Alternative Energy Solutions: Hydro-Electric and Tidal Energy
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Alternative Energy Solutions: Hydro-Electric and Tidal Energy American University Professor Stephen Macavoy Environmental Studies University Honors Capstone Neslihan Yildirim Fall 2012 1 Neslihan Yildirim University Honors in Environmental Studies Capstone Advisor: Professor Stephen MacAvoy and Albert Cheh CAS: Environmental Studies Alternative Energy Solutions In Order To Address the Energy Demands of the World: Hydro-Electric and Tidal Energy Over the last two centuries, industrial activities, deforestation and the burning of fossil fuels have released high concentrations of heat-trapping agents called greenhouse gases (GHGs) into the atmosphere. While a certain amount of greenhouse gas is important to keep our climate warm and livable, these higher concentrations are warming the Earth’s surface to temperatures that threaten life on our planet. Carbon dioxide (CO2) and methane are two GHGs that have increased dramatically due to human activity. With the challenges faced by global warming, the world is faced with the threat of energy demand. The purpose of this capstone is to encourage the use of renewable energy resources in order to best meet those challenges by providing detailed information on the scientific, economic and political backgrounds of two types of renewable energy resources: hydro-electric and tidal energy. The paper addresses several key topics including: How does the renewable energy function? Advantages/Disadvantages, Environmental Effects, Economic Feasibility, Current Electric Power Output/Capacity and Future Projections. Various mediums of research tools were used including energy reports published by the US Department of Energy (latest Annual Energy Outlook Reports) National Renewable Energy Laboratory, US Department of the Interior, and the Department of Energy and Climate Change. The research has indicated that tidal and hydroelectric energy have their advantages and disadvantages in terms of economic feasibility. Although both technologies can provide enough energy to sustain societies, they cannot address the energy demands of the world alone. A multifaceted approach needs to be taken to combine these energy resources with other renewables in order to sustain human lives on a global scale. Hydro-electric and wind energy should be used accordingly to best suit a country’s geographical location since each nation has its own unique natural resources to offer for total electricity output. 2 Introduction The amount of electricity a hydro-electric system can produce depends on the quantity of water passing through a turbine (the volume of water flow) and the height from which the water ‘falls’ (head). The greater the flow and the head, the more electricity produced (Castaldi, 2003). In order to harness the energy from flowing water, the water must first be controlled. Thus, a large reservoir is created, usually by damming a river to create an artificial lake or a reservoir. Water is then channelled through tunnels in the dam. The power of the water that flows through the dam’s tunnels causes the turbines to turn and the turbines in turn make the generators move. The generators are machines that produce the actual electricity. The transformer inside the actual powerhouse takes the AC and converts it to higher voltage current. The engineers control the water intake system. When there is a lot of energy that is needed, most of the tunnels to the turbines are open and millions of gallons of water flow through them. When there is less energy needed, then the engineers can slow down the intake system by closing some of the tunnels. There are also outflow pipelines which carry the used water to re-enter the downstream river for a second time use. Environmentally this is a great way of controlling how much energy is produced since water, essentially does not get wasted. Only the actual amount that is needed is used (US Department of the Interior). During floods, the intake system is helped by a spillway. A spillway is basically a structure that allows water to flow directly into the river or other body of water below the dam, bypassing all tunnels, turbines and generators. Spillways prevent the dam and the community that surrounds the dams from being damaged. The spillways look like long ramps and are generally empty and dry most of the time unless it is needed in case the system is flooded (Water Encyclopedia). The water in the reservoir is considered potential (stored) energy. When the gates open, the potential energy of water flowing through the penstock becomes kinetic energy because it's in motion. The amount of electricity that is generated is determined by several factors. Two of those factors are the volume of water flow and the amount of hydraulic head. The head refers to the distance between the water surface and the turbines. As the head and flow increase, so does the electricity generated. (Tribal Energy and Environmental Energy). 3 Hydro-electric power plants essentially capture the energy released by water falling through a vertical distance, and transform this energy into useful electricity. In general, falling water is channelled through a turbine which converts the water's energy into mechanical power. The rotation of the water turbines is transferred to a generator which produces electricity. An important calculation to consider when constructing and figuring out how much water flow is needed to achieve a certain kW of electric power per hydroelectric power plant is in the following equation/calculation: Power Equation: P = eHQg whereby P stands for the electric power output in KW, e stands for efficiency, H stands for head in meters (how far the water drop is), Q stands for design flow (m^3/s) and g stands for the gravitational constant, 9.81 m/s^2 (Castaldi, 2003). Types of Hydroelectric Plants As mentioned, Hydro-electric power plants can generally be divided into two categories. "High head" and “Low head” power plants. The “High head” are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in a consistent and thus reliable production of electricity, able to meet larger demands. Heads for this type of power plant may be greater than 1000 meters. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system. (RETScreen International) "Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam to channel water, or no dam and simply use the ‘run of the river.’ Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. 4 Hydro-electric facilities with a capacity of less than about 25 Mega-Watts (1 MW = 1,000,000 Watts) are generally referred to as "small hydro," although hydro-electric technology is basically the same regardless of generating capacity (RETScreen International). An equation that best describes the “high head” versus “low head” relationship is Darcy’s Law. Darcy’s law allows an estimate of the average time of travel from the head of the aquifer to a point located downstream. It provides an accurate description of the flow of groundwater in almost all hydro-geologic environments. Henry Darcy was a French engineer who studied the movement of water through sand in 1856 (Division of Water Resources). He found that the rate of water flow through a tube is proportional to the difference in the height of the water between the two ends of the tube, and inversely proportional to the length of the tube. He also discovered that flow was proportional to a coefficient, K, which is called hydraulic conductivity. Although Darcy's Law was based only on slowly moving groundwater in confined aquifers, most of the laws helped develop equations for other aquifer conditions we use today such as hydroelectric energy. The equation for Darcy's Law is Q = - KA[(hA - hB) / L] OR Q = - KA(dh/ dl) where: Q=volume of water flow in ft3/day K=hydraulic conductivity in ft/day A=cross-sectional area in ft2 dh/dl=hydraulic gradient (change in head, dh with distance, dl) The negative sign deals with the direction of flow that is toward the lower hydraulic head. The idea is that groundwater flows or moves from areas of higher hydraulic head to areas of lower hydraulic head as mentioned before (Pinder, 2006). It is based on Darcy’s work that we can estimate the velocity of water or how fast the water is moving between points (as in the case with hydroelectric turbines). Velocity is calculated by using hydraulic conductivity, porosity and hydraulic gradient. The equation used for calculating the velocity of water is as follows: 5 V= (K/n) (dh/dl) where N = porosity, K is a constant of proportionality (a way to relate the reduced flow rate to head loss and length of column) and dh/dl being the hydraulic gradient (Pinder, 2006) "Pumped Storage" is another form of hydroelectric power. Pumped storage facilities use excess electrical system capacity, generally available at night, to pump water from one reservoir to another reservoir at a higher elevation. During periods of peak electrical demand, water from the higher reservoir is released through turbines to the lower reservoir, and electricity is produced. Although pumped storage sites are not net producers of electricity (it actually takes more electricity to pump the water up than is recovered when it is released) they are a valuable addition to electricity supply systems.