So far, the Cambridge solar team has been following the status quo of solar racing – build the fastest car we can – and this entails building a car that looks rather similar to many other fast cars. However, the University of Cambridge has traditionally been the birthplace of novelty and innovation: the jet engine, evolution, the electron  - and we at CUER hope to carry on this tradition by focusing our research on developing new and exciting concepts that harness the power of the sun.

One such promising idea has provided a radical new method of harnessing the sun’s energy with greater efficiency than current silicon solar cells. The concept is based around a simple radiometer – a device that converts light into motion. The radiometer, or light mill, consists of a set of vanes mounted on a spindle. Each vane is a flat plate with one side painted matt black and the other a shiny silver. It works because of the nature of how different types of surfaces behave towards electromagnetic radiation. Dark, matt surfaces will absorb radiation and shiny silvered surfaces will reflect it.

Knowing this, a simple application of Hooke’s Laws of Motion dictates that a greater force will be exerted upon a particle of light that is reflected rather than absorbed, and that hence a greater force will be felt by the silvered vanes. This causes the radiometer to rotate.

So how is this adapted for solar vehicle technology? By running an optimisation algorithm, it is possible to calculate the number of radiometers needed to generate maximum rotational power, length of the arms, area of the vanes and so forth. The results of this optimisation were surprising – by increasing the arm length and vane area (and so increasing the moment applied) it was possible to generate torques and speeds comparable to that of a CSIRO electric motor. This new device has tentatively been named the ‘radiomotor’. Engineering estimates predict it is capable of producing torques as high as 4,000 pMNm using typical Australian sunlight levels.

The radiomotor provides a much more efficient method of converting sunlight into motion for two reasons: the first is thermodynamic, the second is quantum mechanical. The Second Law of Thermodynamics states that any energy transfer will never be 100% efficient – there will always be some energy loss. It therefore is beneficial to minimise the number of energy transfers required to convert sunlight into forward motion. Using solar cells, the light is converted into electrical energy, which is then stored in the battery as chemical energy, converted back into electrical energy and finally, in the motor, converted to kinetic energy powering the car forwards. At each stage of this, energy is lost.

In contrast, the radiomotor converts the light energy directly into kinetic energy and, as a result, a greater proportion of the absorbed light energy is retained, increasing its efficiency above that of solar cells.

The second advantage offered by the radiomotor is that it is capable of utilising energy from the full spectrum of sunlight. Solar cells are strongly limited by their ‘band gap’ – they can only make use of certain photons with the correct energy. In contrast, the principles underlying the operation of the radiomotor apply equally across the entire electromagnetic spectrum, allowing it not only to convert the energy of the visible light, but also that of the infrared and ultraviolet rays.

This basic concept can be enhanced using several well-known technologies. By mounting solar concentrators at a location where they will not shadow the vanes, an increased intensity of light can be focused on the radiomotor, increasing its speed and torque. Initial research has also found that it is possible to use mirrors to capture the reflected radiation and redirect it back towards the radiomotor, so that the energy from the sunlight can effectively be used twice. The motion of the radiomotor can be assumed to be negligible in this context, since the speed of light is so much greater than the rotational speed.

The one disadvantage of the radiomotor is that, unlike solar cells, it does contain moving parts which, in principle, generate friction. Often assumed to be ‘wasted energy’, this heat can in fact be harnessed by using some simple thermodynamic technology. The Stirling engine, originally conceived in the 19th century, is an extremely efficient engine that can be adapted to run on almost any heat source. Its mode of operation is dependent upon the expansion and compression of a working fluid operating at two very different temperature levels.

CUER’s adaptation to this involves mounting small Stirling engines along the axle, where the heat from friction is generated, using that heat to raise the temperature of the working fluid. Since the important variable is the temperature difference rather than absolute temperature, it is possible to get very high efficiencies from even a low rise above ambient temperature, simply by subsequently cooling the working fluid to very low levels. The new concept uses liquid helium to do this. An inert and safe substance, it has the advantage that, as it absorbs the heat from the working fluid and evaporates, it will expand to occupy an enclosed area within the shell and generate an internal lift force – in effect, making the vehicle lighter.

Current calculations based on engineering theory suggest that the incorporation of such a device in a solar vehicle, as a supplement to solar cells, could increase top speeds of the vehicle by as much as 20%. The above graph clearly illustrates the improvement in performance that can be gained by using a radiomotor – a fivefold increase at the highest levels of input.  CUER is currently recruiting students to undertake fourth year projects in this area next year. We hope to build a prototype as proof of concept, followed by the incorporation of the radiomotor into our 2013 vehicle.

Our cars are developed to race in the biennial ‘World Solar Challenge’, the ‘Formula One’ of eco-friendly motorsport, which is a 3000 kilometre race across the Australian Outback. In 2009, we entered the race for the first time as the only UK team. This year, we have developed a much improved car and, in May, we will be unveiling our new design at a special event in Cambridge.

To celebrate our new design, we are launching a competition, for school pupils aged from 6 to 14, to design a car for the future. Unlike our car, the designs will not be restricted to solar power, but we will still be looking for some exciting and innovative ideas (no idea is too silly!).

Entries will be judged, in three age categories, by some of our technical team. We will then shortlist entries from each category and those shortlisted will be invited to our design launch and ‘solar fun day’ in May. Category winners will be announced at this event, alongside our new design, and the overall winning design will feature on our completed car when it races in Australia in October.

Materials and information on the competition are attached and can be found at www.cuer.co.uk/future.

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.

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

It’s possible that this was due to

a) disinterest, or

b) a lack of understanding.

Option (b) is particularly disheartening. Science and engineering are wonderful, exciting, vibrant, ever-changing subjects, and yet so much of that wonder can be drowned out by the increasing complexity of technology. This is a huge barrier for us, since a large part of what we do is geared towards improving public understanding and appreciation of solar vehicle engineering and energy issues.

Therefore, in the spirit of simpler science, we plan on producing a series of articles on the anatomy of a solar car. These articles will explain some basic (and sometimes not-so-basic – I’m looking at you, Navier-Stokes equations…) engineering theory. They will bridge the gap between our engineers and our followers. Anyone can look at a solar car and see that it is crammed with innovation and promise. But when you understand what is really going on – and, more importantly, why - you will be amazed all over again.

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CUER welcomes suggestions from our readers. If there is a particular question you have about solar or renewable technology, let us know and we will try to address it.

To quote their press release, “the Sentience vehicle will calculate and follow an optimal driving strategy. Its control system adjusts vehicle speed, acceleration and deceleration via its adaptive cruise control.” – precisely what Bethany will do as she travels through the outback.

It’s great to see some of these ideas going on to road vehicles and it shows that some of the technology we’re working on is not just applicable to cars in the future, but is feasible right now! This simple principle of saving energy through driving style will apply regardless of the form of energy available, be it petrol, electric charge or solar radiation. What’s also very pleasing to see is that the Transport Research Laboratory has shown that this technology really works. Apparently, “in evening tests on public roads in ‘real-world’ conditions in the vicinity of TRL, achieved mean savings at all times in excess of 5 per cent.”

Of course the major hurdle that this technology will face is our human desire for freedom. If the speed of your vehicle is being controlled by computer, that takes some of the fun out of driving. However it’s quite possible to get around this: the on-board computer can suggest the most efficient driving style and thereby provide some extra ‘fun’ for the driver in trying to drive as efficiently as possible. Something very similar has just appeared in the UK in the form of the Honda Insight’s Eco Assist dashboard.

- Anthony