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 100°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.
Bay of Plenty Energy – 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 Station
Contact Energy – Centennial Drive Station
Tuaropaki Power Company – Mokai
North and South Island power generating plants
Virtual tour of a geothermal power plant
Geothermal power plants
Pratt & Whitney distributed generation plants
Turboden binary cycle power plants
Ormat Geothermal Power Plants
Case studies of Wairakei development
Wairakei Efficiency Study
Poihipi Road Efficiency Study
Case study of Kawerau geothermal development