A fun ending to Year 1 of the Eco Travel Network project here in the Brecon Beacons – The Prospectory’s varied life of eco projects and field trials continues….
Twizonomics 1
A leaflet dropped through my door offering an all inclusive (VAT + installation) price of £3461 for 1.44kWp of solar PV. I already have a 3.98kWp PV, which generated about 3000kWh in the year up to June 2012, so a 1.44kWp would have produced 3000 x 1.44 / 3.96 = 1090kWh of electricity.
I also have a Renault Twizy. While it’s not a “proper” car, it meets most of my local transport needs with the same speed and convenience. Most of my personal travel, and about two-thirds of my annual mileage, is local: round trips of 30-40 miles or less. The Twizy carries two of us for an electricity cost of about 7 miles per kWh, so 1090 kWh of electricity is enough for 7,500 miles. This is more than we do, and more than our battery rental contract with Renault specifies.
So for the cost of a Twizy and a half – just over £10k – I can buy a “solution” to my local travel needs that offers “free fuel” and zero carbon emissions! But what about the other running costs?
As a member of the Eco Travel Network my Twizy costs about £1200 a year in insurance, battery rental, maintenance and roadside recovery. The cost of the electricity to charge it is about £90 a year and is relatively minor. Using the AA’s methodology, at 4500 miles annually, our Twizy costs us 26p per mile in standing charges (excluding finance and depreciation), and 5p a mile in running costs which include fuel, tyres and parking charges. This comes to about 31p a mile in total, and compares well with the 40p a mile the AA estimates are the costs for a small petrol car doing 5000 miles a year.
So there are both environmental and financial reasons for buying a Twizy for local trips, but by itself the financial case isn’t as compelling as fuel costs alone would suggest.
But what if we consider the running costs of a Twizy plus a 1.44kWp PV panel?
This isn’t simple to work out, because a PV system repays your investment in three ways:
1. The government-regulated “feed in tariff” (FiT) which is currently 16p per kWh generated.
2. 3p for every kWh you “export” to the grid (estimated to be half what you generate).
3. Savings on your bill for the electricity you managed to use while you were generating.
You obviously consume more electricity if you have a Twizy, and unless you manage to do all your re-charging while your PV panel is generating, that will increase your annual electricity bill by up to £90. Furthermore, a 1.44kWp PV panel, even at maximum output, doesn’t produce all the electricity a Twizy charger takes in real time. It will, over the course of a year, put back into the grid more than the Twizy takes out, just not necessarily while the Twizy is actually charging.
The annual output of the PV panel, at 1090 kWh, will pay you £174 per annum in FiT. The electricity company assumes you export half your production (i.e. 545 kWh) and pays you 3p a unit for it, which comes to £16 a year. And since your electricity supplier assumes that you export half of what you generate, it seems reasonable to assume that you use the other half. 545kWh, at the rate your electricity supplier charges YOU for daytime electricity, would cost you £76 per annum at the current average price. This puts the total annual financial contribution of your 1.44kWp PV panel to £174 (FiT) + £16 (export) + £76 (reduction on electricity bill) or £266 per annum.
That lowers the annual net running costs for a combined £10k Twizy + PV package by about 6p a mile, to about 26p per mile. This is a third less than the running cost of a small petrol car, and arguably 100% less carbon emissions. More importantly for me is the fact that a solar PV panel, if you can afford one, provides the perfect answer to those who say that electric vehicles are a bad thing because in reality they run on coal. While I can’t honestly claim that my Twizy doesn’t consume any coal-generated electricity, I can claim that it doesn’t require more coal to be burned to keep it running.
Posted in Technology
Tagged eco travel network, Renault Twizy, running costs, solar PV
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Twizyology 3 – Indoor/Outdoor boundaries
Here is a curious psychological phenomenon.
In our Eco Travel Network fleet of Renault Twizys, we have both the doorless and doored versions. Last week, we were alternating between driving one then the other in the pouring rain. The good news is that, as long as the Twizy is moving, you don’t get rained on in either case.
However, one’s naive expectation is that the Twizy with doors (even though it lacks windows) would feel comfortably drier. But that wasn’t our perception. Unfortunately, the rain drips from the roof onto the INSIDE of the doors so the driver is close to two slightly wet surfaces which makes you want to huddle in your seat to keep away from them. This has the effect of making the driving experience feel damper and more uncomfortable than in the doorless Twizy. Here, ironically, the same amount of wetness (in fact a LOT more) is within the same distance from your shoulders and elbows but now it is on the OUTSIDE.
Could this be something to do with our perception of inside/outside boundaries? A small tent doesn’t feel so wet if you have the door open and are watching the rain outside than when you have to zip up completely and cower inside away from the walls….
Posted in Psychology
Tagged doorless, doors, eco travel network, psychology of boundaries, rain, Twizy
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The inherent dodginess of attitudinal surveys
From time to time, The Prospectory has to design and conduct surveys as part of a research project. Surveys can be useful for collecting data on behaviour from a larger sample of people than is possible with free-form interviews or group discussions. But, wherever possible, I would always favour the latter. Free-form techniques are more likely to yield useful and suprising insights and they are also a lot more reliable.
We try to keep survey questions as concrete as we possibly can having learned from experience that the less concrete the question, the less reliable the answers you get. Even when asking about concrete behaviours, it’s more reliable to ask what people actually did yesterday or to describe the last time they did X or Y than to ask them to judge how often they do it in general.
It’s even worse when asking people in surveys to rate their attitudes to any topic. How reliable are their answers? Research suggests not very! People will happily generate and argue for any viewpoint if that is neccesary to justify something they do or have done or a position they believe they hold.
In a recent study, participants completed a survey concerning their moral attitudes. When asked to read through their responses and explain a few of them, a trick was used on 2 of the questions to display the opposite attitude rating to the one they had actually recorded whilst completing the survey.
The experiment then recorded whether particpants would detect the change or whether they would justify and argue for the opposite view of what they had stated on the survey only moments earlier. The sobering result was that 69% of participants failed to detect at least one of two changes. And they often constructed coherent and unequivocal arguments supporting the opposite of their original position. As the authors point out, the results suggest that there is plenty of flexibility in our attitudes and self-attribution and post-hoc rationalisation play a critical part in the views we hold at any point in time.
So, design and interpret attitudinal surveys with care – our brains are highly tuned at generating on the fly reasons why what we just did or said (or think we just did or said) makes perfect sense.
Posted in Psychology
Tagged attitude surveys, post hoc rationalisation, psychology, self-attribution
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Woolly Perspectives
My favourite role as a research psychologist is recording and analysing the language people use when they are discussing something which I’m studying.
Our friend the artist, Pip Woolf is running a fascinating project attempting to mend a badly eroded peat bog on a local mountain top using local wool and local people. Over time, she is drawing lines of wool which look spectacular and are slowly halting the ongoing erosion.
On the last trip up the mountain, I took my digital recorder to try to capture how different volunteers understood or related to the project. Here is the result.
I like the fact that some people talked about the ecological and artistic aspects of the project. Others compared it with other much more “high-tech” projects elsewhere. But for others, it was simply enjoying the horses or hammering in the pegs to hold the wool, the stunning scenery or the chance to meet new people.
People won’t necessarily engage in sustainability projects for sustainability reasons – they may simply do so because it’s fun or different or sociable. I like it.
Posted in Psychology
Tagged art, ecology, language, peat bog, pip woolf, woollenline
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Twizyology 2 – when is a car not a car
As previously, noted, we think the Renault Twizy’s greatest strength is in the fact that it’s not a car – it offers something completely different and fun for travellers who just want to get short distances cheaply and using very little energy. Perfect, for example, for the hilly rural area where we live and work where many people’s journeys are less than 5 miles but up and down very steep hills to the nearest village or town.
We also think that the Twizy’s “not-carness” gives it a chance of disrupting the car market in a way that more conventional electric cars are struggling to do (because they automatically get compared unfavourably with the existing market products).
The most obvious symbol of the Twizy’s not-carness (or so we thought) was that it didn’t bother at all with doors thus making a statement about the kind of short journeys it expected to make. It appeals, for example, to people who might cycle but don’t want the physical effort (especially on our hills) or people who might use a motor scooter but are nervous of driving a two-wheeler and don’t want to get soaked in the rain. The fact that the basic Twizy is completely open-sided makes a statement – this is not trying to be your average car. Think differently! You won’t expect to go far in this, you will need warm clothes and a hat in winter and it’s obviously not intended to be your main car or carry you long distances. Of course, on sunny summer days, it’s delightful to drive along open to the elements especially on our country lanes and open hillsides. But come winter, don your warm coat and hat (which you will need anyway when you get out of any car) and whizz along the few miles to work or the shops. The good news is that you don’t get wet even in our open-sided version – well not until you stop!
But, we discover that most people are choosing the Twizy with doors option. And then unfortunately, psychology cuts in. If you have doors, it feels odd (when it rains) not to have windows which you can close. I have driven both kinds of Twizy in the pouring rain and it certainly feels odder in the doored version not to have windows – I almost found myself reaching out to close them. In our own, open-sided version, it might be pouring with rain just beyond elbow reach but the concept of windows doesn’t cross your mind – how could it when you don’t have any doors?!.
So, maybe inevitably, just months after the Twizy is released, there’s already an add-on accessory of windows and now as autumn approaches, the Twizy owners with doors and windows have started discussing heaters!! Again, the advantage, if you will, of our open-sided version, is you can’t really discuss heaters!
A few Twizy owners are thus turning their vehicles step by step into ‘proper’ cars and the risk is death, effectively, by focus-group-think of a winningly different concept for local, low-energy travel. I hope I’m not right and I also hope those Twizy owners who seemingly really want a car realise that and buy themselves a small, conventional electric car instead.
Once people think of Twizys as proper cars, they lose their challenge to our car-centric mindset. They will start to be compared with small “proper” cars and may risk being written off as somewhat inferior – unless of course padded seats are introduced, proper doors and windows and a greater speed and range. Then, hey presto, you have small quite pricey electric car just like the other small electric cars on the market. We have one such as a community electric car – it’s functional but it offers nothing new or different and is certainly unlikely to challenge the existing market.
The Twizy, as envisaged by Renault, is a challengingly different travelling concept for short journeys. It says something different and offers different values and thereby disrupts our assumptions about car travel and energy. I hope that Renault has the courage to stick with their original idea.
Posted in Psychology
Tagged disruptive, doors, electric vehicles, psychology, Renault Twizy, windows
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Solar miles
As you may have spotted, The Prospectory has a strong interest in the relationship between energy and travel – from both a technical and psychological perspective.
We are interested in how we can all move around using a lot less energy than we currently do and where the energy we require to move us comes from.
Here in the Brecon Beacons, we are lucky to be able to generate a lot of our energy from natural, local sources – mostly rain, some sun and (outside the National Park at least) wind.
The Prospectory itself has a 4kW domestic PV installation and we run a Renault Twizy, “Thierry”, as our own domestic vehicle and in our capacity as Directors and promoters of the Eco Travel Network scheme. Although Thierry can only sometimes charge directly from our PV panels (he tends to recharge a bit too fast), we were curious to know what percentage of the miles he has transported us over the past 2 months (July and August) was effectively generated by our own PV – i.e. pure solar miles.
In those 2 months, our PV panels have generated 722 kWh and according to the stats on our energy use per Twizy run, Thierry averages 8 miles per kWh so, in total, the PV panels have generated 5776 potential Twizy miles. And Thierry, in that time period, has actually travelled 1022 miles. His daily average is 21 miles. (Interesting to note that the national UK average is 23 miles car miles per day – so much for range anxiety!!).
And, rather intriguingly, this is what it looks like on a daily basis over July and August (which were certainly not the sunniest months on record) but each day, there were plenty more solar miles we could have done! And the sunnier it is, the more enjoyable it is driving in an open car.
Travelling green is OK but travelling yellow is a lot more fun!
Twizics 3
Twizics 2 identified air resistance as a major energy drain at high speed, but changing speed also uses energy. If there were no air, drive train or tyre drag, we could maintain a constant speed with little or no energy input, but accelerating and changing direction always takes energy.
Acceleration increases kinetic energy, so you can work out how much energy you need for a given change of speed of a given mass. What’s not so easy to work out is how much we accelerate in the course of a given trip. Driving at high speed usually involves more acceleration and braking, but it’s hard to quantify.
The lower your top speed, the less energy it takes to achieve it, and you also lose less energy when you brake. But should you accelerate gently to your top speed or is it best to accelerate hard? The energy to accelerate a given mass between two given speeds is the same no matter how long it takes you to do it. But the harder you accelerate, the higher your average speed will be over the course of your trip, and that increases the energy used to combat air resistance. So although you will spend less time combating that air resistance if you accelerate quickly, you will use proportionately more energy. So to maximise your range it is best to accelerate gently to your chosen speed and then try to maintain it.
Braking reduces kinetic energy. In a Twizy you get some of this energy back if you slow down using the regenerative braking of the motor rather than the friction pads of the brakes. Friction just converts the kinetic energy into (wasted) heat, while regenerative braking converts some of it back into chemical energy in the battery. But unless we’re going to stay still, we have to accelerate and decelerate to get anywhere, and you can’t really work out theoretically how much energy you would use on an average trip.
The other big consumer of energy around here is hill climbing. Once again, the physics is straightforward: it takes a given amount of energy to raise a given mass against the force of gravity, regardless of how long it takes. But again, the faster you climb the more energy you consume overcoming air resistance. Also, you will need more power even though you need it for less time, so if a hill is very steep, the Twizy’s motor may not be powerful enough to climb it at full speed.
The bottom line is that it is difficult to predict how much energy a Twizy will use on a given trip unless you know the terrain and the speed at all points of the journey.
In an attempt to construct an empirical model, I used a log of a trip I did last year in a fully instrumented electric vehicle. The log records the position (longitude, latitude and altitude) every 5 seconds of a vehicle being driven conservatively along a gently undulating 10 mile route at a maximum speed of 25mph and an average of 16. I did the same trip rather more quickly (maximum speed 35mph average 21) in a Twizy and I know how many Watt-hours of energy that took.
Using this data set I can calculate the overall power consumption by applying the various parameters (drag coefficient, cross-sectional area, rolling resistance, drive train drag, vehicle and occupant mass, gradient). I do not know the true parameter values in all cases, so have made credible estimates. There are several combinations that will produce a “right” answer – i.e. one that corresponds to the actual recorded power consumption on this trip in a Twizy – but the tuning process enabled me to establish which ones are most sensitive.
I then used this “model” to calculate the expected energy consumption for the same trip conducted at a range of “maximum” speeds. The graph below shows the (very simplified) result. Here’s (roughly) where I think the energy goes on a typical ten mile Twizy trip in gently undulating terrain in mid-Wales.
This fits our actual consumption of about 125 Watt-hours per mile at the sedate speeds at which we drive, and the more exciting 170 to 190 Watt-hours per mile that we get when we’re showing off, and all our friends seem to get most of the time!
Twizics 2
Most Twizy drivers will struggle to achieve 125 [wall] Watt-hours per mile and a range of 50 miles in a hilly area like ours. There are two important numbers governing the range and speed of a Twizy – 30 and 50. If you drive a Twizy at 50 mph (its maximum speed) whenever you can, and otherwise as fast as conditions allow, you might get 30 miles. If you drive at 30mph on flat roads whenever you can, and at whatever speed the same power can maintain at other times, you can manage well over 50 miles unless the terrain is against you.
To understand why speed has such an impact on energy consumption, we need to know where the energy goes: some is lost as heat in the conversion from electrical to mechanical energy; a fair bit to the tyres; a fair bit to the drive-train, gears, bearings and brakes; and some to the control systems and (at night) the lights. Most of the large power drains increase roughly linearly with speed, so although you use twice as much to go twice as fast, it shouldn’t affect how far you can go because you take half the time. But above about 10mph you are also subject to air resistance, whose force increases with the cube of the speed, which means that the power to overcome it goes up with the square. So by about 50mph pushing the air aside is accounting for at least half the power expended while at 25mph it accounts for less than a quarter.
The (purely illustrative) figure below models a typical “power budget” you might expect to maintain a Twizy at a constant speed on a flat road.
The graph above is of course over-simplified, and certainly not taken from any official Renault figures! But as a model of Twizy performance it does roughly coincide with what we achieve in practice, and the same model produces the following graph of expected range at a given speed on a flat road.
But of course, our roads are not flat, and we do not drive at a constant speed. We accelerate and we brake, and we climb hills and come back down again. I’ll have a look at the power cost of these in the next article.
Twizics 1 – the Physics of the Renault Twizy
The goal of the Eco Travel Network is to offer people in sparsely populated rural areas more sustainable personal transport. Country people can live as sustainably as city folk in most respects, but the low energy of city public transport systems cannot be matched in sparsely populated rural areas.
Professor David MacKay, in his book “Sustainable Energy without the Hot Air“, measures the energy used by various modes of transport in “kWh per 100 passenger kilometres”. So, for example, he calculates that an average UK car with one person in it uses about 80kWh of energy to travel 100 km: 80kWh/100p-km. An inter-city coach doing 65mph with 50 people aboard manages 6kWh/100-pkm. A rush hour London Underground train manages 4.4kWh/100-pkm. Most rural commuting trips are, by necessity, car trips, so country people are typically using more than 10 times more energy to get around than their city cousins.
Vehicles like the Renault Twizy can change that if they can match the energy per person of urban mass public transport systems. Our Twizy needs 125 watt-hours of mains electricity for each mile we travel with both of us aboard. 100km is 62.14 miles, and 125 watt-hours per mile works out at 7.77 kWh of mains electricity for 100km. This means that a Twizy with only one person in it(at 7.7 kWh/100-pkm) is as energy efficient as a city bus, and with 2 up uses less energy (3.9 kWh/100-pkm) than a rush hour tube train.
The battery charging process on a Twizy isn’t 100% efficient, so 1kWh of mains electricity doesn’t put 1kWh of power into the batteries. Nor is the electrical power in the battery converted completely into mechanical power. Renault hasn’t published either of these efficiency factors to my knowledge, but Tesla claim an impressive 88% “wall to wheel” efficiency for their latest Model ‘S’ Sedan. Let us assume that the Twizy manages a more modest 80%.
That would mean that every kWh of mains electricity translates to 800 Watt-hours of mechanical energy, and our figure of 125 mains Watt-hours per mile translates (somewhat suspiciously!) to exactly 100 Watt hours of mechanical energy for each mile. To see if these figures makes sense, we can note that a 6kWh battery would take you about 60 miles, and require about 7.5 wall kWh of mains electricity. The Twizy charger requires 2.2 kW at full power, and it should therefore take around 3.4 hours to replenish a completely empty battery. Renault’s published figures for the Twizy are about 60 miles maximum range, and 3.5 hours maximum charge time, so we can be reasonably confident that our assumption of 80% “wall-to-wheel” efficiency is not far off.
So what we have in the Twizy is a personal transport system with better energy efficiency than a regular bus, and when carrying two people better energy efficiency than the tube. Interestingly, it is also good for about the same journey length one would typically make for a commuter trip, so vehicles like the Twizy can make rural personal transport as “green” as urban mass transit while offering many of the same conveniences as the most popular form of private transport, the car.
The Prospectory 