To protect environment and for sustainable development, the importance of Renewable Energy Sources cannot be overemphasized. It is an established and accepted fact that Renewable Energy Sources and non-conventional forms of energy will play an increasingly important role in the future as they are cleaner and easier to use and environmentally benign and are bound to become economically more viable with increased use.
Because of the limited availability of coal, there is considerable international effort into the development of alternative/new/non-conventional/renew-able/clean sources of energy. Most of the new sources (some of them in fact have been known and used for centuries now!) are nothing but the manifestation of solar energy, e.g., wind, sea waves, ocean thermal energy conversion (OTEC) etc. In this section, we shall discuss the possibilities and potentialities of various methods of using solar energy.
Winds are essentially created by the solar heating of the atmosphere. Several attempts have been made since 1940 to use wind to generate electric energy and development is still going on. However, technoeconomic feasibility has yet to be satisfactorily established.
Wind as a power source is attractive because it is plentiful, inexhaustible and non-polluting. Further, it does not impose extra heat burden on the environment. Unfortunately, it is non-steady and undependable. Control equipment has been devised to start the wind power plant whenever the wind speed reaches 30 km/h. Methods have also been found to generate constant frequency power with varying wind speeds and consequently varying speeds of wind mill propellers. Wind power may prove practical for small power needs in isolated sites. But for maximum flexibility, it should be used in conjunction with other methods of power generation to ensure continuity.
For wind power generation, there are three types of operations:
1. Small, 0.5-10 kW for isolated single premises
2. Medium, 10-100 kW for communities
3. Large, 1.5 MW for connection to the grid. The theoretical power in a wind stream is given by
For a rotor of 17 m diameter and a velocity of 48 km/h the theoretical power is 265 kW and the practical would be roughly half of this value.
There are some distinctive Renewable Energy Sources end-use features of wind power systems:
1. Most wind power sites are in remote rural, island or marine areas.
2. Rural grid systems are likely to be ‘weak’ in these areas, since they carry relatively low voltage supplies (e.g. 33 kV).
3. There are always periods without wind.
In India, wind power plants have been installed in Gujarat, Orissa, Maharashtra and Tamil Nadu, where wind blows at speeds of 30 km/h during summer. On the whole, the wind power potential of India has been estimated to be substantial and is around 45000 MW. The installed capacity as on Dec. 2000 is 1267 MW, the bulk of which is in Tamil Nadu (60%). The corresponding world figure is 14000 MW, the bulk of which is in Europe (70%).
The average incident solar energy received on earth’s surface is about 600 W/m2 but the actual value varies considerably. It has the advantage of being free of cost, non-exhaustible and completely pollution-free. On the other hand, it has several drawbacks energy density per unit area is very low, it is available for only a part of the day, and cloudy and hazy atmospheric conditions greatly reduce the energy received. Therefore, harnessing solar energy for electricity generation, challenging technological problems exist, the most important being that of the collection and concentration of solar energy and its conversion to the electrical form through efficient and comparatively economical means.
At present, two technologies are being developed for conversion of solar energy to the electrical form. In one technology, collectors with concentrators are employed to achieve temperatures high enough (700°C) to operate a heat engine at reasonable efficiency to generate electricity. However, there are considerable engineering difficulties in building a single tracking bowl with a diameter exceeding 30 m to generate perhaps 200 kW. The scheme involves large and intricate structures involving huge capital outlay and as of today is far from being competitive with conventional electricity generation.
The solar power tower generates steam for electricity production. There is a 10 MW installation of such a tower by the Southern California Edison Co. in USA using 1818 plane mirrors, each 7 m x 7 m reflecting direct radiation to the raised boiler.
Electricity may be generated from a Solar Pond by using a special low temperature’ heat engine coupled to an electric generator. A solar pond at Ein Borek in Israel produces a steady 150 kW from 0.74 hectare at a busbar cost of about $ 0.10/kWh.
Solar power potential is unlimited, however, total capacity of about 2000 MW is being planned. Total solar energy potential in India is 5 x 1015 kWh/yr. Up to 31.12.2000, 462000 solar cookers, 55X104m2 solar thermal system collector area, 47 MW of SPY power, 270 community lights, 278000 solar lanterns (PV domestic lighting units), 640 TV (solar), 39000 PV street lights and 3370 water pumps were installed. Village power plants (stand-alone) of 1.1 MW capacity and 1.1 MW of grid connected solar power plants were in operation. As per one estimate , solar power will overtake wind in 2040 and would become the world’s overall largest source of electricity by 2050.
Direct Conversion to Electricity (Photovoltaic Generation)
This technology converts solar energy to the electrical form by means of silicon wafer photoelectric cells known as “Solar Cells”. Their theoretical efficiency is about 25% but the practical value is only about 15%. But that does not matter as solar energy is basically free of cost. The chief problem is the cost and maintenance of solar cells. With the likelihood of a breakthrough in the large scale production of cheap solar cells with amorphous silicon, this technology may compete with conventional methods of electricity generation, particularly as conventional fuels become scarce. Solar energy could, at the most, supplement up to 5-10% of the total energy demand. It has been estimated that to produce 1012 kWh per year, the necessary cells would occupy about 0.1% of US land area as against highways which occupy 1.5% (in 1975) assuming 10% efficiency and a daily insolation of 4 kWh/m2. In all solar thermal schemes, storage is necessary because of the fluctuating nature of sun’s energy. This is equally true with many other unconventional sources as well as sources like wind. Fluctuating sources with fluctuating loads complicate still further the electricity supply.
The energy content of sea waves is very high. In India, with several hundreds of kilometers of coast line, a vast source of energy is available. The power in the wave is proportional to the square of the amplitude and to the period of the motion. Therefore, the long period (~ 10 s), large amplitude (~ 2m) waves are of considerable interest for power generation, with energy fluxes commonly averaging between 50 and 70 kW/m width of oncoming wave. Though the engineering problems associated with wave-power are formidable, the amount of Renewable Energy Sources that can be harnessed is large and development work is in progress. Sea wave power estimated potential is 20000 MW. Ocean Thermal Energy Conversion (OTEC) The ocean is the world’s largest solar collector. Temperature difference of 20°C between warm, solar absorbing surface water and cooler ‘bottom’ water can occur. This can provide a continually replenished store of thermal energy which is in principle available for conversion to other energy forms. OTEC refers to the conversion of some of this thermal energy into work and thence into electricity. Estimated potential of ocean thermal power in India is 50,000 MW. A proposed plant using sea temperature difference would be situated 25 km east of Miami (USA), where the temperature difference is 17.5°C.
The material of plants and animals is called biomass, which may be transformed by chemical and biological processes to produce intermediate biofuels such as methane gas, ethanol liquid or charcoal solid. Biomass is burnt to provide heat for cooking, comfort heat (space heat), crop drying, factory processes and raising steam for electricity production and transport. In India potential for bio-Energy is 17000 MW and that for agricultural waste is about 6000 MW. There are about 2000 community biogas plants and family size biogas plants are 3.1 x 106. Total biomass power harnessed so far is 222 MW. Renewable Energy Sources are specially designed to meet the growing energy needs in the rural areas for promoting decentralized and hybrid development so as to stem growing migration of rural population to urban areas in search of better living conditions. It would be through this integration of energy conservation efforts with Renewable Energy Sources that India would be able to achieve a smooth transition from fossil fuel economy to sustainable Renewable Energy Sources based economy and bring “Energy for all” for equitable and environmental friendly sustainable development.