TABLE 1. Significant savings were measured at three McDonalds PlayPlace installations of DCV. For a given temperature, measured cooling energy savings at the installation in Sacramento, CA were greater those at similar facilities in the San Francisco Bay area, predominantly because occupancy rates were lower.

In a period of soaring energy costs, responsible facility mangers and engineers are looking for ways to reduce energy use. One approach that is underutilized today is demand-controlled ventilation (DCV). DCV systems save energy by using building occupancy indicators - usually CO2 levels - to regulate the amount of outside air that is drawn in for ventilation. That process reduces the energy penalty of over-ventilation during periods of low occupancy and also gives credit for building ventilation due to infiltration through the building envelope. Despite these advantages, DCV systems are not widely used - primarily because their cost-effectiveness has not been clearly defined, their benefits have not been adequately documented in the field, and design guidelines are rarely available.

Enter the California Energy Commission, which, through its Public Interest Energy Research Program (PIER), funded a set of computer simulations and field testing to determine the conditions under which DCV is most likely to be cost-effective, verify the savings potential, and identify problems that might crop up in the application of DCV (Figure 1).

The overall PIER program aims to decrease building energy use through research that develops or improves energy-efficient technolo-gies, strategies, tools, and building performance evaluation methods. The project was managed for PIER by Architectural Energy Corp. and carried out by researchers at Purdue University and the National Institute of Standards and Technology (NIST). Results showed that significant savings are possible (Table 1).



A (LIMITED) BREATH OF FRESH AIR

The simulation study considered both retrofit and new building designs. In both cases, DCV coupled with an economizer yielded the largest cost savings and best economics compared to an economizer only system. Savings occur because DCV reduces ventilation rates whenever the economizer is not enabled and the occupancy is less than the peak design value. Lower ventilation loads in turn lead to lower equipment loads, energy use, and peak electrical demand.

The analysis found that DCV typically offered payback periods of less than two years (Figure 2). The greatest cost savings and shortest payback periods occur for buildings that have low average occupancy relative to their peak occupancy, such as auditoriums, gyms, and retail stores. From a climate perspective, the greatest savings and lowest payback periods occur in extreme climates (either hot or cold).

The HPHR system did not provide cost savings for most of the cases investigated for California climates. That's because heating re-quirements are relatively low in most of California, and therefore overall savings are dictated by cooling season performance. For savings to occur, the cooling COP of the HPHR system must be high enough to overcome additional cycling losses that come from the primary air condi-tioner compressor, additional fan power associated with the exhaust and/or ventilation fan, additional cooling requirements due to a higher latent removal load, and a lower operating COP for the primary air conditioner compressor because of a colder mixed air temperature. In addition, the HPHR system serves as an alternative to an economizer and so economizer savings that would normally occur in the base case are lost.

The HXHR system had greater operating costs than the DCV system for all cases considered, and the initial cost and mainte-nance costs for an HXHR system are higher than those for a DCV system. The payback period for the enthalpy exchanger was found to be greater than seven years in most cases. Working against the HXHR approach is the fact that there is a penalty associated with increased fan power due to the use of an additional exhaust fan. In addition, as with the HPHR system, the HXHR system replaces the economizer, so economizer savings are also lost with the HXHR approach.

The payback periods noted above were calculated assuming a retrofit application. For new construction, the use of an enthalpy ex-changer or HPHR unit would lead to a smaller design load for the HVAC equipment, which would make the systems look better. As shown in Figure 3, the use of the enthalpy exchanger provides an immediate payback thanks to the accompanying reduction in RTU equipment cost. Although the rates of return for the DCV approach start out negative (due to the initial investment), they surpass the enthalpy ex-changer rates of return within a short time period. In general, the rates of return are higher in hotter climates and for the buildings having higher variability in occupancy. Rates of return for both the HXHR and HPHR systems were negative in the moderate climates, but eco-nomics for DCV were still positive.

The different ventilation strategies also have some different effects on comfort conditions due to variations in humidity conditions. For humid climates, the alternative ventilation strategies provide lower zone humidity levels than a conventional system during the cooling sea-son. DCV typically provides the lowest zone humidities, followed by the HXHR system, and then the HPHR system.



TABLE 2. Installations of DCV at two similar McDonald's PlayPlaces in the San Francisco area provided rapid payback periods.

SIMULATION SHOWS RAPID PAYBACK

The simulation study considered both retrofit and new building designs. In both cases, DCV coupled with an economizer yielded the largest cost savings and best economics compared to an economizer only system. Savings occur because DCV reduces ventilation rates whenever the economizer is not enabled and the occupancy is less than the peak design value. Lower ventilation loads in turn lead to lower equipment loads, energy use, and peak electrical demand.

The analysis found that DCV typically offered payback periods of less than two years (Figure 2). The greatest cost savings and shortest payback periods occur for buildings that have low average occupancy relative to their peak occupancy, such as auditoriums, gyms, and retail stores. From a climate perspective, the greatest savings and lowest payback periods occur in extreme climates (either hot or cold).

The HPHR system did not provide cost savings for most of the cases investigated for California climates. That's because heating re-quirements are relatively low in most of California, and therefore overall savings are dictated by cooling season performance. For savings to occur, the cooling COP of the HPHR system must be high enough to overcome additional cycling losses that come from the primary air condi-tioner compressor, additional fan power associated with the exhaust and/or ventilation fan, additional cooling requirements due to a higher latent removal load, and a lower operating COP for the primary air conditioner compressor because of a colder mixed air temperature. In addition, the HPHR system serves as an alternative to an economizer and so economizer savings that would normally occur in the base case are lost.

The HXHR system had greater operating costs than the DCV system for all cases considered, and the initial cost and mainte-nance costs for an HXHR system are higher than those for a DCV system. The payback period for the enthalpy exchanger was found to be greater than seven years in most cases. Working against the HXHR approach is the fact that there is a penalty associated with increased fan power due to the use of an additional exhaust fan. In addition, as with the HPHR system, the HXHR system replaces the economizer, so economizer savings are also lost with the HXHR approach.

The payback periods noted above were calculated assuming a retrofit application. For new construction, the use of an enthalpy ex-changer or HPHR unit would lead to a smaller design load for the HVAC equipment, which would make the systems look better. As shown in Figure 3, the use of the enthalpy exchanger provides an immediate payback thanks to the accompanying reduction in RTU equipment cost. Although the rates of return for the DCV approach start out negative (due to the initial investment), they surpass the enthalpy ex-changer rates of return within a short time period. In general, the rates of return are higher in hotter climates and for the buildings having higher variability in occupancy. Rates of return for both the HXHR and HPHR systems were negative in the moderate climates, but eco-nomics for DCV were still positive.

The different ventilation strategies also have some different effects on comfort conditions due to variations in humidity conditions. For humid climates, the alternative ventilation strategies provide lower zone humidity levels than a conventional system during the cooling sea-son. DCV typically provides the lowest zone humidities, followed by the HXHR system, and then the HPHR system.



FIGURE 1. The greatest savings and shortest payback periods for DCV occur for buildings that have variable and unpredictable occupancy levels, with high occupant densities at peak occupancy. Field tests in places like Walgreens drug stores (and the McDonald's PlayPlace area shown here) showed DCV to be a cost-effective approach.

VERIFICATION IN THE FIELD

Simulations provide valuable insights, but serve best when combined with validation in the field. In this project, field sites were estab-lished for three different building types in two different climate zones within California. The building types include:
  • McDonalds PlayPlace areas
  • Modular school rooms
  • Walgreens drug stores
In each case, nearly duplicate test buildings were identified in both coastal and inland climate areas. For cooling, greater energy and cost savings were achieved at the McDonald's PlayPlaces and Walgreens than for the modular schoolrooms. That's because these build-ings have more variability in their occupancy than the schoolrooms.

A look at results at two of the test sites - McDonald's PlayPlaces in the San Francisco Bay area that were 15 miles apart - is instruc-tive. The side-by-side test spanned a three-week period in the month of August.

For the first week, one PlayPlace had DCV on and the other had it off. Then, over 11 days in the next two weeks, the DCV status was reversed in each building. Data collected on actual occupancy patterns, building thermal properties, and HVAC equipment perform-ance was fed into a VSAT simulation that predicted energy use for the year. For the Castro Valley site, the annual savings from electricity use was only $2, but the gas savings amounted to $840 for the year, yielding a payback of 1.1 years. Because the Milpitas site had slightly different occupancy patterns and an additional RTU that added to the cost of implementing DCV, the payback there was 3.5 years (Table 2 and Figure 4). The locations were not ideal for DCV, given that DCV has a much greater savings potential in climates with large heating or cooling loads. Even so, the study showed that DCV yielded fairly short payback periods.

There were no substantial cooling season savings for the modular school rooms because the occupancy for the schools is relatively high with relatively small variability. The school sites are also on timers or controllable thermostats, so that 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. As an aside, the results imply that the average metabolic rate of the students may be higher than the value used in ASHRAE Standard 62-1999 to establish a fixed ventilation rate. In fact, at the school room sites in Sacramento, the DCV control resulted in lower CO2 concentrations than the fixed ventilation rate.

Overall, results from the simulations and field tests showed that for most locations throughout the state of California, DCV with an economizer is the most cost-effective ventilation strategy. Partly as a result of this research, the newest version of California's energy code, Title 24, which took effect in October 2005, features a requirement for DCV. All spaces that have a single-zone HVAC system with an economizer, and with an occupant density of greater than or equal to 25 people/1,000 sq ft, must use DCV according to the code.



FIGURE 2. Enthalpy exchangers offer an immediate payback because their use enables the installation of smaller, less costly RTUs. However, the lower operating costs with the DCV approach lead to a higher rate of return.

RECOMMENDATIONS

Several conclusions from the study can help designers decide where and how to implement DCV:
  • The greatest savings and shortest payback periods occur for buildings that have variable and unpredictable occupancy levels, with high occu-pant densities at peak occupancy. Lecture halls, conferences rooms, classrooms, gyms, and retail stores fall in this category.
  • Savings also vary with climate, with the greatest potential savings coming in areas with extreme climates. In California, greater savings occurred in hot, inland climates than in mild, coastal climates. For spaces with constant or moderately variable occupancies with low peak oc-cupant densities, the potential energy savings in mild climates are low. An example of this type of space is a typical office, where there is little potential for savings in mild climates, but savings are possible in more extreme climates (hot or cold).
  • A nonzero minimum ventilation rate will keep non-occupant sources of indoor contamination at acceptable levels. It helps to establish minimum ventilation rates based on the expected types and strengths of pollutant sources.
  • Setpoints for a DCV system need to be low enough to provide adequate ventilation, but high enough to achieve some energy savings. The approach used in this study was to set the upper limit based on the steady-state CO2 concentration expected at design occupancy and a lower limit about 90 mg/m3 (50 ppm(v)) above outdoors to avoid the system turning on and off too often. Other approaches to determining these setpoints may also work.
  • In buildings with an economizer cycle, allow the economizer to override the DCV system when the additional ventilation would provide "free" cooling.
  • Select DCV systems that are able to increase outdoor air intake before the building opens in the morning to deal with concentrations of contaminants that may build up overnight.
  • Always calibrate and maintain sensors according to manufacturer recommendations.
  • Avoid placing CO2 sensors for ventilation control near doors, windows, air intakes or exhausts, or occupants. Do not use a single sensor located in a common return to control ventilation rates for multiple spaces with different occupancies.


FIGURE 4. This figure shows a flow diagram for the modeling approach used in VSAT. Given a physical building description, an occupancy schedule, and a thermostat control strategy, the building model provides hourly estimates of the sensible cooling and heating requirements needed to keep the zone temperatures at cooling and heating setpoints.

Finally, given the availability of the software tool used in this analysis, and the relative simplicity of the simulations, designers should consider performing similar analyses as part of the design process to examine the impact of various DCV design parameters: setpoints, minimum ventilation rates, and operating schedules.