Cross-flow turbines
This impulse turbine, also known as Banki-Michell is used for a wide range of heads overlapping those of Kaplan, Francis and Pelton. It can operate with heads between 5 and 200 m.
Water enters the turbine, directed by one or more guide-vanes located upstream of the runner and crosses it two times before leaving the turbine.
This simple design makes it cheap and easy to repair in case of runner brakes due to the important mechanical stresses.
The Cross-flow turbines have low efficiency compared to other turbines and the important loss of head due to the clearance between the runner and the downstream level should be taken into consideration when dealing with low and medium heads. Moreover, high head cross-flow runners may have some troubles with reliability due to high mechanical stress.
It is an interesting alternative when one has enough water, defined power needs and low investment possibilities, such as for rural electrification programs.
Reaction turbines
Francis turbines.
Francis turbines are reaction turbines, with fixed runner blades and adjustable guide vanes, used for medium heads. In this turbine the admission is always radial but the outlet is axial. Photograph 6.3 shows a horizontal axis Francis turbine. Their usual field of application is from 25 to 350 m head.
As with Peltons, Francis turbines can have vertical or horizontal axis, this configuration being really common in small hydro.
Francis turbines can be set in an open flume or attached to a penstock. For small heads and power open flumes were commonly employed, however nowadays the Kaplan turbine provides a better technical and economical solution in such power plants.
The water enters the turbine by the spiral case that is designed to keep its tangential velocity constant along the consecutive sections and to distribute it peripherally to the distributor. This one has mobile guide vanes, whose function is to control the discharge going into the runner and adapt the inlet angle of the flow to the runner blades angles. They rotate around their axes by connecting rods attached to a large ring that synchronise the movement off all vanes. They can be used to shut off the flow to the turbine in emergency situations. The runner transforms the hydraulic energy to mechanical energy and returns it axially to the draft tube.
Kaplan and propeller turbines
Kaplan and propeller turbines are axial-flow reaction turbines; generally used for low heads from 2 to 40 m. The Kaplan turbine has adjustable runner blades and may or may not have adjustable guide- vanes. If both blades and guide-vanes are adjustable it is described as "double-regulated". If the guide-vanes are fixed it is "single-regulated". Fixed runner blade Kaplan turbines are called propeller turbines. They are used when both flow and head remain practically constant, which is a characteristic that makes them unusual in small hydropower schemes.
The double regulation allows, at any time, for the adaptation of the runner and guide vanes coupling to any head or discharge variation. It is the most flexible Kaplan turbine that can work between 15% and 100% of the maximum design discharge. Single regulated Kaplan allows a good adaptation to varying available flow but is less flexible in the case of important head variation. They can work between 30% and 100% of the maximum design discharge.
The flow enters in a radial manner inward and makes a right angle turn before entering the runner in an axial direction. The control system is designed so that the variation in blade angle is coupled with the guide-vanes setting in order to obtain the best efficiency over a wide range of flows and heads. The blades can rotate with the turbine in operation, through links connected to a vertical rod sliding inside the hollow turbine axis.
Bulb units are derived from Kaplan turbines, with the generator contained in a waterproofed bulb submerged in the flow.
Kaplan turbines are certainly the machines that allow the most number of possible configurations. The selection is particularly critical in low-head schemes where, in order to be profitable, large discharges must be handled. When contemplating schemes with a head between 2 and 5 m, and a discharge between 10 and 100 m3/sec, runners with 1.6 - 3.2 metres diameter are required, coupled through a speed increaser to a generator. The hydraulic conduits in general, and water intakes in particular, are very large and require very large civil works with a cost that generally exceeds the cost of the electromechanical equipment.
The above information should be used in order to choose the turbine.
The potential energy in water is converted into mechanical energy in the turbine, by one of two fundamental and basically different mechanisms:
The water pressure can apply a force on the face of the runner blades, which decreases as it proceeds through the turbine. Turbines that operate in this way are called reaction turbines. The turbine casing, with the runner fully immersed in water, must be strong enough to withstand the operating pressure. Francis and Kaplan turbines belong to this category.
The water pressure is converted into kinetic energy before entering the runner. The kinetic energy is in the form of a high-speed jet that strikes the buckets, mounted on the periphery of the runner. Turbines that operate in this way are called impulse turbines. The most usual impulse turbine is the Pelton.
The higher the head, the smaller the flow.
The hydraulic power at disposition of the turbine is given by:
Ph = Q.gH (W)
Where: Q = mass flow rate (kg/s)
= water specific density (kg/m3)
Q = Discharge (m3/s)
gH = specific hydraulic energy of machine (J/kg)
g = acceleration due to gravity (m/s2)
H = Net head (m)
The mechanical output of the turbine is given by:
Pmec = Ph.n (W)
n = Turbine Efficiency
Methods:
Determining the head:
Head = Highest water level – tailrace water level
The head is the vertical distance from the source of the water down to the turbine. This is usually measured in m. There are various ways to measure it including:
Using a topographic map
If there is a water pipe already installed, you could measure the static pressure with a pressure gauge. The static pressure is when there is no water flowing through the pipe, e.g. all taps and gate valves are turned off
Using altimeters, dumpy levels and other surveying type of equipment.
The penstock (pipe) length is the length of pipe you need to obtain a certain head.
If the site were a vertical waterfall, then the head and penstock length would be equal.
In general terms, high head turbines will cost less and / or produce more power than low head units. You might need to use a few hundred metres of pipe to obtain a suitable head.
In order to achieve desired Head, we store the water by creating DAM on river at specific location as per the geographic & environmental conditions.
Measuring the Flow:
To determine the power potential of water flowing in a river or stream it is necessary to determine both the flow rate of the water and the head through which the water can be made to fall.
The flow rate is the quantity of water flowing past a point in a given time. This is usually measured in litres per second.
Bucket Method
The easiest method for measuring flow rate is with a common 10-litre bucket and a stopwatch. The litres per second flow rate would then be exactly one tenth of the time it took to fill the 10-litre bucket. This method can be employed if you have a narrow opening through a weir or a pipe operating at its maximum flow rate.
Pump Method
If you have a pump, you could determine at what rate you can pump the water out without lowering the water level in the pond.
V-Notch Method
If you have a weir or dam on the water source you may be able to use the V-Notch method of creek flow measurement.
The formula is a rounded form of the very complex formulae required to calculate the water flow. This rounded formula will be correct within ± 1 to 3% up to 300mm of head, ± 2 to 5% 300mm to 500mm of head.
The numbers used are constants for clean-ish water up to 1.05SG @ 15-250C @ 0-300m altitude. So as you can see it is by strict engineering standards, "a thumb suck", but for most intents and purposes it is accurate enough.
H2.5 is made up from the fact that an increase of height of water, increases the area available to flow by the square of the height change and also increases the pressure (head) at the discharge, therefore making more water flow for a given area.
It should be noted that it is difficult to measure the height at the discharge, so measure it say 300mm to 600mm back from the weir.
Cross Section Flow Method
For larger creeks, this method involves the cross sectional area of the flow and the speed of the flow. If you wish to ascertain the flow rate of a stream, when the 10 litre bucket method cannot be employed, you can get a rough idea by measuring the size (cross section) and average flow rate of the stream.
For this method the speed of the mid-stream surface water is measured by timing a float. Choose a part of the stream where the cross section is regular. Measure the cross section by finding the average depth as shown, and the width. Time the float over a short distance to obtain the speed.
The average speed of the whole stream can then be calculated by multiplying the measured speed by:
0.8 for a concrete channel
0.7 for an earth channel
0.5 for a rough hill stream
For streams less than 150 mm average depth, the factor becomes unpredictable and can be as low as 0.25. The flow rate is then equal to the distance that the float travelled multiplied by the correction factor and multiplied by the average depth and width of the stream and then divided by the number of seconds for the float to cover that distance.
If the measurements are taken in metres and the float is timed in seconds, then the result multiplied by 1000 will give you the litres per second flow rate. Overall accuracy of this method is about 80%.