By Paolo Roberto Gomes De Sousa

The Rankine cycle is the most widespread cycle in the world. Used since the late nineteenth century, it was proposed by Scottish physicist and engineer W.J. Rankine, and simultaneously by R. Clausius, a German physicist. The thermal yield of this cycle depends on the average temperature of the supplied and rejected steam. Yield increases as the temperature of the supplied steam increases, or the discharged steam temperature decreases.

A regenerative cycle provides the heating of the boiler feed water via steam extracted from the turbine. This variation of the Rankine cycle reduces boiler fuel consumption and the work of the turbine. It has the objective of increasing overall plant efficiency.

This regenerative cycle has been applied in many power plants. Boilers, heat exchangers and steam turbines have been redesigned with this cycle in mind to achieve higher performance, greater efficiency, and more flexibility and reliability.

Energy and exergy

By simulating plant conditions using Microsoft Excel, it is possible to identify the best application of available thermodynamic capacity, thus obtaining the maximum work and the best overall efficiency of a power plant. This data can then be used to perform an economic analysis to optimize plant operations.

Exergy quantifies the maximum amount of work that can be achieved through the interaction of a system with the environment. An exergetic analysis of a power plant includes the study of the equipment that generates work and heat. This is particularly useful in identifying the maximum useful energy of a system. It is accomplished by modeling of the steam generator, turbine, feed water heaters, deaerator, condenser and pumps. This assists plant owners and operators in evaluating the costs of a thermal system.

The steam turbine supplies heat for the water in a regenerative cycle, as well as power in the form electric energy and mechanical energy. The boiler burns fuel to transform water into steam. This is where the greatest waste of energy occurs.

The boiler feed water heater is where the thermal exchange between the steam supplied by the turbine and the boiler feed water happens. Its design is important in defining the characteristics of the power plant. The pump pressurizes the system. The condensing system, on the other hand, rejects heat from the system and is mainly responsible for system inefficiency. However, the quantity of energy wasted by the condenser is relatively low compared to that of the boiler.

An analysis was carried out concerning the relationship between the boiler feed water temperature, plant efficiency and the power produced by the turbine. When the feed water temperature increases, the turbine produces less power, but the plant becomes more efficient. Raising the temperature of the feed water through steam extracted from the turbine reduces boiler fuel consumption. That cuts operational costs and lessens environmental impact. Each plant has unique operating characteristics. By subjecting any plant to this type of analysis, it is possible to identify the areas of optimum performance profitability.

By running software to simulate the regenerative cycle, a comparison of plant energy and exergy helps to identify how best to run equipment to improve economic efficiency. This simulation showed that the boiler is the biggest area of energy waste in a thermoelectric plant, not the condenser. In addition, net power of the turbine is inversely proportional to the temperature of the water entering the boiler.

Paolo Roberto Gomes De Sousa works for TGM-Weg Group.