SERIES ARTICLE Challenges in the Quest for Clean Energies 4. Other Renewable Resources and Conclusion Sheela K Ramasesha Water, biomass and geothermal are also renewable resources for use in power generation stations. Many countries around the world use these energy resources in their energy mix. The technologies needed for each of these renewable resources and how they are applied is discussed in the article. Their scenario Sheela K Ramasesha is in India is also presented. currently working on renewable energy In the concluding remarks, the role of using renewable energy technologies with primary resources in electricity generation in the light of reduction of focus on photovoltaics at Green House Gas (GHG) emission into the atmosphere is Divecha Centre for Climate Change, Indian Institute of deliberated. The pros and cons of renewable resources are Science, Bangalore. She is discussed in detail and how they should be used responsibly is involved in a wide variety highlighted. The most important step towards reduction of of projects in the field. Her GHG is to reduce consumption of energy in our routine, in hobbies include traveling, music and movies. addition to using renewable sources to generate the much needed electricity. In Part 1 of this series we learnt that our energy-intensive lifestyle has increased the greenhouse gas concentrations in the atmo- sphere that has lead to many disastrous events in the world. Previous articles: 1. Background, Vol.18, No.3, Earth’s surface temperature has gone up by a few degrees centi- 2013. grade resulting in melting of ice and hence rising of sea levels. For 2. Solar Energy Technologies, the generation of power, the main source of fuel has been petro- Vol.18, No.5, 2013. leum and coal, the burning of which has increased the carbon 3. Wind Technologies, Vol.18, No.8, 2013. dioxide in the atmosphere by almost 30% in the last century. Therefore it is imperative that renewable sources like sunlight, wind, biomass and such are used to generate a part of our power requirement. In Part 2 of the series we learnt how solar power can be used to generate electricity without emitting greenhouse gases. Keywords The basic principles behind the photovoltaic and solar thermal Hydroelectricity, bio-energy, were described in detail. Solar parks are set up in many parts of geothermal, renewable energy. the world and the Government of India has started many initia- 1110 RESONANCE December 2013 SERIES ARTICLE tives to support solar technologies. In Part 3 of the series, how Water evaporates wind power can be harnessed to generate electricity and the from the surfaces science behind the wind energy technology were described. There of trees, water are different kinds of wind turbines and each one is good under bodies and specific wind profiles and heights. In India, the government ground to envisages that by 2020, 65 GW of electricity will be generated condense into from wind power. In this part of the series, we will study the other clouds and some renewable resources like hydro, biomass and geothermal power of it returns to the generation technologies. earth in the form of rain. Thus, Hydropower water is a timeless Hydropower is a well-developed and mature technology that is in and renewable use in 159 countries around the world. As per the hydrology resource. cycle, water reaches the earth as precipitation or rain (Figure 1). Some of the water percolates into the soil to create a water table under the ground or runs off on the surface into water bodies like lakes, oceans or ponds. The percolated water will also flow into water bodies with time. Similarly, the snow on the earth’s surface also flows along the ground and percolates to inside the ground. Water evaporates from the surfaces of trees, water bodies and ground to condense into clouds and some of it returns to the earth in the form of rain. Thus, water is a timeless and renewable resource. Figure 1. Schematic repre- sentation of hydrology cycle. RESONANCE December 2013 1111 SERIES ARTICLE Figure 2. Schematic repre- sentation of a hydroelectric plant. Hydroelectricity, as the name indicates, is the electricity obtained by harnessing the power of flowing water. Water is usually stored in dams and led down through large pipes or tunnels to lower levels, as shown in Figure 2. In the course of the fall, the water rotates turbines. The mechanical energy produced is converted to electricity by the generators connected to it. Thus, the potential/ kinetic energy of the water is first converted into mechanical energy and then into electrical energy. The structure that houses the turbines and generators is called the powerhouse. Transform- ers change the alternating current produced by the generators into Hydroelectricity, as current of very high voltage for easy transmission through long the name indicates, is distances. the electricity obtained by The energy potential of a hydroelectric power generation plant harnessing the power site is dependent on the flow and the head. The flow is the volume of flowing water. of water flowing per second expressed in cubic meters per second Water is usually (m3/s). The head is defined as the difference in height across stored in dams and which flow takes place and is expressed in meters (m). led down through Potential (W) = Flow (m3/s) Head (m) 9.81 (m/s2) D (kg/m3) large pipes or tunnels E . (1) to lower levels. In the course of the fall, the Here, 9.81 m/s2 is the acceleration due to earth’s gravity, D is the water rotates density of water and E is the efficiency factor. The density of turbines. water is assumed to be 1. The factor E has to take into account the 1112 RESONANCE December 2013 SERIES ARTICLE inbuilt inefficiencies in the system. The factor E is usually taken to be 0.7. Thus, if water is flowing at the rate of 250 liters/s from a height of 10 m then the power generated can be calculated as, Potential = 0.25 10 9.81 0.7 = 17.17 kW . (2) It is clear from this equation that just fast flowing water without a drop from a height possesses low potential for useful power generation except in the case of large ocean currents. In the example under consideration in (2), if water flows for 1 h, the number of units of electricity generated will be 17.17. This is the theoretically possible electricity output from a hydroelectric plant. But in reality, electricity cannot be generated at the same Thus, if water is rate all through the year. Hence, a term called plant load factor flowing at the rate of (PLF) is defined as, 250 liters/s PLF = 100* (Actual output / Maximum possible output) . (3) from a height of 10 m then the power Higher the PLF, better is the performance of a hydroelectric plant. generated will be This would also amount to more power output so more revenue 17.17 kW and if water from the plant. PLF is affected by non-availability of water, flows for 1 h, the unplanned breakdown and maintenance shut down. number of units of electricity generated The main advantages of hydroelectric power are: will be 17.17. • There is minimal pollution as no fuel is burnt. • The power plant is run by water that is provided free by Nature. • Electricity can be generated only when required by opening the intake or sluice gates. The water can be stored in dams when not in use which means that energy can be stored until it is needed. • Once the dams to store water are built, they last many decades. • The water that is let out into the river after electricity genera- tion can be used for irrigation and other purposes because all of the water is returned to the river. • Relatively low operation and maintenance costs. • The technology is time tested and reliable. RESONANCE December 2013 1113 SERIES ARTICLE Out of the total • Large dams often become tourist attractions. electricity There are some disadvantages of hydroelectric power, namely: production of 20,225 TWh in 2010, about • Dams are extremely expensive to build and must be built to the 3,402 TWh was highest standard of safety. The high cost means that plants from hydroelectric must operate for a long time to become profitable. power. This • The building of dams for hydroelectric power can cause water amounts to over availability problems. Construction of a dam in one location 80% of electricity may mean that those downstream have no control of water generated by all flow. renewable sources. • More importantly, adequate supply of water from rain or snow is required for hydroelectric plants to continue opera- tion. Lack of rainfall in a year can affect electricity production severely. Depending on the location and availability of water resources, many countries around the world have opted for hydroelectric power generation. Out of the total electricity production of 20,225 TWh in 2010, about 3,402 TWh was from hydroelectric power (Figure 3). This amounts to over 80% of electricity generated by all renewable sources [1]. Figure 3. Global electricity generation by fuel, 1973– The World Energy Council calculates the potential global hydro- 2010. power resource at around 16,400 TWh per year, that is, about an Source: IEA. 1114 RESONANCE December 2013 SERIES ARTICLE Figure 4. Share of hydro- power in country’s total power generation. eighth of total global energy consumption [2]. China is the world’s largest producer of hydroelectricity, with the United States, Canada, Brazil, and Russia rounding out the top five countries. In China, hydropower generation has more than tripled during the period of 2000 to 2010.