Diesel Gensets Aim at the Future

Though newer distributed generation options like microturbines, solar, and batteries have grabbed the headlines, diesel remains the go-to option because of its established technology and reliability. But growing competition and new regulations are threatening its traditional role. Here’s what diesel is doing to stay relevant.

If you pay attention to nothing but the headlines in the energy media (including POWER), you can be forgiven for having mostly forgotten about diesel. Staid and reliable, diesel generation isn’t terribly exciting—it’s just there when you need it.

Exact figures for how much diesel generation is in service worldwide are difficult to come by, in part because official statistics from the U.S. Energy Information Administration and the International Energy Agency tend to lump diesel together with fuel oil, but there’s no question there’s a lot of it. Diesel is used extensively for backup generation in developed countries and for primary generation in developing countries where the national grid may be unreliable or simply nonexistent, as well as for island grids, where large power plants are not cost-effective.

Diesel’s advantages are in its simplicity, reliability, fast response, and low cost. Compared to other options such as gas engines, microturbines, and renewable sources such as wind and solar, diesel generators are typically the least expensive in terms of capital outlay. Diesel fuel also has substantially higher energy density than natural gas and other options, which can simplify fuel supply logistics. With simple maintenance requirements and a well-understood, 100-year-old technology, a diesel genset doesn’t need much attention to provide many years of reliable service—an ideal quality for remote areas.

But diesel comes with some drawbacks.

Diesel fuel can be expensive, especially when it must be imported long distances, such as to a remote island. Much of the impetus toward renewable generation on islands such as Hawaii and Puerto Rico, and other areas such as the Caribbean and South Pacific, has come from high electricity costs driven by diesel generation using imported fuel. Hawaii in particular has made the decision to abandon diesel—and all other fossil fuels—by 2045.

Diesel plants built without consideration of this challenge can quickly turn into white elephants, as was the case with the $335 million Tarakhil power plant built by the U.S. Agency for International Development outside Kabul, Afghanistan. Because importing diesel into the country is so expensive and dangerous, the plant has sat mostly idle, managing around a 2% capacity factor since it was completed in 2010, according to a government report released in August.

Another concern that has grown in recent years is emissions. Compared to gas engines and microturbines, let alone renewables, diesel engines have higher levels of particulate, NOx, and SOx emissions. For most of its existence, diesel flew under the Environmental Protection Agency’s (EPA’s) radar, but that came to an end in 2006 with the first national emission regulations under the New Source Performance Standards, which generally required at least a 90% reduction in particulates and NOx for new engines. Those standards have since been tightened even further, and new regulations have also been issued for existing engines.

Efficiency and Environmental Gains

Staying competitive has meant a host of changes for diesel generation over the past few years to improve efficiency and reduce emissions.

Efficiency in a diesel engine is most directly tied to combustion rate, the degree to which the fuel is completely burned during ignition. This is typically a function of how finely and evenly dispersed the fuel is during injection into the combustion chamber. Turbocharging, which forces excess air into the chamber, also improves combustion rate, which is why two-stage turbocharging (with intercooling between the stages) is now common for diesel gensets.

Modern diesel engines use a method known as high-pressure common-rail (HPCR) fuel injection (Figure 1). This method replaces traditional mechanical injection with electronically controlled multiple high-pressure injections during each combustion cycle. Rather than rely on separate injectors controlled by a camshaft, HPCR uses a single system that supplies all the injectors in the engine with a common source of fuel. This allows much higher fuel pressures than a mechanical injection system, which maximizes vaporization of the fuel and, thus, combustion rate.

1. High pressure, high efficiency. Modern high-pressure common-rail diesel fuel systems, such as the Cummins XPI (extreme pressure injection) system shown here, allow for much higher fuel pressures and much more precise and flexible injection of fuel into the combustion chamber. Courtesy: Cummins

In addition, unlike a camshaft that can only control one injection event per combustion cycle, an HPCR system uses an electronic actuator that can manage multiple separate injections at different stages of the cycle. That means far better control of injection timing and characteristics, which can be customized for the demands the engine is called upon to meet while maintaining peak efficiency and lower emissions.

Ultra-low-sulfur diesel fuel (with a sulfur content around 15 ppm) is now the standard for diesel gensets in areas where emissions must be controlled. The use of biodiesel (mostly blended with petroleum diesel) is also growing as a result of the U.S. Renewable Fuels Standard, though this has been somewhat controversial and total U.S. production is still small, around 1 billion gallons a year.

Selective catalytic reduction (SCR) has been successfully used on new diesel gensets to reduce NOx emissions as much as 95%. Another method often used with it is exhaust gas recirculation, which sends part of the exhaust gases back into the combustion chamber. This lowers the adiabatic flame temperature, allowing for lower-temperature combustion and thus lower NOx production.

Better Controls

All this sophisticated technology requires more sophisticated controls, and traditional analog systems are giving way to advanced digital control systems. While small, rarely used backup gensets can survive on analog, larger and more complex systems—especially those providing baseload power—are moving to digital.

Digital controls are essential for the newest gensets that rely on HPCR fuel systems and precise control of ignition and combustion. They’re also necessary where tight emissions compliance is a consideration.

Another advantage is that digital controls can monitor the real-time state of a wide variety of operating parameters and display them on a centralized panel, as opposed to analog systems that rely on less-sophisticated alarms and indicator lights. This allows operators to identify and correct faults much more quickly, leading to more reliable power and less downtime. They also allow for remote monitoring and operation (though proper cybersecurity is necessary), another advantage for gensets that may be located in remote areas.

Though operators must be trained in understanding more sophisticated systems and understanding what may be hundreds of different fault codes, digital systems in general mean more efficient and reliable operation. With diesel gensets typically representing either emergency generation or generation where there may be no grid power to fall back on, these are critical considerations.

Diesel Power Is Still Relevant

A casual observer can be forgiven for thinking that diesel-based power plants are on their way out. Nothing could be further from the truth, though. New ones are still being built, and older ones are being upgraded with modern controls and other technological improvements to boost output, increase efficiency, and reduce emissions.

One such new plant is under construction in a remote area of Saudi Arabia. The United Cement Industrial Co. contracted with MAN Diesel & Turbo to build a 54.5-MW plant that will power its new cement plant between Jeddah and the Al-Sadiya Mountains near the kingdom’s west coast (Figure 2). The plant will be powered by five 11.2-MW MAN 20V32/44CR diesel engines, which feature HPCR technology and sophisticated electronic controls to maintain peak fuel efficiency and low emissions. The cement plant is far from the national grid, so the MAN engines will serve as its sole source of power.

2. Desert power. Four of the five massive MAN 20V32/44CR diesel engines that will power a new cement plant in Saudi Arabia wait for installation in the laydown area. Courtesy: MAN

Diesel was also chosen for its traditional advantages of reliability and rugged technology, which are important for a site that sees 50C heat and regular sandstorms. Though the engines are being adapted for the harsh climate, they need little of the advanced cooling and environmental controls that would be necessary for a gas turbine–based plant. Fuel costs, as well, are much less of an issue in a country with ample fossil fuel resources. This plant is expected to begin operations in 2016.

A larger plant MAN completed in July, the 210-MW Pointe Jarry project on the Caribbean island of Guadeloupe, illustrates how far diesel generation has come in recent years (Figure 3). Compared with the plant it replaced, Pointe Jarry uses 15% less fuel and emits 85% less NOx as a result of improved technology and the addition of an SCR system.

3. Island power. The new Pointe Jarry diesel power plant in Guadeloupe is significantly cleaner and more efficient than the plant it replaced. Courtesy: MAN

Retrofitting more advanced systems onto existing gensets can also reap significant dividends. Wärtsilä recently handled a project in Pakistan at the Kohinoor power plant in Lahore (Figure 4). The 124-MW plant, which operates eight 18V46 Wärtsilä engines, is operated by Kohinoor Energy, one of the first independent power producers in the country. The original turbos had reached their end of life, but rather than simply overhaul them, Kohinoor opted for an upgrade. Replacing the old turbos with new ABB TPL 76C turbochargers resulted in fuel savings of 2.5g/kWh. Reliability and output have also increased. Because the plant could not shut down without having to pay damages to its power purchaser, the upgrades were done one at a time, each taking about 15 days.

4. High speed. Kohinoor Energy in Lahore, Pakistan, achieved substantial fuel savings by upgrading the turbochargers on its Wärtsilä diesel engines. Courtesy: Wärtsilä

Pairing Solar and Diesel

Solar and diesel might seem to be competitors for future generation, but in fact they are proving to be highly compatible options for off-grid applications. Solar-diesel hybrid plants offset the two main drawbacks of each option: The intermittency of solar photovoltaic (PV) generation and the high cost of transporting diesel fuel to a remote operation such as a mine.

Electricity from solar PV in these cases costs at least 50% less than diesel generation, but without battery backup, it does not generate power at night. That’s where the diesel genset—which is cheaper per kilowatt-hour than a battery—comes in, and combining the two can result in substantial savings on power costs. A study by German consulting firm THEnergy estimated that mining projects employing solar-diesel generation could substantially reduce overall costs of power to the mine operators, especially after the first five years (as many costs need to be paid up front).

Fixed solar-diesel hybrid plants have been around for several years (though most are fairly small), but Italian firm Building Energy and Saudi firm SES Smart Energy Solutions announced in June that they were teaming up to develop the first on-field temporary solar-diesel hybrid plant in Saudi Arabia. The first project will be commissioned this fall. The containerized design is portable, modular, and scalable. (For more on hybrid power plants, see “Leveraging Generation Synergies with Hybrid Plants” in the April 2015 issue.)

The recent drop in crude oil prices has likely provided some support for diesel generation, and as battery costs continue to fall, enterprising designers are sure to find efficiencies in combining diesel, renewables, and storage. One example of possible things to come can be found in Texas utility Oncor’s System Operating Services Facility near Dallas (POWER’s 2015 Smart Grid Award winner—see the August issue) which combines diesel gensets, solar PV, battery storage, and a gas-powered microturbine to create a flexible, reliable, fast-responding system.

Diesel generation may not always be the most popular option with regulators, utility planners, and industry pundits, but its many advantages should secure a role for it in the power mix for the foreseeable future. ■

Thomas W. Overton, JD is a POWER associate editor.