Where/How to find an engineer to design AC-coupled system

I now have a 4kw grid-tied system. I would like to be prepared for a grid failure, so need to add on a battery backup. I'm in southern CA. How do I find someone to design and install it?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Where/How to find an engineer to design AC-coupled system

    You have two ways to go...

    One is to find a "high voltage" MPPT Solar Charge Controller and use a transfer switch from the GT central inverter to the high voltage MPPT Charge controller. The only one that I am aware of is the Schneider/Xantrex version that has a max input voltage of ~600 VDC.

    Xantrex XW MPPT 80 Amp 600VDC Solar Charge Controller

    There are a few GT solar central inverters that run at lower voltages... Midnite ha a version that runs at 250 VDC maximum.

    MidNite Solar Classic Charge Controllers and Clippers

    The other options include using TSW type Off Grid Inverters designed to be "back fed" by the GT inverter (Off Grid inverter has to be rated > than the maximum wattage from your GT inverter):

    Sunny Island 5048U 5000 Watt Battery Based (only "native" GT to OG system that does variable battery rate charging--but expensive)
    Xantrex XW Sine Wave Inverters and Accessories (a very capable system. Having a knowledgeable installer is probably a big help--complex)
    Magnum Energy Sine Wave Inverters & Accessories (I think)

    You can use other off grid inverters, but many/most/all (?) of them would require you to setup a dump controller on the battery bank and/or a battery state of charge monitor that turns of the GT Inverter when the battery is full (you should have redundant control--An badly over charging battery is not pretty and can be very dangerous). Also, you may need to install/configure an AC Transfer Switch.

    I suggest that you do some reading and understand the basics... It will help you when interviewing installers if you have basic knowledge. Off Grid/Hybrid systems are a "step beyond" for many GT System installers and they will be (if you are lucky) learning at the same time.

    Are you looking for short term (days/weeks) grid failure or long term (months/years)?

    Also, hybrid systems are not cheap (~$0.45 to $0.75 per kWH for installation and maintenance costs--Electronics will last ~10+ years, typical batteries ~5-8 years or so). If you actually run off grid--You are looking at ~$1-$2+ per kWH (OG systems are less efficient and batteries may not last as long when deeply cycled). Also, for most people, they still need a generator for winter use to supplement solar array during runs of bad weather (and/or severe conservation during the dark times).

    Towards that end, we always suggest doing "extreme conservation" measures first (insulation, energy star appliances, smaller appliances such as laptop vs desktop computer, etc.). It is almost always cheaper to conserve power than to generate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    Thanks much Bill.

    I am looking for a system that will most likely be used for a few days more frequently than months. We have had a couple blackouts recently that were costly when we lost our refrigerated and frozen food. Ice and dry ice worked the second time around, but it was quite expensive as well. I am worried about solar activity that may cause grid failure - the likelihood for that is low, but apparently 2013 has a much higher than usual probability for it happening. I am only looking for enough electrical capacity to run the refrigerator/freezer with solar and have a Honda eu2000i as complementary backup/night-time.

    The process for putting together a gridtie-hybrid system looks iffy since it seems there are no off-the-shelf systems. Like you say, I will have to do some research to improve my understanding, whether I decide to complete the job or not. Your info is a gold mine to get me started!

    Thanks again.
    This forum has so much great info. Thanks to all who put the effort into it.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Where/How to find an engineer to design AC-coupled system

    Hi Steve,

    You are very welcome... and please come back with more questions and/or what you have decided to do.

    A suggestion, since you are looking for "short term" outages--Many times a genset is a better solution. You can use it to power your major loads (fridge/freezer/Air Conditioning/etc.) for ~1/2 the day. And at night/quiet times, make a smaller battery + AC charger (gen/utility powered) + solar panels (if/when you want them).

    That will save you money, fuel/fuel storage, and help with having power for the family (lights, radio/tv, laptop computer, cell/AA battery chargers, etc.) when the genset is not needed.

    Will also give you some experience and help you decide if you every want to go with a full 6kW solar/battery/hybrid GT system later.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    Hello again Bill,

    After taking some time to ponder this, I think I want to approach an electrical engineer to get an estimate on this hybrid backup system. Am I right in interpreting your earlier response meaning that asking a solar installer is the only way to get started? If so, how do you tell whether they are qualified?

    -Steve
    BB. wrote: »
    Hi Steve,

    You are very welcome... and please come back with more questions and/or what you have decided to do.

    A suggestion, since you are looking for "short term" outages--Many times a genset is a better solution. You can use it to power your major loads (fridge/freezer/Air Conditioning/etc.) for ~1/2 the day. And at night/quiet times, make a smaller battery + AC charger (gen/utility powered) + solar panels (if/when you want them).

    That will save you money, fuel/fuel storage, and help with having power for the family (lights, radio/tv, laptop computer, cell/AA battery chargers, etc.) when the genset is not needed.

    Will also give you some experience and help you decide if you every want to go with a full 6kW solar/battery/hybrid GT system later.

    -Bill
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Where/How to find an engineer to design AC-coupled system

    Regarding finding a good installer/contractor... I think you are going to have to call, ask questions (that is why I suggest you get the basic understanding here--so you can verify/understand what folks are telling you), and check references.

    Whether you do it yourself, or use an installer. Only you know your skill set (and willingness to climb on a roof, deal with permits, utility approval if Grid Tied/Hybrid, etc.).

    Me, I am getting older and have family responsibilities--So I use contractors much more than when I was single, younger and life was simpler.

    Certainly the first install--There is a lot to learn. After you have seen/helped on one or two--They are pretty straight forward (although, the configuration programming for the hybrid systems does no seem "obvious" for these complex systems).

    Things that you can help with are measuring your loads and clearly defining what you are expecting (emergency backup power, weeks/months/years of off grid power, etc.). Each will have its own cost/benefits.

    Even if you decide on an installer (hopefully with experience with the type of install you want -- GT/Hybrid/Off Grid), you can still do the power measurements and insulation/efficiency stuff for your home--Which will help reduce solar power costs a lot.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    This problem of installing battery backed capability to a grid-tied system is a thorny problem that the solar industry has yet to solve affordably. Beyond basicly being expensive because of batteries, there is an inherent difficulty of the system wanting to keep the batteries full to be ready for an outage and leaving room for more solar charging which is intermittent of course. The Xantrex XW and the Sunny Island systems can deal with this efficiently, but is why they are expensive. I've toyed with using a dc-dc downconverter feeding an mppt charge controller as a way to connect a high voltage array to a simple off-grid system for use during an outage, but have not seen an off the shelf solution yet. Another possibility is to do some complicated wiring of your array (probably should use high-voltage rated switches) to, in event of an outage, manually reconfigure at least part of it to a lower voltage allowing use of a Midnite 250V charge controller.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system
    solarix wrote: »
    ... wanting to keep the batteries full to be ready for an outage and leaving room for more solar charging which is intermittent of course.

    I would think that as long as you have the grid tie, there is no point in "leaving room for more solar charging", you just sell all of the power once the batteries are full.
    If you have an AC fed battery charger in the mix too, then it does get more complicated, but you still want to keep the batteries full, you just do not want to do so via an inefficient route.
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    another option may be a hybrid inverter/charger. (has gt sell ability at lower efficiency) this can be configured with a portion of your array and will sell any excess power after the batteries have charged. this would feed the ac circuits that you wish to be backed up. design size of battery bank and solar depends on the loads being driven and for how long.

    it is important to know that the highest efficiency will be from the straight gt inverter, but during an outage they are dead in the water unless backfeeding with another inverter to mimic the grid as bb suggested. the grid tied hybrid type inverter/charger will sell and charge batteries, but at a lower efficiency.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    Charge controllers throttle back when the batteries are full to a "float charge". They won't maximize the harvest from the array unless the battery is in the bulk charge zone which is significantly less than full. Its not until you go for the higher end equipment, can you get the capability to handle this. Besides, the OP was talking about an outage situation, where selling back to the grid is not possible. AC coupling two inverters in an off-grid system is not easy.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    i made no reference to charge controllers and i do know what he was asking, but you are correct that backfeeding an inverter can get sticky.
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    I also have the problem of not going back on my promise to my utility to send my power to the grid, so, can any of you guys foresee a system that allows me to connect only during grid failure?
  • vtmapsvtmaps Solar Expert Posts: 3,739 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system
    SteveF wrote: »
    I also have the problem of not going back on my promise to my utility to send my power to the grid, so, can any of you guys foresee a system that allows me to connect only during grid failure?

    My 2 cents: Stick with your generator for grid outages. Increase the size of your grid tie system. Use the extra money you save on your utility bill to buy a few gallons of gasoline for your generator. Avoid batteries.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system
    vtmaps wrote: »
    My 2 cents: Stick with your generator for grid outages. Increase the size of your grid tie system. Use the extra money you save on your utility bill to buy a few gallons of gasoline for your generator. Avoid batteries.

    --vtMaps

    normally i'd agree with only a generator as backup up to a day or so of backup, but during the evening it may not be a good idea to run it if you have neighbors and cops like mine. an inverter with batteries can fill in some of those gaps when the noise would be too much and fuel is running low (you don't want to siphon your car's gas tank do you?) and can afford a more smooth switchover after utility power has been lost. to run either constantly may not be a good idea as one consumes much gas over 48hrs along with its noise and the other has limited capacity resources governed by the battery bank size. blending both and conserving power to be on only at certain times or intervals could work out nicely.

    "can any of you guys foresee a system that allows me to connect only during grid failure?"

    as to this question, i'm not sure what you mean here as during an outage a grid connection is moot as no power source you come up with will be allowed to be active on the grid. you still have sellability during normal operations with either a hybrid or straight gt inverter.
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    @niel
    It's just that if the utility finds my backup system connected they will request the repayment of the rather large rebate I got for the grid-tied system. So, I was just wondering whether a simple, but very visible, connect/disconnect could be in my system circuit to satisfy the utility that I wasn't violating the spirit of the agreement about not having a battery backup.
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    @vtmaps
    Thanks. it's something for me to further ponder. I do not trust our government at this point to be up front about problems that the grid has and I have read too much lately about the fragility of our country's energy transportation system vulnerability.
  • SteveFSteveF Registered Users Posts: 7
    Re: Where/How to find an engineer to design AC-coupled system

    @solarix
    Thanks. That is the type of info I was looking for.
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Where/How to find an engineer to design AC-coupled system
    SteveF wrote: »
    I also have the problem of not going back on my promise to my utility to send my power to the grid, so, can any of you guys foresee a system that allows me to connect only during grid failure?

    What is the agreement with your utility? Is it that you are not allowed to have battery back up or is it that you just need to be selling back extra power? If it is only the later then I would suggest you consider a hybrid or "grid interactive" system as Niel suggested. From what I've read, AC coupled systems can work well but are complicated and expensive.

    With 4KW of pv you could easily change over to an Outback system using 1 GVFX 3648. With this inverter you can sell power to the utility when the grid is available and run on battery power when it is not. Prewired Midnite Solar epanels with this inverter make installation easy and any electrician with solar experience (or with the interest in learning) could do the install. No need for an electrical engineer IMO. The many experts on this forum could help you get the basic design pinned down. You would be replacing your grid-tie inverter with this system. Perhaps you could sell it to defer some of your costs.

    Just MHO of course! Good luck.
  • FatawanFatawan Solar Expert Posts: 71 ✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    It wouldn't be cheap, but Victron has a solution for this--not sure if you need to use their inverters or not--Its Lynx + Quattro + Lithium batteries + inverters. I think the Quattro would even allow you to add a generator to one of the inputs if needed.
    http://www.victronenergy.com/inverters-chargers/quattro/
    http://www.victronenergy.com/upload/documents/Datasheet%20-%20Quattro%203kVA%20-%2010kVA%20-%20rev%2012%20-%20EN.pdf
    http://www.victronenergy.com/dc-distribution-systems/lynx/
    http://www.victronenergy.com/batteries/lithium-battery-24v-180ah/
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    Magnum has a new inverter model for off-grid that is capable of AC coupling for use in a hybrid systems and is quite a bit less costly than the SMA Island or Xantrex XW.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system
    SteveF wrote: »
    @niel
    It's just that if the utility finds my backup system connected they will request the repayment of the rather large rebate I got for the grid-tied system. So, I was just wondering whether a simple, but very visible, connect/disconnect could be in my system circuit to satisfy the utility that I wasn't violating the spirit of the agreement about not having a battery backup.

    This has the feel of a philosophical dispute. If you connect a totally separate battery backup system to your house, then you will have a situation where you sell all of your excess AC production from PV back to the grid, but part of your load on the GTI and grid will be going to a battery charger. No problem there, and none of POCOs business as long as you use an NEC compliant transfer switch setup to go to battery during a power outage.
    Now compare that to the situation in which some of your solar PV goes directly to charge your batteries. From the POCO point of view, in letter and in spirit, the result is just the same.

    What could be different is that POCO does not want you to collect any rebate or incentive based on that part of the system cost which is attributable to the backup function. In that case, if you still use exactly the same components for which the rebate was paid and do not try to collect a rebate on new equipment you add to get the battery backup, you should still be OK.
    The question of what happens if you stop using the equipment that you got the rebate for is one I will leave to the lawyers. :-)
    SMA SB 3000, old BP panels.
  • Robin GudgelRobin Gudgel Registered Users, Solar Expert Posts: 58 ✭✭✭✭
    Re: Where/How to find an engineer to design AC-coupled system

    MidNite does offer a few systems to accomplish the AC coupling, but as you mentioned, they are not cheap. Then again, neither was your GT system. There just ain't no free lunch when it comes to AC coupling. The MAgnum system we offer is about the least expensive system possible. The Xantrex XW inverter is also good and does not require a solid state relay to knock the GT inverter off line. The problem is that you cannot buy an E-Bay inverter and AC couple it. You need a real inverter. Magnum, XW, OutBack or Sunny Island. The inverter is only one part of the expense though. You still need batteries, switchgear and over current protection, solid state relays and don't forget to make it all code compliant. Yup, not cheap. Most installers are not yet up to speed on AC coupling. It is pretty new and not so obvious.
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