P3-1. Demand-Controlled Ventilation Assessment > Outcomes

• The literature review revealed that there is a fairly wide consensus on the best applications for CO2 control. Most discussions of CO2-based DCV mention the following building types as good candidates: public buildings such as cinemas, theaters and auditoria, educational facilities such as classrooms and lecture halls, meeting rooms, and retail and restaurant establishments. However, it is interesting to note that most of the case studies have investigated office buildings and that most reported studies did not include enough information about CO2 readings, sensor locations, and other important data to resolve a number of important questions.
  
  
• Ten demonstration sites were recruited. The initial plan was to recruit 12 sites to serve Projects 2.1 (FDD for Rooftop AC), Project 3.2 (Night Ventilation and Building Thermal Mass), and Project 4.2 (Ventilation Recovery Heat Pumps). Pairs of McDonald's restaurants were recruited in Oakland and Sacramento (with DCV applied only to the play rooms since these are isolated from the interactions of the dining and kitchen HVAC systems). Oakland Unified School District and Woodland Joint Unified School District (near Sacramento) each provided two modular school rooms. Walgreen's agreed to provide two stores in southern California but since each store had 5 HVAC units, the number of sites was limited to 10.
  
  
• A stand-alone ventilation strategy assessment tool was developed to perform the DCV energy simulations. The research team elected to develop a stand-alone assessment software tool, Ventilation Strategy Assessment Tool (VSAT) , to evaluate DCV, heat pump energy recovery (HPHR) (Project 4.2), and enthalpy exchanger heat recovery (HXHR). The primary evaluation approach involved the use of detailed simulations to estimate operating costs and economic payback periods.
  
  
• The cooling and heating season performance of DVC at the quick service restaurants and the modular schools was evaluated for two alternative control strategies. DCV with economizer control (DCV On) and economizer cooling only (DCV Off) yielded the following results:
  
  • The quick service play rooms had greater savings due to larger changes in occupancy compared to the modular school rooms.
    
    
  • In general, inland climates had greater savings compared to coastal climates.
    
    
  • For cooling, greater energy savings were achieved at the restaurant play rooms than for the modular schoolrooms. Primarily, this is because the play rooms have more variability in their occupancy than the schoolrooms.
    
    
  • The largest energy savings were achieved at one of the inland restaurants, which appears to have the lowest average occupancy level compared to the others. The savings in condensing unit energy were 35% and 16% for one of the inland sites and one of the coastal sites, respectively.
    
    
  • The total annual air conditioning cost savings were smaller (23% and 6%, respectively) because the supply fans operates continuously during occupied times for both strategies and fan energy is a significant fraction of the total energy usage.
    
    
  • There were no substantial cooling season savings for the modular school rooms, although the calibrated VSAT model for the Oakland school site does indicate small (about 4%) savings. The occupancy for the schools is relatively high with relatively small variability and the sites are also on timers or controllable thermostats that mean the HVAC units only operate during the normal school day. The schools are also generally unoccupied during the heaviest load portion of the cooling season.
    
    
  • The amount of heating required for the California sites is relatively small and therefore absolute savings are relatively small for application of DCV. However, very large relative savings were estimated using calibrated VSAT predictions. Overall, the total costs for providing heating at these sites is smaller than for cooling, so percentage savings are more important for the cooling cases.

• The indoor air contaminant simulations had the following results:
  
  • The CO2 control cases had higher concentrations than the reference cases based on Standard 62-2001 and proposed addendum 62n. CO2 and VOC concentrations in six space types were simulated for seven ventilation control strategies in four California climate zones. Indoor VOC concentrations were calculated as a means of assessing the impact of CO2 control on non-occupant generated contaminants, for example those emitted by building materials and furnishings. Based on the assumed emission rates, which were not particularly low relative to the limited data from field studies, the average indoor VOC levels during occupancy were always less than 0.4 mg/m3 and less than 0.1 mg/m3 in most cases.
  • The average VOC concentrations, and more so the maximum concentrations, were heavily influenced by the build-up in concentration during unoccupied hours. The buildup during unoccupied hours, in turn, depends on the values assumed for the fan-off infiltration rate and VOC emission rate. As discussed earlier, these elevated concentrations early in the day can be tempered by a nonzero minimum ventilation rate under CO2 control or with an early morning flush-out.
    
    
  • The spaces with more variable occupancy resulted in significant energy savings in all the climates studied. These results indicate that CO2 DCV is not likely to provide much energy benefit in offices in the milder California climates for the relatively stable occupancy patterns used in this study. However in more "severe" climates, the savings in the office space were more significant. The energy savings in the classroom spaces are strongly dependent on the system operating schedule versus the occupancy schedule, and while significant load reductions were seen in this study, application of CO2 DCV in classrooms may require more careful consideration.

• The annual energy load reductions due to the use of CO2 control were significant in most of the cases, ranging from 10 % to 80 % depending on the space type, climate and ventilation strategy. For the office space studied, the reductions are generally around 20 % given the relatively stable occupancy pattern in that space relative to some of the others. Spaces with more variability in occupancy, such as the Conference Room and Lecture Hall, exhibit larger reductions in energy loads. The energy load reductions associated with the use of proposed addendum 62n relative to the ventilation requirements in Standard 62-2001 are as large as 30 % to 50 % in the spaces where the 62n rates are indeed lower.
  
  
• The VSAT simulation study considered both retrofit and new building designs. In both cases, demand-controlled ventilation coupled with an economizer was found to give the largest cost savings and best economics relative to an economizer only system for the different prototypical buildings and systems evaluated in the California climate zones. Figure 5 shows that most inland climate zones had relatively short simple payback periods.

• make better decisions regarding DVC applications,
  
• fine-tune existing installations, and
  
• make early screening available during the design concept phase of new construction and retrofit projects.

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