SBIR-STTR Award

The tether and power generating ground station, are the focus of this proposal. Both these systems have challenges that are unique to the airborne wind energy field. First is the selection and design of the tether itself. This includes the material (e.g. nylon, steel, or composite) and whether to weave in electricity-transmitting wires between the ground station and the flying craft (which would increase its cost and diameter). These wires would be used to power the electronics on the flying craft and facilitate communications with the ground station. All these choices have trade-offs, however. Specifically, the tether will be the main source of aerodynamic drag in the entire system and, thus, one of the key limiting factors in the quantity of electricity produced. The other key system is the ground station itself. This includes several components: the drum and tether management system, electrical generator, and electrical systems that convert noisy renewable energy into smooth, reliable 120V AC for equipment or home use. Conveniently, these converting electrical systems are relatively abundant and a set that fits our exact needs is either commercially available or created with minor modifications. The tether management and electrical generator, however, are much more complex and not commercially available. The tether management system, a combination of custom hardware and custom software, must reliably:-Maintain a desired tension on the tether -Monitor tether tension, reel-out speed, tether-to-ground angle, length deployed, etc. -Maintain reel-out speed to a specific value determined by the wind speed -Monitor and maintain airspeed of the flying craft during launch/landing -Reliably guide the tether onto the drum without crossing layers In aggregate, this creates a complex system of mechanical controls, sensors and software that must work together to maintain consistent flight, maximize power generated, and minimize wear on each component. To maintain consistent tension on the tether requires constantly monitoring and adjusting for differing pull strengths from the flying craft because of changes in wind speed (gusts, lulls) and different aerodynamic lift at each point of the figure-8 flight pattern. To maximize power generated, we do not want any spring or physical braking system to smooth out the tether tension because these forces create drag and friction, wasting energy. Instead, we will use the electrical generator itself as the resistance and rewind motor. A generator and motor are the same physical system, differentiated only by whether electricity is flowing into or out of the device. Therefore, by monitoring the tether tension and reel-out speed, the tether management system can adjust the electrical load (resistance) of the generator to control both variables. The system can also use the generator as a motor to rewind the tether each cycle. In short, by constantly monitoring and maintaining several variables of the system, the tether management system can maximize power generated by clever manipulation of the electrical generator. It must also perform all these tasks flawlessly over the ten-year lifespan of the device while exposed to degrading UV radiation and seasonal weather. Ensuring this level of accuracy and reliability over a decade is a complex engineering challenge. Finally, the electrical generator is novel because it represents one of the biggest advantages our airborne wind energy system has over traditional wind turbines and other airborne competitors. Generators on top of large towers or that are airborne have a premium placed on size and weight for obvious reasons. As a result, they are built from exotic, magnetic materials (e.g. metal-doped ceramics) that are extremely delicate and expensive. They are also physically small. This feeds directly into the fundamental trade-off in generator design: a physically small generator equals higher revolution speeds (e.g. 1,500-2,000 revolutions per minute (r.p.m

Airborne Wind Energy (AWE) is still a relatively new field but is growing quickly. There are approximately 13 other AWE companies worldwide that appear to have the technical competence and realistic understanding of business such that we view them as realistic competitors. As we have detailed above, eWind differentiates itself from them by focusing on following current FAA (or European) flight rules, staying small and competing against the traditional small wind market with a better value proposition and being able to go straight to selling systems to farmers without bureaucratic waivers. There is increasing business focus on the field, there is increasing university funding to study basic problems common to most AWE systems. TU Delft, a university in the Netherlands, is a leader in this field and has even started a fledgling academic department focused on it. This has produced numerous papers and theses dedicated to various problems of AWE (e.g. Ahrens et al. (2014); Fagiano (2009); Haug (2012)). In addition, there have been publications about simplified control systems and prototyping lessons and guidelines (e.g. Fagiano (2012); Fagiano et al. (2013); Fagiano and Marks (2014); Fagiano et al. (2014); Zgraggen et al. (2014)). Unfortunately, much of this work has centered on soft kites with multiple tethers that allow control systems for the airborne device to be placed on the ground. While we agree that this greatly simplifies the control system problem, the cost in drag (and, thus, electricity production) of additional tethers makes the business and financial case for these systems tenuous at best. AWE represents a significant increase in mechanical and control complexity and, thus, must also come with an even greater increase in energy production and value to make financial sense to farmers. Additionally, soft kites (imagine a kite-boarder or para-sail) are very likely to be harder and costlier to maintain in the field for years at a time. In short, rigid frame crafts (such as ours) are more efficient, rugged and easier to launch and land. Conversely, they are less studied, harder to control on a single tether, and less stable, requiring a more robust control system to manage them. The other major category of AWE is called "sky-gen". In this case, the generator is placed on the flying craft itself (as opposed to on the ground) and the electricity is transferred down the tether. There are numerous benefits and disadvantages of both sky-gen and ground-gen. There is a legitimate business case to be made for each one within certain operational regimes. However, while sky-gen is likely better at higher altitudes (i.e. greater than 1000m), ground-gen is better at the altitudes currently allowed by regulations (i.e. below 150m). Companies such as Makani are pursuing sky-gen systems, but because of this height/efficiency tradeoff, they are building utility scale systems that require individual waivers from the FAA to even test. This is not an appropriate model for farmers. As previously detailed in Section 3, we have based our design around the needs of small farmers. We maximize the financial return to farmers while following all current regulations and permitting, allowing the customer to reap the benefit as easily as possible. This focus leads us to the rigid frame flying craft, a ground-gen system and automating the system to minimize farmer involvement. This approach places a greater burden on the research and development side by increasing the design complexity of TED, its controls and the flight software. Previous work on our system has focused on TED, its aerodynamic design and physical control surfaces. This work has already led to a utility patent 9,643,721 titled "Wind Energy Conversion System, Device and Methods" that was issue 05/2017 it covers a flying system with multiple lifting surfaces and side rudders to maximize lift in a given wingspan and improve maneuverability, as well as it's flight path. As noted earlier,