Depletion

Depletion

Electricity Generation at Poihipi, New Zealand.

Electricity Generation at Ohaaki, New Zealand.

Electricity Generation at Wairakei, New Zealand.

Falling electricity production may be boosted through drilling additional supply boreholes, as at Poihipi and Ohaaki. The Wairakei power station has been running much longer, with its first unit commissioned in November 1958, and it attained its peak generation of 173MW in 1965, but already the supply of high-pressure steam was faltering, in 1982 being derated to intermediate pressure and the station managing 157MW. At the turn of the century it was managing about 150MW, then in 2005 two 8MW isopentane systems were added, boosting the station's output by about 14MW. Detailed data are unavailable, being lost due to re-organisations. One such re-organisation in 1996 causes the absence of early data for Poihipi (started 1996), and the gap in 1996/7 for Wairakei and Ohaaki; half-hourly data for Ohaaki's first few months of operation are also missing, as well as for most of Wairakei's history.
Although the usual operation mode of a geothermal generator is steady at full possible power because the fuel is free, Poihipi's operation is constrained by limits on its steam consumption, switching on and off daily as shown by the plot's separation of high and low values. Because the temperature difference between the inlet and outlet of the heat engine is relatively low (as compared to fuel-heated working fluids), power production can be noticeably affected by small variations in temperature of the environmental aspect employed by the cooling unit, especially air-cooled units.