High Altitude Wind Energy Generation (HAWE) is emerging as the new field of excitement in the alternative energy industry. Instead of producing energy from wind turbines mounted on towers on the ground, researchers are attempting to tether wind turbines high in the air to harvest energy from the stronger and steadier winds in the upper atmosphere. Some 22 start-up companies have announced entry into the field and various concept prototypes are being built and tested.
This is a field to watch for people interested in alternative energy.
The contribution of wind energy to global clean energy production has started to rise. In the ten years between 2000 and 2010, the installed capacity of wind turbines increased ten times from 17.4 GW to 175 GW. The actual wind energy produced in 2010 was 340 TWh. Though this represents only about 2 per cent of the world’s energy usage, what is heartening is that wind energy production doubled over the 3 previous years. The expectation is that this rate of growth would be sustained, making wind energy a growth industry.
While wind energy is low cost and does not need the land space that solar photovoltaics need, it still has the problem that power production is intermittent. Wind velocities vary with seasons and even at different times during the day. Wind power, therefore, needs a back-up power source which is usually fossil fuel based. The alternative is to store the energy in batteries, which is expensive. Wind turbine operation also needs a minimum wind velocity of at least 8 kilometers per hour. Such wind speeds are consistently available only in coastal areas, on top of hills or where the terrain forms a natural wind funnel.
While surface wind speeds are affected by the terrain and by the heating and cooling effects of the ground, researchers have found that if we go to heights of 200 meters or more, there are much stronger and steadier winds. The data has been collected up to 20 kilometers height and it shows high wind speeds are available all around the world. These winds have an energy potential 16 times larger per square meter of swept area compared to wind turbines mounted on towers on the ground.
While no HAWE station is operational as yet, many prototypes are being tested out. Most present tests are at the comparatively low height under 1000 meters and generating relatively small amounts of energy, under about 1 MW. HAWE systems are built around kites, hot air balloons and such flying objects which carry a rotating turbine. The kite or balloon is tethered to the ground and the tether carries the cable through which the power generated is transferred to the ground.
One Example of such a system is the Magenn Air Rotor System (MARS) that offers a 4 KW system intended as back-up power or power to feed remote locations. It is a cylindrical light structure, 13 feet in diameter and 40 feet long, that incorporates a 3 dimensional wind turbine. The structure is raised by filling it with helium gas and tethered at a height between 200 feet and 800 feet to suit the optimum wind energy availability at the location. One experimental station has been tried out in North Carolina.
Can this be better?
Joby Energy has designed its HAWE system like a vertical take-off aircraft. The turbine is provided with power from a gen-set for take off and its flight is controlled by computer signals to its various propellers. Once it reaches its operating height of about 2000 feet, the turbine maintains a circular flight path around the tether cable. The power generated by the wind turbine flows through the tether cable to ground based transformers. The Joby system is planned to generate 2 MW, much higher than the MARS system.
Other approaches, notably by Makani , said to be funded by Google, plan a series of flying kites on a single tether that could fly at 600 meter and larger heights and producing 1 MW. A beach at Abu Dhabi is planning to fly some 200 energy generating kites connected to a 60 meter grid of tether posts.
These approaches should push the envelope of understanding of HAWE technology and pave the way for more advanced devices for the future.
At higher altitudes, the wind energy available is higher than at the ground level, but that too is subject to gusts and lulls, causing power production to vary. Such variation would continue to make wind energy a secondary power source that needs back-up primary power.
There are also a number of additional questions to be answered. These include the impact of these flying objects on air traffic corridors, on the migratory patterns of birds, the effect of lightning strikes on the flying turbines etc. Many of these questions will be studied in the prototype plants now being tried out.
Can this be avoided?
One approach that has been tried out on ground-based wind farms can perhaps be applied to overcome intermittent power generation. In Germany, wind farms spread over various parts of the country are controlled from a single central control room. When the power output from any one farm drops due to wind lull, some standby wind turbines are started up in other parts of the country to maintain total power supply to the grid.
The other issues can be addressed by mapping out the air traffic corridors and bird flight paths and pre-determining where HAWE systems may be tethered. For protection against lightning strikes, all the technology used to guard airplanes must be compulsorily applied to airborne wind turbines.
Ground based wind power generation has not been without its problems. Where a large number of wind turbines are located close to each other, the turbulence in air flow has caused reduction of power production from the whole wind farm. There are some researchers claiming changes to crop output from farmlands on which wind turbines are operating. They are attempting to correlate the crop pattern changes to the wind flow patterns caused by the turbines. These problems were not foreseen when wind turbines were first being installed.
Installing wind turbines in the sky could result in such unforeseen future problems. The disturbance to upper atmosphere wind patterns could have impacts on various weather parameters like cloud precipitation, absorption of solar radiation and the like which are not fully understood as yet.
At this early stage of evolution of these concepts, no one is talking cost. While the energy generation costs would be small, the capital cost of building such airborne systems and maintaining them will determine the acceptability of the HAWE systems.
Why are we so critical?
Even in the case of fossil power, the effects of CO2 emissions and global warming were discovered over a 100 years after the first fossil power plants were built. In the meantime, the demand for power grew. Now, when the effects of CO2 emissions are well known, we are forced to continue building fossil fuel power plants as there is no immediate alternative.
The start-up HAWE companies would tend to hype this technology for commercial reasons and it is easy for the world to accept this new field in the belief that any clean power is better than fossil power.
High Altitude Wind Energy Generation
The world needs to continue support to these frontier technologies in our quest for alternatives to the use of fossil fuels. The independent scientific community needs to monitor the development and deployment of this technology. There should be some people constantly playing devil’s advocate so that this technology does not have any negative consequences particularly in relation to rain precipitation or weather pattern modification due to interference with upper air wind circulation.