Why Smart Blinds are More Cost-Effective than Smart Windows for Saving Energy and Combatting Climate Change

The Inflation Reduction Act of 2022 (IRA) includes new and revised tax incentives for clean energy projects.  The bulk of the incentives are earmarked for clean energy generation and storage technologies, but some (thankfully) also favor technologies to increase energy efficiency.  Notable among the energy-efficient technologies supported by the IRA is dynamic glazing technology.

A dynamic glazing, more often called a Smart Window, is a window whose characteristics can be automatically adjusted with changing conditions to significantly reduce a building’s energy consumption and peak power demand.

However, Smart Windows aren’t the only way to do this—and they’re definitely not the most cost-effective way to do it.

The most cost-effective way to dynamically optimize a window’s characteristics is with automated (i.e. “Smart”) blinds.  In fact, Smart Blinds can provide at least a 40%–70% shorter payback period than the EC windows incentivized by the IRA, but unfortunately the IRA provides no incentives to encourage their use.

Benefits of Dynamically Optimizing Window Characteristics

Adjusting a window’s characteristics in response to changing conditions can be advantageous in both nighttime and daytime. However, unlike automated window coverings, Smart Windows provide no benefit over ordinary windows at nighttime, so this post will focus mostly on dynamic shading adjustments in response to changing daylight levels. We refer to this as responsive daylight control.

Responsive daylight control (which we cover in much more detail in a separate post) can provide benefits to building owners/operators (in the form of reduced lighting and HVAC costs), to building occupants (in the form of a healthier and more pleasing visual environment), and to society in general (in the form of reduced demand on the power grid and a reduction in greenhouse-gas emissions).

Of these, the benefits to building occupants are arguably of the greatest value, but most of the interest in Smart Windows is being driven by their potential for energy savings (and the resulting reduction in greenhouse gas emissions)…so that’s the focus of this post.

Smart Windows

It’s generally recognized that exploiting the full energy-saving potential of dynamic window shading requires an active (i.e. electronically actuated) shading device. Active shading devices can be broadly grouped into two categories: motorized window coverings and Smart Windows.

Smart Windows have two big advantages over motorized window coverings: they have no moving parts and a more elegant appearance. They also have another significant advantage over interior window coverings (such as curtains, blinds, and shades): they can block radiant heat from the sun before it passes through the glazing.

There are several active Smart Window technologies, but only two which are reasonably mature and capable of saving significant amounts of energy: ElectroChromic (EC) and Suspended Particle Device (SPD) technology.

EC technology involves a reversible electrochemical reaction (like charging and discharging a battery) that changes the tint of a chemical layer, while SPD technology uses a variable electric field to alter the degree of alignment of rod-like nanoparticles suspended between two sheets. The electrostatic vs electrochemical nature of SPD windows gives them a clear advantage in response time: they can change state much more quickly than EC windows.

However, relatively little information on SPD windows is publicly available, whereas there is substantial public information on EC windows, and only EC windows are incentivized in the IRA. Therefore, this post addresses only EC windows, but there doesn’t seem to be any evidence that the conclusions expressed herein would be significantly different for SPD windows.

EC Windows Have Been Assessed by the Government Services Administration

The U.S. Government Services Administration (GSA) administers a Green Proving Ground (GPG) program aimed at evaluating emerging energy-efficient technologies in operating federal buildings; GPG-033 (issued in November 2017) assessed EC windows in office buildings in Sacramento, CA and Portland, OR. The following resources are available at the GPG-033 webpage:

• A 4-page brief summarizing the overall assessment (GPG-033, 2017)
• A detailed technical report on the assessment in the Sacramento, CA testbed (Fernandes et al, 2017)
• A detailed technical report on the assessment in the Portland, OR testbed (Lee et al, 2016)

These resources provide the most detailed independent technical information on EC windows available in the public domain, and are cited liberally throughout this post.

The First Way EC Smart Windows Save Energy: by Providing a Variable SHGC

A key determinant of a window’s energy efficiency is its Solar Heat-Gain Coefficient (SHGC), which is a measure of how much radiant energy is admitted by the window: the greater the SHGC, the greater the solar heating effects of solar radiation reaching the window.  An SHGC of 1.0 means that all of the solar radiation reaching the window is admitted into the room, whereas an SHGC of 0 means that none of the solar radiation is admitted.

Ordinary energy-efficient windows have a fixed SHGC.  When choosing such a window, the SHGC is specified according to climate and window orientation: cooler climates and northern exposures favor a higher SHGC, while warmer climates and sunnier exposures favor a lower SHGC.

However, the energy dynamics across a given window are always changing with the weather and movement of the sun.  So, choosing a fixed SHGC that minimizes the year-round average daytime load on the HVAC system won’t always minimize the instantaneous load.

That’s where EC Smart Window technology comes in.  An EC Smart Window’s SHGC can be adjusted as conditions change to always minimize instantaneous HVAC loads—and thereby also substantially reduce year-round loads relative to a window with an optimized but fixed SHGC.

The energy savings obtainable this way depend partly on the dynamic range of the SHGC, i.e. the difference between the maximum and minimum SHGCs.  Ordinary energy-efficient windows might be chosen with fixed SHGC of anywhere between 0.2 and 0.6 (depending on climate), whereas a typical EC Smart Window might have an adjustable SHGC range of about 0.1 to 0.4. (Lee et al., Table IV.B.3, page 24). Adjusting the SHGC to 0.1 will increase savings when the HVAC load is cooling-dominated, while adjusting the SHGC to 0.5 will increase savings when the HVAC load is heating-dominated.  However, the benefit of a high SHGC is limited by the fact that admitting too much radiant energy can make the area near a window very uncomfortable.

The second way EC Smart Windows Save Energy: by Providing a Variable VT

As an EC Smart Window’s SHGC is varied, its Visible Transmittance (VT, sometimes also abbreviated T_V) also changes.  Whereas the SHGC determines the amount of admitted solar radiation (much of which is at wavelengths which aren’t visible to the human eye), the VT determines the amount of daylight admitted at visible wavelengths.  While a variable SHGC can reduce energy consumption in a building’s HVAC  system, a variable VT can reduce energy consumption in the building’s lighting system.

Achieving such savings requires that the Smart Window be complemented with a daylight-harvesting lighting control: the Smart Window’s VT is adjusted to maximize glare-free natural illumination, while the lighting control automatically adjusts the lighting to maintain a desired total illumination.  Thus, the lighting controller dims the lights when there’s plenty of natural illumination, “harvesting” the daylight in the form of energy savings.  Achieving energy savings this way is also called daylighting.

Daylighting can be implemented without a Smart Window (or other means of providing a variable VT).  However, most windows must be shaded to avoid occasional glare, and manually operated window coverings are adjusted only infrequently, causing windows to be chronically over-shaded.  Automatically adjusting the VT can significantly increase the average amount of natural illumination relative to manually operated shades, thereby increasing the savings from daylighting by 25–50%.

A typical un-tinted ordinary energy-efficient window might have a fixed VT of about 0.7, whereas a typical EC Smart Window is capable of an adjustable VT as low as 0.03 and as high as about 0.6 (Lee et al., page 23).

However, a VT of 0.03 is often not low enough to block daylight glare from direct sunlight. Therefore, an EC window on a sunny exposure will often have to be augmented with either an adjustable window covering or additional fixed tinting, weakening the advantage of the EC’s adjustable VT.

This was evident in GSA’s EC window testing at the Sacramento, CA testbed, in which the EC windows were equipped with interior venetian blinds. The researchers observed only a “slight reduction in blind use over the course of the study (90% of blinds lowered at the beginning of study; 79% of blinds lowered at the end)” (Fernandes et al, Table ES-2, page 3).

Further, a surprisingly high percentage of blinds were lowered by more than 50% over the course of the test program:

Based on these observations and a survey of the building occupants, the report states that “these results suggest that occupants of the EC floor mostly set the blinds to their original height and that their main concern was protection from glare” (Fernandes et al, page 53).

It should be expected that the additional shading provided by the blinds would drastically reduce the average level of admitted daylight and, hence, the savings in lighting energy. Indeed, the study found a “62% increase in lighting energy use (probably due to issues specific to this demonstration and not attributable to EC technology as a whole)” (Fernandes et al, Table ES-2, page 3; emphasis on “increase” added herein).

Fortunately, savings in lighting energy were observed at GSA’s Portland, OR testbed, but the Sacramento testing shows that glare control can be an issue with EC windows.

Unfortunately, the Savings from a Variable SHGC and a Variable VT aren’t Additive

An EC Smart Window’s SHGC and VT cannot be varied independently, so an EC setting that minimizes HVAC consumption won’t necessarily minimize lighting consumption, and vice-versa (the same is also true of motorized window coverings like Smart Blinds). The following figure shows the relationship between SHGC and VT observed for the EC windows used in the Portland, OR, testbed:

The fact that VT and SHGC cannot be independent varied means that the lighting and HVAC savings are definitely not additive. However, assuming a daylight-harvesting lighting control is present, at least some savings in either HVAC or lighting consumption will always be possible.

Theoretically, the shading can be adjusted to maximize the net energy savings…but that usually doesn’t work out in practice because the occupants’ shading preferences will always trump an algorithmically-selected “optimum” setting.

Savings Will Always be Limited by Occupant Shading Preferences

The most important requirement for any building technology is that it be accepted by the building occupants. So, as with lighting and HVAC systems, occupants must be given at least some control over the window shading.

So, in practice, the actual savings will be determined by the extent to which the occupants’ preferred shading settings match the energy-optimized settings. Generally, occupant preferences will favor lighting savings (which are maximized with glare-free daylight) over HVAC savings (which are maximized with either a fully open or a fully closed setting).

This has three important implications for energy-oriented automated shading (regardless of the type of shading device):

• There is little benefit in complex algorithms to determine energy-optimized shading settings, since they’ll always be trumped by the occupant-preferred settings.
• HVAC savings will account for a smaller proportion of the overall savings than would be expected.
• The overall savings will be significantly less than would be expected just from energy considerations.

While EC Windows Do Save Energy, they aren’t Cost-Effective for Saving Energy

With an understanding of how EC windows can save energy and the constraints on the achievable energy savings, we can move on to whether they’re cost-effective for saving energy.

The maximum payback period for an energy-saving building technology to be considered “cost-effective” is a matter of debate, but is generally regarded as being no greater than 10 years.

The savings provided by an EC window (and, hence, its payback period) will depend on a multitude of installation-specific factors.  These include climate, building configuration (such as the window-to-wall ratio), and the relative efficiencies of the lighting and HVAC systems.  Therefore, a reasonably accurate projection of the achievable savings requires the use of energy simulation software (like the DoE’s open-source EnergyPlus) with installation-specific input parameters.

However, no matter how thorough the modeling, no savings projection can reliably account for the most important variable: occupant preferences. Fortunately, GSA’s EC window testbeds gave occupants control over the shading, so their test results comprehend this key variable.

Here’s what GSA said about the payback period for the windows in their Portland, OR testbed:

“At the GSA national average utility rate of $0.11/kWh and a mature market cost of $61/ft2 (as estimated by the manufacturer) and with the continued need for blinds and their associated costs, payback at the Portland test bed was estimated at 29 years. The incremental difference between installing EC windows and spectrally selective low-e windows was estimated at $37/ft2 with a payback of 13 years.” (GPG-033, “Limited Cost-Effectiveness in General Office Applications,” page 3).

Note that the GSA’s estimated payback for EC windows of 29 years was actually 13 years longer than the estimated payback of conventional (non-Smart) high-efficiency windows in the same installation.

Also, note that the energy savings in GSA’s payback calculations weren’t just due to the new windows per se, but also included the savings provided by a daylight harvesting lighting system. Since daylight harvesting could have been installed without EC windows—at just a fraction of the EC window cost—the payback period of EC windows alone would have been far longer than GSA’s estimated 29 years.

Thus, assuming that GPG’s 2017 assessment is still valid, EC windows might save a significant amount of energy—but they are definitely not cost-effective, especially when compared to non-Smart high-efficiency windows.

Is GPG’s 2017 assessment still valid today?  In a word, yes.  Energy costs have increased, but EC Window costs still haven’t dropped to the “mature market cost” of ~$60/ft2 assumed by GPG.  In fact, the expected payback period is about the same now as it was in 2017, and the tax incentives provided by the IRA are not enough to make the technology appealing enough for mainstream use.

Smart Blinds

Obviously, adjustable window coverings are also capable of modulating a window’s characteristics.  In fact, some adjustable window coverings can save far more energy than any Smart Window technology; these include exterior venetian blinds in hot climates and insulated interior roller shades or cellular shades in cooler climates.

However, a more interesting counterpoint to EC technology is the interior horizontal venetian blind with  motorized slat tilt function, hereinafter referred to as the Smart Blind.

A Smart Blind can’t save as much HVAC energy as some other types of window covering, but is less expensive and can save more lighting energy than all other automated shading devices (including Smart Windows). For these and other reasons, we believe that Smart Blinds will dominate the automated shading market in the U.S.

Like EC Windows, Smart Blinds Offer a Variable SHGC

A Smart Blind is capable of providing an SHGC of less than 1.0 because its slats can reflect some of the solar radiation back out the window.  However, because the blind’s slats are on the interior side of the window, any radiation which isn’t reflected back out the window will heat the room.  Thus, the minimum SHGC of a Smart Blind will necessarily be higher than that of either an exterior blind or a Smart Window.

When used on an ordinary energy-efficient window, an interior horizontal blind might provide a total (window plus blind) SHGC range of 0.2 to 0.4, yielding a ∆SHGC of 0.2. (Ariosto and Memari, Figure 5).

For reference, as previously stated, a typical EC window has a broader SHGC range of 0.1 to 0.5, yielding a ∆SHGC of 0.4.

Thus, the EC window has a minimum SHGC about one-half that of the Smart Blind, and a ∆SHGC about twice that of the Smart Blind.

Therefore, when considering SHGC alone, an EC window could theoretically save twice as much HVAC energy as a Smart Blind.

Like EC Windows, Smart Blinds Offer a Variable VT

Of course, Smart Blinds also offer a variable VT; after all, the whole purpose of venetian blinds is to provide adjustable shading.

For window coverings like curtains and roller shades, the maximum VT is equal to the VT of the window glazing, while the minimum VT can be as low as zero for opaque shading materials.  However, things aren’t so simple for venetian blinds.

For a venetian blind, the VT depends not just on the absolute tilt angle of the blind’s slats, but also on the perspective from which the blind is viewed and whether the incident daylight is direct or diffuse.

If the daylight is diffuse (i.e. without any direct sunlight), a horizontal blind might provide a VT range of between about 0.15 (when the slats are fully tilted) to about 0.8 (when the slats are nearly horizontal).

Thus, when used with a typical view window with its own VT of about 0.8, a typical blind will provide a net (window plus blind) VT range of between 0.1 and 0.64.  A net maximum VT of 0.64 is comparable to that of a typical EC Window, but a minimum VT of 0.1 is higher than that of the 0.03 minimum VT of a typical EC Window.

This would suggest that a venetian blind is less effective than an EC window at blocking glare from direct sunlight—but quite the opposite is true.

That’s because a venetian blind’s slats can be tilted to completely block direct sunlight. Even better, they can still admit a substantial amount of useful, glare-free diffuse daylight at the same time:

This means that a blind actually has two VTs: one against diffuse daylight, and one against direct sunlight:

• When the slats are tilted to block direct sunlight, the effective VT against that direct sunlight can be as low as 0.001—low enough to completely eliminate the risk of glare.
• At the same time—if the slats are tilted only just enough to block the direct sunlight, and depending on the solar elevation—the VT against diffuse daylight can be as high as 0.4, admitting plenty of glare-free natural illumination.

Thus, when used with a daylight-harvesting lighting control, an optimally-adjusted venetian blind should be able to save substantially more lighting energy on a sunny window than an optimally-adjusted EC window.

We’re not aware of any independent testing that confirms this and we haven’t tested EC windows ourselves. However, based on our own testing of responsive daylight control systems with venetian blinds, we know that a blind that is always closed only just enough to fully block direct sunlight does, in fact, admit a substantial amount of glare-free daylight.

Also, as previously cited, GSA found that EC windows actually increased lighting energy consumption (relative to ordinary windows with venetian blinds) in the Sacramento, CA test site (Fernandes et al, Table ES-2, page 3). This means that the EC windows were admitting less daylight, on average, than the previous conventional windows with blinds. This is compelling circumstantial evidence of the advantage of a blind’s ability to admit useful daylight while blocking direct sunlight—or, depending on how you look at it, compelling evidence of a key disadvantage of Smart Windows.

Unlike Smart Windows, Smart Blinds also Offer a Variable U-Value

Smart blinds can also do something else that Smart Windows can’t do: they can modulate a window’s U-value.

The U-value is a measure of how easily heat passes across the window through conduction and convection; the greater the U-value, the poorer the insulation provided by the window.  When choosing an ordinary energy-efficient window for use in a hot or cold climate, the U-value is typically specified to be as low as possible (e.g. via double or triple-glazing and using argon gas or vacuum between the glazings).

As with SHGC, a fixed U-value can’t minimize instantaneous HVAC loads—but a variable U-value can.  Plus, unlike a variable SHGC, a variable U-value can save energy at night as well as during the day.

Unfortunately, horizontal blinds have a smaller U-value range than most other types of window coverings (and, in particular, insulated curtains and roller shades).  The actual U-value range of a blind is determined by many factors, including the type of window and window frame, how the blind is mounted to the window, how closely the blind fits within the window frame, and the design of the blind itself. Ariosto and Memari (2014) provide a good discussion of these factors.

When used with a typical energy-efficient window, the slat-tilt function of a typical horizontal blind can modulate the window’s U-value by only about 10 percent:

That might not seem like much, but it’s better than the 0 percent U-value modulation provided by an EC window.

I’m not aware of any references that provide quantitative projections of the savings achievable through U-value modulation, and the calculations to estimate the savings are rather complicated.  So I’ll leave that topic for another post.

For now, suffice it to say that Smart Blinds can provide at least some HVAC savings through U-value modulation that Smart Windows can’t provide.

EC Windows Won’t Necessarily Save More Energy than Smart Blinds

The following table summarizes the preceding discussion of the energy savings potential of EC windows versus Smart Blinds:

The figure above could be further summarized with this generalization: EC windows offer more potential for HVAC savings (assuming that the EC window’s SHGC advantage trumps the Smart Blind’s U-value advantage), while Smart Blinds offer more potential for lighting savings.

However, one thing we haven’t discussed is the relative value of the HVAC savings versus lighting savings possible with dynamic shading. That’s a complex topic that’s beyond the scope of this post, but here’s a reasonably valid generalization based on the assumption of energy-optimized shading settings (i.e. shading settings that aren’t influenced by occupant preferences):

• In office buildings with fluorescent lighting and in heating-dominated climates, the lighting savings will generally be more valuable than the HVAC savings. Smart Blinds could be expected to save more energy than EC windows in such buildings.
• In office buildings with efficient LED lighting and in cooling-dominated climates, the HVAC savings will generally be more valuable than the lighting savings. EC windows could be expected to save more energy than Smart Blinds in such buildings.

So, there are situations in which Smart Blinds could actually save more energy than EC windows.

Further, the EC window’s advantage in cooling-dominated buildings with LED lighting is less clear when occupant preferences are considered. As previously mentioned, occupant shading preferences are generally more consistent with lighting savings than HVAC savings, so it’s possible that Smart Blinds could actually save more energy in these buildings, too.

In a Best-Case Scenario, How Much More Energy Could EC Windows Save?

For the sake of discussion, let’s ignore occupant preferences and the savings mechanisms in which blinds have an advantage, and instead compare EC windows and Smart Blinds only in terms of SHGC versus VT modulation in diffuse daylight:

Under this best-case scenario for EC windows, they can never save more than twice as much energy as an optimally adjusted Smart Blind—and that 2X advantage would be approached only in buildings whose net energy consumption is heavily dominated by HVAC. Typically, the savings gap (even without considering the blind’s advantages and the implications of occupant preferences) will be much less.

We’ll come back to this 2x upper limit on relative EC savings when we discuss cost-effectiveness later in this post.

Smart Blinds Cost Less than EC Windows

EC Window Costs

A quick internet survey indicates a current average EC Window cost of about $100 per ft² of glazing, while the GSA assessment cited above assumed a “mature market cost” of $60 per ft².

However, these costs don’t reflect any savings from the 2022 Inflation Reduction Act, which provides up to a 30% tax credit for Smart Windows. This would drop the current $100 per square foot cost down to $70. The IRA tax credit is set to expire in 2032, so it’s reasonable to assume that it wouldn’t apply to GSA’s assumed mature market cost of $60. Therefore, for the sake of simplicity, we’ll just assume an EC Window cost of $60 per ft² in both the near-term (with the IRA tax credit) and in the long-term (due to technology maturation).

For reference, conventional insulated glass costs about $10 per ft² of glazing, while more efficient low-e (spectrally selective) windows cost about $20 per ft² of glazing.

Smart Blind Costs

Unlike an EC window, the overall cost of a Smart Blind doesn’t vary directly with the window area, and it also depends on factors other than the window area:

• While the cost of the blind itself does vary directly with window area, the slat-tilt function of even a large blind can be automated with a relatively small motor, and even a small Smart Blind requires the same sensor and control functionality as a large blind. Thus, the automation costs are reasonably insensitive to the window area, so that the overall cost per square foot of window area actually decreases with the size of the window.
• While there is essentially only one EC window technology, there are many blind automation products spanning a range of costs.

Assuming a window area typical of office buildings (and consistent with the window sizes in GSA’s Portland testbed) and an automation cost that’s on the high side of products currently on the market, we estimate an overall Smart Blind cost of $20/ft² of glazing, or only about 33% of the cost of an EC window:

• $8/ft² for the blind itself
• $12/ft² for the costs of automating the blind (i.e. hardware and installation costs for the motor and electronics).

Effective Costs Depend on Purchasing Scenario

The effective cost of an EC window or Smart Blind depends on whether an ordinary window or blind, respectively, would have to be purchased anyway (as in new construction or for a major renovation). In that case, the effective cost is the incremental cost over that of an ordinary window or blind:

• For EC windows, it’s the difference in cost between the $60/ft² EC window cost and either $20/ft² for low-e window or $10/ft² for an ordinary insulated glass window, for an incremental cost of either $40/ft² or $50/ft². Since most new windows are of the low-e type, we’ll assume an incremental EC window cost of $40/ft².
• For Smart Blinds, it’s the difference in cost between the $20/ft² overall cost and the $8/ft² blind cost, for an incremental cost of $12/ft².

However, less than 10% of commercial floorspace is newly constructed or fully renovated each year. In the other 90% of commercial floorspace, the existing windows are still serviceable and don’t necessarily need to be replaced. If the existing windows are replaced for the sake of energy efficiency, the effective EC window cost would be the total cost of $60 per ft² of glazing (versus the incremental cost $40/ft²), plus the cost of the window frame and installation labor (which we estimate at about $20/ft²), for a total of $80 per ft² of glazing.

The energy-saving retrofit scenario is a bit different for Smart Blinds:

• Window coverings are replaced more frequently than windows, so the incremental Smart Blind cost of $12/ft² would apply to more than 10% of commercial floorspace.
• About half of U.S. office buildings are already equipped with blinds which could be retrofitted for automated operation. In these buildings, the Smart Blind retrofit cost would also just be the incremental cost of $12/ft².
• In the remaining buildings—those with no window coverings, or with window coverings which can’t be retrofitted for automated operation—the Smart Blind retrofit cost would be the total cost of $20 per ft² of glazing.

Thus, Smart Blinds cost significantly less than EC windows, with the effective Smart Blind cost ranging from 15% to 30% of the EC window cost depending on the purchasing scenario:

Smart Blinds are Far More Cost-Effective than EC Windows

The salient cost-effectiveness metric for investments in energy-saving technologies is the payback period. Assuming that Smart Blinds save only half as much energy as EC windows (which, as noted above, is a very conservative assumption), and since they cost much less than half as much as EC windows, they will obviously have a shorter payback period.

In fact, depending on the purchasing scenario, Smart Blinds will have a payback that’s only 0.3–0.6 that of EC windows, implying a relative cost-effectiveness of 167%–333% relative to EC windows:

But are Smart Blinds Cost-Effective Enough for Mainstream Use?

The fact that Smart Blinds are far more cost-effective than EC windows certainly suggests that they deserve at least some of the interest and incentives that EC windows are getting.

However, what really matters to the market is not whether Smart Blinds are more cost-effective than EC windows, but whether they’re cost-effective enough to make sense as investments in energy efficiency.

On the positive side, the 17-year paybacks shown in Table 3 aren’t outrageously greater that the typical 10-year threshold for cost-effectiveness in the commercial market. Further, those estimated paybacks neglect any of the energy-saving advantages of Smart Blinds over EC windows, and they’re based on an assumed Smart Blind automation cost that’s on the high side of what’s available today. This suggests that Smart Blinds could, indeed, be cost-effective for energy savings in a building like GSA’s Portland testbed.

On the negative side, there are two big catches with the paybacks of Table 3.

Paybacks are Much Longer When Daylight Harvesting Costs and Savings are Excluded

First, the listed paybacks are for an integrated system that includes both the EC windows or Smart Blinds and daylight-harvesting lighting controls.

Daylight harvesting costs much less than either EC windows or Smart Blinds, and it contributes about two-thirds (our own estimate) of the overall savings. Thus, the payback period for daylight harvesting alone is far shorter than the figures of Table 3—as short as just a few years. This means that purchasers are much more likely to invest in just daylight harvesting, alone, instead of an integrated capability that includes both daylight harvesting and dynamic shading.

So, a more relevant metric for EC windows and Smart Blinds is the payback when the costs and savings from daylight harvesting are excluded. This would approximately triple the paybacks listed in Table 3, yielding about 90–180 years and 50–90 years for EC windows and Smart Blinds, respectively.

As another data point, we’ve separately estimated the median payback period of Smart Blinds alone (when used with a separately costed daylight harvesting lighting control), across the entire stock of U.S. office buildings, at 42 years. The payback could be as short as 10 years in some buildings, but a median payback of 42 years is too long for a mainstream energy-saving technology.

Smart Blinds Need More R&D

The second catch is that automatically adjusting a blind to maximize glare-free daylight isn’t as easy at it might seem. We call this responsive daylight control, and—as described in detail in this post—it’s not just the most valuable form of automated shading, it’s also the hardest to implement effectively.

In fact, no Smart Blind product offers an out-of-the-box responsive daylight control capability, and responsive daylight control is virtually non-existent even in Class-A office buildings with custom-programmed automated shading systems.

However, Smart Blinds are a much more promising avenue to mainstream use of dynamic shading than EC windows. To the extent that the IRA incentives were aimed at combatting climate change, they would be far more effective if they hadn’t been limited to EC Windows, but instead had also applied to all forms of dynamic shading including Smart Blinds.

References

Tim Ariosto and Ali M. Memari. “EVALUATION OF VENETIAN BLIND ATTRIBUTES FOR ENERGY EFFICENCY.” Presented at the 2nd Residential Building Design & Construction Conference – February 19-20, 2014 at Penn State, University Park. <https://www.phrc.psu.edu/assets/docs/Publications/2014RBDCCPapers/Ariosto-2014-RBDCC.pdf>

Luis L. Fernandes, Eleanor S. Lee, Darryl Dickerhoff, Anothai Thanachareonkit, Taoning Wang, Christoph Gehbauer. “Electrochromic Window Demonstration at the John E. Moss Federal Building, 650 Capitol Mall, Sacramento, California.” Prepared for the U.S. General Services Administration by the Lawrence Berkeley National Laboratory. November 2017 <https://www.gsa.gov/system/files/Applied_Research/170920_Electrochromic_Window_Demo_Moss_Building_Sacramento_Final.pdf>

General Services Administration, “GPG-033 NOVEMBER 2017: ELECTROCHROMIC WINDOWS FOR OFFICE SPACE.” <https://www.gsa.gov/system/files/033-Findings-EC_Windows_for_Office_Space-v1.pdf>

Eleanor S. Lee, Luis L. Fernandes, Samir Touzani, Anothai Thanachareonkit, Xiufeng Pang, Darryl Dickerhoff. “Electrochromic Window Demonstration at the 911 Federal Building, 911 Northeast 11th Avenue, Portland, Oregon.” Prepared for the U.S. General Services Administration by the Lawrence Berkeley National Laboratory. June 2016. <https://www.gsa.gov/system/files/Applied_Research/GPG-EC_Windows-Portland-FINAL.pdf>