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  • Running panel dc straight into resistance loads for heating application?

    Was wondering if anyone gas tried this? With slightly used panels running .25 a watt in the Taos area it is not terribly expensive. Was thinking convection could be used to move heat into a gravel wall above the loads to store heat.

  • #2
    Looks like no interest in this topic. Hopefully will have time to try it this summer. Abandoned trying to use standard resistance heaters like oil radiators. Adding enough typical resistance heating loads (1500w) in parallel to get R low enough to keep current below knee and overall power limits of load looks to be cost prohibitive and wasteful. Much less of a problem with much lower water heater Rs so looks like that will be the way to go although I was looking to play with the oil radiators since they are shake and bake. R is just too high. Will strip the insulation off the water heaters and will at least get the thermal mass of the water for my trouble. Thinking add 2x4 frame with hardware clothe and gravel for thermal mass if it works out. Got two from the dump we will see if they leak loads probably shot but those are cheap. That will give me four low value rs that are rated for decent power to play with to set power point. I will have to find the sweet spot of harvesting the panels current when not at full current knee but still being able to get a lot of the full power current since no CC to regulate. Maybe 3/4 of current at knee? Power "wasted" above and below 3/4 current compromise but since its all "free"... Any input as to where most power would be harvested re time spent below max current in normal solar cycle? Hrs at 100% 75% 50% current? Best current setpoint below knee for max power harvested?
    Last edited by 123easy; 12-16-2020, 08:14 PM.

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    • #3
      Originally posted by 123easy View Post
      Looks like no interest in this topic.
      I might have some interest if I could understand the some of the cryptic descriptions you're using actually mean.
      Suit yourself, but if you want to use solar energy for space heat, educate yourself and use passive solar - especially and particularly in Taos.
      Better, cheaper, more reliable, more efficient.
      Been there, done that for a long time.

      Comment


      • #4
        As you see operating the variable current panels with a fixed resistance load
        wastes a lot of potential production. Matching up impedance is an issue, and
        the next problem is finding a way to limit temp, since the AC cut off switches
        on a water heater cannot control DC power reliably. Bruce Roe

        Comment


        • #5
          Originally posted by J.P.M. View Post
          I might have some interest if I could understand the some of the cryptic descriptions you're using actually mean.
          Suit yourself, but if you want to use solar energy for space heat, educate yourself and use passive solar - especially and particularly in Taos.
          Better, cheaper, more reliable, more efficient.
          Been there, done that for a long time.
          Im sorry Im probably a poor comunicator.
          Talking about setting a reasonable power point.
          For instance just a rough in. rough figures.
          This is my understanding. Just seeing if the much greatrer knowledge here says I am incorect.
          5 325 watt panels . that would give a knee of 200v and 45 amps where if voltage increased beyond that do to higher R load no current would flow? So a R of about 6 ohms would still function with the knee at 33 amps this would lock my power in at 1200 watts. At below 33amps no power.

          Im on my third owner built home.. Everyone has used passive solar. Actuallu higher ratio south facing glazings than is considered optimum.

          I actally am a bigger fan of high r values rather than passive solar but thats me .

          I see a lot of the losses that occur from combustion. the cold make up air has to be heated. when its minus 30 it takes a bit.

          I was visiting a friend a bit ago. About 3/4 of the house and shop heated with hot water solar panels most of which he got for free. So pex in the slabs. He programed the zones . Ladder logic hurts my head. Lots of thermocouples to determine when to move the water. Water doesnt move its self so hardware energy and noise there.

          So id like to give this a go. I dont think its unworkable. Every south facing window also loses heat when the sun goes down. One I did had two double pane glazings on each side of the 2x6 framing. I had to leave a crack at the top of the inner glazing otherwise too much heat would be trapped inbetween the two and melt the vinyl. Two double panes showed no heat loss with flir imaging I still like this technique.

          None the less passive solar has limits. It may be in cold climates some combustion heating is unavoidable off grid. I d like to try dc power from photovoltaics to heat. It may not pan out. 325 watt panels are about $100 apiece. I dont see that as being a huge cost over the lifespan of the project. Wood and propane are not exactly cheap and they have to be acquired.

          Heres the first one I built. Using the quad panes. on grid. It wasnt perfect. I learned a lot . The second was better. Im stll learning trying to improve. You never know unless you try. Hoping to learn from the much greater experience here. Already learned a lot. The stickys about high bulk and absorbtion voltages have been huge. Charging batterty banks in the morning and cloudy days with a 2000watt honda clone gen set. . Huge. Thanks SK. Nothing like having your battery bank die after 2.5 years to teach. Hoping to learn more.
          Attached Files

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          • #6
            Originally posted by bcroe View Post
            As you see operating the variable current panels with a fixed resistance load
            wastes a lot of potential production. Matching up impedance is an issue, and
            the next problem is finding a way to limit temp, since the AC cut off switches
            on a water heater cannot control DC power reliably. Bruce Roe
            Thanks for this. Does my 6 ohm 33 amp setpoint make ballpark sense or am I totally missing somthing? Had thought the AC thermal cutouts would work so thats a real barrier not unsolvable, opto solid state relays I would guess but added cost. Thank you!

            Comment


            • #7
              Originally posted by 123easy View Post
              5 325 watt panels . that would give a knee of 200v and 45 amps where if voltage increased beyond that do to higher R load no current would flow? So a R of about 6 ohms would still function with the knee at 33 amps this would lock my power in at 1200 watts. At below 33amps no power.
              I do not understand, 5 X 325 watt panels multiply to 1625 watts, but 200V X 45A = 9000watts,
              something wrong with those numbers. Bruce Roe

              Comment


              • #8
                Originally posted by bcroe View Post

                I do not understand, 5 X 325 watt panels multiply to 1625 watts, but 200V X 45A = 9000watts,
                something wrong with those numbers. Bruce Roe
                I added both the voltages and currents . Parallel and series at the same time quite the trick.

                taking a look look it looks like 12 ohms in one element is about standard not 6

                ' 7 325 watt panels in series 280v / 9A = 31 ohms full power

                two 12ohm elements in series = 24 ohms ohms = 77% knee

                1750 total watts at 77% Each load would have to be rated for 875 watts minimum at 77%.

                4kw 12 ohm 240vac water heater loads are about $10.

                That gives me 450% + margin for DC rather than AC at 875 watts across a 4kw element.

                1750 watts puts me at about half typical water heater wattage. great more water.. And margin.

                Am I close to reality?

                Would 50% of knee (max power point) be better?

                Any chance the AC thermal cutouts would handle 25% of the AC power in DC?
                Last edited by 123easy; 12-17-2020, 07:01 PM.

                Comment


                • #9
                  There is a concept called MPPT (maximum power point tracking) where electronics extracts the maximum power from the panels by adjusting the virtual impedance using switching converters. That is the most efficient way to get power from PV panels. With MPPT, you are lucky to get 20% efficiency. Most of the losses come from getting sun into current, but there is a bit of energy lost in the switching converter as well.

                  Some people argue that it is relatively efficient to pick an output voltage that is 80% of peak voltage and just engineer for that. You won't get 20% that way, but might get 18% if you're lucky. But that still requires an electronic load that adapts as the sun goes up and down. You can't get that level of efficiency over a range of illuminations with just a resistor.

                  Do you have a voltage/current plot for your PV panels at various levels of light, something like these, courtesy Victron Energy? Curves like this tell you how much power you get out at a specific voltage with a specific level of light. In other words, they will tell you how much energy you'll get from the panels with a resistive load, MPPT load or any other load. There are other tools to help you estimate the amount of light at any one instant in your location, such as PVWatts. Together, they will help you know how much energy you get for any system at any instant.

                  7884-pv-voltage-curve_125777129-660x1024.jpg
                  By comparison, you mentioned solar hot water (HW). An HW system can achieve 40% efficiency with little or no smarts. Compared to PV-resistor, HW seems like a much better approach, although higher maintenance and higher cost to install.

                  Another consideration is available roof space. Do you have an estimate for the amount of heat energy you will need? Do you have enough roof space to contain the number of PV or HW panels you need to make a good dent in your heating costs?

                  For what it's worth, in a typical grid-tie PV system, inverter cost is typically less than half panel cost. In other words, it is false economy to leave out the inverter and instead buy more panels. Electronics is well worth the cost because it enables you to get much more out of your panels, and also gets more out of a given amount of roof space.

                  Last thought: do you have power from the grid? If so, a grid-tie PV system will use the grid as storage, collecting energy during the day for you to use at night. That will be more tolerable than just overheating in the daytime and letting the house get cold at night. It also allows you to use grid energy to run a heat pump rather than a simple resistive heater, gaining you another factor of 3 in efficiency.
                  7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

                  Comment


                  • #10
                    Originally posted by 123easy
                    I added both the voltages and currents. Parallel and series at the same time quite the trick.

                    1750 watts puts me at about half typical water heater wattage.

                    Any chance the AC thermal cutouts would handle 25% of the AC power in DC?
                    No way will those AC contacts be happy at a similar voltage, the DC arc will
                    happen even at reduced current.

                    If a typical water heater has a pair of 12 ohm heaters, it could draw 40A or 9600W from a
                    240VAC line, better study up on basic circuits. Bruce Roe

                    Comment


                    • #11
                      Originally posted by bob-n View Post



                      For what it's worth, in a typical grid-tie PV system, inverter cost is typically less than half panel cost. In other words, it is false economy to leave out the inverter and instead buy more panels. Electronics is well worth the cost because it enables you to get much more out of your panels, and also gets more out of a given amount of roof space.


                      .25 a watt now.

                      Im running 9 panels a sunny boy 5000 watt and a midnight 250 mppt now. The 9 325 watt panels were $730. The rest was about $3k.

                      Wheres the false economy? The panels are bulletproof minus age losses. Batteries are consumables. The electronics are like all electronics with a service life.

                      This new one is off grid like what im in now. The wind is crazy. I had a friend bust a large berm with his backhoe. Its several hundred feet long, facing solar south for panels not the roof. Yeah I could get a cheapo inverter then I need a second set of batteries and a CC. I will run a conventional PV systen for normal power usage. SMA 5000 watt 8 l16s . Dont know which CC. Probably two tristar 60A pwms. Because once again panels are cheap. Mppt charge controlers are a lot more and finicky IMO. Ive ran both. Id rather rather install more panels and run pwm. Any way you look at it PWM is less complicated and I say more reliable. Say 12 325 watt panels with two 60 amp pwms instead of the standard 8 -9 panel with a 60 amp MPPT. Yes those are about the same cost. Run the numbers with tristars MPPT vs PWM CCs and .25 watt panels

                      With panels at .25 a watt it changes things.

                      At 81 bucks a 325 watt panel two water heaters worth at the 7 panels is 14 panels or $1100. Call it 1000 watts continuous equivalent . Probably not but lets just say so. So one small space heaters worth but on forever for 20 years $1100 is not even one set of l16s. I was thinking i could store the energy in heat in water and thermal mass. $1100 for twenty years of heat. Thats one fill on a propane tank.

                      Sick of cutting and splitting wood and feeding a stove. Too old for that ****.

                      I actually like High R houses and heating with electric on grid. Too poor for on grid land now and digging running PVS. So trying to get that clean no combustion electric heat from Some DC PV rigs. Yeah baby no fossil fuels totally green. If the chicks dig it who am I to argue?

                      Im not above a little harmless greenwash talk if a beautiful hipster girl is gracing my presence.

                      Have zero interest in a second inverter, battery set, and CC. Just want to pipe the dc in in conduit and dump it across a resistance load.

                      If I can set it up so its just dumping heat into the mass I dont have to watch the charges on the batteries ecetera. I enjoy running a pv system for AC. NO interest in doing it twice I get what i get from it and the passive. Make up whatever else i need to be comfortable with a Rinnai direct vent propane heater or two or three. Insulation is 12" of polyiso except for glass.

                      Frankly id like to do my hot water with DC PV too. These on demand water heaters suck the propane. Using a Rinnai on demand now. At least for a preheat.

                      Im heating with propane in my place now and 3 direct vent rinnais. I let two of those go cold unless I have guests I put two feet of fiberglass in the ceiling. I use way more propane for hot water than to heat with the passive and the high rs.

                      No its not a lot of heat. But with the passive and R72 plus it will be a good bit. Less propane sucking by the direct vents. And its BTUs that I dont have to warm -10 make up air with combustion . Thats huge.

                      In the summer pull the levers on the disconnects.

                      Shake and bake.

                      More time for fishing.

                      Thats how I see it.

                      Flame suit on.

                      It may not pan out. I know that but the numbers here are not high $ to try.

                      Still here to learn
                      Last edited by 123easy; 12-18-2020, 01:15 AM.

                      Comment


                      • #12
                        Thank you for those curves!! thinking maybe 65 % of max setpoint best.

                        Originally posted by bob-n View Post
                        There is a concept called MPPT (maximum power point tracking) where electronics extracts the maximum power from the panels by adjusting the virtual impedance using switching converters. That is the most efficient way to get power from PV panels. With MPPT, you are lucky to get 20% efficiency. Most of the losses come from getting sun into current, but there is a bit of energy lost in the switching converter as well.

                        Some people argue that it is relatively efficient to pick an output voltage that is 80% of peak voltage and just engineer for that. You won't get 20% that way, but might get 18% if you're lucky. But that still requires an electronic load that adapts as the sun goes up and down. You can't get that level of efficiency over a range of illuminations with just a resistor.

                        Do you have a voltage/current plot for your PV panels at various levels of light, something like these, courtesy Victron Energy? Curves like this tell you how much power you get out at a specific voltage with a specific level of light. In other words, they will tell you how much energy you'll get from the panels with a resistive load, MPPT load or any other load. There are other tools to help you estimate the amount of light at any one instant in your location, such as PVWatts. Together, they will help you know how much energy you get for any system at any instant.

                        7884-pv-voltage-curve_125777129-660x1024.jpg
                        By comparison, you mentioned solar hot water (HW). An HW system can achieve 40% efficiency with little or no smarts. Compared to PV-resistor, HW seems like a much better approach, although higher maintenance and higher cost to install.

                        Another consideration is available roof space. Do you have an estimate for the amount of heat energy you will need? Do you have enough roof space to contain the number of PV or HW panels you need to make a good dent in your heating costs?

                        For what it's worth, in a typical grid-tie PV system, inverter cost is typically less than half panel cost. In other words, it is false economy to leave out the inverter and instead buy more panels. Electronics is well worth the cost because it enables you to get much more out of your panels, and also gets more out of a given amount of roof space.

                        Last thought: do you have power from the grid? If so, a grid-tie PV system will use the grid as storage, collecting energy during the day for you to use at night. That will be more tolerable than just overheating in the daytime and letting the house get cold at night. It also allows you to use grid energy to run a heat pump rather than a simple resistive heater, gaining you another factor of 3 in efficiency.

                        Comment


                        • #13
                          I use a commercial rooftop tank/flat plate water heater. it's at a great summer angle, but in winter, the wood furnace takes care of most of the preheat water for the tankless
                          Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                          || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                          || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                          solar: http://tinyurl.com/LMR-Solar
                          gen: http://tinyurl.com/LMR-Lister

                          Comment


                          • #14
                            Originally posted by 123easy
                            Sick of cutting and splitting wood and feeding a stove. Too old for that ****.

                            I actually like High R houses and heating with electric on grid. Too poor for on grid land now and digging running PVS. So trying to get that clean no combustion electric heat from Some DC PV rigs.

                            Have zero interest in a second inverter, battery set, and CC. Just want to pipe the dc in in conduit and dump it across a resistance load.

                            If I can set it up so its just dumping heat into the mass I dont have to watch the charges on the batteries ecetera. I enjoy running a pv system for AC. NO interest in doing it twice I get what i get from it and the passive. Make up whatever else i need to be comfortable with a Rinnai direct vent propane heater or two or three. Insulation is 12" of polyiso except for glass.
                            What will work depends as well on your location. Here at 42 deg Lat temps are below freezing for months.
                            I manage to keep 5000 sq ft warm with solar, with an annual budget of 25,000 KWH. It works because net
                            metering allows accumulating the energy in summer, then getting it back in winter. The power co looks like
                            a free, infinite capacity, zero cost and maintenance battery to me. Real batteries can only serve over some
                            hours, at great cost. The key here is not wasting KWH, and using high efficiency heat pumps year around.

                            Winter here is quite cloudy, some days the panels only put out 10% of their rating just when you need power.
                            If your collection system is not efficient over range, even most of that will be lost. Net metering makes it work,
                            your results may vary. Bruce Roe

                            Comment


                            • #15
                              Originally posted by bcroe View Post

                              What will work depends as well on your location. Here at 42 deg Lat temps are below freezing for months.
                              I manage to keep 5000 sq ft warm with solar, with an annual budget of 25,000 KWH. It works because net
                              metering allows accumulating the energy in summer, then getting it back in winter. The power co looks like
                              a free, infinite capacity, zero cost and maintenance battery to me. Real batteries can only serve over some
                              hours, at great cost. The key here is not wasting KWH, and using high efficiency heat pumps year around.

                              Winter here is quite cloudy, some days the panels only put out 10% of their rating just when you need power.
                              If your collection system is not efficient over range, even most of that will be lost. Net metering makes it work,
                              your results may vary. Bruce Roe
                              Yes thats what I am just starting to understand. I didnt understand the benefits. The Grid interface systems have the worlds best battery. Nice.

                              Comment

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