Week 4 - Hydropower

Question 1

What are the different type of hydropower generators?

Answer 1

Dam (or storage) hydro: 
- Storage: days - months
- Tends to be larger schemes
Run-of-river hydro: 
- Storage: virtually none
- Range of sizes
Pumped storage: 
- Storage typically 8 hours
- Tends to be larger

Question 2

What sizes are hydropower divided into?

Answer 2

Pico/micro: <10kW
Mini/small: 100kW - 20MW
Large: >30MW

Question 3

What are the hydropower components?

Answer 3

• Catchment area, defined by weir locatikn and the topology of the land.
• Weir or Dam
• Penstock, high pressure pipeline, open channel sometimes included.
• Power house, Low pressure tailrace to river

Question 4

What is ‘Head’?

Answer 4

‘Head’ is the vertical distance between water levels behind the weir and the river below the power house.

Question 5

What is the ‘static’ or ‘gross’ head?

Answer 5

The static or gross head (H_G) applies when water is not flowing between the intake and the turbine generator.

Question 6

What is ‘net’ head?

Answer 6

As water flows, the turbine experiences a reduced or ‘net’ head (H_N) due to fiction losses in the penstock ‘head losses’ (H_L)
H_N = H_G - H_L

Question 7

What are the major factors for calculating head loss?

Answer 7

Flow rate and diameter.

Question 8

What does the friction factor depend on?

Answer 8

The friction factor depends on material and whether flow is laminar or turbulent, governed by the Reynolds number.

Question 9

How is friction factor determined?

Answer 9

Usually determined using ‘Moody charts’.

Using Roughness ratio and Reynolds number.

Question 10

When designing pipeline, what is the aim for the head loss?

Answer 10

The head loss is usually below 10% of the gross head.
This is ensured by optimizing the diameter of the pipe. However, there is a trade off in increasing cost as diameter increases for reduced lifetime energy losses.

Question 11

How is River Flow approximated?

Answer 11

River flow is the difference of precipitation (rain and/or snow) and Evapotranspiration (evaporation from soil and plant transpiration).
Q = P - ET

Question 12

What is a Flow Duration Curve?

Answer 12

A Flow Duration Curve is a form of cumulative probability distribution. Each point on curve shows proportion of time that flow rate is exceeded. It is created by ranking flows and determining exceedance percentiles.

Question 13

How is exceedance percentile calculated?

Answer 13

E_P = rank / (n+1)

Question 14

What is the design or rated flow (Q_Tmax)?

Answer 14

Q_Tmax is the design or rated flow at which max power is produced. No more flow can pass through penstock. Design flow typically mean or median annual flow. 30% exceedance common.

Question 15

What is compensation or reserve flow (Q_COMP)?

Answer 15

It is the flow which must remain in river and not go through turbine to preserve life in river.

Question 16

What is the turbine minimum flow (Q_Tmin)

Answer 16

If below this flow, the turbine shuts down. Determined by Q_Tmax and turbine type.

Question 17

Area of a flow duration curve can be estimated by dividing area into columns of defined width centred on known flow rates. How is the available flow in any column calculated?

Answer 17

Q - Q_comp

Question 18

What is the biggest issue when constructing a hydro power generator?

Answer 18

Biggest issue is obtaining credible data, which is very important for ‘bankability’ (i.e. funding) as well as desgin.
30 years of data at the site are required to get a good idea of local climatological norms.
Unlikely to be available so need to measure site for at least a year and benchmark against:
- ‘Analogue’ data from a nearby station.
- Standard flow duration curves based on national estimates.
- Hydrological models to generate flow data fron rainfall and evapotranspiration data.

Question 19

Name some historical uses of hydropower.

Answer 19

• Water wheels for grinding corn.
• Heavy use during the industrial evolution to drive machinery as steam initially treated as backup during drought.
• The need for compact, faster and efficient equipment led to evolution of water turbine (Francis turbine in 1849 and Pelton wheel in 1870).

Question 20

Name some hydropower stations.

Answer 20

• Hoover dam
• Kariba dam
• Itaipu dam
• Three Gorges dam

Question 21

What is an Arch dam?

Answer 21

A dam shaped liked an arch. This gives strength to the construction and uses less material (Cheaper). Used at narrow sites and need strong abutments (substructure at end of dam).

Question 22

What is a Concrete Gravity Dam?

Answer 22

Weight holds dam in place. Uses lots of concrete (expensive).

Question 23

What is a Buttress Dam?

Answer 23

Face is held up by a series of supports. Significantly reduces cost. Flat or curved face.

Question 24

What is an Embankment Dam?

Answer 24

Made of earth or rock. The weight of the st

Question 25

What is an Impulse Turbine?

Answer 25

Impulse turbines use a two stage process to extract energy from water.
The available head is converted into kinetic energy using nozzles from which ‘jets’ of water emerge at high velocity ( v = sqrt(2gH) ).
Then the jets strike the ‘vanes’ on a circular ‘runner’ with energy and momentum transferred to the rotating runner.
The water then falls to the bottom of the case and is discharged.
The interior of the case is at atmospheric pressure and prevents splashing and interference with runner.

Question 26

What is a Pelton Impulse Turbine

Answer 26

In Pelton turbines jets strike the runner tangentially and split into two smooth buckets.
Jets deflect around a smooth curve such that they leave the buckets travelling at angle omega to the original direction. Omega = 180° gives maximum efficiency but ~165° typical in practice.
In an ideal system, energy transfer maximized when runner tangential velocity is half of the jets, u = 1/2 * v (~0.46v in practice).
Up to 6 jets feasible.

Question 27

What is a Turgo Impulse Turbine?

Answer 27

It has (max 2) jets aimed at the runner which hit runner at an angle.
Runner resembles split Pelton runner; diameter 50% smaller for same power.
Runner is cheaper than Pelton wheel.
Can handle greater flows as exiting water doesn’t interfere with adjacent buckets.
Has been successful in South East Asia where monsoon rains bring silt through the machine; wear is much reduced in this design.

Question 28

What is a Reaction Turbine?

Answer 28

Reactor turbines extract energy from changes in water pressure and velocity.
Fluid passes through ring of foil-shaped guide vanes in which part of the available head is converted into kinetic energy.
Water enters the runner along its periphery where the remaining pressure is converted by the specially shaped runner.
The runner chamber is flooded and after the runner water is piped to standing water beneath the turbine to recover as much energy as possible.

Question 29

What is a Francis reaction turbine?

Answer 29

The Francis turbine is similar to a centrifugal pump.
Water enters the spiral case (top) and is directed round the ever decreasing ‘volute’ before guide vanes direct it onto the runner.
Water enters the ‘ring’ of the runner and is directed through a right angle by the blades.
Adjustment of the guide vanes allows operation at range of flows.
Francis turbines come in all shapes and sizes and covering a wide range of flows and head.

Question 30

What is a Propeller/ Kaplan Reaction Turbine?

Answer 30

Propeller turbines are reaction turbines that operate at low head and the flow direction is axial (similar to ship propellers).
There are moveable guide vanes controlling water flow onto the runner from the case (as per Francis).
Kaplan turbine blades can rotate, allowing operation at higher efficiencies under varying flow and head conditions.
Many designs of propeller turbine with fixed blades, guide vanes and housing for the generator.

Question 31

What is a Crossflow Mixed Flow Turbine

Answer 31

Mixed flow (or ‘Crossflow’) turbines are a mixture of impulse and reaction turbines.
The runner resembles a drum on a lawnmower and water is directed onto the runner bu an adjustable vane.
The term ‘Crossflow’ used as water crosses through the runner vanes twice in producing its rotation.
Often called a ‘Banki’ turbine after the Hungarian professor who developed it.
Relatively easy to manufacture and produced in several developing countries to assist their hydropower programmes.

Question 32

What is the Specific Speed for turbines?

Answer 32

It is the speed at which the turbine rotates under 1m net head at 1 m^3/s flow.
N_s = n * sqrt(P) / H_N^1.25
N_s = Specific speed
n = normal speed
H_N = net head
P = Turbine power
Low N_s implies high head and vice versa.

Question 33

Rate the specific Speed for different types of turbines?

Answer 33

Single jet Pelton: 7-27
Multi jet Pelton + Turgo + Crossflow: 27-60
Francis: 60-300
Kaplan and Propeller: 300-900

Question 34

What are the capital costs of hydro?

Answer 34

Hydro costs are highly site specific and dams expensive.
Civil works form two-thirds of total cost (Varies 25 to 80%).
Environmental mitigation (e.g. fish ladders) costly.
Electromechanical components ~10%
Grid connection can be substantial for small hydro
Cost heavily front loaded.

Question 35

What are the benefits of Hydro?

Answer 35

Long lifetime (500-100 years civils; 25-50 year E&M). 
Cost:
- No fuel cost. 
- Low O&M cost
- US Study suggested £1500-£3000/kW
- Low head schemes more expensive as civil works and turbine physically bigger to handle larger flow rates, despite typically shorter penstocks. 
- Unit cost (p/kWh) heavily dependent on discount rate. 
- 8 year payback okay for small hydro. 
- Longer has been acceptable for large strategic schemes. 
Environment:
- No operational greenhouse gas emissions
- Reduced CO2, SO2 or NOx
- Saves finite resources
Non-environmental benefits: 
- Fast operation
- Flood control
- Irrigation
- Transportation
- Fisheries and tourism
- Economic development

Question 36

Name some social and environmental issues of hydro.

Answer 36

• Damage to ecosystems
• Land amenity
• Visual amenity
• Emissions from tropical reservoirs
• Population displacement
• Sensitivity to physical climate change
• Risks associated with large debts