How Does a Cogeneration Plant Work?

May 21, 2014  |  Jim Pretz, Managing Principal & CEO

What is a cogeneration plant and how does it work? The answer lies in another name for the concept – Combined Heat and Power (or CHP). A cogeneration or CHP plant generates both electrical energy (often measured as power) and heat energy.

In its simplest form, this is accomplished by driving an electrical alternator (i.e., an AC power generator) with a turbine or internal combustion engine and capturing the resultant heat (caused by combustion, friction and I2R losses) that would be vented up the exhaust stack or dissipated by a radiator in a simple cycle engine-generator. By converting that heat to useful steam or hot water, a CHP unit can produce combustion efficiencies of 85% or more compared to efficiencies in the mid-30% range for simple cycle generation.

Who can use cogeneration?

Institutions such as universities, industrial process plants, and medical campuses that have a year-round need for both electrical power and steam are candidates for cogeneration. Traditionally, such facilities have purchased electrical energy from a utility company and burned gas, oil, or coal in boilers to produce steam or hot water. But, in areas where the relative cost of purchased electricity versus a readily available fossil fuel like natural gas is favorable, CHP can satisfy much of a facility’s electricity and heating needs at a substantially lower cost than the traditional model.

More about cost

Cogeneration units require substantial capital investments and they introduce some additional ongoing expenses, such as the cost of maintenance contracts offered by several equipment suppliers. However, as the cost of electricity has continued to rise, and as the price of natural gas has plummeted with recent shale gas discoveries, the operational cost savings that accrue from cogeneration can provide a very attractive return on the investment. Moreover, many facilities are facing substantial infrastructure-related capital investments anyway for reasons such as,

  • Compliance with environmental regulations
  • Enhancement of energy security
  • Replacement of antiquated equipment

Cogeneration can be an effective solution to all of the above; and when the operational savings are assessed only against the incremental cost of CHP as compared to investment required for other reasons, the return on investment can make cogen a “no-brainer.”

Energy Security

A CHP unit can enhance the security of the electrical service to a facility in a number of ways:

  1. Quite often the CHP electrical alternator operates in parallel with the local electric utility, and the two sources share the load. In a well-engineered system an interruption of electric utility service will initiate a sequence in which non-critical loads are shed to reduce the total load to a level that can be supported by the CHP alternator. Other relaying and switching will simultaneously function to maintain continuous service to critical loads via the CHP alternator.
  2. In addition, the CHP can be engineered to operate in “island” mode, completely disconnecting critical loads from the utility grid. This can be useful to avoid exposure to storms or utility line switching transients that could otherwise disrupt critical loads.
  3. And because a CHP unit can be designed to operate on multiple fuels – for instance natural gas and No.2 fuel oil – a disruption in the primary fuel supply does not have to put the CHP unit out of commission.Dual fuel capability also enhances the heating energy security provided by a CHP unit. And a unit that is equipped with duct burners and a fresh air fan can continue to produce steam (or hot water) even when the turbine-generator is out of service.

Saving money, improving reliability, reducing emissions

For many facilities, cogeneration is an energy solution that can save money, improve energy reliability and security, and reduce the facility’s “carbon footprint.” An assessment and/or study by a qualified engineering team can help you determine whether cogeneration should play a part in the energy solution for your facility.

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