Hydro Power. Gurpal Sagoo

Transcription

1 Hydro Power At the current rate of energy consumption the World's fossil fuel supply is set to diminish by 2112 [1] unless either alternate energy sources are found or techniques are found to reduce our dependence on fossil fuels. Wind and solar energies are good source to lessen our usage of fossil fuels but not enough energy is generated from these renewable sources; in addition to being unsightly. Water covers 71% of the planet and carries with it phenomenal amounts of potential energy. Hydroelectric power in 2005 provided the world with 19% of its electricity [2] and could be an alternative solution to slow down a growing problem of energy supply. In 1829 Michael Faraday discovered that rotating a magnet in a coil of wire caused charge to flow through the coil, in effect creating a "free" energy resource. He also found that the rate of change of magnetic flux and the strength of the magnet was directly proportional to the induced current. All modern power stations rely on electromagnetic induction to produce electricity by implementing Faraday's observations on a large scale. Although their designs may differ, the principles remain the same, pass water through a turbine, turning a generator and producing electricity. Hydroelectric power stations traditionally use a reservoir of water created by a dam, allowing for a consistent high pressure and flow rate to pass through a turbine. Figure 1 shows a typical hydroelectric dam powerstation. An intake gate controls the flow of water to the turbine while the penstock channel directs the water so that it lands correctly orientated on the blades of the turbine, turning a generator. The turbine is placed at the bottom of the reservoir and through energy conservation, the potential energy of the water, is converted into Figure 1: Schematic of typical hydroelectric power station. [3] kinetic of the generator. The turbines' design changes depending on how great this energy potential is, also known as the head of water. There are two main classifications for turbines; impulsive and reaction. Figure 2 shows a typical impulse turbine. A jet of water hits the bucket shaped blades and under the principle of Newton's Third Law 1 the turbine rotates. Since this type of turbine classically relies on water pressure alone, a high water head is required to turn an impulse turbine efficiently. Turbines Figure 2 also shows a typical reaction turbine. It works differently to an impulse turbine in that it relies only on the flow rate of water. As water is pumped along the blades they spin. In doing so the water pressure pushes the blades around as water falls underneath it. Reaction turbines are more favourable because of their versatility, efficiency, and the requirement of a low head if a dam or reservoir is being used; usually being less than 30m. Figure 2: Diagram comparing impulse and reaction turbines[ 4] 1 1

2 There are many individual designs associated with each of the two types of turbines. Reaction turbines are the most popular, the specific design being the Francis. Figure 3 shows a Francis turbine. Again this needs to be submerged in water and relies on water flow rate, however as the turbine spins faster, negative pressure causes more water to be sucked into the blades. This is due to the fact that water is merely flowing with the blades and as the blades spin the back tail causes water to be pulled into the exit chamber. Most hydroelectric power stations have a dam associated with them. The primary reason for having a dam is flood defence whole turbines control the rate of flow of water and at the same time generating electricity. Dams create large reservoirs that cause devastating environmental consequences to provide a high flow rate and huge amounts of power. To maintain water flow large. In 1967 a 6.3 magnitude earthquake hit Koynanagar, India killing 180 whilst injuring It was believed that the cause of this earthquake was due to the flooding in Maharashtra where the Koyna Dam is. The consequence of flooding lead to induced seismicity eventually leading to a major earthquake. Figure 3: Schematic of Francis turbine with water flow direction [5] Ecosystems and the environment as a whole can be severely disrupted to creation of huge reservoirs; an obvious one being that people need to be relocated if their current home lies in the flooding region of the reservoir. Reservoirs of water tend to have larger surface areas compared to the original river. This causes more water to evaporate to the atmosphere and can have a number of implications. One consequence is a slight increase in global warming. The change in water levels during summer and winter months contributes a great deal in carbon dioxide and methane emissions of a hydroelectric power station. Water in the atmosphere is well known to be the greatest contributor to the greenhouse effect due to its high absorption band at approximately 6.3 microns [6] so increasing the amount of water in the atmosphere implies that more heat is retained. It was, and to some extend still is, believed that hydroelectric power from dams is a source of clean, free, renewable energy. A recent study has shown that this may not be the case. Over dry spring and summer months water levels tend to drop in hydroelectric reservoirs, enabling plant life to grow on the banks on what is considered to be the original water level. Plant life then flourishes over the summer months only to be drowned again when water levels rise again. On doing so the drowning of plant life has a ripple effect; large scale decomposition occurs releasing huge amounts of carbon dioxide and some cases methane into the atmosphere, which is has 4 times the greenhouse effect compared to carbon dioxide. This process potentially happens yearly and essentially offsets the free energy received originally. Hydroelectric power stations are not all bad though. One of the huge benefits of dam power stations is scalability. The UK Environmental Agency recently proposed over 1000 sites for mini dams to provide hydroelectric power capable of powering over 850,000 homes [7][8], which uses the same technology as the Itaipu Dam on the Parana River in Brazil, proving 93% and 23% of Paraguay s and Brazil s total electricity respectively. A large reservoir has many social and economical benefits and steps are being taken to help wildlife. Having a large resource of water means that water can be supplied to remote areas that once couldn t be supplied with fresh drinking water. Serving as a flood control mechanism the dam can 2

3 control the flow of water to the river after passing through the turbines. If water levels rise in the reservoir more turbines can be switched on, increasing the flow of water to prevent over flow or vice versa. The Three Gorges Dam The Yangtze River is not only the most dangerous river in the world but the third longest. Floods have periled China for centuries due to the unpredictable nature of the river. Building a dam on the Yangtze for the purpose of power generation was initially proposed by San Yatsen, back in 1919 however his plans where quickly put down due to political and economical conditions at the time. However crippling floods in 1954 killed around 33,000 people along the Yangzte [9], reviving Yatsen s original idea. The Dam s project was approved in 1992 and construction started in years and 37 billion US Dollars later the Three Gorges Dam is the world s largest hydroelectric power station stretching 2km long and almost 200m high. With 32 Francis turbines a total generating capacity of 26,700MW is achieved easily, enough power to meet 17% of China s growing energy demand. Although the power station itself has been completed, the project is still ongoing with a complex ship lift projected to be completed in The Three Gorges Dam had much controversy surrounding it. Creating a lake half the size of London while flooding a river almost the length of Britain, displaced 1.3 million people sending many into poverty. In addition with large amounts of water, valleys became saturated causing landslides, where in China as many as 10 have caused casualties during the construction of the Three Gorges Dam. The Three Gorges Dam had controversy surrounding it, but in the eyes of China it is considered to be a monumental success and jump started their current position in society as the manufacturing capital of the world. Although their demand for energy is increasing, the Three Gorges Dam provides large amounts of free energy to China, decreasing their need to burn fossil fuels and spend money on importing other fuel. This is a prime example on how a country can become a major player in world economics thanks to a free large scale energy source. Tidal Power Of course hydroelectric power doesn t necessarily mean power from a dam based reservoir. Tidal power comes under the same genre where the Moon s and Sun s gravity pull the world s oceans creating a so called high and low tide twice a day. Figure 4 shows how the ocean s tide changes due to the force from the Moon and Figure 4: Diagram if how Moon affects Earth s tides [10] not the Sun. Since the Sun is a considerable distance away from Earth than the Moon, the Moon s gravitational pull is almost twice as strong as that compared to the Sun. The gravitational force from the moon causes the water to bulge towards it, creating a so called high tide. It can also be seen there is a bulge on the other side of the planet. This is caused from the Earth orbiting the Sun where the water is trying to escape the planet due to the rotational motion of the Earth s orbit. However Earth s own gravity keeps the water from escaping in doing so creates a high tide. It can also be seen that there are two low bulges, where the oceans experience a low tide. This is due to the water displacement where water 3

4 has moved to the high bulges. The process of high and low tides happens twice a day throughout the world. Figure 5 shows a typical tidal barrage cross section where in principle it is simply a mini hydroelectric dam power station. Turbines are placed in a wall blocking the path of the natural ocean tide. As the high tide occurs the rate at which the water moves decreases as it passes through channels in the barrage turning turbines in turn generating electricity; usually being built on big river estuaries. In doing so only specific estuaries can be used making tidal power a geographical dependant energy source as for maximum power generation water movement is to resonate with the modes of the ocean which occurs when the ratio of estuary length Figure 5: Diagram of a typical tidal barrage [11] squared and its depth is proportional to the period squared, in this case being half a day. A comparison to a dam power station is that a tidal power station works with a small head, typically 5m where the tides movement increases the potential of water, much like a reservoir for a dam. In the UK the Severn Estuary is a prime location for a tidal and like the Three Gorges dam it is considered to be the most controversial project to be rationalised in the UK. With tidal movement of 14m, the third largest in the world [12] the Severn Estuary could provide the UK with 6% of its total electrical power. Initial plans in 1987 were to build a barrage stretching across the 1 mile wide estuary, generating on average around 2,000MW all year, peaking at approximately 8,670MW when flow is at maximum [13] Many wildlife conversationalists however quickly dismissed the plans claiming they would destroy the habitats of many specialist species in the planes of the estuary. Many plans since have been submitted for consideration that cater to all, one of particular interest being a tidal lagoon. The principle is similar but instead of using a the entire width of the Severn estuary, a portion is used for power and using these in series along the river would not only generate similar amounts of power, the rivers flow of water would remain unchanged; in theory. However with current budget constraints and residents opposing the unsightly nature of a lagoon it is unlikely that a tidal power station will be built in the Severn estuary. Waves Apart from tidal power and hydroelectric dam power, the motion of waves can be harnessed to produce power from their movement. Inspired by a snake long chained tubes are placed in the ocean. Figure 6 shows the next generation Pelamis wave generator, prototyped in As ocean waves move the cause portions of the generator to move along with the waves. At high waves hydraulic pistons contract naturally do its position and in doing so generating electricity. Throughout the Pelamis generator a series of these hydraulic pistons are placed above and below the structure enabling electricity to be generated not only in both directions of movement but so that twice as much can be generated. Each Pelamis generator can produce 750kW of electricity but being small and compact, dozens of these can produce enough power for a small percentage of the UK s energy requirements. However since this generator has a lot of moving parts, it is prone to failures. 4

5 In September 2008 Pelamis implemented their generator in Portugal generating 2.25MW powering nearly 1,500 homes locally. However just after two months multiple generators sprang leaks which eventually leading to Pelamis to pull their generators from the main grid. Two years later and this ground breaking technology has yet to be successfully installed and maintained on a large scale; however there are plans to install these generators on the north coast, Scotland where seas are turbulent, ideal conditions for the Pelamis generator. Figure 6: Diagram showing overview of the Pelamis generator [14] The problem with all the methods listed above is that they are very unsightly. Figure 7 shows turbines placed underwater, much like upside wind turbines. The problem with these turbines is that they only produce a fraction of the power as other hydropower methods. Subject to the Betz limit 1, the number of these water turbines in a given space is significantly reduced and such they must cover vast areas to have a chance of producing as much power as the other methods listed above. In addition to the Betz limit the speed at which surface water moves is relatively slow compared to deep river currents, and since there is no current infrastructure to install and maintain these turbines safely deep under water these turbines are considered to be experimental with no real world instalments currently. The major benefit from this Figure 7: Artists impression of underwater turbine [15] sand can flow through the river or even sea without a build up of waste. method of power production is that they are hidden and unsightly, in addition to this they do not disrupt tidal flow meaning that marine wildlife and sedimentary This article has put forward many ways in which water can be utilised to produce power which will significantly reduce our alliance with fossil fuels. For the future of this planet to be long lived alternative solutions must be found now. Hydro electric power is the only real alternate energy source that is free, clean and most importantly renewable. Although high levels of investment are 5 1

6 needed in order to install the required infrastructure, this investment should be considered a necessity when considering the future of our planet. References 1. "Energy Policy 2009" Volume 37 Issue 1: Page Renewables Global Status Report 2006 REN21, Environmental Physics Lecture Notes 2010 Peter Doel. 7. Opportunity and environmental sensitivity mapping for hydropower in England and Wales Environmental Agency, UK agency hydropowerschemes The Severn Barrage Project: General Report Paper 57. HMSO Ocean Power Delivery Ltd