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December 9, 2020
Micro Hydroelectric Power Generation Summary
A major industrial water user in New Mexico discharges approximately 3.8 million gal of wastewater per day. The topology of the site provides an elevation difference of about 150 ft between the plant site and the entrance to the municipal sewage line; this flow and elevation difference is sufficient to produce about 40 kW of electrical power using a water turbine/electrical generator set to extract power from the flowing stream.This report includes designs and economic analyses for two distinct cases. One case is based on the written premises of the task; whereas, a second case is based on a real surrogate site, which is Intel’s Rio Rancho (near Albuquerque, NM) plant, which does discharge about 3.8 million gal per day and has about 120 ft of head available for power generation. After analyzing several turbine technologies, the Pelton wheel turbine was determined to be the most economical means for generating commercial electrical power.
Pelton Wheel turbines operate most efficiently with a constant head and flow. Because the wastewater discharge for the task varies from 0.5 – 4 MM gal/day, an integrated study of the flow fluctuations determined that a surge tank of 27,000 gal was required to maintain a steady flow as input to the turbine. The task premises did not include any existing storage for the discharge stream; consequently, a 27,000 gal surge tank was provided for the task premises site. The surrogate site has a surge basin with a surface area of 17,000 ft2 . This surface area requires only a 3 in level change to accommodate 27,000 gal of surge; consequently, no surge tank was included in the surrogate site case.The surge provides the turbine with a steady flow of 2,400 gpm and a constant head of 120 ft. The purchased turbine system selected by CREW has an overall (mechanical + electrical) efficiency of 68%. For the task premises scenario, 40 kW is produced, and for the surrogate site scenario, 30 kW is produced.
The WERC task premises case is most economical with an IROR of 4.3%. This return is marginal for earnings projects under normal circumstances. However, interest rates are now at historically lower levels, and are projected to remain low for several years. The surrogate location IROR is about 2.0%, which is considered as a reasonable return for a minimal risk project with today’s economic environment. This energy recovery initiative is a “Green” project, which inherently lowers the acceptable IROR for environmentally conscious industries.
This project will require about 12 months to complete once funds are available. As energy costs continue to rise, the CO2 level in the atmosphere continues to increase, and the World’s fossil fuels are depleted, reliable new sources of energy will be needed. Hydroelectric power generation is a clean, effective means of generating “green” renewable energy that will continue to be a viable supplement to energy demands long into the future. Any environmentally friendly hydroelectric possibility must be exploited to the maximum. Task 6 addresses the use of hydroelectric power in the most environmentally friendly manner by producing electricity utilizing a high efficiency Pelton Wheel turbine and generator. In 1870, Lester Allan Pelton1 revolutionized hydroelectric power with the invention of the Pelton Wheel, a high efficiency turbine that converts momentum of a water jet stream to mechanical power and, through an electrical generator, electricity. Pelton Wheels operate by
passing a working fluid through a nozzle, which converts pressure energy to kinetic energy. The kinetic energy of the fluid is then converted to mechanical work by impingement of the fluid jet upon the buckets of the Pelton Wheel. The Pelton Wheel drives a rotating shaft, which is connected to the drive shaft of an electric generator. The speed of the Pelton Wheel, at optimum efficiency, operates at a peripheral bucket velocity of ½ the nozzle velocity1, 12; at this optimum condition, the fluid leaves the bucket with minimal velocity.
DESIGN CONSIDERATIONS FOR TASK 6
The design considerations are to:
1. Design a flexible, scalable system using appropriate sponsor input.
2. Address the efficiencies of the hydraulic turbine and the electrical generator.
3. Generate at least 5–15 kW (20–40 is more reasonable) of electric power.
4. Designs were requested for 10–200 ft of head and ½–4 MM gal/day of hydraulic load; however, with adequate surge, head and flow are constant at 150 ft and 3.8 MM gal/day.University of Arkansas
5. Include an economic analysis which provides proof that the project is economical.
a. The task sponsors specified a 5 year project life. However, to receive full benefits of government subsidies, the project life must be 12 years; thus the assumed project life is 12 years.
6. One design consideration for the project was “Ability to handle solid waste”; this was interpreted to mean ‘handling dissolved solids and readily suspendible particulates.’
7. Provide a time-line, from construction to full operation, for the proposed project.
8. Discuss the risks, safety and legal, associated with the design and implementation of the project.
HYDROELECTRIC POWER GENERATION
After surveying the literature and consulting with experts in the field of hydroelectric power generation, a wide variety of turbine/generator combinations were identified that could possibly accommodate the conditions required for this design.
Micro-hydroelectric turbine technologies, for the purposes of this report, refer to any turbine/generator system producing less than 100 kW. Technologies considered for implementation included: Gorlov helical turbines, gravitational water vortex turbines, Francis Kaplan turbines, and Pelton Wheel turbines. Gorlov turbines are helical bladed turbines that are primarily used in large volume, low head situations, such as a river where a dam is not a viable option. The Gorlov turbine is typically
used with large free flowing water sources. Gorlov turbines were rejected for this approach primarily because of the low efficiency (≈35%) which is well below the effectiveness of other micro hydroelectric power generation methods.2 In addition, the
geometry of Gorlov turbines does not fit the inlet and outlet pipe geometry. Gravitational water vortex turbines are a micro hydroelectric technology used at low heads (2.5-10 ft). They create a swirling vortex that is used to drive an impeller. They were rejected primarily because of their inability to effectively handle the high heads (115-150 ft) and inlet and outlet piping particular to this Francis—Kaplan turbines are commonly used in hydroelectric power generation. “Reaction turbines run fully
immersed in water, and are typically used in low-head (pressure) systems with high flow”.5 As the fluid passes through the turbine, the fluid transfers energy to the turbine blades, creating angular momentum that rotates a central shaft and generates electricity.
Francis—Kaplan turbines are highly efficient (up to 90%), can be used at high and low heads, 30–2,100 ft, and are capable of handling high flow rates. These characteristics make the Francis—Kaplan turbines an excellent choice for hydroelectric power generation. Pelton Wheel turbines are impulse turbines that “operate in air, driven by one or more high-velocity jets of water. Impulse turbines are typically used with high-head systems and use nozzles to produce the high-velocity jets”.1 The
momentum of the fluid is then captured and converted to power by a series of precisely designed buckets connected to a rotating shaft. Pelton Wheel turbines are second to the Francis Kaplan turbines in efficiency (80-90%) and are ideal for systems with low flow rates and high heads. After consulting with experts in the field of hydroelectricity, the Pelton Wheel was chosen as the preferred technology. Although the Francis—Kaplan turbine is an efficient solution that meets the demands of the project, Francis turbines are more typically used in large scale operations, such as dams. The relatively small size of the turbine for this project (40 kW) makes the Pelton Wheel the most efficient and economically viable solution for the project.