Micro and small hydro power systems

A run-of-river scheme diverts part of the stream flow through a channel and penstock to a turbine and returns it to the river, with little or no storage.

Layout

  river  weir/intake
   ~~~~>[==]--> canal --> [forebay/
                           settling tank]
                              \  penstock
                               \  (gross
                                \  head)
                         [powerhouse]
                         turbine+gen --> load
                           tailrace--> river

Components

• Diversion weir and intake: raises the water level, directs flow into the canal; trash rack and gate keep out debris and control flow.
• Headrace canal (or pipe): carries water with a small slope to the forebay.
• Settling basin (desilting tank): removes sand and silt to protect the turbine.
• Forebay: small tank at the head of the penstock; gives storage for load changes and has a trash rack, spillway and air vent.
• Penstock: steel, HDPE or PVC pipe taking water down to the turbine; designed for pressure and water hammer.
• Powerhouse: turbine (Pelton, cross-flow, Turgo, propeller, or pump as turbine), governor or electronic load controller, generator, and control panel.
• Tailrace: returns the water to the river.
• Transmission and distribution lines to users.

Classification

Class limits vary by country and textbook. Low-head sites use Kaplan or cross-flow turbines; high-head sites use Pelton or Turgo turbines.

Site investigation decides whether the potential flow and head can be developed safely and cheaply.

Investigations

1. Reconnaissance: study of maps and a field walk to find possible intake, canal, forebay and powerhouse positions and the demand (load) of the nearby village.
2. Head measurement: gross head between intake water level and tailrace level.
3. Flow measurement: dry-season (minimum) and mean flow, from direct measurement or hydrological estimates.
4. Geology and soil: stable slopes, no landslide or leaking ground along canal and penstock; foundation for powerhouse and anchor blocks.
5. Layout and cost: length of canal and penstock, access, flood level of the powerhouse, distance to load.
6. Environment and water rights: impact on downstream users and fish, irrigation and drinking needs.

Head measurement methods

• Water-filled tube (pipe) and pressure gauge: a long flexible tube filled with water is laid down the slope; the gauge reading at the lower end gives the head, $H = p/(\rho g)$. Reading is taken in steps for long drops.
• Spirit level and measuring stick (or Abney level): a level is used to measure the height difference in successive steps along the slope and the steps are added. It is cheap and quite accurate for short drops.
• Altimeter or GPS: readings at the top and bottom give an approximate height difference, with an error of a few metres, so it is used in the first survey only.
• Dumpy level / theodolite: gives the most accurate result in the detailed survey.

The net head is the gross head minus friction and bend losses in the penstock (typically 5–10 % of gross head).

Bucket method

The whole stream is diverted into a container of known volume and the filling time is measured. $Q = V/t$. It is accurate and used for very small flows (below about 10 L/s).

Float (velocity-area) method

Mark a straight length of channel (say 10–20 m), measure the time $t$ for a float to travel it, and the average cross-sectional area $A$. Surface velocity $v_s = L/t$; mean velocity $v = k\,v_s$ with $k \approx 0.8$–0.85. $Q = A\,v$. It is simple but approximate.

Weir method

A sharp-edged rectangular or V-notch weir is built across the stream and the head $H$ over the crest is measured. For a rectangular weir with end contractions $Q = 1.84\,(L - 0.1nH)H^{3/2}$ and for a 90° V-notch $Q = 1.4H^{5/2}$ (m³/s, metres). Accurate for small and medium streams.

Salt dilution method

A known mass of salt is dissolved and poured into the stream at one point, and the conductivity is recorded downstream where it is fully mixed. The area under the conductivity-time curve gives $Q$. It suits turbulent mountain streams where other methods are difficult.

Current meter

A propeller meter measures velocity at several points (usually at 0.6 of depth) in verticals across the section. $Q = \sum v_i A_i$ (velocity-area method). It gives the best accuracy but needs equipment.

Flow duration curve (FDC)

It is a plot of flow against the percentage of time (or days) the flow is equalled or exceeded, constructed from daily or monthly flow records sorted in descending order.

 Q
 |\
 | \___
 |     \____
 |          \______
 +------------------ % time
 0       50      100

For micro hydro the design flow is normally taken at the flow available 90–95 % of the time (dry season) so that power is dependable, and the area under the curve gives the energy available.

(a) Net head

$$H_n = H_g - h_L = 60 - 0.08(60) = 55.2\ \text{m}$$

(b) Hydraulic power at turbine inlet

$$P_h = \rho g Q H_n = 1000 \times 9.81 \times 0.15 \times 55.2 = 81\,227\ \text{W} = 81.2\ \text{kW}$$

(c) Electrical output

$$\eta_o = 0.80 \times 0.92 = 0.736$$ $$P_e = \eta_o P_h = 0.736 \times 81.2 = 59.8\ \text{kW}$$

The scheme falls in the micro hydro range (5–100 kW). Gross hydraulic power was $\rho g Q H_g = 88.3$ kW, so overall efficiency from the gross head is $59.8/88.3 = 67.7\ \%$.

(d) Annual energy

$$E = P_e \times 8760 \times PF = 59.8 \times 8760 \times 0.6 = 3.14\times10^{5}\ \text{kWh}$$

Answer: $H_n = 55.2$ m; $P_h = 81.2$ kW; $P_e = 59.8$ kW; $E \approx 3.14 \times 10^5$ kWh (314 MWh) per year.

(a) Weir

Here $L = 0.6$ m, $n = 2$, $H = 0.18$ m.

Effective length: $L - 0.1nH = 0.6 - 0.1(2)(0.18) = 0.564$ m

$H^{3/2} = 0.18^{1.5} = 0.0764$

$$Q = 1.84 \times 0.564 \times 0.0764 = 0.0793\ \text{m}^3/\text{s}$$

That is 79 litres per second.

(b) Float

Surface velocity:

$$v_s = \frac{20}{25} = 0.80\ \text{m/s}$$

Mean velocity:

$$v = 0.85 \times 0.80 = 0.68\ \text{m/s}$$

Area: $A = 1.2 \times 0.40 = 0.48$ m².

$$Q = A v = 0.48 \times 0.68 = 0.326\ \text{m}^3/\text{s}$$

Answer: (a) $Q = 0.0793$ m³/s (79 L/s); (b) $Q = 0.326$ m³/s (326 L/s).