A paper authored by two Edison Electric Institute (EEI) managers questions the environmental advantages often attributed to distributed generation.
“While DG systems can produce fewer emissions and are cleaner than older central-station power generation, state environmental regulators are starting to focus on DG emissions,” say Louis Harris, manager of retail restructuring policy for EEI and Steven Rosenstock, manager of energy solutions. “There also may be local regulators, especially in those cities classified as nonattainment areas by the Environmental Protection Agency (EPA) that could become more involved as more DG is used.”
The paper notes that at least 90 percent of back-up onsite generation units run on diesel fuel. According to a report published late last year by the Renewable Energy Policy Project (REPP), there were 667,626 diesel generators in the United States in 1998, with a total generating capacity of 108,797 MW. At an annual growth rate of 1.7 percent, according to the report, this number could reach 127,500 MW in 2010.
Yet, say the authors, diesel generators have the highest emissions of any fossil- fueled power plant by far, according to REPP’S Virinder Singh. These generators released nearly 300,000 tons of nitrogen oxides (according to 1996 statistics); overall NOx emissions from fossil-fueled power plants in 1999, according to EPA, were 5.715 million tons. A report by James Lents and Juliann Allison of the University of California, Riverside, confirms that diesel DG systems consistently had the highest emission factors in terms of lb/kWh generated. Lents and Allison compared emission factors for various DG technologies with those of combined-cycle gas turbine systems (CCGT). The CCGT systems had the lowest or second- lowest emission factor for many of the emissions listed. (Only the proton exchange membrane – or direct – fuel cell, which showed 0.00000 values for some emissions, had lower values).
On the other hand, comparing emissions between a natural gas-based DG system and a coal-based plant reveals that the former releases fewer. But gas-based DG functions primarily at peak, while a coal-based plant is used for baseload operations. Currently DG capacity comes at a high cost, while coal generators have very low operating costs: Without major improvements to DG technology, baseload coal power plants will continue to be more economical and efficient.
The EEI paper states that a more meaningful and realistic comparison is to be made between all DG systems and peaking power plants, which are likely to be single-cycle turbines using natural gas or oil. The differences in emissions between gas turbines and gas DG technologies are small. According to the report, newer central-station power plants are usually more efficient than fossil-fueled DG, even considering a reduction in transportation losses. CCGTS typically convert 55-57 percent of the fuel into electricity, while newer technologies are projected to achieve a 60 percent efficiency. Lents and Allison show that fossil fuel-based DG systems convert only 27-44 percent of their fuel into electricity: Of the various technologies, only diesel internal combustion engines are higher than 40 percent. In a draft report released in November 2001, the Regulatory Assistance Project reports similar efficiency values – only solid oxide fuel cells pierce the 40 percent efficiency mark, while microturbines come in last.
Reusing industrial heat for power generation improves the environmental performance of all fossil-fueled DG, but by how much depends on the situation. When combined with the latest DG technologies, CHP will have an impressive air quality and efficiency advantage over the stand- alone system. Whether this is enough to surpass the efficiency of baseload generation depends on many factors, including whether the customer’s need for heat and electricity occurs at the same time, whether the efficiency and emissions of the generation are supplanted, and when the facility operates.
The paper concludes that, in the final analysis, when air pollution issues are considered, only the lowest-emitting DG system with significant waste heat recovery is even marginally competitive with combined-cycle power production. In the U.S., nuclear energy, hydroelectric, wind, and solar central station systems provide more than 30 percent of the electricity needed-emissions-free. Adding DG into the generation portfolio will improve air quality most significantly only if it substitutes for new or existing fossil-fueled generators.