Introduction
This essay discusses a hypothesis of a lighter than air vehicle (balloon) with a capacity to lift up to an altitude of 20km above sea-level (stratosphere). The vehicle is solar powered equipped with a propeller for stabilization and an observation platform to capture video images in case of natural disaster such as floods, bushfires and other natural disasters.
The essay has three objectives; to define and discuss different solar power technologies, in order to derive the most practicable, efficient and effective solar technology, for the assigned task. To discuss on the most appropriate optical camera that can capture earth video images from stratosphere and electric propeller; used to stabilize the vehicle in stratosphere and analyze their power consumption and power storage facilities.
Discussion
Solar energy is radiant light and heat that comes from the sun. The sun radiates an estimated 174 petawatts of solar radiation to earth. Although thirty percent of this radiation is reflected back to mars, the earth’s land, ocean and atmosphere absorbs an estimated 3,850,000 exajoules annually in biomass.
This amount of energy is so immense that one year of this energy (3,850,000 exajoules) is estimated to be double what will ever be harnessed from all of the Earth’s non-renewable resources combined i.e. Oil, natural gas, and coal and all other natural sources of energy.Moreover, one hour of this energy can sustain the entire earth for a year (National Aeronautics and Space Administration: the facts 2009).
Human beings have been utilizing solar energy for thousands of years all over the world. Harnessing the solar energy via different evolving technologies, traditional humans were able to use solar energy for heating, cooking, and drying. Today due to technological breakthrough, the modern man has the capacity to utilize more of the solar energy.
The main breakthrough by modern man was being able to convert solar energy into electricity. Solar energy is commonly used where other power supplies are not available, such as in remote places and in space. It is one of the cheapest sources of electric energy compared to some of the non renewable sources such as coal or oil (Schobert 2002 p. 218).
Solar technologies can be classified into; passive or active, this is determined by how they absorb, convert and supply sunlight. Passive solar technique is more applicable to building designs. Passive solar architecture is mostly applied in building green houses, it includes; creating windows, walls, and floors with solar panel feature or characteristics.
For example, they may be able to harness solar energy and convert it to heat to heat, to supply a building with heat during winter. These systems do not require any external support from other devices for example, electrical devices or mechanical devices. As opposed to passive solar techniques that do not require external support from other devices, active technique requires use of other mechanical and electrical devices for example; use of a converter to convert the electric current.
In Passive solar techniques, materials thermal properties, situated environment in reference to natural circulation of air, and position of the Sun are considered crucial. Passive solar techniques have a higher energy output than passive techniques. In this essay we apply passive solar techniques since they are more effective and applicable to our hypothesis (Schobert 2002 p. 300).
Solar panels are used to capture, generate and supply electricity to demand. Each panels output has a specific rated output of power, which range between (100 to 320 watts). The efficiency of a solar cell is determined by for example, when a 320w, 5% efficient solar panel is compared to a 10% efficient 320watt panel, the 5% efficient solar panel should cover twice the area of the 10% efficient solar panel. This is because there is a restriction on the amount of energy that one solar panel can produce (Schobert 2002 p. 286).
A solar panel is made up of interconnected solar cells, packaged and assembled. Each solar cell converts sunlight to electricity via photovoltaic effect. It is a form of photoelectric cell that contains electrical characteristics i.e. voltage output varies depending on the sunlight exposed upon it. When these cells are exposed to sunlight, they can generate and support an electric current without the need for any external voltage source.
At present, solar cell panels can only convert15% of sunlight hitting them into electricity, at best. The generation of voltage in a material upon exposure to light is what is called photovoltaic effect. For this reason, solar cells are also known as photovoltaic cells (Hantula 2009, p.20).
There are different types of solar cells made out of different materials to satisfy different specifications influenced by efficiency, weight, affectivity and price. All solar cells operate on three main attributes; absorption of light to create either electron-hole pairs or excitons, separation of various types of charge carriers and the separate extraction of those carriers to an external circuit.
The solar cell works in three steps: Photons in sunlight hit the solar panel and are absorbed by semiconducting materials, such as silicon. Negatively charged electrons are knocked loose from their atoms, causing an electric potential difference. Current starts flowing through the material to cancel the potential and this electricity is captured.
Due to the special composition of solar cells, the electrons are only allowed to move in a single direction. An array of solar cells converts solar energy into a usable amount of direct current (DC) electricity. Typically, solar panels require a converter to convert the Direct Current (DC) to Alternating Current (AC) (Jha 2009, p.25).
Most solar cells are made of bulk materials, this includes; thin-films layers, organic dyes, and organic polymers deposited on supporting substrates. They are cut into rectangle shapes of about 240 micrometers thickness in size before they are processed. Others are made from nanocrystals and used as electron-confined nano particles. Silicon is the researched in both bulk and thin-film forms.
The essay will analyze three main types of solar cells typically used, they include; Gallium arsenide-based solar cells, silicon based solar cells and multi-junction cells. There are 4 types of silicon based solar cells; Monocrystalline silicon, Polycrystalline silicon, ribbon silicon and mono like multi silicon. Monoccrystalline silicon cells are made via a crystal growth method called Czochralski process.
This method is used to obtain single crystals of semi conductors. Unlike Monoccrystalline silicon cells, Polycrystalline silicon cells are cheaper since they are made out of cast square ingots. However, they are less efficient. Ribbon silicon is formed by excreting flat thin films from molten silicon. They have lower efficiency than polycrystalline silicon cells.
Mono-like-multi silicon uses existing polycrystalline casting chambers with mono material. The inner sections contain mono-like cells considered to be of high-efficiency while the outer edges are made of conventional poly. The conventional ploy (outer edges) is deducted which results in cheap mono-like cells (Castellano 2010, p.34).
Gallium arsenide-based is a compound of garlium and arsenic compounds. The cell is created via direct reactions from the elements. Solar cells based on this compound can achieve efficiencies of over 32%. When compared to silicon, Silicon is cheaper to process especially because it widely available. Si crystal, makes silicon based cells, it is mechanically resilient in structure with the ability to be grown to very large diameter boules. It is also a decent thermal conductor.
However, in sunlight absorption, Silicon has a much lower capacity for absorption as opposed to Gallium arsenide. While 100 micrometers of silicon is needed to absorb sunlight, the absorption rate of Gallium arsenide is so high that only a fraction of the micrometers thick layer is needed.
Multi-junction solar cells are solar cells designed and produced with p-n junctions. The purpose of the junctions is to reduce one of the largest sources of loss in solar cells, and hence each junction is tuned to a different wavelength of light to improve efficiency.
These cells are also called tandem cells. Their production is very hard due to the size of the materials used especially when extracting the current between the thin layers. There are two methods of producing a multi-junction cell; the first method is where two separate thin film solar cells are wired to each other separately outside the cell, this is the simplest method (Castellano 2010, p.36).
The more second method is where the cells of multiple layers are internally mechanically and electrically connected. This method is harder than the first because during the cells production, the layer have to be carefully matched according to their electrical characters. Among materials used to produce a tandem cell, Gallium arsenide and silicon can both be used. They are most preferred to be used in space due to their high power- weight ratio (Castellano 2010, p.38).
Test outcomes of multi-junction cells and silicon solar cells point to an 18% difference. The performance of multi-junction cells was 43% efficiency while silicon cells achieved25% efficiency under concentrated artificial light. Commercially tandem cells are available at around 40% efficiency under concentrated sunlight. It is important to note that; in real life conditions, outputs of the solar panel are less than theoretical or tested expectations.
This is due to the position of the panel, season, weather changes e.g. cloudy days and other reasons that may affect the amount of sunlight hitting the panel. The maximum power output that can be achieved by a solar panel is measured in Watt-Peak (WP). Even though day to day power out puts vary due to the stated variables, on average Watt-Peaks range between 175W to 235W for average solar panels (Castellano 2010, p.72).
Earlier, we discussed and gave an example; when a 320watt, 5% efficient solar panel is compared to a 10% efficient 320 watt panel, the 5% efficient 320watt panel should cover twice the area of the 10% efficient 320 watt panel. Regardless of size, each solar cells produces 0.5 volts, higher voltage can only be achieved by joining multiple cells to the appropriate.
All these factors considered, the more efficient panel requires covers less space for high voltage while the less efficient solar cell will cover more space for more voltage. This factor will be crucial when considering the appropriate solar cell for our lighter than air vehicle.
The Stratosphere is the second major layer of atmosphere above the earth; it is situated between 10 km and 50 km altitude above the sea level just above the troposphere, and below the mesosphere. Inside the stratosphere, the temperatures are in opposite with the earth’s, warm temperatures are up and cooler temperatures are down.
As opposed to the troposphere which is below it and hence nearer to the earth’s surface, its temperatures are its cooler layers are up and the warm layers are down. The border between the two is marked by where the temperature inversion happens.This border area is called the tropopause. Stratosphere has a temperature of about −3°C / 29.6°F, slightly below water’s freezing point Mohanakumar2008, p.45).
The stratosphere is layered in temperature because the ozone there absorbs high energy UVB and UVC energy waves from the Sun and is broken down into atomic oxygen and diatomic oxygen (O2). Atomic oxygen is found prevalent in the upper stratosphere due to the bombardment of UV light and the destruction of both ozone and diatomic oxygen.
Natural ozone is able to be mostly produced in the mid stratosphere due to minimum UV light passing through it, hence allowing Oxygen and O2 to combine. The mixture of these two elements is the result of high temperature layer found in the stratosphere higher area.
Atomic oxygen is not found in the cooler layer of the lower stratosphere as this area receives very low levels of ultra violet light. Furthermore, no ozone is formed. The reaction of the two elements creates an upper warm layer and a lower cool air.
This set up of the layers eradicates turbulence in this part of the atmosphere hence making it actively stable. Commercial flights cruise at latitude of 9-12km to maintain cabin comfort due to low turbulence in the lower stratosphere, optimized combustion due to the low temperature near the tropopause (equilibrium point) and low density air that reduces drag. This factors are crucial when deciding on the type of propeller and video camera that will be attached to the lighter than air vehicle (Ahrens 2007, p.78).
Despite the lighter than air vehicle depending on hydrogen or helium for lift to stratosphere, the vehicle requires a propeller to maintain stationary when in the stratosphere. Due to the light weight of the vehicle winds and other turbulence effects (especially in upper stratosphere) may push the vehicle off course or even into a spin. The propeller will ensure the vehicle remains stationery and also reduce sway effect as much as possible to allow proper video capture.
The propeller proposed on our vehicle is a; Yuneec Power Drive 200 20 kW (27 hp) electric motor equipped with two lithium-polymer battery packs (13 kg). The propeller is used to run a 115kg kite plane including an average 80kg pilot. The battery packs provide 40 minutes endurance. Considering our lighter than air vehicle will weigh much less than 100kg, an endurance of at least 1hour 20 minutes can be expected (when considering 50% less weight).
The batteries were charged by a 320watt solar panel, for 1.5hrs the panel charged the batteries with enough power to run the motor for 3-5 minutes. This considered, we can conclude that it takes the panel around 12hours to charge the batteries full. Each of the two batteries in the pack is 12volts. Each requires a 13.8volts across the batteries terminal to charge the battery (Abraham et al 2001). A 320watt panel has a capacity of 54.7volts (Raymer 2006, pg.112).
Another propeller proposed to be used in our study is one of the four motors used to power a solar impulse plane. The plane is powered by four 10-horsepower (7 kW) electric motors. The aircraft uses over 11,000 solar cells placed on its wings; power harnessed from the solar cells is stored in two lithium polymer battery packs and used to drive the propellers. The motors can run the plane for one and a half hours on battery power alone.
The aircraft has the capacity to fly for 19hours purely on solar power, charging its batteries in flight. Our vehicle would only require only one of 11ft propellers among the four (7kw) electric motors. On the solar impulse plane, considering its specifications, we can conclude that each electrical motor allocated 2750 solar cells and two lithium polymer batteries (13kg) on the vehicle (Grupp 2012, pg. 57).
To capture video images from stratosphere, a camera; Canon D20 Digital camera is considered. This system is as a result of two technologies synergized in the camera: a high-sensitivity imaging sensor, which has the ability to capture more light and an Image Processor, which actively allows high-speed image processing.
The 12.1 Megapixel sensors synergize with the advanced light reception technology to maximize sensitivity this enables low-light shooting, delivering clear, blur-free images. This expands the usable ISO range to an amazing high of 3200. The camera has a 5* zoom and 28mm wide angle lens. The zoom is operable during video shooting while the wide lens allows the camera to capture a wider area from stratosphere.
Other advantages of the camera are; it is water proof up to 33feet, temperature resistant (freeze proof) from 14°F to 104°F, shockproof up to 5.0 feet, a GPS tracker facility, image quality is ensured with the 12.1 Megapixel. Moreover, the camera is equipped with an Image Stabilization technology. This technology sieves shaky conditions by analyzing camera movement and applies the appropriate rectification methods by automatically applying the appropriate mode settings that facilitate the steadiest possible image.
In stratosphere this technology will ensure video quality shoot even if turbulence is experienced despite using a propeller to stabilize the vehicle. Other technologies will mitigate other possible risks e.g. vehicle may collapse and we may use the GPS tracker to track it on earth, collision, dropping into the ocean and other many risks are mitigated by the camera technology to protect the video output. The camera uses a lithium battery that with a 3hours endurance while shooting video in full power saving mode (Kelby 2012, pg. 114).
Conclusion
After a comparative study of three types of solar cells; Gallium arsenide-based solar cells, silicon based solar cells and multi-junction cells, the study concludes that the multi-junction solar cell are the most suitable for the lighter than air vehicle.
While the Gallium arsenide-based solar cells have 33% efficiency and the silicon based solar cells 25% efficiency, the multi-junction solar cell can achieve an efficiency of over 40%. Due to higher efficiency the multi-junction solar cell will consume 100% less space than silicon based solar cells and around 40% less space than Gallium arsenide-based solar cells.
Moreover, this will also reduce weight significantly considering weight power ratio. Multi-junction solar cells give the best weight power ratio. A 320Watt silicon based solar panel weighs around 18.6 Kg, 25*155*128 (mm) give an output of Nominal power 320watts. A multi-junction solar cell half the size and weight will give the same output (Ewing 2008, pg. 35).
In comparison to a solar impulse plane powered by four 10-horsepower (7 kW) electric motors powered by similar two lithium polymer battery packs is able to maintain flight for 1.5hours running on batteries only. A Yuneec Power Drive 200 20 kW (27 hp) electric motor equipped with two lithium-polymer battery packs (13 kg).
The propeller is used to run a 115kg kite plane including an average 80kg pilot. The battery packs provide 40 minutes endurance. Thus we can conclude among each of the four motors used in the solar impulse plane, individually none is more energy efficient than Yuneec Power Drive 200 20 kW (27 hp) electric motor.
Since considering the 4 motors can only manage 1.5hours of flight in this battery packs, a proper estimate would show that each propeller would only manage a 30 minute flight. This considered, the Yuneec Power Drive 200 20 kW (27 hp) electric motor is better for our lighter than air vehicle.
Considering a 320watt panel 1.5 hours charge can only power a Yuneec Power Drive 200 20 kW (27 hp) electric motor for a maximum of five minutes, to achieve maximum flight (40 minutes) the panel would have to charge the batteries for 12hours.
Hence for this reason, we have to add two more back up batteries, each similar to the two pack lithium polymer battery pack. We will also have to add five more 320 watt solar cells to the lighter than air vehicle. This is because the six 320watt solar panels will have the (328.2 volts) capacity to charge each of the two pack lithium polymer battery in less than an hour to increase flight time.
This would mean initially from earth at full charge, the vehicle would have exactly 2hours of battery life. Since the motor and the video camera which will also use the same back up power (after 3hours), would consume each battery pack in less than 40 minutes. The fastest charger has the capacity to charge a battery in 15minutes, also equipped with functionalities to avoid over charging the batteries. The six 320watt solar panels combined, have (328.2volts) which is enough capacity to charge the batteries in less than half an hour considering the higher capacity of solar energy on earth’s higher atmosphere.
The lighter than air vehicle will maintain an estimated flight time of maximum 14hours, considering 12hours daytime when the solar cells will work. At night time, the vehicle will run on batteries only (2hours capacity) without recharge from the panels. An electrical power grid storage system may be applied to maximize efficiency and increase flight time.
This system will feed the propeller and camera energy when the batteries are full, if the energy is not sufficient the devices can outsource more from the batteries. However, this will reduce battery drain significantly.
Moreover, after the charger blocks the solar panel’s power from overcharging the batteries, the excess power will still be used hence maximizing efficiency. This system may also allow us to increase more solar panels to the vehicle without the burden of having to increase batteries (and their weight).
Reference List
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