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On-Site Cogeneration Systems

Cogeneration is a variation of distributed generation, but it is an often misused and misunderstood term.  A common thought is that cogeneration means running a generator in parallel with the utility grid, or "co-generating".  While this is often done, this is not the meaning of the term. Cogeneration is simply using one energy source to produce two separate forms of energy.  Cogeneration is also known as combined heat and power (CHOP).  In a correctly designed application, one of these forms of energy may be considered "free", since it is a by-product of the production of the primary energy form.  Cogeneration is a method by which an energy user can control and reduce energy costs by utilizing the heat that would normally be wasted and use some or all of it for the thermal requirements of a facility.

Cogeneration is accomplished by utilizing the exhaust gas heat from the internal combustion engine driving the generator (which would normally be lost "up the stack"), the engine cooling jacket water, oil cooling waste heat or a combination of these.  This waste heat can be utilized in a direct application for desiccation, drying or heating various materials or for space heating applications, or may be passed through heat exchangers to produce heated air, hot water or steam.    This "free energy" can either be used in its initial form or may be converted to useable energy for boiler systems, chillers, air conditioning and many other applications, and will contribute significant additional savings to a facility using on-site generation.

Gas Turbines for Distributed Generation

 

Gas turbines (or combustion turbines) are available in a range of sizes. Gas turbines are used mainly above 1 MW, though smaller units – microturbines – are produced in sizes less than 200 kW. Gas turbines can operate in different cycle configurations, including:

• Simple Cycle
• Recuperated Cycle
• Combined Cycle

Simple-Cycle Gas Turbines

Gas turbines can be either a single-shaft machine (with compressor and power turbine on the same shaft) or a split-shaft machine as shown above. The simple-cycle gas turbine includes an air compression section, a burner or combustor (most of which are based on dry low NO_x combustor principles), and a power turbine driving a load such as an electric generator.

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Recuperated Gas Turbines

A recuperated turbine is similar to a simple-cycle gas turbine, except for the inclusion of a special heat exchanger – called a recuperator – that captures exhaust thermal energy to preheat compressed air before the burner. Capturing exhaust energy helps increase electrical efficiency compared to a simple-cycle gas turbine.

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Combined Cycle Gas Turbine

These turbines build on the concept of heat recovery by capturing exhaust energy in a device called a heat recovery steam generator (HRSG). The HRSG may also include a burner to increase the steam output. Steam from the HRSG drives a steam turbine – generating power in addition to the main power turbine. Combined-cycle gas turbine power plants significantly increase power generation output and efficiency compared to a simple-cycle design.

With heat recovery, total efficiency can be high for all cycles. Electrical efficiency increases for recuperated and combined cycle configurations and generally improves as turbine size increases.

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Gas turbines can be classified into four different categories:

• Heavy Duty (frame type)
• Aeroderivatives
• Industrial
• Microturbines

This table shows selected gas turbine manufacturers.

 

Heavy Duty	Aeroderivative	Industrial	Microturbine
ABB	GE	ABB Alstom	Allied Signal
GE	Pratt & Whitney	Solar Turbines	Capstone
Siemens-Westinghouse	Rolls Royce-Allison		Elliott
			NREC

Most DG turbine applications (below 25 MW) use smaller aeroderivative, industrial, or microturbine machines in a simple cycle or recuperated cycle. Microturbine products are being introduced during 1999-2001. Larger "merchant" natural gas power plants (above 25 MW) use heavy duty and larger aeroderivative machines in either a simple or combined-cycle operating mode.

Microturbines...Where Smaller Can Be Better

One of the axioms of the Industrial Age was growth in the size of machinery. Small millstream-powered machinery gave way to larger -- even massive -- and more powerful machines.

Not surprisingly, one of the axioms of the Information Age is a sharp reversal of that trend. Even while machines of all types are becoming more powerful and useful, they are becoming smaller and smaller.

Today's telephones, calculators and computers are a lot smaller and more powerful than their predecessors of 50 years ago. Today, even some electricity generators have followed the trend to smaller and more useful.

Called microturbines, they are small, high-speed generator power plants that include the turbine, compressor, generator, all of which are on a single shaft as well as the power electronics to deliver the power to the grid. Microturbines have only one moving part, use air bearings and need no lubricating oil.

These small power plants could operate on natural gas, diesel, gasoline or other similar high-energy, fossil fuel.

There are several microturbine systems manufactured in the USA.  Among them:

The Capstone MicroTurbine™ System

Applications:

• Peak shaving and load management - Business customers connected to the electric grid can use microturbines to reduce electric costs during periods of peak power usage. Because of the microturbine's compact size, reliability, as well as its low levels of emissions and noise, it can be installed at a number of locations that would be inaccessible to traditional generating applications.
• Cogeneration - Microturbines produce heat that can be reclaimed for secondary uses (e.g., air or water heating or drying applications). Recycling the waste heat for productive uses significantly enhances the cost savings to small and medium-sized businesses.
• Standby or backup power generation - Backup power generation is important to most businesses. Short-term power interruptions can result in lost sales, lower productivity and damage to inventory. The microturbine provides a reliable source of backup power for your business.

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