Power Cycles

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In a simple approach, industry energy requirements can be summarized to heat and power, and as mentioned in the beginning of this chapter, they can be supplied by an external third party, a utility provider, that can sell them separately in various and suitable forms. This approach can simplify tremendously energy management of the plant, allowing owners to concentrate on the core business, but it may imply a huge lost opportunity for energy efficiency.

Since heat and power are merely diverse energy forms, it is possible to produce them simultaneously in a consolidated system, know as cogeneration. This application can significantly increase overall energy productivity and efficiency by boosting the actual amount of useful energy. To select the proper cogeneration system, a heat and power demand map of the industry is needed, describing heat and power load profiles, for different periods and typical production conditions. Information on process waste heat recovery potential also has to be considered, as well as fuel and purchased power, availability and costs. Water availability and environmental restrictions, either for liquid effluents and air emissions, must be taken into account. This data allows evaluation of economical feasibility of various cogen-eration cycles.

A cogeneration or power cycle consists of some basic individual components. They are an energy supply, which in the average industry is a fuel burner, a primary driver (a heat engine) and a heat sink, configured into an integrated system. Depending on the type of cycle, the energy supply and the primary driver can be either one single piece of equipment or a set, while the heat sink can be one single piece or many different pieces of equipment.

The primary driver may be internal combustion engines, gas turbines, a set of boilers and steam turbines, micro turbines, fuel cells and others. They may use a wide variety of fuels, including natural gas, coal, oil, biomass, hydrogen and other alternative fuels to produce power. Commonly, the mechanical energy generated in this primary driver is most likely used to drive a generator and produce electricity, but it can also drive any rotating equipment such as compressors, pumps and fans. Thermal energy leaving the system can be used directly in process applications or indirectly to produce steam, hot water, hot air or chilled water for process cooling.

Ideally, the most energy efficient path is to burn fuel using the highest possible temperature to convert chemical energy into mechanical energy in a gas turbine, an internal combustion engine or a back pressure steam turbine, using subsequently, relatively lower temperature waste heat from the primary driver to match process heat demands. But the decision to follow this ideal path depends on heat and power proportion and investment in the power cycle. Many thermodynamic cycles may be employed in this task, but here the focus will be on the most commonly used ones, Rankine and Brayton and their combination.

The Rankine cycle may use a wide variety of fuels like coal, oil, gas, biomass or nuclear power as the high temperature source, but thermodynamic operation, independent of the heat source, is fairly constant. It produces work by isentropic expansion of high pressure fluid like many other cycles. The working fluid is water or better, steam. Fuel is burned in a boiler, a furnace where heat is released to be transferred to pressurized water contained within a steel structure, either tubes or a drum. The basic premise is that it is easier to make high pressure steam, starting with high pressure water and then heat this water at a constant pressure. Water is basically an incompressible liquid and little energy is needed to compress it to high pressures. The process is controlled simply by steam pressure. Steam generated is expanded in a steam turbine, which usually drives an electrical generator (Figures 10.1 and 10.2). The Rankine steam power cycle is that most commonly used at power plants all over the world and can be considered the foundation of big power generation industry.

While the Rankine cycle uses water, constantly condensing and evaporating, the Brayton cycle is an all gas cycle, using air and combustion gases directly as working fluids. Its primary driver is a single set of air compressor and combustion gas turbine that produces mechanical energy by isentropic expansion of hot flue gas. These turbines operate at temperatures approaching flame temperature around

High Pressure Steam

High Pressure Steam

Figure 10.1 Schematic Rankine cycle.
Figure 10.2 Temperature vs entropy for Rankine cycle.

1300 °C, much higher than inlet temperatures of steam turbines in Rankine cycles (lower than 650 °C). Although this difference might suggest that the Brayton cycle would have a much higher thermodynamic efficiency than the Rankine cycle, the Brayton cycle also wastes energy at a much higher exhaust temperature than the Rankine cycle. Furthermore, since the Brayton cycle uses two mechanical devices, an air compressor and gas turbine, it presents more mechanical irreversibilities, degrading thermodynamic efficiency, which means that it can only be slightly better than the best equivalent Rankine cycle (Figures 10.3 and 10.4).

The Brayton cycle can use only some fuels, mainly gases or light volatile liquids that vaporize fast. The most used fuels are natural gas, residual gases, kerosene and diesel oil. Because of these fuel restrictions, it is less polluting than an equivalent Rankine cycle. On the other hand, while the average Rankine cycle can burn almost any fuel without much preparation, a Brayton cycle may even require previous fuel treatment.

Some other different features can be pointed between Rankine and Brayton cycles, like the need of water treatment for Rankine, and the heat-to-power ratio. Rankine mandatorily produces relatively more heat for each energy unit input than Brayton. The Rankine cycle has unmatched variability of operational load conditions, due to fuel flexibility and the use of the steam turbine. Many optimal operational conditions can be achieved to comply with process heat and power demand, while Brayton, although offering more power operates in a restricted heat-to-power ratio range.

Rankine cycle plants may have a longer life span than Brayton, and all parts are reliable. A well-managed system requires relatively little maintenance. Installation

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