What Turbomachinery does to Avert Climate Change (Part 1 of 2)

Most people complain about climate change, but few take measures to address it. In this article we will see some ways turbomachinery-oriented companies contribute to the well-being of the planet.

  1. Selection and optimization of energy conversion technology (recuperation, proper selection of expander configuration, etc.)

Not all technologies are created equal; where you would use a steam turbine is not necessarily where you would want a gas turbine or an organic Rankine cycle (ORC) instead. Each one of them has its pros and its cons; ORC exist because they do not require as much energy as what is needed for steam cycles, gas turbines have a great power density and an outstanding start-up time (several minutes instead of hours) which makes them great candidates for punctual, unexpected peaks in power demand, etc.

Figure 1 Simple Rankine cycle schematics
Figure 1 Simple Rankine cycle schematics

Now, take the case of a gas, steam or ORC; they all include, in their most basic configuration, a compressing element (compressor or pump), an expander (usually a turbine), a cooling/condensing component and a heating component (boiler, combustion chamber, HRSG, etc.) as one can see on Figure 1 and each of these have an associated efficiency.

This means that their careful design and thorough optimization should be performed in order to maximize the overall performance of the full system. Whether it’s used for power generation or propulsion the result is the same; more power generated for the same amount of heat input (usually the combustion of fuel). However, before starting the full design of the different components the entire system needs to be optimized as well; correct positioning of extractions/inductions, appropriate cooling parameters, use of recuperation/regeneration (see Figure 6), and so on.

Figure 2 Recuperated Rankine cycle
Figure 2 Recuperated Rankine cycle

 

Only when the cycle boundary conditions (and therefore its configuration) are fixed the full design of the components can be performed although some preliminary studies should be undertaken to determine the feasibility of these designs and get an estimation of the components performances. Another goal of such feasibility studies is to determine such things as the estimated dimensions of the components, the configuration of the expander (axial, radial, axi-radial, counter-rotating, etc.) Finally some compromises always need to be done between efficiency improvement and cost of manufacturing, operation and maintenance.

 

 

 

  1. Operation at optimal conditions (design point for overall cycle and each component)
Figure 3 Comparison of efficiency and power rating for axial and radial configurations of turbines
Figure 3 Comparison of efficiency and power rating for axial and radial configurations of turbines

Each energy conversion system whether it is for power generation, propulsion or any other application is designed for a set of operating conditions called a design point. This is where the system will typically be optimum for and where it will be running most of its “on” time. This is why ensuring that the design point (or design points) is accurately defined is critical since operation outside of these defined conditions will lead to additional losses that translate into a lesser power production for the same cost of input energy. Performance prediction of systems at off-design conditions is an essential part of any design task which allows restricting system operation to conditions of high components efficiency. If the pump/compressor is operated at a different mass flow rate its pressure ratio will be different and so will be the efficiency and therefore the amount of power generated by the expander, see Figure 4.

Figure 4 Performance map of a centrifugal compressor showing its efficiency as a function of the mass flow rate for different rotation speeds
Figure 4 Performance map of a centrifugal compressor showing its efficiency as a function of the mass flow rate for different rotation speeds

In our next post, we will continue the discussion of the turbomachinery industry as it relates to climate change. Stay tuned!

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