Method for airborne kinetic energy conversion

Methods and apparatus to harvest renewable energy are provided herein. In some embodiments, a method to harvest renewable energy includes providing an aircraft suitable for untethered flight in an open airspace and an airborne kinetic energy conversion system attached to the airframe, the airborne kinetic energy conversion system comprising a turbine, a generator connected to the turbine, and electrical storage means connected to the generator; flying the aircraft; gaining excess kinetic energy; and converting excess kinetic energy into electricity using the kinetic energy conversion system.

FIELD

The present invention relates to improved energy harvesting and its use on aircraft. In particular, the invention relates to an aircraft employing a hybrid power system for harvesting various renewable energy sources, such as sunlight and wind.

BACKGROUND

Unmanned Aerial Vehicles (UAVs) are unpiloted aircraft that are either controlled remotely or are flown autonomously. UAVs are commonly categorized based on their design and performance spanning the range from small low altitude to large high altitude long endurance vehicles. The UAV technology is taking an increasingly important place in our society for commercial, civilian and military applications. UAVs could provide improved service over existing systems in a large number of applications, ranging from border patrol and coastal surveillance, monitoring of natural disasters, meteorology and cartography to highly flexible telecommunication relay stations. The required endurance may be in the range of a few hours, for example in the case of law enforcement, border surveillance, forest fire fighting or power line inspection. Other applications at high altitudes, such as, for example, communication platform for mobile devices, weather research and forecast, environmental monitoring, may require remaining airborne for days, weeks, months or even years. It is possible to reach these goals using renewable energy sources.

One of the readily available renewable energy sources is sunlight. The use of sunlight as a source of energy for aircraft has many compelling advantages. Sunlight provides about 1000 W/m2at sea level, but reaches more abundant 1400 W/m2at high altitudes also unobstructed by cloud cover. Photovoltaic (PV) cells and modules may be used to collect the solar energy during the day, a part of which may be used directly for maintaining flight and onboard operations with the remainder being stored for the night time.

So far the solar energy has been the only renewable energy source seriously considered for use onboard UAVs. However, the atmospheric environment provides other potentially plentiful and useful sources of energy, for example wind. Unlike solar power, wind power may be available 24 hours a day. Hybrid power systems based on solar and wind power could provide UAVs with more reliable and effective renewable power sources. With advances in efficient and smart power systems, aircraft powered by renewable energy sources may achieve sustained flight at high altitudes for days, weeks and even years.

SUMMARY

Methods and apparatus to harvest renewable energy are provided herein. In some embodiments, a method to harvest renewable energy includes providing an aircraft suitable for untethered flight in an open airspace and an airborne kinetic energy conversion system attached to the airframe, the airborne kinetic energy conversion system comprising a turbine, a generator connected to the turbine, and electrical storage means connected to the generator; flying the aircraft; gaining excess kinetic energy; and converting excess kinetic energy into electricity using the kinetic energy conversion system.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of exemplary embodiments or other examples described herein. However, it will be understood that these embodiments and examples may be practiced without the specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail, so as not to obscure the following description. Further, the embodiments disclosed are for exemplary purposes only and other embodiments may be employed in lieu of, or in combination with, the embodiments disclosed.

FIG. 1shows schematically the main parts of a typical aircraft100, which include a fuselage110, a main wing120, a tail section130and a propulsion system including propeller140. In general, there may be other critical parts and components not shown or hidden from view inFIG. 1. Conversely, in some aircraft certain parts may be missing. For example, all-wing airplanes (i.e., flying wings) may not have separate fuselage and tail. Furthermore, piloted aircraft may also have a cockpit for a pilot. On the other hand, UAVs may have specialized electronics and payloads that enable autonomous flight and operation. This aircraft may be outfitted with means to harvest renewable energy from such sources as sunlight and wind, as described in more detail below. Prior aircraft have been shown to use only sunlight energy as a persistent renewable energy source. In general, various types of engines and motors may be used for propulsion. However, for UAVs and especially long endurance UAVs, it is sometimes beneficial to use electric motors, since they can be powered electrically using renewable energy sources.

In accordance with embodiments of this invention, methods and apparatus are provided for kinetic energy conversion onboard aircraft in free flight. Kinetic energy conversion facilitates extraction of renewable energy from wind as a sole source of energy or an additional source in a hybrid power system. Aircraft able to harness the wind power thus could have access to alternative power sources, such as atmospheric turbulence, wind shears, updrafts, thermals, and others.FIG. 2shows schematically a kinetic energy conversion system (KECS)200comprising a turbine202, a generator (or other type of energy converter204) and means for energy storage206. Means for energy storage206may include an electric battery, a capacitor, a flow battery, a regenerative fuel cell, and other types of electrical storage. In this case mechanical energy provided by the turbine202is transformed by the generator204into electricity and stored for later use. Alternatively instead of the generator204, a mechanical adapter or converter, e.g. a gearbox, may be used with the turbine, from which the mechanical energy may be transferred and stored directly using mechanical storage means, such as flying wheel storage.

In accordance with embodiments of this invention, a wind-powered aircraft may be produced using a KECS.FIG. 3shows schematically at300the main elements of a wind-powered UAV, which comprises an airframe302, electronics304, and a KECS306. Also there may be other elements in a UAV that are not shown inFIG. 3, such as for example a propulsion system. Unlike ground-based and tethered airborne wind turbines, an airborne turbine on untethered aircraft in free flight cannot convert wind energy into electricity directly. Instead, such an aircraft could be wind-powered by harvesting wind energy using a two-step process shown as400inFIG. 4. In this method, an aircraft first gains excess mechanical energy at402, i.e., either the kinetic energy by increasing speed or the potential energy by ascending to a higher altitude. This excess energy may be subsequently transferred to the KECS at404, e.g., by flying faster to drive the KECS' turbine, and then stored onboard in either mechanical or electrical form. The excess potential energy can be defined as the difference between the current potential energy and the potential energy at a cruising altitude. Similarly, the excess kinetic energy can be defined as the difference between the current kinetic energy and the kinetic energy at a cruise speed.

In general, a KECS may be a stand-alone system or an integrated system. In some embodiments, the KECS is seamlessly integrated with other components onboard an aircraft (e.g., the KECS can be a distributed system having one or several shared elements).FIG. 5shows schematically a hybrid system500comprising an integrated KECS502and a solar power system504integrated with propulsion and power train systems. The hybrid system includes a power control electronics block506, which monitors and controls the electrical power flow among other elements, such as photovoltaic (PV) cells508, a generator510, and an electrical storage (e.g., a battery512), as shown by arrows508a,510a,512a, and512b, respectively. The hybrid system500includes an electric motor514controlled via an electronic speed controller (ESC)516. The power to the motor514is provided from a number of other elements in the hybrid system, including as non-limiting examples, a solar power system504comprising PV cells508, a generator510(as a part of the KECS), and an electrical storage (e.g., a battery512). PV cells508provide electrical power when sunlight is available, while the KECS generator510provides electrical power when appropriate wind resources are available. The power generated by the PV cells508and the generator510can be used for powering electronics504and the motor514, as well as for recharging the battery512. The battery512in turn may be used as a power source in the absence of the other two power sources (e.g., PV cells508and the generator510), or when their combined power is insufficient. The power control electronics block504may comprise different parts for interfacing with other elements in the hybrid system, such as a charger for the battery, a maximum power point tracker for the solar system, a speed controller for the generator and so on.

FIG. 6shows schematically another hybrid system600comprising a KECS602integrated with propulsion and power train systems. In this system, the generator and the motor are combined into a single unit604. The electric motor here may be run also as a generator, where instead of consuming it may produce electrical power. Similarly, a propeller (not shown) in this case may be also used as a turbine. In some embodiments, this capability is provided by a specialized electronic component, such as a two-way electronic speed controller (ESC)606. The direction of the power flow between the motor/generator604and the ESC606depends on the direction or the sign of the torque exerted by air onto the propeller. For simplicity, the motor/generator unit604will be referred to as the motor in the following.

In general, a propeller can be characterized by a number of different parameters, including its pitch speed Vp—the speed at which the torque is equal to zero. Thus, the motor in an aircraft moving at speeds lower than Vpmay consume power and provide thrust to the airframe. Conversely, the motor in an aircraft moving at speeds higher than the pitch speed may generate power and exert aerodynamic drag. To illustrate this,FIG. 7shows a plot of the propeller efficiency vs. the aircraft speed. Three different operating modes can be distinguished: propeller (I), freewheeling (II), and turbine (III), which respectively correspond to operating below, near and above Vp. Although, the exact shape of the efficiency curve shown inFIG. 7may vary depending on the particular characteristics of any given propeller, its overall shape is typical. The best efficiency from a propeller in the propeller mode may be achieved operating at speeds of about 0.75 to about 0.95Vp. Similarly, the best efficiency from a propeller in the turbine mode may be achieved operating at speeds of about 1.05 to about 1.25Vp. Thus the transfer from one mode of operation to the other may be accomplished by either changing an aircraft speed or a pitch speed with respect to each other. For example, a propeller maintaining a cruise speed of 0.8Vpmay be turned into a turbine by accelerating an aircraft to a speed of 1.2Vp. The pitch speed is proportional to the propeller's rotation rate (RPM) and pitch. Therefore, the transfer between different operating modes may be also accomplished via changing Vpwithout changing the aircraft speed. For example, the motor may either increase or decrease its RPM to respectively increase or decrease the pitch speed. Similarly, a variable pitch propeller may either increase or decrease its pitch to respectively increase or decrease the pitch speed.

In accordance with embodiments of this invention, a regenerative UAV the energy used by the aircraft can be replenished or regenerated using renewable energy sources) may be produced comprising elements and components shown inFIG. 8. The UAV800may comprise an airframe802, a propulsion system804, and electronics806, which in turn include at least a fuselage802a, a wing802b, a tail802c, a motor804a, a generator804b, a propeller/turbine804c, a battery806a, an ESC806b, and flight control electronics (FCE)806c. Other elements may also be included, for example, such as solar cells as an alternative renewable power source. A feature of a regenerative UAV (e.g.,800) is its ability to recover, convert and store its kinetic and potential energy using the KECS. In embodiments consistent withFIG. 8, the KECS elements are integrated into different systems of the regenerative UAV: one KECS element, the generator804b, is integrated with the propulsion system804and another KECS element, battery806ais integrated with the electronics system806. As described above, a motor can be operated as a generator, so that the motor and the generator onboard the regenerative UAV may be combined in a single unit. Similarly, a single propeller may be used as a regular propeller to provide the thrust and as a turbine to provide excess electrical power.

FIG. 9shows one non-limiting example of a possible implementation of a regenerative UAV900. This aircraft is optimized for long-term endurance and efficient use of all available renewable resources. Its wings902are lightweight (for example substantially less than 50% of the total weight) and may be produced from composite materials, for example those based on carbon fiber, fiber glass, Kevlar®, and others. The wings902may be covered with PV solar cells (shown schematically as904) to efficiently collect and convert solar energy. The wings902have a downward bow shape in order to increase solar exposure at dawn and dusk. Examples of suitable downward bow shapes of the wings902may be found in U.S. patent application Ser. No. 13/46,0146, filed Apr. 30, 2012 by Sergey V. Frolov, et al., and entitled “AUTONOMOUS SOLAR AIRCRAFT”, which is incorporated herein by reference in its entirety. The wings are also shaped to increase soaring efficiency, i.e. the ability to capture and ride thermal and forced-air vertical flows. The shape of the wing902may also reduce aerodynamic interference between the main wing and the tail906, as a well as interference from the propeller908. The wings' airfoil profile may be optimized to produce high lift and low drag at low Reynolds numbers of less than 100,000, i.e. low air speeds. For example, a GM15 airfoil profile may be used.

A single propeller908is located in line with the streamlined fuselage910, which also contains electronics and payloads (not shown). As result, the center of gravity may be lowered significantly below the wings to improve flight stability. Various tail configurations are possible, such as cross-tails, V-tails (shown as906), T-tails, and others. V-tail configuration may be an aerodynamically efficient tail configurations in some embodiments. The UAV configuration described above is particularly suitable for relatively low-weight aircraft with the total weight of less than about 50 kg. Of course, many other UAV designs incorporating a KECS are possible. For example, in the case of larger and heavier aircraft, an airframe with multiple motors and propellers may be preferred, where at least one and preferably all of the available motors are also configured to operate as generators.

A regenerative UAV may have either a fixed or variable airframe. In the latter case, at least some of the airframe elements may alter their relative position in order to change the aerodynamic characteristics of the UAV. For example, a wing may alter its position to either increase or decrease the wing span. Alternatively, it may change its shape, e.g. by bending or unbending of a bowed wing, to shift the relative position of the center of gravity. These changes may favorably affect the operational effectiveness of the hybrid power system. For example, solar-powered flight may favor an airframe configuration with a lower cruising speed, higher stability and higher endurance. On the other hand, a wind-powered flight may favor an airframe configuration with a higher cruising speed and higher maneuverability.

A regenerative UAV equipped with solar cells and a KECS can be powered using both types of renewable energies simultaneously as shown inFIG. 10. In this method shown as1000, the solar PV cells collect sunlight at1004and convert solar energy to electricity at1006. At the same time, the UAV may increase its kinetic energy at1010and convert it to electricity using the KECS' turbine at1012. Both types of renewable energy channels can be used simultaneously to power electronics, the excess energy being stored for later use at1008. In another non-limiting configuration1100shown inFIG. 11the two different types of renewable energies are used at different times. The PV solar cells and modules may be used at daytime to harvest solar energy for immediate use and storage at1102, while the KECS may harvest wind energy at night time at1104. This configuration may reduce the required storage capacity for the battery and thus lower UAV's total weight.

Unlike a solar PV system that can operate only at daytime, a KECS can operate whenever appropriate wind resource is available whether it is night or daytime. A number of different types of wind resources can be identified as suitable for powering UAVs.FIG. 12shows a UAV1202flying along a straight horizontal line through airspace regions marked as A, B and C from right to left. Region B is characterized by having a higher wind speed against the UAV flight direction with respect to regions A and B, i.e. a wind gust. A wind gust in this case increases the relative airspeed and provides excess kinetic energy that may be used by the KECS for conversion to electrical power. Similarly, when flying with the wind (i.e. a tail wind), a drop in wind speed could also provide excess kinetic energy. In general, any air turbulence that causes an increase in the UAV airspeed may be used by the KECS as a renewable wind resource. In some cases, multiple KECS may be used on a single aircraft for better utilization of air turbulence. For example, an aircraft may have two independent KECS installed on opposite sides of a wing, so that each KECS may operate and react differently to local air turbulence events, even when the events on different sides of the wing differ from each other.

The above method of using wind gusts and turbulence as renewable wind energy resource may be generalized as1300as shown inFIG. 13. The method1300comprises the following steps: (1) sensing increased airspeed at1302, (2) starting a generator connected to a turbine (alternatively, operating a motor in a generator mode) at1304in response to higher airspeed, (3) generating electrical power at1306, (4) sensing when airspeed returns to normal at1308, and (5) stopping the generator or exiting the generator mode at1310. This sequence of steps may be repeated indefinitely as indicated by arrows1312,1314.

An updraft or an upward vertical air movement represents another appropriate wind resource. There are various examples of such updrafts in Earth atmosphere. For example, a thermal is a column of rising air created by heating of the ground or sea by the sun.FIG. 14shows a UAV1402flight path across airspace regions marked as A, B and C from left to right. Regions A and C with upward air streams1404and1406respectively may be categorized as thermals. The UAV1402flying through these regions may soar without using its propulsion system, gain excess altitude and obtain excess potential energy. This energy may be released and converted to electricity via an onboard KECS by going through region B without vertical airflows, where the UAV may descend and attain a higher speed necessary for power generation. This procedure may be repeated as needed, e.g. the UAV1402may go in and out of the same thermal and repeatedly use its KECS for power production even in the absence of other energy sources. In the case of large updraft columns and relatively smaller scale UAVs, it may be possible to operate KECS more efficiently even without leaving the updraft region, e.g. operating the KECS while circling inside a thermal column and maintaining constant altitude.

In addition to thermals, other types of upward air streams suitable for soaring may be available, e.g. an updraft produced by winds blowing over mountain tops or into hilltops and steep ridges.FIG. 15shows a UAV1502flying near a mountain ridge through airspace regions labeled A and B. Region B has an updraft that can used for soaring, where the UAV1502may gain excess potential energy. This excess energy may then be converted by the KECS into electricity by flying through region A. This procedure may be repeated as needed.

In accordance with embodiments of this invention, the method of using a KECS in an updraft can be generalized as shown inFIG. 16. The method1600may comprise the following steps: (1) finding and entering a rising air column1602, (2) ascent and gain in altitude1604, (3) exiting the column1606, (4) starting the generator mode in the KECS1608, (5) descent with power generation1610and (6) shutting off the generator1612. The method is cyclical and may be repeated indefinitely as indicated by arrows1614and1616.

Another type of available wind resource includes a wind shear, an atmospheric condition in which two air streams (usually horizontal streams) occur side by side or in near proximity to each other.FIG. 17shows a UAV1700flying through an airspace region with a wind shear comprising two zones: A and B, where the wind speeds are relatively smaller and larger with respect to each other. Using a maneuver called dynamic soaring and depicted inFIG. 17by a dotted line, the UAV1700can gain air speed far in excess of its cruising speed and the wind speed in regions A and B. The resulting excess kinetic energy may be used by the KECS for electrical conversion either inside or outside the wind shear region. For example,FIG. 18shows one possible method of using the wind shear as a renewable energy source. The method1800may comprise the following steps: (1) finding and entering a wind shear zone1802, (2) gaining excess speed1804, (3) exiting the wind shear zone1806, (4) starting the generator mode in the KECS1808, (5) decelerating and generating power1810and (6) shutting off the generator1812. The method is cyclical as other KECS methods and may be repeated indefinitely as indicated by arrows1814and1816.

Wind and other air movements are not the only potential energy resources in the atmosphere that can be exploited by a KECS.FIG. 19illustrates how a UAV1900may gain excess potential energy by flying through a cloud1902and collecting airborne moisture1904as excess weight. Moisture1904present in region A in the form of fog, rain, ice, or snow may be collected to temporarily increase the total weight of the UAV. The excess weight provides excess energy for power conversion by the KECS during descent, after which excess water maybe discarded and the UAV may return to the original position in region B outside the cloud zone. In this case, the UAV may be equipped with a specialized container for onboard water collection and storage.FIG. 20shows an exemplary UAV2000comprising a fuselage2010, a main wing2020, a tail section2030, a propulsion system including propeller2040and a water or moisture collector2050. The water collector2050, e.g., a water-tight plastic container, may be located inside the wing, the fuselage, or inside special cavities outside the airframe. The following method shown inFIG. 21may be used for power generation. The method2100may comprise the steps of: (1) finding and entering a cloud zone2102, (2) collecting moisture and gaining excess mass and weight2104, (3) exiting the cloud zone2106, (4) starting the generator in the KECS2108, (5) descending and generating power2110and (6) shutting off the generator and disposing of excess water2112. The sequence may be followed by ascend and repeat of the above sequence as indicated by arrows2114and2116.

The above methods and flight maneuvers may be executed by either an autopilot in a pilotless mode or a pilot (remote or onboard). A pilotless mode may be attractive for UAVs in some applications. In the case of a pilotless UAV, an aerial vehicle may be equipped with a range of different sensors2202, such as pressure sensors, temperature sensors, airspeed sensors, moisture and humidity sensors, pitot tubes, altimeters, accelerometers, gyroscopes, position (GPS) sensors, light sensors, imaging sensors and others. Sensory data from the sensors2202may be continuously or intermittently provided to an autopilot system2204on board a UAV as shown inFIG. 22, which can process the data, plan the flight path, calculate the actual path, evaluate path corrections and provide necessary data for flight control electronics (FCE)2206. The FCE2206in turn continuously operates on the airframe control surfaces2208to maintain the set flight path and at the same time manages the operation of the KECS2210. The autopilot system2204may be able to automatically analyze the sensory data and recognize the presence of various atmospheric resources suitable for powering the KECS2210, including vertical updrafts and horizontal wind shears. When a particular resource is detected, the autopilot2204may engage an appropriate computer program with a flight algorithm corresponding to such a resource. The autopilot hardware may be preloaded with software containing various programs corresponding to different potential resources. In the absence of any resource, the autopilot2204may initiate an active search algorithm, in which an aircraft may fly a random flight path and look for tell-tale signs of potentially useful atmospheric resources.