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.
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%
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Concept validated: What's this?
|1||Concept not validated||Claims of energy savings may not be credible due to lack of documentation or validation by unbiased experts.|
|2||Concept validated:||An unbiased expert has validated efficiency concepts through technical review and calculations based on engineering principles.|
|3||Limited assessment||An unbiased expert has measured technology characteristics and factors of energy use through one or more tests in typical applications with a clear baseline. |
|4||Extensive assessment||Additional testing in relevant applications and environments has increased knowledge of performance across a broad range of products, applications, and system conditions. |
|5||Comprehensive analysis||Results of lab and field tests have been used to develop methods for reliable prediction of performance across the range of intended applications.|
|6||Approved measure||Protocols for technology application are established and approved.|
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.