A geothermal system exists where water is able to seep into the
ground through cracks and cavities and become heated by the hot
rock deep in the earth. Geothermal systems usually occur where the
earth's crust is relatively thin and fractured.
As the water under the ground heats, it becomes less dense and so
rises, returning to the surface as geysers, hot springs and/or
steam. Large parts of the central North Island - particularly
Taupo, Rotorua and north-east into the Bay of Plenty - contain
geothermal systems. This area is called the Taupo Volcanic Zone.
Some are high-temperature systems and some are relatively low.
High-temperature systems, like those in the Taupo Volcanic Zone,
derive their heat from magma and are closely associated with
volcanism. Low-temperature systems, on the other hand, may be more
closely associated with active faults or areas of extinct
volcanism.
Turning Steam into Power:
The principle of making electricity from geothermal energy is
simple. When hot fluid from deep underground is discharged from a
geothermal well, it comes out either as pure steam or as a mixture
of water and steam. Before Wairakei, Lardarello in Italy was the
world's only developed commercial geothermal resource. It was
referred to as a 'dry field', producing steam only, which did not
require separation of the steam and water before use.
Around the Taupo region, on the other hand, the geothermal
resource is mostly hot liquid. When this fluid is brought to the
surface, it becomes a mixture of about 20 per cent steam and 80 per
cent water, which needs to be separated before the steam can be
used.
The technology for separating steam-water mixtures did not exist
prior to Wairakei. It was a process developed and perfected by Kiwi
engineers in the 1950s, which has been subsequently widely used
around the world. From the wellhead, the piped steam-water mixture
enters a separator. Centrifugal forces and gravity keep the heavier
water spinning around near the bottom of the separator while the
lighter steam collects in the centre and rises to the top, where it
is piped out for transmission to the power station.
Some of the hot water that remains is then directed to a second
type of separation plant, called a flashplant, where the pressure
is reduced, resulting in more steam boiling off from the fluid.
This lower-pressure steam is also piped to the
electricity-generation turbines. The left-over water is either
disposed of back under the ground in a process called re-injection
or discharged into the Waikato River. A small amount is discharged
into the atmosphere as steam.
Wairakei:
The Wairakei station, the world's second geothermal power station,
and the first to utilise flash steam from geothermal water as an
energy source to generate electricity, was first commissioned in
1958 and was completed in 1963. The power station is built beside
the Waikato River 10km north of Taupo, and uses the steam from
wells drilled in the nearby Waiora Valley.
The scheme is based on the tapping of a vast underground water
system that has been heated by very hot, perhaps molten, rocks.
Steam is produced by drilling to release the great pressure on the
very hot water, causing it to boil. The boiling water-steam mixture
is brought to the surface and separated, with the 'dry' steam being
piped to the power station's turbines, and the hot water discharged
into silencers where a drop in atmospheric pressure causes some of
it to flash into the large clouds of steam which produce the
spectacular displays seen in the steam fields. Commissioned in
November 1958, the Wairakei power plant is situated above a large
geothermal system containing water at high temperatures. Currently,
about 5,000 tonnes per hour of fluid is taken from the reservoir.
This is separated into roughly 1,500 tonnes per hour of steam and
3,500 tonnes per hour of water. Dry steam is also taken from
shallow production wells (up to 500m depth) and piped directly to
the turbines. The steam is directed towards the turbines through a
network of pipes around the Wairakei steam field.
Once steam has passed through the turbines, it is condensed within
'direct contact' condensers. This cooling system uses water pumped
from the adjacent Waikato River. After use, the cooling water and
steam condensate is discharged back into the river.
Gases found in the steam supply are pumped from the condenser and
released to the air via gas stacks on the power station roof.
The hot geothermal water is discharged into a system of drains
leading to the Waikato River or it is injected back into the
ground.
Other uses of steam from this steamfield:
Other examples of direct use of geothermal energy can be found
near the Wairakei power station. For instance, the Wairakei
Terraces tourism venture has created silica terraces that have
become a local tourist attraction. The silica-enriched geothermal
fluid from the Wairakei steamfield is piped to a geyser, with the
fluid then cascading over the terraces, creating blues and corals
reminiscent of the pink and white terraces that were destroyed in
the Tarawera eruption of 1886. Another example is the Huka Prawn
Park. This is an aquaculture/tourism/restaurant facility that uses
geothermal heat to commercially breed and grow tropical giant
Malaysian river prawns in captivity. Fully stocked, the 19 large
temperaturecontrolled growing ponds can produce about 32 tonnes of
prawns per year for the prawn farm's restaurant.
The Wairakei Visitor Information Centre is open from 9.00am to
4.00pm weekdays.
There is a viewing platform 2 km up the road from the rear of the
Visitor Centre. NOTE: The Lookout is only open 9am-5pm.
Logging Requirements for this
Earthcache:
Please email the cache owners with the answers to the following
questions and await confirmation before logging online. Do not
post your answers online.
1.a. According to the sign at GZ, what is the temperature
of the geothermal fluid in this steamfield? and b. at what
depth does it exist? and c. in your own words, how is this
geothermal fluid heated?
2. Over the years, how many wells have been drilled in this
steamfield?
3. What is the diameter of the insulated pipelines carrying
the steam to the power station?
4. How fast does the steam travel in these pipelines?
5. Why does the IPENZ recognise this Wairakei engineering
work as an important part of NZ's engineering heritage?
6. What happens every 300m along the steam transmission
lines and why?
7. Take a photograph of you and/or your GPS at the above
co-ordinates, showing the Wairakei steamfield (without any signs in
photo). Upload this photo with your log entry.
Please note, night visits will be disallowed, the Lookout is not
open at night and you won't be able to see the steamfield. Failure
to comply with the logging requirements may result in your online
log being deleted.
The Wairakei visitor information centre is open from 9.00am to
4.00pm weekdays. There is a viewing platform 2 km up the road from
the rear of the Visitor Centre.
FTF honours go to:
Roobi.
