Low Temp Supply Air to Reduce Volumetric Flow Rate
Cooling System Air Supply: Lower-Temperature to Reduce Air Flow vs. Standard-Temperature
Lower design supply temperatures for cooling reduce the air volume required to meet loads and reduces fan energy, duct sizing, and potentially first-installed cost.
Item ID: 203
HVAC--Sensors & Controls
Technical Advisory Group: 2009 HVAC TAG (#2)
Traditionally, the supply air temperature from an HVAC air handler is set to 55°. By lowering the supply air temperature to 45° or 50°, the volume of supply air can be greatly reduced, which saves a lot of electric fan energy. It also allows the ductwork, diffusers, terminal units and other distribution components to be reduced in size, which saves on the installation cost. However, if implementing this strategy requires higher head pressure for the compressor, this can reduce the efficiency of the compressor, so this tradeoff will need to be quantified and compared in cost and energy savings before implementing the design change.
Baseline Description: Supply Air Temperature at 55 degrees
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 many types of 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 is a strategy therefore there are no manufacturers claims.
Best Estimate of Energy Savings:
"Typical" Savings: 2%
Low and High Energy Savings: 1% to 3%
Energy Savings Reliability: 2 - Concept validated
There are a number of trade-offs in analyzing the energy benefits of lower chilled water temperatures. Fan energy and pumping energy savings will be offset by higher chiller energy use. The optimum temperature will depend on the application and is best considered as part of an integrated design approach for a new building. This ET is for climates that can operate cooler temperatures without condensing in the ductwork and at the grilles. Considering 16 different combination of options in an office building (McQuay International, 2013) the best savings was 3% and the optimum result was 2% better.
Energy Use of Emerging Technology:
6.5 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:
This design strategy applies to new large commercial buildings with chilled water systems. For existing buildings, the characteristics of the existing HVAC system limit potential savings.
For the purposes of this analysis CBSA data can be used to make projections of commercial building square footage ( NEEA, 01/1/2014). Using a 1% annual growth rate and 2% replacement rate (3% total), we estimate that over 10 years new construction and major renovations will be 34% of existing commercial square footage: 3,118,000,000 * 0.34 = 1,060,120,000 sf. Applying market shares for existing buildings for conditioned space (85%) and chilled water systems (19%) results in: 1,060,120,000 * 0.85 *0.19 = 171,209,380 sf.
Regional Technical Potential:
0.02 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)
Currently no data available.
Simple payback, new construction (years): N/A
Simple payback, retrofit (years): N/A
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.