This is not another article about Why This Is The End For Nuclear or How Fukushima Made Me Stop Worrying And Love The Bomb. They are already clogging the internet and attracting all sorts of insane online versions of the sandwich board screaming “REPENT. THE END IS NIGH”. Instead, it is an attempt (of which there have also been many) to inject some perspective, some rationality and some information into the energy debate, which has been running for far longer than the past two weeks.

Although CUER is a group firmly focused on renewables, between us we do hold an interest in many other forms of power generation – of which just one is nuclear. CUED runs a particularly popular course on Nuclear Engineering and for many of us it is an eye-opener. The study of a subject at university level is probably as unbiased an exposure as it is possible to get these days and is, one would hope, sheltered from excessive outside influence by Big Oil or Nuclear Shills or Chuck Norris. It’s difficult to lie about something when you must derive it from first principles, and if you try, there is always someone smart enough to spot it. You know – the annoying kid that sits right in the front row and always corrects the lecturer, and who never leaves the university until s/he becomes the lecturer, and suffers the indignity of being called out on every missed minus sign by the next generation of academics. The kind of person who cannot so easily be bought, because if they cared about money, they would have chosen a career path other than academia. The engineers who can be bought (and it’s a good 20% of the intake) usually end up working for Goldman Sachs. It’s a very neat system of quality control.

However – after receiving such an education from people who are usually world experts in the field, it becomes rather frustrating to see misinformation and rumour spread to such a degree as has lately been the case. It gets to the stage where a sort of triage is necessary to weed out the is-this-the-next-Chernobyl crowd, the Sellafield-gave-my-dog-cancer crowd (closely related to the power-lines-gave-me-hemorrhoids crowd) and the why-don’t-we-just-use-cold-fusion-it-really-works* crowd. There are many valid arguments against the proliferation of nuclear power. These are not they.

Everything carries risk. Fossil fuel power carries risk. Coal-fired power stations transmit more radiation to their surrounding environment than do nuclear power stations of the same energy output. Renewables carry risk. While the world’s media was distracted with the panic over impending doom bubbling away at Fukushima Daiichi, it seemed to miss the collapse of a hydroelectric dam which, according to reports, swept away more than 1800 homes and killed many.  As engineers, we can never eliminate that risk, but we can manage it, and we can design for it. This is the approach taken with nuclear power. Discussion of Chernobyl is often irrelevant to modern reactors. Not only was the reactor poorly designed, but the safety systems and procedures in place were not properly followed. Moreover, before 1986, there was no fear of nuclear power – there was no ‘Chernobyl incident’ to act as a focus for public opposition. These days – even the mention of the word ‘nuclear’ is enough to set off an unfounded panic. Anyone trying to convince a government to allow them to build a reactor had better be very sure of themselves. Safety systems within safety systems – all of them passive – that is, that require no activation but will, by default, engage automatically unless they are prevented from doing so – are the norm in the newest reactor designs.

Sadly, these new designs are currently just that – designs – because so far few governments have had the courage to stand up to vehement public opposition to nuclear power. Three Mile Island was one of many nails in the coffin of the US nuclear industry. Although the release of radioactive material following the meltdown was minimal and had little effect on the surrounding environment, the close-to-home incident was enough to set off several enormous anti-nuclear protests. No doubt this was exacerbated by the release of the movie The China Syndrome only days before the incident. The result of this kind of unfounded opposition has been to block the construction of newer, safer reactors and instead prolong the life of older reactors that should be decomissioned, because we need the power.

Many of those holding anti-nuclear views do not necessarily have an engineer’s understanding of other sources of power generation. CUER itself has long documented the struggles with efficiency and the high costs imposed by solar technology. Wind power, tidal power, hydroelectric – all these energy sources are valuable, and should be exploited, but they are not risk-free and neither are they a power generation golden egg. One of the biggest problems with abandoning fossil fuel power generation is that of load following. A power plant does not just sit there unresponsive – it will typically generate a constant baseload and respond to dips and surges in energy demand – the classic “30 million people putting the kettle on at half time” scenario. Currently, fossil fuel power stations are the only power source we have capable of doing this. True, there are potential solutions involving pumped-storage or an international energy grid. These are viable alternatives that should be explored and developed – but that will take time. Nuclear energy is one option – of many – that may help us bridge the gap between our the unsustainable present and the necessarily sustainable future.

Engineers work with scientific evidence, with mathematical modelling, with probabilities and safety factors and we had better make damn sure we get it right, because lives depend upon it. A modern nuclear reactor is not something that has been cobbled together by Acme for an anthropomorphised roadrunner-obsessed canine. There are problems with the nuclear industry – waste disposal, proper regulation, control of radioactive material – these should be faced, addressed and solved. That is what engineers do. The World Trade Centre remained standing for a reasonable amount of time even after it got hit by over 60,000 kg of burning aircraft, because a bunch of engineers thought about the worst possible loading it could suffer, and then overdesigned. Fukushima Daiichi survived an earthquake 10 times stronger than it was designed to withstand. Nuclear power may have started out as a US-Soviet undercover plutonium production racket, but it is well-controlled and well-integrated into our energy mix today.

It is therefore highly frustrating to watch so much science and considered design get thrown out of the window because somewhere along the way a news story about an overheating but still contained reactor snowballs into FUKUSHIMA TO BECOME A NUCLEAR WASTELAND UNLESS CHINA PANIC-BUYS SALT. It is frustrating when offshore wind farms are dismissed because someone on Question Time heard some guy say that they beach whales and kill fairies. It is frustrating when renewable transport alternatives are ignored because global warming is just a conspiracy between all the climate scientists in the world and communist zombies. All these things are branches of the same, scientifically illiterate tree. It seems that so much science policy today is made as a result of bad advice or public pressure – and that is wrong. We need to come up with energy solutions – not just for power generation but for transport – and decisions on what routes to pursue, especially at a time of such restricted funding, should be made rationally.

By all means, dismiss nuclear power as a viable option for the future – but please provide evidence and alternatives. Otherwise, you’re just one more irresponsible internet drone who causes real fear and real detriment by joining in with the chorus of uninformed voices telling anyone who will listen that standing next to a microwave when you’re pregnant will make your baby gay. And that somehow that’s a bad thing.

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*It doesn’t. It really doesn’t.

See here for an interesting look at relative radiation doses.

First off, let’s get the formalities out of the way. ‘MPPT’ stands for ‘Maximum Power Point Tracker’. This may have been mentioned in previous blog posts, possibly in an attempt to clear away the aforementioned fog. It’s not entirely certain why they thought it would help. It’s unlikely that the response to this revelation was “oh, Maximum Power Point Trackers – they track the maximum power point! Of course! It’s all so clear!” No, this is a PR challenge even Ronseal would struggle with.

However, unlike the average Ronseal customer (or perhaps not?) we are in a position to understand the relationship between the photons reaching a solar cell, and the amount of useful energy we can get out of it. However energy on its own is not a useful measure. A solar array could provide 1kJ of energy – in fact, they all will, if you wait long enough – but an array that produces 1kJ in 0.5s is better than one that produces it in 20s.

Power, then, is a better measure of solar cell effectiveness – it tells us how much energy a cell is capable of generating in a given time. The greater the power of the cells, the more energy is available each second to be converted into e.g. forward motion via a motor.

Electrical power is calculated by multiplying the voltage by the current. As we have seen, the voltage of a solar cell can vary depending on what material it is made from. There are also other factors influencing this, temperature being a good example.

The current is a measure of how many electrons flow through a circuit in a given time. Again, this depends on things like temperature, internal resistance of the cell, resistance of the overall circuit, and, of course, the proportion of photons that are creating free electrons in the first place.

Since both current and voltage can fluctuate quite regularly during normal operation, the power must therefore also fluctuate. Current and voltage are related to each other, so that if one changes the other will also change. In a solar cell this relationship is exponential – but the exact nature of that exponential relationship will change depending on factors such as solar intensity and even the unique characteristics of individual cells. This is more clearly expressed in graphical form:

It’s worth noting, from looking at the above graph, the specific relationship between current and voltage. At first, as current decreases gradually, voltage rises rapidly, so I*V (power) will also increase. However, once a certain point is reached, the current decreases more rapidly with small voltage increases. At the point indicated by the black line, the decrease in current outweighs the increase in voltage and the power value no longer increases. In other words, the black line marks the point of maximum power (aha!)

The upshot of all this is:

• Current and voltage vary in quite complicated but predictable ways
• Power generated by the cell therefore also varies, such that it has a maximum possible value.

The question is: can we manipulate the current and/or voltage in response to these variations, such that we are always getting the maximum possible power?

Yes we can! This is what a Maximum Power Point Tracker does. Control circuits within the device allow it to work out the maximum possible power, and what values of current and voltage are required. Both voltage and current are related to a third factor – resistance. By changing the resistance of the circuit through which the current travels, both voltage and current values can be adjusted. An MPPT is capable of artificially manipulating the resistance in order to control the values of voltage and current.

The function of the MPPT is therefore twofold:

• By measuring instantaneous values of current and voltage, it calculates the relationship between them and works out the optimal values of V and I to gain maximum power
• It imposes an electrical load resistance on the solar array, to alter the values of V (and therefore I) to these optimal values, thus ensuring that the maximum possible power is always extracted.

Because the I-V relationships of different solar cells/panels/modules vary, it is sensible to have a large number of MPPTs, each tailored to an individual module of cells. This is what happens on Endeavour – with five separate solar modules, it contains five MPPTs to monitor each one:

In a system such as that used in Endeavour, whereby the solar array feeds a battery, there is a danger that the battery can become ‘overcharged’. Using very unscientific terms:

it’s as though the battery is full up with energy, and can’t accommodate any more. Since the solar array can’t just stop producing energy, the MPPT must exactly match supply with demand so that energy is added to the battery at the same rate it is being fed to the motor. In this way the battery does not become overloaded.

This mode of functioning requires a diversion away from the maximum power point, but the MPPT still has the capabilities to do the job.

An MPPT can also provide additional advantages when used to drive an electric motor directly. Because they can adjust the voltage of the cells, they can be used like a transmission. When starting a vehicle, a high torque (low gear) is required to get it going, corresponding to a low voltage/high current situation. The MPPTs can do this, then raise the voltage to normal levels once the car has got going.

And there you have it: MPPT. Maximum Power Point Tracker.

Does exactly what it says on the tin.

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*After ‘CUER’, that is.

The basic premise behind a solar cell is identical to that behind any type of chemical cell or battery – the separation of regions of different electrical potential. An electric current flows when these regions are connected to each other by an electrical circuit, allowing the negatively-charged electrons to flow towards the positive terminal. In a solar cell, this is achieved by the use of semiconductor materials – a certain class of non-metals that, under certain conditions, can conduct electricity.

The most common solar cells use silicon as their base material.

Silicon has a valence of four – four electrons in its outer shell. It can create a bond using each electron, so there are four possible bonds available. In its pure form, therefore, silicon atoms can bond with each other to create a single crystal structure in which every electron is used up in a bond. However, when silicon is doped with impurities – other atoms such as phosphorous or boron – that have different numbers of outer electrons (5 for P, 3 for B), things change.

Phosphorous forms four bonds with silicon but the fifth electron cannot be used, so with the addition of energy (e.g. heat) these extra electrons can be knocked loose of their atoms. Once this happens we have a source of free-moving charged particles capable of carrying an electrical current. The phosphorous atoms left behind become positively charged ions*.

Phosphorous-doped silicon is an n-type material – ‘negative’ – because of this oversupply of electrons. Boron-doped silicon is p-type – ‘positive’**. Boron, with only three outer electrons, cannot form the fourth bond with silicon. There is an electron deficit, also known as a ‘hole’. When an extra electron is provided to form that bond (resulting in an ‘electron-hole pair’), a negative ion is formed.

When n- and p-type materials come into contact, the excess n-side electrons move towards the p-side to bond with the holes. The electrons/holes closest to the boundary are the first to form pairs. However, once this region around the n/p boundary accumulates electron-hole pairs, it becomes more difficult for electrons to cross that boundary and reach the free holes on the other side. This is because the electrons that bond with holes on the p-side form negatively charged ions, and leave behind positively-charged ions on the n-side. The p-side ions repel electrons, and the n-side attracts them.

The result of this is an electric field that opposes the initial movement of electrons, so that electrons can only flow from p to n and not the other way about. This zone  is known as the depletion zone, and acts as a ‘barrier’ separating the n- and p-sides.

The importance of photons

Where does light come in? Light is made up of photons – individual light particles. Each photon has a fixed wavelength, and a fixed amount of energy available that corresponds to that wavelength. Photons can be absorbed by electrons and impart that energy to them. If the energy is great enough, the electrons can break free of their bonds.

These electrons may then be influenced by the electric field of the depletion zone and travel to the n-side of the material, while the ‘hole’ remains on the p-side. As a result there is a build-up of electrons on the n-side and an excess of holes on the p-side – a potential difference exists between the two sides. When an external current path is provided (i.e. a circuit linking n to p) the electrons flow through the circuit to the p-side, driven by this voltage.

This is where the electric current comes from. The photons absorbed by the solar cell constantly produce free electrons that then travel through the circuit to the p-side, where they may form an electron-hole pair before once again being liberated by the absorption of a photon. In this way a solar cell provides a potentially unlimited supply of energy.

Efficiency

The problem with this basic silicon cell layout is one of efficiency. Sunlight is made up of a range of wavelengths and therefore contains a variety of protons with different energies. A very specific amount of energy is required to release an electron from its bonds. This energy is different for every material and is known as the band gap energy for that material.

The second important point is that a photon represents a quantum of energy – a single packet that cannot be divided up – so an electron has no choice but to absorb the entire energy of a photon. Certain photons won’t have enough energy to free an electron. Any photons with insufficient energy have no useful impact. They pass through the solar cell. Those photons with an energy greater than the band gap do release electrons, but the extra energy is imparted to the electron as kinetic energy, which rapidly becomes wasted as heat.

These effects together account for around 70% of energy loss.

Using a material with a lower band gap would mean that fewer photons are totally wasted. Unfortunately, this has the result of reducing the voltage of the cell.

There are also problems connecting the external circuit to the cell since silicon is not a good conductor. We want to make it as easy as possible for the free electrons in the n-side to reach the electrical contacts and flow through our circuit to the p-side. This is done using a metallic contact grid. Nevertheless the resistance of the semiconductor material (through which the electrons must travel  to reach the contacts) and the presence of the grid blocking out some photons lead to extra losses.

Finally, not all electrons released from their bonds may travel to the n-side – they may recombine with another hole on the p-side – in which case the energy of the photon is wasted.

Multijunction Cells

The World Solar Challenge has two sub-categories of vehicle corresponding to the type of solar array they use. Vehicles using silicon cell technology (such as Endeavour) are evaluated separately from those using Gallium Arsenide cells or similar (such as Nuna). This is because GaAs cells are multijunction cells, and are therefore much more efficient.

As mentioned above, one problem with solar cells is that they have a specific band gap – a fixed energy required from photons to release electrons. This causes a lot of photon energy to be wasted, if photons have too much or too little energy. A multijunction cell attempts to remedy this by using two or three different semiconductor materials, all with different band gaps. They are layered on top of each other, with the highest band gap at the top (so that photons with lower energies can pass through and be absorbed further down).

The overall result of this is that multijunction cells can generate electrical energy from a greater range of wavelengths of light – fewer photons are wasted – so they are much more efficient. However, due to difficulties with manufacturing, they are also much more expensive, about 100 times more so than silicon.

Because of this, the WSC awards a prize for Best Silicon Cell Finish, since it recognises that these vehicles are at a disadvantage compared with multijunction cell users. However, in the 2011 race, the rules have been changed to reflect the disparity between Si and GaAs cells. Any car using GaAs cells will be restricted to half the array area of a silicon car.

Solar Concentrators – the importance of intensity

Intensity of light is a measure of how many photons strike a given area of cells in a given amount of time. In a solar cell, the number of electrons available to do electrical work is proportional to the number of photons absorbed by the cell. A higher intensity of light will produce a bigger current, so the aim is to gather as many photons per second as possible. This can be done by increasing the total area of the solar array – but this is restricted in the WSC.

Instead, a solar concentrator may be used. The Michigan solar car Continuum is a particularly good example of this. Parabolic reflectors collect light and focus it onto a much smaller area – increasing the intensity by a huge amount.

The alterations in the WSC regulations mean that some teams may decide to keep a 6m^2  car, comprising 3m^2 of concentrators and 3m^2 of gallium arsenide cells.

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*An ion is a charged particle. A phosphorous atom is electrically neutral, since it has fifteen protons in its nucleus, each with a positive charge, and fifteen electrons in orbitals around it, each with a negative charge. When an electron is lost from phosphorous in a solar cell, these charges no longer balance out and it has an extra positive charge, turning it into an ion. The equivalent happens when boron gains the extra electron, becoming a negative ion.

**It’s important to note that these materials don’t actually have an overall charge initially. The ‘n’ and ‘p’ simply refer to the fact that one has too many electrons to form bonds, the other too few. It’s very misleading. This is why I quit electronics.

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Image Credits

1. Electronics Tutorials
2. Images Scientific Instruments
3. The Future of Things

It is probably because of this everyday familiarity of the concept – or at least the language – of aerodynamics that BMW chose a slightly different approach in their latest ad campaign, in an attempt to sound hi-tech:

Transcript:

Is this the car of the future?

It’s made of lightweight materials.

It uses fuel…intelligently.

And…

It manipulates…the wind.

Really? It manipulates the wind? That’s clever.

Although most people are indeed familiar with words like ‘aerodynamic’, ‘streamlined’, ‘air resistance’, ‘drag’ (we’ll leave ‘wind manipulation’ for a bit) – what do they actually mean? Why should a torpedo be more aerodynamic than a brick? What is the science behind our instinct to streamline things?

We’ll start with ‘air resistance’, otherwise known as ‘aerodynamic drag’. There are actually two main sources of drag – skin friction drag and pressure drag. Both can ultimately be traced back to a single phenomenon known as the boundary layer.

The boundary layer exists as a result of the viscosity of air. At the surface of a body, the velocity of the air molecules must be zero (relative to the body), due to frictional forces. This is known as the “no-slip condition”. In the free stream, the air is moving at a (mostly) uniform velocity. Therefore, somewhere between the surface and the free stream, there must be a gradual increase in velocity. This region is known as the boundary layer.

The boundary layer characteristics (thickness, type) are related to a measurement known as the Reynolds number, which itself is derived from three factors:

- the density of the fluid

- the speed at which the fluid travels relative to the body

- the distance along the body the fluid has travelled.

A boundary layer may be laminar or turbulent. Briefly, a laminar boundary layer occurs at low speeds and densities, and can be thought of as consisting of horizontal motion of air molecules, in neat ‘layers’ of increasing velocity. Turbulent boundary layers are associated with high speeds and densities and – obviously – are more turbulent, displaying large-scale vertical and horizontal unsteady motion. It is very difficult to keep a boundary layer laminar, as even very small discontinuities in a surface can be sufficient to cause transition to turbulence.

Skin friction drag arises from friction – shear forces – between the molecules in the boundary layer and the surface of the car. It depends on factors such as the total surface area of the car and the type of surface – a smooth surface will produce less drag than a rougher surface. Why? Because a rough surface will cause protrusions into the boundary layer, disrupting it and increasing the frictional force. It is for this reason that solar car teams go to such lengths to encapsulate their cars, forming a smooth uninterrupted surface. Additionally, turbulent boundary layers result in higher skin friction drag, so maintaining laminar flow for as long as possible is also important. Encapsulation helps with this, too, since with many solar vehicles, the discontinuity presented by a solar array can often cause transition to turbulence.

Back to the brick. It has a pretty rough surface, so we know that it will have a fairly high level of skin friction. We can smooth that out, so that it has a perfect surface, but it’s still unlikely that it will match up to a streamlined torpedo. A brick is not streamlined. It’s a brick. It’s got lovely perpendicular sides and right angles, which is great for house-building but won’t get it a cameo in Das Boot. This is where pressure drag comes in.

Sharp corners on a body present an ‘obstacle’ to boundary layers. The pointier the corner, the more of a problem it can be. This is to do with the pressure distribution around a moving body, caused by those corners. As an example, here is the pressure distribution around a sphere in inviscid (frictionless – so no boundary layer) flow:

Essentially, a concave curve away from the body represents an area of high pressure, and a convex curve towards the body represents an area of low pressure. This is due to Bernoulli’s Principle, since a directional change in airflow causes a change in velocity. The greater the degree of curvature, the greater the pressure difference, so on a body with sharp corners, such as a brick, the pressure variation over the body will be more extreme.

Now, a boundary layer on the front half of such a body has no problem, because the air is travelling from an area of high pressure to an area of low pressure – along a positive pressure gradient – and so is ‘pushed’ along. On the back half of the sphere/brick, however, this changes. The air is now travelling from low to high pressure – an adverse pressure gradient that opposes the airflow – and this has the effect of reducing the velocity of the air in the boundary layer – so much so that, eventually, it starts to go backwards. The boundary layer separates from the body. Instead, an area of unsteady, chaotic flow is formed – the wake.

This causes a problem. Looking back at the pressure diagrams, we can see that the area of high pressure at the nose of the body is balanced out by an equivalent area at the rear. If the boundary layer separates, this area is no longer present, replaced by a lower-pressure wake. The result? Pressure drag:

We can’t eliminate pressure drag, but we can reduce it. This is done by encouraging a much gentler adverse pressure gradient (APG). How? By reducing the sharpness of corners, blending everything into everything else – by streamlining. The boundary layer will separate further along the body and the area of the wake is reduced, along with the corresponding drag.

So, let’s streamline our sphere-brick:

Unfortunately for us, it no longer looks like a brick. Instead, it looks quite similar to the body of a solar car or a cross section of a wing – both of which belong to a class of objects known as aerofoils. It has lost its symmetry because it is the back half – the APG – that is causing the most problems.

Pressure drag tends to be the dominant source of drag, so teams will trade off a greater surface area for a smaller wake.

Now, aerodynamics does not end there. Even more complex things happen when you duct-tape glue fairings and a canopy onto that solar car body shape. The presence of the road has a strong aerodynamic effect on the vehicle. There are books written about the aerodynamics of spinning wheels. Wings are a whole other story entirely. Once the sound barrier is broken (unlikely, in a solar car) the rules are almost completely rewritten.

However, hopefully this has opened a window onto the challenges faced by the Aero Team. Air (and any other fluid) is such a wonderfully complex substance that the mathematics involved is incredibly nasty:

As a result, most of our aerodynamic research is qualitative – not dealing in numerical values other than experimental ones. CFD modelling is one tool that allows us to predict how exactly the air will behave around a car, but – with so many factors involved – it can never be completely relied on. Wind tunnel testing is absolutely fraught with interfering variables – the most obvious being that while we can scale down our car, we can’t scale down the boundary layer that goes with it, so our results will never be completely accurate.

Finally, from reading this article, you should have concluded (rightly) that it is very difficult to build a vehicle that doesn’t manipulate the wind in some way. It’s very difficult to build anything that doesn’t. And that therefore, BMW has wasted three very expensive seconds of airtime on informing us that its all-new car-of-the-future range has the same aerodynamic qualities as a brick.

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For those who are keen to explore further:

• Stunning flow visualisation photographs
• Vortex streets (for those wondering what the circular pattern at the rear of the aerofoil image is – see Satellite Eye on Earth for some really amazing examples.)
• Introduction to Reynolds Numbers
• More on boundary layers

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Image Credits

1. Nasa Glenn Research Centre
2. Aerospace Web
3. Aerospace Web
4. Coilgun Systems
5. Plus Magazine, Supersonic Bloodhound - Ben Evans

Solar Impulse

This is a particularly interesting project for many reasons, not least of all because air travel is a major concern for many environmental reasons, whilst it is regarded as contributing not all that much CO2 compared to other sources (power generation and sea freight) it does release emissions directly into the upper atmosphere, where it can have a damaging effect, thus to raise the profile of sustainable technology in this way is admirable, and not all that dissimilar to our aims with our solar powered car entry into the Global Green Challenge (http://www.globalgreenchallenge.com.au/).Furthermore, to an aerospace engineer such as me, this project has very many exciting challenges.  For instance, the wingspan is quoted at around 61 metres in length, that is about the same as an Airbus A340! This is done for a very good reason, high aspect ratio wings (wingspan >> chord length) have a lower induced drag term, and so require much less power; this is why gliders have such long narrow wings (it is seen less on powered aircraft since the wings are used to store fuel and take much greater loads).The project seems to be advancing well, with the website stating that load testing on the main wing spars has been completed, this is very important, since with wings of this length the self weight of the wings will generate a significant moment, and once the wings are loaded there is the possibility of large tip deflections, leading to potential fatigue problems.The large wing area will provide the space for the solar array, although the plane will only be travelling at around 70 mph - not a lot faster than our car - so it will require considerably more energy (around 30kW compared to our 1kW).  Batteries will also be a key part, as they will need to store enough energy for overnight, and will need to be as light as possible (the all up weight of the aircraft is meant to be below 1,500 kg), but I’ll leave discussion of solar cells and batteries for somebody better qualified than myself.It is also interesting to look at previous solar powered planes. NASA have built a variety of unmanned solar and fuel celled vehicles (check out the NASA Pathfinder Aircraft), as early as the mid 1980′s.NASA Pathfinder Plus

Once I’ve read more on this subject I hope to bring you some more details, for now I hope this has further whetted your appetite for the development of solar powered transport.

- Mike