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Summary

Displacement Ventilation

HVAC Air Distribution: Displacement Ventilation vs. Overhead Diffusers

An air distribution system which supplies low velocity air, typically at floor level. Supplied air rises slowly and is extracted from the ceiling, as opposed to intentionally mixing the air.

Synopsis:

In a displacement ventilation (DV) system, cool ventilation air is supplied horizontally at the floor at very low velocities and delta-t's.  Therefore, this strategy works well in cooling dominated climates. Temperatures are stratified throughout the space, with cool air pooling near the floor and warmer air rising and eventually being extracted near the ceiling.  The cool air at the floor is drawn to the heat loads (people and equipment) by vertical convection currents. It is believed that for certain climates and buildings, DV can provide energy savings and comfort advantages versus traditional overhead mixed air systems. This is in part due to supplying the needed ventilation air at the occupied level instead of at the ceiling where it may get short circuited back to the air handler.

The advantages of DV can include a reduction in cooling energy, longer free-cooling periods, better air quality than traditional HVAC systems, and a reduction in ventilation air requirements. These benefits are very climate and building specific, and DV may not be appropriate outside of relatively mild climates with low cooling loads. Hybrid technologies are being explored, such as DV with cooled ceilings, which may be appropriate for more extreme climates.

In order for a DV system to function well, ceilings must be at least nine feet high in order to produce the required convective currents. Because the cooling air is being supplied near the occupants, those nearest the supply grilles may feel chilled. For more extreme climates, buildings with DV require a separate heating system for the perimeter. Even in milder climates, the building must achieve a very tight envelope in order to rely solely upon the DV system for heating. The system also requires that the conditioned spaces be densely and steadily occupied, with low levels of disturbance which could disrupt the air flows. This makes DV desirable for many school environments, but perhaps inappropriate for other settings. While there have been many claims of energy savings, calculations for DV savings have not yet been validated by an unbiased testing agency.

Displacement ventilation has been widely used in Europe over the past three decades as an energy efficient approach to provide improved indoor air quality, compared to overhead mixing systems. For research on displacement ventilation, see (Center for the Built Environment, 2014)

Energy Savings: 25%
Energy Savings Rating: Concept validated:  What's this?
LevelStatusDescription
1Concept not validatedClaims of energy savings may not be credible due to lack of documentation or validation by unbiased experts.
2Concept validated:An unbiased expert has validated efficiency concepts through technical review and calculations based on engineering principles.
3Limited assessmentAn unbiased expert has measured technology characteristics and factors of energy use through one or more tests in typical applications with a clear baseline.
4Extensive assessmentAdditional testing in relevant applications and environments has increased knowledge of performance across a broad range of products, applications, and system conditions.
5Comprehensive analysisResults of lab and field tests have been used to develop methods for reliable prediction of performance across the range of intended applications.
6Approved measureProtocols for technology application are established and approved.
Simple Payback, New Construction (years): 2.9   What's this?

Simple Payback is one tool used to estimate the cost-effectiveness of a proposed investment, such as the investment in an energy efficient technology. Simple payback indicates how many years it will take for the initial investment to "pay itself back." The basic formula for calculating a simple payback is:

Simple Payback = Incremental First Cost / Annual Savings

The Incremental Cost is determined by subtracting the Baseline First Cost from the Measure First Cost.

For New Construction, the Baseline First Cost is the cost to purchase the standard practice technology. The Measure First Cost is the cost of the alternative, more energy efficienct technology. Installation costs are not included, as it is assumed that installation costs are approximately the same for the Baseline and the Emerging Technology.

For Retrofit scenarios, the Baseline First Cost is $0, since the baseline scenario is to leave the existing equipment in place. The Emerging Technology First Cost is the Measure First Cost plus Installation Cost (the cost of the replacement technology, plus the labor cost to install it). Retrofit scenarios generally have a higher First Cost and longer Simple Paybacks than New Construction scenarios.

Simple Paybacks are called "simple" because they do not include details such as the time value of money or inflation, and often do not include operations and maintenance (O&M) costs or end-of-life disposal costs. However, they can still provide a powerful tool for a quick assessment of a proposed measure. These paybacks are rough estimates based upon best available data, and should be treated with caution. For major financial decisions, it is suggested that a full Lifecycle Cost Analysis be performed which includes the unique details of your situation.

The energy savings estimates are based upon an electric rate of $.09/kWh, and are calculated by comparing the range of estimated energy savings to the baseline energy use. For most technologies, this results in "Typical," "Fast" and "Slow" payback estimates, corresponding with the "Typical," "High" and "Low" estimates of energy savings, respectively.

Status:

Details

Displacement Ventilation

HVAC Air Distribution: Displacement Ventilation vs. Overhead Diffusers

An air distribution system which supplies low velocity air, typically at floor level. Supplied air rises slowly and is extracted from the ceiling, as opposed to intentionally mixing the air.
Item ID: 18
Sector: Commercial
Energy System: HVAC--Sensors & Controls
Technical Advisory Group: 2009 HVAC TAG (#2)

Synopsis:

In a displacement ventilation (DV) system, cool ventilation air is supplied horizontally at the floor at very low velocities and delta-t's.  Therefore, this strategy works well in cooling dominated climates. Temperatures are stratified throughout the space, with cool air pooling near the floor and warmer air rising and eventually being extracted near the ceiling.  The cool air at the floor is drawn to the heat loads (people and equipment) by vertical convection currents. It is believed that for certain climates and buildings, DV can provide energy savings and comfort advantages versus traditional overhead mixed air systems. This is in part due to supplying the needed ventilation air at the occupied level instead of at the ceiling where it may get short circuited back to the air handler.

The advantages of DV can include a reduction in cooling energy, longer free-cooling periods, better air quality than traditional HVAC systems, and a reduction in ventilation air requirements. These benefits are very climate and building specific, and DV may not be appropriate outside of relatively mild climates with low cooling loads. Hybrid technologies are being explored, such as DV with cooled ceilings, which may be appropriate for more extreme climates.

In order for a DV system to function well, ceilings must be at least nine feet high in order to produce the required convective currents. Because the cooling air is being supplied near the occupants, those nearest the supply grilles may feel chilled. For more extreme climates, buildings with DV require a separate heating system for the perimeter. Even in milder climates, the building must achieve a very tight envelope in order to rely solely upon the DV system for heating. The system also requires that the conditioned spaces be densely and steadily occupied, with low levels of disturbance which could disrupt the air flows. This makes DV desirable for many school environments, but perhaps inappropriate for other settings. While there have been many claims of energy savings, calculations for DV savings have not yet been validated by an unbiased testing agency.

Displacement ventilation has been widely used in Europe over the past three decades as an energy efficient approach to provide improved indoor air quality, compared to overhead mixing systems. For research on displacement ventilation, see (Center for the Built Environment, 2014)

Baseline Example:

Baseline Description: HVAC with Electric Heating
Baseline Energy Use: 10.5 kWh per year per square foot

Comments:

The 2009 Commercial Building Stock Assessment gives the actual electrical building EUI's 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). Since this technology can be applied to many types of non-office commercial buildings, a baseline energy use of 10.5 kWh/sf-year is assumed ( NEEA, 12/21/2009)

Manufacturer's Energy Savings Claims:

Comments:

No manufacturer's claim. This is a design strategy.

Best Estimate of Energy Savings:

"Typical" Savings: 25%
Low and High Energy Savings: 0% to 45%
Energy Savings Reliability: 2 - Concept validated

Comments:

Savings will vary depending on the base comparison HVAC system, the application, and the climate. A paper on displacement ventilation in the ASHRAE Journal indicates that model simulations of DV systems showed lower cooling equipment energy use than mixed air systems (30-75% less) and higher supply and return air fan energy consumption (35%-50%). Overall energy reductions of 13-45% were reported for restaurants and small office buildings.( Hamilton, Roth, & Broderick, 09/2004)

However, (Novoselac, 2002) shows results from three studies modeling the differences between variable air volume systems and displacement ventilation with cooled ceilings. He found that boiler energy savings is offset by increased fan energy use. The results show the displacement ventilation systems may or may not save energy. 

For the purposes of estimating savings potential, 25% savings is used, assuming that this design strategy will be applied in cases where it offers an advantage.  

Energy Use of Emerging Technology:
7.9 kWh per square foot per year What's this?

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.))

Comments:

This is basically a passive system so should last 20 year or more.

Technical Potential:
Units: square foot
Potential number of units replaced by this technology: 92,453,065
Comments:

This measure is a design strategy for air distribution that can be best applied to new construction or major renovations. It is not a likely retrofit to existing buildings. It is most suited to office buildings, education buildings and other public spaces like libraries, theatres, and assembly halls. CBSA data ( NEEA, 01/1/2014) includes projections of commercial building square footage that show an annual growth rate of approximately 1%. 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. Assuming the next 10 years of construction represents the new construction/major renovation potential results in 34% of the existing commercial square footage (3,118,000,000, (NEEA, 2014)) or 1,060,120,000 total square feet. Applying market shares from existing buildings, consider only conditioned square feet (85%), electrically heated space (27%), and offices, education, and Assembly (38%). This results in the potential for use of this technology of 92,453,065 sf.

Regional Technical Potential:
0.24 TWh per year
28 aMW
What's this?

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)

First Cost:

Installed first cost per: square foot
Emerging Technology Unit Cost (Equipment Only): $6.94
Baseline Technology Unit Cost (Equipment Only): $6.25

Comments:

Baseline rule of thumb is about $2,500 per ton with one ton serving about 400 sf yielding a baseline system cost of $6.25/sf.   The ASHRAE Journal article reports that DV system first cost is 5-17% more than mixing ventilation systems (Hamilton, Roth, & Broderick, 2004).  Assuming a mid-level incremental cost increase of 11% indicates a cost of $6.94 when the displacement ventilation is included. 

Cost Effectiveness:

Simple payback, new construction (years): 2.9

Simple payback, retrofit (years): N/A

What's this?

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:

ASHRAE, 01/01/2009. Best Practices for Datacom Facility Energy Efficiency
American Society of Heating, Refrigerating and Air-Conditioning Engineers

Schild Peter, 01/01/2004. Displacement Ventilation
Air Infiltration and Ventilation Centre

Morton Blatt, 05/01/2006. Applications Guide for Off-the-Shelf Equipment for Displacement Ventilation Use
California Energy Commission

Atila Novoselac, 06/01/2002. A critical review on the performance and design of combined cooled ceiling and displacement ventilation systems
Energy and Buildings , 34

S Hu, 07/01/1999. Comparison of energy consumption between displacement and mixing ventilation systems for different U.S. buildings and climates
Massachusetts Inst. of Tech.

1 ASHRAE, 09/01/2009. Standard Benchmark EUI
1 , 1

NEEA, 01/01/2014. Total Pacific Northwest Building Stock Based on Preliminary Numbers from the 2013 Update to the CBSA
Northwest Energy Efficiency Alliance

CADMUS, 12/21/2009. Northwest Commercial Building Stock Assessment (CBSA): Final Report
Prepared by the CADMUS Group for the Northwest Energy Efficiency Alliance

Center for the Built Environment, 11/13/2014. Research on Displacment Ventilation
College of Environmental Design, University of California, Berkeley
Special Notes: Develop new and improved guidelines, tools and resources for system designers and operators of buildings using displacement ventilation (DV).

Hamilton, Roth, & Broderick, 09/01/2004. Displacement Ventilation
ASHRAE Journal

Rank & Scores

Displacement Ventilation

2009 HVAC TAG (#2)


Technical Advisory Group: 2009 HVAC TAG (#2)
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