With the exception of FloDesign/Ogin, shrouded turbines have fallen out of favor with inventors as well as the media. The buzz in 2013 was all about kites or as their promoters prefer to call them Airborne Wind Energy Systems (AWES). Likewise, seed capital for new wind energy inventions has shifted from DOE to Silicon Valley with Google’s investment in Makani’s flying wind turbine.
Note: The following is adapted from Chapter 7, Novel Wind Systems in Wind Energy for the Rest of Us by Paul Gipe.
Kite proponents in North America and Europe exhibit a cult-like fervor, claiming kites are the wave of the future because kites will be more productive and less costly than the wind turbines we have now. Google’s investment has only stoked their certainty. Google couldn’t be wrong, could it? In this, kites–as a new technology—are no different than DAWTs or VAWTs or any other wind invention that wins high-profile backing. VAWTs have celebrity endorsers. DAWTs have DOE. Kites have Google.
Most wind engineers don’t give kites a second thought. The typical reaction is, “You must be kidding?” As with DAWTs, kites and other flying wind energy systems are not found in any engineering textbooks on wind energy. However, kites and kite promoters have won media attention and with that political attention—and money–soon follows. For this reason, some analysts have felt it necessary to give kites more than a cursory glance.
No one has done more to analyze the realistic prospect of using kites to generate electricity than wind energy analyst Mike Barnard. His interest in kites and other wind turbine inventions is that of an advocate. He wants wind energy deployed. Anything that stands in the way of expanding the use of wind energy today is ripe for his critical analysis. His approach is cool and level-headed. He’s not one to fall for hype.
Barnard segregates kites into several categories based on their wings, where they generate electricity, how they fly, and at what altitude they fly.
- Soft wing, hard wing, lighter-than-air
- Generation on the ground or in the air
- Single tethers or multiple tethers
- Crosswind flying or static flying
- High-altitude flying or low
When we think of kites, we naturally think of soft wings. EnerKite is just one of many examples of soft wing kites that use a fabric parasail. Google’s Makani, on the other hand, uses a tethered fixed-wing plane that flies to its position. Magenn and its successors propose helium filled blimps that carry the generator to altitude. Makani and Magenn were designed to generate electricity on board and transmit the electricity to the ground via a tethered power cable. EnerKite and the others like it spool and unspool a reel attached to a generator on the ground. Makani flies across the wind as do most soft wing kites, the Magenn blimp rests at its operating position. Some propose flying their kites at altitudes common to commercial aircraft, others at heights not much different to commercial wind turbines today.
No kite company has built anything more than a prototype. EnerKite, for example, has produced a proof-of-concept 30 kW model that may be suited for niche applications in remote locations, such as for military use. Makani, for all its flash, has only developed a prototype. While some have made sophisticated measurements under trial conditions, none of the kite companies have published performance results measured under standardized conditions so that their results can be compared to conventional wind turbines. Until kite proponents prove their claims, they remain just that–claims.
Kites, like other novel wind turbines, have several technical limitations to overcome before they are anything more than a novelty. One of the most obvious problem is the tether whipping around the sky. Flying a kite at anything much more than the height of a conventional wind turbine requires an aircraft flight exclusion zone. This alone is enough to ground most kite concepts.
If the kites can’t be flown high enough, because of potential conflicts with aircraft, to take advantage of increased wind speeds at height, then they don’t offer any yield advantage over conventional turbines.
For brief use at remote sites, manual operation of the kite may be sufficient. But for anything approaching commercial use, the kite must be automatically flown into the air, automatically flown once the kite becomes airborne, and automatically retrieved when necessary. Automatic flight hasn’t been demonstrated on anything more than a prototype scale.
Barnard sums up his take on kites after an exhaustive analysis.
“The potential energy available in the wind flowing high above our heads is alluring, and harvesting it with tethered flying wings has great appeal, but as soon as you start engineering an airborne solution to harvest that energy, the compromises strip away the potential bit-by-bit until it just isn’t viable in any incarnation so far attempted. And it’s clear that many of the current organizations in the field were started at best with optimistic assessments regarding safety and aviation authority approvals.”
By early 2014, the longest that a tethered kite has stayed airborne is two weeks. For comparison, conventional wind turbines at good sites operate more than 6,000 hours per year or 35 weeks. While they may not be in continuous operation during that time, conventional wind turbines are ready and available to generate electricity 98% of the full 8,760 hours in a year. That’s a lot more than two weeks.
It will take years of steady development if kites are ever to compete with conventional wind turbines. They are a long way from that today.