Charge Controllers & Grid-tie Inverters [RE-wrenches]

[Ray Walters]

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HI;

I've been playing with this problem with my own EV (144 v nom). The 
losses of the dc-ac-dc are not as bad as you might think. I was charging 
the other day at a little better than 80% efficiency. That was from my 
friend's DR2412 through a 100 ft long 14 awg extension cord to my step 
up transformer and then to a K&W charger. That's a lot of stuff, and we 
read his E meter out of his batteries, and my E meter into my batteries. 
(84%) We could have done all kinds of stuff to improve that: shorter 
thicker extension cord, Manzanita Micro power factor corrected charger, 
etc. My previous charger from Manzanita had efficiency better than 95%, 
it just didn't like dirty grid power around here.
The only other game would be to create your own control system with a 
voltage controlled relay. EV suppliers have special relays with flash 
protectors and other weird stuff to control the very hard to control 
high voltage DC. You'd still need the AC charger for finish and float 
charging.
 Another possibility would be to undersize the voltage of the array 
string, so that it could bulk charge the batteries at the modules MPPT, 
and then as battery voltage rose their output current would just peter 
out. Then you'd just use the DC-AC-DC charging for the last 20% to 30% 
to finish the charge. You might add another module to the string in the 
summer and drop it in the winter... but then the EV batteries need 
higher voltage when its cold...... (and PV to battery mismatch could be 
as inefficient as the original charging method.)
I'm just throwing ideas I've had out....
Another idea I considered was to split the vehicle battery bank into sub 
groups with a voltage an MX-60 could handle (60 v battery nominal max I 
believe). But then that's a whole lot of loot on controllers...
Playing with Vehicle Integrated PV (VIPV) definitely has me looking at 
lower voltage, smaller machines at the moment. (48 volt motorcycle I 
built is a blast)
This is a question thats actually going to come up more as our good old 
USA  progresses away from oil. There is even proposals to use EV battery 
banks to help with grid peaking, and then buy back at a lower TOU rate. 
Get ready ya'll, Grid tie PV was fringe stuff 20 years ago, too.

Ray


 Peter Parrish wrote:

>Thanks Darryl for the reply. 
>
>The AES works with low voltage strings.  Currently there are 4 Shell SP-75
>("12 volt") modules per string. Vmpp around 60 volts and Impp around 4 A.
>And there are four of these four module strings feeding a single AES
>inverter.
>
>I have asked the customer for the operation and maintenance manual for the
>Geo. Don't have it yet.
>
>- Peter
>
>-----Original Message-----
>From: Darryl Thayer [mailto:daryl_solar at yahoo.com] 
>Sent: Sunday, June 11, 2006 12:19 PM
>To: RE-wrenches at topica.com
>Subject: Re: Charge Controllers & Grid-tie Inverters [RE-wrenches]
>
>
>Hi Peter
>As I understand it there is no charge controller that
>works at that high an input voltage, the voltage of
>your string inverters.  As I understand it the
>switching devices generate a lot of heat.  However VFD
>variable frequency drives work at those voltages.  
>
>What is the battery voltages of the GEO?
>Darryl
>
>--- Peter Parrish <peter.parrish at calsolareng.com>
>wrote:
>
>  
>
>>The same customer that wanted a production meter has
>>also indicated an
>>interest in having a solar chager for his GEO (sp?)
>>electric car. He was
>>thinking of a system distinct from the two-inverter,
>>grid-tie system he
>>already has. Of course, the way he charges the GEO
>>is via a AC/DC
>>charger, which he appreciates involves a dual
>>conversion (DC-AC-DC)
>>which wastes power.
>> 
>>I told him a seperate off grid system would have two
>>problems: (1) the
>>PV modules would not be eligible for a rebate, (2)
>>when the GEO's
>>batteries were fully charged he would be not be
>>using the availble
>>generation capacity of the seperate sytem.
>> 
>>So I proposed looking into placing a charge
>>controller in parallel with
>>one of his inverters, and so long as the PV string
>>sizing matched the
>>inverter input range and charge controller input
>>range, we might have a
>>solution. The customer would not have to add more PV
>>modules and he
>>could charge his GEO from his solar resource
>>directly. 
>> 
>>How might the inverter and charge controller work
>>together, expecially
>>when the PV DC power availble is low or the
>>batteries need a healthy
>>charge? Could we err on the size of a small charge
>>controller, to limit
>>contention between the two units?
>> 
>>Thanks in advance for help on what is admittedly an
>>odd-ball request.
>> 
>>
>>Peter Parrish, Ph.D. President
>>NABCEP Certified PV Installer (Cert. No. 031806-26)
>>    
>>


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