Exploring onsite power generation technology and equipment options

Power outages can disrupt industrial production, destroy data, result in lost materials, and cost a manufacturing plant a boatload of money in lost revenue or spoiled products. Near-constant power is no longer a convenience — it is a vital necessity.Although utility power delivery has become more technologically sophisticated, utility power outages have increased in many areas of th...

By Jack Smith, Senior Editor, Plant Engineering Magazine April 15, 2002
Standby power
Standby power and limited peak shaving
High-hour peak shaving
High-hour peak shaving with heat recovery
Diesel generators
Lean burning gas engine generators
Gas turbines
Fuel cells
Energy products

Power outages can disrupt industrial production, destroy data, result in lost materials, and cost a manufacturing plant a boatload of money in lost revenue or spoiled products. Near-constant power is no longer a convenience — it is a vital necessity.

Although utility power delivery has become more technologically sophisticated, utility power outages have increased in many areas of the country — with some areas experiencing more than 50 outages a year. Outages can last from a few seconds to hours. A target of electric power reliability for many users is 99.9999% uptime. This amounts to downtime of about 30 seconds a year.

One of the primary reasons to install onsite power systems is to improve the reliability of electric service to the site. An onsite power system can provide backup or standby power, load management, peak shaving, base loading, or primary power.

Standby power

Standby power is needed only when utility power fails. Critical loads can be protected by an uninterruptible power supply (UPS) in combination with a diesel generator. Their high reliability and low cost make them the technology of choice for standby power. Portions of a plant’s load can be easily segmented into critical and noncritical loads requiring backup. Automatic transfer switches can be programmed to cut power to noncritical loads. This design reduces the amount of onsite generation needed for any given installation.

Standby power and limited peak shaving

When peak demand for power is created only during limited times of the year, such as in the coldest winter months or during summer air conditioning season, a diesel generating system may be the best choice if total operating hours do not exceed 300 hr. By cutting a plant’s peak electric demand during critical times, considerable savings can accrue on electric utility costs.

High-hour peak shaving

In plant applications with daily, nonseasonal peak demands for electricity, considerable cost savings are possible using a lean-burning gas engine generator to supply the peak portion of electricity. A typical installation may require the generator to run for a portion of each day, depending on the plant’s load. The economic benefits depend on the local utility’s rate structure.

High-hour peak shaving with heat recovery

In plant applications where there is a simultaneous need for electric power and heat, a lean-burning gas engine generating system coupled with heat recovery can be an economical solution. Heat requirements can be for hot water or low-pressure steam. Air conditioning needs can be met using an absorption chiller. In this type of application, the onsite generator would run constantly to provide all or a portion of the plant’s electric needs. The byproducts of hot water or steam for processing or space conditioning would also be produced.

Lower rates from the utility can often be negotiated by flattening a plant’s peak electric power needs. Microturbines and larger gas turbines can be used in this kind of application, provided there is a source of free or low-cost fuel, or a requirement for compactness or noise abatement.

Diesel generators

Diesel generators have been employed for more than a century for standby electric power. They range in commercial size from 200 kW to more than 2 MW. Their advantages include:

  • Wide availability

  • Low purchase cost

  • High reliability and longevity

  • Rapid load assumption (from start to full load in about 10 sec)

  • Excellent load following characteristics

  • High overall energy efficiency (approximately 40%)

  • Low maintenance.

    • Individual units can be run in parallel for installations up to 20 MW. Uncontrolled emissions are high in nitrogen oxides and particulates, limiting their use to less than 300 hr per year in most areas. However, employed as standby generators or in limited peak shaving applications, diesel generators continue to be the most popular choice of onsite electrical power generation.

      Some diesel generators are available with particulate traps and catalytic after treatments. These offer lower emissions than uncontrolled gas turbines.

      Lean burning gas engine generators

      Natural gas burning reciprocating engine-driven generators range in size from about 300 kW to 20 MW. They are suitable for onsite power generation, high-hour peaking operation, or combined heat and power (CHP) applications (Fig.1.). Advantages of lean-burning gas engine generators are:

    • Very low emissions

    • Excellent fuel economy

    • Rapid availability

    • Proven reliability

    • Low purchase cost.

      • Lean-burning gas engine generators are especially suited for CHP applications, where there is a need for both electricity and year-round heat or cooling using an absorption chiller. By using both the electricity and the waste heat from the exhaust and engine water jacket, energy efficiency can exceed 80%.


        Microturbines range in size from 30 kW to 60 kW. They are best suited to applications where there is a source of free or low-cost fuel such as well head gas, bio-gas, coal seam methane, or flare gas. Advantages of microturbines are:

      • Compact size

      • Low emissions

      • Fuel flexibility

      • Low noise

      • High-quality waste heat recovery.

        • When both the electricity and the waste heat are used, overall efficiency is good.

          Gas turbines

          Operated primarily by utilities, the gas turbine has evolved into a workhorse in industry and has become the premier electric generation system for peak and intermediate loads. They are available in sizes ranging from several hundred kW to hundreds of MW.

          A gas turbine uses combustion to produce a high-temperature, high-pressure gas working fluid. It induces shaft rotation by impingement of the gas upon a series of specially designed blades. The shaft rotation drives an electric generator and a compressor for the air used by the gas turbine. Many turbines also use a heat exchanger called a recuperator to impart turbine exhaust heat into the combustor’s air/fuel mixture.

          Advantages of gas turbines are:

        • Great power-to-weight ratio compared to reciprocating engines

        • Smaller than their reciprocating counterparts of the same power

        • Easy to operate.

          • The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoints. Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load.

            Fuel cells

            Fuel cell technology is still emerging. They can be highly reliable and efficient when the waste heat is also used onsite. A fuel cell is an electromechanical system that consumes fuel to produce electricity. The main chemical reaction is not combustion. However, there may be sources of combustion, such as reformers/fuel processors, used within the fuel cell system.

            A phosphoric acid fuel cell has a phosphoric acid electrolyte separating its anode and cathode terminals (Fig. 2). The electrolyte allows positive hydrogen ions to pass through, while blocking electron flow. The hydrogen ions combine with air at the cathode side in an exothermic reaction, which produces heat and water. An inverter, which changes the dc to ac, is connected across the anode and cathode terminals to allow current flow. Typical power output is approximately 200 kW for this type of fuel cell.

            Phosphoric acid fuel cell advantages include:

          • Commercially available (200 kW units)

          • Proven operation on natural gas, hydrogen, landfill gas, and methane

          • Low emissions

          • High availability

          • Potential for a federal rebate

          • Quiet operation

          • High reliability.

            • Phosphoric acid fuel cell disadvantages include:

            • High cost of platinum catalyst component

            • Requirement for external reformer to separate hydrogen from fuel supply

            • Poisoning of reformer by CO, NH 3 , and S

            • High purchase cost

            • High maintenance cost

            • Inability to instantaneously react to step loads.

              • Applications suitable for fuel cells include sites with free fuel byproducts, such as digester gas from wastewater treatment plants, methane gas from cattle rendering plants, and hydrogen from plastics and petroleum facilities.

                Plant Engineering magazine extends its appreciation to Baldor Motors and Drives, and Cummins Power Generation for the use of their material in the preparation of this article.

                Energy productsSwitchgear saves space

                System VI Switchgear combines Vista UDS with the versatility of switchgear in a space-saving, multisection package. This presents an option for applications through 34.5 kV requiring 16-kA symmetrical short-circuit duty. Vista gear includes 600-amp load-interrupter switches and fault interrupters.

                S&C Electric Co.


                Write 421 on PE card

                Surge protector installs in main panel

                Surge Free 4 05 XT Series ac power line protectors install at the main service panel to prevent damage to equipment from transients, lightning, and surges. The unit has an Ipeak of 400 kA for extraordinary performance during catastrophic transient events. Multiple, heavy-duty current pathways and solid copper bus bar construction provide minimal impedance and enhanced current sharing.

                MCG Surge Protection


                Write 423 on PE card

                Automatic standby power

                PSS Series Packaged Standby Generators offer a practical and economical solution to emergency power problems. When a power failure occurs, the system, when combined with an automatic transfer switch, automatically transfers the electric load to the generator set. When power is restored, the load is transferred back to the utility line. The engine can use LP or natural gas.

                Winco, Inc.


                Write 422 on PE card

                Switch automatically transfers to standby power

                Model RMT single-operator and Model RMTD dual-operator automatic transfer switches for 600-V applications are available in 2, 3, or 4-pole configurations for emergency/standby power systems. These switches are labeled for use with any circuit breaker with instantaneous trip and offer front-accessible wiring and rapid arc quenching.

                Russelectric, Inc.


                Write 424 on PE card