|
Initially geothermal power plants used only the
separated steam from geothermal steamfields, passing
it through a condensing steam turbine. Separated
water was either reinjected, or in the case of
Wairakei, discharged to the Waikato River. Condensing
steam turbines are still used for many new plants,
especially in the larger sizes, and for higher
temperature resources. The Wairakei, Ohaaki and
Poihipi plants are of this type.
An option is to use the separated brine, which
may still be at a temperature in excess of 130°C,
to boil a secondary working fluid to drive other
smaller turbines in a closed cycle. This is usually
referred to as a binary plant. The two small Bay
of Plenty Electricity plants at Kawerau are of
this type, as is the new plant installed at Wairakei.
The Ngawha plant is similar, but uses the steam
in a binary unit as well. Plants of this type
are particularly suitable for lower temperature
resources and/or resources with higher gas contents.
More recently, geothermal combined cycle plants
have been installed utilising a steam turbine
with binary plant heat exchangers acting as the
condensers, and additional binary plants operating
on the separated brine. The Mokai and Rotokawa
plants are of this type.
A potential variation on the binary cycle concept
is the Kalina cycle which uses an ammonia-water
mixture rather than an organic fluid such as isopentane
as the working fluid. While some of these plants
have been constructed internationally, the technology
has yet to be applied in New Zealand.
Whichever plant type is selected, the relatively
low steam temperatures and pressures mean that
the efficiency of conversion of heat to electricity
is low compared to fossil-fuel fired plants. Overall
conversion factors of less than 15% are usual
even for modern geothermal plants operating on
a good geothermal resource, which is why direct
use is more efficient.
There are economies of scale in geothermal generation
developments. The recently constructed plants
in New Zealand, and the individual units within
them, have been significantly smaller than would
routinely be considered optimum on resources of
similar sizes internationally. This has led to
higher unit costs. The main driver for the small
sizes has been conservatism over sustainability,
enforced through the regulatory process.
Links
Generators
Bay
of Plenty Electricity - Kawerau Geothermal
Todd
Energy - Kawerau Geothermal
Mighty
River Power Geothermal Stations
Top
Energy - Ngawha Geothermal Power Station
Contact
Energy - Geothermal Power Stations (pdf brochure)
Contact
Energy - Te Mihi
North and South Island power generating plants
http://www.elec.canterbury.ac.nz/research/powersystems/nzpower/north.shtml
Virtual tour of a geothermal power plant
http://www.calenergy.com/html/aboutus4.asp
Geothermal power plants
Ormat
Geothermal Power Plants
http://geoheat.oit.edu/bulletin/bull26-2/art3.pdf
http://jardhitafelag.is/papers/PDF_Session_01/S01Paper079.pdf
Case studies of Wairakei development
http://www.geothermie.de/egec-geothernet/ci_prof/australia_ozean/new_zealand/a_brief_history_of_the_wairakei_.htm
http://wwwrcamnl.wr.usgs.gov/rgws/Unesco/PDF-Chapters/Chapter9-9.pdf
http://www.ipenz.org.nz/heritage/itemdetail.cfm?itemid=84
Wairakei
Efficiency Study
Poihipi
Road Efficiency Study
Case study of Kawerau geothermal development
http://www.geothermie.de/egec-geothernet/ci_prof/australia_ozean/new_zealand/kawerau_geothermal_development.htm
See Other Geothermal Energy
Content
|
