of a few minutes at a time but adding up to about 8 hours a year. This is fatal for microprocessors which are upset by millisecond disturbances. As Yeager puts it, we will need power delivery systems with switching operations that reach the speed of light.

The challenge which the EPRI presents is to provide a minimum of 1000 kWh of electricity per year to everyone in the world by 2050, about the same as the US in the 1920s, remembering that by then the estimated global population will be 9-10 billion. This would be the equivalent of tripling the world's generating capacity which translates to building a 1000 MW power station every two days.

The revolution which will make this vision possible is the shift from mega power plants and creaking national grids to much smaller dispersed grids. Large overland grids are inefficient and expensive to maintain. They are subject to frequent failure and even at the best of times incur up to 10 per cent line losses. In the UK it is claimed the grid is well over its 30 year replacement date.

Above all, the answer is to devote massive resources to the development of renewable energy technologies to harness a mere 1/15 000th of the energy of the sun. Solar energy promises unlimited free meals. The Royal Commission on Environmental Pollution sees:

a shift from very large, all-electricity plant towards smaller and more numerous combined heat and power plants. The electricity distribution system will have to undergo major changes to cope with this development and with the expansion of smaller scale, intermittent renewable energy sources. The transition towards a low-emission energy system would be greatly helped by the development of new means of storing energy on a large scale.

22nd Report, Energy, the Changing Climate, p. 169, Stationery Office, 2000

This ties in with the Washington Worldwatch Institute which states that 'An electricity grid with many small generators is inherently more stable than a grid serviced by only a few large plants.' It will of course be the perfect way to exploit renewable energy. So-called intelligent grids which can receive as well as distribute electricity at every node are already emerging. Another Yeager suggestion is that we create DC microgrids which, he says, will, 'eliminate much of the imperfections in the sine wave that creates the upsets for microprocessors - those millisecond or nanosecond disturbances' (Electrical Review, 10 October 2000, p. 27). Most significant of all is his prediction that in the future most of our electricity will come from millions of micro-turbines, solar panels and hydrogen powered fuel cells. He is one of many who believe that the fuel cell is the power source of the future.

As indicated earlier a fuel cell is a reactor which combines hydrogen and oxygen to produce electricity, heat and water. In effect it is a continuously regenerating battery in which the chemical equivalent of combustion takes place to release energy.

The route to electricity from sewage is normally via the digestion process which produces biogas which, in turn, powers conventional generators. Now there is on the horizon a system of producing electricity directly from a microbial fuel cell (MFC). Researchers at Pennsylvania State University have a developed a device which serves dual roles of generating electricity and, at the same time, performing the function of a sewage treatment plant. The bacteria in normal sewage treatment use enzymes to oxidise organic material and in the process release electrons. The MFC is a cylinder with a central cathode rod surrounded by a proton exchange membrane (PEM). A cluster of graphite anode rods surrounds the cathode. Bacteria become attached to the anodes causing the organic waste to be broken down into electrons and protons. A charge separation occurs with protons allowed to pass through the PEM to the cathode but not the electrons. These are diverted to power an external circuit. The circuit is completed to allow the protons and electrons to recombine at the cathode to produce pure water (Figure 21.1).

Water y

Cathode

Anode

Cathode

Anode

Figure 21.1

Microbial fuel cell (derived from New Scientist)

This is the first MFC designed specifically to process human waste. As the first prototype, there is considerable further research and development to be undertaken and it may take another 20 years for it to achieve the scale of output which would make it commercially viable (New Scientist, 13 March 2004, p. 21).

The holy grail of energy is the fuel cell that creates power with absolutely no polluting emissions. That will happen when the electrolytic process to split water into oxygen and hydrogen is driven by zero carbon renewable energy systems. But, if you are already producing carbon-free electricity why incur an efficiency drop by creating hydrogen? The obvious answer is that it is the way of ensuring continuity of supply. Most renewable systems are intermittent and hydrogen supplies the so-called flywheel effect smoothing out the peaks and troughs. It is also transportable so has universal application.

Producing hydrogen by the electrolyser method is fairly energy intensive and not carbon free unless it involves carbon neutral generation systems. A less carbon intensive alternative being developed is the hydrogen generator fuel cell (HGFC). Its fuel is a mixture of ethanol and water. Ethanol (alcohol) is produced by the breakdown and fermentation of crop waste or fuel crops. The ethanol and water are mixed with air and then heated to 140°C causing the mix to vaporise. The gas then passes over a catalyst (rhodium and cerium oxide) which increases the temperature to 700°C and breaks down the ethanol into hydrogen, carbon monoxide and carbon dioxide. Some of this heat is used to heat up the incoming mixture. The gases pass to a cooling chamber reducing the temperature to 400°C. They then pass across a second catalyst (platinum and cerium oxide) which causes the carbon monoxide to react with the hot water producing carbon dioxide and hydrogen in a 50-50 split. The carbon dioxide (CO2) balances that absorbed by the biological waste during growth.

This system could be scaled up to supplying grid-scale fuel cells by using a combination of agricultural waste and dedicated rapid rotation energy crops. The initial heating process could be assisted by evacuated tube solar thermal collectors (Figure 21.2).

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