ece 7800: renewable energy systems topic 15: micro-hydropower systems spring 2010 © pritpal singh,...

ECE 7800: Renewable Energy SystemsTopic 15: Micro-Hydropower Systems

Spring 2010

© Pritpal Singh, 2010

Introduction Hydroelectric power systems form

almost 20% of the world’s energy production. This is mostly very large systems, typically > 30MW plants. In many African countries, hydropower accounts for >90% of the power generation capacity, e.g. Tana River hydropower station in Kenya.

The focus of this topic is on small-scale hydroelectric power generation defined as < 100kW plants.

Micro-Hydropower System An example of a micro-hydropower

system in which higher elevation water in a river is partially diverted through a penstock and through a powerhouse is shown below.

http://www.eere.energy.gov/consumer/your_home/electricity/index.cfm/mytopic=11060Source:

Power Output of a Micro-Hydro Plant There are three contributions to the

power generated by a micro-hydro plant:

1) Potential energy

2) Pressure energy

3) Kinetic energy

Power Output of a Micro-Hydro Plant (cont’d)

The three contributions can be expressed in a per unit of weight basis as:

Energy head = z + p + v2

γ 2g

where z = elevation above a ref. ht., p is the pressure, γ is the sp. weight of water, v is the average velocity and g is the acceleration due to gravity (9.81 m/s2).

Power Output of a Micro-Hydro Plant (cont’d)

In calculations associated with micro-hydropower systems in the US, you may find mixed units used. The below table shows useful conversions between units.

Power Output of a Micro-Hydro Plant (cont’d)

Example 4.3

Power Output of a Micro-Hydro Plant (cont’d)

A simple dimensional analysis (see text) can be used to show that the power theoretically available from a hydroelectric site, P, of head, H, and water flow rate, Q, is given by:

P = γQH

Substituting the appropriate units gives:

P(W) = 9810 Q(m3/s) H(m)

= Q(gpm) H(ft)

5.3

Power Output of a Micro-Hydro Plant (cont’d)

The previous equation does not differentiate between high head, low flow and low head, high flow conditions. However, a high-head, low flow site can use smaller-diameter piping and smaller turbines which results in a much cheaper implementation.

Power Output of a Micro-Hydro Plant (cont’d)

The head, H, in the previous equation is the gross head (HG) because it does not include the pipe losses. The net head, HN, is the gross head minus the pipe losses.

Pipe Losses The frictional losses in a pipe vary as

the square of the flow rate and with the diameter of the pipe as shown in the below figure.

Power Output of a Micro-Hydro Plant (cont’d)

Taking the pipe losses into account as well as the efficiency, e, of the turbine/generator, the power output can be expressed as:

P(W) = e Q(gpm) HN(ft)

5.30

= 9.81 e Q(m3/s) HN(m)

Power Output of a Micro-Hydro Plant (cont’d)

Example 4.4

Optimum Flow Rate In the previous example, the losses are

very significant and a larger diameter pipe would be better. However, if the diameter of the pipe is constrained, there must be an optimum flow rate - too fast, pipe friction losses increase; too slow, power output reduces.

The head loss, ΔH, is the difference between the gross head and the net head. The net head can be approximated by:

ΔH = kQ2

Optimum Flow Rate (cont’d) The power delivered is given by:

P=cHNQ = c(HG- ΔH)Q

= c(HG – kQ2)Q

For max. power, this equation needs to be differentiated and set equal to zero. The result is that the optimum flow rate occurs when:

ΔH = ⅓HG

Optimum Flow Rate (cont’d)

Example 4.5

Measuring Stream Water Flow To measure the water flow in a stream,

a barrier (weir) can be established and the rate of flow over the barrier can be measured (see text).

Turbines Turbines convert the mechanical energy of

the impact of the water to rotational energy to turn the shaft of an electric generator. Three types of turbines are available:

1) Impulse turbines – high speed jets shoot onto buckets along the circumference of the wheel.

2) Reaction turbines – pressure difference across blades creates torque to turn wheel.

3) Overshoot waterwheel – not used for electricity generation because their rotational speed is too slow.

Pelton Wheel Turbines Pelton wheel turbines (a type of

impulse turbine) are the most common type of turbine used in micro-hydro systems. A four-nozzle Pelton wheel turbine is shown below:

Pelton Wheel Turbines (cont’d)The flow rate to a Pelton wheel turbine is controlled by the nozzles. When the water exits the nozzle, its pressure head is converted to kinetic energy. We can therefore write,

HN = v2 =>

2g

A nozzle diameter can be determined from the flow rate and Q and flow velocity v.

NgHv 2

Pelton Wheel Turbines (cont’d)Since Q=vA, where A is the nozzle diameter, we can calculate the jet diameter, d, for a Pelton wheel turbine with n nozzles using:

Solving for jet diameter gives:

2

42 ndgHvAQ N

n

Q

gHd

N4/1)(

949.0

Pelton Wheel Turbines (cont’d)

Example 4.7

Pelton Wheel Turbines (cont’d)

Source: http://www.ex.ac.uk/~sritchie/hydro/factfiles/factfile2%20-%20Pelton%20Wheel.pdf

http://www.eere.energy.gov/consumer/your_home/electricity/index.cfm/mytopic=11120Source:

Kaplan Turbines In the case of low head, high flow rate

installations, reaction turbines are preferable to impulse turbines. The commonest type of reaction turbine is the Kaplan turbine. Larger units have adjustable runner blades which offers the advantage that high efficiency can be maintained even in partial load conditions and there is little efficiency drop due to head variation or load.

Kaplan Turbines (cont’d) A picture of a right-angle propeller

drive turbine is shown in the figure below:

Kaplan Turbines (cont’d)

http://www.vatech-hydro.at/view.php3?f_id=4719&LNG=EN

Kaplan Turbines (cont’d)

A fixed-blade turbine runner 15 feet 10 inches (4826 mm) diameter rated 28000 HP (20880 kW). In 59 years of service this runner produced 7.5 terawatthours of energy. This runner was installed at Manitoba Hydro's Great Falls generating station. This runner is on display at the Manitoba Electrical Museum, Winnipeg.

http://en.wikipedia.org/wiki/Image:HydroelectricTurbineRunner.png

Electrical Aspects of Micro-Hydro Large hydroelectric systems feed into a grid network. However, small scale micro-hydro systems are usually stand-alone, providing power to a small village or individual ranch/home.

A stand-alone system is usually sized to meet the average base load and then peak loads, e.g. motor starting currents, etc. are met by batteries. Since water flow does not change significantly, the battery bank can be quite small (little autonomy is required unlike a stand-alone PV system).

Electrical Aspects of Micro-Hydro (cont’d)

An electrical block diagram of a micro-hydro system is shown below:

Case Studies Examples of microhydropower

systems for three applications will be presented next. The two case studies are for:– Derendingen, Switzerland– Ifugao, Philippines– Waslala, Nicaragua

Derendingen, Switzerland

For the first time an archimedes screw is installed in Switzerland. The plant has a head of 1.15m at an average flow of 1000l/s. The picture above shows the site before the plant was built. At the right hand you can see the new railtrack bridge, at the spot covered by the shadow, the plant will be  erected.

Derendingen, Switzerland (cont’d)

In the picture on the left  you can see the Archimedes screw hanging on  the hook of a mobile crane. It shows the inlet of the screw. The nearest part is the case of the gear and generator unit. The screw has three helix blades at a diameter of 1600mm and weights 5800kg. The outlet of the screw can be seen in the picture at the right. The tube is not all around, this way the circular shape can by made more accurate.

Derendingen, Switzerland (cont’d)

The screw is put in place. All the concrete walls tower only 5cm above the upper water level, so during high-water, the water has a free running area.

Derendingen, Switzerland (cont’d)

The screws tilt angle of 22°

Derendingen, Switzerland (cont’d)

After putting the screw in place, the floor of the plant house also has been installed. The house will absorb the sound, assisted by a cabinet around the gear and generator unit. It is not that loud that it would actually be needed, but the neighbors are very sensitive. On top of the screw you may see the gear and generator unit. The water in front of the microhydro is not dammed up yet because there is still a lot of work to do. On the right hand side you see the bypass channel, in the middle the screw channel. At left there is enough free running area for high-water. 

Ifugao, Philippines

Power House at Maggok 1

Power House at Maggok 2

Waslala, Nicaragua

Waslala, Nicaragua Turbine

Waslala 180 kW Power Plant

Useful Sources for Micro-Hydro Info.• http://www.eere.energy.gov/consumer/

your_home/electricity/index.cfm/mytopic=11050• http://www.microhydropower.com/• http://www.microhydropower.net/