SmartEnergyViews

MIT News Office

Imagine windows that not only provide a clear view and illuminate rooms, but also use sunlight to efficiently help power the building they are part of. MIT engineers report a new approach to harnessing the sun's energy that could allow just that.

The work, to be reported in the July 11 issue of Science, involves the creation of a novel "solar concentrator." "Light is collected over a large area [like a window] and gathered, or concentrated, at the edges," explains Marc A. Baldo, leader of the work and the Esther and Harold E. Edgerton Career Development Associate Professor of Electrical Engineering.

As a result, rather than covering a roof with expensive solar cells (the semiconductor devices that transform sunlight into electricity), the cells only need to be around the edges of a flat glass panel. In addition, the focused light increases the electrical power obtained from each solar cell "by a factor of over 40," Baldo says.

According to the article, three of the inventors have launched a spin-off company to bring the technology to market, hopefully within three years. Other reports say the technology won't necessarily be suitable for windows, but might work as a skylight.

Pacific Business News reports that a new state law in Hawaii requires all new single-family homes built after Jan. 1, 2010 to have solar hot water heaters. Home sites that do not receive enough sunlight can use some other renewable energy source. Homes may also have gas-powered tankless "instant" water heaters as long as the home has one other gas appliance.

Solar hot water heaters are far less expensive than solar PV (photovoltaic) electric systems, and the payback period is much shorter. According to the DOE, water heating represents up to 17 percent of national residential energy consumption.

FindSolar.com, operated by solar industry interests and the DOE, has a handy Solar Estimator that calculates an estimated cost for a home solar PV system and provides a list of contractors in your area. It uses some curious formulas that I don't quite understand to calculate positive ROI, but it provides some interesting insights.

Some manufacturers also have calculators, including BP Solar USA, Sharp Electronics, and Kyocera Solar. And the PVWatts.org calculator estimates how much solar power you can generate for any given U.S. location based on a number of variables.

Where I live in Tennessee is listed at the high end of "good" on the "solar scale" of available solar energy. But a 3 or 4 kW system that costs $40K or so will only generate about $35 per month worth of electricity. That's a payback of about 95 years!

(Most of the calculators reduce the payback period based on tax deductions if you finance the system with a home equity loan. I don't understand how that works. It seems like the additional interest v. paying cash would make the system cost more over its life, not less. I'm not very good at math, though, so maybe someone can explain that?)

Folks living in Florida (where we lived for a while) can get a much better deal, because a) they get slightly more solar radiation, but more important b) they have a state rebate incentive that will pay for half the system. Plus they require utilities to provide "net metering" (meaning you can sell excess power back to the grid). The payback period is reduced by half or more, but it's still a stretch.

Conclusion: While good for the environment (assuming "green" manufacturing practices), residential solar PV systems are still too expensive and not yet cost effective. Expect this to change, however, as R&D produces better and cheaper solar panels and manufacturing ramps up.

For you folks in the TVA (Tennessee Valley Authority) service area, here's a free online energy audit. It's fairly detailed, and the more information you provide the better recommendations it can make. Complete it by the end of June 2008 and get a free home energy savings kit complete with some CFL bulbs and other goodies.

You can enter up to 12 months of utility bill info, but I put in just two, one for August and one for February. It did a pretty good projection on the other months based on that. Anyway, have a couple of utility bills handy when you start the survey. (P.S. The survey asks for gas consumption in "therms." Ours is billed in CCF. The conversion factor is 1 CCF = 1.0250 therms. Or you can just enter the CCF and it's close enough.)

Bonus: If you register and complete the survey, TVA will send you an energy conservation kit that includes two compact fluorescent light bulbs, outlet and light switch gaskets, filter whistle, two faucet aerators, hot water temperature gauge, home thermometer, and a "How to Save" brochure. (Through June 30, 2008, while supplies last.) That's a pretty good deal.

Customers of TXU Energy (a Texas utility company) can get a free iThermostat that hooks up to the internet.

It's similar to programmable thermostats that let you have different settings for the days of the week and times of the day, except it communicates wirelessly with a bridge device connected to your home network so you can program it from anywhere.

Details here.

Got our first full-month utility bill since installing our new energy efficient front loading washer/dryer.

As compared to the same period last year, it appears we may have reduced total water consumption by about 21%. Water is so cheap it isn't much in terms of cost savings, but every little bit helps in a drought.

It looks like we may have reduced our electricity usage some, too, possibly owing to less drying time. Compared to the previous month it's about a 13% reduction, but it's hard to compare month to month (average temperature, holiday cooking, etc.)

But the really good news is that CFLs, energy efficient washer/dryer, and other conservation measures have reduced our electricity consumption by 31% as compared to the same period last year. Last year's average temperatures during that period were 50.6 (high) and 31.7 (low), compared to 46.9 and 27.1 for this year, so the savings may actually be higher.

There's more stuff we can do to conserve, though, so we'll keep chipping away at it.

LED Christmas lights save money and energy. We got some. They have strings of 50 at K-Mart on sale for $6.99. They also have them at Home Depot, Lowes and Target.

They look great, and use less than 5 watts per string as compared to about 25 watts for a string of 50 mini lights or 350 watts for a string of 50 traditional C9 lights.

Parts of this won't apply to everybody, but we found that insulating our pipes under the crawl space (and setting the timer on our hot water recirculating pump) resulted in savings.

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Just got our first full month electric bill since switching out our most frequently used light bulbs with CFLs (6 bulbs).

Along with other conservation measures, it looks like we had about a 15% savings compared to the same period last year, which for us works out to about $26 for a 30 day billing period at the current rates.

So basically, the CFLs have already paid for themselves (which doesn't seem possible, but there you go). Our savings may be higher because we work mostly from home so we are here 7X24 so our lights are on longer. Your mileage may vary.

It should also be noted that the average daily high temperature for June 2007 was about 2 degrees higher (88.5) as compared to June 2006 (86.6), so it's likely we ran the AC a little more this year than last.

We mentioned previously that dimmable CFLs are hard to find. I couldn't find any locally, but a good source on the internet for dimmable CFL bulbs is TopBulb.com.

The bulbs are a little expensive ($24 ea. for the ones we bought), but they work and they will pay for themselves in energy savings over the 10,000 hour rated life of the bulb (if they last that long).

We got 15w (2700K color temp.) replacements (made by Technical Consumer Products, also known as "Springlamp" brand) for two 60w kitchen lights. Actually, we were using 54w long-life bulbs, so the 15w CFLs are a little brighter. They work fine with the dimmer but won't dim as low as incandescent (the CFL dimmable range is 100% to 20%).

We've been looking in to compact fluorescent bulbs (CFLs) to reduce energy consumption for lighting. Here's what we've learned so far.

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