In recent days, many people find themselves spending time and resources on uncovering the best solution to optimize the power generation cycle. Until recently, 80% of power plants worldwide (whether fossil fuel, nuclear, or clean technology) used steam as its main working fluid and while it is still the most common option, today’s power plants are finding another fluid to use.
Although supercritical CO2 study began in the 1940’s, it was disregarded as an alternative fluid option because it was expensive to explore and steam was still perfectly reliable at the time. Nowadays due to increasing quantity and quality demand in power, researchers are looking into the possibility of replacing steam with supercritical carbon dioxide. The discover of this property, increases the incentive of exploring the technology further. This year, the US Department of Energy is awarding up to $80 million towards projects to build and operate a supercritical CO2 plant.
Steam turbines are not just restricted to conventional or nuclear power plants, they are widely used in combined cycle power plants, concentrated solar thermal plants and also geothermal power plants. The operational requirements of a steam turbine in the combined cycle and CSP’s means that they operate under transient conditions. Even in conventional steam turbines, the market requirements are changing with requirements for faster and more frequent start-up which can result into faster deterioration of the equipment and reduced lifespan. During the startup phase, significant heat exchange takes place between the steam and the structural components that include the valves, rotor and casing. The accuracy of the life prediction is strongly affected and dependent on the accuracy of the transient thermal state prediction .
Though the expansion of steam takes place in the nozzles and blades, the influence of the leakage steam during the startup phase is significant with steam expanding through the labyrinths resulting in expansions, condensation, and increased velocities which may even reach supersonic levels. During cold start, the flow is minimal, the temperature of the metal is at room temperature and heat exchange happens between the steam and metal parts resulting in thermal stress.
One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio. Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1). This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.
For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds. This results in a more responsive engine and overall higher boosts at lower RPMs. However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system. This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine. The same effect is present for larger turbochargers, only in reverse. They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.
In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo. One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series. The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.
To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of the gas turbine.
With this in mind, during the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.
As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate. Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness. Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.
There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces. Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow. Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces. Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.
When deciding on a new product line, manufacturers of turbomachines and their engineering teams must often decide whether to rescale a product that they already manufacture or to begin a full design process for a completely new machine. For example, a producer of 5 MW axial turbines wants to start manufacturing 10 MW turbines, does it make sense to create a brand new design from scratch or to simply scale up the 5 MW turbine they already produce to a similar 10 MW version? To answer this question, many considerations have to be taken into account, the general answer however is, that it is almost always a better idea to start a new design.
Improved Design Technology
Many manufacturers wrongly believe that by simply scaling their current product that they will save not only on design costs, but that they can leverage their existing manufacturing capabilities to stamp out a similar product. What is not factored in however is the progress of design technology and theory since their original machine was first conceptualized. The result from a simple scaling process will simply be a less optimized and efficient machine for any use as compared to a new configuration using the latest in design software. Increasing software sophistication and computing power are constantly pushing the boundaries of efficiency while minimizing operating costs. Simply put, your competitors will have designed a superior product compared to yours.
AxSTREAM 3D Blade Design Software
When was your current machine designed? Many older machines were created using materials that by today’s standards are simply not capable of operating at the extreme conditions (mostly temperatures) required today to attain the energy efficiency requirements set up by ever increasing regulations. Depending on materials used, the optimal blading structure, bearings, etc. geometries would be significantly unique. If one were to simply scale up their current product, they would either be using old materials or have inefficiently designed machine components for a different material. In either case, their scaled machine will be inferior to a configuration that was conceptualized and optimized from scratch.
Another very significant aspect of machine resizing is that it is not a straight forward process; if you want to double your power generation in a turbine for example you are not going to be doubling the blade size or mean diameter, for example, even when considering the same boundary conditions (inlet pressure and temperature, as well as, outlet pressure, rotation speed, and so on). For each specific set of conditions, fluid, rotation speed, mass flow rate, etc. a unique flow occurs inside the different blades. Changing one parameter will lead to changes in the flow and therefore result in inefficiencies, as it is what happens in off-design conditions (the machine is not operating at its maximum performance). This is why flow similarity parameters become relevant.
Machine Purpose and Type
One of the obvious questions to ask is, what is the purpose of my new machine and how much larger (or smaller) will I need it to be? If the new machine is intended for use with a completely different fluid, a new design will be optimal as different fluids interact in unique ways with varied rotor and stator configurations.
The machine type that you are considering is also critical to the decision. Different turbomachines do not scale in similar fashion with increase in size. For instance, radial turbines are usually not as efficient as axial turbines when one starts to approach the 2 MW range. In this instance the ideal solution is for a complete redesign since a smaller scale version that the manufacturer may have had would not be configured to operate at higher power ranges efficiently.
One of the most prominent steps of complete gas turbine design is turbine-compressor matching. There are three major components to a gas turbine: compressor, combustor, and turbine. Although all of the components are designed individually, each of the components needs to correspond within the same operating condition range since all are integrated into one cycle. Consequently, an optimal design of each component must fit the requirement of other component’s optimal parameters. Corresponding operating points for each component must be found at equilibrium with the engine, thus the overall performance of gas turbine can be reached within the defined range of parameters.
The idea behind component “matching” process is to find flow and work compatibility between corresponding components. Based on the mechanical constraints, gas generator speed and firing temperature of a gas turbine have limitations depending on: ambient temperature, accessory load and engine geometry. The match temperature chosen should be the ambient temperature which reach both upper limits at the same time. Pressure ratio needed to allow a certain gas flow is also one of the most prominent parameters that has to be taken into consideration. Designers need to make sure that the gas flow through the power turbine from gas generator satisfy the pressure ratio needed for compressor power requirements. Gas generator can easily show an altered match temperature due to some conditions i.e: reduction in compressor efficiency (due to fouling, etc), change of thermodynamic properties of combustion product, gas fuel with lower or higher hearing value, etc. Match parameters of an engine could also be altered by changing the flow characteristics on the first turbine nozzle.
Using characteristic map/curve as well as thermodynamic relationships of turbine and compressor, calculations can be performed to identify the permitted operating range. It must be taken into consideration that all calculated value must match the value from map data.
Trying to find the fastest solution for this step? SoftInWay’s turbine-compressor matching feature in AxSTREAM could help you cut engineering time and simplify the process. Combining performance maps of turbine and compressor, making it easier for the user to determine points of joints operations.
Take a look into AxSTREAM’s to learn more about this.
At the beginning of my studying of the peculiarities of supercritical CO2 (S-CO2) cycle I was wondering: why do scientists involved in this area state that highest temperature limit for the cycle is about 650-700 ˚C. In turn, the inlet temperature in the first stages of gas turbines handles the temperatures about 900 ˚C without cooling at similar pressure levels as for supercritical CO2 Turbines. As a result the following question rose in my mind – why the temperature magnitudes of 900 ˚C are not achievable in S-CO2 turbines?
As a next step, some investigations were performed with the aim to reveal the essence of such a temperature limit. Eventually the result was quite obvious but rather interesting. The density of S-CO2 is significantly higher than the density of combustion products at the same pressure and temperature magnitudes. This fact means that stresses at static vanes and rotating blades are significantly higher than in gas turbines vanes and blades at the same conditions. Therefore the maximum allowable temperature for S-CO2 turbine will be respectively less with the same high temperature material. However, you might say that there is another way to solve the problem with stresses, namely, increasing the chords of blades, leading edge thickness, trailing edge thickness, fillets etc. This approach would lead to such blades shape and turbine cascade configuration that their aerodynamic quality becomes very low so the gain in efficiency at cycle level will be leveled.
Interested in learning more about our research, and how using the AxSTREAM turbomachinery platform, we were able to study these phenomena?
Geothermal energy is known to be a reliable and sustainable energy source. As the world gives more attention to the state of the environment, people lean towards using more energy sources which have little to no impact on nature. Where it is true that currently no other energy source can outperform fossil fuel due to its energy concentration, geothermal energy is a good prospect as a temporary substitute until a better form of energy supply is found.
There are two types of geothermal power sources; one is known as the steam plant and the other is the Binary cycle. Binary cycles have the conceptual objectives of: high efficiency — minimizing losses; low cost to optimize component design; and critical choice of working fluid. This particular type of cycle allows cooler geothermal supply to be used, which has a huge benefit since lower temperature resources are much more common in nature.
The way a binary cycle works can be explained using the diagram shown above. Since the temperature of geothermal source is not high enough to produce steam, hot water is fed into a heat exchanger. From there, secondary liquid with lower boiling water than water i.e. isobutane, absorbs the heat generated. As the steam of secondary liquid moves the turbine, electricity will then be produced. This whole process repeats in a cycle since the secondary fluid will then condense back to its liquid state and being used for the same process.
From the process described above, it can be seen that binary cycle is a self-contained cycle — ‘nothing’ goes to waste. This fact leads to the potential of having low producing cost energy source from binary power cycle. That being said, due to the lower temperature, the conversion efficiency of the geothermal heat is also considerably low. Consequently, Carnot efficiency of such process is lower than most power cycles. Large amount of heat is required to operate a binary cycle, leading to a better and larger equipment. Not only that since a bigger amount of heat energy has to be let out to the environment during the cycle, a sufficient cooling system must be installed. Although the production cost is found to be lower, the investment cost for installation would be very expensive. Then, the main question to this particular technology implementation would be how to improve the quality of production and economic feasibility?
First, one of the main aspect of binary power cycle is to overcome water imperfection as a main fluid. Consequently choosing optimal working fluid is a very essential step. Characteristic of optimal working fluids would include a high critical temperature and maximum pressure, lower triple-point temperature, sufficient condenser pressure, high vaporization enthalpy, and other properties.
Second, it was studied on multiple different events that well-optimized ORCs perform better than Kalina cycles. The type of components chosen in the cycle also affect the cycle performance quite substantially, i.e plate heat exchanger was found to perform better in an ORC cycle in the geothermal binary application compared to shell-and-tube. Addition of recuperator or turbine bleeding also have the potency to improve the overall performance of a binary cycle plant. It is important to model multiple thermodynamic cycle to make sure that the chosen one is the most optimized based on the boundary conditions. While designing ranges of thermodynamic cycles, it is common that the cycle is modeled based on ideal assumptions. For binary cycle in geothermal application, plant efficiency would be the most important parameter. In order to achieve a desired plant efficiency, both cycle efficiency and plant effectiveness should be maximized.
Additionally, pinch-point-temperature between condenser and heat exchanger is a substantial aspect to pay attention to, even the smallest change of in temperature is considered a significant change. Thus, including this parameter is a very important aspect.
This particular cycle has many potentials which haven’t been explored. Enhance the advantages of your binary power cycle using our thermodynamic tool, AxCYCLE.
Almost every car produced nowadays is propelled by a Reciprocating Internal Combustion Engine (RICE). Fueled by gasoline or diesel, these engines have pistons inside the cylinders which move up and down, compressing and expanding the mixture. They are connected to a crankshaft that converts the movements into a rotational motion to turn the wheels that move the car.
Big engine makers are constantly researching and developing to make engines lighter, more powerful, more fuel efficient, and more environmentally friendly. But isn’t there a better way to power the automobile Industry?
After WWII, the gas turbine (GT) engine (turbojet) was a trend for aircraft propulsion. A few companies did some research and explored the idea of using a GT to power a car. The GTs mentioned here are evidently not turbojets, but turboshafts where almost any power is used from exhaust. Instead there is a power turbine activated by the combustion gases that would be connected to a gearbox and consequently to the wheels.
The first company to ever build a GT car was Rover in 1950 with the JET 1. A few years later GM also built a number of futuristic prototypes called the Firebirds.
While some companies came up with GT cars, it was Chrysler that invested the most in this concept, spending a lot of time and money doing R&D for almost 20 years (from 1950 to 1970).
For the first time ever in 1963, more than just a prototype came out and fifty-five cars were built and given to people to try as a daily mode of transport. Although reviews were generally good, the project did not go any further.
The car used the A-831 GT, a dual spool, and free shaft engine with an output of 130 horse power, weighing 410 lbs. It comprised a single stage centrifugal compressor rotating at a maximum of 44,600 rpm (CR=4:1), the air, after leaving the compressor, would go through 2 regenerators working as heat exchangers using hot gases from the exhaust to increase temperature before the combustion to reduce fuel consumption. From the combustion chamber, the gases travelled by a single stage axial turbine that activated the compressor and the accessories and posteriorly through a variable geometry power turbine nozzle, to control the amount of gas that would go through, before the free single stage axial power turbine that was connected to a Torqueflit, 3 speed automatic transmission.
Chrysler ended up destroying all but nine of the cars. Today they are in museums or in Jay Leno’s garage.
Why didn’t a car with a well-reviewed engine and a futuristic concept stick? Why are GTs present in so many industries but not in Automotive? They’re faster, simpler, have a better power-to-weight ratio and require less maintenance.
While they have advantages, however, they also have some disadvantages. Some of the Chrysler car users mentioned a lack of engine brake, lack of support when maintenance was needed and noise. This could easily be solved, and Chrysler did fix some of this issues. What ultimately killed the project was the low throttle response in comparison to RICE and fuel consumption. GTs are very fuel efficient for high speeds with constant throttle, but cars operate at relativity low speeds with a big vary of throttle. This has a big impact in the GT fuel efficiency. Although the company tried to resolve this issue, the 1970’s oil crisis made the scenario even worse.
It’s possible that soon electric hybrid vehicles will mean the GT finally becomes a viable power source for cars. Whether braking or accelerating, the micro gas turbine runs at a relatively constant rpm and generates electricity to be stored in batteries. Those batteries are connected to electric motors (4 in the Jaguar C-X75 case, one on each wheel) that run the car. Two known prototypes are the Jaguar C-X75 using two 70kW micro turbines produced by Bladon Jets, and the Capstone CMT 380 using a single 30 kW micro gas turbine