Matt Deady, Professor of Physics, Director of the Physics Program, Bard College
Evaluating the Viability of a Micro-hydro Site
For a micro-hydro installation to be feasible, certain stream conditions must be met.
The amount of power that can be generated at a hydro site depends on three factors:
Power = (head) × (available flow rate) × (system efficiency) .
The head is the vertical drop the water will go through from entering the micro hydropower system to exiting it. For instance, it could be the height of the dam the unit will be installed on, or the elevation difference between the turbine’s water input and output.
The available flow rate starts from the total flow rate of the stream, but would be restricted by environmental or legal conditions on how much water can be diverted from the stream as well as subject to seasonal variations in water flow.
The system efficiency represents the losses in the power generation system due to factors such as friction in the water flow, turbine, or generator.
Calculating the possible power from a stream site is a straight-forward application of physics, and is detailed in the Maximum Hydropower from a Site page. Without factoring in system efficiency, here are some combinations that yield 5 kW of power:
However, micro hydropower systems do not have 100% efficiency. The water loses energy running through the pipes; the interaction with the turbine blades loses more energy; the generator connected to the turbine does not convert all of the energy into electricity. The overall efficiency of such systems is generally close to 50%, which means that the head and flow numbers in the table above would each yield 2.5 kW of electric energy.
Since stream water flow varies so greatly over the course of a year, good estimates of possible power production in all seasons should be made to provide a realistic evaluation of the economic viability of the site. The National Water Information System is one resource to help dam owners understand seasonal and annual variability in their site’s flow.
Different types of micro-hydro turbines are better suited for various flow and head conditions. In rough terms, impulse turbines need a high head to give the water jets high velocity, and low head situations are better suited to reaction turbines. A more detailed discussion of choosing a turbine is given in the Types of Turbines post.
Judging Energy Usage
It can be very confusing to try to sort out the numbers about energy, so let’s put it on a scale we can more easily relate to – one home. The average U.S. home consumes 11,000 kiloWatt-hours (kW-hr) of electricity in a year, and the rate of usage varies over the course of a day and over the span of the year. Since there are 8760 hours in a year, a micro-hydro plant would only have to generate an average of 1.3 kW of power to provide all of the electricity for a single home, on average.
A useful comparison can be made with solar power. A single rooftop solar panel produces about 300 Watts of power under full sunlight conditions, so an array of 20 panels on a roof produces a maximum of 6 kW of power. But the sun is not always shining maximally on the solar panels (or shining at all), so accounting for daily and seasonal variations gives an effective amount of sunlight that would be as if the panels got 4 or 5 hours of full sunlight each day. That translates to an effective average power production of 1.1 kW over the course of the year, a little less than the home consumes.
Judged in this scale, micro hydropower may be a reasonable energy source.