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The generation of electricity at the Wairakei Power Station is achieved through four different turbine-generator types. This variety of machines is necessary because geothermal fluid is a natural product and therefore varies.

Inside Station B of the Wairakei Power Station showing a Mixed Pressure steam turbine. The steam turbine is grey and red with two steam control valves in front of it. The generator is light green and its exciter is smaller and to the far right. The most striking feature is the bright green and massive condenser beneath the turbine. It is this condenser that makes such turbines economic, efficent and workable. Waikato River water enters the condenser through the two central pipes and is sprayed into the steam to cool it, creating a vacuum that pulls the low pressure steam through the turbine - Image: Heurisko Ltd.

Generator G4 is painted green and the Intermediate Pressure steam turbine that drives it is painted red. The exciter that produces the magnetic field in the stator of the generator is on the right and is blue - Image: Heurisko Ltd.

The ways in which the geothermal fluid varies are

• the mass flow rate, pressure and volume of steam available
• the proportion of steam to hot water
• the ageing and use of the geothermal field
• improvements in bore technology
• environmental factors

The variety of these turbine-generator pairs is shown in the table below. Note there is presently a total of 10 generators currently operating with an 11th 'Binary Plant' under construction.

Generators G7, G8 and G9 with their low pressure steam turbines (painted grey and red) to drive them. Notice the maintenance in the foreground being done on the exciter for the stator's magnetic field - Image: Heurisko Ltd.

The Types of Wairakei Turbines

1. Low Pressure Turbines

In a Low Pressure turbine the steam enters in the centre and flows out towards the ends through five stages of blading, before being drawn into the condenser below.

The term 'Low Pressure' means that the steam is only at 1.1 atmosphere pressure (0.1 bar gauge pressure). Although this is just above normal atmospheric pressure, the steam is able to drive the turbine because the condenser creates a vacuum on the output side, effectively sucking the steam through.

Wairakei Power Station has 4 Low Pressure steam turbines that all drive 2-pole generators at 3000 rpm.

2. Intermediate Pressure turbines

Wairakei Power Station has 2 Intermediate Pressure steam turbines that both drive 2-pole generators at 3000 rpm.

The steam enters the turbine at 3.5 Bar of pressure and flows from the front towards the rear, pushing against 3 different stages of blading.

A single condenser in a mixed pressure turbine is painted green to show it contains cool river water. The water enters through the two pipes and is sprayed into the round chamber - Image: Heurisko Ltd.

3. Mixed Pressure Turbines

In a Mixed Pressure Turbine the same turbine is driven by one steam pressure and then another.
Wairakei Power Station has 3 Mixed Pressure steam turbines that drive 4-pole generators at 1500 rpm.

The first pressure is Intermediate Pressure steam at 3.5 Bar, which enters the turbine in the centre. The steam then pushes against 8 consecutive rows of blades, as it travels towards the front of the turbine. The pressure of the steam when it exhausts this inner casing is about 1.1 Bar, it is then turned around and flows back towards the generator between the inner and outer casings. Low pressure steam is added through another set of throttles. The steam goes through another 7 sets of blades before exhausting into the condenser.

4. High Pressure Turbines

The original High Pressure turbines have been removed from the Wairakei Power Station because the high pressure steam (12 bar or atmospheres pressure ) is no longer available from the geothermal field. These turbines are now used for generation at the Ohaaki Power Station.

5. The Binary Plant

The Binary Plant is still under construction and will use heat energy in waste hot water to drive two 8 MW generators. However the binary plant needs about 2MW of power just to run its own pumps and fans so the overall generation is 14MW.

As the condensers are air cooled, the weather will have a big effect on output. They will be able to generate about 4MW more on a frosty night than a hot summer's day.
The Binary Plant uses hot water to heat the low boiling point (29°C) liquid Iso-pentane The Iso-pentane gas is used to drive the turbine and generator.
This is done by:

1. pumping the Iso-Pentane through a heat exchanger, at high pressure (11 Bar)
2. allowing the Iso-Pentane to boil
3. using the Iso-Pentane gas to drive a turbine
4. cooling and condensing the Iso-Pentane
5. repeating the process

The Binary plant has these advantages:

• an extra 14 MW of power is generated which is enough to supply about 10,000 homes.
• cooling of the waste water makes is easier to remove dissolved contaminants such as Arsenic, Lithium and Silica
• it does not affect the supply of geothermal water used by others.

Condensers

Condensers rapidly condense steam that is leaving the turbine. This causes a vacuum to form that effectively 'sucks ' the steam through the turbine, while it is being 'pushed' from the input side.

A condenser doubles the power output of the turbines and generators. At Wairakei the condensers are simple large open chambers into which river water is sprayed.

The cooling of the steam to form water releases a great deal of heat energy, mainly because it involves a change of state from gas to liquid. It takes 40kg of cool Waikato River water to condense each kg of steam.

The vacuum in a condenser is maintained using a Barometric Leg, which is a large column of water beneath the condenser through which the cooling water drains into the Waikato River. This allows the water to leave the condenser without it having to be pumped.

Non Condensable Gases

To create a complete vacuum in a condenser all gases must be removed. This is difficult however because gases such as oxygen, carbon dioxide and hydrogen sulfide are dissolved in the geothermal steam and cooling river water.

As the pressure drops within a condenser it encourages these dissolved gases to come out of solution and become free gases. This can commonly be seen when a cap is removed from soft drink bottle and the bubbles of carbon dioxide escape leave the drink. The bubbles remain as gases because the temperature is too high for them to condense into liquids. This explains the term Non Condensable Gases or NCG's for such gases. NCG's reduce the condensers efficiency at Wairakei, so they are pumped from the top of the condenser and ejected into the air above the power station.

At Wairakei about half of the non condensable gases are from the steam. The rest is water vapour and gases that were dissolved in the cooling water. In particular the two gases in the geothermal steam, carbon dioxide and hydrogen sulfide, are partly soluble and about half are absorbed into the cooling water and discharged to the river.