Abstract
The following research investigates the use of multiple power sources for an unmanned blimp for extended operations. A blimp is unique for a long endurance platform as it does not require propulsion to remain aloft. Therefore, it fills a niche in low to medium altitude long endurance unmanned air vehicles. This ability can be utilized for the monitoring of sensitive environmental areas, disaster sites, and coastal patrols. The problem is how to power them with redundancy to perform the desired task properly. It can be accomplished through the utilization of multiple independent power sources. The surface area of the gas bladder is utilized for the placement of high-efficiency lightweight, flexible solar cells typically used in satellites. Power is further enhanced with the use of battery storage and a small internal combustion hybrid electric power source to supplement the system in times of increased current draw. Finally, depending on the size and intended purpose of the vehicle, recommendations are made as placing all of the systems on the platform may not be feasible or suitable depending on its purpose.
Introduction
Endurance of unmanned systems is critical for the success in specific missions. Unmanned platforms are particularly suited for long endurance scenarios. The problem is how to successfully implement a power system that allows the platform to perform as desired. Dirigibles, more commonly known as blimps, are uniquely suited for long endurance operations (Prisacariu, 2018). Their only limitations are the availability of power systems and reserves. The problem is how to implement and utilize them effectively, although blimps are lighter than air vehicles, as with any type of aircraft, weight is a critical component of stability. What is unique about a blimp, is that it does not need forward thrust to maintain altitude. This ability to ride the prevailing winds presents some unique opportunities for power management and propulsion (Ribeiro, 2017). It is, of course, predicated that the intended direction is in the same as the wind. Still, if the vehicle must go into the wind, then significant power may be required due to the surface area the craft has because of the aerodynamics of the gas bladder (Prisacariu, 2018).
A blimp is a lighter than air vehicle because it has a sizable lightweight bladder that contains a gas that is lighter than the surrounding air and counteracts the force of gravity. This gas is contained in the envelope that has a significant volume. Typically, helium is used for safety reasons, hydrogen may be substituted since the platform will be unmanned, but hydrogen has inherent dangers as it is flammable so precautions must be taken when using it (Prisacariu, 2018). A hydrogen-filled envelope has the advantage of being able to be replenished from the ballast water by breaking it down to its elemental units of hydrogen and oxygen with electricity (Rusdianasari, 2018). Electrolysis is energy-intensive, so a way to recharge the batteries rapidly to extend mission times must be developed. Instead of using a single source of power and energy reserves, multiple faceted approaches are used to extend the range and utility of the platform with existing technologies to minimize development times and reduce costs (Boukoberine, 2019).
Weight is the critical factor when designing a power source for a lighter than air vehicle; it must also give the desired endurance required for the intended mission of the platform. By utilizing a three-pronged approach, the limitations can be overcome to increase efficiency and extend the duration the unit can remain on station or patrol. The use of solar panels, lightweight batteries, and a hybrid internal combustion / electric power source to maintain charge and provide power and propulsion to the onboard systems can accomplish this. Recent advances into materials have also led to the development of affordable ultracapacitors, which may supplement the batteries as their ability to deliver current is not restricted by internal resistance (Shah, 2018).
The use of the advanced lightweight systems is combined to allow the vehicle extended range and power options. The system can also be scaled to fit a variety of different sized blimps based upon the gas envelope and weight capacity allowing the increase of capabilities. All things being equal, the logic must control the power system in the most efficient manner, depending on its mission and weather conditions (Ribeiro, 2017). If necessary, all three sources can be utilized to meet the power requirements of the craft, depending on the conditions set forth by the operators. The utilization of all three systems increases the capabilities of the vehicle, depending on how it is configured for a given task. The capture of solar energy, combined with lightweight, efficient storage, and an internal combustion hybrid powerplant is the most suitable combination of power sources to provide the energy required to operate a long-endurance unmanned airship. These methods must be balanced with the objectives of the operators to offer the most flexibility in the system without added complexity (Boukoberine, 2019).
Power Requirements
To increase reliability and reduce costs, the vehicle uses electric motors with propellers for propulsion and maneuvering. This system is easy to maintain and troubleshoot while delivering enough power to control and maneuver the vehicle. The use of electric motors allows the installation of multiple energy sources with different and separate methods of power generation and storage (Papa, 2017). Each system must be capable of powering the unit separately, this ensures redundancy and increases utility, but they must also be able to supplement the other components in times of increased current draw. For example, blimps do not require any forward motion to remain aloft so they can drift with the wind currents if necessary. They will not always be going with the wind and will be required to fly upwind, or with a crosswind, this requires increased power to maintain course (Prisacariu, 2018). During these maneuvers, the power draw may exceed the capability of a single system such as solar and must, therefore, be supplemented with battery power.
Not only does the propulsion and maneuvering system draw power, but so does the payload, command, control, and communication components of the airship. Each system may be able to meet these needs on their own, but a reduced capacity on one system will require the addition of the other to supplement. The airship uses a combination of solar, battery, and a hybrid internal combustion generator to power the vehicle. The redundancy allows an extended range and rapid recharging of the cells at night or during reduced light conditions. Using the batteries for storage reduces fuel consumption. It ensures the hybrid generator is mainly used for recharging but, in times of increased load, can add to the power capacity for propulsion. Still, fuel quantity is finite, so the use of the system must be limited (Papa, 2017). Airships offer the unrivaled ability for endurance, but the available energy supply must be matched to the payload and lifting capacity of the gas bag for maximum efficiency. When these items are balanced for mission requirements, an effective platform is developed.
Lightweight Solar
Solar power is the primary energy source for the airship; the gas bladder offers an opportunity to place enough collectors to capture the light from any angle (Ghosh, 2017). The problem is the envelope size is finite, and the weight of the solar cells must be considered as not to interfere with the cargo capacity of the vehicle. For this reason, typical cells will be too heavy if enough, are placed on the bag to power the vehicle. An alternate source of efficient, lightweight, and flexible collectors must be used for maximum efficiency, solar cells typically used on satellites meet all of those criteria (Cappelluti, 2016). Not only are they lightweight and flexible, but they offer a 40% improvement in energy collection over a similar standard cell.
The cells must be placed directly on the gas bag to collect the light, and because of the compound curves, energy collection will vary along the length of the bladder (Ghosh, 2017). The advantage of this is that the vehicle does not need to be oriented in a specific direction and attitude to collect energy. While performing the assigned task, it can passively recharge the batteries while powering the propulsion and payload. The disadvantage of solar collectors is that it requires light, which is not available all the time except for certain latitudes during specific times of the year. Areas such as those above the Arctic and below the Antarctic circles receive 24 hours of daylight during their respective summers, tasks such as environmental monitoring can be accomplished using solar power alone. Summer is a relative term, and temperatures can still be quite cold, so the materials will need to remain flexible in that environment. Utilizing solar cells from the space sector accomplishes this, they are designed to operate in this extreme environment while withstanding large temperature swings and have a higher current output than typical consumer-grade cells (Cappelluti, 2016). They are also more costly, so similar cells should be acquired that are not space rated but exhibit the same qualities; this will keep costs down while giving the same benefits.
Hybrid Engine
In addition to the solar cells, there should be an alternate method of power generation. Advances have been made in the miniaturization of hydrogen fuel cells to provide electricity. Still, they are complicated and require high-pressure vessels that add weight and reduce lifting capacity unless the gas bladder is enlarged (Gadalla, 2016). Although fuel cells offer high energy density, they are still a relatively new technology, advances have been made in miniaturization, but they add complexity (Belmonte, 2017). Therefore, the obvious choice is the addition of a small hybrid power plant that recharges the batteries and can add to the available energy reserves. A small single or twin cylinder internal combustion engine connected to a generator can accomplish this; the heaviest component will, of course, is the fuel supply as it weighs an average of 7 pounds per gallon depending on which type of engine is selected (Cirigliano, 2018). A spark ignition engine will require more volatile fuel than diesel; the operational environment should determine the engine selection as either type of power plant has specific advantages.
Spark ignition allows easier starting and familiarity for the operators while a small diesel is more suitable for the constant speed required for electrical generation and is more fuel-efficient. A starter motor is not necessary as voltage can be fed into the generator to start the engine, reducing component count. The ability to charge the batteries rapidly may not be needed, so not every platform will require the use of the hybrid engine package, or it could be made smaller depending on the requirements of the operator. Any system added to the vehicle is a weight that reduces the amount of lifting and payload capacity. The efficiency of the solar cells combined with battery size may not require the installation of the power plant, in areas where constant sunlight is available, and it is environmentally sensitive. The fuel my pose a hazard for scientific research so it can be exchanged for increased storage capacity reducing the chance of contamination of pristine environments if there is a failure or from affecting instruments that monitor pollution as the exhaust from the system has the potential to skew the collected data.
Energy Storage
Theoretically, the solar cells are going to supply enough power for the airship, when a surplus of power is available, it will be stored within lithium polymer cells and possibly ultracapacitors (Shah, 2018). Both types of energy storage offer benefits, including lightweight, high energy density, and reliability. The ultracapacitors will be used primarily for starting the hybrid engine as they deliver current at a much higher rate than the batteries without damage and can recharge rapidly. If need be, their energy can be used to power other systems on the vehicle as a backup in case of problems with storage or power generation. The ultracapacitors are relatively lightweight compared to batteries of a similar size and will not adversely impact the cargo capacity of the airship.
Batteries require that the weight is balanced against cargo capacity; if longer endurance is needed, accommodations are made for the addition of extra cells at the expense of the payload or fuel. Lithium polymer is the best choice for this application due to its energy density and weight. Cells can be stacked in series to increase voltage and capacity. Storage can be adjusted to suit mission parameters if other factors are compensated for to maintain the optimum weight of the vehicle. Liquid fuel for the hybrid engine is another method of energy storage, albeit chemical. This energy source will only be used on an as-needed basis due to the limited supply available, as it must be replenished on the ground during regular servicing.
Limitations
Blimps, although highly efficient, have limitations that affect the tasks that can be accomplished with them. Long endurance missions require the energy to complete them, and solar is available during daylight hours. Still, for nighttime operations, the battery must be drawn upon so it must have enough capacity, or the hybrid engine will have to be used consuming the fuel. The aerodynamics of the craft itself is also a factor; it requires more energy to head upwind or crosswind while maintaining a set course than it would be to drift with the wind. Station keeping, depending on the method used, will shorten the duration it will be available to remain on task as the hybrid power plant may need to be brought online for additional power. Replenishment of the lifting gas may have to be done during a mission, carrying a tank of helium to replace that which naturally escapes will be too heavy. Therefore the ability to crack the water and separate it into hydrogen and oxygen may be required. The process of electrolysis is energy-intensive but can be done on a small scale to increase the endurance of the platform, using the ballast water as the source of the hydrogen (Rusdianasari, 2018).
The blimp must operate above any obstructions that it may encounter, in urban areas, unpredictable wind currents could drive the craft into buildings or towers, this places restrictions on the altitude where it can safely operate in those environments. The platform would be better suited for wilderness, coastal regions, or areas without significant infrastructure (Klemas, 2015). Its design may limit where it can be deployed, but where it can operate, it has advantages over other platforms in endurance and the ability to loiter. The blimp is more susceptible to the elements, and care must be exercised to operate the vehicle within limits imposed by the design of the airship. Still, if deployed in a suitable environment, it will excel at the assigned task (Prisacariu, 2018).
Recommendations
Simplicity breeds reliability; therefore, the use of proven technologies combined to meet the necessary power requirements of the platform provides an advantage over the use of emerging ones. Solar, battery, and a hybrid generator are the best choices to power the platform. As with any airborne system, weight must be the primary concern, the use of lightweight components sourced from other platforms will accomplish this. Flexible solar cells for satellites offer high efficiency with a minimal weight penalty. They can be placed on the gas bladder to capture light at any angle and increase efficiency (Cappelluti, 2016). They are durable and have already been tested in space, proving their reliability. Energy storage must be as light as possible while still providing reserves to accomplish the mission. Lithium batteries are the best choice for this because of their energy density. However, they can be supplemented with supercapacitors for short duration high current draw situations such as the starting of the hybrid engine to recharge the batteries or to provide supplemental power (Shah, 2018). The hybrid powerplant is sized according to perceived need and operational environment, diesel will give the best economy, but there are not many small size engines of this type available so a spark ignition will be a universal fit for familiarity by the operators and ease of use.
Conclusion
The use of unmanned airships is currently limited, their advantages are being rediscovered, and their ability for long endurance low to medium altitude flight is unrivaled by any other platform when compared to the energy required to remain aloft. Advances in battery, solar, and hybrid engine technology can be combined to power the airship to make it a useful alternative platform. The primary concern is the power to weight ratio as the size of the gas bladder limits blimp payloads. Capacity can be increased by a larger envelope, which can be utilized to capture more solar energy, but this also has a larger sail area and drag, which will require more energy to maneuver and be subject to controllability issues with higher winds. Engineers will have to take into consideration all these factors when designing the vehicle and size it appropriately while considering the intended purpose. Blimps are a vehicle that can fill a niche that requires slow speed airborne units with the ability to loiter for long periods with minimal energy expenditure.
References
Belmonte, N. L. (2017). Case studies of energy storage with fuel cells and batteries for stationary and mobile applications. Challenges. doi:http://dx.doi.org.ezproxy.libproxy.db.erau.edu/ 10.3390/challe8010009
Boukoberine, M. N. (2019). A critical review on unmanned aerial vehicles power supply and energy management: Solutions, strategies, and prospects. Applied Energy. doi:10.1016/j.apenergy.2019.113823
Cappelluti, F. G. (2016). Novel concepts for high-efficiency lightweight space solar cells. E3S Web of Conferences. ESPC. doi:10.1051/e3sconf/20171603007
Cirigliano, D. F. (2018). Diesel, spark-ignition, and turboprop engines for long-duration unmanned air flights. Journal of Propulsion and Power. doi:https://doi-org.ezproxy. libproxy.db.erau.edu/10.2514/1.B36547
Gadalla, M. Z. (2016). Analysis of a hydrogen fuel cell-PV power system for small UAV. Hydrogen Energy, 6422-6432. doi:https://doi.org/10.1016/j.ijhydene.2016.02.129
Ghosh, K. G. (2017). Power generation on a solar photovoltaic array integrated with lighter-than-air platform at low altitudes. Energy Conversion and Management, 286-298. doi:10.1016/j.enconman.2017.10.039
Klemas, V. V. (2015). Coastal and environmental remote sensing from unmanned aerial vehicles: An overview. Journal of Coastal Research, 1260-1267. Retrieved from http://ezproxy.libproxy.db.erau.edu/login?url=https://search-proquest-com.ezproxy.libproxy.db.erau.edu/docview/1713957927?accountid=27203
Papa, U. P. (2017). Conceptual design of a hmall hybrid unmanned aircraft system. Journal of Advanced Transportation. Retrieved from https://link-gale-com.ezproxy. libproxy.db. erau.edu/apps/doc/A545147732/AONE?u=embry&sid=AONE&xid=57c0b6d3
Prisacariu, V. C. (2018). Analysis performances of UAV airships. Scientific Bulletin “Mircea Cel Batran” Naval Academy, 21, 180-189. doi:10.21279/1454-864X-18-|1-030
Ribeiro, C. G. (2017). A platform for autonomous path control of unmanned airship. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 4737-4747. doi:10.1007/s40430-017-0891-9
Rusdianasari, B. Y. (2018). HHO gas generation in hydrogen generator using electrolysis. IOP Conference Series. Lampung Selatan, Indonesia: IOP Science. doi: https://doi.org /10.1088/1755-1315/258/1/012007
Shah, N. C. (2018). Supercapacitors in tandem with batteries to prolong. Electronics, 7(1). doi:http://dx.doi.org.ezproxy.libproxy.db.erau.edu/10.3390/electronics7010006
Patrick Carroll 