An Overview of Renewable Energy Sources

Shahidul I. Khan

Professor, Department of Electrical and Electronic Engineering

and

Director, Centre for Energy Studies

Bangladesh University of Engineering and Technology

Dhaka 1000

E-mail:  shahidul@eee.buet.ac.bd  

 Introduction

“Energy is Eternal Delight”- William Blake

A source that is constantly or cyclically replenished, including direct solar energy and indirect sources such as biomass, wind, or hydropower is called Renewable Energy Source [1]. Conventional renewable energy sources are solar, wind, geothermal, hydro and biomass and some of the advanced alternative energy sources are ocean thermal energy generation (OTEC), biomass fuels, wave and tidal power, fuels cells and magnetohydrodynamic (MHD) conversion.

Solar Energy

The availability of solar radiation is extremely high in some localities of the world. There are major advantages to using solar energy for a variety of energy needs including electrical generation and space heating. The technology is also readily available and easily installed in most countries of the world. Furthermore, the lifetime costs of solar energy is cheaper than for conventional energy sources. In some areas, any excess energy produced during the day may be sold to local utility companies or stored for use at night and on cloudy days.

Other sources of energy are not considered as available, clean, abundant, and inexhaustible as the sun's energy that reaches the earth. The rate at which solar energy arrives at the top of the atmosphere is a constant for all practical purposes (the Solar Constant) and the U.S. National Aeronautical and Space Administration's (NASA) standard value for the Solar Constant is 429.2 Btu per sq. ft per hour. The rate of receipt of solar energy on the ground surface depends on its orientation with reference to the sun. The rate is maximum when the surface faces the sun. In principle, it should be possible to convert all the solar energy received on the ground directly or indirectly into other forms of energy such as heat and electricity, which can then be utilized for various purposes.

Wind Energy

Wind energy is actually a form of solar energy, because winds arise primarily from temperature differences on the earth's surface because of unequal exposure (or absorption of) solar radiation. The basic design of most wind turbines (or wind mills) consists of blades (or vanes) radiating from a central axis (hub), which may be horizontal, which is the more familiar design or vertical. When the wind blows against the vanes, they rotate about the axis, and the energy of the rotational motion is converted into electrical energy by connecting the wind turbine to an electric generator. Windmills were used extensively in Europe in the 18th and 19th centuries, especially for milling grain and pumping water.

Hydroelectric Energy

Water power has been used on a local scale for thousands of years, but it was only at the beginning of the 20th century that large-scale damming of rivers commenced for generation of hydroelectricity. Hydroelectricity is and will remain the predominant form of renewable energy in the world. Countries and areas with mountainous or plateau regions make for prime locations of hydroelectric plants and have the greatest potential for hydropower. Since the 1700s, kinetic energy in the form of falling and flowing water of rivers and streams has been used to produce electricity in small- and large-scale hydroelectric plants Large-scale hydroelectric projects are those in which high dams are built across rivers to create large reservoirs, and the flowing water is controlled as it falls and spins turbines producing electricity. Small-scale hydroelectric projects have a low dam, with little or no reservoir capacity, built across small streams or rivers. The natural flow of water is used to generate electricity, but as such, can have seasonal water flow problems, which limit production. Pumped-storage hydropower systems are used to supply peak-demand energy, and are of the falling water variety. In this case, water is pumped uphill to a secondary reservoir during times of low-energy demand or high water flow times, and is released during times of peak-energy demand. Hydroelectric power plants fall into two main categories: (1) run-of the-river plants, and (2) storage plants, which depend on water stored in a reservoir or lake created by building a dam across the river at a suitable location (e. g., the TVA hydroelectric plants, USA). Run-of-the-river plants usually do not have facilities for storing substantial quantities of water; the power generated may thus be variable, as it depends almost entirely on the stream flow, which changes with seasons. Storage plants, on the other hand, can operate in accordance with power demand rather than stream flow. Reservoirs also are important for flood control and irrigation, in addition to the generation of electric power.

Brazil and Paraguay (1997) owned has the region's largest operating hydroelectric plant, the 12,600 MW Itapd project. Bangladesh has the lone hydroelectric power station of 230 MW capacity at Kaptai, Chittangong and a few micro hydro power station in hilly regions.

Ocean Energy

Tidal power

Another potentially abundant source of energy for producing electricity from flowing water is to engineer the use of the daily oscillation of ocean water levels as a result of gravitational attraction among the earth, moon, and the sun on a daily or bi-daily basis. The energy contained in flowing water as it rises or falls due to ocean tides [2-4] can be converted into rotational energy in a hydraulic turbine and then finally into electrical energy by means of an attached generator. Several unique advantages of tidal energy are: (1) tides occur with predictable regularity (two high tides and two low tides everyday), and (2) the power source has little if any serious or unmanageable environmental drawbacks, (3) it is free energy, (4) operating costs are low, and (5) little if any air pollution or land disturbance. Some of the major limitations, of course, are that: (1) tidal power is generated only twice a day, thus continuity of source may be a problem, but this is also a limitation with solar or wind energy; (2) construction costs are high; and (3) concerns exists involving storm damage and seawater corrosion of facilities and machinery.

The maximum amount of electrical energy that can be generated from the tides depends on the tidal range or more precisely the difference in water levels between successive high and low tides and the volume of water flowing through the turbine. A favorable site for a tidal power plant should have certain unique characteristics. The site should have a large tidal range of at least 30 feet or 10 m and suitable topographic features that permit enclosure of large areas by small dams. Likely sites are, for example, estuaries and bays, which might generate some environmental concerns. In the open ocean, the tidal range is commonly 2-3 feet (0.6-0.9 m) but in some bays and estuaries it maybe as high as 50 feet (15 m) or more which is ideal for tidal power plants. Such high tidal ranges, how­ever, occur only at a few ideal places in the world such as the Bay of Fundy on the Atlantic Coast of Canada near Maine (50 ft), the Severn River estuary in England (45 ft), or the Rance River estuary in France (40-44 ft) A small plant built at St. Malo on the northern coast of France in 1968 was generating around 160 MW of commercial power. Also, a small unit was generating an output of 20 MW at Anapolis Royal, Nova Scotia, on the Bay of Fundy, where tides have about a 16-feet differential.

Wave power

The waves falling on a mile of beach contain an estimated 65 MW of power, but this power source can be very difficult to harness. However, several countries including Japan, England, France, and USA were extensively experimenting with the idea of producing electricity from wave power in 1986. In 2001, the ocean's wave energy was successfully being tapped for commercial purposes, using the Limpet (land-installed marine powered energy transformer) System developed by a renewable energy company (Wavegen) in Scotland. The system collects wave energy in a chamber and the power of the waves in the chamber is transformed into electricity with a turbo generator, which uses a turbine whose blades turn in the same direction no matter which way air flows across the blades, thus continuing to turn as the waves rise and fall. The system is based on at least 20 years of research dating back to the 1980s [5].

Geothermal Energy

Some of the heat contained inside the earth can and is being used around the world for heat generation. Under special circumstances, heat from inside the earth now reaches the surface by conduction through rocks or, in a few special cases, by outward flow of heated waters, or molten rock. The only way to speed up the process is to use better heat conductors or to speed up the flow of heated water. Replacing rocks with better conductors seems unlikely. In some instances, deep holes are being drilled, and rocks are shattered as in the oil industry, by controlled blasts or hydro-fracturing, then the process is to circulate water which is heated by the earth's heat, recovered, and used to generate electricity. Energy developments such as these have enormous practical difficulties. Experts, therefore, do not count all the average heat in rocks inside the earth as a potential resource of energy. Particular circumstances do offer a good deal of promise for geothermal power. There are 4 special types of geothermal resources. They are classified as hydrothermal, geopressured, hot-dry rock, and magma systems. There may be possibility of extracting energy from abandoned on-shore gas field [6] in Bangladesh.

Ocean Thermal Energy Generation (OTEC)

Because ocean water stores a tremendous amount of heat from the sun, there is a thermal gradient existing in ocean water. The temperature of the ocean water decreases with depth and as such the thermal gradient is a direct function of the penetration of solar radiation. The difference in temperature between the warm surface water and the cooler water at depth can be utilized to drive a special turbine and generate electricity. Since the sun warms the ocean waters, they constitute a virtually inexhaustible source of energy. Moreover, unlike direct solar energy, ocean energy would be available continuously rather than only in the daytime, especially in warmer tropical climates.

The introduction of the development of ocean thermal energy to produce power was conceived in France in 1881 and verified in 1929 in an installation off the coast of Cuba. After some unsuccessful tests the idea was abandoned. There has been renewed interest in the OTEC technology and concepts since the early 1970s and Japan has successfully run a prototype power station.

Energy and the Millennium Development Goals

Renewable energy sources can successfully be utilized for world poverty reduction. Currently, some 2.4 billion people in developing countries lack modern fuels [7] for cooking and heating and about 1.6 billion people do not have access to electricity. In South Asia, only 30 per cent of the rural population has access to electricity compared with 68 per cent of rural poor in most of the developing countries in the Asian and Pacific region. The MDGs are a set of time-bound, measurable goals for combating poverty, hunger, disease, illiteracy, environmental degradation and gender inequality: all these goals require or relate to energy services. The MDGs may not be met unless rapid progress is made in extending efficient and affordable energy services to the poor in support of productive economic activities or social development (Appendix).

Changing the Paradigm

Energy services for poverty reduction are less about technology and more about understanding the role that energy plays in people's lives and responding to the constraints in improving livelihoods. In the past, dissemination programmes have tended to concentrate on the supply of energy, such as electricity or petroleum, or on energy technologies, such as solar equipment or improved stoves. The required paradigm shift is from technology-driven energy use to a development-focused framework. Energy needs should be considered within the overall context of community life, and energy policies and projects should be integrated in a holistic way with other improvement efforts relating to health, education, agriculture and job creation. Policies, programmes and projects should start from an assessment of people's needs of different rural communities vary widely, and finding appropriate technologies and effective implementation strategies can be very site-specific.

While it is accepted worldwide that energy is a basic need for survival and a key input to social and economic development, about 2.4 billion people in developing countries still lack access to clean, affordable and reliable sources of modern energy, and the majority of these people live in the Asian and Pacific region. An amount of energy roughly equivalent to 7 per cent of the world's electricity production today could cover basic human needs. In an age of apparently advanced technological and management skills, the world has failed in this relatively modest challenge.

While energy services are directly associated with the quality of life and level of development, the amount and quality of energy consumption has a co-relation with poverty, deprivation, social seclusion, access to knowledge and achievements, health, livelihood and security. The wider disparities in various regions, countries and communities are also correlated with the disparities in gaining access to energy services.

Nearly 50 per cent of the world's population still relies on the traditional fuel source-the direct combustion of solid biomass for fuel. The continued dependence on biomass combustion, without technological improvements, makes people face irrecoverable costs that lead to a deteriorating quality of life for women, children, families and communities.

Strikingly, traditional biomass fuel use in rural areas by the poorest sectors makes it clear that the majority of people in many countries have not been able to access better energy carriers to meet their needs, and few institutions have taken the lead in providing energy services to these sectors.

To ameliorate the situation, the world leaders at the World Summit on Sustainable Development agreed "to improve access to reliable and affordable energy services for sustainable development, sufficient to facilitate the achievement of the MDGs, including the goal of halving the proportion of people in poverty [8] by 2015, and as a means to generate other important service that mitigate poverty.

Conclusion

A comprehensive description of renewable energy sources is described in this lecture. It is sincerely hoped that the renewable energy will have positive impact in eradiating poverty and illiteracy in Bangladesh. The Centre for Energy Studies, BUET can prove services to nation on energy related issues.

Acknowledgement

Some of the information provided in this lecture is from the book [1] “World Energy Resources” by Dr. Charles E. Brown.

References

[1]        Charles E. Brown, World Energy Resources, Springer, Berlin 2002.

[2]        Shahidul I. Khan and M. U. Mahfuz, “Prospect of Tidal Energy in Bangladesh”, in conf. Proceeding, World Renewable Energy Congress (WREN)-VII, Colonge, Germany, June 29-July 05, 2002, pp. in CD.

[3]        M. Hassan, K. M. Mahfuz, M. A. Haque and Shahidul I khan, “ A New Concept of Micro-Hydro Power Generation from Tide”, presented at the International Conference on Electrical Engineering, Khulna, October 23-24, 2002.

[4]        Shahidul I khan, K. S. Kannan and M. S. Majid “Prospect of Tidal Energy in Malaysia”, in conf. Proceeding, World Renewable Energy Congress (WREN)-V, Florence, Italy, 1998, pp. 2425-2428.

[5]        The Futurist Magazine, World Future Society, May-June 2001 issue, p. 2

[6]        Debendra Guba, Herbert Henkel and Gunnar Jacks, ‘Abondoned On-Shore Deep Wells-A potential Geothermal Energy Resources for Rural Bangladesh”, a seminar presentation on 8 January 2006 at Petrobangla Seminar Hall, Dhaka.

[7]        United Nations Department of Economic and Social Affairs, "The Energy Challenge for Achieving the Millennium Development Goals", New York, United Nations, 2005, available on line at http://sea.un.org/un.

[8]        United nations, “Report of the World Summit on Sustainable Development”, Johannesburg, South Africa, 24 August-4 September 2002.

Appendix

Table A1 Energy services and the MDGs