Estimating Head and Flow A full feasibility study will needed to generate accurate head and flow rates, but it can be helpful to get a very rough and ready estimate of this before making your enquiry. Head The head is the height difference between where the water would enter the hydro power system and it would leave it, measured in metres. Typically this could be the height of a weir or the vertical drop of water over a water wheel. If a waterwheel is an overshot type, the head will be the same as the wheel diameter. If the site is undeveloped and has no old watermill structures present, then the potential head would be between where the hydro intake screen would be and where the water discharged from the turbine would return to the watercourse. With hydro it is very important to get as much head as you possibly can, as more head means more power for not much more cost. Depending on how much flow you have, the minimum amount of head required for a viable hydro system varies. If you have low head and low flow, then installing a hydro system won’t be very cost effective. Typically a head in excess of 1m is the minimum requirement. Flow This the amount of water that can be passed through the turbine, measured in cubic meters per second. If is very difficult to work out how much flow there is at a site, and establishing this forms a large part of the feasibility studies that we offer. The table below shows photographs of ‘typical’ rivers and streams to help you estimate how much flow you have. You need to estimate the average annual flow, and not the flow on the wettest year of the year. River Categories Category Example Description / Flow Minimum Head Needed 1 Very small stream 0.01 m3/s to 0.05 m3/s 10m - 50m 2 Small Stream 0.05 m3/s to 0.25 m3/s 2.5m - 10m 3 Stream 0.25 m3/s to 1.00 m3/s 2 - 2.5m 4 Small River 1.00 m3/s to 2.00 m3/s 1.5m - 2m 5 River 2.00 m3/s and greater 1m - 1.5m Once you’ve estimated the head and the average flow, you can estimate the power output as detailed below. Power The maximum power of the hydro system can be roughly estimated as: Head X Flow X 5 = kW The head is in metres and the flow in metres-cubed per second. The micro-hydro power systems supplied by Hydro Generation typically range from 5kW - 250kW. A system smaller than 10kW however is unlikely to be cost effective. Example 1 A River which is as large as the photo shown in Category 5 above. Estimate the average flow rate to be 4 m3/s. The head at the site is 2 metres. Therefore the estimated power output would be: Power (in kW) = Head X Flow X 5 = 2 (metres) x 4 (m3/s) x 5 = 40 kW This is a reasonably good size for micro-hydro system. Example 2 A fast-flowing stream in a back garden, but no significant drop apart from the slope of the land, which falls 2 metres over a 100m length. From the table Category 1 above estimate the average flow rate at 25 litres/second (0.025 m3/s). The power output would be: Power (in kW) = Head X Flow X 5 = 2 (metres) x 0.025 (m3/s) x 5 = 0.25 kW This is not enough power to make a micro-hydro project economically viable.