Discovering Appropriate Cogeneration Opportunities

Discovering Appropriate Cogeneration Opportunities

5.10.04

Wade Troxell, Professor/Director, Colorado State University

Patrick Dawson, Student, Mechanical Engineering, Colorado State University

Free electricity. Sound enticing? To thousands of commercial applications "free" electricity has become a reality. Utilizing the concept of cogeneration, the simultaneous production of a thermal and electrical load from a single thermodynamic process. Cogeneration, or Combined Heat and Power (CHP), is not a new concept, (it can be traced back to medieval smokejacks) but it is a concept that is far too often overlooked. New, packaged CHP systems are now designed and sold to be more easily installed and quickly producing at preexisting facilities. These packaged systems are sold in a variety of forms, including diesel gen-sets, fuel cells, combustion gas turbines and coal engines, and all utilize the waste heat given off in the electricity production process. With cogeneration commercial facilities can potentially experience:

Higher power reliability
Lower electrical and thermal costs
Less vulnerability to grid failure or sabotage
Environmental benefits due to reduction of power supplied by utility

Despite all of the advantages that CHP offers, it is not practical for every situation, and each application must be carefully analyzed to discover the appropriateness. There are some basic questions that should first be addressed to decide if cogeneration suitably fits the given application. They are as follows:

How is the thermal load distributed at the facility?
- CHP works best when the thermal load (hot or cold) is centrally distributed, therefore no existing thermal system needs to be removed or installed to properly utilize the waste heat
Is the electrical load constant? Or at least have a baseload?
- For cogeneration to be feasible the device should be operating continuously so that all of the electrical power will be utilized and the device can operate at maximum efficiency.
Can the waste heat from the CHP device be properly utilized?
- For maximum system efficiency the waste heat needs to be fully consumed.
How is the thermal load currently supplied at the facility, and how will it be supplied with CHP?
- The choice of CHP technology depends heavily on this due to fuel cost volatility, and installation of new fuel systems.
Do the local electrical costs support the production of on-site power?
- Even though the efficiency of most cogeneration systems are significantly higher than utility power plants, the cost to produce on-site electricity can still cost more based on different fuel sources.

An example of an appropriate CHP situation can be demonstrated in a case study that was performed at a large, industrial application. In this specific application, there was a pre-existing district heating system, which satisfied the thermal load distribution question. The thermal load must simply now be generated from a different source. The system provided steam from three centrally located natural gas fired boilers, and experienced a maximum steam demand at ~ 190klbs/hr and a summer minimum at 15 klbs/hr. This required a very flexible thermal system, therefore a gas turbine was selected due to the equivalent fuel system and the large thermal load range that can be gained with the use of supplementary duct burners.

The electrical load was then examined to find the appropriate power generation capacity of the turbines. The system experienced an alternating electrical consumption trend with a peak at 16 MW and an offset baseload of 8 MW. Therefore, two 3.5 MW turbines were chosen to provide an overall 7 MW to the system. The use of two turbines adds redundancy to the system, which in turn creates reliability.

The turbines have a steam capacity of ~17 klbs/hr, therefore only on the very hottest summer days would the waste heat not be fully utilized. With the addition of supplemental duct burners to raise the exhaust temperature the turbines could create a combined maximum of ~ 216 klbs/hr of steam, consequently having the capability to appropriately supply the thermal load with minimum waste.

Finally, the cost of supplying the power on-site was compared to purchasing power from the utilities. Currently the application was purchasing power at an average of 4.5 cents per kilowatt-hour from the local utility. This price is currently one of the lowest in the US and still offered an estimated electrical savings, based on the two turbines operating continuously, of ~ $2.1 million per year. There was a 17% increase in fuel usage though, which resulted in a net savings of approximately $1.4 million dollars per year. It is also important to note that while the savings of this system are susceptible to fickle fuel prices the application had no plans to replace the natural gas fire boilers in the near future and would only experience a 17% increase in fuel to offset savings.

In addition to all of the financial benefits of cogeneration for this application there were also environmentally friendly advantages. For example, the NOx emissions of the district heating plant would be reduced from 60 tons/year to 36 tons/year. Also, there would be avoided emissions from the coal-powered local utilities. The estimated simple payback period of installing this system was a mere 3.5 years.

As can be seen from this random case study, when the conditions are apt the choice of implementing cogeneration is an obvious one. Each application must be properly analyzed and some basic questions first answered, but if the situation is right rewards can include substantial environmental benefits, increased power reliability, and significant financial savings.

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