Natural Ventilation for Mixed-Mode Conditioning
Cooling System: Maximize Natural Ventilation Cooling vs. Conventional Setback and Ventilation
Utilizing natural ventilation (driven by wind and stack effect rather than fans) for nighttime precooling of a building, preferably one with extensive exposed masonry structure in a climate with significant daily temperature swings
Item ID: 541
HVAC--Other HVAC Systems
Technical Advisory Group: 2014 Commercial Building TAG (#9)
Average TAG Rating: 3.83 out of 5
TAG Ranking Date: 03/17/2014
TAG Rating Commentary:
- Should be useful in many Northwest climates; many design and integration challenges....depends on people and/or automated control
- Excellent potential through automated operation in windows and skylights.
- Support of this measure through resources like the Energy Studies in Buildings Laboratory is critical to implementation. While passive and simple in concept, this is a complex and nuanced design strategy that often requires advanced modeling to optimize that isn't available in all design firms
- A proven but underutilized strategy in this region. Would be a terrific measure to incentivize in concert with thermal mass or phase change materials.
- Again a great concept but limited applicability.
Mixed-mode conditioning uses natural ventilation to perform cooling during feasible times with mechanical air conditioning available to supplement cooling when outdoor temperatures are high. Natural ventilation can also be used for nighttime cooling to maintain comfortable building space temperatures while minimizing the use of mechanical (refrigerant-based) cooling, at least in climates with a larger daily temperature range in the summer. This is not a new technology, but rather is a refinement of what architects have known for centuries but too many forgot after the advent of air conditioning.
Mixed-mode conditioning is not suitable for retrofit as significant design work would be required (Navigant). Instead of letting space temperatures rise to 80ºF overnight and then cool in the morning, natural ventilation cools the inside air and then the thermal mass to delay or eliminate the need for costly mechanical cooling the next day. It is generally accomplished using some combination of natural forces; wind and the buoyancy of warmer air. Architectural features of a building can make better use of these forces with:
To optimize energy savings, occupants can be signaled when to operate the windows by their work stations avoiding costly mechanical automation. Because natural ventilation uses unfiltered outside air, it is not suitable in locations with high humidity, pollution, or high noise levels (Navigant). Cost-effectiveness significantly improves when you can eliminate—or at least substantially downsize—the HVAC cooling equipment. The US DOE Building Technologies Office estimates that mixed mode conditioning could reduce cooling energy consumption by 20% (Navigant).
- An unobstructed floor plan
- A thinner building perpendicular to the wind
- Exterior features to guide breezes into operable, protected windows by exposed masonry (uncovered slab floors, columns, and metal deck ceilings) that acts as thermal mass
- An atria, wind turbines, chimney, clerestory, or cupola to draw warm air up through the building
Baseline Description: Large Office Building with No Natural Ventilation
Baseline Energy Use: 6.6 kWh per year per square foot
The 2009 Commercial Building Stock Assessment gives the actual electrical building energy use index (EUI) for various types of heating and cooling systems (Table D-EA5). Office buildings with electric heating and cooling have an EUI of 20.1 kWh/sf/year. Office buildings with no electric heating or cooling use only 8.2 kWh/sf/year, indicating that the combined HVAC heating and cooling energy use is 11.9 kWh/sf/year. For all commercial buildings, the corresponding numbers are 19.9 and 9.4 kWh/sf/year, respectively for a heating and cooling use of 10.5 kWh/sf-year.
Commercial buildings with electric cooling and with no electric heating have an electrical EUI of 16.8 kWh/sf-year (14.8 for office buildings). This indicates that the heating load for all categories of commercial buildings is about 3.1 kWh/sf-year (19.9-16.8) with a cooling load of about 7.4 kWh/sf-year (10.5-3.1). The corresponding electrical EUI for office buildings with electric cooling with no electrical heating is14.8 kWh/sf-year which indicates a space heating load of 5.3 kWh/sf-year with a corresponding cooling load of 6.6 kWh/sf-year (11.9-5.3).
Since this technology can be applied to new large office buildings, a baseline cooling energy use of 6.6 kWh/sf/year is assumed (NEEA,12/21/2009).
Manufacturer's Energy Savings Claims:
This isn't a product with a manufacturer, so this field is intentionally left blank. This technology is a design strategy.
Best Estimate of Energy Savings:
"Typical" Savings: 20%
Low and High Energy Savings: 5% to 50%
Energy Savings Reliability: 1 - Concept not validated
This is more of a design strategy and not a technology that can be retrofitted onto existing structures. Energy savings would vary widely depending upon a host of variables, including climate zone and diurnal temperature swings, internal heat gains, solar exposure and southeast plus south glazing area, economizer and chiller performance etc).
The US DOE Building Technologies Office estimates that mixed mode conditioning could reduce cooling energy consumption by 20% (Navigant). One commercial building case study found a realized energy savings of 24.5% for a commercial building while another researcher found that buildings with mixed-mode cooling could save 5% to 50% on yearly energy usage. It was also noted that buildings only conditioned with natural ventilation were impractical (Navigant).
Energy Use of Emerging Technology:
5.3 kWh per square foot per year
Energy Use of an Emerging Technology is based upon the following algorithm.
Baseline Energy Use - (Baseline Energy Use * Best Estimate of Energy Savings (either Typical savings OR the high range of savings.))
Potential number of units replaced by this technology:
Natural ventilation or mixed-mode cooling isn't feasible to apply as a retrofit. This measure is more of a design strategy rather than a technology and maximizing use of natural ventilation is best applied to new construction or major renovations. The best market appears to be large and medium size office buildings (Katipamula, 2010).
CBSA data ( Ecotope, Inc., 01/01/2014) includes projections of commercial building square footage. For office buildings this growth rate is approximately 1.5%. If we assume existing buildings have a 50 year life, then 2% of buildings will be replaced or have major renovations in a given year. Thus in any given year, new construction or major renovations will be ~3.5% of the existing building stock. Assuming the next 10 years of construction represents the new construction/major renovation potential results in 41% of the existing office building square footage: 411,000,000 * 0.41 = 168,510,000.
Regional Technical Potential:
0.22 TWh per year
Regional Technical Potential of an Emerging Technology is calculated as follows:
Baseline Energy Use * Estimate of Energy Savings (either Typical savings OR the high range of savings) * Technical Potential (potential number of units replaced by the Emerging Technology)
Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $5.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $0.01
The equipment needed for natural nighttime ventilation is primarily to open and close windows, skylights, and louvers and the controls to operate those closures as needed based on indoor and outdoor temperature, wind and rain. In addition to equipment, some architectural features may need to be added or modified, such as wing walls, more exposed masonry structure and chimneys. The wide variety of equipment and strategies needed to achieve effective nighttime ventilation cooling vary widely based on climate as well as building site, orientation, layout, construction and occupancy. However, Mount Angel Abbey in Oregon utilizes natural nighttime ventilation cooling well enough that they were able to eliminate mechanical cooling so building costs were break even to 9% less than for a similar conventional building (Churchill, 2006 Pg 7).
Costs also vary with each project due to building layout (lot size), occupancy, design, features, and climate. NSF/IUCRC (2004) estimated the cost of implementing mixed-mode conditioning systems to be $5.00/sf for new commercial construction (Navigant, for the US DOE Building Technologies Office, 2012).
Simple payback, new construction (years): 42.0
Simple payback, retrofit (years): 42.2
Cost Effectiveness is calculated using baseline energy use, best estimate of typical energy savings, and first cost. It does not account for factors such as impacts on O&M costs (which could be significant if product life is greatly extended) or savings of non-electric fuels such as natural gas. Actual overall cost effectiveness could be significantly different based on these other factors.
Reference and Citations:
The key section is on passive cooling.
High Performance Classroom
Oregon Department of Energy