Some ideas -- electric cars -- were impossibly futuristic until they happened. Others -- like user-generated videos or free e-mail accounts -- were things that the public didn’t know it wanted until they existed. Others, like prepaid electricity meters and eBooks, are making a comeback.

Below is my list of technologies that should’ve, would’ve and perhaps could’ve been contenders if circumstances were different. Instead, they are just highly touted technologies that will likely land on the scrap heap of history along with the Stanley Steamer, the Nuclear Airplane and all the members of Ratt.

A caveat: I didn’t include technologies on the list -- fusion, osmotic pressure gradients, or modular nuclear -- that are still in the development stage but could have a massive potential. Conversely, I’ve left out the pure crazies. The list below contains the ones that almost made it.

1. Stirling Engines. Stirling engines -- which try to convertsolarheat into power via a piston and a mass of hot air or hydrogen -- have nearly all of the hallmarks of an intriguing idea doomed to obscurity. Stirlings have a long history (first created by Scottish inventor Robert Stirling in 1816), potential applications in a variety of fields, a legion of technical papers, and a long line of exuberant fans.

Stirling Energy Systems and Tessera Solar hoped to build solar thermal parks based around Stirling’s 25-kilowatt SunCatcher. The SunCatcher exhibited a whopping 31 percent efficiency, higher than other solar thermal technologies.

Stirling engines, however, also suffer from a major problem: competing technologies work better. The SunCatcher at its core is powered by a rapidly moving piston: try keeping that operating flawlessly in the desert for 25 years at a stretch. Stirling engines also remain the only solar thermal technology without an inherent mechanism for storing heat. The declining price of solar panels damaged this technology's potential, as well. At the end of 2010, the companies gave up on the idea.

2. Portable Methanol Fuel Cells. For nearly a decade, I’ve had meetings with companies touting direct methanol fuel cells (which produce electricity from a sealed tub of methanol) and the same question always comes up.

Why would a consumer want to carry around a little jug of flammable liquid to charge up their camera or phone when they could just 1) carry some spare batteries or 2) plug it into a charger?

You never get a straight answer. It’s as if these companies think most Americans spent their teen years in a KISS tribute band and are pretty comfortable lugging around lighter fluid. While large-scale methanol fuel cells may carve out a market in forklifts and industrial equipment (where the economics and applications are totally different) the portable market looks dead.

Nonetheless, the prototypes have all been great, particularly the fuel-cell-powered TV from Toshiba.

3. The Three-Wheeled Car. First, there was the Dymaxion, a three-wheeled car with a tail fin from Buckminster Fuller. It crashed during its public debut in 1933.

Then came the Messerschmitt. During the early '50s, the aircraft maker of the Third Reich was not allowed to make planes. It took the cockpit from a bomber and made a three-wheeler. When allowed to return to planes, it dropped cars.

More recently, Aptera and Venture Vehicles have struggled to bring three-wheelers to market. Three-wheelers cost less, have tighter turning radiuses and handle better, and can get better mileage. (Zap also has a three-wheeler.)

So what’s the problem? They look like something out of Dr. Seuss. Car consumers will go for cute (Volkswagen) but not freakish.  

4. Grease Diesel. Making America energy independent by running our cars off the drippings from the deep-fat fryer in back of the Long John Silver’s has been a dream of many entrepreneurs. Americans love fast food, and the freeways of this fine land are dotted with IHOPs that could also serve as service stations.

But believe it or not, fuel consumption far outpaces the feedstock waiting behind Jimboy’s Tacos. Even if you collected all of the inedible tallow, yellow grease, brown grease and animal fat laying around slaughterhouses, you might only have enough fuel for 1 billion gallons of diesel. The U.S. consumes around 60 billion gallons a year at most.

We can get fuel from grease traps, but it won’t make a huge dent.

5. Portable Solar. I have four portable solar chargers at home. They have been used, collectively, twice. Technically, small solar chargers embedded into phone cases and other accessories work. The problem comes in human behavior. Few of us spend the requisite eight hours a day outside needed to trickle-charge a cell phone. They make great demos for school projects, but fail on the utilitarian front. BIPV is better, but not by much: how many contractors are going to put a solar skin on a building that may not last as long as the structure itself? And what about shading? Double doomed points if the plans include dye-sensitized solar.

6. Foot Power. How many people does it take to power a light bulb? 60. A human foot step generates 1 watt per square foot, so you need 60 for an incandescent.

PowerLeap, which is trying to commercialize piezoelectrics, estimates its technology costs $100 to $200 per square foot compared to $1 to $6 for laminated flooring.

Piezoelectrics and kinetic energy actually could work in a few environments. EnOcean and Brother Industries have created, respectively, light switches and TV remotes that can operate on the energy harvested from finger movements. At the other end of the spectrum, VyCon has a flywheel that harvests energy from cargo cranes. In both situations, energy production dovetails nicely with a local, immediate use.

But the marquee attraction -- foot power -- in this field is a tough sell. (Editor’s note: piezoelectric materials generate an electric field when bent; kinetic devices convert movement into electricity. While the underlying mechanisms aren't exactly the same, you can lump both into the power-from-motion category.)

7. Thin Clients. The three-wheeled cars of computing. Thin clients consume far less power than standard computers: some, like N Computing, have released desktop units that only consume 1 watt of power. The performance has also improved to the point where few corporate employees would notice the difference between a thin client and a desktop.

Utilities have begun to offer rebates to companies that swap out desktops for thin clients. And some companies have reported an uptick in thin client sales with the spike in power prices.

Unfortunately, most employees are about as enthusiastic about thin clients as they are about adult diapers. Culturally, they seem to be a step down.

To top it off, the opportunity for thin clients that has arisen in the past few years with spiking energy costs is being eclipsed by smart phones and tablets. Look how many companies are making iPhone apps or tablets. IT departments will spend the next few years concentrating on this rather than a conversion to thin clients.

Thin clients have merit, but their timing is awful.

8. Small Wind Turbines. They cost more than solar panels, but they also don’t produce power as consistently, make more noise, and feature far more mechanical parts that can break. Oh, and the neighbors will tell planning commissions that they are a hazard.

9. The Tall Tower.  Dan Zaslavsky at Israel’s Technion has proposed planting hollow towers in hot, dry places that can capture moisture and high winds. Because the air will pick up speed as it swirls down the tower, it will be fast enough to crank turbines by the time it hits the bottom. Dry communities get cool and humid air.

Did we mention that it measures a kilometer high and nearly 400 meters across?

EnviroMission has proposed something similar for the U.S. and Australia. The Empire State Building is only 373 meters tall.

10. Battery Swapping in Cars. It looked fantastic...in 2006. Consumers don’t have to worry about range anxiety with a swappable battery. Unfortunately, the declining price of batteries, the emergence of rapid in-car chargers like the 6.6-kilowatt monster on the Ford Focus, and DC charging have all begun to make battery swapping inconvenient and less attractive economically. This one still might work -- battery swapping ensures your car stays younger longer, like Dorian Gray, in comparison to a conventional EV or even a gas car. It could be a hit in the taxi/bus/fleet car market. But it's locked in a race against time in the consumer market.

Another technology that may never take off: services that allow you to roam and get any charges forwarded to your power bills. Who cares. Just buy it like you buy Slim Jims and milk at 7-11.

Honorable mentions

11. The MDI Air Car. The balloon with four wheels.

12. EEStor. Class, discuss.

13. The Nano Fridge from Cool Chips. You’ve got to love any high-tech company with a lab in an undisclosed location in Russia and a logo that consists of a dolphin holding a pickaxe.

14. Wave and Tidal Devices. Water is 800 times more dense than air, so wave and tidal devices should be capable of generating more power than wind turbines, but these devices have to live in harsh environments and work flawlessly.

15. Electrochromic Windows. Windows that dynamically tint might work, but they have to compete against cheaper passive films. Hopeful, but an uphill battle. These are at the center of a large controversy in the green building space. Soladigm, one of the startups in the area, raised $10 million more to top out a third round worth $40 million today.