Geothermal double-flash plant with interstage reheating: An updated and expanded thermal and exergetic analysis and optimization

• A double-flash geothermal power plant employing a reheater is presented.
• The plant is 3–5% more efficient than a double-flash plant.
• The problems of silica precipitation and reservoir dry-out were analyzed.
• For a 250 °C reservoir the plant converts 46.5% of geofluid exergy into power.
• The cost of the reheat system should be rapidly repaid due to extra generation.

The concept of a double-flash geothermal power plant with an interstage reheater is updated and extended. The conceptual plant holds a thermodynamic performance advantage over a conventional double-flash plant over the range of reservoir temperatures studied from 220 to 320 °C, with the advantage increasing as the temperature increases. An exergy assessment is performed on the whole plant with a detailed examination of the reheater. The effect on performance of the terminal temperature difference in the reheater is found to be small. The trend in performance of the new plant is determined as a function of rising wellhead quality in the event that the reservoir begins to “dry out” yielding higher steam fractions at the wellhead. The potential for silica scaling in the lower-temperature components is examined and found to be of concern. The exact optimized separator and flash temperatures are compared to the simple rule-of-thumb, the “equal-temperature-split” rule, which turns out to be quite accurate. A conventional double-flash plant using a turbine designed to remove moisture after each stage is also examined and found to be theoretically roughly equal to the conceptual plant in terms of power performance. The extra power that can be generated by a reheat plant is monetized to compare with the extra cost involved in the more complex plant relative to a conventional double-flash plant. The projected payback is relatively short indicating that the proposed plant should be both thermodynamically and economically worthwhile on condition that some means of controlling silica precipitation is adopted.