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The design of solar-powered cars and their challenges and prospects for the future
As concern grows over the course of the 21st century for the welfare of the planet, there is an increasing need for alternative sources of energy to polluting fossil fuels used since the Industrial Revolution. In 2014, the total fuel consumption of the UK alone was 193.4Mtoe1 and road transportation accounted for 21% of this total consumption1. Solar power, is one of a number of possible renewable fuel solutions.
1.CAPTURING SOLAR ENERGY
With current technology, solar energy can be converted to thermal (solar heating cells) or electrical energy. Solar-powered cars designed up until now use photovoltaic cells to directly convert solar energy to electricity.
The photovoltaic cell structure:
1: A photovoltaic cell
Photovoltaic cells work by using the properties of semi-conductor materials. A thin layer of semi-conductor material is treated so that it produces an electric field to make one side of it positive and one side negative.
The structure of a semi-conductor is shown in figure 2. Silicon is the most commonly used semi-conductor in PV cells. Silicon forms four covalent bonds with other silicon molecules to make a macromolecular lattice. As we approach absolute zero, as with insulating materials, all the
electrons in the covalent bonds are ‘stuck’ in their fixed energy levels and cannot move to carry charge. However, once minimum energy is applied (this minimum energy being known as the ‘band gap’), electrons can move out of
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the bonds and move freely to carry charge. The treatment of the semi-conductor which gives it the polarity previously described, means the ‘free’ electrons are attracted to the positive side and provided electrical conductor materials e.g. copper wires, are connected across the two sides, an electrical current will flow.
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Figure 4: Produced from data sources quoted in bibliography
1: Stanley E. Manahan, ‘Environmental Chemistry’ (6th edition, 1994)
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The efficiency of PV cells therefore depends upon the band gap of the semi-conductor as the semi-conductor will only be able to absorb wavelengths of sunlight which are shorter than the longest wavelength providing the photons with this minimum energy. Consequently, the efficiency using a cell with a single semi-conductor layer, known as a ‘single junction’ PV cell, is only around 11-12%1.This is only slightly higher than geothermal energy production which has the worst conversion efficiency of all sources of energy.
Finding solutions to the efficiency problem
Research is ongoing into the potential success of ‘multi-junction’ PV cells. This consists of layering up different semi-conductors with different band gaps in order to maximise the total percentage of sunlight absorbed.
The structure is shown below; possible semi-conductor materials include gallium arsenide, amorphous silicon and copper indium diselenide.
The different semi-conductors would be layered up in order of their ability to absorb light of increasingly long wavelength. Cell 1 therefore captures the highest energy photons e.g. silicon, band gap 1.17eV2, leaving lower energy photons to be captured by lower band-gap cells 2 and 3.
Experimental multi-junction cells have reached conversion efficiencies of 35%3. This is getting closer to the efficiency of other renewable resources, as shown in figure 4.
4: Comparison of conversion efficiency of different fuels
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Figure 4: Produced from data sources quoted in bibliography
1: Stanley E. Manahan, ‘Environmental Chemistry’ (6th edition, 1994)
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2. ADAPTING A VEHICLE TO SUIT SOLAR ENERGY
Another issue for photovoltaic cells is their lack of ability to store the electrical energy they produce since, by design, PV cells provide a continuous flow of current due to the applied polarity across them. This need for storage ties in with the reliability of PV cells since, like wind-generated electricity, the reliability of solar-generated electricity is diminished due to the intermittent intensity of sunlight therefore a source of energy in those off-peak times i.e. a way needs to be found to store solar energy.
a) Solar hydrogen
The first solution attempts to use hydrogen as a fuel for cars, sticking to the regular internal combustion set–up of a car. Solar generated energy is used to electrolyse water:
2H2O + solar-generated electrical energy → 2H2 (g) + O2(g)
The hydrogen gas product can then be combusted without pollutant products (water is considered a greenhouse gas but there is potential for it to be condensed with minimal temperature due to its high melting point of 273K):
2H2 (g) + O2 (g) → 2H2O (g)
5: An internal combustion engine adapted for hydrogen
One of the benefits of hydrogen combustion engines, is that the wide flammability range of hydrogen means it can combust in a lean mixture i.e. one where the stoichiometric air-fuel ratio is lower than theoretically ideal.1
However, the enthalpy change per mole of hydrogen combusted is only a fraction offossil fuel equivalents:
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Figure 6: Produced from data sources quoted in bibliography.
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b) An alternative solution is to store solar-generated electricity in batteries for use when light intensity is low.
Photovoltaic cells can be arranged to provide any desired voltage.
An individual photovoltaic cell can form part of a larger structure of other PV cells connected in series or parallel in order to achieve the terminal voltages desired. These structures are called modules. Multiple modules can form an array. These arrays are often referred to as photovoltaic or solar panels.
7: Photovoltaic macrostructure
Most previously modelled solar vehicles run on 80-170 volts1. Therefore a circuit could simply be made up of a number of identical solar modules in series. This can be proven by Kirchoff’s Laws:
Total voltage in a series circuit = sum of all the individual component voltages.
Total current in a series circuit = same at any point in the circuit loop
However, in practice, the fault with this series arrangement is that if one module or even PV cell is exposed to no or very little solar light, the current through it will decrease and, in turn, the current through the whole array will decrease or an effect called ‘hotspot heating’ may arise due to the high resistance produced by the low current cell.
There are two solutions available to this problem; bypass diodes, where each cell is connected in parallel with a semi-conductor diode so that current from working cells flows through the diode instead of the shaded cell. Blocking diodes work in a similar way but are in series with the cells and in reverse polarity to prevent current flowing back a different way if terminal voltage drops below the EMF of the battery being charged.Alternatively, cells can be arranged in parallel.
8: Blocking and bypass diodes
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In order to store the electrical energy, the whole circuit must therefore be connected to a battery which can be charged up. The most common batteries are made of nickel-cadmium or lithium ion as the standard lead acid batteries used by fossil-fuel powered cars are too heavy. These batteries are thenused to power electric motors in vehicles.
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The use of batteries can be modelled in two ways around the design of a car:
a) Only the battery itself is actually fixed to the car
This is really a natural progression on from the electric cars now widely available in the automobile market.
The car itself runs on an electric motor which is powered by a battery. Static photovoltaic panelsare set up in areas where maximum solar energy can be absorbed. The car then has a built in charging socket and leads to allow current to flow through the circuit and charge up the battery. This is the model that current commercial solar vehicles such as the Nissan Leaf use.
10: Static solar charging and a real life model in the Nissan Leaf
This approach to solar vehicles has proven success, particularly in the developing world. In rural Kenya, most residents survive with light from kerosene lamps and candles which are hazardous as potential fire risks and while many have mobile phones, they walk miles to get them charged up. ToughStuff is a company which sells small PV modules which can be fixed to roofs to charge phones or power lights. As of February 2011, 140,000 ToughStuff PV modules had been sold in Kenya as well as many ‘Business in a Box’ packs distributed in collaboration with NGOs such as Christian Aid to aid small business in rural communities. By 2015, ToughStuffaimed to distribute 33 million products to low income populations1.
b)The whole circuit is integrated into the vehicle itself
In other words, a photovoltaic panel is simply wired into a similar circuit to that of an electric car. The panel would be in series with the motor of the car:
11: Integration of a solar panel
When light intensity is high enough, the current flows directly to the electric motor and at the same time flows through the battery and charges it up for times of low light intensity.
Figure 10 & 11: (adapted) Leaf)
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MODELLING A VEHICLE AROUND AN INTEGRATED PHOTOVOLTAIC CIRCUIT
The integrated photovoltaic circuit offers more flexibility due to the panel being attached therefore length of journey a solar vehicle can go before it needs recharging is less of an issue. This is important as in a UK Department of Transport survey entitled ‘Public attitudes towards electric vehicles’, 40% of participants said that recharging was one of the most important issues that they would consider when buying an electric vehicle1.
a)Incline
If a solar array was being used to power, for example, a house, it would be advisable to orientate the panel to the same angle as the latitude of the house. For example, London has a latitude of 51⁰ therefore, on a house, the angle of solar array which would absorb maximum sunlight would be 51⁰ to the horizontal. This is known as the ‘array tilt angle’.
12: Array tilt angle
Similarly, the solar azimuth angle would have to be considered. This is the compass direction from which sunlight is coming; true south,i.e. azimuth = 0is considered to be the optimum facing position for maximum absorption.
13: Azimuth angle
These factors present a challenge for solar vehicles which are constantly moving. For short distances it makes no difference to efficiency of output as the change in latitude would be negligible. However over long distances these angles could change. A solution to this is a solar tilt rack which allows the elevation angle and azimuth to be adjusted manually; these are normally fixed to the roofs of buildings but could be integrated into a car design similar to the way a roof box is fixed on.
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14: Solar tilt rack and a possible mechanism of screws and roof rails to allow it to be attached to a car like a roof rack.
e.g. A car travelling in London1:
Latitude = 51⁰, altitude = 5m, declination = 43⁰
Azimuth = = 2.7⁰
Based on this, the tilt rack should be at 51⁰ to the horizontal and facing 2.7⁰ east of true south. This would allow the panel to absorb the maximum intensity of 2.7kWh/m2/day.2
b) Aerodynamics
One problem that could clearly arise from the optimisation of tilt angles of the array is its effect on aerodynamics. A large, flat panel of solar cells will have a high surface area and simply attached at an angle will result in a high resistive drag force on the car. This does not bode well considering the conversion efficiency of a solar cell. A possible solution to this is to encase the array within a polycarbonate case thus the transparency would allow absorption. Polycarbonate is a good choice as it has a low density2 of 1.2x10-5 kgm-3but is shatterproof and resistant to both organic and inorganic chemicals and therefore often used in windows.
Therefore, a curved shape would be favourable to minimise the surface area in contact with the air per second to minimise drag force such as is used in Formula One racing cars:
15: F1 car with notable curvature of the bonnet
c) Materials for construction
Ideally, a solar vehicle is very light as, based on the equation v = , at a high mass more KE energy is required to achieve the same velocity and since a solar vehicle relies on, as we have
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previously discovered as less efficient and reliable source of energy than fossil fuels, it is beneficial to keep mass low.
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For that reason, solar cars are usually made of carbon fibre or fibre glass and titanium composite materials which are often used in the construction of fighter jets.
Materials such as carbon fibre and titanium composites have a high strength to weight ratio (or specific strength) compared to other strong materials such as steel.
If the Young’s Modulus of steel and a carbon fibre composite are compared using the equation
E =
E (steel) = 205GPa whereas E (carbon fibre) = 142GPa however the specific strength of steel is only 0.073Nmkg-1 compared to carbon fibre which is 1.08Nmkg-1 which is almost 15 times greater than steel1.
Titanium and carbon fibre composites have a high cost due to their manufacture which involves high processes such as fractional distillation of crude oil for carbon fibre.2 This is potentially a downfall for solar-vehicles as despite their renewable source of energy; non-renewable resources such as crude oil have to be used to manufacture the materials made for them. By 2030 it is predicted that 1.85x1017J of energy will have gone into carbon fibre production2. However, in less time, worryingly, the world has consumed an estimated 2.9x1012 times that figure of energy2.
THE FUTURE
Overall, we have seen that there is both potential for solar vehicles to be built and a gap in the market for them too. Indeed, some companies have delved into the possibilities for solar cars however, there is perhaps more to be to done to make them as practical as fossil-fuelled cars. Unfortunately, one of the main hindrances to the mass production of solar vehicles is cost incurred by the materials required to make them and the advanced technology within them. However, as with all advances in technology, it is likely that with ongoing research, more advanced solar technology will be available to the mass market. For example, in 1945, only 10,000 households in the USA owned a television set, by 1960, it was 60 million; a 6000% increase4. Therefore, with a looming energy crisis, perhaps it is inevitable that we will see more cars reliant on renewable energy.
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BIBLIOGRAPHY
The UK government website for National Statistics: Energy consumption
Stanley E. Manahan, ‘Environmental Chemistry’ (6th edition, 1994)
‘Introduction to Building’: Osbourn and Greeno (4th edition, 2012)
The workings of photovoltaic cells:
Data for figure 4:
Hydrogen combustion:
Integrating PV cells:
Data for figure 5:
Solar car adaptions:
Case study of use of solar energy:
Angle calculation:
Material statistics: The Engineering Toolbox, The British Plastics Federation
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1Megan Kendall