The Geothermal Power Development at Wairakei, New Zealand

Abstract

Until recently the only country to generate power on a substantial scale from geothermal heat has been Italy, but New Zealand is now doing this successfully at Wairakei near the centre of the North Island. The field from which heat is obtained covers only a very small part of an extensive belt—about 150 miles long by 30 miles wide and bounded by active volcanoes—in which a variety of thermal activity occurs 69 MW of plant, forming the first stage of development, had been commissioned by March 1960 and a 123 MW extension is now under construction. The total authorized installation of 192 MW is expected to be completed early in 1963. The largest of the generating sets now being installed are 30 MW mixed pressure machines. About 10 per cent of the energy requirements of North Island is now being supplied from Wairakei and in a year's time this contribution should be about 20 per cent The Wairakei bores yield a mixture of hot water and steam, and the paper describes how the two phases are separated from one another at the wellheads. The first stage plant is supplied with bore steam only, but in the extension now in hand limited use will be made of flash steam from the hot water. A further plant extension has been tentatively planned to raise the installed capacity to 282 MW, the intention being then to generate about one-third of the total output from flash steam. The bores are mostly about 2000 ft deep and after separation at the wellhead the steam is delivered to the power station at two working pressures—180 lb/in2 g and 50 lb/in2 g. The hot water is being transmitted separately at a somewhat higher pressure The paper, which deals briefly only with geophysical theories as to the origin of the heat and the mechanism of its upward transfer to the ground surface, is primarily concerned with the engineering problems of extracting electrical power from the steam and hot water. The site, the bores, the plant and equipment and the buildings are first described and some of the principal design problems are then considered. These problems include the choice of suitable materials resistant to corrosive steam and the techniques of transmitting large quantities of steam and hot water over a considerable distance and of flashing part of the water into steam on arrival at the power station. After dealing with design considerations the paper proceeds to describe the control methods and the safety precautions adopted. An account of operating experience to date then follows and estimates of cost are given The station provides base load at a very high load factor, the output being independent of seasonal or diurnal variations, and the development has come at a time when the hydraulic resources of the North Island are approaching full exploitation. Despite much practical experience there is as yet little firm knowledge regarding the origin and extent of the geothermal heat. Although it is believed that the heat resources of the Wairakei area are ample it was thought prudent to write off all capital costs over a period not exceeding 20 years, and allowance has been made for writing off the bore-hole costs over a shorter period The New Zealand Ministry of Works are responsible for drilling and other steam winning work; the generating plant is installed and operated by the State Electricity Department. The Wairakei geothermal development is of considerable scientific as well as commercial interest and scientific investigations are undertaken by the New Zealand D.S.I.R