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The fields of energy conversion and propulsion systems stand at the forefront of technological innovation, driving advancements in transportation, aerospace, and sustainable energy solutions. In the quest for optimized performance and efficiency, researchers are increasingly turning to holistic modeling approaches that bridge the gap between 0D and 3D simulations. The integration of simulation levels offers a more comprehensive understanding of complex systems by combining simplified, yet accurate, 0D models with highly detailed 3D representations. In this exploration of holistic modeling, we’ll look into how combining 0D and 3D perspectives is essential for designing, analyzing, and optimizing various systems such as steam power plants and rocket propulsion.
Holistic modeling combines simplified 0D models with detailed 3D simulations, allowing engineers and scientists to get a complete picture of how different components interact within systems. This approach considers both the big picture and the finer details, providing a more faithful view of how systems work in the real world.
Energy conversion and propulsion systems operate as thermodynamic cycles, which engineers must design, evaluate, and optimize. The engineer’s process requires simulation of how a new or existing system and its components perform under different conditions. However, given the sheer multitude of components involved in such cycles, software tools are absolutely necessary for studying cycle operation and component interaction. AxSTREAM System Simulation is SoftInWay’s software tool that enables engineers to study and optimize cycles using methods such as Design of Experiment and Monte Carlo. The platform provides a virtual lab where one can tweak and test different setups to find the most desirable ones, both at steady states and in transient conditions. Read More
A heat pump serves as an alternative to gas or electric boilers, relying on the production of heat. Unlike boilers, a heat pump doesn’t generate heat but extracts energy from the air, water, or ground.
Figure 1 – Example of Heat Pump Installation. Source.
Heat pumps and electric boilers both draw power from the mains electricity supply, yet heat pumps exhibit higher efficiency. This efficiency is contingent upon the conversion efficiency, measured by the Coefficient of Performance (COP), of a specific heat pump. The COP represents the ratio of heat energy received to the electricity consumed, particularly in the operation of the pump’s compressor unit. Notably, a heat pump consumes 3-6 times less electricity than an electric boiler with the same output.
Even in challenging conditions, such as an outside air temperature of -25°C, heat pumps excel in providing heating. Simultaneously, they achieve a high COP – generating 2-5 kW of heat or cold (depending on the type of heat pump) per 1 kW of electricity. This starkly contrasts the lower efficiency of gas and electric boilers.
Heat Pump Use Potential
The economic (rising energy costs) and environmental (effects of climate change) aspects of heat pumps should also be noted when discussing heat pumps. Heat pumps make it possible to utilize renewable heat resources such as geothermal, solar thermal energy and recovered heat from the urban environment. In addition, heat pumps maximize the decarbonization potential of renewable electricity sources (such as wind and solar) by converting them into renewable heat. In combination with thermal storage and electric boilers, heat pumps provide flexibility and security to the building life-support system, offering daily, weekly, and seasonal flexibility. Read More
The open Brayton cycle is commonly used in gas turbine engines for power generation, aircraft propulsion, and industrial processes. The Brayton cycle, named after American engineer George Brayton, who proposed it in 1872, converts fuel energy into mechanical work. The Brayton cycle became widely used in practical applications following the development of the gas turbine in the 1930s. Gas turbines were used extensively during World War II, and their efficiency and reliability improved significantly.
The Brayton cycle has numerous advantages for energy applications. For one, engineers can design it compactly, making it a strong fit for tight enclosures. With gas turbines being a common engine driver, the Brayton cycle is popular in modern applications. Since Brayton cycle engines are particularly efficient in gas turbine engines, they are useful for power generation, aircraft and marine propulsion, industrial processes, and more.
Additionally, Brayton cycle motors produce few emissions thanks to their efficient exhaust gas treatment systems, making them a greener choice than other fossil fuel systems. Coupled with their high efficiency from the isentropic compression and expansion processes that minimize energy loss, engineers continue to develop Brayton thermodynamic cycle systems to this day.
There are two main types of a Brayton cycle: open and closed. In an open Brayton cycle, the working fluid is continuously supplied to and exhausted from the system, and the process operates in an open loop. On the other hand, in a closed Brayton cycle, the working fluid is contained within the system and circulates in a closed loop. The schematics of a simple open-cycle gas turbine utilizing an internal-combustion process and a simple closed-cycle gas turbine using heat transfer processes are both shown in figure 1.
Figure 1: Open cycle (left) and closed cycle (right) for operating a Brayton cycle (https://web.mit.edu).
An ideal Brayton cycle consists of the following four processes:
Isentropic compression: air is compressed to high pressure and temperature via an isentropic process that is adiabatic and reversible.
Constant pressure heat addition: high-pressure air is fed into a combustion chamber where fuel is added and ignited.
Isentropic expansion: high-temperature gases from the combustion chamber expand in the turbine in an isentropic process, producing mechanical work.
Constant pressure heat rejection: the exhaust gases are expelled into the atmosphere.
Renewable energy is a topic which has gained significant traction in recent years. Unlike fossil fuels, which are finite and contribute to environmental degradation, renewable energy provides a cleaner, healthier, and more sustainable path forward for meeting our energy needs. Energy storage systems refer to technologies that store energy for later use, allowing for a more flexible and reliable energy supply from renewable sources such as solar and wind.
There are a wide variety of energy storage systems that enhance the power generation capabilities of renewable power plants. The most familiar system may be hydropower, with over 95% of today’s energy grid storage being held by pumped hydropower. When electric demand is low, “turbines pump water to an elevated reservoir using excess electricity. When electricity demand is high, the reservoir opens to allow the retained water to flow through turbines and produce electricity” [5]. Thanks to its performance, pumped hydropower has dominated the energy grid storage market for years. However, other emerging technologies are gaining notoriety, including compressed air energy storage, which will be the topic of today’s blog.
Have you ever wondered what happens to the air when you blow up a balloon? Well, some clever people have figured out how to use that air to store electricity. It’s called compressed air energy storage (CAES), and it’s basically like having a giant balloon underground that you can fill up with air when you have extra electricity and let it out when you need more. Sounds simple, right? Well, not quite. Some challenges are involved, like keeping the air from getting too hot or cold, and making sure it doesn’t leak or explode. But if done right, CAES systems can help us use more renewable energy sources like wind and solar, and reduce our dependence on fossil fuels.
Fig – 1. Green Energy Conservation [4]Compressed air energy storage is a technology that stores excess electricity as compressed air in underground reservoirs or containers. When electricity is needed, the compressed air is heated and expanded to drive a turbine and generate power. CAES can help balance the supply and demand of electricity, especially from intermittent renewable sources like wind and solar. There are two main types of CAES: diabatic and adiabatic. Diabatic CAES dissipates some of the heat generated during compression to the atmosphere and uses natural gas or other fuels to reheat the air before expansion. Adiabatic CAES stores the heat of compression in a thermal storage system and uses it to reheat the air without additional fuel. Adiabatic CAES has higher efficiency and lower emissions than diabatic CAES but also higher costs and technical challenges. Read More
As human-beings, our differences are what makes us unique (if I may quote the Seek Discomfort crew – “What makes you different is what makes you beautiful”). For turbomachines, this sentiment also rings true. We design different turbomachines because we have varied roles, needs and constraints for them. To that effect, there is no universally best turbine, compressor, or pump. Therefore, figuring out which set of “skills” a turbomachine should have is the key role of a design engineer so that they may effectively capture and estimate performances of the machine they will work on early on while having the certitude this is the best that can be done.
Generative design is one of these recent buzzwords that characterizes an approach to the design of components (or systems) that has been around for quite some time already. Rather than producing one geometry for one value of each input (such as boundary conditions, flow coefficients, number of stages, etc.), generative design allows you to create thousands of designs within minutes that you can review, compare, and filter to select the one that best suits your needs. Let’s look at an example of an axial turbine design process comparing traditional preliminary design vs. generative design.
Approach 1 or what most companies call Traditional Preliminary Design, is to look in textbooks and previous examples of what a given turbine for that application “should” look like. It may involve things like using Ns-Ds diagrams, load-to-flow diagrams, blade speed ratio vs. isentropic velocity ratio correlations, scaling/trimming existing designs, etc. These have served their purpose well enough, but they have their limitations which make them fairly challenging to really innovate. Such limitations include previous experience/data being restricted to a given fluid, relative clearance size, given configuration, lack of secondary flows, etc. A summary of a traditional preliminary design workflow (familiar to too many engineers) is presented below.
Figure 1 Summary of traditional preliminary design workflow
Now, we know that changing (ahem, improving) your workflow is not always easy. But growth happens through discomfort and switching to a generative design approach does NOT mean rebuilding everything your team has done in the past. What it effectively gives you is the confidence that the input parameters you finalized will provide not only the desired performance but the best ones that can be achieved (and it saves time too…a lot of time). From there, you can use these inputs in your current design software or you can continue the design process in our design platform, AxSTREAM® (meaning you can add generative design capabilities upstream of your existing workflow or replace parts/all of that workflow depending on what makes the most sense for you). You can pay your engineers to do engineering work, instead of visiting online libraries and guessing input parameters in hope they will find the needle in the haystack. Or, with generative design, you kind of look for haystacks and shake them until the needle falls off.
So, how does this work in AxSTREAM, you may ask? Very well, I may reply :D. Read More
Update – February 28, 2023: AxCYCLE is our legacy software and is replaced by AxSTREAM System Simulation. System Simulation was born out of the union of the legacy AxCYCLE and AxSTREAM NET software packages.
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”.
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
Despite the deepening understanding of the essence of gas-dynamic processes and the development of computational methods, simpler design methods such as scaling and trimming remain in demand in turbomachinery engineering. The main advantage of these approaches over design from scratch is simplicity and its inexpensive nature due to the small-time expenditure and lower demand from computational resources. Good predictive accuracy of the performance and efficiency of the resulting machines is based on the use of an existing machine with well-known characteristics as a prototype.
Conversely, using the prototype imposes restrictions on the use of scaling and trimming methods. It is almost impossible to get a new design with pressure and efficiency higher than that of the prototype. Also, in cases where it is required to obtain performance that is significantly different from the prototype, the inherent reliability of the original prediction may be insufficient.
Scaling Method
Easy to apply and general, valid, scaling laws are needed for design and application engineers. The scaling laws are needed for the purposes of:
Predicting the full-scale performance machine from model test data obtained from a scaled machine
Obtaining a family of machines with different performances on the basis of one well-tested machine
Experimental performance and efficiency testing on a full-size model of large machines such as fans to ventilate tunnels and mines or to move combustion air and smoke gas in power plants may be impractical due to the high energy costs and geometric limitations of the experimental stand. In these cases, a scale model is used. And although complete similarity is not maintained, for example, in terms of the Reynolds number, the correction factors in most cases are well known and the prediction accuracy is high.
The method involves the implementation of the flow path of the designed fan or compressor on a scale to the prototype. This means that all linear dimensions (e.g. diameters, blade chord, axial length, etc.) must be multiplied by the scaling factor (SF). The angular dimensions (e.g. blade angles at inlet and outlet, stagger angle, etc.) remain unchanged.
When scaling, it is assumed that parameters such as Pressure Ratio, circumferential velocity (U), and axial velocity (Cz) are equal for the designed machine and the prototype. Thus:
The condition of equality of the Reynolds criteria is not ensured, since the designed compressor and the prototype do not have the same diameters of the rotor with the equality of other parameters that determine the number of Rew. This design guarantees the practical accuracy of the calculated characteristics, provided that the gas movement in the flow path is turbulent. It is known that for “physical” values of the Reynolds numbers
the flow remains turbulent and the inequality of the Reynolds numbers of the designed compressor and the prototype has little effect on the gas-dynamic characteristics.
To determine the efficiency of low-pressure fans, a well-known formula is usually used:
An example of obtaining a stage of an axial compressor by the scaling method is shown in Figure 1.
Figure 1 – Axial Compressor Stage Scaling
The disadvantages of the scaling method include the need to change the rotor speed. This can be relevant for industrial installations, where the rotation speed is often limited and tied to the frequency of the electrical network current. Additionally, the need to change the overall dimensions can be a limiting factor, especially if it is necessary to increase productivity significantly, and the installation location for the turbomachine is limited. In some cases, maintaining full geometric similarity is impossible for technological or constructive reasons. For example, the minimum value of the tip clearance may be limited by the operating conditions of the rotor (not touching the rotor against the housing) or the impossibility of obtaining a small clearance if the scaling is carried out from a large prototype to a small model.
Due to concerns about air travel’s impact on climate change, research and development into electric aircraft has been ongoing for several years. Within the last decade several startups as well as larger corporations have been developing electric or hybrid electric aircraft (Ros, 2017). The ultimate goal is to Conduct long (>500 miles), full-electric commercial flights with large aircrafts capable of carrying 100’s of passengers, but this will require at least 5-10 more years of development. Luckily, smaller electric aircraft designed for short-range flights (<500 miles) with anywhere from 1-20 passengers have already been tested successfully utilizing electric batteries, a hybrid-electric system and even a hydrogen fuel cell. With these advances, emission-free air travel is closer than you think.
Electric Aircraft
Examples of full-electric aircraft designs include the Airbus E-Fan 1.0 and E-Fan 1.1 (Airbus Group), shown in Figure 1. These two-person aircraft utilize two ducted, variable-pitch fans, shown in Figure 2. Each fan is powered by a 30-kW electric motor. The motors are powered by several lithium-ion battery packs stored in the wings. While the aircraft only provides an hour of flight time, the batteries can recharge in approximately one hour and can be easily be swapped in and out.
There are several reasons besides climate change why electric aircraft should be developed from a business perspective (Figure 3). Short and mid-range regional flights make up a significant portion of all flights around the world. The current flight range of electric aircraft is limited to these short and mid-range fights. Additionally, shorter flights spend relatively more time taking off and landing than cruising at high altitudes, which makes shorter trips less energy efficient. While short, regional flights are economically unattractive for large commercial aircraft, a smaller aircraft with less fuel consumption may provide a valuable alternative. Read More
As we covered in our previous blog about fuel cell systems, a large contributor to their efficiency is the compressor that is selected for it. But what are the different kinds of compressors, and which one is best for a specific system?
Compressors have a wide variety of designs and types, which differ in pressure and performance, depending on the kind of compressed fluid. Compressors are also classified according to the type of work: dynamic and positive displacement. Figure 1 shows the types and classification of compressors.
Figure 1: Compressor Types. Source: Dongdong Zhao, “Control of an ultrahigh-speed centrifugal compressor for the air management of fuel cell systems” 5 Jun 2014, p. 8.
Figure 2 shows a comparison of various types of compressors according to several criteria: generated pressures, occupied volume, lubrication requirements, compressor weight, and pressure ripples at the outlet.
Figure 2: Comparison of Compressors. Source: Dongdong Zhao, “Control of an ultrahigh-speed centrifugal compressor for the air management of fuel cell systems” 5 June 2014, p. 13.
As can be seen from the comparison above, we can conclude that centrifugal compressors offer a number of advantages over its positive displacement counterparts:
Lightweight;
Small volume;
Only the bearings require lubrication;
Creates a sufficiently high pressure (1.5…6 bar);
Has high efficiency (80…82%); and
Has a fairly wide performance range.
Next, we will consider the application of the centrifugal compressor in the fuel cell system. Read More
Alternative energy based on the use of fuel cells is gaining more and more popularity and is increasingly being used in the automotive, aerospace, and energy industries as well as other sectors of the economy.
What is a Fuel Cell?
Fuel cells (FC) are electrochemical devices which convert the chemical energy of a fuel directly into usable energy – electricity and heat – without combustion. This is quite different from most electricity-generating devices (e.g., steam turbines, gas turbines, reciprocating engines), which first convert the chemical energy of a fuel to thermal energy via combustion, then into mechanical energy, and finally to electricity.
Fuel cells are similar to batteries containing electrodes and electrolytic materials to accomplish the electrochemical production of electricity. Batteries store chemical energy in an electrolyte and convert it to electricity on demand until the chemical energy has been depleted.
Fuel cells do not store chemical energy. Rather, they convert the chemical energy of a fuel into electricity. Thus fuel cells do not need recharging, and can continuously produce electricity as long as fuel and an oxidizer are supplied.
Today, there are two types of electrolytes used in fuel cells: acid or alkali. The type also depends on the chemical reactions that take place in the element itself. Read More
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