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Cost Analysis of Indoor Air Abstracts
Abstract
A simplified methodology is defined that can be used by indoor air quality (IAQ) diagnosticians, architects/engineers, building owners/operators, and the scientific community, for preliminary comparison of the cost-effectiveness of alternative IAQ control measures for any given commercial or institutional building. Such a preliminary analysis could aid the user in initial decision-making prior to retaining experts (such as HVAC engineers and building modelers) who could conduct a rigorous evaluation.
This preliminary methodology consists of text, logic diagrams, and worksheets that are intended to aid the user in:
- assessing which IAQ control option(s) might be applicable in the specific building being addressed;
- designing alternative control measure [involving increased outdoor air (OA) ventilation, air cleaning, or source management steps], and developing rough installed and operating costs for these measures;
- estimating the approximate effectiveness of the alternative control measures in reducing occupant exposure to contaminants of concern; and
- comparing the cost-effectiveness of the alternative control measures under consideration, to aid in selection of the optimal control approach.
In this report, the term "cost-effectiveness" refers to the incremental increase in annualized cost per unit reduction in exposure by the building occupants. "Exposure" is the number of person-hours per year during which the occupants are exposed to a unit concentration of the contaminant of concern; in this report, the units of exposure are (mg/m3)-person-hr/yr. The most cost-effective control approach is the one offering the lowest annualized cost per unit reduction in exposure.
Abstract
A cost comparison has been conducted of 1 m3/s indoor air cleaners using granular activated carbon (GAC) vs. photocatalytic oxidation (PCO), for treating a steady-state inlet volatile organic compound (VOC) concentration of 0.27 mg/m3. The commercial GAC unit was costed assuming that the inlet VOCs had a reasonable carbon sorption affinity, representative of compounds having four or more atoms (exclusive of hydrogen). A representative model PCO unit for indoor air application was designed and costed, using VOC oxidation rate data reported in the literature for the low inlet concentration assumed here, and using a typical illumination intensity. The analysis shows that, for the assumptions used here, the PCO unit would have an installed cost more than 10 times greater, and an annual cost almost 7 times greater, than the GAC unit. It also suggests that PCO costs cannot likely be reduced by a factor greater than 2 to 4, solely by improvements in the PCO system configuration and reductions in unit component costs. Rather, an improved catalyst having a higher quantum efficiency would be needed, increasing reaction rates and reducing illumination requirements relative to the catalysts reported in the literature. GAC costs would increase significantly if the VOCs to be removed were lighter and more poorly sorbed than assumed in this analysis.
Abstract
A series of computer runs has been completed using the DOE-2.1E building energy model, simulating a small (4,000 ft2) strip mall office cooled by two packaged single-zone systems, in a hot, humid climate (Miami). These simulations assessed the energy penalty, and the impact on indoor relative humidity (RH), when the outdoor air (OA) ventilation rate of the office is increased from 5 to 20 cfm/person in this challenging climate to improve indoor air quality. One objective was to systematically assess how each parameter associated with the building and with the mechanical system impacts the energy penalty resulting from increased OA. Another objective was to assess the cost and effectiveness of off-hour thermostat set-up (vs. system shut-down), and of humidity control (using overcooling with reheat), as means for reducing the number of hours that the office space is at an RH above 60% at the 20 cfm/person ventilation rate.
With the baseline set of variables selected for this analysis, an OA increase from 5 to 20 cfm/person is predicted to increase the annual cost of energy consumed by the heating, ventilating, and air-conditioning (HVAC) system by 12.9%. The analysis showed that the parameters offering the greatest practical potential for energy savings are conversion to very efficient lighting and equipment (1.5 W/ft2) and conversion to very efficient cooling coils (electric input ratio = 0.284). If the increase to 20 cfm/person were accompanied by either of these conversions, the 12.9% HVAC energy penalty for the increased OA rate would be eliminated; the modified system at 20 cfm/person would have a lower annual HVAC energy cost than the baseline system at 5 cfm/person. Other parameters offering significant practical potential for energy savings are: conversion from packaged single-zone units to a variable air volume system; conversion to cold-air distribution (minimum supply air temperature = 42 °F); or improvements in the glazing or in the roof resistance to heat transfer. If the OA increase were accompanied by any one of these modifications, the 12.9% penalty would be reduced to between 2 and 7% (the modified system at 20 compared against the baseline at 5 cfm/person).
According to the DOE-2.1E model, the increase in ventilation rate could be achieved with an 85% reduction in the number of occupied hours above 60% RH, compared to the baseline system at 5 cfm/person — with only a $19/year increase in energy cost — if the economizer were eliminated. That is, most of the elevated-RH hours in the baseline case were predicted to be the result of economizer operation. If the control system were modified so that it controlled the humidity as well as the temperature in the office space, all of the elevated-RH occupied hours would be eliminated, at an energy cost of $90/year.
Neither economizer elimination nor humidity control would address unoccupied periods, when most of the elevated-RH hours occur. Building operators concerned about biological growth at elevated RH should consider operation of the cooling system during unoccupied hours, perhaps with the thermostat set up, rather than system shut-down off-hours. Off-hour set-up from 75 to 81 °F would add only $10/year to energy costs, and would provide some modest reduction in unoccupied elevated-RH hours. Set-up to 79 °F would provide a greater reduction, at an energy cost of $38/year.
DOE-2.1E underestimates the number of elevated-RH hours because it does not address the moisture capacitance of building materials and furnishings, or re-evaporation off the cooling coils when they cycle off with the air handler operating. As a result, the performance of the RH reduction steps above may be overestimated, or the costs of the steps underestimated.