Howard wrote:dspp
In your haste to decry all legacy car manufacturers
you have neglected to answer my question about the 9 Guildford buses overloading the local grid. You may not know much about marketing cars
but, having read your posts on other threads, I do respect your views as an expert on power distribution. Is this indicative of a problem in supplying a concentration of BEVs? Or is it just a one-off issue affecting the local situation in Guildford.
regards
Howard
I've done some sums to understand generic bus fleets. I won't bore you with all the URL references. Your Guildford video
https://www.bbc.co.uk/news/av/business- ... o-electric .
I've used a typical Volvo B7 such as the Volvo B7RLE and the 7 is because these use 7-litre diesel engines. These are very typical of the single decker city buses you see around the world.
41 litre per 100 km is the average fuel consumption in city traffic (33% idle and average speed of 19 km/h) of all Volvo B7R and Volvo B7RLE chassis delivered since the introduction of Euro IV in 2006.
Using TfL data a typical bus does 97 miles per day, or 156 km/d. That fits well with the ranges stated in the video.
So a typical bus uses 64 litres of diesel per day.
If this was being burnt in a diesel engine that would mean that you have to insert 10kWh of chemical energy in, to get 3.3kWh of kinetic energy out, with the rest being wasted as exhaust heat & acoustics.
For approximating purposes lets use 90% conversion efficiency for a BEV. For the time being I will ignore charging efficiency as I think there is another factor that is more important.
That means that a typical bus will consume (i.e. discharge from its battery) 234kWh of electrical energy each day in doing 97 miles of work. That energy needs recharging overnight.
There were 9 buses in the Guildford fleet per your post.
So 9 x 234 = 2106 kWh to be recharged overnight. Let's assume 10pm until 6am for recharging, so an 8hr recharge period. The 10pm start is usefully well into the low load period, and the 6am is before it starts rising, so this is the lowest point in the typical UK load cycle.
2106/8 = 263 kWh/h will be the AVERAGE recharge rate, and I'll return to the average bit in a moment.
Recall that a typical house single phase 100A supply of 240V can supply 100 x 240 / 1000 = 24 kW maximum continuous
So a 415V three phase supply of 400A will give a max of 415 x 400 x rt3 (1.732) / 1000 = 287 kW maximum continuous (this is about the equivalent of 10 house supplies, but no diversity factor)
So if the bus depot had a dedicated 400A three phase supply of 415V then it could deliver 287 kW continuous, i.e. 287 kWh every hour, or 287 kWh/h. Compare that with the 263 kWh/h required on AVERAGE and you would think that would be OK. In fact there is probably just about enough spare capacity to cope for 10% of charging efficiency losses, but probably not for other losses in the system, but I don't think losses are the real issue here.
But that ignores the tapered charging issues. If you look at a battery charge curve the starting charge rate is typically twice the average, so one would really need an 800A supply if one were to plug in all the buses simultaneously at 10pm. It is a fairly typical 'stupid' use case to assume a small fleet would start charging essentially simultaneously. Even a little bit of staggering would help greatly.
Frankly providing a 800-1000 A additional supply to a local bus depot is doable, but a little forethought would suggest putting in a new 11kV supply to this area, as the depot will not be the only bit of this area of Guildford that is running hot. You need to recall that most of these areas of England had their electrical system origins back in the days of DC cabling, and are horrendous antiques that are well overdue replacement - just like most of the UK's electrical infrastructure. My guess is that this set of buses would have pushed them over the edge and precipitated a need to reinforce the local area. By way of comparison my little factory has about 600A of 415V 3ph as its supply, and we don't have an 11kV dedicated feed.
That is why in this instance there is a Tesla storage solution going in to avoid the local upgrade - one sees the same thing going on in inner city USA where they are putting in storage rather than building/renewing their inner city power stations. [edit:] As a fagpacket approximation the Tesla storage will reduce the approx 800A peak additional 10pm load to a 131A average additional daytime load. Add in a bit of dual-supply into the overnight charge (i.e. from both the storage battery and the regular supply) and that could easily bring the avge daytime increase below 100A. Start to do these numbers and you can see how a little bit of localised storage can avoid the need for expensive grid cabling upgrades etc.
Of course the fact that the UK's electrical infrastructure is 50-years old on average and so needs replacement in any case (in fact it is long overdue) will not be a reason for the anti-BEV luddites to desist. Instead they will wail & moan & gnash their teeth and (probably) insist that it is all the nasty EU's fault.
Meanwhile my customers around the world are getting on with it at a goodly pace.
By the way the more global data that I am playing with suggests that on average about a 1.5x increase in the electrical network is required. In some hotspots (such as this Guildford bus depot) that will be more like 2x, but in very large sections it is 1x (i.e. no upgrade required). That 1x includes a lot of UK housing. Whether it is cheaper to put in localised storage in the 2x areas to avoid the need to do grid reinforcement is a localised cost-benefit-analysis. My guess is that this was as much a technology demonstrator project in this particular instance, though there may also be technical issues associated with the legacy cabling as in so much of the UK.
There is a lot of value in the Tesla storage play.
regards, dspp