| Abstract |
Developers have an incentive towards increased size to gain benefits from economy of scale - for example, US$850/kW for 110 MW units compared with US$1,000/kW for the same station size with 55 MW units. Currently, the largest geothermal turbines in single cylinder, double-flow configuration are 110 MW at 50 Hz and 77.5 MW at 60 Hz. These are of equivalent size, after accounting for the different operating speeds. Increased size presents challenges for both the designer and the developerÃs engineer to ensure that both the developer and the operator maximise the return on their investment over the life of the plant. These challenges arise largely as a result of increased wetness through the steam path, together with increased stresses and corrosion fatigue potential from longer last row blades. This paper discusses the problems exacerbated by large sized units and how these can be alleviated by selection of appropriate materials and design configuration. Optimisation of the main design parameters for the power plant and steam-field, in conjunction with the supplier, also has a major influence. This involves selection of the steam inlet pressure, design cooling tower wet-bulb temperature, cooling tower approach temperature and condenser pressure. Optimisation of the steam-field, leading to selection of the steam inlet pressure, needs to consider also the likelihood of resource run-down of pressure and flow and the policy with make-up wells. Some recent examples of large size units are reviewed. At Darajat in Indonesia, the rated size is 81.3 MW, with capability to 101.7 MW. Wayang Windu in Indonesia is rated at 110 MW. Recent inspection of the three 77.5 MW turbines at Malitbog in The Philippines revealed these to be in excellent condition after operation since 1997, with minimal scaling, no significant erosion and no blading defects. It is concluded that the technology exists to further increase size, even before resorting to the more expensive titanium last row blades. However, the limit may be imposed instead by transportation restrictions of large/heavy items, or transmission limitations. |