WHY, IN THE PAST 25 YEARS, WERE THERE NO NEW COMMERCIAL VAWTS ON LAND?
There are three basic reasons:
1. The VAWTs cannot be built very tall to catch the high wind speed over land at high altitude! (See Fig 4 for an illustrative drawing). This is due to the VAWT design that the two blades and center mast are rotating. There will be too much inertia force to contain, and it would not be cost-effective to build a very tall VAWT!
2. At the annual mean wind speed of 6 m/s, the energy efficiency of VAWTs are only 70% of that of HAWT. The annual mean wind speed over land in Denmark is 6m/s at the height of 50 meters (see Fig 5), where the VAWTs are not nearly as energy efficient as the HAWTs at this annual mean wind speed.
3. The VAWTs are held up with guy-cables. This is one of the main reasons that VAWTs are at low costs; but, the guy-cables take up a lot of land space, and the farmers do not like them.
WHY VAWT AT SEA NOW?
1. Wind at sea are strong at low altitudes, we do not have to build a tall VAWT to catch the high wind at sea! The low cost/kW VAWTs can catch strong wind at sea at the height of 15 to 60 meters.
2. VAWTs are energy efficient at high mean wind speeds of over 8 m/s, with energy efficiency of over 90% of that of HAWTs. The energy efficiency of a variable-speed VAWT (like the Sandia 34-m VAWT), the energy efficiency can reach 93% of that of HAWT at annual mean speed of 8.5 m/s.
3. There are plenty of high wind surfaces at sea! The space will not be an issue!
4. VAWT can be easily tilted down, assembled on the beach, has a low center of gravity, towed to site for installation, and towed back to shore for repairs & maintenance, all at low costs!
5. VAWT’s generator is at the bottom, where the size and weight of the generator are not important. Therefore, we can use large, heavy, high-voltage generators.
6. VAWT is omni-directional and has a low center of gravity. For a floating-VAWT for deeper water application, these characteristics are very import, and will make it much cheaper to build a floating platform to support the VAWT.