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Doing some thought experiments on 'Solar Charging'. Photovoltaic Solar and Rechargable batteries seem to be a match. Solar Cells produce DC current and Rechargeable batteries eat DC current. Yet discussions I see in and around the forums when it comes to discussing solar charging keep mentioning 'inverters' which are the devices for converting DC current to AC current, which would then be passed to the EVSE(charging station)to be fed to your EV(such as the volt) which uses an internal rectifier to convert it back to DC current to feed to the batteries. So we have:

Solar(DC) => Inverter(AC) => EVSE(AC) => Rectifier(DC) => Batteries

What is the lossage in the Inverter&Rectifier these days? The amount of power lost?
 

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90 - 95% efficiency for each step. Your diagram also misses the Volt's DC-DC converter between the rectifier and the batteries and losses due to the wiring. Most of the concern is with maximizing the efficiency of the solar panels. In order to be able to charge the Volt even when the Sun is not out, you have to be able to "store" the power in the grid (spin the meter in reverse) so you have to invert it.

Just flip the Volt over and lay it on the front lawn to catch some rays ;)
 

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90 - 95% efficiency for each step. Your diagram also misses the Volt's DC-DC converter between the rectifier and the batteries and losses due to the wiring. Most of the concern is with maximizing the efficiency of the solar panels. In order to be able to charge the Volt even when the Sun is not out, you have to be able to "store" the power in the grid (spin the meter in reverse) so you have to invert it.

Just flip the Volt over and lay it on the front lawn to catch some rays ;)
Wasn't Solyndra's solution to have the solar collectors facing 360 degrees...no flipping required :) Even if DC to DC charging was available, solar panel power would still have to be rectified. And household current plus appliances are AC.

Solar Inverter and Rectifier Lossages, rhymes with; solar cooking your breakfast sausages. Gotta go don't 'cha know! I'm out don't pout.
 

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Discussion Starter #4
90 - 95% efficiency for each step. Your diagram also misses the Volt's DC-DC converter between the rectifier and the batteries and losses due to the wiring. Most of the concern is with maximizing the efficiency of the solar panels. In order to be able to charge the Volt even when the Sun is not out, you have to be able to "store" the power in the grid (spin the meter in reverse) so you have to invert it.

Just flip the Volt over and lay it on the front lawn to catch some rays ;)
I guess what I'm trying to figure out, is it worth it to try and 'mainline' and EV's rechargable batteries from solar panels or other DC sources. Theoretically yes, and you might capture some more energy BUT, if you don't put some extra circuitry to regulate the flow as the sun comes&goes in there you could end up damaging the batteries.
 

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My system takes energy from the solar panels, charges my main house batteries, and those are inverted to AC again to charge the Volt, which I only do when I'm not really incurring the round-trip loss to the house batteries, or at least minimizing it.

Solar panels ARE diodes, and in this case they are forward-biased, which is what limits their voltage output at the point at which the cell diodes turn on (if that didn't happen, there'd be another limit - how short the photon wavelength).

At any rate, once you start loading solar panels, they become almost constant current devices (assuming non changing sun conditions of course). So there's a voltage, which is temperature dependent, at which they make peak power. Not only is this voltage temp dependent, it's also insolation dependent. Modern charge controllers incorporate a very efficient switching supply to match this max power point to whatever the house batteries are at - which itself varies all over depending on state of charge, temperature, and net in-out flow of power. Doing that - while it does incur some diode and switching and inductor losses - gives you nearly 30% more out of a given set of panels in most conditions. You get most of that gain when you most need it, too - when the batteries are low voltage yet the panels are cold and in overcast sun.

It's insanely better than just a direct "efficient" hookup is, and I've tried both. So there, the extra electronics are way worth it even though there's some loss in them, because there's even more lost harvest just loading down the panels to a lower voltage where the power is dropping because the current out of them isn't going up as fast as you're lowering the voltage on them in a direct-connect.

This can't be "fixed" as easily as you might think. You have to have the excess panel voltage to handle the other worst case, where they are hot (reduces the max power voltage point a lot) and in low sun. So there will always be times when without that conversion you're wasting possible power harvest.

There's a similar switcher in the Volt that takes the input AC and rectifies it and then adapts the output voltage to whatever the batteries want at the current it wants to put into them as is. This limits at some desired current drawn from the power company as well, so you don't fry things when the battery is near-dead and accepts charge at a lower voltage than when it's charged. This charger therefore needs to be more constant-current than constant voltage. Doing that with a switcher is FAR more efficient than just having a dropping current source...

So no - even theoretically you don't get more "mainlining" because you don't get the max power available from the PV panels doing that even in the best case of perfect matching for a *single* set of conditions of panel condition and battery condition.

Remember, there's a difference between the oversimplified "press release" theory, and the real deal, almost always. It's easy to "violate" the oversimplified version without actually having anything non-theoretical going on, once you know the real story.
Edit: Here's the curve for a typical PV setup.
mpp.jpg
 

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One way to up this efficiency is to grid tie the solar and use net metering. With no true battery charging losses all the available power in the solar panels is translated via a mppt to AC inverter and either the power is used directly by the loads available on the homes meter side of the connection or the meter is "run" backwards for a credit for later use. In this way all the power gets used and no losses are incurred with an intermediate battery step. The net metering then provides all the power back to the consumer on demand. This storage is more than short term like batteries in most cases and is usually rated on a monthly or annual basis, in effect storing power in January for use in June. As nice as an off grid system seems, it is very expensive per kWh in additional hardware costs, battery charging losses and solar power not consumed with no place to go when the batteries are full and no loads are available. That being said, it is nice to have full control of your power source with no games from the "man" but it has its price.

In my case the usage in the summer months more than outstrips my solar setup capability. I need like 120 kWh on really hot days for the AC but the solar system only gives about 70 kWh of production on those same days. But as of the first of May we have 2070 kWh in the credit column for on peak TOU. I suspect that by the end of May if it doesn't get too hot we will be at 2500+ kWh for the June through September high consumption months.

This is the best of both worlds, you get 100% efficient "storage", the power is available when the rates are the highest and our need is the greatest. With a TOU plan and some creative load shifting, with a medium size solar system one get get a pretty good ROI by clipping the tops off or completely replacing the TOU peak rate power.

PS those crazy plans in California that pay back net metering in dollars can actually be a money making venture, where really high priced tier payback is then used to offset the costs of large off peak consumption. A relatively small system can make a net zero dollar bill.
 

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I thought I would update this thread with new information and technology available today. I was at the 2014 Solar Decathlon in Irvine, Ca and a number of the contestants were using solar panels with a DC-DC charger for the EVSE. They promoted it as more efficient than the DC-AC-DC transmission as there's about a 8-15% loss in efficiency at each inversion. As I'm looking into buying a 4.56kW rooftop solar array and a 2016 Volt, I'm starting my research for a DC-DC EVSE.

As far as California Net Metering, each utility has a maximum cap for Net Metering V1.0 and the whole state will switch to Net Metering 2.0 after July 1, 2017. From what I understand PG&E and SDG&E will reach their cap in a few months, but SCE will likely not reach the cap for another year. The agreement is grandfathered for 20 years (the estimated life of the solar array). Between overbuilding the system size and time-of-use rates, it can be quite profitable for the owner. However, legal and social issues arise as a surplus energy owner is not being taxed for retail energy production and the owner is using the utility company's grid system while the company is losing money from each owner's usage. For this reason, some utility companies will not agree to inter-tie or connect grossly oversized solar arrays and some have started a minimum $10 monthly charge. There have been proposals for each solar owner to pay a fee based on system capacity and to reduce the rate that the utility companies pay for surplus electricity. These changes will be incorporated in Ca Net Metering V2.0. California is weighing the impact of each proposal as Nevada's and Arizona's policy change has greatly affected the solar and utility industries. I believe California is a net energy importer as we have mostly natural gas power plants. We do not get a significant amount of energy from hydro-electric, nuclear, or wind.
 

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There are very noticeable differences in efficiency when you stick strictly to running DC rather than converting it back and forth from AC to DC. In fact, some people have started to run their entire home off of 12V, though that seems to be more of a tiny home thing. The problem is, in order to do that, you need both a functioning battery back up system and 12 VDC appliances, which are not yet common. With brushless DC motors on the rise, this might become a more compelling route if you are truly interested in living off the grid.
 

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While DCFuser is exactly correct with needing the DC-DC conversion at the input charger stage, there are gains to be had with loosing the AC conversion step. Solar is fickle, we need a storage point to buffer power in vs power out, but minimizing the conversion losses is always the goal. At a given power, low voltage like 12V means high current, and high wire cost (heavy gauge), while high voltage allows low current, low wire cost, but higher cost electronics. Given the constant cost reductions in semiconductors, compared to the cost of copper, you see where we're going.

12VDC infrastructure is a convenient low cost and efficient means to distribute low power, but in the power range we're interested in (1-100KW), High Voltage DC has only recently started to make sense. Tesla PowerWall is an interesting step, HVDC could be the future for residential distribution.

I have a small 12VDC power distribution system in my house (in parallel to the normal AC). 2x 100W panels feed a 12V battery bank (through a mppt charge controller). The battery bank provides 12V to security lights, cameras, internet and network equipment, and device charging. It also powers a fish tank filter and light. During a sunny day, the panels provide 100% of needed power, and a little extra to float the batteries. Without sun, utility power takes over. Without utility or solar, the batteries kick in.

Certainly this is a hobby project, cost of such just can't compete with the $0.10/KWHr utility power, but it does provide backup, security, and is interesting learning for a larger scale 120VDC (or higher) distribution system, that I may eventually plan.
 

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The problem with DC only system is what do you do with the excess power produced? No DC load eek no production. With the AC conversion and net metering you get a credit for the power (how it is credited varies a bunch). I produce way more than I can use in Jan. but consume it back in June. No battery based system can provide that. Low voltage DC has problems transmitting enough power before the amperage climbs sky high. High voltage DC it is more critical to have faulting systems in all the wiring because of the ability to sustain an arc.
 
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