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Can you offer some advice on refrigeration?

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  • Can you offer some advice on refrigeration?

    We have an off grid house on an island and our Dometic RGE400 fridge is giving us a hard time this year (pilot wont stay lit) and honestly, I've never liked the unit in the first place. I would really like to go solar. We already have a small solar setup, and I upgraded the charge controller in anticipation of a larger system to run a fridge. Hopefully I can present the challenges I have, and you can steer me right.

    The reason I don't like the propane fridge are because it is too small (8 cubic feet) and that temperature regulation is challenging. This is a 4 bedroom house where we stay for a week or 2 at a time, so we need to bring enough food for everyone for that long. Once you load that thing up, not only does it take a while to get down to temp, but on really hot days, it struggles to get there at all. I think we exceed it's capacity and expect too much from it. We PACK it full, and there is nothing but convection to circulate air.

    I see two options. My preference, mostly due to familiarity, is a typical residential fridge run off an inverter. I've done the math, and the size fridge I need wants about 900 watt hours/day. The benefit of this fridge is good temp regulation, large size, and price of the fridge itself if it needs replacement ($500-$600). Down side, I need an inverter (psw) and larger solar array/batteries than other methods.

    My other option is a 12/24v fridge. I know nothing about them except that there is one other one on the island. I have not asked them how they like it. The downside is the cost. One similar in size to the $600 conventional one is over 2k and does not seem to have a significant wattage reduction at about 650 watt hours/day. (although after reading that, I realize there is loss through the inverter as well) My wonder is, how well do they work at regulating temp and how do they like being packed full? Is it similar to a conventional fridge or my current propane fridge? The benefit is lower energy use but not by a dramatic amount IMO.

    My other loads are cell phone chargers and some LED lights. A worst case scenario of 300 watt hours if the house is packed. (we also use a lot of kerosene lamps). I have the benefit of only needing to do this during the summer months and really only loading the house up during july and august. The house is used from may to september, shuttered and closed after that.

    It appears to me that initial cost will be about the same. I need to upgrade my solar no matter which route I choose, but obviously more if I go 120v. One benefit is that I have a local supply of panels at .50/watt. I really don't want to sacrifice on size and better temperature regulation would be great to have. What advice can you give? I've read a lot on here about this. Obviously what I'm hoping for is confirmation that MY way is the best way. lol. That being said, I take advice when it is given so I appreciate what knowledgeable people have to say.

  • #2
    search for solar chest type refrigeration. vertical front load units will run the compressor each time you open the door, chest type will cycle much less often and for a shorter run time. chest types can also maintain temp for many hours w/o power.

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    • #3
      I like where your head is at, but that presents another challenge. Getting the "design team" to accept a chest fridge instead of a standard upright. The women still want it too look like a "regular" kitchen. I totally see the advantages, but I just don't think it would fly, aesthetically.

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      • #4
        In that case expect to at least double the batteries necessary. and if you chose to run an inverter to power a 120v unit, expect another 50% increase in battery size.

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        • #5
          IMO spending the money for a solar / battery system to handle 1300wh a day for only a few months out of the year could be a big expense and more than likely a waste of money.

          It is probably best to get a new refrigerator if the old one is not working but I would then get a 2000watt iinverter type generator to run it. At least you can take it with you when you leave and you won't have to worry about a big battery system staying charged when the house is closed up. Even if you take the batteries when you leave you still have to keep them charged and they will still slowly die and have to be replaced in a few years.

          Both AC and refrigeration loads are not practical powered from batteries if other power sources are available.

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          • #6
            I have done a lot of math, and discussed that part of this in another thread, but I feel that for under 2k, I can build a system that will power this reliably. (at 120v) Clearly less if I use I 24v fridge. Maybe that is a lot but I think it's reasonable. I'm done with propane, and I'll be doing either the 24v or 120v fridge. It really comes down to which one is the better performer. If a 24v fridge will operate similarly to a 120v unit, and I can get a big enough one, I'll do it. It just seems that the 120v units are nicer. But if it just comes down to cost, in my mind I'd rather spend the money on a larger solar setup and a less expensive fridge, than a small solar setup and a really expensive, cant get parts/service fridge. That's what I have now with the propane one and it's been annoying just trying to get parts and service. If a $500 fridge dies, so what. Get a new one. But right now I'm spending 220 bucks on a part in hopes that I diagnosed it right to fix a 1300 fridge that in reality just doesn't do what we need. Is that the same situation with the 24v refrigerators?

            I see two roads. Spend 2k on a fridge and $500 to upgrade the solar or spend $500 on a fridge and $2k to upgrade the solar. (but also a higher battery replacement cost periodically). I'm just weary of a 24v fridge. Should I be? IDK. Decent sized solar setups are common on the island. One house is doing battery replacement this year. He has I think 12, Trojan T105 batteries and I think he said he got about 10 years out of them. He said he adjusts the controller to just keep them happy over the winter. There are a few houses with some serious power. One has a whole building dedicated to batteries/controllers etc. He has multiple arrays on multiple roofs as well as I think 4 wind turbines. I don't need quite that much.

            Other power sources are not available here. We do have a 2k watt inverter generator that we use for power tools during construction and for vacuuming (I looked into sizing the solar for a vacuum cleaner until I found out what they need!) but running it just to run a fridge would be a bit obnoxious.

            Since we all like pics, here is the house in question.

            House.jpg
            Attached Files

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            • #7
              It appears your power needs require a battery, but are dominated by the fridge. Using a really large battery won't
              reduce long term battery costs substantially, you just buy a more expensive battery less often.

              What wears out a properly maintained battery, is the amount of energy in charging and discharging. Perhaps if
              you could arrange for the fridge to run mostly when the sun is up, the battery exercise would be minimized and
              battery life maximized. Put in a second, low temp fridge thermostat that is switched on when solar energy is
              available. Run hard in the solar day, freeze some state change storage (ice packs). When the sun sets, default
              to a somewhat higher set thermostat, try to keep the door closed, and thermally coast through the night. A chest
              style with really good insulation will help. Bruce Roe

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              • #8
                To safely generate 1300Wh a day you will need a battery system that is about 220Ah at 24Volt or 440Ah at 12Volt. You can make that using 4 x 6v 220Ah batteries or 2 x 6v 440Ah batteries.

                To keep that battery system happy you need about 1/10 the Ah rating in charging amps which is about 530 watts (22a x 24v = 528w) or ( 44a x 12v = 528w) or 600watt if you round off.

                That would require an MPPT CC rated 30Amp for the 24v battery system or 50amp for the 12v battery system.

                Then add a PSW inverter (12v or 24v depending on your battery system) to run the fridge.

                You can try to get pricing on that equipment but I believe you will easily exceed that $2k you estimated. Based on a number of existing systems it will cost you about $2000 to $3000 / kWh of generation.

                It can be done but unfortunately it costs to build a balanced solar / battery system
                Last edited by SunEagle; 06-13-2017, 01:07 PM. Reason: added last sentence

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                • #9
                  Originally posted by SunEagle View Post
                  To safely generate 1300Wh a day you will need a battery system that is about 220Ah at 24Volt or 440Ah at 12Volt. You can make that using 4 x 6v 220Ah batteries or 2 x 6v 440Ah batteries.

                  To keep that battery system happy you need about 1/10 the Ah rating in charging amps which is about 530 watts (22a x 24v = 528w) or ( 44a x 12v = 528w) or 600watt if you round off.

                  That would require an MPPT CC rated 30Amp for the 24v battery system or 50amp for the 12v battery system.

                  Then add a PSW inverter (12v or 24v depending on your battery system) to run the fridge.

                  You can try to get pricing on that equipment but I believe you will easily exceed that $2k you estimated. Based on a number of existing systems it will cost you about $2000 to $3000 / kWh of generation.

                  It can be done but unfortunately it costs to build a balanced solar / battery system
                  I think that's pretty much exactly what I had planned. I was planning 4, 6v 230 AH batteries. I already have a 30 amp MPPT controller, and at $.50 a watt, I was planning to run 4, 250watt panels (if that works, voltage wise. I can go up to 100v on my controller) . In my other thread we discussed over sizing the array to take advantage of days of lesser production since my battery bank is on the small side. On full production days, my charge controller would limit the charge and I would just not be taking full advantage of the array size. My controller says I can go up to 2340 watts without damage.

                  Here is where you may hate me. I just bought 2 of these batteries a couple months ago. Even though all 4 will not be exactly the same age, I'm hopeful that adding 2 more of the same battery to my bank, given that they have had only about a month of VERY light use, will be OK. Then I have 230 ah at 24v. I paid 220 for both batteries so I expect to pay 250 for 2 more since I dont have cores to trade. $500 for 1000 watts of panels from the guy in my building and I'm at $750. I figured add in 250 to 300 for misc wires/ mounts etc. Then maybe $300 to $500 for a psw inverter. I'm thinking 500 watts may be enough, but I'm prepared to to to a 1k if I need to. Unless I'm missing some glaring cost, that keeps me under the 2k. It helps that I already have my controller and 2 batteries and I did not figure that into the cost. Does this seem reasonable, or am I missing something?

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                  • #10
                    But the real question is, are 24v refrigerators as good as a typical 120v one? I'd still probably need the 2 batteries, but I would not go nearly as aggressive on the array if I ran the 24v fridge.

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                    • #11
                      Originally posted by Basketcase View Post
                      But the real question is, are 24v refrigerators as good as a typical 120v one? I'd still probably need the 2 batteries, but I would not go nearly as aggressive on the array if I ran the 24v fridge.
                      Hard to say. Some of those 24v fridges are pretty good but there are more 120v to choose from. You will just need an inverter for the 120v but when you add the cost of a 24v against a 120v with inverter you probably will spend less on the 120v fridge combo.

                      As for 1000 watts of panels. You may over run your 30Amp MPPT since it can probably only take about 800 watts for a 24v system so be careful not to push your existing CC.

                      You can find a 24v psw rated 600watt Samlex inverter for less than $300. That 600w should have a surge rating of about 1200w which should be enough to start a standard fridge. I would get the fridge first before I got the inverter to make sure I don't under size it.

                      Now for the batteries being a mix of old and new. While it is not a good idea to do that you might get away with it but don't expect to get the same life time out of your mixed bag compared to all new batteries.

                      As for investing all that money into a solar /battery system. I guess it comes down to what someone wants and needs. At least you have neighbors that should help guide you with their experience for their systems.

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                      • #12
                        So it seems like I'm not that far off then? I've tried to do my own research and only ask questions to fill in the blanks. I appreciate everyone's input.

                        In regards to the array size. I certainly don't want to overdrive the system. This thing also needs to be able to be left alone for a week or 2 at a time. I based my information on what I read in the manual for my controller. The controller was recommended to me by this board. I've inserted it below. Loosely translated it seems to say that if the array is sized larger than rated power, that max rated charging to the battery will be longer. It also seems to imply that I can go to up to triple the rated power without side effects. Do I interpret this correctly?

                        Controller specs.JPG

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                        • #13
                          Well based on that spec sheet it looks like you can actually install a pv array up to 2340w for a 24v system. So there must be some type of limiting device to keep from overloading that 30A CC using more than 780watts.

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                          • #14
                            That's great. That's exactly what I was hoping for. Since panels are so cheap, I feel like I can keep my battery bank smaller because it will be charging longer throughout the day, and even on bad days I should have plenty of charging. I'm trying to make it idiot proof so when I'm not there and there are people using the house, it will just work. Thanks for all the info. I'm excited to get moving on this.

                            In regards to these panels. I don't have specs on them, but perhaps its easy enough to generalize? They are 250 watt panels. I know I can't run 4 in series but the guy said 2 in series would keep me below my 100 volts. So then I would series/parallel my panels?

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                            • #15
                              Originally posted by Basketcase View Post
                              That's great. That's exactly what I was hoping for. Since panels are so cheap, I feel like I can keep my battery bank smaller because it will be charging longer throughout the day, and even on bad days I should have plenty of charging. I'm trying to make it idiot proof so when I'm not there and there are people using the house, it will just work. Thanks for all the info. I'm excited to get moving on this.

                              In regards to these panels. I don't have specs on them, but perhaps its easy enough to generalize? They are 250 watt panels. I know I can't run 4 in series but the guy said 2 in series would keep me below my 100 volts. So then I would series/parallel my panels?
                              Yes. Wire the 2 series sets of 2 in parallel.

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