Reciprocating engines (Figure 1) drive the vast majority of on-site generation. They are mass-produced by many manufacturers around the world, cost less than other distributed generation (DG) technologies, and have a fully developed sales, maintenance, and repair infrastructure. All of these factors, combined with market familiarity, decreasing exhaust emissions, extended service intervals, and long engine life, continue to make reciprocating engines the most commonly used DG technology.

Figure 1: Reciprocating engines

Reciprocating engines have been the main option for distributed generation for the past few decades. Technological advances are improving their appeal as a low-cost, reliable option. Pictured here is a 334-kilowatt lean-burn natural gas generator set from Cummins, designed to have high fuel efficiency and low emissions for peaking, prime power, and combined heat and power applications.
Figure 1: Reciprocating engines

All reciprocating generators have two main components: An internal combustion engine that burns diesel, propane, natural gas, or gasoline and an electrical generator that converts the shaft power of the engine into electricity. Electrical conversion efficiencies for natural gas–fired reciprocating engines in the 5-kilowatt (kW) range are about 24 percent. For larger engines in the 250-kW and higher range, efficiency can exceed 33 percent. If thermal energy is recovered from the exhaust gas and the engine cooling jacket and put to use, overall system efficiency can approach 80 percent.

What are the options?

Voltage. Many small businesses operate on the same voltages found in residences, namely 120 and 240 volts (V). If your business has only single-phase voltages or if you want backup power only for single-phase circuits, you’ll want to purchase a single-phase generator.

Larger businesses and office buildings will often be supplied with three-phase power, and some circuits will operate at 277 or 480 V in addition to 120 V. If your business has circuits at these higher voltages, you’ll most likely want to purchase a three-phase generator.

Power. Reciprocating generators are available in a very broad range of power output, from less than 1 kW to several hundred kW or even multi-megawatt units. Generators of just a few kW are typically portable, have an attached fuel tank, and are intended to supply power for just a few hours. Above about 7 kW, generators tend to be semi-permanently or permanently installed and connected to a separate fuel supply, such as a diesel or propane storage tank or natural gas supply line. Businesses often choose to connect only the most mission-critical circuits to a backup generator, so the power level you need for your business may be well below your building’s peak power level.

Fuel. Reciprocating engines can be fueled by diesel, natural gas, propane, or even gasoline for the smallest units. Diesel-fueled reciprocating engines are more common than natural gas–fueled engines, because they tend to be more dependable and somewhat less expensive to operate than those that run on natural gas or propane. Most older diesel units are used strictly for backup power during grid outages, due to diesel engines’ higher emissions. Many air districts will only allow these units to run for a limited number of hours per year. Generators that burn propane or natural gas tend to produce fewer emissions but have more maintenance requirements, and they can be sensitive to both fuel pressure and volume, so it’s important to ensure high-quality fuel supplies for these units.

Dual-fuel engines are growing in popularity. These units run primarily on natural gas, but a small amount of diesel fuel is still required to ignite the mixture. Emissions are reduced nearly to the level of natural gas engines. These units can be operated on 100 percent diesel fuel at times when natural gas is not available, but can’t run exclusively on natural gas because they typically lack spark plugs. A number of engine manufacturers make dual-fuel units, and existing diesel generator sets (gensets) can also be retrofitted to dual-fuel at a reasonable cost.

Emissions. In May 2004, the U.S. Environmental Protection Agency (EPA) finalized a comprehensive rule intended to reduce emissions from nonroad diesel engines. The new engine standards will reduce particulate and nitrogen-oxide emissions by 90 percent. Closely linked to these engine provisions are new fuel requirements that will decrease the allowable levels of sulfur in fuel used in nonroad diesel engines by more than 99 percent.

Manufacturers of diesel engines and generators have responded to these regulations by adopting emissions controls from highway diesel engines to nonroad applications. These improvements will continue as stringent new emissions standards come into effect in the coming years. These regulations, which are being phased in over a four-year period that began January 2007, require new stationary diesel engines, including engine generators, to comply with a tiered timing structure of emissions allowances. Generators are rated from Tier 1 to Tier 4, based on the systems’ engine horsepower rating, with most non-emergency diesel engine generators required to achieve compliance with the most stringent requirements by 2012.

Emergency standby generators (those generators that only operate when normal power is lost) typically operate for very few hours per year, so the EPA exempts them from the most stringent Tier 4 regulations, requiring Tier 2 and Tier 3 compliance instead. However, if the engine generator is used to support the load when normal power is present, it will be required to comply with Tier 4. This includes applications such as peak shaving and operating during storm conditions. The full text of the EPA regulations (PDF) is available online; many state and local municipalities have additional, more stringent, regulations.

Combined heat and power. Some generators can be packaged or retrofitted with combined heat and power (CHP) systems, which recapture waste heat from the generation process. There are four sources of usable waste heat from a reciprocating engine: exhaust gas, engine-jacket cooling water, lube-oil cooling water, and turbocharger cooling water. CHP systems convert this heat into useful thermal energy, in the form of steam or hot water, for use in nearby buildings. Generally, the hot water and low-pressure steam produced by CHP systems based on reciprocating generator sets meet low-temperature process needs; provide space heating and potable-water heating; and run chillers that provide cold water, air conditioning and dehumidification, or refrigeration. Making use of what would otherwise be waste heat boosts the system’s overall efficiency considerably—typically up to 70 to 80 percent of the energy content of the fuel is utilized.

The U.S. Department of Energy provides a variety of information resources on its CHP Deployment page, as well as links to CHP Technical Assistance Partnerships.

How to make the best choice

Determine your power needs. The best way to choose the right size of generator is to identify the end uses you want the generator to supply, and simply add up the power consumption of those end uses. You’ll typically find the power draw of each individual piece of equipment on its nameplate. If a significant fraction of the load that the generator will supply is made up of motors, be sure to explain this to your generator vendor, because motor-startup current requirements may necessitate purchase of a larger generator. Also, be sure to consider whether you want to purchase additional generator capacity for future expansion.

Familiarize yourself with state and local regulations. Many state and local governments have regulations that restrict the hours of use, emission, or allowable noise levels for generators. Before purchasing a generator, be sure to learn about all the regulations your generator will have to comply with.

Select a quality service plan. Maintenance requirements and frequency vary from one engine generator manufacturer to the next. Service intervals typically range from 2,000 to 4,000 operating hours; regular maintenance generally involves changing the oil, oil and air filters, spark plugs, and spark plug wires and adjusting valves. Choose a dealer who offers a service plan that includes yearly visits. Dealers typically offer warranties that cover parts and labor for at least two years from date of purchase, but pay attention to the provisions of the warranty: some are comprehensive, but some cover only certain parts of the generator. Pricing for a service plan will vary according to the size and type of generator you select.

What’s on the horizon?

In addition to limits on the overall noise created by generators, some municipalities are enacting ordinances that limit noise at specific frequencies. Recent advances in insulating materials are being incorporated into exhaust mufflers and into the sound-attenuating enclosures that allow generators to comply with these ordinances.

Who are the manufacturers?

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.

All content copyright © 1986-2020 E Source Companies LLC. All rights reserved.