May-June 2004

What's Up With Reciprocating Engines

In the inaugural issue of Distributed Energy (Nov/Dec 2003), this column featured an overview of the five major types of onsite power generation technologies. Beginning with this issue, the next five columns will take a closer look at each major technology in turn, focusing on the good news in research and development trends and what's projected to happen with costs and performance over the next few years. 

For this column on reciprocating engines, I invited Timothy J. Callahan, a leading engine expert at the Southwest Research Institute, to join me in bringing you the latest. Read on to find out what this technology offers you today—and what it promises for the future.

Reciprocating engines are a well-established technology featuring equipment that is durable and able to use a variety of fuels, with relatively low capital costs and widely available service networks. Thanks to their flexibility, these engines are found in numerous distributed-energy sites throughout the world. A single engine can be tailored to meet the requirements for a variety of applications. Although diesel engines generating electricity on a standby or an emergency basis are commonly found, natural-gas engines providing baseload electricity in distributed-power applications are on the rise.

Engine-based combined heat and power  (CHP) systems also are gaining. As of 2003, there were approximately 1,250 reciprocating engine­based CHP installations in the United States, more than 45% of all domestic CHP sites (Energy and Environmental Analysis Inc., 2003). Most sites are relatively small; although engine-based CHP totals more than 1,180 MW of electrical capacity, it represents less than 2% of all US CHP capacity. Individual engine-based CHP installations range from 30 kW to more than 40 MW, with many larger projects featuring multiple units. A typical commercial application for reciprocating engine CHP, for example, is a hospital or health care facility with a baseload electric demand of 1 MW. In such an application, multiple 200- to 300-kW natural-gas engine-generator sets could be utilized to meet the electric demand, with approximately 1.6 MW of thermal heat available from engine exhaust. 

There's no doubt that over the last 20 years, manufacturers have been producing engines with steadily improving efficiency and brake mean effective pressure, or BMEP, a measure commonly used to evaluate the reasonableness of engine-performance figures and to compare engines, defined as the average (mean) pressure, which, if imposed on the pistons evenly during each power stroke, would produce the measured (brake) power (for additional details, see http://www.epi-eng.com/ET-BMEP.htm), with the current state of the art for gas engines at 20 bar BMEP and about 44% thermal efficiency. Among other studies, Callahan's research documenting progress for four specific engine models illustrates this trend clearly (Figure 1).

It is currently quite difficult to obtain high efficiency without concurrently increasing NOx emissions, however. In fact, this turns out to be a key driver of engine research and development efforts. "While the economics of power generation places a premium on engine efficiency, environmental regulators continue to exert downward pressure on permissible exhaust emissions [specifically NOx]," says Callahan. "This means that the most important overall challenge for this technology is to improve the trade-off between efficiency and emissions."

Several public/private partnerships have sprung up that are dedicated to addressing the efficiency/emissions challenge. Although competition among manufacturers by itself will lead to improvements in engine performance, Callahan notes, these initiatives are sharing research and pooling resources with the objective of speeding along the necessary technical advances faster than would be the case given typical competitive pressures.

• The Advanced Reciprocating Engine Systems (ARES) program, which Callahan helped create in 1998, brings together leading engine manufacturers, US national laboratories, universities, and other research organizations under US Department of Energy sponsorship. The companies are cooperating with the goal of achieving 50% thermal efficiency and NOx emissions as low as 0.1 g/bhp-hr. These goals represent improvements of 30% and 95% in efficiency and emissions, respectively, from today's averages.

Because the efficiency/emissions equation varies significantly with engine size, Callahan points out that it's important to frame the discussion of performance in the context of engine size. "Recent market survey data show that almost 80% of gas engines ordered are in the 1- to 2-megawatt size range, with 15% in the 2- to 3.5-megawatt category," he says. "It's logical to focus on engines that are less than 3.5 megawatts when discussing future targets" (from Diesel and Gas Turbine Worldwide's 27th Annual Power Generation Order Survey, as presented in the October 2003 issue of Diesel and Gas Turbine Worldwide).

Figure 2 shows the ARES 2010 goals and intermediate targets relative to the current state of the art for gas engines of less than 3.5 MW.

• The Advanced Reciprocating Internal Combustion Engine program (ARICE) is a technology development and demonstration program begun in 2001 targeting 0.01 g/bph-hr. NOx emissions at 46% thermal efficiency levels by 2010. It is funded in part by the California Energy Commission, with the expectation that the target NOx level will become the regulatory limit in California if the program is successful since it will demonstrate a new best-available control technology upon which to base a new standard. There are currently three major projects being carried out under this program, with leadership from several national laboratories and the industry.

In fact, collaboration between these two programs is underway, with participants exploring how best to leverage all of the resources available to develop stationary reciprocating engines to meet environmental and other challenges (visit www.energetics.com/Pages/default.aspx for details and for links to ARES and ARIC Web sites).  Such collaboration could speed things up even more, delivering significantly improved engines into the hands of customers sooner than originally envisioned.

What's in It for Me?

What is it that energy customers can expect in the future from reciprocating engines in distributed-energy applications? Callahan and others [as reflected in the "Reciprocating Engines" chapter of the National Renewable Energy Laboratory report entitled Gas-Fired Distributed Energy Resource Technology Characterizations (US Department of Energy, October 2003)] anticipate that research and development investments by manufacturers, government, suppliers, and other entities will yield the following bottom-line benefits. 

• Lower capital costs. The main route to lower equipment costs for engines is through increased power density (i.e., higher kilowatt output for a given engine size). Higher power density means lower cost per kilowatt of capacity because for a similar amount of material and labor cost, a manufacturer can produce more power from a given engine base (and customers then can pay less to obtain a given kilowatt of output). Efficient utilization of exhaust energy offers a promising avenue to boosting power density.

Another route to lower capital costs is through prepackaging. With more effective packaging and integration of systems and controls, the cost of basic engine packages eventually could fall by as much as 25%. Standardized designs for systems and auxiliary components, along with preassembly of modular units at the factory, could significantly lower installation costs, especially for smaller systems. When less customized integration is needed, related engineering, construction, and project management costs also decline.

• Lower operations and maintenance (O&M) costs. Companies are working to simplify equipment service requirements and lengthen the intervals between required servicing. They're also trying to minimize the attention that the equipment demands from operators and service personnel. Use of ceramics and other advanced materials, improved lubricants, and improved engine components can contribute to achieving these goals. Engine designs for longer lives (more time between overhauls) and more controlled wear also will contribute to reduced O&M costs.

• Reduced emissions. Advanced combustion technology and improved sensors and controls are combining to help engines achieve emissions levels as low as 10 ppmv NOx (at 15% oxygen), comparable to NOx emissions from natural-gas turbines. The objective is to achieve these levels while improving fuel efficiency and perhaps without any aftertreatment for engine exhaust. Companies also are pursuing more cost-effective avenues for treating exhaust to control NOx, carbon monoxide, and volatile organic compound emissions.

• Higher efficiencies. With the use of high-temperature materials (e.g., thermal barrier coatings), more advanced engine sensors and controls, and improved combustion practices, higher fuel efficiencies can be realized with corresponding operating cost-savings for facilities. Improved controls will allow for very lean combustion that optimizes both efficiency and emissions. Increased BMEP and engine speed will increase power output and correspondingly decrease cost per kilowatt of the larger engines.

• Increased heat recovery for CHP applications . Use of low-temperature cooling circuits and improved thermal management will allow more heat to be recovered from engine waste-heat streams, yielding more efficient and cost-effective CHP systems.

The Numbers

A National Renewable Energy Laboratory report on distributed-energy technologies (also cited in this column in the Jan./Feb. 2004 issue) ventures to project evolutionary technology advances and corresponding equipment performance and cost projections. Using five engine systems, the authors estimate both standalone and CHP system performance and cost figures for 2005, 2010, 2020, and 2030, based on a set of technology and learning-curve advances, including those listed previously. Figure 3 shows estimates for four key characteristics: total installed cost, O&M costs, electric efficiency, and NOx emissions for two of the representative systems in power-only mode.

But Wait!

Figure 3 depicts a steady pace of improvement in performance and reductions in cost that is typical of evolutionary product development. But keep in mind the good possibility for revolutionary technology advances that such experts as Callahan look to as a result of the ARES and ARICE programs. Taking into account the current economic and environmental climate, Callahan is confident that these initiatives, along with independent efforts, will continue to thrive and progress toward their goals. "Customers can not only count on reciprocating engines to meet their onsite power needs today, but they also can expect steadily improving efficiency and environmental performance for years to come, with a good probability for more revolutionary change along the way."

Callahan invites readers with questions about the future of reciprocating engine technology to contact him at tcallahan@swri.org.