Mechanical Nighttime Ventilation Cooling
Cooling System: Mechanical Nighttime Ventilation Cooling vs. Conventional Setback and Ventilation
A system that uses an air-side economizer to automatically ventilate a building at night to cool the building's thermal mass and reduce the need for compressor-based cooling the following day.
Item ID: 102
HVAC--Other HVAC Systems
Technical Advisory Group: 2010 HVAC TAG (#3)
Average TAG Rating: 3 out of 5
TAG Ranking Date: 06/29/2010
TAG Rating Commentary: 1. This is probably programmed already for most DDC system to provide night purge of the building using cool outside air at night.
2. This just involves getting smarter controls on economizer infrastructure already required.
3. In the 1980s, DOE2 simulations suggested there often wasn't a net savings, due to increased fan energy. Needs to be studied. Today VSDs may change that, but we today have generally higher supply pressures as well. A great concept, but needs to be done with care. Eventually we may have actuated openings in the walls... that would be nice.
4. Not sure the economics are there
5. Great residential technology for the Northwest. Can eliminate the need for compressor-based air conditioning where cooling is only needed sporadically. An upcoming control modification will allow the system to meet ASHRAE 62.2 standards. Caution: I am biased.
6. Night-time flushing of commercial facilities can be an effective way to lower the temp of the building mass. It will help meet the cooling load the next day, with less mechanical cooling required.
This is a cooling strategy, so would not be an energy saver during the heating season. Furthermore, during swing months, it is important to manage the strategy such that morning warm-up is not required as a result.
Perhaps this strategy is in the category of "best practice control strategies," and therefore part of a broader technology package."
Technical Advisory Group: 2009 HVAC TAG (#2)
Technical Advisory Group: 2014 Commercial Building TAG (#9)
Technical Advisory Group: 2015-1 Commercial HVAC TAG (#11)
Average TAG Rating: 2.8 out of 5
TAG Ranking Date: 03/10/2015
TAG Rating Commentary:
- Most applicable East of the Cascades. Very simple to implement.
- This control strategy doesn't fit well in utility incentive programs.
- We already looked at this in the non-HVAC TAG last year.
- This is going to be very climate- and application-specific. Need to have rather cool night temperatures, or move much more air than ventilation function if night temps are moderate instead of cold. Worth some modeling to see where in the BPA region it would have value.
- A regional analysis should be performed to understand the potential savings with a focus on North West climate.
Integrated nighttime ventilation cooling, also known as nighttime flush or night-purge, uses an air-side economizer during the night time to cool the building mass during hot summer days. The building mass consists of building materials, the foundation, and furniture. During periods of hot weather, pre-cooling this thermal mass during nighttime or early-morning hours can reduce compressor-based (DX) system operation and energy consumption. Nighttime ventilation cooling systems are equipped with a control scheme that opens the outside air dampers in the air handler to bring in low-temperature outdoor air to the conditioned space, thus decreasing the need to use HVAC system cooling the following day. This ET focuses on commercial buildings that are occupied during the day, but unoccupied at night. Ideally, these buildings would have direct digital HVAC system control.
The energy savings can range from about 10-30% of the cooling demand, with greater savings from buildings having more thermal mass, such with radiant floors and exposed structure, located in climates with large diurnal temperature swings during the summer.
Mechanical nighttime cooling isn’t a new technology, but it has a small adoption rate. Depending on the manufacturer of the HVAC system, it may be implemented by: flipping dip switches on the existing controller, adding a board to the existing controller, or replacing the controller. It will cost about $500-$2,500. Major manufacturers of HVAC equipment can provide reliable controls and expertise for this upgrade. Natural nighttime ventilation cooling is similar but uses architectural features to take advantage of breezes and the stack effect rather than HVAC fans; see ET #541 “Natural Ventilation for Mixed-Mode Conditioning” for details.
Baseline Description: Standard cooling load of commercial buildings
Baseline Energy Use: 7.4 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 (leaving non-HVAC end uses such as lighting and plug load) 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 energy 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 is 14.8 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 commercial buildings, a baseline cooling energy use of 7.4 kWh/sf/year is assumed (NEEA,12/21/2009).
Manufacturer's Energy Savings Claims:
Currently no data available.
Best Estimate of Energy Savings:
"Typical" Savings: 10%
Low and High Energy Savings: 5% to 15%
Energy Savings Reliability: 2 - Concept validated
Energy savings are highly climate dependent and behaviorally driven. Ideal climates for ventilation cooling have high daytime temperatures, large daily temperature swings (>25°F) and relatively dry conditions. Savings are improved when occupants use low (~67°F) minimum space temperature limits. Building design is also a factor. Buildings with slab floors and exposed masonry columns and beams provide thermal mass that can increase energy savings.
One study used modeling to estimate 17-39% savings in Seattle (Braun, null Pg 404), but other studies suggest a range of 5-18%, in cities with less temperate climates. It is estimated that energy savings in the Northwest would range from 5-15%, averaging 10% of cooling loads. This study indicates that energy savings are actually higher percentage in more temperate climates (that also have large diurnal temperature swings) because building envelope heat conduction is a smaller fraction of total cooling loads, more dominated by internal gains.
Energy Use of Emerging Technology:
6.7 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 technology could technically be used for virtually any commercial building that is unoccupied at night. The total of the entire commercial building stock in the Northwest that is equipped with an economizer is used for this analysis less the square footage associated with hotel/motels, hospitals, and other health care facilities. The numbers are taken from preliminary updated numbers from the 2013 update to the Commercial Building Stock Assessment (CBSA) using the estimates for 2014 (before the update was completed -- from early January, 2014) multiplied times the percentage of commercial space that is conditioned based on the 2009 CBSA times the percentage of building square footage that is equipped with an economizer ( 44.3% from Table C-HS20 of the Cadmus "Northwest Commercial Building Stock Assessment", for NEEA, December 21, 2009). The potential square footage that can utilize this ET is thus 2,640,946,000 sf of total conditioned space less hospital, health care and hotel/motel space (56.2 +141.6 +135.3 million sf) x 44.3% = 1,022,375,778 sf.
Regional Technical Potential:
| || Total Commercial Floor space || % Conditioned || Conditioned space |
| Source, units || (NEEA, 2014) (s.f.) || (NEEA, 2009 App C, Table C-GB4) || (s.f.) |
| || 3,118,000,000 || 84.7% || 2,640,946,000 |
0.76 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): $0.04
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $0.00
The cost of augmenting the control intelligence of an existing air-side economizer to perform nighttime ventilation in addition to conventional economizer cooling depends on the brand and model of the existing system. For some, it can be as simple as modifying dip switches with guidance from the manufacturer, which could be about $500 in labor. For others, it will require the modification or replacement of the control board, which could cost up to $2,500. The first costs above assume a 10-ton rooftop unit serving 40,000 square feet of office space. A total installed cost of $2,000 will be assumed ($0.04/sf for equipment and $0.01/sf for labor for a unit serving 40,000 sf of floor space).
Simple payback, new construction (years): 0.6
Simple payback, retrofit (years): 0.8
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.
Payback will be better in hotter climates with large diurnal temperature swings, for building structures with exposed masonry; when economizer controls can be modified rather than replaced; and for buildings with lower allowable nighttime temperature limits.
Integrated nighttime ventilation cooling, also known as nighttime flush, uses an economizer during the night time to cool the building mass during a series of hot summer days. This is an alternative to conventional HVAC controls, which will allow space temperatures to drift upward during the night, typically from 72 to 80°F. This control strategy lowers the nighttime space temperature to the lower allowable limit, typically about 67°F. When properly implemented, this technique can decrease the HVAC system cooling requirements the following day. The fan energy used in nighttime cooling is exceeded by the reduction in fan and compressor cooling energy consumption during the day.
This strategy can also be implemented in homes and in commercial buildings utilizing natural ventilation (E3T #541 Natural Ventilation for Mixed-Mode Conditioning) but this assessment focuses on commercial buildings with direct digital control and an economizer, which will present or at least be part of all HVAC systems less than five years old and at least three tons in capacity.
The energy savings will be more with buildings:
- Having greater thermal mass, such as with slab floors and exposed building structure (columns and beams), especially with horizontal surfaces, in particular floors, as cool ventilation air will tend to fall to the floor level
- In climates with large diurnal temperature swings in the summer, such as east of the Cascade Mountains
- With few or no nighttime occupants or occupants who are amenable to cooler temperatures
- The building has large areas of exposed internal thermal mass, not covering floors with carpets and coverings, walls with cupboards and panels, or ceilings with acoustic tiles and drop-panels
- Overcooling is avoided to preclude the need for subsequent re-heating or thermal discomfort the following day
Nighttime ventilation cooling isn’t a new technology, but it has a fairly small adoption rate. Depending on the manufacturer of the HVAC system, it may be implemented by: flipping dip switches on the existing controller, adding a board to the existing controller, or replacing the controller. Major manufacturers of HVAC equipment can provide reliable controls and expertise for this upgrade. Some commissioning is recommended with this installation, given the percentage of commercial sector air-side economizer installations that don't operate optimally and some that may not operate at all.
Standard practice is a conventional heating system with direct expansion (DX) cooling. The heating system may be a furnace, heat pump, or a boiler hot water air handler. The HVAC system energizes the DX cooling when cooling is called for.
Commercial versions of the technology were available before 2000. About 20,000 systems have been installed in Northern California, mostly by one contractor/manufacturer (Beutler Corporation, Smart Vent Products, Inc.). An advanced variable speed version was developed, too. This can be employed on any equipment that already has an economizer. To retrofit systems that do not already have an economizer, an outside air (OSA) duct sized for 100% OSA would need to be added along with the associated controls. There are packaged controls/damper boxes available from any commercial fabrication shop, but it’s advisable to have clear equipment specifications as part of utility incentive programs to ensure successful implementation.
With a nighttime ventilation cooling system in operation, occupants will enjoy a thoroughly ventilated space in the morning. With conventional control systems, the mechanical system may provide little or no ventilation at night if there aren't any occupants in the building, so building material outgassing and other indoor air quality issues may be a consideration. The space will also be more comfortable in the morning of a hot day because the temperature of the air and thermal mass will start out at about 67°F and slowly rise. With a conventional cooling system, the air will have just recently been cooled down from 80°F to about 75°F and the thermal mass will remain at a warmer temperature.
End User Drawbacks:
Night-flush ventilation is effective when it cools a building’s thermal mass. It is most effective when a building has large areas of exposed internal thermal mass. This means not obscuring floors with carpets and coverings, walls with cupboards and panels, or ceilings with acoustic tiles and drop-panels. Thermal mass for night-time cooling is most efficient in horizontal surfaces, in particular floors, as cool ventilation air will tend to fall to the floor level. Using natural ventilation for cooling also requires a relatively unobstructed interior to promote air flow. This technology is effective with HVAC systems using chilled beams (ET #316 “Active Chilled Beam Heating and Cooling”) and when ceiling fans are installed. Overcooling should be avoided as it may result in subsequent re-heating or thermal discomfort the following day. Additional potential drawbacks include:
- Incremental cost
- Lack of easy-to-use modeling tools, particularly for energy compliance
- Lack of understanding of good and bad applications
- Local HVAC maintenance personnel inadequately familiar with the installation who attempt to perform it with a customized installation rather than a more reliable version provided by the HVAC manufacturer
Operations and Maintenance Costs:
This should not impact operation and maintenance costs. The periodic recommissioning will be performed along the other uses of the air-side economizer hardware and controls.
Anticipated Lifespan of Emerging Technology: 15 years
The thermal mass has an unlimited life. The life of the controls is comparable to that of conventional electronic controls – about 15 years.
Natural nighttime ventilation, which precludes the need for fan energy use but provides significantly less cooling effect
Conventional economizer controls, which allow space temperatures to climb to the top of the allowable range (~80°F) rather than cooling them down to the bottom of the allowable temperature range (~67°F)
Reference and Citations:
Measure Guideline: Ventilation Cooling
Building America, Building Technologies Program, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy
Alternatives to Compressor Cooling
California Energy Commission
Non-Compressor Cooling Alternatives for Reducing Residential Peak Load
American Council for an Energy Efficient Economy
Alternatives to Compressor Cooling, Phase V: Integrated Ventilation Cooling Final Project Report
California Energy Commission
Total Pacific Northwest Building Stock Based on Preliminary Numbers from the 2013 Update to the CBSA
Northwest Energy Efficiency Alliance
Northwest Commercial Building Stock Assessment (CBSA): Final Report
Prepared by the CADMUS Group for the Northwest Energy Efficiency Alliance
Evaluating the Performance of Building Thermal Mass Control Strategies
Co-authors: Kent W. Montgomery and Nitin Chaturvedi discuss details of modeling these systems and their performance in five sites around the country.
Autodesk Sustainability Workshop