Natural Ventilation for Mixed-Mode Conditioning

Summary

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

Synopsis:

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:

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

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).

Energy Savings: 20%

Energy Savings Rating: Concept not validated What's this?

Level	Status	Description
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, New Construction (years): 42.0 What's this?

Simple Payback, Retrofit (years): 42.2 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.

TAG Technical Score: 3.18

Details

Natural Ventilation for Mixed-Mode Conditioning

Cooling System: Maximize Natural Ventilation Cooling vs. Conventional Setback and Ventilation

Item ID: 541

Sector: Residential, Commercial

Energy System: 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.

Synopsis:

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 Example:

Baseline Description: Large Office Building with No Natural Ventilation
Baseline Energy Use: 6.6 kWh per year per square foot

Comments:

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:

Comments:

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

Comments:

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 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.))

Technical Potential:
Units: square foot
Potential number of units replaced by this technology: 168,510,000
Comments:

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
25 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): $5.00
Emerging Technology Installation Cost (Labor, Disposal, Etc.): $0.01
Baseline Technology Unit Cost (Equipment Only): $0.01

Comments:

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).

Cost Effectiveness:

Simple payback, new construction (years): 42.0

Simple payback, retrofit (years): 42.2

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:

Autodesk, 04/14/2014. Sustainability Workshop
Autodesk
Special Notes: The key section is on passive cooling.

Greg Churchill, 12/18/2006. High Performance Classroom
Oregon Department of Energy
Special Notes: Page 7

Rank & Scores

Natural Ventilation for Mixed-Mode Conditioning

2014 Commercial Building TAG (#9)

Technical Advisory Group: 2014 Commercial Building TAG (#9)
TAG Ranking: 1 out of 44 Technologies (2014 Commercial TAG strategies ranked separately)
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.

Technical Score Details

TAG Technical Score: 3.2 out of 5

How significant and reliable are the energy savings?
Energy Savings Score: 3.0 Comments:

Reliability is dependent upon occupant engagement. To improve this number, I would recommend inclusion of some means of alerting occupants to the actions required to maintain a comfortable space - software, apps, lighting, or other means.
This strategy is a great idea in concept, particularly in locations that have high mechanical cooling loads in the baseline and dry enough air to have a strong diurnal cycle (air that cools down quickly at night after a hot day). The challenge is coming up with a method that doesn't increase the consumption for heating. Operable windows and actuated exterior louvers may leak more air in cold weather than the alternative (fixed pane windows and insulated walls), and in the case of louvers may also have a far lower U-value. So the success may vary with the climate and the relative efficiencies of the heating and cooling systems.

How great are the non-energy advantages for adopting this technology?
Non-Energy Benefits Score: 3.8
Comments:

Fresh, clean air upon arriving in the morning after a nighttime of flushing.
Higher IAQ--more fresh air--to be enjoyed by the occupants during hot weather. The challenges I mention here about avoiding increased heating requirements due to leaky building openings in the winter becomes a mute point if the occupant insists on operable windows for individual control and fresh air. This technology also has a negative non-energy benefit in the area of security. It's not likely that a customer will leave first floor windows open at night unless there are bars across them.
Improvement in IAQ and consequently productivity can be a good motivator.

How ready are product and provider to scale up for widespread use in the Pacific Northwest?
Technology Readiness Score: 2.9
Comments:

Is there any gap in design skills in the region to support this strategy?
"Needs technical support (e.g. via groups like Energy Studies In Buildings Laboratory, which is already providing this service) to assist individual projects with tuned solutionsNeeds technical toolkit / resources to assist design / construction teams in wide scale implementation"
It seems more guidance on which buildings types and constructions this is best suited for is required.
Here we have challenges to the designers and to the developers. Successful natural ventilation requires a sophistication in design that isn't yet widespread. Creation of openings in the envelope that are automatically actuated will depend on reliable and reasonably priced actuators and control systems. Creation of openings that seal tightly in cold weather will also be challenging. Creation of openings that have reasonably low U-values when shut will also be a challenge if dampers are used. Natural ventilation for daytime use can be implemented with manual controls. If natural ventilation has been selected for daytime use, its extention to nightime use wont be difficult if the system is automated not manual. The use of natural ventilation (day or night) depends on a reasonably clean ambient air (not right next to a freeway) and secure surrounding area at least for the lower floors. The system would also need to include adequate response to high winds and rain.
Key architectural and climate constraints

How easy is it to change to the proposed technology?
Ease of Adoption Score: 2.8
Comments:

As a retrofit technology, this can be difficult to achieve given the limitations of window openings and sufficient floor area to accommodate ventilation stacks.
Benefits from customized, project-specific approaches & design solutions
It doesn't seem like there is good resolution on how to address the cold morning temperatures.
The occupant is likely to appreciate it. Whoever pays the energy bills may be less adaptable if the heating bills are high. The developer on spec is likely to hesitate. The owner-occupied developer is likely to consider this if it's a specialized building where sustainability is a strong impulse.
I am not sure about how many buildings are a good target for this technology in the Northwest.
Much easier in new construction

Considering all costs and all benefits, how good a purchase is this technology for the owner?
Value Score: 3.4
Comments:

The proposal for narrow buildings and open floor plates is challenging. The developers want to maximize the rentable square footage per square foot of land, and a narrow building has high surface to volume ratio, driving up envelope costs. A narrow building also needs lower U-values to perform as well as a fat building in the heating season--another increase in envelope costs.
In the right parts of the country (high elevation, big diurnal temperature swings, low humidity, relative high cooling demand) this seems like a nearly ideal fit. But northwest climatic conditions are more of a stretch here -- good in some places but not in others...

Completed:
5/2/2014 9:11:19 AM
Last Edited:
5/2/2014 9:11:19 AM