Single-Shaft Combined Cycle Power Plant: a Great Invention or an Elaborate Joke?


A combined cycle power plant (CCPP) uses both steam and gas turbines which increases the efficiency up to 50 percent compared to a simple-cycle plant. Conventional CCPP applications use separate gas and steam turbines and route the waste heat from the gas turbine to the nearby steam turbine to generate extra power. In recent years, an alternative design for a CCPP has been developed with single-shaft rotors.

So, what are the drawbacks and advantages of single-shaft CCPP design? Is it both possible and (more importantly) a good idea to have a single-shaft CCPP? To answer that we need to look at how one would work.

The typical steam and gas turbine rotors for a conventional CCPP application (high power ~200MW) are presented in Figure 1. The first power train (gas turbine) consists of a generator, compressor, and gas turbine parts. The second power train (steam turbine) contains high-intermediate and low-pressure turbine rotors and another generator.

Separate Gas Turbine and Steam Turbine Rotors
Fig. 1 – Separate gas turbine and steam turbine rotors (AxSTREAM RotorDynamics models)

In a single-shaft application, only one generator would be driven by the gas-steam-turbine power train. An optimal variant would be to have the generator between the gas turbine and a steam turbine as shown in Figure 2. Read More

Gas Turbine CFD – Driving Innovation with Data and Insight

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally Written By Chad Custer – February 2, 2021

Once upon a time in a world without gas turbine CFD simulation.

Manager: The design team came up with a new blade concept, but they need to know the maximum possible temperature in the machine.

Test engineer: Anywhere in the whole machine?

Manager: Yes. And for any operating condition the machine might get used for. How long until you can have those results to the team?

Test engineer: Uhh

Terms like “virtual prototype”, “simulation testbed” and “digital twin” have become so common that you may dismiss them as buzzwords. However, to me, these terms not only still have meaning. These words do drive how I look at simulation. Read More

Waste Heat Recovery

During industrial processes, an estimated 20 to 50% of the supplied energy is lost, i.e., by dumping the exhaust gas into the environment [1]. The waste heat losses and the potential work output based on different processes including but not limited to the ones shown in Figure 1. Does it REALLY have to be thrown away? Sometimes yes, other times no. In this blog post, we will focus on the “no” through a process  called “Waste Heat Recovery”.

Waste heat losses and work potential of different process exhaust gases - Image 1
Figure 1: Waste heat losses and work potential of different process exhaust gases [US Department of Energy [2]]
Some well-known examples of waste heat recovery processes are found in turbochargers in cars or a heat recovery steam generator. One simple structure of application is when a heat exchanger is fed with the exhaust gas of a turbine, therefore being cooled down before being released into the air. This heat exchanger is part of a secondary (bottoming) cycle where another turbine provides additional power output without having to burn additional fuel. This heat exchanger is part of a secondary cycle where another turbine provides additional power output. Read More

Design of Waste Heat Recovery System based on ORC for a Locomotive Gas Turbine

This is an excerpt from a technical paper, presented at the Asian Congress on Gas Turbines (ACGT) and written by Abdul Nassar, Nishit Mehta, Oleksii Rudenko, Leonid Moroz, and Gaurav Giri. Follow the link at the end of the post to read the full study!


Gas turbines find applications in aerospace, marine, power generation and many other fields. Recently there has been a renewed interest in gas turbines for locomotives. (Herbst et al., 2003) Though gas turbines were first used in locomotives in 1950 – 1960’s, the rising fuel cost made them uneconomical for commercial operation and almost all of them were taken out of service. The diesel locomotives gained popularity and presently locomotives are operated by diesel engines and electric motors. The emission levels in diesel locomotives have raised concerns among the environmentalists, leading to stringent emission norms in recent years. One of the solutions to reduce emission for these locomotives is to switch to LNG fuel which requires huge investment in upgrading the engines to operate with LNG. The other alternative is Gas Turbine based locomotives and this has gained renewed interest with RZD and Sinara Group of Russia successfully operating LNG based Gas Turbine-electric locomotives. Fig. 1 shows the GT1-001 freight GTEL from Russia, introduced in 2007. It runs on liquefied natural gas and has a maximum power output of 8,300 kW (11,100 hp). Presently, this locomotive holds the Guinness record for being the largest gas turbine electric locomotive (Source: Though there have been a lot of improvements in gas turbines, the thermal efficiency is still not very high unless the exhaust heat is efficiently utilized by a bottoming cycle.

Fig. 1 Russian GT1_001 gas turbine locomotive

Converting the gas turbine into a combined cycle unit, with a bottoming steam cycle, is employed in case of several land-based and marine applications; however, such an option is not practical in a locomotive gas turbine due to the requirements of steam generators, steam turbines and other auxiliaries. The next best alternatives are to utilize either an organic Rankine cycle (ORC) or a supercritical carbon dioxide cycle (sCO2) to extract heat from the exhaust of the gas turbine and convert it into useable energy in the bottoming cycle (Rudenko et al., 2015; Moroz et al., 2015a; Moroz et al., 2015b; Nassar et al., 2014; Moroz et al., 2014). Supercritical carbon dioxide cycles, operating in a closed-loop Brayton cycle, are still in research phase. There is not much practical experience in deploying an sCO2 unit for propulsion gas turbines even though there is considerable research currently in progress. Hence, the obvious choice is to incorporate an ORC based system which is compact, modular and easy to operate. The same concept can also be implemented in any gas turbine application, be it a land-based, power generation, or marine application. Read More

Optimization of the Closed Supercritical CO2 Brayton Cycle with the Detailed Simulation of Heat Exchangers

Recently scientists and engineers have turned their attention again to carbon dioxide as a working fluid to increase the efficiency of the Brayton cycle. But why has this become such a focus all of a sudden?

The first reason is the economical benefit. The higher the efficiency of the cycle is, the less fuel must be burned to obtain the same power generation. Additionally, the smaller the amount of fuel burned, the fewer emission. Therefore, the increase in efficiency also positively affects the environmental situation. Also, by lowering the temperature of the discharged gases, it is possible to install additional equipment to clean exhaust gases further reducing pollution.

So how does all of this come together? Figure 1 demonstrates a Supercritical CO2 power cycle with heating by flue gases modeled in AxCYCLE™. This installation is designed to utilize waste heat after some kind of technological process. The thermal potential of the exhaust gases is quite high (temperature 800° C). Therefore, at the exit from the technological installation, a Supercritical CO2 cycle was added to generate electrical energy. It should be noted: if the thermal potential of waste gases is much lower, HRSG can be used. More information on HRSG here:

Any cycle of a power turbine installation should consist of at least 4 elements : 2 elements for changing the pressure of the working fluid (turbine and compressor) and 2 elements for changing the temperature of the body (heater and cooler). The cycle demonstrated in Figure 1 has an additional regenerator, which makes it possible to use a part of the heat of the stream after the turbine (which should be removed in the cooler) to heat the stream after the compressor. Thus, part of the heat is returned to the cycle. This increases the efficiency of the cycle, but it requires the introduction of an additional heat exchanger.

The heat exchangers used in the sCO2 cycle are of three basic types: heaters, recuperators, and coolers. Typical closed Brayton cycles using sCO2 as the working fluid require a high degree of heat recuperation.

Supercritical CO2 Power Cycle with Heating by Flue Gases
Figure 1 – Supercritical CO2 Power Cycle with Heating by Flue Gases

Having examined this scheme and examined the process in detail, we can draw the following conclusions about the advantages of this cycle which is demonstrated in Figure 2: Read More

Combined Power Cycles: What Are They and How Are They Pushing the Efficiency Envelope?

Combined cycle power plants have introduced a significant increase in efficiency compared to simple cycle power plants. But what is a combined cycle power plant and how does it work?

What is a Combined Cycle Power Plant?

In simple terms, a combined cycle power plant is a combination of more than one type of cycle to produce energy. A combined cycle plant consists of a topping and bottoming cycle with the objective to maximize the energy utilization of the fuel. The topping cycle normally is a Brayton cycle based gas turbine while the bottoming cycle is a Rankine cycle based steam turbine.

Gas turbines are used because this equipment can very efficiently convert gas fuels to electricity with the choice of using different fuels. Recently, the simple cycle efficiencies of gas turbines have improved considerably. As an example, standard fossil fired Rankine cycles with conventional boilers have an efficiency in the range 40–47% depending on whether they are based on supercritical or ultra-supercritical technology. By utilizing waste heat from the heat recovery of the steam generators to produce additional electricity, the combined efficiency of the example power plant would increase to 60% or more. Combined cycles are the first choice if the goal is to generate maximal energy for a unit amount of fuel that is burnt.

Why Don’t all Power Plants Use Combined Cycles

You might be wondering why not all plants are based on the combined cycle. The primary reason is fuel availability. Not all regions are blessed with the availability of gas that can be easily utilized in a gas turbine. Transporting gas from one location to another, or converting a fuel to gas specifically for operating a gas turbine, may not be the best economic decision. The technological expertise required in maintaining a gas turbine is another challenge faced by gas turbine operators. A typical combined cycle plant is presented in Figure 1.

Schematic of a combined cycle power plant created in AxCYCLE
Figure 1. Schematic of a Combined Cycle Power Plant Created in AxCYCLE

The key component of the combined cycle power plant apart from the turbines is heat recovery steam generator (HRSG). The major objective is to convert maximal heat from the exhaust gas of the gas turbine into steam for the steam turbine. The HRSG, unlike the conventional boiler, will operate at a lower temperature and is not subject to the same temperature as the boiler furnace. The exhaust gas from the gas turbine is directed through the tubes of the HRSG wherein water flowing through these tubes, observes heat and converts into steam. The temperature of the live steam is in the range of 420 to 580 C with exhaust gas temperatures from the gas turbine in the range of 450 to 650 C. A supplementary burner could be included in the HRSG, but adding a supplementary burner reduces the overall cycle efficiency. Read More

Turbomachinery CFD Simulation: Art in Motion

This is an excerpt from the Siemens Blog. You can read the full version here.

Originally Written By Justin Hodges - July 14, 2020  

Turbine blade simulation juxtaposed with turbine blade art. The resemblance is uncanny! 

Believe it or not, there is some true art in turbomachinery CFD simulation. From the creamer in your coffee to the tumbling of flow through a small waterway. There is something palpable with intrigue when observing fluid flows in our everyday routines. As computational fluid dynamics practitioners, we are fortunate to have a unique opportunity. That is to simulate and observe these same curious fundamentals of turbulence and fluid flow until our heart is content.

Read More

Vertical Pumps: What Are They, Where Are They Used and How To Design Them?


Vertical pump designs are similar to conventional pumps, with some unique differences in their applications.  Pumps use centrifugal force to convert mechanical energy into kinetic energy and increase the pressure of the liquid. Vertical pumps move liquids in the vertical direction upwards through a pipe. All pumps pressurize liquids, which are mostly incompressible. Unlike compressible gases, it is impossible to compress liquids, therefore the volumetric flow rate can not be reduced. Therefore liquids are transported by pumping and the inlet volume flow rate is equal to the exit volume flow rate.

Vertical centrifugal pumps are simply designed machines, and have similarities to their horizontal counterparts. A casing called a volute contains an impeller mounted perpendicularly on an upright (vertical) rotating shaft. The electric drive motor uses its mechanical energy to turn the pump impeller with blades, and imparts kinetic energy to the liquid as it begins to rotate. These pumps can be single stage or multistage with several in-line stages mounted in series.

The centrifugal force through the impeller rotor causes the liquid and any particulates within the liquid to move radially outward, away from the impeller center of rotation at high tangential velocity. The swirling flow at the exit of the impeller is then channeled into a diffusion system which can be a volute or collector, which diffuses the high velocity flow and converts the velocity into high pressure. In vertical pumps, the high exit pressure enables the liquid to be pumped to high vertical locations. Thus the pump exit pressure force is utilized to lift the liquid to high levels, and usually at high residual pressure even at the pipe discharge.

Applications of Vertical Pumps

An “in line” vertical pump is illustrated in Figure 1 (Reference 1), where the flow enters horizontally and exits horizontally and can be mounted such that the center line of the inlet and discharge pipes are in line with each other.  This is a centrifugal pump with a tangential scroll at the inlet that redirects the flow by 90 degrees and distributes it circumferentially and in the axial direction into the impeller eye. The discharge is a simple volute that collects the tangential flow from the impeller exit, and redirects it into the radial direction.

in line Pump - Figure 1
An “in line” Vertical Pump. Source

Figure 2 shows a vertical pump that has a vertical intake that directs the flow straight into the eye of the pump rotor. At the impeller exit, the tangential flow is collected by a volute and diffused in an exit cone. An elbow after the exit cone redirects the flow into the vertical direction to lift the liquid to the desired altitude. (Reference 2). Read More

Back to Basics: What Makes a Good Pump?

Everyone is familiar with pumps, but how many people really think about how much depends on this ubiquitous invention? The scope of pump applications is wide: distribution and circulation of water in water supply and heat supply systems, irrigation in agriculture, in the oil industry, in fire extinguishing systems, etc.

A pump is a hydraulic machine designed to move fluid and impart energy to it. A schematic diagram of a simple pumping unit is presented below.

Figure 1 Pumping Unit Diagram
Figure 1: Pumping Unit Diagram
1 – intake valve; 2 – suction pipeline; 3 – vacuum gauge; 4 – pump; 5 – manometer; 6 – check valve; 7 – gate valve; 8 – pressure pipeline

Positive Displacement and Dynamic Pumps

According to the principle of operation, pumps can be divided into two main groups: positive displacement and dynamic. In positive displacement pumps, a certain volume of the pumped liquid is cut off and moved from the inlet to the pressure head, where additional energy is supplied to it. In pumps with dynamic action, the increase in energy occurs due to the interaction of the liquid with a rotating working body.

The most widely used pumps are centrifugal pumps which are of the dynamic type. The principle of centrifugal pumps uses a rotating impeller to create a vacuum in order to move the fluid. The impeller rotates within the housing and reduces pressure at the inlet. This motion then drives fluid to the outside of the pump’s housing, which increases the pressure.

These pumps benefit from a simple design and lower maintenance requirements and costs. This makes them suited to applications where the pump is used often or continuously run.

Figure 2 Centrifugal Pump
Figure 2a: Centrifugal Pump
Centrifugal Pump Designed using AxSTREAM
Figure 2b: Centrifugal Pump Designed using AxSTREAM

In most cases, the pumps are electrically driven, but if the pump is of high power and high speed, then these pumps are driven by steam turbines. Read More

Pump Rotor Dynamics – from Residential Pools and Human Hearts to Heavy Duty Industry Applications

You rarely find a rotary machine with a wider range of applications than pumps. These machines acting in a single role can be installed both to supply the water to a garden pool and move the crude oil in pipelines.

And even more, the same simple pump can substitute the functions of the human heart by moving the blood through it.

Fig. 1 - Left ventricular assist device - a tiny pump moving the blood in the human body
Fig. 1 – Left ventricular assist device – a tiny pump moving the blood in the human body [1]
Although the heavy duty industry applications of pumps are less delicate at first sight, they can still generate similar effects of this unique nature which is inherent only to this type of machine and should be studied carefully when executing rotor dynamics calculations. Read More