short circuit thermostat

plan was to use six to eight panels in series, increase to twelve later. wire directly to 30 gallon 120AC water heater. Home depot had a sale on water heaters. The thermostat will turn off when temperature hits the correct setting. all sound good except. one problem I forgot, DC arching. Normal existing thermostats on water heaters react slowly. How to stop the arching in the thermostat contacts?? on opening the circuit. when temperature has been reached. When we used DC motors we used a system called double back contacts, but that is possible when using a high speed drop relay, How to use that principal on a water heater, any suggestions would be appreciated. as it is now the thermostat burns up in a few cycles.
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Comments

  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: short circuit thermostat

    Welcome to the forum "tikidog".
    Have a look at solid state relays. They don't arc.
  • mike95490mike95490 Solar Expert Posts: 8,002 ✭✭✭✭
    Re: short circuit thermostat

    Conventional water heater thermostats are designed for AC, DC power is MUCH harder to control, so have the thermostat control a SSR (Solid State Relay) rated for your volts/amps/ AC or DC
    Powerfab top of pole PV mount | 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 ,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: short circuit thermostat

    Except if he's trying to do this with PV + thermostat only then there is no stable power source to run any type of relay with.

    Frankly this is why it isn't done.
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    My first ever post on here.... I avoid forums as much as I can, but this is a topic hitting close to home.

    First, anybody who wants to do this probably has either grid power or an off grid solar power system that can be used for control power. That isn't the problem. The relay is no problem.. Crydom. There is a guy who invented some black box that is supposed to take care of everything, but my BS meter squealed when I read his info and besides, it is a lot of money. Add to that, will he still be around when the thing smokes?

    I am trying to figure out how to do the same thing in a house that will go up in the spring. We are living in an RV parked inside of a barn with the solar system for a future house done. The issues seem to be pretty straight forward, but as I have found in the past, this may not necessarily be true.

    I plan to use a big standard electric tank and power only the bottom element. The challenge is to figure out what will happen when powering a resistance element from a variable voltage source like solar panels and no battery or controller. Impedance is easy to find. Theory says that if we get the voltage high enough by series wiring the panels, we should get a decent amount of BTU's and it should be a simple manner of measuring amps with a clamp on, multiply that by the volts, get watts... Turn that into BTU's and then know how much heat I should get. However, it would be nice to be able to predict this accurately. Theory tells me around 1000W will give me the 50 degree rise on the 40 gallons or so, double what we need most days. However, until I hook it up, I don't really know exactly what will happen. Inefficiencies, varying impedance & voltage, etc. and I think I'll need about 1200 watts of panels. Is there anybody out there who has actually done this?

    Going further with this: I plan to run the output from the electric tank to a small propane tank, then in a loop around the house with a 1/40HP circ pump controlled by an aquastat set at maybe 110F so the water in the line is always hot. The electric tank will be set as high as we decide is safe, maybe 160F. The propane tank will be set as low as we can stand it, maybe 105F, so it will only run if the sun isn't doing the job. I'll put a manual over ride switch or timer on the pump for saving energy at night. Of course super insulation needs to be added to the tanks and the house loop also needs to be done well. I did this circ loop before in our last home and it was wonderful to have hot water in the shower right away, plus for saving water.
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    Welcome to the forum Bob!

    We do have a couple people that have been using Heat Pump based water heaters... In theory, as long as the ambient temperatures are over ~50-55F, they are 2-3x the efficiency of a resistance based system. And, if I recall correctly, the Geo Stream from GE draws around 500 Watts (don't know the starting current, but probably like a refrigerator with a 1,200+ Watt minimum AC inverter).

    If you don't want to do batteries for heating water (I like avoiding batteries too), I have been thinking about what you have asked. I wonder if a Linear Current Booster (such as used for DC well pumps) would work well (or at least better) than a direct connected solar panel/array?

    A 1,000 Watt array would work well with a minimum rated LCB of:
    • 1,000 Watts * 0.77 panel+controller derating * 1/24 volt nominal = 32 amp rated output minimum
    A quick search shows an interesting two element heater... Series/parallel ratings for 12/24/48 volts rated configurable to over ~1,000 Watts. At $170, not cheap.

    I can find a 6kWatt heater @ 208 VAC ($76) from Granger Supply... Does not look very interesting. Running on 24 or 48 VDC:
    • P= V2/R = I2R
    • R=V2/P = (208 VAC)2/6,000 Watts = 7.21 Ohms
    • P=(24volts)2/7.21 Ohms = 80 Watts
    • P=(48volts)2/7.21 Ohms = 320 Watts

    Linear Current Boosters are not cheap (and there are not many around for off the self projects)... Perhaps talk to Solar Converters Inc about their products/recommendations:

    http://www.solarconverters.com/index.php/products/83-power-pump-tracker-linear-current-booster

    Perhaps Sun Pumps would be interesting to talk with too:

    http://www.solar-electric.com/wind-and-water-products/sodcwapu/sun-pumps/sopuco/pcbsepuco/pcbcoforbrpu.html

    In general, the Linear Current Boosters are not cheap ($300-$600 for the few that I found, more if you have to parallel several for 32 amp current output)... So, it becomes a question of running a larger solar array and a Midnite solar MPPT charge controller ($610 or so) with the "waste not/want not" feature controlling the DC relay and an appropriate water heater element.

    The Midnite also has the "new" Wizbang Jr. battery monitor/shunt system--Would let you "over panel" your array, and limit charging current to the battery bank (to 13% rate of charge, or whatever you need)--Excess power (using the waste not mode--I do not know the details) to divert excess array energy to the water heater (cycling the AC load via silicon relay--usually much easier/cheaper).

    But, if you are already looking at a MPPT charge controller, why not look at using a ~4.5 kWatt 240 VAC electric water heater element on your 120 VAC inverter (~1.1kWatt).

    And here you are back at 120 VAC loads and AC Inverter system. And wondering if a GeoSpring water heater from GE or Geyser with 2-3x the heating efficiency is a better deal in the long run (cold/dry air exhuast is great in a hot/humid climate. In cooler climates, will need to vent outside air to prevent over cooling room where unit is installed).

    My 2cents anyway.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    What I said:
    "The challenge is to figure out what will happen when powering a resistance element from a variable voltage source like solar panels and no battery or controller."

    Why all of the complications? I know, the old "wasting your watts" argument. I think that went out the window when panel prices dropped below a buck a watt. My goal is to do this with simple off the shelf things, nothing fancy to buy or to break. Easy to fix; not requiring a rocket scientist. One of the things I like to warn people about going solar: You are your own utility, so you better know how to keep the power on and you better have spare parts on hand. I may use the aux contact in my Classic to switch it on & off, but may just let the sun do it. I can buy about 1200W of solar panels shipped for less than $1200. They may be crap, but for this it doesn't really matter. If I cut that cost by 50% buying expensive toys, I would be going in the hole. It seems to me that with a Vmp of around 110 and amps around 10.5, I should be able to decently drive the standard 4500W element. Not at 4500W, but the water in the tanks and pipes is the battery. And the voltage will go up when the panels are cold. If it takes all day to heat it, no problem. This is the same as a solar electric battery system, about 5 hours during winter and we are charged 95% of the days. Over sizing either is how we get through cloudy days and then recover without burning fuel. We are successfully doing this, so why not do the same with water heat?

    By the way, this thinking extends to our water system. We could drill a $15K well & hope that the water is drinkable. Instead, we are building a 10,000 gallon underground rain water cistern and we are not buying any $2000 filtering system. We drink rain water now and have been doing so for 3 years as we develop our property, learning more each year. This year we watered a big garden and lived on rain while collecting from only 1/2 of our barn roof. We have tanks full of filtered rain sitting on the floor in our solar heated barn and expect to make it till spring. This is in semi arid eastern Montana, where our solar conditions are better than anybody believes.

    I found an old RV water heater element today. It tests at 10 ohms, we'll just see what happens when I hook it up while suspended in a bottle of water. Tomorrow if the sun cooperates I'll be testing at a neighbor's where I set up a 24V construction power system recently. May not work, we'll see. Then I will be basing my plans on facts, not opinions. I have asked many people about this before coming on here. It just flabbergasts me that I cannot find anybody who has tried it.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: short circuit thermostat

    Possibly because when you look at the number of variables involved you come up with inevitably inconsistent results for the system which translates to a random value of doing it.

    Or as engineers say "you plug in the numbers and it adds up to not worthwhile".
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    OK... I always assumed that the coupling of the output of a Current Source type Solar Electric panel to a pure resistance heater would be highly inefficient.

    However, I could be wrong and it may work "good enough" to be useful (solar panels being so much cheaper than a decade ago). And if we can do this without expensive hardware and batteries, why not.

    So, using PV Watts hourly output for an mid April sunny day, and a end of December Winter sunny/so so days, what would the actual panel output connected to a ~1,000 Watt (0.91 Ohm) water heater element look like (Vmp~30 volts). Fixed array, tilted to Latitude with 77% rated output from 1,300 Watt array (i.e., 1,000 Watt typical output):



    Vmp=30 volts; Imp-array=43 amps; (1,300 watt array). 1,000 Watt heater R=0.91 ohms at 30 volts


    1,300 Watt array @ 0.77 derating











    Date



    Watts
    AH/H
    WH of Heat


    1974
    4
    16
    07:00
    4
    0.13
    0.02


    1974
    4
    16
    08:00
    276
    9.20
    77.02


    1974
    4
    16
    09:00
    557
    18.57
    313.70


    1974
    4
    16
    10:00
    756
    25.20
    577.89


    1974
    4
    16
    11:00
    880
    29.33
    783.00


    1974
    4
    16
    12:00
    924
    30.80
    863.26


    1974
    4
    16
    13:00
    939
    31.30
    891.52


    1974
    4
    16
    14:00
    933
    31.10
    880.16


    1974
    4
    16
    15:00
    833
    27.77
    701.60


    1974
    4
    16
    16:00
    637
    21.23
    410.28


    1974
    4
    16
    17:00
    361
    12.03
    131.77


    1974
    4
    16
    18:00
    58
    1.93
    3.40





    WH total

    7100

    5,633.61








    Watts of Solar heat


    roughly a 7.1 hour sunny day SF California



    Assume Vmp~fixed, Amps * Solar Energy Available


    WH of Heat=I^2 * R for Watts<1,000 watts












    Good Winter Day




    AH/H
    WH of Heat


    1974
    12
    22
    08:00
    2
    0.07
    0.00


    1974
    12
    22
    09:00
    316
    10.53
    100.97


    1974
    12
    22
    10:00
    598
    19.93
    361.58


    1974
    12
    22
    11:00
    773
    25.77
    604.17


    1974
    12
    22
    12:00
    865
    28.83
    756.54


    1974
    12
    22
    13:00
    872
    29.07
    768.83


    1974
    12
    22
    14:00
    812
    27.07
    666.67


    1974
    12
    22
    15:00
    657
    21.90
    436.45


    1974
    12
    22
    16:00
    397
    13.23
    159.36


    1974
    12
    22
    17:00
    44
    1.47
    1.96





    WH total

    5336

    3,856.52











    So So winter day




    AH/H
    WH of Heat


    1974
    12
    21
    09:00
    34
    1.13
    1.17


    1974
    12
    21
    10:00
    45
    1.50
    2.05


    1974
    12
    21
    11:00
    210
    7.00
    44.59


    1974
    12
    21
    12:00
    70
    2.33
    4.95


    1974
    12
    21
    13:00
    86
    2.87
    7.48


    1974
    12
    21
    14:00
    386
    12.87
    150.65


    1974
    12
    21
    15:00
    678
    22.60
    464.79


    1974
    12
    21
    16:00
    435
    14.50
    191.33


    1974
    12
    21
    17:00
    87
    2.90
    7.65





    WH total

    2031

    874.66




    Excluding 77% derating for Solar Array NOCT Value, wiring drop, some dust on panels, the additional matching losses from Array to Resistance heater with variable solar current output (assuming Vmp-array is roughly fixed, and Imp varies with solar insolation:

    April = 5,634 collected /7,100 avail = 0.79 = 79% resistance heater conversion efficiency
    Dec Good = 72% conversion efficiency
    Dec So So = 43% conversion efficiency

    So--Overall, it is actually much more efficient than I thought it would be--But for cloudy days, the conversion efficiency goes way down.

    If you are still interested, can run the modeling from PV Watts hourly output over the entire year for your location and see what the end results would be (or month by month, etc. numbers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: short circuit thermostat

    So HBob , why not buy a solar collector for heating water as I have done & use the solar to heat water & a small PV to run the pump. My summer water is 130F with a mixing temp valve & my winter is much hotter because I use a jacket on my wood airtight stove, but still with a mixing valve .

    If you run a loop hot water feed , make sure you don't come off the loop feed up , always extract sideways (less heat transfer ) Mine ""was"" , (newer house) now 22* down then 22* flat & the to the feed. Rancher house on crawl , with guest rooms & on-suites .


    VT
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    I guess I failed to say why... It gets down to way below zero here. Using water heating panels is more efficient, but the cost of the glycol loop, heat exchanger and storage tank, second pump, etc. would pay for well over 1000W of electric panels and all of the things that go with them. Not to mention all of the additional things to go wrong. This did not make any sense a few years ago. Things change.
  • gww1gww1 Solar Expert Posts: 963
    Re: short circuit thermostat

    I am not sure if this is of interest to this thread or not.

    http://www.fieldlines.com/index.php/topic,148385.0.html

    Good luck
    gww
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    I did the same modeling for the entire month of December for Lewiston Montana:
    • Lewiston Montana, 1,236 Watt array, Vmp=30 volts, Vertical panels facing south. PV Watts December, 0.81 derating
    • Optimized for Winter/Snow climate
    • 0.96 Ohm Water Heater, ~864 Watts @ 30 Volts
    • Watt rated maximum????
    • AH/H = WH/H / 30 volt Vmp
    • WH of Heat = AH/H * AH/H * 0.96 Ohms
    • Daily efficiency is already derated solar panel output (81%) into resistance heater.

    Daily Summary based on real hourly data for "typical" December (24=Midnight daily summary from spread sheet). WH/H is total WH available per day from array. Note Total/Day is total WH per day into the resistance heater.:



    Year
    Month
    Day
    Time
    WH/H
    Total/Day
    Daily %Eff


    1969
    12
    1
    24:00
    5,663
    4,416
    78%


    1969
    12
    2
    24:00
    3,681
    1,967
    53%


    1969
    12
    3
    24:00
    4,839
    3,435
    71%


    1969
    12
    4
    24:00
    1,965
    646
    33%


    1969
    12
    5
    24:00
    1,726
    563
    33%


    1969
    12
    6
    24:00
    5,935
    5,031
    85%


    1969
    12
    7
    24:00
    735
    111
    15%


    1969
    12
    8
    24:00
    1,673
    564
    34%


    1969
    12
    9
    24:00
    2,864
    1,724
    60%


    1969
    12
    10
    24:00
    2,924
    1,662
    57%


    1969
    12
    11
    24:00
    4,466
    3,256
    73%


    1969
    12
    12
    24:00
    2,296
    1,169
    51%


    1969
    12
    13
    24:00
    4,449
    2,945
    66%


    1969
    12
    14
    24:00
    1,600
    349
    22%


    1969
    12
    15
    24:00
    1,868
    630
    34%


    1969
    12
    16
    24:00
    3,699
    2,388
    65%


    1969
    12
    17
    24:00
    3,312
    1,966
    59%


    1969
    12
    18
    24:00
    2,252
    978
    43%


    1969
    12
    19
    24:00
    2,709
    1,322
    49%


    1969
    12
    20
    24:00
    1,516
    496
    33%


    1969
    12
    21
    24:00
    339
    24
    7%


    1969
    12
    22
    24:00
    3,366
    1,987
    59%


    1969
    12
    23
    24:00
    3,393
    2,355
    69%


    1969
    12
    24
    24:00
    2,339
    982
    42%


    1969
    12
    25
    24:00
    5,065
    3,769
    74%


    1969
    12
    26
    24:00
    2,043
    725
    35%


    1969
    12
    27
    24:00
    1,337
    413
    31%


    1969
    12
    28
    24:00
    3,333
    1,963
    59%


    1969
    12
    29
    24:00
    1,173
    301
    26%


    1969
    12
    30
    24:00
    5,888
    5,223
    89%


    1969
    12
    31
    24:00
    656
    103
    16%




    For a 1,236 Watt south facing, vertical array--Is this a useful amount of power?

    December, models out to 53,436 WH of electric heat per month (182 331 BTU).

    And 88,834 WH from panels (already 81% derated--Useful energy).

    That would take a few gallons of oil to heat the water (depending on efficiency of water heater). Or ~10 gallons of gasoline for a generator.

    Tankless propane water heater at 80% Efficiency. Propane is ~91,500 BTU per gallon:
    • 182,331 BTU * 1/0.80 tankless eff * 1/91,500 BTU per gallon = 2.5 gallons of propane
    I am not a fan of tankless water heaters. But, it seems to be a lot of work/money for winter hot water use.

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    I did my testing; as well as I could in poor conditions. The sun did come out very late, but I never saw the potential output from the panels I was using. Only got up to about 20% of that. However, I was able to determine that the panels will run at very close to Vmp, supplying what ever amps the panels can produce to the heater. The idea that MPPT or any other complicated thing is needed for this is ridiculous. Cold days will make for higher voltage and the amps accepted by the heater will go up in direct proportion to that, so cold days will provide more heat without MPPT. The only thing I would say is that exceeding the amps that the heater will accept, (using Vmp & Ohms law calculating) would be wasteful. Otherwise, I expect better efficiency than we get with a charging system. Resistance heaters are 100% efficient.

    So, I am figuring our water use, adding the heat needed for a 50 degree rise and allowing for a 10 - 20% loss per day through the insulation. I am putting up about 1200W, running free #6 cables that were left over from a job and we'll see in a year or so if it does what I expect. Everything else I have done with solar heat has worked as I expected, even when a mechanical engineering friend from my old life argued. The secret is that he was trained by ASHRAE, a council made up of manufacturers of heating & cooling equipment. What do you suppose their agenda is?

    You may not get this, but I have no interest in getting that technical. The other thing is that where I am located, away from the mountains out toward the prairie we have better solar conditions that I see in any charts. Lewistown is bad compared to here. Today it clouded up early, yet we were able to use over 4KW from our 2000W array. It could have been more, but I like to be cautious. My method of load shedding is to switch our horribly inefficient RV fridge over to electric when the sun is good. I only ran it half of the day, so we could end the day with the Trimetric showing 100%. By the way, I have been running the new 2030 all summer long and helping Ralph with feed back. He has been making constant modifications. I have version #8... It is done, perfect. The interesting thing he added is Rpc (returned % of charge). What you put back in comparison to what you used. I typically see over 120%. Your average person would be thinking overcharging. It is perfect. I am so sick of people who say either that overcharging is bad or that we never get charged with solar that I could scream. My Crowns run at better than 1.28 most of the time. I have seen 1.295. So every time I hear of somebody saying 1.265 is full I want to throttle them. And the last thing... I have my CEF set at 98% and it is pessimistic.

    One last thing.... Why go to all of this expense & work? Because it is the right thing to do and I hate generators.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: short circuit thermostat

    You do know PV's are a current source, right? Once they reach their maximum current, that's it; the cold weather advantage of higher Voltage is not gained without an MPPT function.

    Your biggest problem is an utter lack of thermostats/switching equipment meant to operate on high Voltage DC.

    This is one of those projects that sounds so simple when you lay it out on paper, then generates headaches when you try to get it to work reliably under the varying conditions experienced.

    Nothing like a stable Voltage source to make things work right.
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    Bob,

    As Cariboocoot/Marc says--The solar panels are current sources--Vmp for a first approximation, is a fixed value. Of course, it changes with temperature (rises below ~25C and falls above that temperature).

    I believe you are (for this thread) looking for a Winter hot water solution (avoid solar thermal+water+antifreeze+pumps+etc). So, sizing the system to give optimum results in cold weather (Vmp-array>=operating voltage of the resistance heater--picked 30 volts and a 1,236 Watt array.

    The spread sheet is setup to model 1,236 Watt array with Vmp-array=30 volts and a 0.96 Ohm water heat element. No MPPT or anything electronic.

    Of course, a larger array will give more current, however, the solar array will begin to clip the current (current will begin to drop as voltage exceeds Vmp drop across the resistor, and would drop to Voc at zero current--Which is not going to happen here). So, a larger array is only going to give better cloudy weather performance.

    Note that in the above design, I optimized the operating point for that particular heating element to be at 30 volts and ~864 Watts or ~28.8 Amps. And that falling temperature/rising Vmp-array will not change the output of the heating element in full (or less) sun. In fact, Imp falls slightly with cold cells (not very much, but it does fall).

    Also did the array vertical to keep it snow free.

    I can model a 2x larger array, but you asked for a ~1kWatt array--So that is what I tried.

    Sorry, I don't know where your installation will be... I can do a different location--See if PV Watts has something closer to your location and I can run the numbers there.

    I am a little confused... Are you disagreeing with my modeling of the array+heating element? Or would you have me choose different conditions (different location, different size array, oversize the array, change tilt, different month(s))?

    Regarding choices (battery bank, MPPT controller, Linear Current Booster) vs a simple panel-heater connection. You can see from the "efficiency" of [email protected]/[email protected] that there is a quite a difference between effective power transfer conditions.

    When the panels are under full sun, the transfer is basically 100%. However, if the array wattage drops to 1/2, the transfer efficiency falls to 50%--So you lose a bunch of energy (specifically because the array is running at less voltage:
    • 864 Watts; Vheater = sqrt(P/R) = sqrt (864w/0.96o) = 30 volts (full sun)
    • 432 Watts; Vheater = sqrt(432w/0.96o) = 21.2 volts (1/2 sun)
    So you can see that we wasted almost 8 volts from the array--And since Power=V2R, the drop in voltage hurts power output even more.

    The choices (optimum cost of array/available heating element at ~30 volts):
    1. Cheap: Array-Heater (no battery bank--Yea! But huge losses in transfer efficiency in less than clear sky)
    2. Bit more cost: Array--PWM Controller--Battery Bank-Heater (loss of MPPT improvements in cold weather for array--~10-15%)
    3. Much more costly: Array-MPPT Controller--Battery Bank--Heater (same as #2 with MPPT controller; ~10-15% improvement in sub zero weather)
    4. #2 or #3 with Heat Pump Water Heater (Probably not useable in sub ~55F weather/room)
    5. Wild Card: Array--Linear Current Booster--Heater (may or may not work, LC Boosters are near non-existent in retail)
    Can play with controllers in #2/#3 (basically battery based system) -- Using Midnite controller + digital relay in Waste Not Mode--Or get a Xantrex/Schneider C40 or similar PWM controller set up in Diversion Load mode.

    And while #1 is horribly inefficient in less than full sun (mornings/late afternoons) and cloudy weather (1/2 solar power, only 1/2 that makes it to heater). However--The overall efficiency hit is not "terminal" with poor sun angle/weather because when you have lost "good sun" on the array, there is not much power anyway. 1000 Watt array getting 500 Watts of sun and feeding 250 Watts to heater--250 Watt loss is still less than the original 500 Watt loss due to poor weather/less than optimum angle.

    So--I believe my spread sheet model is "accurate enough" based on the conditions I tried. 88 kWatt from December solar array vs 54 kWatt*hours into the electric heater (~61% December average efficiency)... Is any of the other solutions (#2-#5) up the efficiency to harvest an additional 34 kWH (assuming 100% transfer efficiency to electric heater)?

    Only you can answer that.

    Personally, adding the 1,000 Watt array to the battery based system, and using the heater as a diversion load is probably the technically superior design. However, even that only gives you an additional ~1.7 gallons or so of propane worth of hot water.

    Where we keep coming back to saying that generating heat from solar electric panels is usually not very cost effective. Especially in Winter when there is simply much less sun.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat
    Your biggest problem is an utter lack of thermostats/switching equipment meant to operate on high Voltage DC..

    Not true. Crydom makes SS relays, up to 40A,150VDC with various control voltages. I'll figure that out later, but my plan of using 3-24V panels in series gives a max of about 135V in cold conditions here, so no problem. Remember that you don't need to run the element at 240V unless you need fast recovery. This is not the goal at all here. I suppose the theory is that MPPT would increase the wattage during cloudy conditions, but that is a difficult thing to prove. I don't believe it is worth the money, as I have felt for many years. If more panels are cheaper than complicated & expensive controllers that could fail, I think it is a no brain decision. How many panels & mounts can I buy for what a Classic costs? Farmers pump water without fancy controllers. This is a similar idea. Whatever volts & amps the panels provide will be turned into heat. The voltage does vary with the clouds and the temp. Cold will provide more, with or without MPPT. I have to tell you that after all of my "fun" with MidNite, that would be the last thing I would do. But that is a whole other story that I won't go into here.

    The other comment about this "sounding so simple" is exactly why I went over to a neighbor's 24V PWM system yesterday and spent 3 hours testing in varying conditions. I am no longer talking theory. I came back with a gallon jug of hot water that was magically heated with poorly matched element & panels. I verified it in the field. Exactly how I figured out years ago that MPPT is not all it is cracked up to be. It may say new guy under my name, but I have been doing this for 14 years now. If I bragged about my systems under the signature it would end with "no generator ever used here". This is still true.

    I want this separate for several reasons, but #1 is simplicity. #2 is that my solar electric is pretty much maxed out. These are my last words on the subject until after I have installed it and that may be a year from now. The only reason I am going to all of this trouble now is stumbling into a deal on some odd panels. I don't even have a water heater yet or a house to feed it with.... Next summer is the goal.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: short circuit thermostat

    Let us know how it all turns out.
  • CraziFuzzyCraziFuzzy Registered Users Posts: 19
    Re: short circuit thermostat

    You could always use a small 12V AGM sealed battery and a tiny '12V solar charger' and use that 12V as your control voltage for the SSR. Run that through the water heater's thermostat and to the 'coil' of an appropriate SSR, the the solar loop through the 'contact' of the SSR. Cheap and reliable, and you could use the 12V to feed an LED light or something as well. I know you said 'no batteries', but this is a very simple arrangement, and would be hands-off for many many years.
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    What you missed is that I already have a solar system for my house. I have the 120VAC control power that really should be used with an AC rated thermostat, as most people do. The no batteries comment means no new batteries.... Nothing to make an MPPT controller work. As little complication as possible. The KISS principle.
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    From a 50,000 foot level, how do you want the system connected. I guess the "no batteries" confused me.

    You want ~1,000 Watts at what voltage (AC/DC) at the water heater element? You only want power to the water heater thermostat when the batteries are over ~80% SOC, or something else?

    Use the electric water heater as a preheat to a tankless propane heater (for example)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HandyBobHandyBob Banned Posts: 31
    Re: short circuit thermostat

    Go back to post #5 & read the last paragraph. I've said it all and it is going to work. 3 panels in series, about 110V & more when cold.
  • CraziFuzzyCraziFuzzy Registered Users Posts: 19
    Re: short circuit thermostat
    HandyBob wrote: »
    Go back to post #5 & read the last paragraph. I've said it all and it is going to work. 3 panels in series, about 110V & more when cold.

    No doubt it will work. It won't be very maximized in power recovery though, without something back-regulating panel voltage. That's the purpose of the MPPT circuitry. With that fixed resistance load, at lower sun levels, production will be VERY small. By actually increasing the resistance (that's what the MPPT controller would do in this instance), panel voltage would be allowed to increase, to a point where the actual power produced would be higher. Solar panels are not like other voltage and/or current sources, in regards to I/E curves. Allowing as much current as possible to flow out of them does not necessarily make the most power.
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    Yep... I modeled 30 volts Vmp-array into a 0.96 Ohm heating element for ~1kWatt of heating in my previous posts. You could do ~120 volts into a ~14.4 ohm resistor for 1,000 Watts nominal, and it will give you the same results.

    A solar array (mostly a current source with a peak output voltage limit) driving a resistive load will be less than 100% efficient at converting electrical power from the solar array at less than rated sun (100% sun = 100% power, 50% sun = 25% power).

    Unfortunately, the worse sun on the panel (weather, orientation for fixed panel), the lower the transfer efficiency. It would be much better to use the solar array to charge the battery bank (relatively fixed battery bank voltage with "matched" Vmp array) would be more efficient. Of course, if you draw power from the battery bank (night/poor weather), you have the losses of the battery bank (~80-98% efficient, depend on battery type, how it is cycled/recharged, etc.).

    No "great" solution, just different solutions, each with its own positive and negative issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CraziFuzzyCraziFuzzy Registered Users Posts: 19
    Re: short circuit thermostat

    And I'm sure it's been mentioned, but if you are trying to use the sun to hear water, moving electrons is rarely going to be the most efficient method.
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: short circuit thermostat

    Thinking about your panels to a DC heater element with thermostat or whatever your using to stop a boil over ? How are you going to open a DC 110V line without an ARC & fire ?
    That had me thinking of what your proposing.

    VT
  • BB.BB. Super Moderators, Administrators Posts: 28,139 admin
    Re: short circuit thermostat

    Thermostat has been discussed... Solvable.

    The issue Bob was trying to avoid with thermal systems is anti-freeze and maintenance. He lives in a hard freeze/cold climate and the simplicity of solar electric plus the savings of plumbing+antifreeze+maintenance was a draw (I can understand that).

    At least during winter, lack of sun in his location makes any solar difficult (vs a few gallons of fuel per month worth of heat per month).

    A hybrid water heater (heat pump) would have been interesting--But probably not warm enough in winter for the heat pump to work (efficiently). Plus not a cheap solution either (plus the AC inverter+Battery bank+solar array system required to support the heat pump).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tikidogtikidog Registered Users Posts: 5
    Re: short circuit thermostat

    Several have asked how I plan to do that, Remember the old days when DC was everywhere, I am using the same old technique to stop arching as is used on electric trains and old trucks. Its worked for many years, old electric cars used it too. Actually I'm not using dc element but using the existing 1400 watt, 120 ac that came with the tank. keep in mind that this all for the numbers. Nothing in concrete yet and so far no hot water heat worth mentioning. I wanted to see how well it would/would-not work before I built it .If I can find a way to use solar directly and keep in KISS I will have won the fight to save money over the long run. . Grid tie to me is a ????? and is not part of the real issue with solar and conservation. It saves no money. I am on my fourth set of batteries, this last group are Trojan T105. the lead used alone would put me in carbon footprint hell for the next twenty years. But this new relay I am using is great. it has four sets of break contacts. so far NO arching.
    That said , It looks by the numbers I am running that fuel cell is the direction this must go to be powerful enough to resistance heating. That will cost more than my house to install. for a 3 Kw generator it starts at $22,000.
  • CraziFuzzyCraziFuzzy Registered Users Posts: 19
    Re: short circuit thermostat

    Also remember that arcing will not be much of a concern switching a purely resistive load. It is the inductive kick generated by the coil in a motor that causes the high voltage arcing commonly associated with DC switching. In all, this should work fine, but I think in regards to your desire to KISS, that you'd get better returns simply adding these panels to your existing array, running heating of of your battery DC, and limiting your preheat to daytime hours by either monitoring battery voltage or a timer. This way, you avoid adding more load to your inverter, you harvest only side that is in excess of your needs, and you don't put any extra cycling on the batteries in doing so.
  • CDN_VTCDN_VT Solar Expert Posts: 492 ✭✭✭
    Re: short circuit thermostat
    BB. wrote: »

    -Bill

    Thanks Folks for the replys , Funny forum software is not letting us know Who is replying to Whom .
    Well I get what most are saying but in the 70's , Brill Coaches (electric buses Da-bus)in BC hydro 600VDC had arc's with no motor rotation or connection. I do believe above 80Vdc and in humidity DC can arc.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: short circuit thermostat
    CDN_VT wrote: »
    Then again , what do I know , were flying that thing into the ground pulling a barge off the candled ice & im the tech second seat !

    You know quite a bit.

    DC can create a sustainable arc above 15 Volts. It does not require a motor or induction coil to do so. Multiple series interrupts are a way of handling the problem, but they do not eliminate the arc or the possibility it may start a fire.

    One thing is for sure: if the people trying this have any degree of success they will crow about how they know so much more than engineers despite their success being 100% dependent on pure luck. But when they burn the place down they'll never post pictures of the disaster.

    Now if you'll excuse me I'm going to feed my diplomas through the paper shredder and become an Internet Expert.
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