Energy Use and Emissions
One of the benefits of using natural ventilation as part of the cooling and ventilation strategy for a building is a likely reduction in electricity demand. A reduction in electricity consumption translates to reduced emissions due to avoided electricity generation, assuming the electricity is not generated from renewable resources such as wind or solar. To begin to understand the benefits of reduced consumption of electricity, consider the reduction in emissions of nitrogen oxides (NOx), sulfur dioxide (SO2), carbon dioxide (CO2), and mercury (Hg). According to a recent benchmarking report conducted by the Natural Resources Defense Council (NRDC), Public Service Enterprise Group (PSEG), and the Coalition for Environmentally Responsible Economics (CERES), power plants are responsible for about 23% of NOx, 67% of SO2, 40% of CO2, and 33% of Hg emissions in the United States (CERES et al., 2002). As shown in Table 4, these major pollutants result in acidification of lakes and streams, degradation of built infrastructure, crop and forest damage, harm to aquatic ecosystems, lung and heart disease, respiratory damage, premature mortality, and toxicity to humans (CERES et al., 2002). Emissions reductions per mega-Watt hour (MWh) of electricity generation are also shown in Table 4 to illustrate the environmental value in each unit reduction of electricity consumption.
The values shown in Table 4 represent average emission rates of the largest 100 electric power generation companies in the United States, which account for approximately 90 percent of reported electric industry generation and emissions. Another possibility for reducing emissions is to generate electricity from more efficient or completely renewable resources, such as wind or solar, which results in zero operating emissions. Co-generation plants, which produce electricity, steam, and sometimes chilled water, are also more efficient that traditional electric power plants; thus electricity from co-generation plants results in less emissions than those reported in Table 4. The East Campus Project is considering the use of electricity from the MIT Central Utilities co-generation plant. MIT could also consider buying “green tickets,” a premium on regular electricity market rates that essentially pays for the added cost of electricity from renewable sources, from the local electric utility to result in zero emissions from the electricity needed to run a cooling and ventilation system.
Initial estimates for the cooling load of the new Sloan building are shown in Table 5. To present a conservative “back of the envelope” estimate for the potential reduction in electricity needed for cooling, and subsequent avoidance of emissions, assume that a natural ventilation strategy could provide the cooling needed for half of the load in the transitional months of April, May, and October. This conservative strategy could be likened to people opening their windows on half of the warm, yet breezy days of these three months. Using a standard coefficient of performance (COP) for the air conditioning equipment of 3 to convert from cooling load to electricity requirements, the estimated avoidance of electricity is 62 MWh over these three months, or 7 percent of the total estimated electrical energy requirements (including lighting, HVAC, etc.) for the building in these three months. As shown in Table 5, estimated emissions savings are on the order of 200 lbs NOx, 400 lbs SO2, 90,000 lbs CO2, and <0.0001 lbs Hg, assuming average emissions rates per MWh given in Table 4.
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