At the 2019 ASME/Turbo Expo held in Phoenix, Arizona, S Can Gülen, a Bechtel fellow at the Bechtel Infrastructure & Power, spoke about the fine print in OEM claims of world record efficiencies achieved by gas turbines and combined cycles. Presenting a paper titled, “Disappearing thermo-economic sanity in gas turbine combined cycle ratings: a critique”, Gülen said projected efficiencies often do not take into account one or more parasitic loads. He said often the high efficiencies are just features of high steam conditions obtaining from the higher turbine inlet temperatures. He also said the high vacuum in the bottoming cycle and state-of-the-art last stage blades in steam turbines play crucial roles in securing the necessary output and efficiencies.
Gülen spoke about showcase power plants having favourable site conditions that may not be obtainable in typical customer sites. Below are excerpts from his paper.
There is an optimal combination of TIT and cycle PR for maximizing gas turbine specific output and combined cycle efficiency. These two parameters control the exhaust gas temperature, which in turn controls the bottoming cycle efficiency (as dictated by the second law of thermodynamics).
Current state-of-the-art, as quantified by net LHV combined cycle efficiency at ISO conditions with “new and clean” equipment, practically achievable only in “showcase” power plants is:
About 62% with 1,600°C TIT class gas turbines;
About 61% with 1,500°C TIT class gas turbines.
For cost-effective designs in accordance with local laws and regulations governing stack and other emissions, water conservation and environmental protection, rationally achievable efficiencies are at least one percentage point below those cited above (significantly lower for units with supplementary firing in the HRSG). Practically achievable net LHV efficiency (let us repeat, only in “showcase” power plants with uniquely advantageous site conditions and with no cost spared) with next-generation 1,700°C class gas turbines is 63%.
Claims should contain (i) diligently enumerated assumptions (especially steam cycle conditions and plant auxiliary load) and (ii) thermodynamically self-consistent set of parameters, i.e., output, heat rate or efficiency, exhaust temperature and flow (not exhaust energy) along with cycle PR and TIT or TIT per ISO2314 standard (not the “firing” temperature).
In particular, for simple cycle ratings, the following assumptions must be listed: 1. Fuel gas temperature and pressure (so that one can get a feel for the size of the gas booster compressor if needed) 2. Inlet/exit losses (especially for advanced machines with model-based control (MBC)) 3. Presence of cooling air cooling.
For combined cycle ratings, the following assumptions must be listed: 1. Steam cycle (throttle pressure and steam temperatures) 2. Condenser pressure and the LP turbine configuration (including LSB length) 3. Gas turbine output and exhaust parameters (flow and temperature) 4. Items excluded from the plant auxiliary load.
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