A micro-hydro power system needs a sufficient amount of falling water to be available in order to be feasible. Mountainous and hilly sites are best suited for this type of renewable energy. To figure out the amount of power that is possible from your water source you need to know the head and flow of your stream. The head is the vertical distance of the falling water. While the flow is the speed the water flows at.
A micro-hydro power site usually falls into either a low or high head category. A higher head is better due to needing less water to produce energy as well as the equipment being cheaper than those with a low head. A change in elevation that is less then 10ft (3 meters) is categorized as low head. Anything with a vertical drop less than 2ft (.6 meters) will make a micro-hydro power system not possible. Though if you have as little as 13” of water depth you are able to utilize a submersible turbine, which was originally designed to power scientific instruments being towed behind exploration ships.
There is both a gross and net head that needs to be calculated. The gross head is the vertical distance between where the water enters the penstock, pipes that convey the water under pressure, to where the water exits the turbine. You calculate your net head by subtracting the friction that is caused by the piping and the turbine itself.
While the best way to get an accurate gross head is to have a professional survey of your desired site, you can do a rough estimate yourself. You can use the hose-tube method by taking stream-depth measurements across the width of the water supply you intend to use. Once you know where you intend to place the beginning of the penstock and the turbine you can follow the direction below.
The Hose-Tube Method is done by:
- Make sure you have all supplies needed: Someone to help, 20ft to 30ft (6 to 9 meters) small diameter garden hose, Funnel, Measuring tape or yardstick
- Stretch the hose down the water channel from desired entrance to the penstock (usually the highest elevation)
- One person place the funnel into the hose upstream as close to the surface as possible
- At the downstream position have the other person lift their end until the water ceases to flow from it.
- Then measure the vertical distance from the surface of the water to the end of the hose. This is your gross head for this section of the waterway.
- Then move the funnel end of the hose to where the measurement was taken and once again stretch your hose down the water channel and repeat steps 3 thru 5 until you reach your desired position for the turbine.
- Once you have completed your measurements for each section, from entrance to pipes to exit from turbine, add them together for a gross head of the site chosen.
- To be conservative in your measurements it is best to subtract 1 – 2 inches (2 – 5 centimeters) from each measurement before adding to account for water that can continue to flow after both ends are level.
The flow of your waterway can probably be found at public sources; such as a U.S. Geological Survey, the U.S. Army Corps of Engineers, the U.S. Department of Agriculture, your county’s engineer, or local water supply of flood control authorities. If the flow is unavailable from these sources you can also do a rough estimate at the site yourself. There are two simple methods for this:
1. The bucket method which involves damming your stream to divert its flow into a bucket or container. The rate at which the container fills is the flow rate. If you used a 5 gallon bucket and it was filled in one minute then your flow rate would be 5 gallons a minute.
2. As long as the water isn’t fast flowing and/or over your calves you can use weighted-float method. This involves measuring the depths of the waterway across its width. To do this you will need: a helper, tape measure, yardstick, weighted-float (a plastic bottle halfway filled with water will do), stopwatch, and graph paper. Then to calculate the flow for a cross section of the waterway at its lowest water level you need to:
- Find the most uniform depth and straightest stretch of the waterway
- Measure the width of the waterway at the narrowest point
- Use the yardstick vertically to measure the depth at 1ft increments. You may wish to use a string stretched across to mark the increments.
- Plot the measurements on the paper to give you a cross section diagram of the waterway
- Calculate the area of each section by determining the areas of the rectangles (area = length × width) and right triangles (area = ½ base × height) in each section
- From the section you measured mark a point at least 20ft upstream
- From there release your weighted-float and time how long it takes to reach your measured part of the waterway. Be careful to not let the weighted-float drag on the streambed at anytime.
- To get your flow velocity divide the distance between the two points by the seconds it took the float to travel. Doing this multiple times and using the average will you give you a better measurement
- Multiply the velocity average by the cross-sectional area of the stream
- Finally you need to account for the roughness of the bed of the waterway. You will need to multiple the results by either 0.6, for many rough stones on the bottom, 0.7, for only small to medium stones on the bottom, or 0.8, for a smooth sandy type bottom.
Once you have the flow and head calculations you can estimate the power outage for a standard microhydropower system, which has about 53% efficiency. To do this you multiply the net head by the flow then divide by 10 to get the output in watts.
net head [(feet) × flow (gpm)] ÷ 10 = W
Caution: Please remember that flowing waterways will have variable flows throughout the year. So taking the measurements at the waterways lowest average for the year can ensure that enough energy output is available to support your energy needs.
When you are considering this alternative for your personal energy system you need to consider the power output that is possible, the price, and legal issues, such as water rights and permits. These issues taken as a whole will help decide if this renewable energy is for you. Considering the low impact on the environment, the ability to build it yourself with locally available parts, and the fact that it has been used for thousands of years makes this natural energy stand alone in todays search for alternatives to the conventional power supply.
Tags: calculate energy from water, depth measurements, energy system, gross head, micro hydro, microhydro, off-the-grid, personal energy, Renewable Energy, submersible turbine, water turbine