The Requirements for Sustainable Energy Sustainability issues The environmental consequences of continuing growth in economic activity in developed countries have become apparent firstly from the consideration of the use of natural resources, and secondly from the damage to global, regional, and local environments from emissions This, and the future projections for population and economic growth in developing countries require re-evaluation of the position of humanity in the global environment and the established approaches to development. This is especially apposite to the consumption and provision of energy, considering the thermodynamic irreversibility of combustion processes, the rapid depletion of fossil fuels, and the effects of emissions such as global warming and acid rain. The Brundtland report (1987) defined sustainable development as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs" and advocated integration of environmental considerations into all aspects of economic and development policy in order to meet this goal. Sustainable development issues therefore include: Ecological sustainability: Preventing damage to major biological life cycles, maintaining ecosystems and biodiversity; Sustainable resource use: Resource use should not threaten ecological sustainability and should be within the renewable capacity; Sustainable waste management and pollution prevention: Generation of waste and release of pollution should not threaten ecological sustainability and should be within the absorption capacity of the receiving environment; Development of a sustainable society: This is concerned with social inclusion and economic development to improve the quality of life for all, both in the short and long term. Thus, in energy projects we are primarily concerned with providing essential energy needs with minimum resource use, waste, and harmful emissions; and must strive to achieve this within renewable limits as a matter of urgency.

Concepts and systems Sustainable energy CHP systems Biofuels Potential Applications Fuel Cells Gibbs energy Reformers Fuel cell performance Control and ancillaries Electricity and heat transfer Installation and operation Commercial availability Economics Environment Environmental aspects Index of technical reviews

Sustainable development and the "Triple bottom line"

Future energy needs in the developed world

An economic growth rate of 2% over 50 years equates to a compound increase by a factor of 2.7, and a 3%growth rate to a factor of 4.4. Primary energy consumption will increase with economic growth unless energy is used more effectively (conservation) or generated and transmitted more efficiently. Many of the easy improvement measures have already been undertaken, and improvements in electricity generating efficiencies using combustion engines are limited by the second law of thermodynamics. Hence, with economic growth, further incremental improvements are unlikely to maintain levels of energy consumption within present levels, let alone achieve the required reductions in the consumption of fossil fuels.

A progressive approach is required to increase the use of renewable energy sources and efficient conversion technologies dramatically (as well as considering the other non-fossil fuel source: nuclear power), to reduce the consumption of non-renewable fuels despite increasing energy consumption. The developed world must take a lead in this as we consume 80% of the world's energy resources, and have the infrastructure to support the technical development required.

Future energy needs in the developing world

The developing world includes 80% of the population, who currently consume only 20% of the energy. At present the energy per capita in the developing world is therefore 1/16 of that in the developed world. The population is increasing and economic growth is required for the wellbeing of the population. Assuming an increase in energy per capita, only to 1/4 of that of the developed world's current consumption, and no further increase in population, gives an increase in consumption of energy of the developed world by a factor of 4, and an increase of world energy by a factor of 1.6, again assuming the increase in the developed world's consumption doesn't happen. An increase in local energy consumption by a factor of 4 is unlikely to be met by traditional means (mainly the combustion of fuel wood) for three reasons:

• The likely catastrophic effect it would have on the environment due to deforestation
• The movement of population to the cities and the health effects of local air pollution
• The increasing requirement for energy in the form of electricity rather than heat

Development of energy infrastructures will therefore be required, which will hopefully be combined with other improvements in infrastructure such as sanitation. However, we know that the world cannot sustain an increase of the use of fossil fuels by 1.6, and technologies such as nuclear power are not suitable where the infrastructure is insufficient to support it. Hence development and the widespread use of alternative or intermediate technologies for the developing world is required. These must make effective use of local resources taking into account other demands on these resources, and where possible should be within the renewable capacity. In summary, the technologies must be:

• Effective
• Accessible
• Affordable

Local energy projects

Energy losses in electricity generation and transmission constitute the largest element of energy consumption, and provision of electricity transmission systems is expensive. The electricity generation efficiencies of centralised power generation can be increased using combined cylcle plants, but the overall efficiencies will not be as high local combined heat and power schemes which recover most of the waste heat.

The domestic sector is the second largest end user of energy, being second only to transport. Here, heat and power must be provided to the dispersed locations of individual dwellings within the community.

Biofuels are available and used most effectively locally. Energy from waste derives from human or farm animal wastes, which are associated with human settlements or communities. Hence these fuels are the ideal fuels to meet local domestic energy needs.

Potential technologies

Biofuels

As stated above, biofuels such as landfill gas, sewage gas, and biogas from farm slurry are ideal fuels to meet local energy demands, being renewable and available locally. Their use may also be beneficial for reasons other than providing energy, such as reducing releases of methane and providing fertiliser. However, the quantities available are limited so they must be used efficiently and supplementary fuel or energy sources are likely to be required.

Liquid biofuels such as ethanol and plant oils are high value fuels and are primarily used for transport applications.

Biomass and solid wastes may be gasified to create a higher premiim fuel, or combusted in high efficiency boilers or furnaces to increase the energy yield.

Combined heat and power

Biofuels and energy from waste may be used in prime movers to provide electrical power and heat. Heat is recovered from that produced by the inefficiency of the prime mover to give a high overall efficiency even if the electrical generation efficiency is low. However, electrical energy is a premium form of energy, and premium fuels are required for the prime movers. It may be more effective to use higher electrical efficiency prime movers to convert the premium fuel to electricity and use lower grade fuels to provide heat by direct combustion.

Fuel cells

Fuel cells have the potential to convert the energy from biogas, gasified fuels, and liquid biofuels to electricity more efficiently than any other prime mover, as their efficiency is not limited by the Carnot efficiency. They are fully scaleable and hence may be considered for small scale local applications where combined cycle plants are not feasible.

Hydro

Hydro electricity is a low cost renewable but available sites are limited by the necessity for a sufficient head, and consistently high flow rate of water. Flooding large areas and water use can have severe adverse environmental impacts if not properly planned. Small scale hydro is the most promising way forward. Pumped storage and tidal power are variations of hydro power. The sites for tidal barrage schemes are very limited, although individual water turbines may emerge for very localised applications.

Wind

Wind power is increasing but is very sensitive to wind speed and must work in conjunction with other energy sources to ensure continuity of the supply. Wind power is usually connected to a large electricity network so that the energy generated during optimum wind conditions can be utilised, and to overcome stability of supply problems. Larger scale wind turbines are more efficient and are proving successful, albeit with slightly higher electricty generation costs than using cheap fossil fuels. Small scale wind mills have been used to provide low torque motive power for many centuries but small scale electricity generators have not been effective.

Wave

There is a massive amount of wave energy available, which is more consistent than wind power. However, means of capturing this energy and transmitting it to the users reliably and economically have not been achieved.

Solar

Solar energy is renewable but the amount of energy available from solar thermal and photovoltaic systems depends on the climate and is usually very limited, especially with the latter which are currently very inefficient and expensive, both in financial terms and the energy required to manufacture.

Geothermal

Geothermal energy is available at a large number of locations but sufficient grade heat is available at much fewer locations, and extraction may be difficult.

Conclusion

All energy sources have limitations on their use and many of the renewable energy costs are greater than the current low prices of fossil fuels. However, we should not wait until shortage of fossil fuels pushes prices up before introducing significantly more renewable energy worldwide. A mix of all the technologies is required to move towards sustainability.

Globally, biofuels are likely to provide a major renewable energy contribution, and fuel cells offer the most efficient means of converting the limited quantities of biofuels to electrical energy.

References

1. DTI Energy statistics: Energy in Brief

2. Technology of biogas production & application in rural areas; World energy conference 1989
3. Green energy, biomass fuels & the environment; United Nations Environment Programme 1991