Design Consideration of Micro Hydro Power Plant ~ Renewable Exergy

Micro-hydroelectric power is both an efficient and reliable form of a clean source of renewable energy. It can be an excellent method of harnessing renewable energy from small rivers and streams because it requires very little or no reservoir to run the turbine. The water will run through the turbine and goes back to the river or stream, and can use for other purposes. The Micro-hydroelectric power plant has minimal environmental on the local ecosystem. The power generation of MHP is between 5-100 kW. To Design an MHP, there are many considerations to be taken into account:

1. Flow Duration Curve (FDC): The flow duration curve is a cumulative frequency curve that shows the percent of time specified discharges were equaled or exceeded during a given period. It combines in one curve the flow characteristics of a stream throughout the range of discharge, without regard to the sequence of occurrence. The choice of turbine type, size, and speed are based on the net head and maximum water flow rate.

2. Flow Rate Measurement: To measure the water flow rate involves the measurement of the cross-sectional area of the river and the mean velocity of the water through it.

Cross-sectional Area (Ar)

Where a = width of top river (m)

b = width of bottom river (m)

= the average height of water in the river (m)

Velocity (Vr)

The velocity can be measured by a floating object, which is located in the center of streamflow. The time (t) in seconds elapsed to traverse a certain length (L) in the meter is recorded.

The surface speed (m/s) is given as

To estimate the average flow speed (Vr), the above value must be multiplied by a correction factor that may vary between (0.6) and (0.85), depending on the water course depth and their bottom and river bank roughness (0.75 is a well-accepted value).

V_r = 0.75 * V_rs

Discharge (Q)

Q = A_r * V_r

Q=water flow rate in m³/s

3. Weir and open channel

Q = 1.8 * (W – 0.2h) * h^1.5

Where W = Wier width (m)

h = Wier height(m)

W = 3h

4. Trash rack

To prevent the trash from getting entry into the entrance bars at certain spacing (called trash rack) are placed in a slanting position.

5. Penstock

Penstocks are used to conveying water from the intake to the powerhouse. The internal penstock diameter (D_p) can be estimated from the flow rate, pipe length, and gross head as

Where n_p = Manning’s coefficient

Q = Water flow rate (m²/s)

L_p = Penstock length (in m)

H_g = gross head (in m)

The wall thickness of the penstock depends on the pipe materials, its tensile strength, pipe diameter and the operating pressure. The minimum wall thickness is recommended as:

Where D_p = penstock diameter (in mm)

t_p = minimum penstock thickness (in mm)

6. Head measurement

The gross head (H_g) is the vertical distance between the water surface level at the intake and at the tailrace for the reaction turbines and the nozzle level for the impulse turbines. The modern electronic digital levels provide an automatic display of height and distance with a measurement accuracy of 0.4 mm.Once the gross head is known, the net head (H_n) can be computed by simply subtracting the losses along its path, such as open channel loss, trash rack loss, the inlet to penstock loss, gate or valve loss, and penstock friction loss.

7. Turbine Power

Where P_t = power in watt generated in the turbine shaft.

ⲣ = water density in kg/m³

H_n = net head in m

Q = water flow rate in m³/s

g = acceleration due to gravity

ƞt = turbine efficiency

8. Turbine speed

Where ⍵= turbine speed in rad./sec.

Pt= turbine power

Pl = load power

B = turbine and generator friction torque coefficient in N.m/(rad./sec.)

J = moment of inertia of the whole rotating system in kg/m²

When

⍵=constant means operation is steady

If ⍵≠constant means operation is not steady and governor must be installed so that turbine output power match with generator output power.

Where N = turbine speed in r.p.m

N_s = specific speed of tubine

H_n = net head in meter

P_t = turbine power in kW

9. Turbine selection

Once the turbine power, specific speed and net head are known, the turbine type, the turbine fundamental dimensions and the height or elevation above the tailrace water surface that the turbine should be installed to avoid cavitation phenomenon, can be calculated.