Energy Savings with DCV

Most analyses of DCV have assumed buildings without DCV have minimum ventilation rates equal to the minimum requirements in standards, which is often incorrect. These analyses also assume that DCV systems operate as intended, with no error in measurement of CO2 concentration. Emmerich and Persily [3] review several case studies of DCV and these studies report energy savings of 8% to 80%. Brandemuehl and Braun [4] used simulations to estimate the energy savings of DCV for four building types and twenty US climates. The buildings had constant volume HVAC systems. Predicted reductions in heating energy use ranged from approximately 26% to 100%, with the high percentage reductions for buildings that required little heat. The reductions in electricity for cooling when employing DCV were 20% or less.

The analysis of DCV energy performance most relevant to California was performed by Persily et al. [5]. They employed simulations to estimate total ventilation-related energy loads with and without DCV for six building space types and four California Climates (Bakersfield, Los Angeles, Sacramento, San Francisco).

When complying with Title 24 standards (Figure 1), the percentage reductions in ventilation heating and cooling load range from -17% to 0% for the office building, 64% to 72% for the conference room, 32% to 43% for the lecture hall, 40% to 60% for the classroom, 13% to 25% for the portable classroom, and 52% to 76% for the fast food restaurant.

General Office Space

DCV is not required in general office spaces under the California Building Energy Efficiency Standards (Title 24)While measured data on the minimum ventilation rates in existing offices are limited and subject to substantial measurement error, the available data indicate that, on average, minimum ventilation rates dramatically exceed code requirements. These high average minimum ventilation rates suggest that significant energy savings might be possible in general office spaces if DCV was employed to control ventilation rates.

To assess the energy savings potential, Hong and Fisk [6] employed simulations of a typical medium size office building for the range of climate zones of California to estimate the energy savings potential and cost effectiveness of DCV for general office spaces. The modeling utilized national-level data indicating that average minimum ventilation rates in offices are 13 or 38 L/s-person (28 or 81 cfm per person). The two very different average minimum ventilation rates are the result of different measurement protocols; however, both values exceed the minimum ventilation requirement in California offices which is approximately 8 L/s-person (17 cfm per person). 

The modeling and cost analyses indicated that DCV would generally be cost effective for general office spaces in California if existing office buildings have a 38 L/s per person (81 cfm per person) minimum ventilation rate (Figure 2). DCV was not found to be cost effective if the typical minimum ventilation rate without DCV is 13 L/s per person (28 cfm per person). Results of the energy modeling indicate that energy savings potential of DCV is largest in the desert area of California (climate zone 14), followed by Mountains (climate zone 16), Central Valley (climate zone 12), North Coast (climate zone 3), and South Coast (climate zone 6). 

This figure shows predicted cost effectiveness, expressed as net present value (NPV) per square meter floor area, for application of demand controlled ventilation in a medium size office building in five California climate zones. Results are provided assuming minimum ventilation rates without demand controlled ventilation of 13.2 and 38.2 L/s per person (28 and 81 cfm per person). 

For more information, see Hong and Fisk [6]