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.