Analysis of hot spots in boilers of organic Rankine cycle units during transient operation

• The thermal stability of cyclopentane, in an organic Rankine cycle unit, is analysed numerically.
• The case study is the Draugen offshore oil and gas platform in the Norwegian Sea.
• A transient analysis is performed to identify hot spots during load variations.
• Guidelines for safe and reliable operation of organic Rankine cycle units are suggested.

This paper is devoted to the investigation of critical dynamic events causing thermochemical decomposition of the working fluid in organic Rankine cycle power systems. The case study is the plant of an oil and gas platform where one of the three gas turbines is combined with an organic Rankine cycle unit to increase the overall energy conversion efficiency.The dynamic model of the plant is coupled with a one-dimensional model of the once-through boiler fed by the exhaust thermal power of the gas turbine. The heat exchanger model uses a distributed cross-flow physical topology and local correlations for single- and two-phase heat transfer coefficients.The results indicate that severe load changes (0.4–1.0 MW s−1) can lead to exceedance of the temperature limit of fluid decomposition for a period of 10 min. Ramp rates lower than 0.3 MW s−1 are acceptable considering the stability of the electric grid and fluid decomposition. It is demonstrated that the use of a spray attemperator can mitigate the problems of local overheating of the organic compound.As a practical consequence, this paper provides guidelines for safe and reliable operation of organic Rankine cycle power modules on offshore installations.