Energy efficiency has been called the low-hanging fruit for reducing carbon emissions, because it actually pays for itself. But it can be difficult to get people to take simple steps to save energy, and it's hard to maintain those savings over time. For example, people offset the savings from a new efficient refrigerator by changing their habits or buying a plasma TV.

A company called OPOWER has what appears to be a successful strategy for dealing with this human element of energy efficiency. It has increased participation in energy efficiency programs from about 5 percent to 80 percent for the utilities that have used it so far, and the savings appear to be sustained.

So far, since its founding in 2007 the company claims its program has saved over 90 million kilowatt hours of electricity. The company has even attracted the interest of the White House--last week President Obama dropped by, highlighting the company as a source of the green jobs he hopes to help create.

The company's approach is based on the idea that people want to fit in. OPOWER first lets people know how their energy use compares to that of their neighbors. Then for each billing period the company gives them a single tip that they can act on, also connected to what they're neighbors are doing, such as "Most people in your area keep their AC at 78 degrees." They also tell people how much energy they will save.

It's obvious how this could work with people who are using more electricity than average. But for those using less than average, the company also compares them to their "efficient" neighbors to motivate change.

The company makes use of public information (such as from the tax assesor's office and weather data) as well as third-party demographics to tailor results. If a person is renting, OPOWER doesn't recommend insulating the house, for example. It also has software that looks at energy use patterns to identify likely sources of energy waste. Power spikes during hot days greater than that shown in neighboring houses could suggest someone has an inefficient air conditioner.

It seems like a promising approach. But it's not going to solve the world's energy problems on its own. On average the program has cut energy consumption by a modest amount--about 2.5%.

A promising approach to reducing the cost of solar power is moving forward with the construction of an installation in Indianatown, FL, that will combine a field of solar concentrators with a natural gas power plant.

Today The New York Times has an update on the project, which the utility Florida Power and Light (FPL) announced almost two years ago, and which we wrote about here. When completed later this year, the power plant is expected to generate up to 75 megawatts of power by making use of turbines at the natural gas plant, which itself has a 3,600 megawatt capacity.

The solar concentrators generate steam, which can be used to drive the turbines. Using existing turbines and generators can greatly reduce the cost of a solar power plant. FPL says the current project reduces costs by 20 percent, according to the Times.

Similar natural gas-solar hybrid projects are being built in Egypt, Morocco, and Algeria. There's also an effort to pair solar concentrators with coal fired power plants. Abengoa, the Spanish based company that's building natural gas plants in Morocco and Algeria, is working with Xcel Energy in Colorado to build solar coal hybrid test facility. According to Abengoa, such hybrids could cut the cost of solar power by 30 to 50 percent to as low as 6 cents per kilowatt hour, which is competitive with many fossil fuel power plants.

Transonic Combustion's fuel injection system. Credit: Technology Review.

A conference put on by the new Advanced Research Projects Agency for Energy (ARPA-E) this week was packed with companies exhibiting intriguing approaches to clean energy. I'll be looking into some of them in more detail in upcoming stories, but here's a few that caught my eye.

Transonic Combustion, based in Camarillo, CA, has developed a gasoline fuel injection system that can improve the efficiency of gasoline engines by 50 to 75 percent, beating the fuel economy of hybrid vehicles. A test vehicle the size and weight of a Toyota Prius (but without hybrid propulsion) showed 64 miles per gallon for highway driving. The company says the system can work with existing engines, and costs about as much as existing high-end fuel injection.

American Superconductor, of Devens, MA, is developing massive 10 megawatt wind turbines that are only possible with the use of extremely lightweight superconducting generators. (Today's turbines typically generate around 2 to 3 megawatts.)

A group out of Michigan State University is developing a natural gas electricity generator for use in hybrid vehicles. The goal: give natural gas cars the same driving range as conventional gasoline cars, making way for their wide adoption.

Oscilla Power, based in Salt Lake City, UT, plans to start testing a novel wave power generator. Wave power is notoriously difficult to harness because of the damage waves can cause to mechanical systems. Oscilla has found a way to use an inexpensive iron-aluminum alloy to generate electrical current, without the need for any moving parts.

Palo Alto Research Center (PARC), spun out of Xerox PARC, is developing a new form of refrigeration that could be three times as efficient as existing forms. It's based on thermoacoustics, a technology that works for cooling at extremely low temperatures (such as for liquefying gases), but hasn't been used for cooling at room temperature (what you need for household refrigeration). The company thinks it's found a way around previous limits to the technology.