Sometimes it helps to think big and be unencumbered by practical realities. A Yale professor gave Fred Smith a middling grade on a paper that outlined Federal Express. Conversely, Xerox execs didn't see a huge future in personal computers.

There's also an engineer I keep hearing about that saw the power of cell phones back in the ‘80s. He leased three high peaks in Arizona from the federal government for 99 years for $1 a year. His family will collect royalties from carriers for decades.

Then there is the other side of the coin: voice recognition, pen-based computing, atomic-powered airplanes. The vision was there but the pieces didn't cooperate. The Segway: great idea, until you realize there is a very finite number of postal carriers on the planet.  

Here are my top 10 greentech big concept companies. Some may make it. Many won't, but they certainly are entertaining

1. Citizenre. This is the ultimate do-it-yerself outfit. Citizenre's goal is get around suppliers and middlemen to bring solar to the masses. The company doesn't just want to make solar panels – it wants to build a 500-megawatt factory, one of the largest solar factories on earth. Along with manufacturing, Citizenre wants to put solar panels on roofs, rent them to consumers in a power provider-like arrangement, and come up with demand response programs to curb consumption. Oh, it also wants to make its own silicon (or whatever substance it chooses to make its panels out of). That's what, six companies in one? Sales will mostly be conducted through multi-level marketing.

The company has already missed a few self-imposed deadlines (like having factory plans in place) and has not updated its Website since 2007. The last news event occurred this summer when Citizenre announce it and cable-level celeb Ed Begley, Jr. parted ways.  

"The end vision is the same, but the implementation stages are evolving," said a spokeswoman. "This is a complex and ambitious plan and subject to a number of moving parts."

2. Better Place. There are a lot of reasons this company's plan to build networks of charging stations shouldn't work, and you hear them from critics in the back of the room every time founder Shai Agassi speaks. Building charging stations will be expensive. The number of plug-in cars that can support this will probably be low for years. There is limited intellectual property and plug-in station owners will probably insist on standards.

Consumers may also balk at the battery-swap concept. There are electric buses in which mechanics swap out depleted batteries for fresh ones, but the bus owner owns all the parts. Here, you'll be putting a stranger's battery in your car. Did the previous owner get rear-ended in the parking lot of a Beyoncé concert? Is it damaged? Will you blow up? Batteries are touchy.

But here's what they have going for them: Agassi knows how to sell stuff. You can't rise to become one of the top guys at SAP, where he came from, without knowing how to talk people into buying stuff they may not really need, or not need for while. There's a good chance that he can convince state, local and some national government to pay the billions required to put in this network. Charging stations will become the sports stadiums of the 2010s. Even it if fails, it succeeds.

3. DayJet (Formerly Jetson Systems). Created by Citrix Founder Ed Iacobucci, DayJet's goal has been to provide on-demand airline flights between secondary hubs – Savannah, Ga., Fresno, Calif., El Paso, Texas – that aren't served well by the major airlines. These kind of flights can free up traffic and congestion. The company even hired mathematicians from the former Soviet Union to figure out pricing and routes (see CNET story).

"It's all about the long tail," Ed told me back in 2005 when he unveiled the company. I loved it, particularly because I really didn't know that that meant. I went to the Website to get an update. They discontinued passenger service in 2008.

Runner up: The Air Scooter personal helicopter. You fly it yourself, straight into the side of the house.

4. Sunlight to Petrol. This project, out of Sandia National Labs, for turning carbon dioxide into liquid fuels could work. And it if does, it revolutionizes society. People will brag about their bad breath: I am the Saudi Arabia of Carbon Dioxide! But it's going to take some work. Sandia estimates it could be 15 to 20 years away.

In this, heat from the sun drives a device called a Counter Rotating Ring Receiver Reactor Recuperator. This breaks the carbon-oxygen bond in carbon dioxide to form carbon monoxide and oxygen. Carbon monoxide could then be mixed with water and/or other substances to generate methanol or synthetic petroleum.

Transforming carbon dioxide into something useful generally takes quite a bit of energy, but it's possible. Some startups are working on similar projects.

5. Artificial Meat. Peta is giving away $1 million in a scientific contest to people who can devise a way to grow meat in petri dishes with animal stem cells. The science will be difficult and the getting consumers to accept artificial meat will require one of the greatest marketing projects of all time. And Motor City Madman Ted Nugent will probably go after the winner with a crossbow. But it would massively allow humanity to curtail the consumption of fossil fuels, fertilizers, water and agricultural land.

Then again, I saw dinosaur-shaped chicken nuggets in the store the other day. Maybe the sales job won't be so tough.

6. Nautilus Materials. When people hear this business plan they freak. Nautilus mines for minerals in holes and digs in the seabed. It's working on a project off the coast of New Guinea.

7. The LiftPort Group. The organization wants to build an elevator to space with carbon nanotubes. You can buy tickets now for the maiden voyage in 2031. Laughable yes, but even if part of this works it could be a tremendous demonstration of how carbon nanotubes, the miracle material from the labs, could be used in construction.