Rankine cycle power augmentation: a comparative case study on the introduction of ORC or absorption chiller

Abstract

The search for more efficient processes to save resources and avoid further environmental damages is mandatory in the current society. Increase in power and efficiency of already existing power plants play important role in the transition to more efficient cycles. This work compares two different thermal arrangements that can be fitted in already existing Rankine cycle power plants to improve their power and efficiency. These arrangements are possible when fuel sulfur concentration is reduced to meet stricter SO x emission legislation. The first thermal arrangement is composed of an absorption chiller and a chilled water storage tank designed to make use of the energy remaining in boiler exhaust gases. Thermal accumulation enables the use of an absorption chiller to produce chilled water during the period of 24 h. This chilled water is used in the condenser during peak hours to increase power output and the efficiency of the plant. The second option is fitting an optimized organic Rankine cycle (ORC) also powered by the exhaust gases from the boiler. Results show that fitting an ORC using n-butane as a working fluid represents a gain of 409.3 kW on net power and 0.42% on efficiency against 55.92 kW and 0.07% obtained using the absorption chiller. Therefore, from a thermodynamic point of view, it is more interesting to use optimized ORCs than absorption chillers for existing power plants operating under the operational conditions evaluated.