The amount of electrical energy that can be produced from a hydro power station is governed by the amount of water that is available and the difference in height between the source of the water and the level of the hydro turbine.

The following formula for Power in Megawatts from Flow in cumecs (m3/s) and Head in metres (m) demonstrates the relationship.

Power (MW) = Efficiency x gravity/1000 x Head (m) x Flow (m3/sec)

• The peak turbine efficiency is around 94%
• Generator efficiency is around 97%
• Transformer efficiency is around 99.5%

Overall efficiency from the hydro station is therefore around 90% (being 0.94 x 0.97 x 0.995 = 0.91)

Specifically for the Bogong Project

• Mean Operating Water Level in the Headpond at McKay Creek Power Station is RL 1069.0 m AHD
• Tailwater level at Junction Dam under full flow is RL 644.0 m AHD
• Maximum available head is therefore 1069.0 – 644.0 = 425 m
• Maximum Flow from MKPS = 39m3/sec (140 Megalitres/hour)
• Therefore Maximum Theoretical Power available from BOPS is
• 39 (Flow) x 425 (Head) x 9.81/1000 = 162 Megawatts based on 100% efficiency
• Actual Power available allowing for all sources of losses is
• Combined efficiency of turbine/generator/transformers is 90%

Hydraulic Losses:

= Friction loss = 12.027m
= Form losses = 2.187 m

Nett Head = 425.0 (Max H) – (12.027 + 2.187) = 410.8m

Actual Power = 0.90 x 39 x 410.8 x 9.81/1000 = 141.5 MW 

Which is why BOPS is nominally rated at 140 MW capacity.

Sources of hydro-generator efficiency losses.

The Francis Turbine at Bogong is 95.2% efficient (Compared with the normal 94%) due primarily to very small variation in head and flow and confirmation of hydraulic efficiency via a scale model test.

The efficiency of other hydro machines is significantly impacted by the range of heads and flows under which they are required to generate.

For example the 150 MW (180 MW max) station at the base of Dartmouth Dam is required to generate with heads varying between a max of 170 metres and 118 metres depending on the dam water level.

• At full head: Power = 0.80 x 9.81/1000 x 170 (H) x 137 (Flow) = 153 MW
• At min head: Power = 0.50 x 9.81/1000 x 118 x 130 = 75 MW

The efficiency is also affected by changes in the flow rates through the turbines. Lower flows generally result in lower efficiencies.

Power from full tap flow in Bogong Site Office

• Flow: 1.7 litre kettle filled in 10 seconds giving a flow of 0.17 litres per second = 0.00017 m3/sec
• Head: Assume 30 m head to break pressure tank
• Efficiency: Assume 80%
• Power output: 0.8 x 9.81/1000 x 0.00017 x 30 = 0.00004 MW = 40 Watts  (i.e. enough power to run one 40 watt light bulb)
• If left on all year this would use almost 5.4 Megalitres of water.
• Flow units: 1m3/sec = 1000 litres/sec = 1000 kg/sec = 3.60 Megalitres/hour = 86.40 ML/day

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