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July 17, 2008

Green Gadgets: The Fan, Not AC

This is another article in our "Green Gadgets" column written by Shane Conder, a guest blogger.

This gadget will blow you away.

It's mid-summer. We're sitting in our upstairs office. The humidity is medium-high. It's 80 degrees. My wife asks me to turn off the fan because she's cold. Cold? With an indoor temperature of 80 degrees?

That's right. That's the effect wind chill can have with a simple floor fan. I'm guessing the fan was making her feel like it was in the mid to upper 60s. That's a 12-15 degree difference in feeling. Is that true?

As it turns out, there are multiple things going on. First, wind chill is the apparent cooling of the air temperature as wind passes over your skin and causes faster evaporation from your skin. (As an example, spray a water bottle at your skin with some wind and you'll feel even cooler.) Second, the heat index is the temperature you feel when you combine humidity with the actual air temperature.

Wind chill is basically when the temperature feels lower than it really is and is typically only useful when the real temperature is below 50 degrees Fahrenheit. The heat index is basically when the temperature feels higher than it really is and is typically only useful when the real temperature is above 80 degrees Fahrenheit.

Both measurements can have a dramatic effect on how cold or hot you feel. For instance, with a real temperature of 84 degrees and the relative humidity at 80% it will feel like it's 94 degrees. Likewise with wind chill. If it's really 20 degrees Fahrenheit out and the wind is blowing at only 10 miles per hour it will feel like it's 9 degrees.

So what does this have to do with fans during the summer? If you can run a fan that helps dry the air around you some by more rapidly evaporating and removing liquid from your skin you'll feel cooler for two reasons. The first is from the water evaporation around you. The second is from the drying effect this will ultimately have. The net effect of both is that it will now feel as if it's cooler than the real temperature rather than warmer. I believe this effect goes up as the humidity increases.

The typical measurement found around the web is that running an overhead fan can make an 80 degree room feel as if it was 72 degrees, which is typically considered a comfortable room temperature.

What about the electric cost?  If you don't already have air conditioning, you will be adding a small cost. An overhead fan likely uses between 50 watts and 100 watts, on high. At the worst case, running one 24 hours a day will use 73 kWh in a month. That's a little over $12 a month at our rate of 16 cents per kWh.

That sounds like a lot? Well, compare that to a full house air conditioner. A typical household air conditioner can run at 3500 watts. They won't run all the time, of course, but they will keep the entire house cool-even rooms you aren't using. Calculating the actual expected energy use of an air conditioner is actually quite complex. We'll simplify by assuming it can keep your house cool by running for 8 hours each day. Insulation and outside temperature will affect this number. However, if we assume that, the air conditioner will cost about $136 per month at 16 cents per kWh.

It's also said that raising the thermostat from 72 degrees to 78 degrees in the summer can lower the energy consumption by 10 to 20%. At these rates, this would save us $13-$27 per month. That's enough to pay for two fans running all the time! We don't have an air conditioner, though, and our warmest room on our top floor rarely goes above 85 degrees. Our floor fan, though is very powerful and creates a strong wind that makes even 95 degrees feel quite comfortable. It's cost? 300 watts. At 8 hours per day, and our energy costs, that's under $12/month. Still far, far cheaper than running an air conditioner.

Still too expensive? There are some solar powered desk and floor fans available, such as this fan and clock combo. A pure solar powered fan likely won't be as powerful as a floor fan or overhead fan, but it might be enough for one person at a desk.

Try out of a fan. You really may be blown away by how comfortable you feel with it on.

Posted by shane at July 17, 2008 11:13 AM