# [EVDL] DC versus AC for Solar PV



## EVDL List (Jul 27, 2007)

I know that many EV users consider charging their cars from a solar PV
system on the roof of their house. I believe that almost all current
systems convert the DC solar PV power to AC with an inverter, which,
while it is about 95% efficient, is the only solar PV component that
is prone to maintenance and replacement.


What would be the advantages of using solar PV in a pure DC system,
where all the appliances in the house use DC power and the EV is charged
using DC power? What sort of DC-to-DC converter is needed, and
is it more efficient, less expensive, and/or less maintenance prone than
with an AC inverters?

Thanks,

-- 
Larry Gales
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.sjsu.edu/mailman/private/ev/attachments/20120602/4e897c4d/attachment.html 
_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to [email protected]x.xxx.edu only.
| Multiple-address or CCed messages may be rejected.
| UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
| OTHER HELP: http://evdl.org/help/
| CONFIGURE: http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

Larry,
At low power levels the DC route is much more efficient
than the detour via AC.
For example, I have a 12V battery that feeds an internet
modem with WiFi radio, an internet telephone and a
conference phone - all directly connected at 12V DC.
Total power draw at 1 Amp is 12 Watts. The 42 Ah battery
will last almost 2 days if power disappears.
The normal way of powering these 3 devices was originally
with AC adapters. Maybe the AC power draw was not so much
higher - maybe only 15 Watts AC, but to get this small
amount of AC, an inverter may have a 50% or less efficiency.
So, it draws more than 30 Watts from the battery, so the
same battery now lasts less than 16 hours, possibly *much* less.

On the other hand, if the power draw is high then the
problem with running 12V is that current is high and the 
resistance of the lines causes a large power drop.
This can cause the DC case to become less efficient than
AC conversion, due to the higher voltage and consequently
lower current.

Of course, if you match the PV panels to your EV pack so
they can charge without any conversion, then you can make
it reach the highest efficiency by avoiding the detour
via AC, but your EV must allow direct DC input and be
able to interrupt that current safely when charging
completes (this can be as simple as a low resistance
FET transistor, controlled by the BMS).

Hope this clarifies,

Cor van de Water
Chief Scientist
Proxim Wireless Corporation http://www.proxim.com
Email: [email protected] Private: http://www.cvandewater.com
Skype: cor_van_de_water XoIP: +31877841130
Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Larry Gales
Sent: Saturday, June 02, 2012 2:16 PM
To: SEVA; Electric Vehicle Discussion List
Subject: [EVDL] DC versus AC for Solar PV

I know that many EV users consider charging their cars from a solar PV
system on the roof of their house. I believe that almost all current
systems convert the DC solar PV power to AC with an inverter, which,
while it is about 95% efficient, is the only solar PV component that is
prone to maintenance and replacement.


What would be the advantages of using solar PV in a pure DC system,
where all the appliances in the house use DC power and the EV is charged
using DC power? What sort of DC-to-DC converter is needed, and is it
more efficient, less expensive, and/or less maintenance prone than with
an AC inverters?

Thanks,

--
Larry Gales
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
http://lists.sjsu.edu/mailman/private/ev/attachments/20120602/4e897c4d/a
ttachment.html
_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to [email protected] only.
| Multiple-address or CCed messages may be rejected.
| UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
| OTHER HELP: http://evdl.org/help/
| CONFIGURE: http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to [email protected] only.
| Multiple-address or CCed messages may be rejected.
| UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
| OTHER HELP: http://evdl.org/help/
| CONFIGURE: http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

I do not have personal practical experience myself, but can 
confirm what Cor posted about using a pure DC system at low 
current load levels.

In the late 1990's after my retrofit to add lots of goodies
to my S-10 Blazer EV (like a mod to the battery racks to 
be able to also handle the taller T-145 batteries), when I
changed out my T145 pack because it no longer had the 
range I needed to do the occasional long runs to prove it 
could be done, there were still a couple 'good-enough' 
batteries amongst the 22 T145 battery pack that still had 
some serviceable life left in them.

At the time, my co-worker at the Sunnyvale hp site, had a 
small rustic cabin on his very remote N. Cal. property he 
had bagged in a land auction years before (from some goofy-
fool who the Feds busted for growing dope).

I gave him two of my tired (for my purposes) T145's, and 
he took them up to his cabin to use with his cheap little 
PV system (the PV panel was so low an amperage, he did not
have a charge controller on it).

He used it not with an inverter, but purely for late night 
DC blub lighting. No refrigerator load or anything like 
that, everything else was low-tech (the guy was a 
survivalist Y2k nut).

Those old T145s worked fine for him for several years. Not
complaining, he did mention that the DC system voltage
would sag if the lights were left on all night (an issue
with a DC system design, not the batteries).

So, even with a low current draw, expect the same voltage
and current drooping when the SOC capacity goes lower than
50%. I would notice the same effect when I drove my Blazer 
EV that had a DC design: as the SOC reduced, so did the 
pack voltage and thus driving performance (that's only 
normal).


{brucedp.150m.com}



-


> Cor van de Water wrote:
> > Larry,
> > At low power levels the DC route is much more efficient
> > than the detour via AC.
> ...


----------



## EVDL List (Jul 27, 2007)

I'd agree with Cor -- at low power levels -- 50 watts or less, go straight
DC. But at higher power levels, it can often be easier to do AC.

Inverters are around 97% efficient for the best grid-tie only ones now --
part of this is that they are using high voltage (200 to 500 volts DC)
instead of 12 to 48 volts DC. And, a 10 to 15 year warrantee is standard.
They aren't expected to go out every 2 or 3 years like back in the 80's.
I've been doing a lot of system maintenance recently, and it's actually
more common for me to find problems with the solar panels than the
inverters (mostly damage from squirrels, and raccoons, though actual solar
panel failures are becoming more common as the panels are made cheaper and
cheaper). There are certain brands of inverters that I do find failed all
of the time, but the better brands are very rare to have issues.

If you've got a DC battery based system (non EV) then you have a 48 volt
battery bank (or 24 or 12, but those are fairly rare any more). The big
electronic component is the charge controller, which has all of the maximum
power point tracking equipment in it, rather than having it in the inverter
like with a grid-tie only system. That takes the voltage of the solar
panels (from 60 to 200 volts DC) and converts it down to the battery
voltage of 48VDC nominal (44 volts to 60 volts operating). They are
upwards of 97 to 98% efficient usually, and in my experience are fairly
trouble free... at least as good as the good inverters. Then... of course,
if you have loads that are not 48 volts DC (which is most often the case)
you have a separate inverter which takes the battery voltage and converts
to 120/240vac at around 92 to 95% efficiency depending on the model and
manufacturer.

The issue with doing PV direct at high voltage directly into the EV is
handling the charge control and maximum power point tracking. The voltage
of a PV array varies about 20% from summer to winter because of the
temperature of the panels... and this is in addition to the battery voltage
varying between a discharged and full pack. So, if you design the PV array
to be slightly higher than the full pack voltage, in the summertime with a
hot array... it will end up being at least 25% to 30% higher than the pack
starting (discharged) voltage in the wintertime with a cold array. You
just lose that extra potential voltage by hooking the array to the lower
voltage pack. Almost all battery based PV systems use a DC-DC converter in
the charge controller, and maximum power point tracking algorithms to allow
the PV array to operate at it's best voltage, then convert that down to the
battery voltage at ~97% efficiency, instead of just losing that 20+% in
some times of the year. The 2 or 3% loss is better than having a voltage
mismatch loss which varies from a few percent at the best case to 25 to 30%
at worst case, during the year..... though it does introduce more expensive
electronics in there. The widespread use of MPPT charge controllers is
only in the last 5 to 10 years, as DC-DC converters became cheap and
reliable enough. The issue with doing this for an EV bank is that the
highest voltage equipment that I commonly use is for a 60 volt nominal
battery bank. I guess if you reconfigured your EV bank into 48VDC pieces
for solar charging, then back to the higher voltage (96, 144) for EV use,
you could use existing MPPT charge controllers.

The other problem with doing DC direct is that, at least for me, my car is
sitting at my house most of the night, but tends to be out doing errands
and such during the day, when the sun it out. Running that sunshine into
the grid is a more efficient (since I get 100% efficient net metering -
-something you never get with a real world battery) way to store it than
doing stationary batteries then dumping back into the EV batteries.

Z






> Cor van de Water <[email protected]> wrote:
> 
> > Larry,
> > At low power levels the DC route is much more efficient
> ...


----------



## EVDL List (Jul 27, 2007)

Thanks for all this useful information. I was thinking in terms of a
wholly off-the-grid situation in which all electrical appliances are
located around a small central core, as opposed to being distributed
throughout the house.

I was thinking of this as something we might need to do in the future if
our economic system starts to become unstable and costly and people are
forced to rely more on their own resources.

-- Larry Gales



> Lee Hart <[email protected]> wrote:
> 
> > On 6/2/2012 4:15 PM, Larry Gales wrote:
> > > I believe that almost all current systems convert the DC solar PV power
> ...


----------



## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> 
> > On 6/2/2012 4:15 PM, Larry Gales wrote:
> > > I believe that almost all current systems convert the DC solar PV power
> ...


----------



## EVDL List (Jul 27, 2007)

> Robert MacDowell wrote:
> 
> > On Sun, Jun 3, 2012 at 10:16 AM, Lee Hart <[email protected]> wrote:
> > =
> ...


----------



## EVDL List (Jul 27, 2007)

That's a pretty impressive return on investment. What does that come to in
terms of kwh? I would guess somewhere around 0.03 pence or about $.05?
Which 3 months of the year was that - hopefully a good representation of
typically cloudy dreary weather?

Peri =


-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Martin WINLOW
Sent: 04 June, 2012 1:39 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] DC versus AC for Solar PV




> Robert MacDowell wrote:
> 
> > On Sun, Jun 3, 2012 at 10:16 AM, Lee Hart <[email protected]> wrote:
> > =
> ...


----------



## EVDL List (Jul 27, 2007)

Peri,

It's actually only about 1400 kWh and is not very typical as we had an unus=
ually sunny spring this year. This was from mid Feb to date. The reason the=
income is so high is due to the Feed In Tarrif that we have in the UK (and=
in other parts of Europe... and elsewhere?) to encourage people and busine=
sses to buy into micro-generation - solar, wind and hydro being the most co=
mmon. This is all part of the UK's commitment to reduce carbon emmisions a=
nd increase the proportion of renewable energy generation.

The deal is that for retro-fitted, residential arrays you get 43p for every=
solar kWh generated. Then you get another 3 pence or so from your electri=
city supplier if you are generating more than you are using - and you save =
whatever the cost of the electricity that you would otherwise be taking off=
the grid at the going rate - around 13p/kWh during the day here at the mom=
ent - a bit more if you have a split tariff ('Economy 7' as we call it over=
here, tho there are more complex tariffs available as well). And all this=
guaranteed (by the G-ment) for 25 years and index linked (to inflation).

Unfortunately, the scheme was rather more successful than the G-ment though=
t it would be and they have being trying to reign in cost of it all - and h=
ave been taken to court over this already. It is coming down to about 21p/=
kWh soon and I expect it will go altogether next year, if not sooner.

Interestingly, the price of solar has fallen dramatically on the back of th=
e FIT scheme here in the UK. I have been interested in PV all my life and =
have kept in touch with prices for yonks. PV modules have dropped to half =
the price they were 2 years ago and are still going down. So although the =
FIT is nice (3 times better return on investment than the best bank rate at=
the moment) the traditional payback time of 20-25 years has fallen down to=
about half that without the FIT and about 3-4 years with it.

Regards, Martin Winlow
Herts, UK
http://www.evalbum.com/2092
www.winlow.co.uk




> Peri Hartman wrote:
> 
> > That's a pretty impressive return on investment. What does that come to =
> in
> ...


----------



## EVDL List (Jul 27, 2007)

> Lee Hart <[email protected]> wrote:
> 
> > On 6/3/2012 8:34 PM, Robert MacDowell wrote:
> > > And even more than that, solar is by nature a "peaking unit" which means
> ...


----------



## EVDL List (Jul 27, 2007)

On Tue, Jun 5, 2012 at 3:33 PM, Robert MacDowell <[email protected]> wro=
te:
> ...
> So, same ampacity as the interurban coach but with a worse case maximum
> rating. (neither the substation nor the trolley wire would be capable =
of
> delivering 10,000 amps.)
>
> I wonder why the fuses got so much smaller and why it was no longer
> necessary to have arc chutes or blowouts.
> ...

I would guess it's the difference between 1500VDC and 400-600VDC.

A sand-filled cylinder that can quench an arc at 400-600VDC may fail
or explode at 1500VDC. All of the arc's energy is absorbed by the fuse
as heat. If that's too much energy, the fuse may rupture and let the
arc spread to nearby metal. Instead using air and magnetic blow-outs
with arc chutes should avoid that.

-Morgan LaMoore

_______________________________________________
| Moratorium on drag racing discussion is in effect.
| Please take those discussions elsewhere. Thanks.
|
| REPLYING: address your message to [email protected] only.
| Multiple-address or CCed messages may be rejected.
| UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub
| OTHER HELP: http://evdl.org/help/
| CONFIGURE: http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> Lee wrote:
> > Modern DC-rated fuses, circuit breakers, contactors, and switches do
> > exist. But you won't find them at the local hardware store any more!
> >
> ...


----------



## EVDL List (Jul 27, 2007)

>> Modern DC-rated fuses, circuit breakers, contactors, and switches do
>> exist. But you won't find them at the local hardware store any more!



> Robert MacDowell wrote:
> > I'd love to know where to get them.
> 
> I collect them when the opportunity arises, and have perhaps a dozen of
> ...


----------



## EVDL List (Jul 27, 2007)

They aren't necessarily cheap, but if you need new DC rated circuit
breakers, check out Midnite solar... they make a wide variety of 125VDC,
150VDC, and 300VDC rated circuit breakers, from 3 amps up to 250 amps, with
proper boxes for them all too.

Z



> Lee Hart <[email protected]> wrote:
> 
> > >> Modern DC-rated fuses, circuit breakers, contactors, and switches do
> > >> exist. But you won't find them at the local hardware store any more!
> ...


----------

