Grid tied back up generator: DC coupled?

One of the interesting features built into modern hybrid inverter/chargers is the ability to use power frequency shift power control to manage AC coupled inverters. The Xantrex system is binary in that it enables/disables their AC coupled GT inverters. SMA's Sunny Island and Sunny Boy inverters seem to have a linear scale to their power control with frequency shift. Both have problems with an external back up generator: (a) the generator is not the grid, so it can not be back-fed surplus power and (b) it controls the power frequency, marginalizing FSPC by the hybrid inverter.

What if the generator were DC coupled to the batteries of the hybrid? Now, the hybrid can generate the AC and use FSPC to manage the AC coupled GT inverters. The generator start/stop can also be controlled by the hybrid based on demand and/or battery state. However, the obvious solution is to use a 120V/240V generator set and a 120V/240V charger. But the intermediate 120V/240V step is unnecessary and reduces efficiency through multiple conversions. What about a generator set that is designed to directly charge batteries? The telecom industry has been doing this for years. Here is one example unit that I have found through a quick Google search: portable gasoline -48V DC generator for telecom applications

Has anyone tried this? Any thoughts? Potential pitfalls?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,747 admin
    Re: Grid tied back up generator: DC coupled?

    The DC genset you pointed to seems to be a pretty fuel efficient unit. Might be a good alternative.

    I don't see any major issues other than that is still a pretty large genset (11kW DC).

    I am a believer in smaller off-grid systems and using smaller gensets.

    But if you have the need for larger amounts of power--it does look interesting. But that genset is still gulping down more than 1 gallon per hour of fuel at full power. Lots of fuel storage needed for longer term use.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Grid tied back up generator: DC coupled?

    In such a situation the DC generator would be just like a PV controller injecting charging current into the batteries. The generator would have to provide voltage and current regulation for battery charging just like a PV controller.

    The only issue is DC generators are usually more expensive then AC generators, primarily because of commercial volume is much lower and the competitive price pressure is not there to keep prices low.

    As to efficiency, you have the DC to AC XW inverter conversion loss. A straight AC generator pass through would be more efficient for AC loads. For battery charging the DC generator direct would be more efficient.

    For PV power, it is more efficient to AC couple a GT inverter to XW output then run a PV controller to charge batteries. For AC coupling of GT there is only the conversion loss of the GT inverter for AC loads. For PV controller to battery to XW inverter AC you have conversion loss of the PV controller (similar to GT inverter) plus the conversion loss of the XW inverter converting DC to AC.

    In both cases, if there is no grid connection to consume excess power, if the power cannot be used either for AC loads or battery charging the PV potential generation is lost.
  • Dr. StrangeloveDr. Strangelove Solar Expert Posts: 49
    Re: Grid tied back up generator: DC coupled?

    Bill, I had picked a smaller model genset as an exemplar until I realized that it was pull start. Part of the design of theis system is the automatic fall back to generator power. However, if the generator will be supplying DC to the inverter(s), it seems that it should be rated to the max load of the inverter(s), say a pair of SI 5000US, about 10kW.

    Concerning the efficiency: I would think that difference between generator DC -> internal inverter -> AC -> loads versus generator DC -> hybrid inverter -> AC -> loads would be a wash, assuming the genset inverter is as efficient as the much more expensive hybrid inverter. The DC delivered above the needs of the load would directly charge the batteries, rather than pass through the AC -> inverter charger -> DC path, with higher efficiency.

    The concern about availability and price remain. On the plus side, telecom equipment is designed for very high reliability and uptime, compared to consumer grade equipment. There will be a price differential for that alone, but at least you do get something for it.

    Has anyone tried this approach?
  • VicVic Solar Expert Posts: 2,976 ✭✭✭✭
    Re: Grid tied back up generator: DC coupled?

    Dr. S,

    I have not tried this.

    And realize that you are using this as an example, but looks to me as if this unit has a settable DC output, which is "regulated", but a CC function is lacking, and would not EQ most batteries even HOT ones, but for your use, this might be fine as a Bulk charger (and you may be thinking of AGMs).

    Other units may have more features, and so on.
    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: Grid tied back up generator: DC coupled?

    I have a Alpha Tecnology 48 volt DC generator I have been thinking of using on a 24 volt system if I ever get to it. I am planing on using an MX 60 charge controler to charge the batterys. This was talked about last year and a lot of people here think it will work. I have 2 unused sw inverters. If I could sell them I probably would get a Magnum inverter and AC couple one of my extra GT inverters I have. Sale prices+ $1350 for the SW 4024 and $1650 for the SW 5548. If anyone interested give me an Email. Otherwise if I don,t sell my sw-4024 I am going to go ahead and use it. Will set the inverter charger to use the grid during the most productive hours of my GT inverters. :Dsolarvic:D
  • Dr. StrangeloveDr. Strangelove Solar Expert Posts: 49
    Re: Grid tied back up generator: DC coupled?

    Vic, I was thinking along the lines of the genset being used for bulk & absorb charging as well as powering the hybrid inverter. The particular unit I picked does give me the willies because all its labeling appears to be in Chinese.

    Thank you Solarvic for the Alpha Tech reference, these are labeled in English and have propane and "natural gas" jets which I plan to use for my application. They appear to have a separate power input for the starter battery, which eliminates a known limitation of the Generacs. They look really nice. Would you happen to have any other references for DC generators?

    I had hoped that these DC gensets would be configured as at least 2 stage chargers, offloading some of the battery management. But I think I'll need to think about the entire system interaction, and how a single extended outage might cause the interplay of multiple cycles of battery charge & discharge and generator start, run and stop.

    Thanks again.

    Andrew
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: Grid tied back up generator: DC coupled?

    One of the appealing things about a DC gen setup is that your loads will always be powered by a nice clean sine wave from your inverter and not a dirty wave from an AC generator.
    On the other hand, you're then 100% reliant on your inverter, if that fails, you have no backup. Many of the newer inverters also have a gen "boost" or "assist" feature where they can add to the existing power of an AC gen, which is another feature you'd lose out on with a DC unit.

    With regards to availability, what about the combined generator/welder units that they sell? Have seen these with selectable DC voltages in the range 22 - 40V ish. One of the modes is Constant Voltage which would suit DC charging. Some of the units seem to offer 120V/240V and DC welding. The best of both worlds, if it works ;)
  • solarvicsolarvic Solar Expert Posts: 1,053 ✭✭✭✭
    Re: Grid tied back up generator: DC coupled?
    Vic, I was thinking along the lines of the genset being used for bulk & absorb charging as well as powering the hybrid inverter. The particular unit I picked does give me the willies because all its labeling appears to be in Chinese.

    Thank you Solarvic for the Alpha Tech reference, these are labeled in English and have propane and "natural gas" jets which I plan to use for my application. They appear to have a separate power input for the starter battery, which eliminates a known limitation of the Generacs. They look really nice. Would you happen to have any other references for DC generators?

    I had hoped that these DC gensets would be configured as at least 2 stage chargers, offloading some of the battery management. But I think I'll need to think about the entire system interaction, and how a single extended outage might cause the interplay of multiple cycles of battery charge & discharge and generator start, run and stop.

    Thanks again.

    Andrew
    I bought my alpha tech generator off Ebay new a few years ago. One telecom co bought out another and decided that they weren,t going to use them. The generator part has a Koehler motor and generator and Alpha tech. has an ECM that controls starting and monitoring the batterys. It has its own starter and 12 volt charging system for the starter battery. The one I got is 48volt DC. The geneerator can charge 96 amp at 62 volt. Since I already have a SW-4024 inverter and a Exeltech 1100 24 vdc inverter I plan on having a 24 volt system. So will probably charge the 24 volt system with a outback charge controler. I will just manually start the generator when I need the generator. Also don,t want the extra expense of the extra batterys need for a 48 volt system. I started a post here. If you look under my solarvic name you can find it. How to charge 24 volt with 48 volt generator. Might be interesting to you. I have a gas well with free gas to power the generator. I get a lot of power outages because the power co don,t keep the trees trimmed like they used to. Usually I get at least 2 power outages on average for every month over the year. I don,t know what you can buy on of these alpha tech for but have seen them on ebay a few times. The ECM has a lot of dip switches where you can customize the programing of the generator system. :Dsolarvic:D
  • Dr. StrangeloveDr. Strangelove Solar Expert Posts: 49
    Re: Grid tied back up generator: DC coupled?

    Solarvic, I found your old thread. I plan to go with a 48V system (though the the 96V option on the 7.5kW Alphagen is intriguing - but it has NEC implications as well as a lack of 96V inverters). The ECM sounds like it is pretty full featured for battery charging. I am jealous of your gas well! "Natural gas" here is no cheaper than the electric monopoly. Ooops, did I mention they just got another rate increase through the sleepy PUC? Residential power will now be about $0.31/kWh.

    So WTF is up with Alpha and password protection of their manuals on their web site? Are they supposed to be a secret or something? Or do their mar/com people want more fodder for their cold-call lists to show how productive they've been? I can't figure out what a CE-3x, CE-9x, PN-4xL or PN-6x is supposed to be from the available documentation. Why is the CE-3/9G not compatible with the PN-6x? Sure, they throw out that mumbo-jumbo in their marketing collateral but they don't bother explaining what it's all about - mar/com people drive some of us engineers nuts! Why can't they use the same terms their own engineers use?

    I also noticed that Alpha Tech is the same outfit that bought up Outback. How's that working out? I have connections in Bellingham, but the Alphagen genset operation appears to be in Burnaby, BC.

    Edit: My mistake, the Alpha Tech web site is nearly as fractured as the Schneider site. If you look up their AlphaGen products for TV cable plant support, the page is far more useful and informative, with non-password protected PDF documents.
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