Vertical Head Height
Head height is bound to be an important factor because the greater vertical height that the water is supposed to fall, the greater the potential energy available in the water to transform into usable power. Head is the vertical height distinction between where the water penetrates the turbine generator housing up to the spot where the water comes into the intake pipe or penstock.
The vertical head height of the site can be assessed electronically via GPS or gauged from geological maps or the internet via Google Earth or Google Maps data. The vertical gap between contour lines on a map hinges on the mapping available in the site area and no mistake! However, the utilization of data to affect directly the elevation difference between the upstream entry point and the downstream exit point will offer you the head height.
For low compression micro-hydro schemes that utilize canals or ditches, the head height would be rather small. Similarly, a much higher head height the water flow and pressure would be strong. Again the units used, feet or meters are your options but must coincide with the ones used for the flow rate measurements.
Keep in mind that high head heights may demand lengthier pipe runs from the river to the turbine increasing cost and friction losses as the water flows along the pipework.
How Much Power in the Water
Having determined through calculation above the flow rate of the water passing a specific point in a given time and the vertical head height through which the water needs to fall, the theoretical power (P) within the water can be determined as:
Power (P) = Flow Rate (Q) x Head (H) x Gravity (g) x Water Density (ρ)
Where Q is the volume flow rate passing through the turbine in m3/s, H is the effective head height in meters, g is the acceleration due to gravity at 9.81 m/s2 and ρ is the density of water, 1,000kg/m3 or 1,0kg/liter.
Then the maximum theoretical power available in the water is proportional to the product of “Head x Flow”, as the pull of gravity on the water and the water density is always a constant. Therefore, P = 1.0 x 9.81 x Q x H (kW).
A water turbine is not excellent so some input power is slip within the rotation of the turbine blades because of the friction and water leakage but by meticulous design, these losses can be undoubtedly decreased to a small percentage. Most of today’s water turbines have an efficiency rating of between 80 and 95%, relying on the type, reaction or impulse so the efficacious power of a micro hydropower scheme can be given as:
Available Power from a Micro Hydro System
Where: η (eta) is the efficiency of the turbine being used.
So for example, a low-pressure micro-hydro scheme operating at 85% efficiency with a head height of 10 meters and a water flow rate of 500 liters per minute past a fixed point would deliver a power rating of approximately:
Power (P) = 0.85 * 9.81 * 0.00833 * 10 = 1007W or 1.0kW
As: 1,000 liters is equal to 1m3, so 500 liters is equal to 0.5m3. One minute is equal to 60 seconds, then a flow rate of 0.5m3 per minute is equal to 0.00833 m3 per second.
Now, 1.0kW may not seem much, but this equates to over 8.7kWh ( 1.0 * 24 * 365 ) of free hydroelectricity annually. When power is comparable to the product of “Head x Flow”, increasing both two elements and the efficiency of the hydro system would lead to a growth in the generated power on condition that the available water supply is reasonably steadfast throughout the year.
Also, the mechanical exchange of power from the rotating turbine to an electrical generator, or alternator, such as belt drives, gearboxes, chains, etc, will cause additional diminutions and allayed overall efficiency to perhaps as low as 50 or 60%.