Mini-Hydroelectric (AC-Direct) Engineering Design Analysis
At this point you have determined from a preliminary resource evaluation that the stream or river that you are considering has enough hydroelectric potential to warrant consideration of a mini-hydroelectric power generation facility to supply either a residence, a small community, or to generate power to sell back to a regional power grid. The very smallest AC-Direct systems (practically speaking) are 1.5 kilowatt, but this amount of power generation is inadequate as a sole source power supply, and so with a system this small, micro-hydroelectric is usually more appropriate for a residential power supply. Typically, self-standing mini-hydroelectric power generation is not worthwhile at a power generation potential of less than 10 kilowatts. However, if you are selling power back to a local power grid, the economics become favorable for much smaller systems.
2) Determine Optimal Turbine Type. There are a variety of turbine types that vary in efficiency as a function of head and flow rate. The graph shown below is a manner of broadly determining which turbine is most appropriate for different combinations of head and flow. Use the information gathered during your hydrologic evaluation to see which turbine type is most appropriate for your application. In most instances, in accordance with a series of first principle physics precepts codified as Murphy's Law, the hydrologic circumstances are likely to fall in a borderline between two or more different types of turbines, and if that is the case, then the ensuing engineering analysis should be undertaken with each turbine type to exhaust all possibilities and not exclude what could emerge as the most appropriate technology for your particular application. Broadly speaking, mini-hydroelectric applications include high-flow, low-head applications, for which the Pelton turbine is typically the most appropriate, and high-flow, low-head applications, for which the XXXX turbine is typically the most appropriate.
2) Pipeline Sizing and Head Loss Determinations. Having determined the best turbine for the conditions present, next you will want to determine the pipeline size and material that will work best for your application. Possible pipeline materials include a variety of plastics and a variety of steel. The diameter of the pipeline chosen will be determined by the flow rate to be used and by the friction losses experienced, which vary as a function of flow rate and pipe characteristics. The material used for the pipeline may vary as a function of a variety of environmental factors, but typically the controlling variable is cost of the pipe and installation. Once you have settled on a pipeline diameter and material, use the head loss charts provided by the manufacturer to determine the head loss for the pipe. You can also use the empirical Hazen-Williams head loss equation, applying the roughness coefficient that is specific to each type of pipe that can be supplied by the pipe manufacturer or distributor. Once you have determined the total pipeline head loss, then subtract this from the vertical drop between the point of water capture and the point of power generation and calculate anew the hydroelectric potential based on the design flow rate settled upon. If the hydro potential has dropped to beneath your target, then increase the pipeline diameter to the next size and repeat step 2.
3) Determine Generator/Governor/Turbine. Having settled on the best turbine type and having settled on pipeline characteristics, the next step is to source equipment. You will need to consult with manufacturers/distributors for the turbine that you have determined will work best. Some companies offer a factory assembled combination of turbine, generator, governor, and control panel, and in other cases, you may elect to fit these together and balance them yourself. You will need the advice of the turbine manufacturer to determine options that will work best with the equipment selected. It is always easiest to defer to a manufacturer that does puts the three together in one skid mounted system. Canyon Industries offers custom mini-hydroelectric plants that are tailored to your combination of actual head (factoring in head losses) and design flow rate. Their units start at around $15,000, whereas the actual hardware can be cobbled from separate sources for less than half that amount for a comparably sized unit. Putting the three things together, however, is not like assembling a model airplane as there are no instructions. Do not try to do this on your own unless you are handy and have a considerable understanding of mechanics, gears, power assemblies, and that sort of thing.
4) Design Water Intake. Rivers are systems of incredible raw power. Enormous volumes of sediment bed load are transported during storms. A water intake system must be capable of withstanding the force of storms and must simultaneously be resistant to the clogging that can occur during the low-flow transport and deposition of silt and sand. In many cases, a small reservoir is likely to be the most practical solution for balancing the opposing forces of high and low flow circumstances. However, permitting restrictions may not allow for impoundment of streams, and water works of this nature can become financial black holes. The water intake is arguably the most important part of any hydroelectric power system and is the least quantifiable. Optimal water intake design will vary mostly as a function of stream width, depth, slope, sediment size, and depth to bedrock.
5) Design Generator Housing and Water Discharge Structure. The hydroelectric generator must be placed in a housing of some sort and this must be located near enough to the river to return the water that is diverted for hydroelectric generation back into the river but far enough away so as to be safe from periodic flooding.
6) Transmission Design. Power transmission can be achieved with above ground aluminum cable or can be undertaken with buried copper cable. Either way, the distance and cable size must be designed as a function of the output voltage and the distance. In many cases it is necessary to use combinations of step-up/step-down power transformers to transmit the power in a high voltage that has much lower line losses than transmission of low voltage power. The design process is often iterative with different sizes of cable and transformers to achieve the combination that is most economically practical. The last time I checked world spot prices, copper was 20 times more expensive than aluminum, so expect dramatic differences in cable between the two options for power transmission.
7) Grid-Tie Electronics and Metering. Consult with your local power company for restrictions and criteria related to selling power back to a local grid.
8) Economics. Steps 1-7 will ideally result in a procurement/installation budget. Based on the costs involved, a wide variety of economic analysis models can be used to determine how long it will take for the capital costs to be recovered in savings from power consumption. Beyond the materials and installation costs, you will need to know the current price that you pay per kilowatt-hour from your local power company as well as the price that they will pay you for any power that you sell back to them in the case of grid-tie systems. The result of this analysis is the number of years required to make your investment pay off. Discounting relativistic factors like the personal importance to you, personally, of reducing your carbon footprint, or the gee-whiz value of cool things, a rational economic determination will hinge around the return on investment that you expect from alternate investment models to decide whether it is a good idea or not to proceed with the installation.
9) Getting It Done. Step 9 is where you learn that you underestimated the costs of installation but at the end of the process feel much better about yourself.
10) Showing it Off. This is the fun part and may make your underestimation of the installation costs seem amusing after the fact.
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