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Posts tagged ‘green’

Does Your Building Pass the Lick Test?

Cate Leger

Energy codes have done a good job of moving California architecture to more energy efficient buildings.  However, green building needs to also include green building materials.

Back in the 1990s a great deal of attention in the green architecture community was placed on materials.  Materials were researched and compared for their toxicity, life cycle impacts on the environment and human health, carbon footprint, sustainable yield, cultural impacts of extraction and more.  However, as concern about the climate has grown and disagreements sprang up on the ‘greenness’ of various building systems and materials, the focus has generally shifted to energy efficiency and more recently shifting to 100% renewables.

While CALGreen has incorporated standards to limit off gassing of some toxic chemicals, there are many human and environmental health issues of materials these standards don’t address.  Here are some excellent resources to fill the gaps:

  • Healthy Building Network’s HomeFree offers a short, easy to use specification focused on indoor air quality and health. Healthy Building Network also has extensive database on chemical content and environmental impacts for thousands of products and great articles explaining their research. https://homefree.healthybuilding.net/reports
  • The Living Building Challenge, a third party green building rating system similar to LEED, has a comprehensive Red List of chemicals to avoid and a database with materials meeting those standards. https://living-future.org/declare/declare-about/#the-red-list
  • The Green Science Policy Institute provides guidance on avoiding toxics in the home. They also have a series of short videos breaking down the toxic chemical landscape into 6 classes, allowing the layperson to understand the general problems.  http://greensciencepolicy.org/
  • The Forest Stewardship Council provides the most comprehensive third party certification for sustainably harvested wood.

What we build with also has a significant impact on the climate.  The embodied energy or the carbon footprint of building materials varies widely.  Using low carbon materials for construction is critical in addressing climate change because the greenhouse gas emissions savings are accrued earlier, at the time of construction, when they have the biggest impact.  Prioritizing low carbon materials can significantly reduce the lifetime carbon footprint, and in some cases even reverse it!

Metal and plastics in general have a very high carbon footprint.  Concrete, while lower in embodied energy per pound, is used in such great quantities that its global warming impact tends to dwarf that of other materials used in construction.  Blowing agents used in some of the foam plastic insulations have such high carbon footprints that their addition to a building can negate the operating energy savings for decades.  For more information, The New Carbon Architecture by Bruce King offers inspiring examples and details of low carbon construction.  https://ecobuildnetwork.org/projects/new-carbon-architecture

If the health of workers, occupants and our planet are not reason enough to use healthy building materials, fire is yet another one. The toxicity of building materials and furnishings can be amplified when they are burned. Smoke that engulfed the North Bay – and blanketed the entire Bay Area – was laced with dangerous chemicals: dioxins, furans, hydrogen cyanide and heavy metals, threatening the health of all.  Sadly, firefighters are known to have some of the highest rates of cancer.  Frequent exposure to these kinds of chemicals may be a contributing factor.   Many of these dangerous chemicals are also left behind in the ash.

Energy Return on Energy Invested (EROEI)

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Judhajit Chakraborty, Assoc. AIA, LEED® AP, WSP Built Ecology

In February 2016 I attended the Net Positives Conference organized by the International Living Futures Initiative (Living Building Challenge) in San Diego, CA and I was very impressed with the quality of both the presenters and the presentations. Unlike the Greenbuild it was more of a closed group conference with about 350 attendees, digging deep into the water and energy problems of the world and providing far-fetched but definitely achievable solutions.

In this article I would like to mention one presentation that I really liked. On a conceptual level it talked about a term which is not generally used while talking about energy efficiency in the built environment. This is Energy Return on Energy Invested (EROEI) and this term has been in use in the world of Energy Economics and Carbon Economics for a long time now. But now as high performance design is looking more towards embodied energy (total energy from source – production + transportation), having a basic understanding of EROEI is important.

So, what actually is this Energy Return on Energy Invested (EROEI)? Mathematically it is a simple ratio: ratio of the amount of usable energy acquired from a particular resource to the energy expended to acquire that energy.green1

The higher the EROEI, the more “profitable” the energy resource (from an energy standpoint). If the EROEI drops below 1, it means that it takes more energy to produce the usable energy than is contained in the finished product. In other words, EROEI is a measure of the amount of energy society gets, as a ratio of the amount of energy we put in to get that energy. Net energy available is thus the amount of energy left for a modern society to function (Food production + support and education of people + healthcare for people + arts, entertainment, sports and other amenities) after spending the initial energy to get that energy. The more it costs to extract the energy, the less net energy is left for use. This leads to the energy cliff exponential graph to the left.

green2The dark gray area is the net energy available for a society to run smoothly and the light gray area is the energy used to procure that energy. According to this graph, a minimum EROEI of 14 is required for a smooth functioning of society. The graph also shows EROEI’s of major fuel sources. The conventional fossil fuels have the highest EROEI’s and that is why they are continuing to sustain our society. Even a reduction of 50 to 25 EROEI (which means gradual depletion of natural resources and hence more energy required to extract) have an inconsequential effect on the percent energy out. However that percentage changes sharply when unsustainable means of fuel is being brought into practice (corn ethanol, shale oil, bitumens etc). This also means that going forward, as our natural resources gets depleted, we as a society need to make smarter choices and invest in the right form of energy. Alternative clean fuel sources such as PV, nuclear and wind play a huge part in this. They have low EROEI’s but they don’t consume natural resources. However, currently these fuel sources are nowhere close to replacing the conventional fossil fuels. It can make good economic sense for an individual — or a society — to invest in a renewable energy producing system as long as the system can be maintained and operated for a long time.

As we debate our future energy choices and policies, energy return on investment should be an important part of such discussion. This gives us a “reality check” as we figure out where to put our energy investments.

Green: Emerging Technologies and Resources for Photovoltaics

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Judhajit Chakraborty, Assoc. AIA, LEED AP WSP Built Ecology

1. ClearView Power™: PV’s are getting transparent. Yes that’s right, past MIT researchers and current entrepreneurs, Miles Barr, Ian Millard, Vladimir Bulovic and Richard Lunt are making transparent solar cells that could turn everyday products such as windows and electronic devices into power generators— without altering how they look or function. How? Their new breakthrough in solar cells absorb only infrared and ultraviolet light, thereby only letting visible light pass through the cells unobstructed, so our eyes don’t know they’re there. They estimate that this technology of using PV coated windows in a skyscraper could provide more than a quarter of the building’s energy needs without changing its look. They’re now beginning to integrate their solar cells into consumer products, including mobile device displays and have started a venture called Ubiquitous Energy (http://ubiquitous.energy/).5

Current versions of the ClearView transparent PV cells transmit more than 70% of the visible light, which is within the range of tinted glass now used in the windows of buildings. But their tested efficiency is very low—only about 2%, but they are working on it. In theory, their design should realistically be able to reach over 12% efficiency, comparable to that of existing commercial solar panels. This will be challenging, but they believe they can do it by carefully optimizing the composition and configuration of the PV materials. The other challenge is the longevity of PV. In commercial applications such as window coatings, the solar cells need to continue performing well for many decades. With many industries tackling the same issue, the team believes that this engineering problem should be solved in the coming years, and their solar cells should be guaranteed to have a commercially viable lifespan. And this may well be a game changer in the PV industry.

62. Project Sunroof by Google: Until now you had used Google for comparing air-fares, hotel fares and for umpteen searches, now Google lets you weigh the costs and benefits of installing solar panels on your rooftop through its new online tool ” Project Sunroof” (https://www.google.com/get/sunroof#). This new tool, now available for select cities in CA, AZ, NV, CO, CT, NY, NJ, NC and MA, requires a user to enter the home address and it computes how much sunlight hits your roof in a year. It takes into account:

• Google earth’s database of aerial imagery and maps
• 3D solar radiation modeling of your roof
• Shadows cast by nearby structures and trees
• All possible sun positions over the course of a year
• Historical cloud and temperature patterns that might affect solar energy production

Project Sunroof then computes the savings by using the current solar industry pricing data to run the numbers on leasing, taking a loan, or buying solar panels for your house to help you choose what’s best for you. Not only that, it also compiles the following incentives to calculate the final PV cost.

• Federal and state tax credits
• Utility rebates
• Renewable energy credits and net metering

Project Sunroof is a faster, simpler way of sizing up possible pros and cons of solar than calling someone for a site evaluation or using the more complex calculator offered by the U.S. Energy Department

Green: Red List Building Materials and Living Building Challenge

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Judhajit Chakraborty, Assoc. AIA, LEED AP WSP Built Ecology

First of all, I wish all the readers and members of AIA East Bay a very happy and prosperous 2016.

“Monumental Green,” the theme of Greenbuild 2015 held at Washington D.C. paid a lot of emphasis on the environmental impact of materials on buildings and building environmental standards like WELL, SITES, Living Building Challenge (LBC), PEER and I am going to write about Red List building materials and how LBC has been leading the way in material disclosure and transparency.

Red List Materials, as the name suggests, are materials that contain chemicals that have been labeled as harmful and detrimental to living creatures, including humans and/or the environment. These lists have primarily been developed by green building rating system developers and some big architecture firms like Perkins and Will. These lists are typically developed from chemical hazard lists published by several government agencies, such as the Environmental Protection Agency (EPA), European Union Commission on Environment, and the California Department of Toxic Substances Control.pg. 5 green 1

Of all the environmental building standards that prohibit red list materials in any building project, the LBC has been more rigorous in implementing this and has developed their own Living Building Challenge Red List. Following is the LBC’s Red List materials which are banned from any project adhering to LBC certification:

■ Alkylphenols
■ Asbestos
■ Bisphenol A
■ Cadmium
■ Chlorinated polyethylene and chlorosulfonated polyethlene (CSPE); HDPE and LDPE are
Chlorofluorocarbons (CFCs)
■ Chlorobenzenes
■ Chloroprene
■ Chromium VI
■ Chlorinated polyvinyl chloride
■ Formaldehyde (added)
■ halogenated flame retardants (HFRs)
■ Hydrochlorofluorocarbons (HCFCs)
■ Lead (added)
■ Mercury
■ Polychlorinated biphenyls
■ Perfluorinated compound
■ Phthalates
■ Polyvinyl chloride (PVC)
■ Polyvinylidene chloride
■ Short Chain Chlorinated paraffin
■ Wood treatments containing creosote, arsenic or pentachlorophenol
■ Volatile organic compounds (VOCs) in wet applied products

Following the development of this list, LBC developed the DECLARE product labeling program and includes a database of materials which are LBC compliant. Here is the database.

Sound that Chills! – Thermoacoustics

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Juhajit Chakraborty Assoc, AIA, LEED AP WSP Built Ecology

Energy Efficient Building HVAC systems have been gaining grounds in the United States since the last decade. One of the areas that have achieved significant improvement is the refrigerant emissions. We all know that refrigerants contribute greatly to global warming. A project cannot be LEED certified if they use CFC based refrigerants. Almost all state and local building codes have banned CFC use and mandates use of only low ozone depleting potential (ODP) and global warming potential (GWP) refrigerants. But they still contribute to global warming. Refrigerants are an integral part of the HVAC process as they transport heat thru continuous compression and expansion cycles.

Xerox-owned Palo Alto Research Center (PARC) has developed a technique to enable sound or thermoacoustic cooling technology for air conditioning applications. In a nutshell, what they have been develop-ing is an air conditioner but it works like a loudspeaker. Interesting, isn’t it?

What is Thermoacoustics?
It is the interaction between temperature, density and pressure variations of acoustic waves. A thermoacoustic HVAC system compresses and expands gases with high intensity sound waves. When compressed, heat is generated because of the pressure applied and during expansion, cooling happens. More so like the chill we get when a carbon dioxide cartridge or canister is suddenly discharged and the gas is allowed to expand. The thermoacoustic system can potentially allow the compressor to complete 10,000 compression/expansion cycles, way more than a mechanical compressor system. Thermoacoustic compressors have been traditionally used mainly in labs and cryogenic cooling to turn on atmospheric gases like nitrogen into chilly liquids and therefore they usually perform best under extreme conditions and not efficient for commercial use for maintaining ambient temperatures (70°F -75°F) – this is what PARC has developed a prototype system, for ambient temperatures. With this technology hitting the com-mercial market, not only will result in zero emissions, but also it will save tremendously on energy. Cooling applications represent 25% of all electricity use in the United States, consuming over seven quadrillion BTUs of energy and generating nearly 600 million metric tons of CO2 emissions annually. If widely used, this prototype technology can save up to 13% reduction in energy consumption in US, double the efficiency of air conditioning and reduce CO2 emissions by 311 million metric tons annually.

How does it work?
Gases are first inserted into a tube filled with mesh membranes which are called regenerators. As the sound wave passes through the regenerators, a low-pressure, low-temperature/high-pressure, high-temperature gradient begins to form. One end gets hot while the other gets cool. Heat exchangers are then used to extract and exploit the cooling or the heating as required. The prototype’s increased efficiency comes from the electromechanical couplings generated in the “transducers” (device that converts variations – in this case pressure and temperature) through the interconnected thermoacoustic chambers in series such that unused energy is efficiently passed from one chamber to another. The image below shows a basic system (left) which is used in labs and the improvised prototype developed by PARC for ambient temperatures (below).

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Green: Daylight Redirecting Film

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Judhajit Chakraborty, Assoc. AIA, LEED AP WSP Built Ecology

We all know the benefits of day-lighting and how important it is in our designs. Buildings with abundant natural light have been shown to:

• Increase employee productivity in offices
• Boost retail sales in stores
• Improve student test scores and attentiveness in schools
• Decrease rates of absenteeism
• Improve patient recovery times in hospitals and health centers
• Reduce energy costs

There are many strategies to effectively day-light a space without compromising too much on glare. Worth mentioning there are two categories of glazing in a unit – Vision glazing mostly for views and protected by blinds, and daylight glazing mostly for daylight which, with effective strategies, can be blind free.

October_ArchNews DRAFT pg 1-5_Page_5_Image_0002The image on the left shows the daylight and vision glazing positions of a single glazing unit. Also on a fully glazed surface, the glazing from the floor level to 1’ is the least useful in terms of both views and day-lighting. The image also shows a typical light-shelf for redirecting light to the ceiling for daylight.

This article is about a new product which acts as a light-shelf but it has the power to throw light much deeper into the space. The product also known as the Daylight Redirecting Film or DRF, is a film consisting of many micro-prisms which optically redirects over 80% of the light incident on the glazing upward. There is another diffusing film which distributes light uniformly over greater ceiling depths. In doing so, the film: reduces glare and discomfort caused by direct sun, redirects natural light to as much as 40 ft, extends daylight zone to 8ft for every feet of daylight glazing with DRF, and provides up to 50% lighting energy savings over a comparative baseline. The DRF technology is very new and there are only a couple of players in the market, the most prominent among them is 3M. The image below shows how the 3M Daylight Redirecting Film works.

 

October_ArchNews DRAFT pg 1-5_Page_5_Image_0003As shown in the image to the left, the assembly includes a double glazing unit or an IGU (Insulated Glazing Unit) and the DRF film is placed on Surface no 2 as shown in the enlarged image. The assembly comes with a diffusing film on Surface 3 which distributes light uniformly towards the ceiling. Extensive tests have been done on this DRF technology especially with the 3M film at the LNBL lab and at the newly constructed Net Zero Sacramento Municipal Utility District (SMUD) building. All the reports are available upon contacting any 3M rep.

As promising as this product seems to be, there are some limitations; since the film is applied on Surface # 2, then there is no scope for any low-e coatings as they also go in Surface 2 and the technology is such that currently the film can only go on even numbered surfaces. Upon asking 3M reps mentioned that they will be launching a product early next year which will combine both the films into one. Also in terms of simulation, there are some challenges with predicting glare as no daylight calculating engines are robust enough to smoothly analyze the micro-prisms. But nevertheless, this is a product worth trying out.

Green: The Flexlab at LBNL

The Worlds Most Advanced Building Efficiency Test Bed

Judhajit Chakraborty. Assoc. AIA, LEED® AP WSP Built Ecology

Judhajit Chakraborty. Assoc. AIA, LEED® AP WSP Built Ecology

As I was thinking about what to write for this month’s article, I realized that I’ve never written about the FLEXLAB at the Lawrence Berkeley National Laboratory. I toured a few months back as part of the International Building Performance and Simulation Association (IBPSA)-United States’ San Francisco Chapter’s event. Believe me, it is by far the coolest building lab I have ever seen. Not surprisingly, it is the world’s most advanced bed for testing building efficiency. I am sure many of the readers would be interested to know that such a facility exists at a stone’s throw distance and that also they can request to tour the facility (Ask me for the right contact).

 

So what is FLEXLAB?

This Department of Energy (DOE-funded) lab which started operating last year is the most flexible, comprehensive and advanced building simulator in the world. It lets stakeholders test energy efficient building systems either as a standalone system or an integrated system under real-world conditions. The test beds can test a whole range of systems including different HVAC systems (VAV, Chilled Beams, Radiant systems etc), lighting systems (with a plethora of control options), building envelope (glazing and shading systems), control systems, plug loads – in any combinations.

Green3Users can test different scenarios and alternatives, perform Return on Investment (ROI) and life cycle cost analysis which produces data thereby ensuring the efficiency of the project much before the construction or retrofitting even begins. Did I forget to mention that you can simulate different orientations as well? Yes, the test beds are motorized and can rotate to optimize orientation angles.
When I visited the FLEXLAB, one of the test beds was an office, which looked like any other corporate office with desks and chairs arranged across a well-lit floor. But then instead of humans, there are slender poles of different heights that constantly monitor temperature and light levels. There are some metal tubes stationed near the workstations and they emit heat, the same amount as a human body. It was really an amazing set up to understand in real time how your building will perform before you spend millions on it.

Green4What does the FLEXLAB facility offer?

As mentioned before, the FLEXLAB allows stakeholders to evaluate their project’s performance in terms of energy efficiency and return on investment by: optimizing integrated systems; ensuring occupant comfort and user friendliness; analyzing cost benefits; training the building operators and facility managers; and build confidence among the building industry in new building technologies.
Who is using it?

Green2Until a month ago, Webcor and Genentech was using the test bed to inform Genetech’s new 250.000 sq. ft office in South San Francisco. They were using the same ventilation system, glazing and interior modules for all the tests as it was in the design. Together, the FLEXLAB facility helped to deliver this state-of- the-art building that will set a new bar for energy efficiency and thermal comfort. ■

Solar PPA/Solar Lease for Your Photovoltaic system – Think before you leap.

Judhajit Chakraborty By Judhajit Chakraborty, Assoc. AIA
Last month I was working on a project where we designed a renewable energy system for a client and upon the client’s request, we looked into various financing options of those renewable energy systems. This included state and local utility incentives, solar loans, and solar PV leases, which are also known as Power Purchase Agreements (PPA’s).

This article looks into various myths and misunderstandings regarding PPA’s. Though they claim to have zero down payments and cover all maintenance costs over the lease period, there are many hindrances and disadvantages of which consumers should be aware before embarking towards the PPA route.

1: With a solar lease, one might think of it as affordable solar energy with zero down payments.
However, upon calculating the payments on that zero down solar lease or PPA one will find the payments over the 20-year lease period (which is the general duration of a PPA lease) will triple the cost for a solar system when compared to purchasing a system.

2: Selling a home with a solar lease or PPA—the lease can be easily transferred to your home buyers.
However, It has been reported recently by homeowners and even real estate brokers that it has been more difficult to sell homes with a solar lease or PPA attached to it. It is a very complicated process which is being made even more complex by the agreement clauses and language. Be sure to consult with experts before signing the agreement.
http://stovallteam.com/leased-solar-panels-may-create-dilemma-for-sellers/

Also, home buyers must qualify to assume lease with an excellent credit rating generally more than 700, otherwise in order to sell the home, the lease has to be paid off and/or disclose the solar lease obligation when selling the home.
http://www.bakersfieldcalifornian.com/business/x65918383/Solar-panels-may-create-dilemma-for-home-sellers

Consider if a seller has 12 years of remaining lease payments on a 20-year lease on an outdated 8-year old solar system—what home buyer would want to be burdened with this when they could buy a brand new solar system for tens of thousands less? Of course, that would be a hard sell. And that would affect the home sale considerably. Many home buyers who thought of a PPA as a viable option to increase their home’s value during the recession are victims of this 20-year term.

3: A solar PPA lease is the only option for a zero-down payment solar installation plan.
Today there are many solar loans options available (5-20 yrs) which are “zero down.” For example, there is the FHA Title 1 Zero Down which is more convenient to get than a PPA agreement and one can keep the Federal Tax Incentive and all the advantages of applicable case rebates which cannot be attained with a PPA. Another example is the new PACE (Proper Assessed Clean Energy) programs that are cropping up all over the country that allow financing the PV system and its payback through property taxes.

What are the advantages of a PPA and which building types have the most advantage by going through the PPA lease? Certainly not residential. In my experience, I have found this type of leasing is ideal for:

1: State and Federal facilities which in most cases do not have the funding to outright buy and install a PV system but want to promote sustainability, renewable energy production in their facilities and without any headache for their maintenance as they are taken care of by the leasing company.

2: Commercial facilities for similar above-mentioned sustainability and maintenance reasons.

Solar PPA leases still dominates the PV market today, but their future looks bleak as both PV and its installation is becoming less expensive. PPA’s rose into prominence because maintenance was a hassle, but now that maintenance is much improved there are more attractive solar loans now through which one can own their own PV system, really adding value to their homes.

Solar PPA/Solar Lease for Your Photovoltaic System – Think before you leap.

Judhajit Chakraborty, Assoc. AIAby Judhajit Chakraborty, Assoc. AIA

Last month I was working on a project where we designed a renewable energy system for a client and upon the client’s request, we looked into various financing options of those renewable energy systems. This included state and local utility incentives, solar loans, and solar PV leases, which are also known as Power Purchase Agreements (PPA’s).

This article looks into various myths and misunderstandings regarding PPA’s. Though they claim to have zero down payments and cover all maintenance costs over the lease period, there are many hindrances and disadvantages of which consumers should be aware before embarking towards the PPA route.

1: With a solar lease, one might think of it as affordable solar energy with zero down payments.

However, upon calculating the payments on that zero down solar lease or PPA one will find the payments over the 20-year lease period (which is the general duration of a PPA lease) will triple the cost for a solar system when compared to purchasing a system.

2: Selling a home with a solar lease or PPA—the lease can be easily transferred to your home buyers.
However, It has been reported recently by homeowners and even real estate brokers that it has been more difficult to sell homes with a solar lease or PPA attached to it. It is a very complicated process which is being made even more complex by the agreement clauses and language. Be sure to consult with experts before signing the agreement.

http://stovallteam.com/leased-solar-panels-may-create-dilemma-for-sellers/

Also, homebuyers must qualify to assume lease with an excellent credit rating generally more than 700, otherwise in order to sell the home, the lease has to be paid off and/or disclose the solar lease obligation when selling the home.

http://www.bakersfieldcalifornian.com/business/x65918383/Solar-panels-may-create-dilemma-for-home-sellers

Consider if a seller has 12 years of remaining lease payments on a 20-year lease on an outdated 8-year old solar system—what home buyer would want to be burdened with this when they could buy a brand new solar system for tens of thousands less? Of course, that would be a hard sell. And that would affect the home sale considerably. Many homebuyers who thought of a PPA as a viable option to increase their home’s value during the recession are victims of this 20-year term.

3: A solar PPA lease is the only option for a zero-down payment solar installation plan.

Today there are many solar loans options available (5-20 yrs) which are “zero down.” For example, there is the FHA Title 1 Zero Down which is more convenient to get than a PPA agreement and one can keep the Federal Tax Incentive and all the advantages of applicable case rebates which cannot be attained with a PPA. Another example is the new PACE (Proper Assessed Clean Energy) programs that are cropping up all over the country that allow financing the PV system and its payback through property taxes.

What are the advantages of a PPA and which building types have the most advantage by going through the PPA lease? Certainly not residential. In my experience, I have found this type of leasing is ideal for:

1: State and Federal facilities which in most cases do not have the funding to outright buy and install a PV system but want to promote sustainability, renewable energy production in their facilities and without any headache for their maintenance as they are taken care of by the leasing company.

2: Commercial facilities for similar above-mentioned sustainability and maintenance reasons.

Solar PPA leases still dominates the PV market today, but their future looks bleak as both PV and its installation is becoming less expensive. PPA’s rose into prominence because maintenance was a hassle, but now that maintenance is much improved there are more attractive solar loans now through which one can own their own PV system, really adding value to their homes.

This ” Green Column”article was first published in the April 2015 issue of ArchNews

Daytime Radiative Cooling — Passive Cooling During Day

Judhajit Chakraborty, Assoc. AIA

Judhajit Chakraborty, Assoc. AIA

By Judhajit Chakraborty, Assoc. AIA

First of all I would like to wish all the readers a very happy 2015.

We all know or have heard about nighttime radiative cooling. It is a type of cooling to the night sky based on the principle of heat loss by long-wave radiation from the earth’s surface to the atmosphere. Clear starry skies help in achieving more radiative cooling. However, higher levels of humidity will slow down the process. This passive cooling strategy has been used a long time. Green roofs, roof ponds, etc use this strategy to keep houses cool at night. But what about daytime, especially in arid/semi-arid regions?

Not long ago, an urban heat island effect study in downtown Phoenix, AZ showed building façade temperatures at a whopping 170-180°F, about 60°F higher than the ambient air temperature and much of the radiation and re-radiation from the building façade stays there, with no where to go as the air temperature is hot too and it cools down only at night. Unless, there is a material smart enough to reflect all the radiation to the outer space which is very cold, 3°F above absolute zero or about -270°F. Dr. Aaswath Raman and his team from Stanford University came up with such a material. They invented a material that would radiate all sorts of radiation (direct, indirect, infra-red) to the outer space during daytime thereby resulting a surface temperature of about 10°F cooler than the ambient air temperature. The other cool part of it: the material radiates sunlight at a wavelength between 8-13 microns and that is basically a transparent portal to the outer space. So, in a nutshell, this smart material directly radiates sunlight to the outer space and without being obstructed by anything, atmosphere, clouds, nothing.

So, what is that material? It consists of four layers of silicon dioxide which is sandwiched with three layers of hafnium dioxide. Each of the seven layers has a precisely defined thickness ranging from 13 to 688 nanometers (billionths of a meter). It is also backed by a layer of silver 200 nanometers thick, which will act as a mirror.

Dr. Raman’s team computed the thickness of those layers for reflecting the entire solar spectrum while, at the same time, getting rid of infrared light at the frequency which can most easily escape from Earth into outer space. And then they built a mock up, to see if their theory works.

Well, it did. They mounted the sandwiched material on a silicon wafer to keep it flat. It was then held in a specially
designed box made of Mylar, polythene, polystyrene, acrylic and wood, to minimize the conduction of heat into it from its surroundings, and then left outside on a sunny, but a cold ,California day. The photonic sheet settled down to a temperature 10°F cooler than its surrounding air temperature. Thermal isolation was necessary as without that, the temperature difference would disappear—but the result would have been a slightly cooler surrounding.

A long journey still awaits for turning this discovery to a commercially available building material. Dr. Raman and his team, however, have taken the first cost reduction step by working out that hafnium dioxide (which is expensive) can be replaced with titanium dioxide (which is cheap). They will probably need to replace the silver, too—though the cost of silicon dioxide, or sand, is cheap. The material will work only on those parts of a building (mainly the roof) that have a clear view of the sky, and thus of outer space.

Though it will not replace air conditioning completely the idea of cooling down the building mass (roof) passively during daytime will save a lot on air conditioning costs when in USA, more than 15% of a buildings electricity is devoted to cool a building. Passive cooling now has become a little cooler! Cheers to Dr. Raman and his team at the Stanford University. ■