题目内容
Renewable EnergyA．In the past century, it has been seen that the consumption of non-renewable sources of energy has caused more environmental damage than any other human activity. Electricity generated from fossil fuels such as coal and crude oil has led to high concentrations of harmful gases in the atmosphere. This has in turn led to many problems being faced today such as ozone depletion and global warming. Vehicular pollution has also been a major problem.B．Therefore, alternative sources of energy have become very important and relevant to today’s world. These sources, such as the sun and wind, can never be exhausted and therefore are called renewable. They cause less emission and are available locally. Their use can, to a large extent, reduce chemical, radioactive, and thermal pollution. They stand out as a viable source of clean and limitless energy. These are also known as non-conventional sources of energy. The following section outlines several different types of renewable energy.C．Solar energy is the most readily available source of energy. It does not belong to anybody and is, therefore, free. It is also the most important of the non-conventional sources of energy because it is non-polluting and, therefore, helps in lessening the greenhouse effect.D．Solar energy has been used since prehistoric times, but in a most primitive manner. Before 1970, some research and development was carried out in a few countries to exploit solar energy more efficiently, but most of this work remained mainly academic. After the dramatic rise in oil prices in the 1970s, several countries began to formulate extensive research and development programmes to exploit solar energy.E．When we hang out our clothes to dry in the sun, we use the energy of the sun. In the same way, solar panels absorb the energy of the sun to provide heat for cooking and for heating water. Such systems are available in the market and are being used in homes and factories. Solar energy can also be used to meet our electricity requirements. Through Solar Photovoltaic (SPV) cells, solar radiation gets converted into DC electricity directly. This electricity can either be used as it is or can be stored in the battery. This stored electrical energy then can be used at night.F．Another type of renewable energy is hydro power. It is one of the best, cheapest, and cleanest sources of energy, although, with big dams, there are many environmental and social problems. Small dams are, however, free from these problems. This is in fact one of the earliest known renewable energy sources, in the country (since the beginning of the 20th century). In fact, for the last few hundred years, people living in the hills of the Himalayas have been using water mills, or chakki, to grind wheat. Besides being free from the problem of pollution, small hydropower plants are also free from issues and controversies that are associated with the bigger projects, namely affecting the lives of thousands of people living along the banks of the rivers, destruction of large areas under forest, and seismological threats.G．New environmental laws affected by the danger of global warming have made energy from small hydropower plants more relevant. These small hydropower plants can serve the energy needs of remote rural areas independently. The real challenge in a remote area lies in successful marketing of the energy and recovering the dues. Local industries should be encouraged to use this electricity for sustainable development. It is a technology with enormous potential, which could exploit the water resources to supply energy to remote rural areas with little access to conventional energy sources. It also eliminates most of the negative environmental effects associated with large hydro projects.H．The sea could also provide immense power—that is ocean thermal, tidal and wave energy. On an average, the 60 million square kilometre of the tropical seas absorb solar radiation equivalent to the heat content of 245 billion barrels of oil. Scientists feel that if this energy can be tapped a large source of energy will be available to the tropical countries and to other countries as well. The process of harnessing this energy is called OTEC (ocean thermal energy conversion). It uses the temperature differences between the surface of the ocean and the depths of about 1000m to operate a heat engine, which produces electric power.I．Energy is also obtained from waves and tides. In some countries such as Japan small scale power generators run by energy from waves of the ocean, have been used as power sources for channel marking buoys.J．Another renewable energy resource is biomass which is derived from the carbonaceous waste of various human and natural activities. It is derived from numerous sources, including the by-products from the timber industry, agricultural crops, raw material from the forest, major Darts of household waste and wood.K．Biomass does not add carbon dioxide to the atmosphere as it absorbs the same amount of carbon in growing as it releases when consumed as a fuel. Its advantage is that it can be used to generate electricity with the same equipment of power plants that are now burning fossil fuels. Biomass is an important source of energy and the most important fuel worldwide after coal, oil and natural gas.L．Traditional use of biomass is more than its use in modern application. In the developed world biomass is again becoming important for applications such as combined heat and power generation. In addition, biomass energy is gaining significance as a source of clean heat for domestic heating and community heating applications. In fact in countries like Finland, USA and Sweden the per capita biomass energy used is higher than it is in India, China or in Asia.M．The category of renewable energy should also include geothermal energy. We live between two great sources of energy, the hot rocks beneath the surface of the earth and the sun in the sky. Our ancestors knew the value of geothermal energy; they bathed and cooked in hot springs. Today we have recognized that this resource has potential for much broader application.N．The core of the earth is very hot and it is possible to make use of this geothermal energy (in Greek it means heat from the earth). These are areas where there are volcanoes, hot springs, and geysers, and methane under the water in the oceans and seas. In some countries, such as in the USA water is pumped from underground hot water deposits and used to heat people’s houses.O．The utilization of geothermal energy for the production of electricity dates back to the early part of the twentieth century. For 50 years the generation of electricity from geothermal energy was confined to Italy and interest in this technology was slow to spread elsewhere. In 1943 the use of geothermal hot water was pioneered in Iceland.P．The last type of energy involved here is co-generation. Co-generation is the concept of producing two forms of energy from one fuel. One of the forms of energy must always be heat and the other may be electricity or mechanical energy. In a conventional power plant, fuel is burnt in a boiler to generate high-pressure steam. This steam is used to drive a turbine, which in turn drives an alternator through a steam turbine to produce electric power. The exhaust steam is generally condensed to water which goes back to the boiler.Q．As the low-pressure steam has a large quantum of heat which is lost in the process of condensing, the efficiency of conventional power plants is only around 35%. In a cogeneration plant, very high efficiency levels, in the range of 75%-90%, can be reached. This is so, because the low-pressure exhaust steam coming out of the turbine is not condensed, but used for heating purposes in factories or houses. Since co-generation can meet both power and heat needs, it has other advantages as well in the form of significant cost savings for the plant and reduction in emissions of pollutants due to reduced fuel consumption. In ancient times, people used geothermal energy by cooking in hot springs.（）
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Making Robots More Like UsA．On a recent morning Natanel Dukan walked into the Paris offices of the French robot maker Aldebaran and noticed one of the company’s humanoid (类人的) NAO robots sitting on a chair. Mr. Dukan, an electrical engineer, could not resist. Bending over, he kissed the robot on the cheek. In response the NAO tilted its head and touched his cheek. It is certainly a very French application for a robot, but the intimate gesture by the ＄16,000, two-foot robot, now being used in academic research labs and robotic soccer leagues, also reflects a significant shift.B．Until recently, most robots were carefully separated from humans. They have largely been used in factories to perform repetitive tasks that required speed, precision and force. That generation of robots is dangerous, and they have been caged and fenced for the protection of workers.C．But the industrial era of robotics is over. And robots are beginning to move around in the world. More and more, they are also beginning to imitate—and look like—humans. And they are beginning to perform tasks as humans do, too. Many of the new generation of robots are tele-operated from a distance, but are increasingly doing tasks independent of direct human control. Moreover, the new robots are designed not just to replace but to collaborate with humans.D．The idea that robots will be partners of humans, rather than stand-ins or servants, is now driving research at universities and industrial laboratories. This year, new United States industry standards for robotic manufacturing systems were published, underlining the emergence of the field. The standards specify performance requirements that will permit human workers to collaborate with robots directly, and they reverse manufacturing guidelines from 1999 that prohibited "continuous attended operations" requiring humans to be in close contact with robots that were deemed unsafe by the industry.E．Today’s robot designers believe that their creations will become therapists, caregivers, guides and security guards, and will ultimately perform virtually any form of human labor. (Robots that can think on their own—that is, perform with high levels of artificial intelligence—have yet to arrive.)F．The key to this advance is the new robots’ form. Their humanlike appearance does more than satisfy science-fiction fantasies. Roboticists say they are choosing the human form for both social and technical reasons. Robots that operate indoors, in particular, must be able to navigate a world full of handles, switches, levers and doors that have been designed for humans. Roboticists also point out that humans have an affinity (亲和力) for their own shape, easing transitions and making collaboration more natural. Creating robots in humanoid form also simplifies training and partnerships in the workplace, and increases their potential in new applications like caregiving.G．It is still unclear how well these new robots will be accepted by society, for they raise fundamental questions about what it means to be human. However, rapid improvements in computer vision, processing power and storage, low-cost sensors, as well as new algorithms (算法) that allow robots to plan and move in disordered environments, are malting these new uses possible and in the process changing the nature of robotics.H．And so on factory floors around the world, a new breed of robot is being manufactured by companies like Rethink Robotics of Boston, which makes a humanoid robot for simple factory automation tasks, and Universal Robots of Odense, Denmark, which makes a dual robot-arm system designed for doing more traditional factory applications, but without cages.I．In Dr. Kemp’s Healthcare Robotics lab at Georgia Tech, a five-foot robot named Cody, which is able to sense forces on its arms and has a base that allows it to move gracefully, is being used as a dance partner for both experienced human dancers and patients in physical therapy. "This is a way that robots can be used for fun, interactive exercise in restoration," Dr. Kemp said. "We can also use it as a tool to understand whole body physical interaction between people and robots."J．At Carnegie Mellon University, Manuela M. Veloso, a professor of computer science, has developed a series of mobile robots she calls CoBots to perform tasks like delivering marl, guiding visitors to appointments and fetching coffee. She calls it "symbiotic autonomy," since the robots also rely on humans. For example, because they don’t have arms, they can’t operate elevators, so they have been programmed to wait and ask for human assistance. If they get lost, they stop, call up a map of the building on their computer screens, interrupt a passing human and say, "I am lost, can you tell me where I am The robotics community calls the idea cheating," Dr. Veloso said, "but it’s not. It’s the secret to real autonomy."K．To function in the real world and to be safe, robots must have a radically different design from factory robots, which are based on "stiff" actuators (制动器) capable of moving with great speed to a precise position. The new robots have "obedient actuators," which respond to external forces by yielding in a natural fashion. The original research into this area of what is now known as "soft robotics" began in the mid-1990s at the Massachusetts Institute of Technology, with work by Gill Pratt, who was exploring walking robots, and Matthew Williamson, then a graduate student and now director of technology development at Rethink Robotics.L．The research was not initially focused on solving the problem of human interaction, but the scientists soon realized the implications, recalled Dr. Pratt, who is now the project manager for the Defense Advanced Research Projects Agency’s Robotics Challenge, an upcoming contest that is intended to advance robotics technology to be used in natural disasters and other emergencies. "It actually started with numerically controlled machine tools," he said—using computer-controlled robots to perform milling tasks.M．For those manufacturing uses, what mattered was the precise positioning of the robot limb. However, Dr. Pratt was focused on developing walking robots that could move in the natural world, and force was more significant than precision to meet that challenge: "There the position of the limb didn’t matter so much, but what mattered was how hard was the robot pressing on the world, and how hard the world was pressing back on the robot," he said.N．The solution was to put something elastic, like a spring, between the motor and the joint. These are now described as series elastic actuators, and the technique of installing them is now widely used as a low-cost solution for robots that are both nonthreatening to humans and able to move more quickly and lightly in the natural world.O．Dr. Pratt recalled an incident when the researchers first realized that series elastic actuation was the key to freeing robots. While working on an early humanoid robot named COG, in a project led by Rodney Brooks, the founder of Rethink Robotics who was then director of the M.I.T. artificial intelligence lab, they were demonstrating how the robot could do tasks like writing with a pencil and paper. However, there was a bug in the software, causing the robot’s arm to repeatedly bang the table. Dr. Brooks decided it was an opportunity to demonstrate the safety of the technology. He placed himself between the table and the arm, which began hitting him.P．Roboticists describe this original approach as "passive obedience." Today there are other approaches—"active obedience"—that use software and sensors to blend speed and precision of more rigid robots with operations that are safe around humans. Roboticists are creating robots with humanlike appearance as humans tend to be attracted by their own shape.（）
Making Robots More Like UsA．On a recent morning Natanel Dukan walked into the Paris offices of the French robot maker Aldebaran and noticed one of the company’s humanoid (类人的) NAO robots sitting on a chair. Mr. Dukan, an electrical engineer, could not resist. Bending over, he kissed the robot on the cheek. In response the NAO tilted its head and touched his cheek. It is certainly a very French application for a robot, but the intimate gesture by the ＄16,000, two-foot robot, now being used in academic research labs and robotic soccer leagues, also reflects a significant shift.B．Until recently, most robots were carefully separated from humans. They have largely been used in factories to perform repetitive tasks that required speed, precision and force. That generation of robots is dangerous, and they have been caged and fenced for the protection of workers.C．But the industrial era of robotics is over. And robots are beginning to move around in the world. More and more, they are also beginning to imitate—and look like—humans. And they are beginning to perform tasks as humans do, too. Many of the new generation of robots are tele-operated from a distance, but are increasingly doing tasks independent of direct human control. Moreover, the new robots are designed not just to replace but to collaborate with humans.D．The idea that robots will be partners of humans, rather than stand-ins or servants, is now driving research at universities and industrial laboratories. This year, new United States industry standards for robotic manufacturing systems were published, underlining the emergence of the field. The standards specify performance requirements that will permit human workers to collaborate with robots directly, and they reverse manufacturing guidelines from 1999 that prohibited "continuous attended operations" requiring humans to be in close contact with robots that were deemed unsafe by the industry.E．Today’s robot designers believe that their creations will become therapists, caregivers, guides and security guards, and will ultimately perform virtually any form of human labor. (Robots that can think on their own—that is, perform with high levels of artificial intelligence—have yet to arrive.)F．The key to this advance is the new robots’ form. Their humanlike appearance does more than satisfy science-fiction fantasies. Roboticists say they are choosing the human form for both social and technical reasons. Robots that operate indoors, in particular, must be able to navigate a world full of handles, switches, levers and doors that have been designed for humans. Roboticists also point out that humans have an affinity (亲和力) for their own shape, easing transitions and making collaboration more natural. Creating robots in humanoid form also simplifies training and partnerships in the workplace, and increases their potential in new applications like caregiving.G．It is still unclear how well these new robots will be accepted by society, for they raise fundamental questions about what it means to be human. However, rapid improvements in computer vision, processing power and storage, low-cost sensors, as well as new algorithms (算法) that allow robots to plan and move in disordered environments, are malting these new uses possible and in the process changing the nature of robotics.H．And so on factory floors around the world, a new breed of robot is being manufactured by companies like Rethink Robotics of Boston, which makes a humanoid robot for simple factory automation tasks, and Universal Robots of Odense, Denmark, which makes a dual robot-arm system designed for doing more traditional factory applications, but without cages.I．In Dr. Kemp’s Healthcare Robotics lab at Georgia Tech, a five-foot robot named Cody, which is able to sense forces on its arms and has a base that allows it to move gracefully, is being used as a dance partner for both experienced human dancers and patients in physical therapy. "This is a way that robots can be used for fun, interactive exercise in restoration," Dr. Kemp said. "We can also use it as a tool to understand whole body physical interaction between people and robots."J．At Carnegie Mellon University, Manuela M. Veloso, a professor of computer science, has developed a series of mobile robots she calls CoBots to perform tasks like delivering marl, guiding visitors to appointments and fetching coffee. She calls it "symbiotic autonomy," since the robots also rely on humans. For example, because they don’t have arms, they can’t operate elevators, so they have been programmed to wait and ask for human assistance. If they get lost, they stop, call up a map of the building on their computer screens, interrupt a passing human and say, "I am lost, can you tell me where I am The robotics community calls the idea cheating," Dr. Veloso said, "but it’s not. It’s the secret to real autonomy."K．To function in the real world and to be safe, robots must have a radically different design from factory robots, which are based on "stiff" actuators (制动器) capable of moving with great speed to a precise position. The new robots have "obedient actuators," which respond to external forces by yielding in a natural fashion. The original research into this area of what is now known as "soft robotics" began in the mid-1990s at the Massachusetts Institute of Technology, with work by Gill Pratt, who was exploring walking robots, and Matthew Williamson, then a graduate student and now director of technology development at Rethink Robotics.L．The research was not initially focused on solving the problem of human interaction, but the scientists soon realized the implications, recalled Dr. Pratt, who is now the project manager for the Defense Advanced Research Projects Agency’s Robotics Challenge, an upcoming contest that is intended to advance robotics technology to be used in natural disasters and other emergencies. "It actually started with numerically controlled machine tools," he said—using computer-controlled robots to perform milling tasks.M．For those manufacturing uses, what mattered was the precise positioning of the robot limb. However, Dr. Pratt was focused on developing walking robots that could move in the natural world, and force was more significant than precision to meet that challenge: "There the position of the limb didn’t matter so much, but what mattered was how hard was the robot pressing on the world, and how hard the world was pressing back on the robot," he said.N．The solution was to put something elastic, like a spring, between the motor and the joint. These are now described as series elastic actuators, and the technique of installing them is now widely used as a low-cost solution for robots that are both nonthreatening to humans and able to move more quickly and lightly in the natural world.O．Dr. Pratt recalled an incident when the researchers first realized that series elastic actuation was the key to freeing robots. While working on an early humanoid robot named COG, in a project led by Rodney Brooks, the founder of Rethink Robotics who was then director of the M.I.T. artificial intelligence lab, they were demonstrating how the robot could do tasks like writing with a pencil and paper. However, there was a bug in the software, causing the robot’s arm to repeatedly bang the table. Dr. Brooks decided it was an opportunity to demonstrate the safety of the technology. He placed himself between the table and the arm, which began hitting him.P．Roboticists describe this original approach as "passive obedience." Today there are other approaches—"active obedience"—that use software and sensors to blend speed and precision of more rigid robots with operations that are safe around humans. The new industry standards for robotic manufacturing systems has changed the earlier guidelines that forbade continuous attended operations.（）
Making Robots More Like UsA．On a recent morning Natanel Dukan walked into the Paris offices of the French robot maker Aldebaran and noticed one of the company’s humanoid (类人的) NAO robots sitting on a chair. Mr. Dukan, an electrical engineer, could not resist. Bending over, he kissed the robot on the cheek. In response the NAO tilted its head and touched his cheek. It is certainly a very French application for a robot, but the intimate gesture by the ＄16,000, two-foot robot, now being used in academic research labs and robotic soccer leagues, also reflects a significant shift.B．Until recently, most robots were carefully separated from humans. They have largely been used in factories to perform repetitive tasks that required speed, precision and force. That generation of robots is dangerous, and they have been caged and fenced for the protection of workers.C．But the industrial era of robotics is over. And robots are beginning to move around in the world. More and more, they are also beginning to imitate—and look like—humans. And they are beginning to perform tasks as humans do, too. Many of the new generation of robots are tele-operated from a distance, but are increasingly doing tasks independent of direct human control. Moreover, the new robots are designed not just to replace but to collaborate with humans.D．The idea that robots will be partners of humans, rather than stand-ins or servants, is now driving research at universities and industrial laboratories. This year, new United States industry standards for robotic manufacturing systems were published, underlining the emergence of the field. The standards specify performance requirements that will permit human workers to collaborate with robots directly, and they reverse manufacturing guidelines from 1999 that prohibited "continuous attended operations" requiring humans to be in close contact with robots that were deemed unsafe by the industry.E．Today’s robot designers believe that their creations will become therapists, caregivers, guides and security guards, and will ultimately perform virtually any form of human labor. (Robots that can think on their own—that is, perform with high levels of artificial intelligence—have yet to arrive.)F．The key to this advance is the new robots’ form. Their humanlike appearance does more than satisfy science-fiction fantasies. Roboticists say they are choosing the human form for both social and technical reasons. Robots that operate indoors, in particular, must be able to navigate a world full of handles, switches, levers and doors that have been designed for humans. Roboticists also point out that humans have an affinity (亲和力) for their own shape, easing transitions and making collaboration more natural. Creating robots in humanoid form also simplifies training and partnerships in the workplace, and increases their potential in new applications like caregiving.G．It is still unclear how well these new robots will be accepted by society, for they raise fundamental questions about what it means to be human. However, rapid improvements in computer vision, processing power and storage, low-cost sensors, as well as new algorithms (算法) that allow robots to plan and move in disordered environments, are malting these new uses possible and in the process changing the nature of robotics.H．And so on factory floors around the world, a new breed of robot is being manufactured by companies like Rethink Robotics of Boston, which makes a humanoid robot for simple factory automation tasks, and Universal Robots of Odense, Denmark, which makes a dual robot-arm system designed for doing more traditional factory applications, but without cages.I．In Dr. Kemp’s Healthcare Robotics lab at Georgia Tech, a five-foot robot named Cody, which is able to sense forces on its arms and has a base that allows it to move gracefully, is being used as a dance partner for both experienced human dancers and patients in physical therapy. "This is a way that robots can be used for fun, interactive exercise in restoration," Dr. Kemp said. "We can also use it as a tool to understand whole body physical interaction between people and robots."J．At Carnegie Mellon University, Manuela M. Veloso, a professor of computer science, has developed a series of mobile robots she calls CoBots to perform tasks like delivering marl, guiding visitors to appointments and fetching coffee. She calls it "symbiotic autonomy," since the robots also rely on humans. For example, because they don’t have arms, they can’t operate elevators, so they have been programmed to wait and ask for human assistance. If they get lost, they stop, call up a map of the building on their computer screens, interrupt a passing human and say, "I am lost, can you tell me where I am The robotics community calls the idea cheating," Dr. Veloso said, "but it’s not. It’s the secret to real autonomy."K．To function in the real world and to be safe, robots must have a radically different design from factory robots, which are based on "stiff" actuators (制动器) capable of moving with great speed to a precise position. The new robots have "obedient actuators," which respond to external forces by yielding in a natural fashion. The original research into this area of what is now known as "soft robotics" began in the mid-1990s at the Massachusetts Institute of Technology, with work by Gill Pratt, who was exploring walking robots, and Matthew Williamson, then a graduate student and now director of technology development at Rethink Robotics.L．The research was not initially focused on solving the problem of human interaction, but the scientists soon realized the implications, recalled Dr. Pratt, who is now the project manager for the Defense Advanced Research Projects Agency’s Robotics Challenge, an upcoming contest that is intended to advance robotics technology to be used in natural disasters and other emergencies. "It actually started with numerically controlled machine tools," he said—using computer-controlled robots to perform milling tasks.M．For those manufacturing uses, what mattered was the precise positioning of the robot limb. However, Dr. Pratt was focused on developing walking robots that could move in the natural world, and force was more significant than precision to meet that challenge: "There the position of the limb didn’t matter so much, but what mattered was how hard was the robot pressing on the world, and how hard the world was pressing back on the robot," he said.N．The solution was to put something elastic, like a spring, between the motor and the joint. These are now described as series elastic actuators, and the technique of installing them is now widely used as a low-cost solution for robots that are both nonthreatening to humans and able to move more quickly and lightly in the natural world.O．Dr. Pratt recalled an incident when the researchers first realized that series elastic actuation was the key to freeing robots. While working on an early humanoid robot named COG, in a project led by Rodney Brooks, the founder of Rethink Robotics who was then director of the M.I.T. artificial intelligence lab, they were demonstrating how the robot could do tasks like writing with a pencil and paper. However, there was a bug in the software, causing the robot’s arm to repeatedly bang the table. Dr. Brooks decided it was an opportunity to demonstrate the safety of the technology. He placed himself between the table and the arm, which began hitting him.P．Roboticists describe this original approach as "passive obedience." Today there are other approaches—"active obedience"—that use software and sensors to blend speed and precision of more rigid robots with operations that are safe around humans. For a robots to move in the natural world, force counts more than precision.（）
Renewable EnergyA．In the past century, it has been seen that the consumption of non-renewable sources of energy has caused more environmental damage than any other human activity. Electricity generated from fossil fuels such as coal and crude oil has led to high concentrations of harmful gases in the atmosphere. This has in turn led to many problems being faced today such as ozone depletion and global warming. Vehicular pollution has also been a major problem.B．Therefore, alternative sources of energy have become very important and relevant to today’s world. These sources, such as the sun and wind, can never be exhausted and therefore are called renewable. They cause less emission and are available locally. Their use can, to a large extent, reduce chemical, radioactive, and thermal pollution. They stand out as a viable source of clean and limitless energy. These are also known as non-conventional sources of energy. The following section outlines several different types of renewable energy.C．Solar energy is the most readily available source of energy. It does not belong to anybody and is, therefore, free. It is also the most important of the non-conventional sources of energy because it is non-polluting and, therefore, helps in lessening the greenhouse effect.D．Solar energy has been used since prehistoric times, but in a most primitive manner. Before 1970, some research and development was carried out in a few countries to exploit solar energy more efficiently, but most of this work remained mainly academic. After the dramatic rise in oil prices in the 1970s, several countries began to formulate extensive research and development programmes to exploit solar energy.E．When we hang out our clothes to dry in the sun, we use the energy of the sun. In the same way, solar panels absorb the energy of the sun to provide heat for cooking and for heating water. Such systems are available in the market and are being used in homes and factories. Solar energy can also be used to meet our electricity requirements. Through Solar Photovoltaic (SPV) cells, solar radiation gets converted into DC electricity directly. This electricity can either be used as it is or can be stored in the battery. This stored electrical energy then can be used at night.F．Another type of renewable energy is hydro power. It is one of the best, cheapest, and cleanest sources of energy, although, with big dams, there are many environmental and social problems. Small dams are, however, free from these problems. This is in fact one of the earliest known renewable energy sources, in the country (since the beginning of the 20th century). In fact, for the last few hundred years, people living in the hills of the Himalayas have been using water mills, or chakki, to grind wheat. Besides being free from the problem of pollution, small hydropower plants are also free from issues and controversies that are associated with the bigger projects, namely affecting the lives of thousands of people living along the banks of the rivers, destruction of large areas under forest, and seismological threats.G．New environmental laws affected by the danger of global warming have made energy from small hydropower plants more relevant. These small hydropower plants can serve the energy needs of remote rural areas independently. The real challenge in a remote area lies in successful marketing of the energy and recovering the dues. Local industries should be encouraged to use this electricity for sustainable development. It is a technology with enormous potential, which could exploit the water resources to supply energy to remote rural areas with little access to conventional energy sources. It also eliminates most of the negative environmental effects associated with large hydro projects.H．The sea could also provide immense power—that is ocean thermal, tidal and wave energy. On an average, the 60 million square kilometre of the tropical seas absorb solar radiation equivalent to the heat content of 245 billion barrels of oil. Scientists feel that if this energy can be tapped a large source of energy will be available to the tropical countries and to other countries as well. The process of harnessing this energy is called OTEC (ocean thermal energy conversion). It uses the temperature differences between the surface of the ocean and the depths of about 1000m to operate a heat engine, which produces electric power.I．Energy is also obtained from waves and tides. In some countries such as Japan small scale power generators run by energy from waves of the ocean, have been used as power sources for channel marking buoys.J．Another renewable energy resource is biomass which is derived from the carbonaceous waste of various human and natural activities. It is derived from numerous sources, including the by-products from the timber industry, agricultural crops, raw material from the forest, major Darts of household waste and wood.K．Biomass does not add carbon dioxide to the atmosphere as it absorbs the same amount of carbon in growing as it releases when consumed as a fuel. Its advantage is that it can be used to generate electricity with the same equipment of power plants that are now burning fossil fuels. Biomass is an important source of energy and the most important fuel worldwide after coal, oil and natural gas.L．Traditional use of biomass is more than its use in modern application. In the developed world biomass is again becoming important for applications such as combined heat and power generation. In addition, biomass energy is gaining significance as a source of clean heat for domestic heating and community heating applications. In fact in countries like Finland, USA and Sweden the per capita biomass energy used is higher than it is in India, China or in Asia.M．The category of renewable energy should also include geothermal energy. We live between two great sources of energy, the hot rocks beneath the surface of the earth and the sun in the sky. Our ancestors knew the value of geothermal energy; they bathed and cooked in hot springs. Today we have recognized that this resource has potential for much broader application.N．The core of the earth is very hot and it is possible to make use of this geothermal energy (in Greek it means heat from the earth). These are areas where there are volcanoes, hot springs, and geysers, and methane under the water in the oceans and seas. In some countries, such as in the USA water is pumped from underground hot water deposits and used to heat people’s houses.O．The utilization of geothermal energy for the production of electricity dates back to the early part of the twentieth century. For 50 years the generation of electricity from geothermal energy was confined to Italy and interest in this technology was slow to spread elsewhere. In 1943 the use of geothermal hot water was pioneered in Iceland.P．The last type of energy involved here is co-generation. Co-generation is the concept of producing two forms of energy from one fuel. One of the forms of energy must always be heat and the other may be electricity or mechanical energy. In a conventional power plant, fuel is burnt in a boiler to generate high-pressure steam. This steam is used to drive a turbine, which in turn drives an alternator through a steam turbine to produce electric power. The exhaust steam is generally condensed to water which goes back to the boiler.Q．As the low-pressure steam has a large quantum of heat which is lost in the process of condensing, the efficiency of conventional power plants is only around 35%. In a cogeneration plant, very high efficiency levels, in the range of 75%-90%, can be reached. This is so, because the low-pressure exhaust steam coming out of the turbine is not condensed, but used for heating purposes in factories or houses. Since co-generation can meet both power and heat needs, it has other advantages as well in the form of significant cost savings for the plant and reduction in emissions of pollutants due to reduced fuel consumption. Ocean thermal energy conversion can generate significant amounts of power from temperature differences of the ocean.（）
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