How to Calculate the Amount of Available Power for a Mini Hydro
- 1). Determine the flow. The flow of a stream varies daily and seasonally as a result of environmental, climatic and human factors. It is important to be aware of the nature of these fluctuations at a site. The Oregon Department of Energy recommends sourcing flow data from the local offices of the U.S. Geological Survey or the U.S. Department of Agriculture. If no such data exists, it is necessary to conduct a site survey and measure the flow.
- 2). Determine the head. Head quantifies the pressure of falling water and is a function of the vertical distance that the water drops and the characteristics of the hydro system. The gross head is the maximum available vertical drop in the water. Typically, the actual head seen by the turbine, called the net head, will be slightly less than the gross head due to losses incurred as it moves through the system.
Hydro installations are typically classified as low head (less than 10 feet vertical drop) or high head (more than 10 feet vertical drop) according to the Oregon DOE. High head is desirable for mini-hydro as fast flowing water on its own holds sufficient energy for small-scale power production. For a given output, the greater the head, the less flow is required, allowing for smaller, more efficient turbines to be used. - 3). Calculate the available power. The following formula can be used to estimate the available power:
Power = Gross head x Flow x System efficiency x Gravitational constant
The system efficiency is a function of the turbine and generator efficiencies, which are in turn determined by the hardware and the head and flow. In general, a low head, low flow turbine is less efficient than a high head, high flow turbine. According to the Oregon DOE, overall system efficiencies typically range between 40 percent and 70 percent. The average efficiency of well-designed mini-hydro systems is typically in the region of 55 percent.
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