Gas or Diesel?
Whichever internal combustion technology you use, pressure to increase efficiency is spurring technological advances and decreasing costs.
By Ed Ritchie
As distributed energy continues to grow in commercial applications, internal combustion engines remain the dominant driver of generation sets. And despite the argument for more sustainable methods such as wind, solar, and fuel cells, internal combustion has the advantage economically. When paired with high efficiency combined heat and power applications, they’re more than likely to maintain a commanding market share for years to come. But that doesn’t mean manufacturers are resting on their laurels. In fact, research and development is producing impressive results, and those results are making distributed energy more accessible then ever before.
To start, let’s look at some recent innovations at Cummins Power Generation, Fridley, MN. In November 2011, the company launched its new C3000 Series generator set, offering a power range of up to 3.5 MW. Based on the QSK95 engine, the high-speed diesel targets markets such as data centers, hospitals, and utilities.
“The latest technology and architecture, in terms of emission controls noise, efficiency, and reliability—these are part of what we term a super system, which includes design of the switchgear and transfer switches and networking into critical building loads,” says Gary Johansen, executive director of Global Engineering, Cummins Power Generation
The C3000 Series illustrates some of the competitive challenges faced by genset manufacturers. Customers want higher power density ratings—a quality that could be generally described as higher efficiency, but that higher efficiency must be accompanied by lower noise and lower emission levels. Whether it’s diesel or natural gas, Johansen believes that today’s reciprocating engine technology measures up to the task, and he cites a new range of military gensets introduced to armed services customers as an example. The gensets are unique because they meet very stringent noise standards regulations without any nonmetallic noise mitigating materials.
By employing acoustic analysis to achieve lower decibels with their metallic components, Johansen explains that they were able to achieve high performance in terms of noise capability while also meeting “other requirements for maintenance and survivability under various environmental factors that come to play in a battlefield situation.” The new Advanced Medium Mobile Power Sources deliver about 21% better fuel efficiency over previous models, and are nearly 20% less in cost. Moreover, they’re lighter and more reliable.
“That’s one example,” adds Johansen, “but if you think about telecom and small gensets being mounted with a cell tower on the top of an apartment building in India for example, you see the need for a very keen focus on noise attenuation.”
A very keen focus might be understating matters somewhat. In October 2011, Cummins unveiled its Acoustical Testing Facility.
“We’re very excited, and it’s a big investment and commitment for us, but it’s very critical as a customer attribute,” says Johansen. “When you look at other facilities where you’re placing gensets next to a luxury home or small office building, or in just about every market that we are in, there is a sensitivity to acoustic performance.”
Johansen sees a bright future for diesel engines in the standby environment or any application as the prime power driver because they’re dependable, and the power density is good in terms of meeting a footprint requirement.
“They’re cost-effective, and if you look at the emissions reductions and efficiency improvements in the last 20 years, it’s really nothing short of remarkable,” he notes. “So we continue to have a high level of optimism and do combustion research and development to maintain an industry leading position. It’s a core competency of our business.”
Helping Out After the Big Quake
Dependability is certainly a key attribute of modern diesels, agrees Jeff Crisman, sales manager at Enercon Engineering, Atlanta, GA, a designer and manufacturer of diesel generator set enclosures, switchgear, and controls for utility grids. But availability is another key factor, as in the case of making it possible to deliver 32 gensets in eight weeks.
In July of 2011, Enercon fulfilled a rush order for 32 power generation units for an emergency power contract between Tokyo Electric Power Company (Tepco) and APR Energy, Jacksonville, FL. The generator sets are powered by MTU 16V 4000 diesel engines and are providing electricity to Yokosuka and Hitachinaka, two cities on Japan’s eastern shore where quake damage devastated much of the infrastructure. The units are housed in modified ISO shipping containers, 40 feet long by 8 feet wide by 9.5 feet high, sound attenuated not to exceed 85 A-weighted decibels, or db(A), at 3 meters. Fuel tanks are custom manufactured, UL-142 certified, approximately 550 US gallons (usable) capacity.
Turbocharger selection can vary depending on transient and altitude requirements.
Generator sets for use with coal mine methane are designed to respond to fluctuations in gas composition.
Higher speed microprocessing and improved control algorithms allow for improved transient response capabilities and quicker adjustments.
|Photo: Rolls Royce
Rolls Royce Denmark central heat facility.
The units are located on the site of a damaged nuclear facility, but noise levels were still an issue.
“No one wants to hear the generators operating, even though they want the power,” says Crisman. “Typically, a big two-megawatt unit has a sound level of 113 decibels, but we get it down to 85 decibels at 100 feet or 15 meters. If you have 10 units running in one area, every time you add a unit you add three decibels to the ambient sound level in that area.”
Vibration is another point of concern. Although Enercon uses isolation techniques to counter vibration, the fact that the containers were mounted on truck trailers allowed the trailer tires to handle the task. Another technique involves mounting the fuel tank beneath the genset so it absorbs some of the sound and vibration.
Speaking of fuel, Crisman notes that typically, a 2-MW genset burns approximately 157 gallons an hour at full load, and units are usually designed with 8 to 12 hours worth of fuel.
“It’s usually 1,250 gallons,” says Crisman, “but in a situation like Japan, the diesel arrives by truck, and they keep a big reservoir and pipe it to each unit’s tanks with fuel transfers and duplex transfer pumps on the units, so if they’re not using the fuel can be transferred out. Placing fuel tanks under backup generators is common, and if you drive around to the back of a Wal-Mart or Lowes, you’ll see the backup generator, and there’s usually a large fuel tank underneath it.”
Those underground tanks are finding less use as natural gas grows in acceptance for the standby industry, according to Michael Kirchner, technical support manager at Generac Power Systems, Waukesha, WI. Kirchner says that in commercial market spaces such as convenience stores and larger retail operations, there’s a growing trend in natural gas as a fuel.
“When we’re in the smaller applications under 150 kilowatts, natural gas is prevalent because it’s very cost-effective,” says Kirchner. “Historically it’s been dominated by diesel, but it’s changing due to the preference for sustainability and less fuel complications because you don’t have the hassle of maintaining diesel fuel onsite.”
Kirchner sees a similar trend occurring with commercial standby systems in the range of 1 MW, and higher. For Generac, much of the transition can be credited to the company’s use of paralleling technology, a method of using smaller units, or modules, of natural gas powered engines and connecting them together to build higher total output capacities. Until about 10 years ago, running in parallel required third-party equipment and complex integration, but by replacing the third-party equipment with hardware integrated into the genset’s control system, module sizes of 300 kW are convenient for creating systems to satisfy loads from 300 to 1,200 kW, and, cost-effective.
“We’re talking order of magnitude differences in cost,” says Kirchner. “Typically one-megawatt natural gas engines were two or three times the cost of diesel, so it’s really been a deterrent. By optimizing the 300-kilowatt module with paralleling technology to supply these larger loads, natural gas is maybe a 50% premium over diesel, so now all of a sudden it’s a realistic choice; whereas, two and a half to three times the cost was not. And for many companies there is the issue of running as green as possible, plus the elimination of diesel fuel maintenance.”
For sites that keep diesel onsite for the security of maintaining independent reserves, or to satisfy government regulations, running gensets with bi-fuel capability is another solution.
“We use a compression ignited engine that runs on about 75% natural gas and 25% diesel, and that keeps diesel fuel to a manageable amount,” explains Kirchner. “If you bring in three days of diesel fuel, the cost to buy and frontload all that is a capital expenditure. Whereas bi-fuel allows you to bring in 12 to 24 hours of diesel and now it’s more manageable, with less fuel onsite and easier to turn over the fuel to burn some of it as it ages. Then you can get the extended runtime by blending with natural gas in the engine.”
Handling a variety of fuels is just one example of how generator sets have changed dramatically over the last 10 years to 15 years, notes Mike Devine, VP of technology, Caterpillar, Lafayette, IN.
“It used to be that you could take a gas genset, and for 50 or 60 Hertz, and it was simply a matter of changing the speed on the engine and changing the spark plug or something like that to get a little better efficiency, but it all worked out well. Today, we are asking engines to do so many things, in addition to emissions control there is higher efficiency, longer life, and better overall owning and operating costs.”
One of the most dramatic areas of technology that Devine has observed is the overall use of electronics in engines.
“It has made a huge difference, just as it has in an automobile,” says Devine. “Now we have hardened electronics that are designed to operate the engine in any atmosphere conditions. In the past, changing the timing on the engine electronically was a big deal, but today we’re monitoring and adjusting timing on individual cylinders. So the ability to manipulate the engine and to keep it within safe operating ranges, while optimizing fuel efficiency and emissions levels, has been a huge part of what’s going on.”
Turbo-chargers, and even spark plugs, have seen similar upgrades. Moreover, fuel systems are handling a wide variety of challenges. Along with the issues of efficiency and emissions, there’s the whole spectrum of contaminants. Then, too, an engine must run efficiently even when the volume of energy in the gas fluctuates. Sensors, for example, can help the engine operate in a certain environment.
“It’s what we call charge density control, and we see it having a lot of affect on gases that are changing, because even pipeline quality natural gas doesn’t remain exactly the same,” says Devine. “It may come from two or three different sources and even though they’re minor percentages such as the methane number varying from a 78 to 75 or 73, the engine has to be smart enough to maintain its efficiency and emission controls.”
The control systems on reciprocating engines have also made great strides in the area of microgrid operations, he adds.
“We’ve been doing small situations like universities and even small communities in various parts of the world for a long time, and we’re seeing changes in controlling grids with electronics and the ability to collect data. If I’m doing combined heat and power at a university, there could be steam and hot water and cooling systems, plus power, and the information SCADA system shows the demands that are going to be placed on the engine The engine technology has kept up with that, and in many cases, the engine can actually operate with those systems, or you can take a generator set and control system and operate a complete power facility from the engine control panel.”
Whether it’s diesel, natural gas, or even heavy fuel oil, users can expect their engines to run well for a long time, according to Graham Gate, vice president, Energy Systems, Rolls-Royce, Bedford, UK. Gate joined the Rolls Royce Diesel Power Business in 2006 as VP of Projects, then became the business unit manager in 2008, and he’s impressed with the track record of internal combustion engines.
“We have some in India that are now approaching 30 years old and still in operation. They have had plenty of spare parts, but they’re still there, and they’re pumping oil as part of key infrastructure,” says Gate. “For Bergen Engines, the real technology advantage is about gas engines and natural gas. They are very clean running—cleaner than diesel or heavy fuel.”
Gate notes that customers appreciate the fact that reciprocating engines offer efficiency and reliability in a familiar technology that’s relatively straightforward to maintain. And when that technology is used in a combined heat and power application, the efficiency gains can be significant.
“We have engines in a glass factory in Italy, and the heat is taken off to heat the factory and also provide heat for glass bending because they mold windshields there,” he says. “That gives them an efficient operation up into the 70 and 80s for the recovery of that energy. Another very interesting line that these engines fit into is the greenhouse market where you get a threefold benefit. You get electricity production, and you can use the hot water off the jacket and store it, and it’s circulated through the greenhouses to keep them warm. Then you scrub out the carbon dioxide from the exhaust emissions, and it is circulated around the greenhouses, which adds a significant boost in product growth.”
Even higher-efficiency performance is possible with district heating, and Gate notes that in Denmark district, heating serves whole areas of the country and urban environment, and the internal combustion is very much in the running because they’re flexible and the efficiency doesn’t change over the power range. A typical plant in Denmark runs for about 5,000 hours through the winter and shuts down in the summer, yet these plants remain economically viable because they have the ability to double their overall efficiency by maximizing use of the waste heat.
“One Danish installation claims 92% efficiency for 5,000 hours a year, with 16 megawatts of heat and 12.3 megawatts of electricity,” says Gates.
Unfortunately, the US lags far behind Europe and Danish countries in district heating, according to Dr. Bryan Willson, professor and director, Engines and Energy Conversion Laboratory at Colorado State University. The Engines and Energy Conversion Laboratory is a comprehensive research/teaching facility, with emphasis on engines, fuels, and energy conversion technology.
“District heating and combined heat and power are quite common in Europe, and it pains me that we don’t have enough of it in the US,” says Willson.
The Colorado Energy Lab has a 1.8-MW, Caterpillar 3516C engine with an efficiency rating in the low 40s, but Willson notes that it can be used in a combined cycle just as a gas turbine could. And, by using the waste heat to create steam, the Cat would get an additional 5% more efficiency, yet there would still be half the heat that is unused.
Willson adds, “If you were to locate those engines in dense areas such as office buildings, hospitals, and universities, you’re generating power onsite and have additional for the electric grid, and you can utilize the waste heat for heating in winter or cooling in the summer.”
As the market for peaking or firming of renewables on the grid expands, Willson sees a growing demand for reciprocating engines of 10 MW and higher.
“You can’t consider having a hundred engines running, so you want those engines in bigger chunks within the 10- to 20-megawatt range,” says Willson. “Also there are economies of scale. I’m hearing that 20 megawatts is a good size, and that a fast response time is desirable from these larger engines. And manufacturers appear to understand that and that the value of a resource increases with the speed at which it can be dispatched. This is an area where engines have a real advantage over turbines, because the challenge with turbines is getting high efficiency.”
Willson notes that another obvious opportunity for reciprocating engines is in providing distributed energy to natural gas drilling sites. “This is where we would expect to see further development, and it makes sense that natural gas be used in gas fields rather than hauling in diesel engines and the fuel required to run drilling operations. Natural gas avoids that additional traffic, and it’s cheaper and cleaner.”
Natural Gas Heating Up
According to Volker Schulte, technology leader at Jenbacher A.G., natural gas will surge in popularity for onsite power generation.
“Historically, we focused on gas engines and gas combustion,” says Schulte. “Now as you look more at what’s going on in the industry and the dynamics, you see gas moving out of the niche category, and it’s going to become the major fuel source of the 21st century. You see it for a large-scale power generation and combined cycle power plants, but you also see it in more of the distributed power settings and within segments of five to 10 megawatts.”
Research is a high priority at Jenbacher, and Schulte notes that because the company is part of GE, it has access to sophisticated tools and methodologies, in terms of analytics, design, and testing. From an engineering design standpoint gas engines need to employ methodologies and tools like finite element and analysis for thermal analysis of components like cylinder heads. Overall, the level of sophistication has risen dramatically, and Schulte adds that his engineering team, which has grown over the years in the context of gas engines, but when combined with the GE energy team and the whole portfolio of products, there are more than 16,000 engineers available within the company.
Increasingly stringent efficiency requirements get applied to what some might consider to be the heart of the engine—the cylinder head. Volker describes it as sitting in the middle of everything, and, while it has to withstand high peak firing pressures, it also must contribute to the performance of the engine by guiding the gas flow into the combustor. This relates to the demands of flow optimization, a task that requires mechanical work for material selection and thermo mechanical stresses. All told, cylinder heads and the systems surrounding them are subject to extreme temperature and pressure requirements. However, if the engineering related to cylinder heads isn’t impressive enough, consider the electrical control system
Electronics are meeting difficult challenges in the control of transient operations, explains Schulte.
“You get many more applications where you actually do not just have the need for steady state constant speed operation integrated in parallel with the grid. Now many more applications are in island mode, or you have your own microgrid and you have to deal with several engines together having to handle load shedding in a big way. For example, there’s a lot of applications in textile factories in Pakistan, and they control the factory with these type of engines, but the load fluctuates a lot depending on the operations of the factory and power consumption of the machines. So the load will drop or pick up very quickly, and the engines have to manage these types of transients.”
The challenge is in having the engines remain online without tripping or shutting down during very strong transient load demands. Jenbacher is solving the problem with control system logic, a technology that also works for stabilizing so-called “zero-voltage ride-through” for grid load.
“It’s really about fast controllers and fuel controllers,” says Schulte. “These are very fast, and they are mounted on the engine, because it’s usually not able to do it fast enough if it’s somewhere in the control room, which is something like 100 meters away.”
Ed Ritchie is a writer specializing in energy, transportation, and communication technologies.