Coal, Cogeneration, Gas

Generating Power to Increase Efficiency

Issue 11 and Volume 118.

alt   By Sherif Youssef, P.E., Philadelphia Gas Works

Combined heat and power (CHP), also known as co-generation, is the simultaneous production of useful thermal energy and electric power. The technology is receiving increased consideration by consulting engineers and architects as a viable option for customers who want to generate their own electricity. Generally, a CHP integrated mechanical system refers to a power generation process in which thermal heat is recovered and used to drive heating, hot water, or cooling through an absorber. CHP involves a facility’s entire energy delivery system. This useful thermal energy is essential for the success of the application.

Modular designs for CHP systems allow for easy expansion. Onsite generation can be equipped with dual-fuel capabilities of natural gas and/or oil. This helps increase the reliability of the system and reduces its operating costs. To qualify for a discounted or interruptible rate, an alternate fuel should be used as a backup in case of interruption. Oil, propane, and electricity have all been used as backups for engines or turbines. Turbine efficiencies are in the range of 30 percent, and overall thermal efficiency can reach 70 to 80 percent when heat recovery is added to the equation.

The power consumption requirement for a commercial customer is always greater in the summer than in the winter. Because electricity cannot be stored, utilities build generating capacity to supply the maximum summer load requirements. To recoup this investment, a utility must charge for the excess capacity it generates to meet summer requirements. Through CHP, customers can avoid the need for excess capacity and have a flat electric demand all year long. CHP can also help the customer become more energy independent, reduce the end of system load levels, reduce system losses, and improve voltage.

CHP begins with a power generation process in which thermal heat is recovered and used to drive cooling and/or heating equipment. Various technologies are available to generate onsite electricity. These include internal combustion (IC) engines, gas turbines, microturbines, steam turbines, and fuel cells. Facilities or businesses must develop feasibility studies which are unique to their energy demands and can identify technologies that are appropriate for their needs. Possible technologies include:

  • IC engines – The reciprocating engine market is growing robustly, averaging 27 percent annual growth in sales over the last 10 years. Dual-fuel engines used in co-generation applications can be run on natural gas, and light fuel oil engines represent a reliable technology that is easily serviced and maintained with relatively low capital costs.
  • Gas turbines – Gas turbines can be operated in several different configurations. They can be combined with a heat recovery steam generator (HRSG) to form a system in which both gas and steam turbines are used in a combined cycle. Gas turbines are relatively inexpensive to own and operate.
  • Microturbines – If noise level and avoidance of oil is a concern, microturbines can be an attractive option. Some products offer flexible modular systems, but microturbines are generally more expensive than IC engines.
  • Steam turbines – Steam turbines provide fuel flexibility and come in multiple configurations. They can be used alone or in conjunction with gas turbines in a combined cycle system. The primary energy of the new steam is used in the turbine to generate electric power and is subsequently used to generate process heat or steam.

Customers should evaluate the feasibility of a co-generation project for their facilities. Examples of facilities that might be good candidates for co-generation include universities, hospitals, hotels, nursing homes, prisons, large apartment complexes, and industrial manufacturers that have thermal needs such as drying, melting, or sealing.

There are several factors to consider when exploring the suitability of co-generation for a facility’s needs. These factors include average costs of electricity, heating fuel costs, fuel flexibility, operation and maintenance costs, and potential impacts of regulatory emissions. Facilities should determine whether or not they need substantial amounts of thermal energy. Other factors such as the number of hours of operation and the location of the facility may impact decisions about a project.

In the last couple years, reliability has driven additional interest in CHP systems. CHP systems can be a potential solution for organizations wanting to diversify energy options and reduce costs. It can also help customers meet their long-term energy and power goals.

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