Weather Report: February 2004
Using VLI Data To Analyze CO2 Ventilation Control SavingsWith more and more buildings "going for the green," owners and their consultants are constantly looking for ways to pick up more points by reducing energy consumption. One such measure gaining popularity is ventilation control through carbon dioxide (CO2) monitoring. And there's no better tool out there than VLI data to provide a quick analysis of this system without having to model an entire building.
Although it's not toxic, CO2 can be dangerous in high concentrations because it is an asphyxiant (displaces oxygen). Concentrations exceeding 35,000 ppm can cause shortness in breath. Above that level, the central nervous system begins to shut down. Ambient CO2 levels are usually in the 350 to 375 ppm range. The recommended CO2 concentration level for an occupied space is about 0.1%, or 1,000 ppm (refer to ASHRAE Ventilation Standard 62-89).
Building ventilation systems must continuously introduce a sufficient amount of outside air to maintain the CO2 concentration below this level. The above ASHRAE standard provides the minimum amount of outdoor air required per person to accomplish this for various spaces and activity levels. These recommendations are based on the calculated CO2 production per person plus a small margin of safety (see Appendix D of the standard). As an example, the recommended minimum ventilation rate for a typical office space is 20 cfm/person.
A traditional ventilation system (constant volume) will provide a constant minimum amount of outside air to a building or space whenever the space is "occupied." When the space is "unoccupied," the outside air dampers are closed. A CO2 ventilation control system (variable volume) will vary the amount of outside air being supplied to the space to maintain the concentration of CO2 below the desired level.
The first step in modeling this system is to establish the ventilation load profile, which would be based on a known occupancy schedule. Figure 3 provides a ventilation profile (% of outside air being conditioned each hour) for an assumed occupancy schedule for a constant volume system compared to a variable volume system with CO2 control.
Next, we generate VLI data based on these profiles. For those of you who may be new to this column, VLI data (ventilation load index) is the calculated sensible and latent heating and cooling energy per unit cfm of outside air required to maintain a constant temperature "leaving the coil." If we assume a temperature of 55°F, the sensible cooling VLIsc is calculated for each hour that the outside air temperature is above 55° and the sensible heating VLIsh is calculated for every hour below 55°. The latent cooling VLIlc is calculated whenever the OA humidity ratio is above the humidity ratio associated with 55° at saturation. And for humidified spaces, the latent heating VLIlh is calculated for each hour that the OA humidity ratio is below the humidity ratio associated with a defined space %rh condition.
To generate VLI data based on a variable load profile similar to Figure 3, these hourly calculations are simply multiplied by the % load defined for the hour. Figure 4 provides an example summary of this calculation based on the assumptions indicated. In this particular example, CO2 control would result in about a 50% reduction in ventilation energy costs compared to the constant volume system (depending on location). These savings could be significant given the comparatively small costs required to install CO2 sensors and associated controls (assuming a DDC system already exists). As the demand for more energy savings intensifies and the accuracy and dependability of CO2 sensors improve (a subject for another day), CO2 control will likely become a fairly common cost saving measure. ES
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NOTES - This report is generated from raw data furnished by the National Weather Service (NWS). Normal max/min and degree day values are from the historical record provided by the National Climatic Data Center (NCDC). ASHRAE design hours are number of hours that meet or exceed the 1997 ASHRAE Fundamentals design conditions. The cooling ventilation load index (VLI) is the total (sensible + latent) energy in ton-hrs/cfm required to maintain 55°F discharge air temperature. Likewise, the heating VLI is the sensible heat energy in Therms/cfm required to maintain 55°. The humidification VLI is the amount of water in gal/cfm required to maintain 30% rh at 70° space temperature.