OSA WATER WORKS

Micro-Hydroelectric Engineering Design Analysis

Micro-hydroelectric power generation is often the most appropriate scale hydroelectric technology for residences and small commercial applications.  It is particularly attractive for remote installations where there is no electrical grid to tie into or in cases in which bureaucracy or other issues make such a connection impractical.  The steps that continue presume that you have gone through the preliminary resource evaluation and determined that micro-hydroelectric is most appropriate to your particular circumstances. 

1)  Turbine Selection.  If your point of power generation is more than 500 feet from where you plan to use the power, then you will need to consider an induction turbine, which produces an output in 480-Volt or higher that can be transmitted to your battery bank without significant line losses.  If your turbine location is adjacent to the point of use, then a standard micro-hydroelectric turbine assembly with 110-Volt output that is changed in situ to DC power with a rectifier is likely the most appropriate alternative.  In the latter case, output can be put out to 12-, 24-, or 48-volt systems, and it will be necessary to select a turbine output voltage in accordance with the power inversion equipment used to make your micro-hydro output useable with conventional household electronics.  In the case of turbines separated from the point of use, the high voltage transmitted from the turbine must be transformed at the point of use to charge batteries, and the transformer used must be sized in accordance with the battery bank configuration and inversion voltage used.  In addition to these factors, each hydroelectric turbine manufacturers have capacity ratings that are relevant to design.  While the wattage ceiling for individual units vary as a function of the output voltage, more than one turbine can be deployed in parallel so that a cascading series of turbines increases overall power as a geometric function of the number of turbines used.  That said, for water flow rates so high that multiple turbines can be powered, it is arguably best to deploy a single mini-hydroelectric plant instead.  However, deployment of multiple micro-hydro turbines makes it possible to capitalize on flow rates that vary seasonally from small flow rates able to power only a single wheel during periods of low flow to substantially greater power supply during storm runoff or during wetter times of the year when more water is available.  There are two rating factors in turbine selection that directly affect the ensuing system design that must be carefully considered in advance for optimal results:

A)  The electrical output voltage
B)  The corresponding electrical output wattage.

Since power (watts) = voltage (volts) * current (amps), the turbine rating described above can also be described in terms of a single variable instead of two.  And many hydro people refer to the amperage of a system.  While this is interchangeable at the head of the design process, voltage and wattage become quintessential design variables in the next electrical step of the process, so turbine ratings are important to know and understand in advance.  No two turbines are quite alike, and to design with one in mind and install a different type may lead to unpredictable results.

2)  Example.  Consider the case of 100 feet of effective head and a useable flow rate that varies between 100 gpm in the dry season and 400 gpm in the wet season.   Application of the Power Formula reveals that this water source, presuming a turbine efficiency of 50%, has a wattage range of 900 watts - 3.6 kilowatts.  A Harris hydroelectric turbine has a ceiling of 2.5 kilowatts in 48-volts.  In 24-Volts it peaks at 1500 watts and in 12-Volts at 750 watts.  In our case, a 12-watt system would be an under-utilization of the resource.  Yet, an expensive 48-Volt system would not fully capitalize on winter resources.  However, at 24-Volts, the turbine would achieve full dry-season utilization with 40% capacity to spare.  And two such units in parallel would achieve full wet season utilization with 30% capacity to spare.  Beyond this simplistic example, variable output can be achieved from variation in the number of nozzles (1, 2, and 4) as well.  The point is that turbine selection must be made very carefully on the basis of the water flow and head available, and that this design linkage has a cascading relevance to all subsequent parts of the analysis.

3)  Water Intake.

4)  Generator Housing.

5)  Pipeline Design.  Having determined the intake and generation points and the range of water flow rates to be used, the pipeline must be designed accordingly.  For micro-hydroelectric applications PVC is the gold standard in remote locations.  High density polyethylene (HDPE) is an option that may work best in some industrialized settings.  Rarely does galvanized iron compete economically for a role in micro-hydroelectric pipelines, but the appropriate piping material will vary as a function mostly of economics and specific performance requirements.  Once the material is determined, the diameter must be determined.  The design flow rate should pass through the pipe without causing excessive pipe losses.  Since pipe losses directly reduce power consumption, the appropriate pipe diameter for a given flow will be greater for longer transmission distances.  Once head losses are determined for a preliminary pipeline diameter, the Power Equation must be re-worked with the newly calculated Head.  If the resulting power generation potential is not adequate for the power demands calculated, then select the next largest diameter pipe and repeat the design step until settling on the pipeline diameter that is optimal for the design flow rate and the pipeline distance.

6)  Design Inverter Capacity.  The value in Column H of the spreadsheet under the line heading Total Daily Consumption provides an estimation of the amount of power required for all the appliances listed in Column H to be operated simultaneously.  This will require a bit of an iterative process.  While a conservative design would be to assume at peak capacity everything is on at the same time, this may amount to an instantaneous power demand of several kilowatts, and the larger the inverter the more expensive.  For modestly anointed homes (no AC, gas stove, hybrid clothes dryer, gas hot water heater, and DC-refrigeration), an inverter greater than 3.5 kilowatts may be somewhat excessive.  However, please note that a 3.5 kilowatt inverter may not even carry a fully electric clothes dryer alone, hence the need to plan for the use of energy efficient or hybrid appliances.  Also, stoves, hot water heaters, and clothes dryers are typically 240 volt, which requires two inverters, essentially doubling the cost.  Besides the wattage of the inverter there is one other design decision that will also bare upon the cost of the battery bank and will affect system performance.  A decision must be made as to the input voltage of the inverter, 12-, 24-, or 48-volts.  A design analysis of this decision is beyond the scope of this discussion, but in general terms, smaller systems can be done in 12-volt.   High yield systems should be planned in 48-volt, and for most mid-range applications, 24-volt is the most reasonable input voltage.

7)  Design Battery Capacity.  The battery bank allows the use of more power at one time than the turbine is generating.  However, unlike in solar power systems, the battery bank does not have to satisfy a power draw when no charging source is present.  Solar systems have no input at night.  Hydro systems have input 24 7, so the battery bank acts more as a capacitor and less as an actual power source than in solar or wind systems, where three days of independent battery storage capacity is typically recommended for a robust design.  For a hydroelectric power supply the battery bank can be sized by applying a rule-of-thumb 200 amp-hours in battery capacity for every kilowatt of inverter rating.  For the 3.5 kilowatt inverter mentioned above, this would correspond to a battery bank of 700 amp-hour capacity.  This would be achieved by a deployment of 8 425 amp-hour L-16 6-volt batteries in two parallel series of four batteries each.  Input inverter voltage directly affects battery bank size as the number of batteries is directly proportional to the design voltage used.  A 48-volt system requires twice as many batteries as a 24-volt system, which in turn requires twice as many batteries as a 12-volt system.

8)  Charge Controller.  Sizing of the charge controller is done according to the charging source and varies as a function of the Pelton's amperage output.  For the system described in the example above and presuming a 24-volt system, 100 gpm and 100 feet of head have a potential amperage of 37.5 at 24 volts.  To provide a 20% margin of error, a charge controller of 45 amps or more would be required. 

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