Perhaps the biggest recent item of news around the University has been the completion of the Green Heart, the new parkland in the centre. This has brought to an end many months of highly disruptive, noisy and unpleasant building work, and replaced it with a pleasant arrangement of grass, walkways, and bright lights. It is no understatement to say that the cost has been controversial, with Comment having already covered this topic in issue 1501.

I write not so much to repeat the point as to draw attention to one particular part of the Green Heart – the unusual collection of triangle paving that makes up the Pavegen system, installed at the end of the bridge section, next to the stairs. This unusual section generates power from the footsteps of people going over it, which it is claimed will be sufficient to charge mobile phones on the various benches.

The manufacturer of this system, Pavegen, features some glowing quotes from many prestigious media sources including the BBC, Forbes, and Bloomberg on their website. According to Pavegen, Forbes claimed that ‘Pavegen wants to become the Tesla of walking’ – a rather bold claim. And indeed, the idea of a paving system that not only withstands footfall but also generates power is an attractive one.

One must bear in mind, however, that things which seem too good to be true probably are. While it is true that the system produces 2 to 4 joules of energy per footstep, this figure with no context is utterly meaningless. According to Tesco a single 51 gram mars bar contains 228 kcal, or 953,952 joules of energy. In other words, just 1 gram of Mars Bar contains over 4000 times more energy than the Pavegen system generates from a step.

Perhaps food is an unfair comparison to make, since food (especially chocolate bars) do tend to be quite energy-dense. Let us instead consider the Pavegen system over a whole day. On a typical day, the Green Heart blog claims that 4000 people will walk over the section of paving. If we are generous and assume that each person takes 2 firm steps, and that the day is 8 hours long, that gives us a power of 1.1 Watts. Again, for context, even an energy efficient LED light bulb needs a power of 15 Watts.

Let us now compare this to another form of renewable energy – solar. According to theecoexpert one should expect to generate about 200 Watts from a mere square metre of photovoltaic panels, in good light. The installation cost for 1 square metre could not be found, but for 8 square metres on a roof the estimated cost is £1500 – £3000. By way of comparison, Pavegen’s founder, Laurence Kemball-Cook, claimed in May 2018  that the price for a square metre of Pavegen flooring could drop to ‘around £500 within the next few years’. It is likely that the University paid far more than quoted here for its system – the first tiles cost a whopping £20,000 per square metre. Even taking the lower figure, this would make the same area cost around £4000, £1000 more than the solar panels while producing (at least during daylight) over 100 times less power.

This is not where the problems end with the Pavegen system, however. If you have been to the area where the tiles reside, you will notice the substantial gaps of normal paving around the system. It is entirely possible, and indeed I have witnessed it on several occasions, to step right around the system, generating no power whatsoever.

Another potential problem could occur around maintenance. Unlike photovoltaic panels which have no moving parts and require almost no maintenance, the Pavegen system is made up entirely of moving parts, which are all potential failure points. Now it may be that the components have been made so strongly that they will not need replacement, but if they do, this will require at least closing off the path, causing at least some inconvenience to Green Heart users.

The numbers just do not stack up – it would seem that solar panels would have been a far better investment.