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Do androids dream of electric cars?

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PDR
1338198.  Thu Dec 05, 2019 9:51 am Reply with quote

dr.bob wrote:

All the discussions that have so far taken place about the power output of the solar panels in Dundee are based on incomplete information. PDR and I have disagreed about how effective this installation would be, but neither of us really knows how these solar panels would work in real life.


We may not know, but we can get some good indications. In solar power there is a parameter called "annual energy production per peak capacity" (kWh/kWp). It's a measured rather than calculated parameter for a specific location that establishes the total energy a perfectly-oriented solar panel of a given size will collect over an extended period, so it takes into account day/night/cloud/rain/ice/temperature/smog etc. Somewhere there is a table of these values for cities that I was looking at the other day, but I'm damned if I can find it again. The nearest thing I can find now is from the Energy Saving Trust website where it says:

Quote:
A 4kWp system in Scotland can generate about 3,400 kilowatt hours of electricity a year


If we normalise this to a kWh/kWp number:

kWh/kWp = 3400/4/8760 = 0.097 or roughly 10%

Annoyingly the website is vague, so I'm going to take the best case assumption that this number includes the ~25% efficiency of the solar panel and any DC-AC conversion losses (typically 2-8%, needed to feed the grid but not necessarily needed to feed battery chargers). If this assumption is wrong then the following will overstate the matter by a factor of four, but I don't think it is. If we apply this number to our 2,000kWp car park roof array we then get a daily output of:

2,000*24*10%= 4,800kWh per day

Which for a 20 charger system gives us a daily 240kWh per charger - enough for around 11 hours of full power charging with w 22kW charger, or under five hours of full power charging with a 50kWh charger. So the numbers we were playing with are in the right ballpark.

I take the point about trickle charging and not moving cars around, but that doesn't appear to be what they are offering. As Suze mentioned, the Dundee website is explicit on this point:

Drive Dundee Electric wrote:
To ensure other EVís get a chance to charge we have enforced the following conditions:

30 minute max stay when charging at rapid charger. No return within 4 hours.
3 hour max stay at fast charger. No return within 4 hours.


So the "park it all day/night and trickle charge" option doesn't seem to be on offer.

All of that seems quite encouraging for a small number of chargers but still shows it would need external power for both overnight charging and if more chargers were added. I am going to stick my neck out a bit over that kWh/kWp number, however. These numbers usually assume the array remains correctly oriented (ie at 90 degrees to the incoming sunlight) and it's not clear whether that original 3400kWh/yr is based on that number. It usually is because commercial arrays often have "active alignment" (ie they continually change their azimuth angle as the sun rises and sets). The Dundee installation appears to be fixed-angle, and in Scotland the solar azimuth varies from ~25degrees in the winter to over 70 degrees in midsummer. That's a huge variation which could reduce the effective array area by up to 20%.

PDR

 
Alexander Howard
1338199.  Thu Dec 05, 2019 10:02 am Reply with quote

When cars can be set to self-drive, then the cars and charging stations will be adapted so that unattended cars can be charged. An obvious dodge for a driver would be to avoid paying for parking by telling the car to drive round and round the ring road all day, and charge itself when it runs down. That will change the economics.

At the same time, parking charges could be brought down because a car left without a driver nor passengers can be parked / stacked in a space only as wide, long and high as itself.

 
dr.bob
1338205.  Thu Dec 05, 2019 10:55 am Reply with quote

PDR wrote:
We may not know, but we can get some good indications.


Just be careful to bear in mind that that's all they are: indications. Either of our figures will only ever be best guesses and may not be an accurate indication of reality.

PDR wrote:
Quote:
A 4kWp system in Scotland can generate about 3,400 kilowatt hours of electricity a year


If we normalise this to a kWh/kWp number:

kWh/kWp = 3400/4/8760 = 0.097 or roughly 10%

<snip>If we apply this number to our 2,000kWp car park roof array we then get a daily output of:

2,000*24*10%= 4,800kWh per day


Please bear in mind that this figure is missing an important point that I've already pointed out twice now. I'll make it a third time in the hope it stays towards the front of people's minds:

"Any solar installation has to cope with the peaks and troughs of unreliable sunlight. In particular, it has to cope with the maximum power output of the system on a clear summer's day. If it can't handle this load, then energy will be wasted. "

By all means discuss the average output of the solar array but for certain aspects, for example the size of the batteries required to store excess energy, it's vitally important that you consider the maximum output of the solar array on a sunny midsummer's day.

PDR wrote:
Which for a 20 charger system gives us a daily 240kWh per charger - enough for around 11 hours of full power charging with w 22kW charger, or under five hours of full power charging with a 50kWh charger. So the numbers we were playing with are in the right ballpark.

I take the point about trickle charging and not moving cars around, but that doesn't appear to be what they are offering. As Suze mentioned, the Dundee website is explicit on this point:

Drive Dundee Electric wrote:
To ensure other EVís get a chance to charge we have enforced the following conditions:

30 minute max stay when charging at rapid charger. No return within 4 hours.
3 hour max stay at fast charger. No return within 4 hours.


So the "park it all day/night and trickle charge" option doesn't seem to be on offer.


Yes it is. I think perhaps you need to re-read what's already been discussed on this thread since I addressed this point over a week ago back in post 1337478 when I highlighted a newspaper article that makes it clear (to me at least) that the new chargers are not subject to the 3 hour limit.

The primary thrust of the BBC article I originally quoted was that these new chargers are designed to address a big problem with EVs, to wit how do people without off-street parking charge their EVs. The plan is for each charger to be used by one person during the day (charging while they're at work) and a second person during the night (someone living locally charging when they're home from work).

Given that planned use case, I don't see that's it's very useful to talk about how long a particular type of charger can run at full power. A much better way of looking at it would be to say "these chargers will be servicing 40 cars per day (20 chargers servicing 2 cars each), so how much charge would each car receive?"

PDR wrote:
All of that seems quite encouraging for a small number of chargers but still shows it would need external power for both overnight charging and if more chargers were added.


No it doesn't.

I've used the exmaple of a Nissan Leaf before since it's a popular type of EV. There are a variety of models available on the market from the early models equipped with a 24 kWh battery to more modern models which now carry a 40kWh battery.

Let's just consider the more modern cars, since battery size is only going to increase. Let's assume your figures are 100% correct and that on average, the Dundee solar panel array is likely to generate a daily 240kWh per charger. Given that only 2 cars per day will be attached to these chargers, the cars can only draw 80kWh per day, even in the extremely unrealistic scenario that every car arrives at the charger with completely empty batteries. This will only use one third of the power generated on average. Still less on a sunny midsummer's day.

I don't know which EV has the largest battery, but I did a google search to find which one has the largest range (as I figured that'd be a good clue). Google tells me that the Tesla Model S has a range of 335 miles and wikipedia tells me this car has a 100kWh battery. If you can find an EV with a larger battery, please let me know.

Even this huge car, given the use case of charging two cars per day, will draw a maximum of 200kWh per day per charger (again assuming the unrealistic scenario where every car arrives with completely flat batteries). That means that your figures for the average power output of the solar array will still produce enough power to run the chargers all day, all night, and still have 40kWh per charger per day left over.

 
PDR
1338211.  Thu Dec 05, 2019 11:25 am Reply with quote

dr.bob wrote:

Please bear in mind that this figure is missing an important point that I've already pointed out twice now. I'll make it a third time in the hope it stays towards the front of people's minds:

"Any solar installation has to cope with the peaks and troughs of unreliable sunlight. In particular, it has to cope with the maximum power output of the system on a clear summer's day. If it can't handle this load, then energy will be wasted. "



I don't think it does miss it, as evidenced by the subsequent discussion which still uses the same 2,000kW array sizing. In the case we're discussing the array size is not driven by an output requirement - it's driven by the available roof area (it's not actually stated, but you and I have both made this same assumption for similar reasons). The maximum output of the array (the 2,000kW number) drives the maximum number of chargers which can simultaneously be used at full power. If there are too many chargers being used then you would design the system to throttle back the chargers, and if there was an excess compared to the current demand then the system design would use that to charge batteries (as described for this system) or other energy storage facility (pumped water, flywheels etc).

So I'm not clear which aspect you feel overlooks the peak array power issue - could you clarify?

PDR

 
cnb
1338213.  Thu Dec 05, 2019 12:21 pm Reply with quote

It looks like both PDR and dr.bob are assuming that this charging station is not connected to the national grid. None of the articles I can find about it suggest this to be the case. I'd expect it to be designed to use the grid in extreme conditions.

On a sunny day, any surplus that can't be immediately used to charge cars will charge the local storage batteries, but once those are full any further surplus will be exported to the grid.
On a dull day or at night, any excess demand will initially be delivered from the local storage batteries, but when they are empty any further demand will be met from the grid.

During the parts of the year when dull days are common and demand is expected to exceed solar supply, it would probably be sensible to charge the local batteries from the grid overnight when electricity is cheaper, and use that stored energy to charge cars in the daytime.

 
barbados
1338214.  Thu Dec 05, 2019 12:58 pm Reply with quote

I may be missing something, and I'll confess my "expertise" is one of fixing shit that breaks rather than designing shit that works, but my limited understanding is that when you do design such a thing you look at the most efficient output when you have the least efficient input - you aim low and shoot high so to speak. If that is not the case, then it could be the reason why I am finding fault in dr.bob's synopsis. Surely if you are designing the system in question, you would look to work to maximum output with minimum input and work from there. We know for example that on a fully 100% efficient sunny day, the chargers will operate at 100% availabilty for (if memory serves) between the hours of 11 and 1pm, and that output would resemble a bell curve across the day, decreasing after 1pm by a similar amount to the the increase in the morning. But, we know that they don't occur every day. On most days there is some form of cloud cover, and if the output is quoted as xKWh, that measure needs to be an average across an average day. So when you suggest I am wrong dr.bob (as you did in post 1338170) you are incorrect. You can disagree with my understanding, but unless the design of the system is one that I see as arse upwards, then you need to justify why you think that
Quote:
"Any solar installation has to cope with the peaks and troughs of unreliable sunlight. In particular, it has to cope with the maximum power output of the system on a clear summer's day. If it can't handle this load, then energy will be wasted."

because that either doesn't address the suggestion that you are disagreeing with, or it is an incorrect thing to say - because you would need to work to an average input - not a peak.

 
PDR
1338222.  Thu Dec 05, 2019 5:09 pm Reply with quote

cnb wrote:
It looks like both PDR and dr.bob are assuming that this charging station is not connected to the national grid. None of the articles I can find about it suggest this to be the case. I'd expect it to be designed to use the grid in extreme conditions.


We did cover that a couple of pages back - the issue we've been discussing is more about how much contribution the solar power would make. I think we've concluded that at peak power the assumed 2,000sqm PV system would be able to deliver around 500kW* into the 20 chargers - 25kW per charger assuming 100% switching efficiency. Also based on one source we have an indication that the 24/7/365 average output for the same array would be around 200kW or 10kW per charger.

I think this is where we then differ. I observe that the common chargers run at 22kW and 50kW, or up to 120kW (I think) for a Tesla, and people would expect to be able to charge at those sorts of rates. DrBob feels that people would typically want to leave their cars on the chargers for longer periods at lower charge rates. My view would need a lot of additional electricity from the Grid whereas DrBobs would not. Both would need additional electricity to significantly increase the number of charging points.

PDR

* This is assuming 25% efficiency in the PV panel itself, which is more than they currently achieve but probably not an unreasonable assumption for those of the near future

 
dr.bob
1338240.  Fri Dec 06, 2019 6:06 am Reply with quote

PDR wrote:
So I'm not clear which aspect you feel overlooks the peak array power issue - could you clarify?


Of course. I was merely trying to make it very clear that the figures you were using were those for the average power output and not the peak power output. I hope we can agree those two numbers are very different and I was just wanting to make sure that anyone who came back to this thread at some point in the future and saw those numbers was not going to use them to make some unwarranted conclusions. There was probably no need for me to make sure of that, but I was just planning ahead.

So, in summary, it wasn't a criticism, merely a (probably unnecessary) clarification.

PDR wrote:
I observe that the common chargers run at 22kW and 50kW, or up to 120kW (I think) for a Tesla, and people would expect to be able to charge at those sorts of rates. DrBob feels that people would typically want to leave their cars on the chargers for longer periods at lower charge rates.


I'm a little confused by this statement, so it's my turn to ask for clarification. I'm particularly confused by your use of the word "typically". As far as I can see, this could mean one of three things.

1) When talking about public EV chargers in general, I feel that the most common way of using them would be for people to leave their cars on the chargers for longer periods at lower charge rates.

2) When talking about the specific installation in Dundee mentioned in the BBC article at the start of this recent discussion, you feel that they will generally be used by people charging their cars at full power for short periods of time.

3) Some other meaning that I haven't fully understood.


If you meant 1) then you're very much mistaken. I would invite you to re-read what I've previously written on this thread because I'm pretty sure that, whenever I've spoken about people leaving their cars on public chargers for long periods of time, I've been careful to make sure that I've only been speaking about the specific installation in Dundee mentioned in the BBC article. The reason for this is that the BBC article makes it clear this is the way the installation is designed to be used in order to address the problem of people without off-street parking (if I've not been clear on this, I'd be interested to know what language I used to give you this impression so I know what to avoid saying in future).

When considering public chargers other than the specific installation in Dundee, of course I assume they will generally be used for short times at full power since that's how they're designed to be used.


If you meant 2) then I'm confused why you think the chargers would be used in this way. The BBC article makes it clear that the installation is designed to be used in order to address the problem of people without off-street parking and is intended to be used by some people parking all day while at work, and others parking all day while at home at night. For sure, some people might leave work early and free up a charger that is then taken up by someone else, or go out in the evening and find that their place has been pinched (much like losing a parking place outside your house). But surely these events would be the exceptions rather than the rule, so maybe you could clarify why you think the specific installation in Dundee would be used in the manner in which you describe.


If you meant 3), then please clarify what I've missed or misunderstood.

 
PDR
1338244.  Fri Dec 06, 2019 6:11 am Reply with quote

I guess I missed that you were only refering to future users of this specific installation. Apologies for the misunderstanding.

PDR

 
barbados
1338247.  Fri Dec 06, 2019 6:44 am Reply with quote

Don't take this as a criticism dr.bob, but I think the confusion has occured over a lack of clarity about what is actually being discussed.

There have been a couple of occasions where I have responded thinking you are talking about something specific, and you have meant something generic, and likewise when I have thought you are talking in general, it has transpired that you are talking about a specific thing (not just on this thread).

It's probably a two way confusion - it is one of the difficulties of the written conversation (along with the timings of the conversation). If we three were sat in a cafe over a cup of coffee (other less appetising drinks are available) the confusion would be addressed more immediately. Unfortunately we aren't in that position so maybe we need to look at all possible meanings to the posts (indeed as you ahve above) and say when you aren't sure what the meaning is -"I'm not sure what you mean, but if it is (a) I agree. If it is (b) I disagree because I think (c).

 
dr.bob
1338248.  Fri Dec 06, 2019 6:55 am Reply with quote

Good idea.

Along with that, I think it's probably worthwhile to read through any posts we make before hitting the "Submit" button and have a think to ourselves if what we've written could be misinterpreted in some way. If it could, then it's probably worth rewriting slightly to clarify the point we're trying to make.

That may sound like a pain in arse and extra work, but hopefully it'll help to avoid at least some of the extra work involved in clearing up confusion after the fact.

 
barbados
1338289.  Sat Dec 07, 2019 5:25 am Reply with quote

It will.
If I can just go back a couple of posts, Iím unsure why you think that the benchmark for the solar panels should be in optimum conditions.
Would you clarify why you think we shouldnít use the average daily sunshine over a day of unbroken sun?
It seems similar to suggesting that inside the polar circles would be ideal because of the white night phenomenon that occurs during the summer months, while you discount the polar nights of the winter months.

 
dr.bob
1338426.  Mon Dec 09, 2019 10:47 am Reply with quote

barbados wrote:
If I can just go back a couple of posts, Iím unsure why you think that the benchmark for the solar panels should be in optimum conditions.
Would you clarify why you think we shouldnít use the average daily sunshine over a day of unbroken sun?


The benchmark depends on what you're considering. If you're discussing the total power output over the course of a year, for example to estimate how many cars in total your system will charge, then the average sunshine figures are the correct ones to use. If, by contrast, you're discussion the maximum power output, for example to estimate how large the batteries will need to be to store any excess power, then the peak output in optimum conditions is absolutely the correct figure to use.

It all depends on what is being discussed, which brings up another interesting point about these discussions. The problem with having discussions on a talk forum is that replies can sometimes be separated by several days. This often makes it difficult to remember the precise details of what is being discussed even for those directly involved in the discussion. For a third party joining the discussion, it's even more difficult.

This combination of problems can often result in people arguing at cross purposes since they are, in fact, discussing subtly different things without realising it. Going back to the original posts to figure out what is actually being discussed can often be long-winded, difficult, and time consuming.

 
barbados
1338438.  Mon Dec 09, 2019 12:17 pm Reply with quote

That makes sense now -thank you.

 
cnb
1338456.  Mon Dec 09, 2019 5:26 pm Reply with quote

dr.bob wrote:
If, by contrast, you're discussion the maximum power output, for example to estimate how large the batteries will need to be to store any excess power, then the peak output in optimum conditions is absolutely the correct figure to use.


It was the discussion of peak power output and the possibility of waste that made me think that you were ignoring the grid connection.

In a grid-connected facility, you wouldn't install a battery capable of storing all the excess production on an optimal day, as large parts of that expensive battery's capacity would sit unused most of the time.

Optimally sizing the battery would require a financial calculation involving the cost of battery capacity, the cost of capital, the rate at which surplus energy can be sold to the grid, and the day and night rates at which energy can be purchased from the grid.

 

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