Recent changes in technology and in government regulations have changed cogeneration energy system possibilities. Micro-turbine generators have become available that are not only reasonably priced, but also competitively efficient in converting gas to electricity. Micro-turbine generators that have recently become available have gas-to-electric conversion efficiencies of 25-28%. This has not previously been the case. In government, recent legislation has deregulated the power generation industry in some locations. Previously, only regulated electric utilities were permitted to sell electricity. Now, in some states, anyone is permitted to generate electricity for their own use or to sell.
The purpose of this study is to examine the environmental effects as well as the economics of micro-cogeneration energy systems. The goal of implementing such a system would be to reduce total energy resource consumption, thereby increasing operating efficiency and reducing carbon emissions, while simultaneously economically benefiting the facility. Considered herein, a micro-cogeneration energy system consists of micro-turbine driven generator(s) and exhaust heat exchanger(s). The terms "cogeneration energy system" and "cogenerator" are used interchangeably. The micro-turbine driven generators run on natural gas and provide electricity and a high-temperature exhaust. The prefix "micro-" refers to a power rating less than about 500 kW. The heat exchangers transmit the thermal energy of the exhaust to a hot water storage tank. Switchgear is also required for connecting the system to the facility without interfering with the grid. For control, and as a safety measure in this study, electricity is drawn into a sub-circuit of the facility either from the power plant or the turbine, but never both simultaneously.
The application of a cogeneration energy system was studied at an athletic club in Atlanta, Georgia. This application was chosen because it has a very high thermal hot water demand, and the water demand from day to day is nearly constant throughout the year. It was found that an 84-kW micro-turbine generator system could meet most of the hot water demand. It was assumed that the exhaust thermal energy would be used to meet their hot water demand, as opposed to their space heating demand. An athletic club has no space heating demand in the summer, so the cogenerator would not be run. It is not economical to have costly equipment sit idle. Contrarily, the water heating demand is large throughout the year.
Sized to meet the hot water load, the turbine generator system would only produce a small percentage of a typical health club’s electrical demand if it were run at full load during business hours. That being the case, the optimum operating strategy for the club would be to use the cogeneration system to meet their hot water demands and only supplement their electrical needs. This operation strategy is called "thermal tracking." The cogenerator is run with the intent to satisfy the thermal demand of a facility, and the electricity it produces is supplemental.
"Electric tracking" describes the strategy for which a cogeneration system would be chosen to satisfy all of the electrical needs of a facility, thereby rendering the heating capability of the system supplemental. Such a facility could be completely independent of their local electric utility. For athletic clubs, that much electrical production would correspond to a thermal output much larger than what can be utilized.
In a third optimum, a system can also be designed to be operated only when it is economically beneficial to the facility using it. This is called "economic tracking." Calculations would be performed to find an effective break-even point, i.e., at what cost of electricity does it become economically impractical to run the cogenerator? When the incremental cost of purchased electricity falls below that break-even point, the facility would not run the cogenerator. However, if the incremental cost of purchased electricity is higher than the additional cost of natural gas used to run the cogeneration energy system, it is wise to run the cogenerator.
Sometimes the most beneficial operating strategy overlaps with another. For the athletic club studied here, it turns out that it is always economical to run the cogenerator. Hence there is a 100% overlap between thermal tracking and economic tracking. However, running the cogenerator full time throughout the year exceeds the thermal load. Further, it may not be safe to run the system when the facility is closed and no staff members are on duty. Therefore, thermal tracking is taken to be the optimum for an athletic club.
An economic analysis is also necessary in order to quantify the economic benefits of cogeneration. A statistical analysis is necessary to size the cogenerator for probable hot water thermal loads. It is not practical to design the system for an assumed constant hot water load. An environmental analysis is also needed to demonstrate the effect a cogeneration system would have on carbon emissions. These analyses are carried out in Chapter III.
The two main components of the cogeneration system, the micro-turbine generator and the heat exchanger, are described in Chapter IV. The exact specifications of the micro-turbine generator selected are also mentioned in this chapter.
The operations data taken from an athletic club are discussed in Chapter V. Utility usage records were obtained and analyzed. Monthly patterns were noted of electricity and natural gas use. This chapter also reports 24 randomly selected days of daily utility usage recordings and one business day of hourly recordings.
The analyses in Chapters II and III were used and developed to predict what size cogeneration system would be ideal for the athletic club. These results are reported in Chapter VI. The net savings and pay back period of the system were also found. Further, the environmental effects of the cogenerator are noted. Both environmental and economic benefits were found. The economic benefits accounted for a sales tax of 7% in Georgia.
The different factors that affect the economic simple pay back period of the cogeneration system are noted in Chapter VII. These factors include the statistical profile of the facility’s thermal demand, the size of the cogenerator, and the electric rate structure the power utility uses to bill the facility for electricity purchase. The fact that it is profitable, not costly, to reduce carbon emissions when cogeneration technology is utilized is reemphasized. Cogeneration technology is also helpful in the matter of conserving natural fuel resources. The manner in which businesses decide whether or not to invest in new equipment is considered. The effects government can have on carbon emissions reduction by deregulating utility companies and by offering incentives are discussed in this chapter. Unfortunately, many national energy and environmental strategists and business owners are not aware of the already-existing technologies that make it possible to profitably reduce emissions. Most likely, business owners would implement cogeneration systems if they were aware of its economic benefits. Recommendations are made for future analyses that can be applied to other facilities.