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Are fossil fuels required if an off-grid home has resistive loads?

  • 1.  Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 05-14-2023 02:05 PM

    I just finished reading the book "Photovoltaic Design and Installation for Dummies" by Ryan Mayfield. Overall, it seems like a good book. I noticed a surprising paragraph in it. 

    "One type of load that should never be placed on a battery bank is anything that uses electricity to generate heat (these are called resistive loads). That rules our water heating, space heating, and electric stoves (incandescent lights also fall into this category). In a stand-alone, battery-based system, petroleum-based fuels generally support resistive loads, with propane being one of the most common in off-grid applications. For people connected to the grid with a utility-interactive, battery-based system, these loads can still be present; you just can't back them up with the batteries."

    Does this mean that anyone who has electric heating, incandescent lighting, electric stoves, or space heating has to have a generator run by propane or another petroleum fuel if they are off-grid? 

    Thanks in advance for your answers to this question. 

    Best regards,

    Kat



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    Kat Friedrich
    Editor in Chief
    American Solar Energy Society
    Monona WI
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  • 2.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Chapter Leader
    Posted 05-15-2023 09:15 AM
    You would want to place a load controller between the batteries and DC electric heater. You could also run 120-volt AC electric heaters through an AC-DC inverter.

    Sent from my iPhone




  • 3.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 05-15-2023 10:51 AM

    To go to your last line directly, NO!

    That is an odd paragraph. The only way I can excuse it is by saying they were being very generalized all over the place.
    Ironically resistive loads are not the hellcat for off grid. Its inductive loads, IE anything with motors. And within that sub group, SIZE matters. Your fridge has a motor but its inductive "Hit" is small enough that most inverters won't "hiccup" on it.

    When a motor starts, in the first split instant, there is a huge current surge as compared to the current draw after it is up to speed. It is this current spike that can cause big voltage drops on the inverter and send its voltage mins into the toilet, potentially causing error trips faults, IE loss of backup power.

    SO if there should be anything on his naughty list, it should be inductive loads, not resistive loads, which are basically uniform.
    Now, giving him the benefit of the doubt, resistive heating tend to be large, so I would say his implied heating nono is based on just load size as opposed to the "structure" of the load.

    Including incandescent lights is a bit of a stretch, unless you have a ton of them. Again its cumulative.

    So technically speaking, resistive loads (Heating) can be dealt with IF YOU HAVE a big enough capacity from the inverters AND the battery bank.
    Commonly speaking, they are the ones people generally look at last, because they are big and money is usually factor. SO, people usually shoot for going after the really important ones first, water pump (Yes I know, motor) fridges, microwave, lights, communications, etc... and maybe set the heating really, really low or use low wattage foot heaters (little box cubes)., as opposed to a central heating system.
    Hope that helps....



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    william fitch
    Owner
    www.WeAreSolar.com
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  • 4.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    ASES Life Member
    Posted 05-15-2023 02:05 PM

    I'm going to disagree with Bill on this one.  The off grid guys I know (through the Outback Power user forum) all pretty much use their systems to run well pumps, A/Cs (many are in warm climates), even large power tools.  The secret is in having an inverter sized large enough to handle the starting current.  And many have soft start motors that reduce the inrush current. Most off grid batteries can handle large surge currents as well, if they are sized appropriately.

    Resistive loads for heat are a steady current draw on a battery and will quickly reduce your battery "fuel tank".  That's why the push to heat pumps and HP water heaters.  

    In short, resistive loads are not a good choice if you want to live off grid, unless you have a large solar resource with a huge battery. 



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    Mike Curran
    Retired EE
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  • 5.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 05-15-2023 02:22 PM

    I am not sure we really disagree that much.
    Big enough inverters can eliminate all load issues, as can a big enough battery bank for total power. Basically my preference for res over induct. is steady VS not. It is always easier to handle a steady load than one that is not, aside from the total power needed over time. Inductive loads (Industrial) are the common enemy even with grid stability. Industrial arc furnaces, huge, huge motors, etc.. 
    Ask res customers who are on the same feeder lines for foundries and steel plants, etc.. It isn't pretty. Things have gotten better than they used to be, line stabilization, etc.., but steady res loads do not have these issues.



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    william fitch
    Owner
    www.WeAreSolar.com
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  • 6.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Posted 05-15-2023 02:31 PM

    Technically it is possible to run 100% on solar+battery in an off-grid application with all types of load. The problem is economics as battery capacity tends to be large and expensive in off-grid installations. It is advisable to update the numbers every 2-3 years as the cost of batteries is falling! 

    Regarding the specific case of resistive or heating load, the concern may be the size rather than the type of load. A rice cooker for example with 500 watts taking 30 minutes to cook will consume 0.25kWh which seems manageable if you have a 10 kWh home battery.
    For an off-grid space heating, heat pump, solar thermal, good insulation are options to consider to optimize system cost-efficiency performance. Incandescent lighting ! i am not sure if they are not banned, or why it is needed. If it is for decoration, there are LED that mimic incandescent filament while saving energy.

    I don't know the full context or date of the writing, however, I think that nowadays it is possible to eliminate completely the need for fossil fuels in off-grid applications without a lot of financial burden.



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    Elias OUEDRAOGO
    Business Developer
    Future Energy Company
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  • 7.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 05-21-2023 05:39 PM
    Edited by Kat Friedrich 05-21-2023 05:41 PM

    Thanks for your responses, everyone. 

    I was going to recommend this book as a shelf book for the ASES office, but based on your comments, I think it may have technical issues. It seems to advocate for undersizing solar power systems and relying on utilities and generators more than I hear other people discussing. 

    What are some other books that you think provide a better general overview that really gets into the nuts and bolts of how to install and design PV systems? 

    As someone who is editing Solar Today, I want to be familiar with instructions for the process down to the nut/bolt level, even if I am not planning to ever install solar panels hands-on myself. 

    Thank you,

    Kat



    ------------------------------
    Kat Friedrich
    Editor in Chief
    American Solar Energy Society
    Monona WI
    ------------------------------



  • 8.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 05-22-2023 05:42 AM

    WOW! I think that is a 7 course meal. 

    There's production, load and storage. It is really that simple. However saying that "truth is a bit like saying to put a man on the moon, build a rocket, put man in rocket, fire rocket and wait until the return. Oversimplification.

    The advise should be to over size PV arrays not the opposite, on that single point, money available.

    Electric power systems like many complex systems are just that. Complex. You really can not distill down a whole complex engineering area to a few sound bites, IF YOU WANT "in the weeds" understanding. My KISS idea is simple but it does NOT ATTEMPT to get into the weeds. Not its objective. 

    I think if you want a really good understanding of electric power production and its intertwining's, it requires allot of reading across many authors and a lot of time. Its a bit like trying to understand how to be a good cook from one recipe book. Probably not going to happen. For me understanding electric power came from the understanding of electricity from early Science, right up through Physics and beyond. You have to understand the fundamentals other wise there will be misunderstandings and misconceptions along the way.

    I wish I had an easy compact answer for you. I do not.
    I suppose the best idea I can say is start with ACTUALLY fully understanding electricity at its core. Go from there. That IS THE ENERGY.



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    william fitch
    Owner
    www.WeAreSolar.com
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  • 9.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Posted 05-22-2023 12:21 PM

    This book is quite easy to read to grasp the overall first-hand knowledge about solar PV for homes. It emphasizes that all great designs start with a dream! Once you dream to go 100% solar, you will go beyond the analytical reasoning to find ways that make your dream come true

    On the book website, you will find beginner-friendly tools for designing off-grid PV systems, by inputting the types of appliances, battery size, location….

    http://www.solarelectricityhandbook.com/solarcalculator.aspx



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    Elias OUEDRAOGO
    Business Developer
    Future Energy Company
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  • 10.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Posted 01-01-2024 02:49 PM

    I've had a remote Alaska off-grid facility for 27 years and consider resistive loads too demanding and inappropriate. I heat with wood when needed (only on site for March and summer) and have a supplementary propane space heater that is minimally operated. I have a 48v/1200 ah battery bank (16-600ah Sureties) and not had a need since 2014 to run any back up genset to charge the batteries. Each battery cycle rarely hits 25-30% drawdown. Avoid drawn down of 50% or more.  We have two 10 module azimuth tracing arrays for a total of 4.4kW. The trackers are not activated in March. Ample sun/diffused sun and albedo from the snowpack is ample for any electronics, shop equipment and kitchen appliances, LED lighting. 



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    Frederick Dure
    Investment Manager
    Boreal Wilderness LLC
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  • 11.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    ASES Life Member
    Posted 01-01-2024 04:51 PM

    "Does this mean that anyone who has electric heating, incandescent lighting, electric stoves, or space heating has to have a generator run by propane or another petroleum fuel if they are off-grid?"

    No.  If you have enough solar resource and a large enough battery, any amount of extra loads can be supported off grid.  But it's much more efficient to use efficient electric appliances.  With today's available technology, anything that heats with resistance can be replaced with something better, and minimize the need for a huge battery and solar array to charge it.



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    Mike Curran
    Retired EE
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  • 12.  RE: Are fossil fuels required if an off-grid home has resistive loads?

    Silver
    Contributor
    Posted 01-02-2024 02:21 PM

    @Mike Curran, that is what I was thinking too - and that is what prompted me to ask the question. 

    I appreciate everyone's responses.



    ------------------------------
    Kat Friedrich
    Editor in Chief
    American Solar Energy Society
    Monona WI
    ------------------------------