Getting Some Shade

September 13, 2015
November/December 2015
This online-only article is a supplement to the November/December 2015 print edition of Home Energy Magazine.
Click here to read more articles about HVAC

You’re uncomfortable. You feel streaks of sweat starting to inch down your forehead, back, and neck. The sun is relentless. Frantically, you look around for relief. The only thing in sight is a tree. You dash toward it and step into its dark shadow. You sit down and relax as the shade wraps you in a cool embrace.

We experience the discomfort of heat quite frequently. As humans, we’re very bad at tolerating extreme temperatures. That’s why we invented fans and air conditioning (A/C). These inventions use the power of electricity to keep our indoor spaces cool and comfortable. Unfortunately, the use of electric power has serious environmental implications. Today, most of our power is generated by burning fossil fuels. This leads to emissions of CO2, a major greenhouse gas. According to the latest data from DOE, each household in the United States spends approximately 700 kilowatt-hours of energy per year on A/C. This leads to 100 million tons of CO2 being released into the atmosphere. That averages out to approximately 2 tons of CO2 per air conditioner. The consequences of our air-conditioning technology are tremendous. A cleaner alternative would significantly improve our planet’s health.

Inside Window Temperatures

Inside Window Temperatures
Figure 1. This graph shows the temperature recorded at each window between the hours of 2 pm and 8 pm. Temperatures were recorded at half-hour intervals. Data were taken on a west-facing window in Park City, Utah. Weather was sunny, with a high of 80.4°F and a low of 48.9°F.

In recent years, scientists have introduced a slew of renewable-energy solutions, including PV solar panels and wind turbines, which can be used to power air conditioners without emitting CO2. These technologies, however, are expensive and it takes years for the energy savings they yield to make up for the energy consumed by their production. We would reap greater benefits from a simpler solution.

The sun’s primary mechanism for heating up our homes is the window. The sun’s rays easily pass through clear glass to bring its scorching heat inside our living rooms. Years ago, humans put blinds on their windows to prevent the sun’s rays from making things too uncomfortable. But that doesn’t work very well. The sun can still shine directly on the clear glass and transmit its energy into the home. The heat may be concentrated around the window initially, but it steadily transfers to the interior of the building. Heat energy is unaffected by a few window blinds, and proceeds to warm up the home as if nothing were there. Clearly, indoor window blinds do little to reduce the need for air conditioners.

So how do we solve this home-cooling conundrum? The answer is as easy as stepping into the shade! All we have to do is find a way to bring the shade into our homes. This is simple: Put your blinds outside the window! This effectively shades the window from the sun’s rays and prevents it from heating up the house. It’s not nearly as advanced or innovative a solution as A/C, but it definitely works. In fact, it has a major advantage over our man-made cooling technologies: It doesn’t consume energy. It’s an easy fix and one that a few—but not nearly enough—are using.

So how much energy can you save by putting your blinds outside the window?

Calculating Energy-Saving Potential

Many complex mathematical processes go into developing a complete model of the heat transfer phenomenon that occurs across a window. I’ll spare you the details. The fundamental process for estimating how much energy employing external window blinds can save is actually quite simple. We can use the following equation to calculate the heat flux (the heat transfer rate per unit area) of energy passing through the window:

q” = (t)(q”solar)(cos(A))

where q” is the heat flux passing through the window; t is the transmissivity of the window glass; A is the angle at which sun’s rays hit the window; and q”solar is the heat flux of the sun. These factors determine the rate of heat energy passing through the window. Now the transmissivity of glass is about 0.9. The sun has a heat flux of about 1,000 watts per square meter. And at a standard daytime hour, the sun might hit a window at an angle of about 450. If we put these values into our equation, we produce a result of 450 watts per square meter transferred across a typical window. The addition of outdoor window blinds changes the calculations in a simple way. We go back to our equation for heat flux. All the values remain the same except that we must account for the porosity of the outdoor blinds. A typical set of blinds might have porosity of 0.2 (letting in 20% of outside radiation). We simply multiply our original result of 450 by 0.2 and get a new value of 90 watts per square meter.

Our calculations reveal that without outdoor window blinds, about 450 watts per square meter of energy are let into the house. With outdoor window blinds, only 90 watts per square meter are let in. The difference is 360 watts per square meter. Now let’s say an average window is about 1 meter by 1 meter in surface area, and an average home has about eight windows. That means that the total energy blocked from entering the building would be 360 x 8 = 2,880 watts or 2.88 kilowatts. However, this number applies only when the sun has a clear shot at your window. We’ll make a conservative estimate and say that, on a typical summer day, the sun has a clear shot for only two hours (Your window, of course, experiences more than two hours of sun exposure each day, even if it’s not a “clear shot,” making our calculations an underestimate.)

There are 94 days between the summer solstice and the autumnal equinox of 2015. (Your window also experiences sun exposure during other seasons, further shrinking our estimate.) So that’s a total of 2 X 94 = 188 hours of intense sunlight. We multiply that by our 2.88 kilowatts to get approximately 540 kilowatt-hours. That’s roughly how much sun energy you prevent from heating up your house over the course of one year by installing outdoor blinds.

Now, as I mentioned earlier, the latest data from DOE indicate that in one year, the average American household consumes about 700 kilowatt-hours of energy for the purpose of cooling. But if you install outdoor blinds, you eliminate the need to expend 540 kilowatt-hours of energy, or about 77% of 700. Thus, this model shows that outdoor blinds have the potential to reduce energy consumption on A/C by 77%!

Simply placing blinds outside the window instead of inside drastically cuts down the amount of energy you must expend on cooling your house. Now imagine if every household in the United States decided to put their blinds outside. We could reduce energy expenditure on cooling to a negligible amount with that one simple change.

Experimental Data

A simple experiment can test the energy-saving potential of outdoor window blinds. I conducted the experiment using a room with three west-facing windows. One window has outdoor blinds. One window has indoor blinds. One window has no blinds. I set up a thermometer right next to each window to show how much heat is being let into the room by that window. I then recorded the temperature reading next to each window every half hour from 2 pm to 8 pm. This covers the time period when sun intensity is at its highest and the data show significant trends. The results of the experiment confirm the dramatic effect of outdoor blinds on indoor room temperature. See Figure 1.

learn more

See more on the energy used for A/C.

For a comparison of the benefits of renewable technology with those of passive solutions to summer heat, see Schaeffer, John. The Solar Living Source Book. Gabriola Island, BC: New Society Publishers, 2015. The 30th anniversary edition is available for purchase.

See more on the movement of solar energy through windows.

The temperature beside the window with outdoor blinds was consistently several degrees lower than the temperature beside the window with indoor blinds. It was even farther below the temperature beside the window with no blinds. The maximum difference between the temperatures beside the window with indoor blinds and that with outdoor blinds was 10°F. This difference was present for about an hour. The average difference between the two windows for the total six hours was about 7.2°F. This is a significant temperature difference, one that would have a considerable impact on the need for A/C. It confirms the finding of the model described above that outdoor blinds have the potential to dramatically reduce energy spent on A/C.

Outside Is In

It should be noted that numerous factors can alter how much energy will actually be saved in any particular household. Actual savings will vary with the number and size of windows in the house, the amount of sunlight received by each window, and the porosity of the installed blinds. This article just suggests the energy-saving potential of outdoor blinds in an average household. But the results of the experiment described in the article are very impressive.

So if you’re looking for an easy way to help keep your clients comfortable in the summer, lower their utility bills, and alleviate the energy consumption crisis on our planet, put the blinds outside!

Everest Fang is a high school senior in Salt Lake City, Utah, and a part-time research assistant at the University of Utah math department. In the summer of 2014, he worked as a lab research assistant at the University of Utah College of Engineering.

The modeling discussed in this article was conducted under the supervision of Kent Udell, professor of mechanical engineering at the University of Utah, who specializes in thermal energy.

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