731 Celcap

Lee (1988) reports that the Naval Civil Engineering Laboratory developed a cogeneration analysis computer program known as Civil Engineering Laboratory Cogeneration Program (CELCAP), "for the purpose of evaluating the performance of cogeneration systems on a lifecycle operating cost basis." He states that "selection of a cogeneration energy system for a specific application is a complex task." He points out that the first step in the selection of cogeneration system is to make a list of potential candidates. These candidates should include single or multiple combinations of the various types of engine available. The computer program does not specify CHP systems; these must be selected by the designer. Thus, depending on the training and previous experience of the designer, different designers may select different systems of different sizes. After selecting a short-list of candidates, modes of operations are defined for the candidates. So, if there are N candidates and M modes of operation, then NxM alternatives must be evaluated. Lee considers three modes of operation:

1) Prime movers operating at their full-rated capacity, any excess electricity is sold to the utility and any excess heat is rejected to the environment. Any electricity shortage is made up with imports. Process steam shortages are made-up by an auxiliary boiler.

2) Prime movers are specified to always meet the entire electrical load of the user. Steam or heat demand is met by the prime mover. An auxiliary boiler is fired to meet any excess heat deficit and excess heat is rejected to the environment.

3) Prime movers are operated to just meet the steam or heat load. In this mode, power deficits are made up by purchased electricity. Similarly, any excess power is sold back to the utility.

For load analysis, Lee considers that "demand of the user is continuously changing. This requires that data on the electrical and thermal demands of the user be available for at least one year." He further states that "electrical and heat demands of a user vary during the year because of the changing working and weather conditions." However, for evaluation purposes, he assumes that the working conditions of the user-production related CHP load-remain constant and "that the energy-demand pattern does not change significantly from year to year." Thus, to consider working condition variations, Lee classifies the days of the year as working and non-working days. Then, he uses "average" monthly load profiles and "typical" 24-hour load profiles for each class.

"Average" load profiles are based on electric and steam consumption for an average weather condition at the site. A load profile is developed for each month, thus monthly weather and consumption data is required. A best fit of consumption (Btu/month or kWh/month) versus heating and cooling degree days is thus obtained. Then, actual hourly load profiles for working and non-working days for each month of the year are developed. The "best representative" profile is then chosen for the "typical working day" of the month. A similar procedure is done for the non-working days.

Next an energy balance or reconciliation is performed to make sure the consumption of the hourly load profiles agrees with the monthly energy usage. A multiplying factor K is defined to adjust load profiles that do not balance.


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