Anyone who has ever read Heat Magazine, or read about Heat Magazine, will be aware that its USP (or, more accurately, SP) focuses on multitudinous close-up shots of cellulite and slightly flabby bits and otherwise un-photoshopped imperfections of celebrities whose day job is to make the rest of us feel bad because they can afford airbrushing. Usually with big red circles around the offending areas.
This post is a bit like that, only with solar cars.
It started (as, sadly, so few things do) with a trip to the Science Museum in London, where we spotted this:
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Being the geek that I am, I sometimes like to sit down and watch Discovery Channel’s “How Stuff’s Made”. Last week, the program was on tortilla chips, combat knives and mattresses. Anyhow, this blog post will be like that, but CUER themed, and the “Stuff” in question is the canopy.
Aerodynamics is not a precise science. If it were, then all F1 cars would look and perform the same (given the same engine etc), and there would be no need for the teams to spend millions of pounds on wind tunnel testing and computational fluid dynamics (CFD), and Adrian Newey would be out of a job. However, it isn’t, and that makes designing an aerodynamic piece of bodywork somewhat of an art form, where the intuition of the designer can make a great deal of difference. Just like a good artist knows how to paint a house to make it look, well, like a house, so a good designer knows how to draw a form that is aerodynamic.
So, when CUER recognised through our own wind tunnel tests that the 2009 canopy wasn’t very good (it was shedding two big vortices), a fourth year project student turned solar car designer …
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Those ardent followers of not only our blog but also our twitter feed (@cuer2011) will have started to hear many names float about in reference to the battery. In deference to their loyalty, and also because it makes a reasonably interesting blog post about batteries, here is a breakdown of the components of CUER’s battery.
We begin with the smallest unit in a battery – the cell. This is a single lithium-iron-phosphate (LiFePO4) cell. A cell is essentially a block of chemistry – in its simplest form, it contains two metal electrodes and an electrolyte (here, LiFePO4). What we often refer to as a battery (e.g. Duracell, Energizer) is actually just a single cell. They work by undergoing oxidation and reduction reactions at the two electrodes; these reactions are complementary and require a flow of electrons. It is this flow of electrons that provides the ‘electricity’ from the cell.
Here is one of our cells:
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CFD stands for Computational Fluid Dynamics and is the source of all those beautifully-rendered, streamlined pictures generated and displayed whenever someone talks about aerodynamics. But what does it actually take to generate those pictures, and is that really the whole story?
The equations governing fluid flow are some of the most complicated and difficult to solve in any engineering discipline. The only way to actually write out an analytical solution by hand is to simplify the problem heavily (inviscid, irrotational, 2D planar, Newtonian etc.). The problem here is that these solutions only work as rough approximations to real life in very limited circumstances. The aerodynamic profile of a car, for instance, is not an easily-defined shape for which you can derive a solution on two sides of A4.
The above equation is a vector notation version of the Navier-Stokes equation governing fluid momentum and is highly nonlinear if all terms are considered.
To get around this we make approximations in a different way. Rather than treating the whole volume as containing the fluid in question, you break it down into tiny cells and consider each one individually.
If you consider a tiny cell such as a triangle, tetrahedron or simply …
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Vortices are beautiful.
As aerodynamicists, we strive for neat airflow. We want gentle pressure gradients and flow patterns that follow tidy, straight, parallel lines; boundary layers that stay nice and thin and never, ever separate; everything behaving as it should. Chaotic, spinning, turbulent flow is bad. It complicates the airflow, it increases drag – it mucks about.
But – vortices are beautiful:
Even the mathematics that describes them is beautiful – although this is a slightly more contentious claim, since beauty is in the eye (or the i) of the beholder, and never more so than when derived from first principles.…
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Following on from the article on solar cells, we continue to disperse the dense fog surrounding electrical engineering to reveal the technology behind CUER’s most-loved acronym* – the MPPT.
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 …
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A solar cell is fascinating, in that it is one of only a few machines that operate with no moving parts. This gives it a potential edge over other energy-generating technologies, which have to contend with energy losses and maintenance costs due to constant motion (e.g. turbines) and heat transfer (e.g. biofuels, nuclear).
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.…
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Aerodynamics (more generally known as fluid dynamics) is one of the more ‘user-friendly’ aspects of design. Most people have some instinctive understanding that cars (and anything else that moves) need to be ‘streamlined’. We don’t need to run a simulation to conclude that a torpedo is more aerodynamic than a brick. It’s obvious, we say. It’s common sense.
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:
One of the things that we felt was lacking on the original CUER blog was interaction with our readers. Although we built up quite a good regular readership, we got very few comments or feedback (perhaps our writing was just that good?)
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 …
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