Evaluation of the Near Source Air Quality Impact of Distributed Generation

Distributed generation (DG) has been adopted in California because of its potential to supply a significant fraction of increased power demand in the future. DG offers several advantages over conventional centralized power plants. The relatively small size of a DG unit (usually less than 5 MW) allows location of the unit in the immediate vicinity of the area that requires the power. This independence from a large power supplier results in 1) reliability of power supply, 2) potential cost control because the DG unit is tied to the community that it services, 3) reduction of transmission losses, and 4) increased efficiency when waste heat is utilized for local heating and cooling needs. However, the emissions from a DG unit can impact the air quality in the populated area where it is located. This report summarizes the results from a project designed to estimate the local air quality impact of DG growth in the South Coast Air Basin. This project takes advantage of emission inventories developed by UCI (Samuelsen et al., 2005), for several DG growth scenarios in the South Coast Air Basin (SoCAB). We first developed and applied a systematic method to allocate the 5 km by 5 km gridded emission inventory from UCI to individual DG units within each grid. Then, a state of the art dispersion model, AERMOD (Cimorelli et al., 2005) was used to examine the air quality impact of the DG units in these emission inventories. The meteorological inputs for AERMOD were constructed from observations made at 26 meteorological stations maintained by the South Coast Air Quality Management District (AQMD). Finally, we examines the air quality impact of using distributed generation (DG) to satisfy future growth in power demand in the South Coast Air Basin, relative to the impact when the same power is supplied by expanding current central generation (CG) units. In these two scenarios, the emission factors for both the DGs and the CGS are assumed to meet the California standard for new sources. The impact of decreasing boiler emissions by capturing the waste heat from DGs is not examined. The impact from some micro turbines and fuel cells, which are located on the ground but have relative low emissions, is not examined either. The results from this study indicate that 1) Meteorology plays a major role in determining both the maximum hourly as well as annually averaged concentrations associated with DG units, 2) Because of the interaction between buoyant plume rise and meteorology, ground-level impact does not always increase with size of the DG: a 10 MW unit can have a smaller air quality impact than a 1 MW unit. These results suggest that siting of DG units has to pay attention to meteorology to reduce air quality impact. Simulations with AERMOD suggest that DG growth in the SoCAB is likely to have its greatest impact on NO2 and PM levels, which are already close to or exceed either NAAQS or CAAQS in several areas in the SoCAB in 2007. Areas near Central LA meteorological station will exceed the California NO2 annual standard if any future generating capacity is located in the area. The air quality impacts of the two alternate scenarios indicate that the shift to DGs has the potential for decreasing maximum hourly impacts of power generation in the vicinity of the power plants. The maximum impact on hourly concentrations is reduced from 24.5 ppb to 6 ppb if DGs rather than CGs are used to generate power. However, the annually averaged concentrations are likely to be higher than the scenario in which existing CGs are used to satisfy power demand growth. Future DG penetration will add an annual average of 0.1 ppb to the current basin average of 20 ppb, while expanding existing CGs will add 0.05 ppb to the existing level