The TurboCore platform Series: TurboGenerator

The Dynamo TurboCore was developed to expedite the deployment of our flagship product, an electric power solution for powering artificial lift systems in the Oilfield—we will be piloting it this summer. Based on this product we will be able to show our partners how to design and develop their own systems around the TurboCore. Simultaneously we will be releasing our 1500 Series Turbocore, which as the name implies, is a scaled up version of the 700 series and is capable of producing over 2x the power.

TurboCore System Diagram

TurboCore System Diagram

A turbogenerator, as the name implies, is a turbine that produces electric power. The Turbogenerator is created by taking the baseline gas generator and bolting on several components. Hot pressurized air coming out of the TurboCore is converted into mechanical work through a second turbine, called the power turbine. This turbine is attached to a shaft which drives a generator. With the 1500 series TurboCore, the shaft power output for this basic turbo-generator is approximately 15kW.

Gas generators have a few key characteristics that drive their performance. One that is important here is the load line. The load line relates engine speed (RPMs) to engine pressure and airflow—and ultimately total power output. As fuel is added to the engine RPMs increases as does total power. Eventually the gas generator will reach peak power. The factors that limit fuel input and power output for a particular set of hardware include shaft speed limit, temperature limits, and compressor stall limits The limiting factor in any particular situation depends on the full system design and the expected operating environment.

Adding a power turbine is tricky business. Just as turbocharging a traditional car engine changes how it behaves, adding a power turbine requires significant design considerations. This is because adding a power turbine adds flow restriction and back pressure to the gas generator. The increase in backpressure results in a shift in the operating line of the gas generator—ultimately impacting the gas turbine system performance and total available power.

1500 Series TC Operating Line

1500 Series TC Operating Line

When designing a gas turbine there are a lot of considerations to be made, from optimizing blade shape to optimizing the critical parameters of the gas generator for different applications. At Dynamo we’ve created a suite of tools to help us analyze gas turbine system behavior and find proper match between compressors and turbines to get the desired performance and operating characteristics. An example of how the operating line is impacted by a power turbine is shown in the figure to the left, and represents one of the more complex analyses.

These sets of tools allows Dynamo to quickly evaluate different applications and use cases for the TurboCore, including the implementation of several independent loads and their effect on system performance.  Using these tools, the Dynamo development team has repeatedly demonstrated the ability to rapidly develop solutions around the TurboCore, with design to prototype validation times under nine months.

The TurboCore platform Series: TurboCore Platform

Over the past few months the Dynamo Team has been working on deploying our first generation TurboCore 700, a turbine engine platform for the next generation of remote power products.  Our platform is designed for three things: reliability, flexibility, and modularity.  There are several ways to interface with our platform, from our software & controls API to interfacing with the physical hardware of our product—and we’ll be working to make this as seamless for designers as possible.

The reason we are doing this is simple.  With all of the resources and talent of the world at our disposal, we know that there are experts out there who know far more about their customers’ needs than we will, and we want to empower them to leverage our product.  Every week we get a request for adapting our hardware for specific application, including water desalination, flameless heat, & residential combined heat and power.  While we wish we could tackle all these problems, we can’t.  As turbine experts, we can teach you to push the envelope of imagination with our products.

At Dynamo we are working on developing a world class turbine platform. We are focusing our in-house design efforts on a unique generator solution built around this technology; we also provide these components as kits for our development partners.  But at the core of the platform is our gas generator—it’s the heart of the engine that drives the performance & flexibility of the unit.  The gas generator is designed to run across a broad range of environmental conditions, with a wide range of fuels, and to do so reliably for thousands of hours—allowing end users the freedom to employ the turbine when and where they need it.  The turbomachinery is interchangeable, allowing us to provide different gas generators sized for specific applications, at a cost that is comparable to traditional reciprocating engines found in the market today.

P1020090-BThe gas generator alone only produces hot, pressurized air, and additional components are needed to put it to work—such as the addition of heat exchangers, generator, auxiliary turbines.  We do these ourselves in building our generator product.  To facilitate the mechanical design, we offer a Hardware Development Kit, which contains everything from CAD models, drawings, specs, and reference designs for products we have worked on.

Adding components to an engine will affect its performance, and we realize that it may be difficult for those without a turbo-machinery background to understand the effects.  To make it easier to work with the TurboCore platform, we offer training and certification programs that teach you how to integrate this unique engine in a variety of products.  In some respects, providing this information is a double edged sword.  We are describing to potential competitors how we engineer and build our products, and we are giving away a wealth of applications we could build ourselves, but we taking on this risk both get our technology to those who need it and to motivate ourselves to continue to provide higher performance & more innovative turbines.

Beyond the hardware platform, we also offer a software platform and API, which allows end users to interface directly with our control system.  We build the control system to ensure that the engine can be started and operated reliably and safely; this is no trivial task, considering the broad range of fuels we expect to use, and the broad range of environments we will have to operate in.  However, our goal is to also provide end users with the ability to configure and operate the device as they see fit.  We are still working on making this system more robust, and anticipate releasing an API in a year.

This is our brief plan on getting TurboCores into as many hands as possible, and we are hoping those of you who have the will and the imagination can work with us to bring this game changing technology to the marketplace.  To demonstrate this approach, the next post will describe a reference design we put together over the summer for a customer.

The TurboCore platform Series: Building a successful platform

Building a platform technology is more than just declaring it as such.  Products do not exist as platforms for the sake of being a platform, but must have some intrinsic value, which can be built upon and ultimately enable new and superior products and services.  Platforms must also have clear points of stability and vicissitude.  Lastly, platforms must have clear processes for enabling collaboration between different parts of the ecosystem—be it internal manufacturing teams, or application developers that clearly articulates how to best leverage a platforms strengths.

The purpose of developing a platform, as indicated in the last post, is predicated on either reducing cost or building value within an ecosystem.  Regardless, the underlying product itself must have some native value, which can be leveraged and grown.  In the previous post vehicle chassis were a strong example because they were the foundation for both cost reduction and were valuable in providing the structure which ultimately housed the vehicles built from them.  In the Apple example, the mobile hardware is the platform, on top of which new applications could be built that uniquely leveraged mobility.  A platform with a unique value proposition fosters a strong ecosystem—tautologically, a strong platform begets itself.

Because the platform is really a foundation for everything that is built on top of it, there must be features of the platform which are immutable, delivering a core set of values.   This can range from the physical footprint (think x86 chipset platform) to how it is used (think Facebook).  There must also features that can be modified or tweaked in a controllable manner to allow users of the platform to adapt it to their own needs.  Often times these modifications exist beyond the physical (or digital) product produced by the source company.  In the example of vehicle chassis, variations include physical pieces of equipment added to create the final product, from the engine and drive train installed on the chassis, to the trim on the interior.  This alludes to early channels for platform technology, namely the OEM. OEMs take core pieces of hardware, like CPUs or Engine, and build a variety of products around them, such as Servers and PCs or trucks and generators—however the interface between the core technology and the whole products are the same.  The customer ultimately derives value from consuming the whole product for their specific needs, but the platform is what provides the foundational value for those end products, be it computational power or horse power.

The last key is defining how the platform interacts with the ecosystem around them.  It should be clear that the interface itself sits and dictates the boundaries of what is core to the technology, and what is mutable.  In software this can be as simple as providing an API or SDK; hardware standards are more complex, with physical properties that must be taken into account.  Regardless, these interfaces define the strength and flexibility of a platform.  If the interface is poorly defined or does not allow partners to leverage the underlying technology, the platform will wither.  Part of understanding the interface is understanding how to communicate it to ecosystem partners; depending on the complexity of the underlying platform, a simple API may be sufficient, but more sophisticated systems may require training, education, and joint technology development.  Partners themselves may need to be qualified or their products screened before being released to the world.  Building a robust process for defining that interface is an exercise in trust between the technology provider and the ecosystem, and is an integral part of building a world class platform.

The TurboCore Platform Series: What Drives Platform Development

All organizations face limits in resources, but the envelope of the possible can be drastically expanded by building on a powerful platform.  The cost of development and manufacturing can be reduced (you do not need to re-invent the wheel), and an ecosystem can be developed with partners, customers, and collaborators who will drive adoption and innovation in a product.

Platform technologies are an abstraction of a technology or product that allows other systems, products and processes to be built on top of it.  Platform technologies can be broken down into two types: external platforms and internal platforms.    Internal platforms are very common, and exist as a way to reduce manufacturing costs.  External platforms are customer and partner facing, allowing for ways to build value beyond the confines of the business.   Said another way, platforms either shrink the pie [cost] or grow the pie [value], and in a duplicative turn of phrase, can both grow and shrink the pie simultaneously.

Examples of internal platforms include designs implemented to reduce the cost of several products built by a single company.  Examples includes the VW vehicle chassis platform & Apples IPhone / IPod architecture.

VW’s vehicle chassis platform allows the company to use the same underlying components to build several models of car (think Passat, Jetta, etc) utilizing a fixed set of engines (1.4L, 2.0L, 2.4L etc), across several brand lines (VW, Seat, Audi).  The result is reduced cost, reduced employee training, and higher quality products; at the same time the components and features that truly segment the products and customers can be tailored to drive value for the business.

If you take a look at the last few generations of Apple hand held devices, the IPhone and IPod specifically, you’ll notice a few things.  A lot of the materials and components are the same, and they are arranged in the same way on the device.  Functionally the phone has superior features, including the phone, a better camera, etc.  There are also different levels of phone and ipod, typically with increasing levels of storage that further differentiate the products.  Beyond sharing design techniques this has a lot to do with using a shared manufacturing platform to build different products with different price points.

Examples of external platforms include designs implemented to grow ecosystems around a product, and to layer on to the value of the products built by the initiating company. To use parallel examples, this includes Lotus’ coupe rolling chassis platform and of course the Apple iTunes.

Lotus is a very different animal in the automotive industry, and is well known for performance cars and race heritage.  Since they do not have the same volumes of production that a company like VW, they have used their chassis platform a different way.  Most recently they lent the platform for the Lotus Elise—a solidly designed two seater chassis born from the racetrack—to other manufacturers as what they called a “rolling chassis”, meaning it had the frame, suspension, wheels, but no engine, bodywork, or other accoutrements that define a vehicle.  These products went out to GM, Citroen, and most notably Tesla for their roadster.  The result was that manufacturers were able to layer on value to the underlying platform, and Lotus could leverage their investment in design to sell more vehicles than they could have by themselves.

The platform built by Apple using iTunes and the iPhone is another example of allowing others to leverage the strengths of a product to build new and unique products and services.  This external platform has become a significant addition to the underlying hardware, and is often a point of differentiation with Apples competitors.  Furthermore, the applications built on the Apple platform extend far beyond Apple’s core competency, and control, yet Apple is still able to monetize this by sharing a percentage of sales made through the Apple Store.

There is actually a third type of platform not discussed here, but it is related to both internal and external platforms, and is built by a consortium of customers and vendors.  It is called a standard—it’s a way to take a set of products and technologies and build a common interface to allow greater value creation for the ecosystem as a whole.  Common Standards include IEEE’s 802.11, which gives us the standard for using WIFI, another is the 36” racking standard which nearly every server in the world is physically sized to.  Whether platforms are built to support the internals of a business, build layers of value above a business, or standardize an industry, it is clear that platforms are key to driving business value.

The TurboCore Platform Series: Technology Origin Story

In one of our original blog posts (What is Dynamo Series, pt 3), we alluded to the innovation being developed here at Dynamo—that we were developing a platform technology, based on turbo-machinery, to revolutionize small power products.  We are going to spend the next few months showing the strengths of this platform, but first we will give you a little background on where this technology came from.

Dynamo was founded by two turbine engineers who had looked long and hard at the status quo for building turbines.  We had firsthand experience working on the assembly lines for aircraft jet engines (in one of the first factories in the US to build jet engines, we might add).  And we can confirm that all of your assumptions around building these turbines are probably true.  Modern manufacturers work with super metals with esoteric names, like Inconel, Rene, and Waspaloy.  They have machines that are 20 feet tall and can cut complex dovetails into solid disks of nickel with greater than 0.000,01” of accuracy (we call that tenths in the industry), and they have measurement tools to match.  When you are pushing the envelope of engine performance, you need every tenth you can get.  There is significant technology innovation being developed as well to improve manufacturability and product quality, from a machine that would friction weld shafts at high speed to novel ceramic composite matrix forming technology.  A lot of work goes into building these parts; it’s not uncommon for a part to have a buy-to-fly ratio of over 90% (that means from the raw stock metal, only 10% is left over in the finished part).

As amazing this sounds, we also learned how 20th century the manufacturing process was.   For a lean assembly line, there was not much of a line.  Assemblies were put together by hand on mobile carts; the carts were moved around the factory floor to stations, where one type of work or another would be performed (e.g. welding, fastening, plumbing, etc).   As often as not, engines would move back and forth between stations depending on the exact engine that was being built.  The average time to assemble a small engine was two months.

On the parts level of manufacturing, there were other things that didn’t strike us as terribly modern.  We called our business a “lean pull” manufacturing business, but the reality was that we built components in batches, and “lean pull” just meant we kept inventory in a holding pattern depending on what the assembly team told us to deliver in the next two weeks.  We also did not have entirely fungible labor, and would spend a good deal of our planning time figuring out which machinist could make which parts on the given machines we had working that day.  This combined with a metrics-driven culture resulted in some creative accounting.  Sometimes we would build extra inventory when times were slow, just to keep labor working;  I remember a few times we would “hold” unsalvageable components that didn’t pass their drawings check for a few weeks until we could “hide” the single reject when a large batch of inventory came through so it wouldn’t impact our metrics for that week.  A large part of this seemed to be the fact that one out of ten of any batch would need to be re-worked at some point because the tolerances required by the parts were not met by the manufacturing process.

When there wasn’t a standard way to tell if a part was not conforming to the manufacturing requirements, we had to take the specimen to Al.  Al was a living library with 30+ years’ experience making components for turbines—not an engineer by training, but a master manufacturer.  His workspace, on the second floor, was filled with rejected components.  Every one or two weeks I would bring a component to Al, show him the drawings and we would describe why we thought there was a problem.  Al would gnaw his pen (which he also used to mark up the drawings), rub his brow and ask you to leave the part on his desk.  You were to return the next day to hear his verdict on whether the part should be kept, reworked, or scrapped—and you took his word as gospel.

By contrast, I want to describe another engine factory for you; our founding team had the opportunity to tour a truck engine factory in North Carolina that was similar in scope to the turbine factory we worked at.  This factory converted raw inputs to fully built, tested, and shipped engine in a week; and it did it at a rate of an engine every 5 minutes.  While we did not have the same hands-on experience as we had at the turbine manufacturer, the differences were immediately clear.  There was, for one, an assembly line!  Engines would move down a conveyor belt; each station had a 5 minute step before an engine would move to the next station.

Even with this strict timing and specialized stations, each engine was built-to-order, with seamless inventory management in the background operation.  Be it a different cam-cover or turbo-charger, the inventory was pulled to the specific station, and refilled as local supply ran low.  Part of this was achieved with crude robots, where parts were delivered by following a set of colored lines on the ground from one side of the factory to the next.

What really inspired us, however, was that the diesel company was also building tens of thousands of small turbines as part of this process.  Turbochargers are not the same as aircraft jet engines by any stretch of the imagination, but they do have a lot of technology in a small package.   They have high speed bearings that must survive the constant loading and unloading of a diesel engine; and they have many little features that contribute to performance and life.  When we compared the diesel manufacturer to our experience with turbines, we realized something.  The products these two companies were building were for very different markets.  By necessity, the turbine had to be built with critical alloys, exacting requirements, and a high rejection rate—partly because they are high performance products, and partly because so few were built a year (<500).  In some ways, each engine was its own special production.  The diesel units on the other hand are built in a cost competitive market, and where over 60,000 engines would be built—the manufacturing learning curve is also much faster with many more samples to work with.

But this also opened our eye at Dynamo.  After seeing these two models, we asked the question “What if we built turbines the way they build diesel engines?”  The result is a new way of thinking about the supply chain, of how the engine is built and assembled.  It’s a new way to think about what the final product will cost, and how many we can build in a year.  The other challenge is a market challenge; if we want to build 60,000 turbines, we have to find someone who wants to buy them.   Luckily in the small power market, there are always people looking for something more reliable, more fuel flexible, and smaller than what they have today.  In order to access all these customers, however, we had to also think of our product as a platform that could be easily adapted as needed for unique applications.

The Bakken—Part 3: Existing Generators

After speaking to operators and service personnel in the field, we learned a few astounding facts about how essential the reliability of the power generator is to production yields.  Specifically, generator problems account for at least 90% of downtime in upstream operations.  Generators will go down at any time of day, but most operators only find out during their daily site visit—which is why most of the leasing companies we visited were on the phone starting with the first shift around 8am.  Most operators expect you to send out a service technician within two hours of that first call (mostly because that’s how long it takes to drive across the field), and they all want the generator back up in three hours.  We also learned that there is a huge surge of generator orders around the coldsnap of the year.  In an environment where temperatures routinely range
Diesel Fuel Gelled on a Fuel Filter

from -40°F to 80°F, weather is a killer for these generators.  Diesel freezes at -22°F, and if the generator shuts down, so does the lubricating oil.

Diesel generators also have another unique problem:  diesel theft.  One of the smaller operators we visited told us they had lost $350k worth in stolen diesel in the last 3 months (or an estimated $3M for the year).
E&P operators aspire to use flare gas to run their generators, but the natural gas generators adapted for this purpose have higher failure rates.  Most of these NG generators have propane available on site for backup, but the primary cause of failure is of course due to the fact that flare gas has inconsistent fuel quality, which beyond causing engine shutdowns also shortens their life.  One supplier told us he only expects his fleet of generators to last 12-18 months before he will have to overhaul or replace them.  The quality of flare gas is so poor, one major generator vendor won’t sell their generators without first evaluating a test sample for examination in the lab.  If the results come back as poor, they will not provide a generator.  If the results come back positive, they will sell the product without a warranty.  In addition, the availability of these generators is less than 90%.
Needless to say, the systems used by the industry are subpar.  Technologies do exist to bring improve the availability of flare gas burning generators, but the costs don’t currently justify their implementation, and the sub-par availability is the biggest barrier to the implementation of conventional reciprocating generators.  The technology we are developing here at Dynamo is built to solve the numerous problems here.

The Bakken—Part 2: Wellpad Configuration

While we are talking about the oil field, we should probably talk about what the oil field is.  Upstream operations mostly take place at a wellpad.  Modern fracked wells require relatively large amounts of space, as shown in this picture on the left.   This is done to accommodate all the equipment needed to drill and complete a well.  “Completion” is a process whereby rock is perforated and stimulated (fractured & cleaned up). Once completed, a well enters production phase—as represented by the picture on below.
Oil wells are not static entities either; once drilled their production decreases over time.  Just as a juice box becomes harder and hard to drink from as you pull out all the fluid, the same is true with oil coming out of the ground.  After some time, usually a few months, artificial lift is installed to keep the oil flowing.  Artificial lift is a generic term for a pumping unit, but usually takes the form of a pump jack, like the ones below. Also on site you’ll find tanks for holding oil and water, heater treaters (basically a system to separate oil from water and associated gas), flare stacks, and of course an onsite generator.  While all of the equipment onsite is necessary, without a generator all activities on site come to a stop.
Early on, while the Bakken was being developed, (for political, legal, and strategic reasons) most wells were drilled with only one well on any given pad.  Today operators are doing what’s called infill drilling, where they packing together wells as densely as possible.  Without going into a detailed description of the geology and science of drilling and fracking, one thing is clear: there is more than one well per pad being drilled today.  This is important because each well typically needs 50-75kW of power.  Numbers vary, with as few as three wells per pad but we saw one pad with nine wells on site.  Each operator has their own secret number which is highly correlated to the reservoir engineering that is going on below the ground.
Kodiak Oil & Gas Corp
Howard Weil Energy Conference, March 2014


After the first six months (as shown from the chart on right), a typical well will be producing $30,000 in oil a day (or $1,250 an hour).  I think it is interesting to note that on that well pad with 9 pumpjacks, we saw one generator, which rented for roughly $20,000 a month.  Each hour of downtime from that generator at that wellpad amounted to $10k—and a good generator averages a day and a half of downtime a month.  More on this next time.

The Bakken—Part 1: Boomtown

Image source: UND EERC
The Dynamo Team went out to Williston, ND last month to see firsthand the shale revolution that is changing
the energy world.  The reason to come to Williston is that it is at the center of the action. Landing in Williston was uneventful, save that the airport was probably the size of our incubator, Greentown Labs.
A boomtown is an amazing place, where at first the world seems like any other you are familiar with, but after a while you realize it is actually different.  Very, very different.

Getting out of the Airport, Williston seems like any other midwest American town of ~14,000 people: streetlights, cars, gas stations, and local restaurants that were never displaced by big national chains.  But once you drive half a mile beyond the airport, you notice the odd things.  There are far too many trucks on the road.  Not SUVs, but 18 wheelers, and cement trucks, and tankers—in fact, you barely see any other types of vehicles on the road.  These trucks always seemed to be on the road, morning or evening; we would later hear them screaming by as we tried to sleep in our hotel.


As you drive from the airport, along Highway 2, you see the roads are lined, not with strip malls like you would see in suburban America, but office buildings of service companies; names like Schlumberger, Baker Hughes, Cameron, Caterpillar, & Weatherford streak by in your peripheral vision.  Of the houses you did see, you wonder why the houses are so small and packed together in this mostly un-inhabited county.  You notice the flash of light from the sides of metal buildings still under construction, and you wonder why there needs to be so many trailer parks for this little town.  As you will later learn, Williston has the highest rent in the US, and some believe that the population of Williston swells to 75,000 people in the summer—and they are all here to work on drilling oil.
We arrived at our hotel to find it was still under construction.  Workers were painting the walls and lining Ethernet as we checked in.  When we asked the front desk where we could grab some dinner, she exclaimed “Applebee’s just opened up down the street.”  As we drove to dinner (we opted for something more local than Applebee’s) we saw Pumpjacks right in town—an integral part of the urban landscape.
We had arrived in a boomtown, where oil wells and buildings, services were being built out at a lightning pace, but where talent and the houses for them to live in could not be found fast enough.  We had arrived in a town where at every table sat groups of people with the word oil on their lips.  We had arrived in a town where something new and different was taking place; where people came to be a part of the tidal wave that would bend history.

Getting Electricity from a Jet

After the Turbocore, there is a dynamo (or as we call it these days, a permanent magnet generator).  The generator that is attached to the Turbocore produces electricity at 1 kHz, much too fast for conventional equipment to use it, so we pass the power through a rectifier to make stable DC power across a DC link.  After the DC link, an inverter changes the power to a more standard form of AC power – in this case 60 Hz standard utility power – so that it can go into the grid, power a compressor, power a beam pump, or be used for other applications.  A DC converter could also be installed in lieu of the inverter for DC applications.  This is a typical power conversion architecture for microturbines.

While this architecture is a little more complex than your household Honda generator, it gives us product flexibility and reliability.  The DC link is electrically simple, and is a good place to create modularity in the engine.  Components on the left can be changed independently of components on the right.  This means it’s easier for us to cost effectively provide 120V 1-phase power or 3 phase 240V power or even 48 VDC by changing a few parts.  On the other side of things, it’s easier for us to make upgrades to the underlying hardware—the engine and the generator—without sacrificing electrical quality.  In fact, multiple inverters and multiple generators can be attached to both sides of the link, providing end users with a wealth of options for power.

While we are talking about electrical, and what that means for the end user, we do want to talk about why the Dynamo Turbocore has two turbines.  We could have gone with a single turbine: it’s cheaper, there are less parts and less engineering to be done with a single shaft engine.  However, we went with a split shaft design because we realized doing so would result in a more stable and more reliable engine whose performance would be less sensitive to changes in the application. 

A two-shaft engine has two main subsystems: the gas generator and the power turbines.  The gas generator includes the compressors, the combustion chamber, and the turbines that power the compressors; the remaining turbines are mechanically connected to an electric generator and are called power turbines.  There are three main advantages for doing this.

The first advantage of a two-shaft engine is that the second turbine, which spins the electric generator, can be designed to operate at a lower RPM, which results in less stringent performance requirements for the turbo-machinery, the electric generator and the power conversion unit.   This holds true for mechanical loads as well, which also see significant advantages from lower gear ratios and lower speeds.  

The second advantage is that the power turbine is a constant power device, which is exhibited as a significantly superior torque characteristic versus engine speed compare to a single-shaft engine.  For a single-shaft engine, the available torque decreases to zero as the speed of the engine drops; for a two-shaft engine, the available torque increases as the speed decreases.  The torque characteristic for a two-shaft engine t is also superior to that of a reciprocating engine, which has a relatively flat torque curve.  The torque advantage is important during back starting heavy loads. 

Lastly, the Turbocores are controlled to provide consistent power to the power turbine; the gas generator is essentially de-coupled from the power demands;  the gas generator can be throttled up and down faster and the compressor is not limited by the load on the turbine, and can generally operate near their efficiency point.  The control system can be more robust—there is less compromise between keeping the engine operational and preventing a brown-out.  Big power generating turbines where the load varies over time are generally of this split shaft configuration.  As a corollary split shaft engines are easier to start, with less thermal loading to the turbine system.

It is for these reasons and others we went to a split shaft design for our Turbocore product.