More diversion load stuff

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Joe_B
Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
Hi guys,

The recent "what do I do with excess energy" thread here got me thinking, I have a 40 gallon LPG water heater that is killing me on propane bills. My PV system produces extra power and usually once my batteries are charged, my controller goes into float mode and unloads the panels. I have an XW system with 2 XWSCC's.

So I am thinking of installing a water preheater tank as a dump load and have a few questions.

1. I am planning on using a large solid state relay to switch my 24V battery system to the water heater element. I am using the XWSCC aux output to control the relay. I will set the aux output to connect the battery when the controller reaches the float voltage. Anyone see any issues with this?

2. I was looking at 24 volt heater elements and they go for anywhere from $30 to over $100. The heater I am looking at installing is a 20 gallon, 110V, 2KW. The element I was looking at runs on 24V at 20A for 480W. Here is the question, If I hook 24 volts to the existing 110V/2KW element, it should run at one quarter of the power or 500W. Should I just buy a 24V element or hook 24VDC to the original equipment?

The heater will be within 5 feet (wire distance) of the battery bank, I am planning to use a 60 amp SSR (Crydom D06D60) and 4 ga welding cable to hook things up, the SSR will live in the XWPDP.

So what do you think? Am I doing anything stupid? I can get the water heater from HD for about $270, the relay goes for about $100 I have all the rest of the stuff except for the 24V element if i have to buy one. so for less than $400 my numbers say this should be worth it considering I am burning about $100 in propane each month.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: More diversion load stuff

    How much excess worth of power do you have per day/month (kWH?).

    It would seem if you do not have much power, then the costs would not make it worth the money.

    If you have a lot of power, then looking at heat pump type water heater would then be interesting (roughly 2x more heat from same input energy as a resistance heater).

    Other way would be a thermal solar array and run the circulation pump with your current DC battery bank (although, a nominal solar PV panels connected to a DC circulation pump would work too.

    Back to your setup--you have a fuse/breaker on that line? Resistance heaters in hot water tanks can fail with a dead short.

    Also, you should not need to limit yourself to DC rated elements... Get a much larger AC element and derate the wattage by:
    • Element Watts * (Vbatt)^2 / (Velement)^2
    You may save some money.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff

    Thanks for the information Bill, I currently have six SW235's on the roof (1 SCC) and they are producing about 6.5 KWh per day in full sun with this daylight time. I expect to get about 8 KWh or so at peak summer time. I have 840 AH of trojan L16REB batteries at 24V.

    My issue is this, I do not have sell enabled and I do run grid support. If I do not switch the grid support voltage setting depending on sunny or cloudy days, I lose a lot. I run the house in grid support mode at night so the batteries ramp down. During the day, the batteries get charged and then the controller loafs at the house load, I can see that I am wasting about 750 watts when this happens.

    So what I do is if it looks like it is going to be cloudy, I set my grid support voltage to 32V which is Xantrex code for "only go into grid support when the batteries are at 100%". In thiss mode, my batteries are fully charged and then the house takes the load. If it looks like it will be sunny, I set the grid support volts to 24.6V and let the batteries discharge at night and recharge during the day, obviously harder on the batteries.

    So what I would like to do is leave grid support volts set to 32V which will essentially put my batteries in a float mode but during sunny days when the power would normally be wasted I will dump the energy into the water thereby hopefully lowering my propane demands.

    As for the wiring, I have will use a 75 amp DC breaker.

    Thanks, I thought resistors are resistors but just needed to hear it about the element. You just saved me some money. I am assuming that running DC through the OEM thermostat is a bad thing, right?
  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
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    Re: More diversion load stuff

    I have a SSR running off my MX60. The relay energizes a regular house outlet powering an oil filled electric heater, or dehumidifier, or distiller.

    Why not just run the water heater as bought...120vac. The SSR will turn on and off when needed (very fast too) using inverter power. You can have a hold or delay on the aux energizing too. This would be the cheapest way and avoid low voltage heating element. You could always go that way if you found that the 120vac way didn't work.

    Ralph
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff
    Ralph Day wrote: »
    I have a SSR running off my MX60. The relay energizes a regular house outlet powering an oil filled electric heater, or dehumidifier, or distiller.

    Why not just run the water heater as bought...120vac. The SSR will turn on and off when needed (very fast too) using inverter power. You can have a hold or delay on the aux energizing too. This would be the cheapest way and avoid low voltage heating element. You could always go that way if you found that the 120vac way didn't work.

    Ralph

    Hi Ralph,

    I do not want to load my inverter if I dont have to. With an extra 2KW load on the inverter, I could overload things if I turn on other stuff. The XW4024 is only good for 4KW. Since the heater is so close to the batteries, its more efficient to do it with DC. Also seems like I can just run the 2KW OEM element at 24 volts and get about 500 watts of load. Win-win!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: More diversion load stuff

    I am wondering if running a straight AC water heater would be better than trying to run a 24 volt conversion--You are only loosing the 5-10% of the XW inverter losses.

    And, running the AC contactor on DC can be an issue... It is probably not designed or rated for DC--but you may be running substantially less DC current--so it is possible that it could work out OK (welded/arcing contacts being the big issue).

    A $100 gallons of propane per month towards what? Cooking, domestic hot water, domestic heating?

    I am guessing that you are probably paying 2x or more for the same "btu" content as I do for natural gas... I run around $20-$60 per month (summer/winter) to heat, cook, hot water, cloths washing/drying (clothes drier in winter).

    Assuming 750 watts * 6 hours a day of lost power = 4.5 kWH per day "lost"...

    That would be around:
    4.5kW per day * 3,412.3 BTU/kWH = 15,355 BTU's per day "worth of electricity"

    A gallon of propane contains around 91,600 BTU per gallon... 30 days of 100% electrical conversion:
    • 15,335 BTU per day * 30 days / 91,600 BTU per G = 5 gallons per month (towards resistance heating)
    So, 5 gallons per month propane savings with a resistance based electric water heater (I assume that I understood your numbers correctly).

    If you got a Heat Pump type water heater, that would be around 10 gallons per month savings...

    If I got the numbers wrong--you can "adjust" them to fit your needs.

    In the end, try to estimate where your fuel costs are going (hot water, cooking, washing clothes, space heating, etc.) and run the figures one what would make sense... Conservation (insulation, double pane windows, low flow shower heads, use thermos cooking for stews and soups, etc.) vs where you would like to spend your money on the technical side (solar thermal vs heat pump water heater, etc.).

    Is it worth it to you?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff

    Well Bill, if you are going to apply logic and math to this.....

    Seriously, I am using propane pretty much exclusively for DHW. I have a cook top but it hardly ever gets used.

    Now to the numbers:

    I am presently using about 40 gallons of propane every 2 months and paying $5.96 per gal. Last bill was $209.00 for 2 months.

    So if I use your numbers and not factoring in conversion losses, I am looking at saving $29.75 each month. With this information, the install will pay for itself in a little over a year. Seems like a reasonable payback.

    The reason I do not have a heat pump water heater is that I dont want to load my inverter with it. I only have 4KW available and sometimes I get close, so I have propane with pilot, no electrical load.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: More diversion load stuff

    It appears you can set the GE GeoSpring to Heat Pump Mode only, which:
    How long should my GeoSpring water heater run in order to reheat the water in the water heater?
    • The recovery time is a function of a number of different factors including the amount of hot water used, the temperature of the incoming water, the ambient air temperature, heating source, etc. The GeoSpring heat pump can recover 8 gallons per hour (at 68°F ambient air temp) while standard heating elements can recover about 22 gallons per hour. The difference is, the heat pump only draws about 550 watts of energy compared to 4,500 watts from standard heating elements. So if an average shower uses 16 gallons of hot water, the heat pump would run about 2 hours to reheat the water and use about 1.2 kilowatt hours of electricity. For the same 16 gallons of water, a standard electric water heater would take about 48 minutes at 4,500 watts of power which is about 3.2 kilowatt hours, almost three times the energy. Therefore, although the heat pump runs longer to reheat the water, it still ends up using approximately 62% less energy. Since most consumers use hot water in varying amounts followed by long periods of little or no usage, the heat pump has more than sufficient time to reheat the water and satisfy the consumer’s hot water demands. If more than the average amount of water is used, GeoSpring is uniquely designed to switch from heat pump to resistance heat and back again, to provide the same volume of hot water as a standard electric water heater. In the winter time when ambient air temperatures are likely to be lower, the heat pump will have to run longer to heat the water.

    I don't think you are "wasting" Propane... At $6.00 per gallon (eek!), I would be looking for alternative fuels (diesel, gasoline, etc.)... Propane has a lot less heat per gallon than either of those other options.

    you are paying:
    • $6/0.0916 MBTU = $65.50 per MBTU
    • Natural Gas ~ $12 or so per MBTU (10xTherm) (PG&E)
    • Diesel ~ $4.50 per gallon / 0.135 MBTU per gallon = $33.00 per MBTU
    And, I would seriously look at Solar Thermal panels for DHW and Space Heating--If you are into plumbing DIY--Not a bad alternative.

    Solar Shed and other Solar Thermal Links

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff

    Yeah Bill,
    Geospring $1500, crappy little preheat tank $268
    Cant deal with it.

    As far as the DC on the thermostat, I was thinking of putting a snubber cap across the contacts just for the heck of it.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: More diversion load stuff

    I think somebody around here got one for under $1,000 with Sears (?) and the federal tax credit...

    Use this fuel cost calculator and plug in $5.95 per gallon for propane and $0.13 per kWH for electricity (or whatever you are using):
    $38.09 per Million BTU of Heat delivered to home
    $3,618.55 per year for normal home for Electric

    $98.90 per Million BTU of Heat delivered to home
    $9,395.50 per year for normal home for LP (Propane) Gas

    Add in that the heat pump hot water is 2x the efficiency of electric resistance heaters (assuming ~>55F ambient temperatures), you would get ~5x the amount of hot water with electricity as you do with traditional electrical resistance heaters.

    Even if electricity is 2x, you still come out way ahead (then have to look at the payback time on the expensive hot water heat pump system).

    That propane is scary costly. :cry:

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
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    Re: More diversion load stuff

    Heater coils are a resistive type load, Look at getting a cheap MSW inverter. Then all of the safety and temperature controls will work as is.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff
    n3qik wrote: »
    Heater coils are a resistive type load, Look at getting a cheap MSW inverter. Then all of the safety and temperature controls will work as is.

    Well actually I still have the old prosine 2.0 inverters I was using before I installed the XW. I guess I could just hook the prosine to the batteries and use the aux to enable/disable it.

    ETA: Woops never mind, those are 12V.....
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
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    Re: More diversion load stuff
    Joe_B wrote: »
    Yeah Bill,
    Geospring $1500, crappy little preheat tank $268
    Cant deal with it.

    You might be able to get a GEO Spring a lot less than $1500.00. Sears is always running sales and they have a web site that you can find all available rebates. I got mine for $1295.00 from sears on an internet sale and 0 interest on my sears card for a year. Power co gave me $300.00 rebate and I got s 30% tax break off that. It has a2.3 efficiency rateing and mine has only used 218 KWH since instalation In oct 15 2010. I only run in in the max effiency mode and never run out of hot water. They also have 2 regular heating elements. 2 other plus.s are they dehumidife the air and help to cool house in summer. :Dsolarvic:D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
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    Re: More diversion load stuff
    solarvic wrote: »
    mine has only used 218 watts since installation

    Vic, you mean 218 watts peak/average power to run the heat pump?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarvic
    solarvic Solar Expert Posts: 1,071 ✭✭✭✭
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    Re: More diversion load stuff
    BB. wrote: »
    Vic, you mean 218 watts peak/average power to run the heat pump?

    -Bill
    Bill I goofed!!!!!!!! It was 218 KWH since instalation in Oct 15 2010.

    Vic
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff

    OK, So I have all the low voltage stuff wired up, Breaker, SSR/heat sink, controller. and have two honking wires ready to wire up to my 400/800 watt element. I am now searching for a tank, the temptation is to just pony up $250 for a home depot special (20 gallon point of use electric heater). But before I do this I thought I would ask the experts on the forum if there are any other alternatives for a tank (cheaper)? :D
  • raydias
    raydias Solar Expert Posts: 68 ✭✭✭✭
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    Re: More diversion load stuff

    I am looking at doing a similar design. i got luck one day at Lowes and picked up a 28gal water heater on clearance for $74.

    A couple of other ideaa I was toying with:

    http://forum.solar-electric.com/showpost.php?p=79075&postcount=14

    and

    http://forum.solar-electric.com/showpost.php?p=81345&postcount=33

    The last one i am almost ready to test. waiting on a pump.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
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    Re: More diversion load stuff

    There are plenty of small hot water heaters at Sears Outlet.......Cheap !! I think you can have it delivered to a store and save the shipping.

    http://www.searsoutlet.com/d/products.jsp?md=srh_md&stxt=hot+water+heater
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: More diversion load stuff
    Joe_B wrote: »
    ...Also seems like I can just run the 2KW OEM element at 24 volts and get about 500 watts of load. Win-win!

    No, you will only get 80W of load.

    (2000W/120V=16.67A; 120V/16.67A=7.2ohm; 24V/7.2=3.33A; 24V*3.33A=80W)

    Before you buy water tank, test your SSR with air cooled resistor load. You can make your own resistors by coiling steel wire.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff
    AntronX wrote: »
    No, you will only get 80W of load.

    (2000W/120V=16.67A; 120V/16.67A=7.2ohm; 24V/7.2=3.33A; 24V*3.33A=80W)

    Before you buy water tank, test your SSR with air cooled resistor load. You can make your own resistors by coiling steel wire.

    Yeah, figured that out a few days ago, I have obtained a 24 volt dual heater, each element is 1.25 ohms which gives me either 500W or 1000W at 28.8 volts.

    ETA: I already tested it all with a bucket of water, My relay and wiring drops about .95 volts so the element sees just about 28 volts. Each element provides about 618 watts. The relay is impressive, looking at only about .3 volts across it at 20 amps, keeping the loss low is key, all the impedances are so low even the connections make a big difference.
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: More diversion load stuff

    You can also record total DC energy going to your diversion load with one of these. If you going to drive your SSR with PWM, you may need few electrolytic filtering capacitors connected in parallel with SSR power input, for smoothing out current spikes that may mess up measurement.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff
    AntronX wrote: »
    You can also record total DC energy going to your diversion load with one of these. If you going to drive your SSR with PWM, you may need few electrolytic filtering capacitors connected in parallel with SSR power input, for smoothing out current spikes that may mess up measurement.

    Thanks, I plan on using my Victron BMV-600 to monitor the battery and I have set up an X-10 input to monitor when the element has power. Looks like I will have about 2 to 3 KWH per day to dump into it.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
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    Re: More diversion load stuff

    Am a bit late to this thread. And, have scanned every post, but may have missed a reference to the following ...

    Hi Joe B, I have an XW SCC, it has older XW which has been impossible to update without the implater thingie, so the functions of Aux may have changed. But my XW does not have a direct Aux function corresponding to CC in Float.

    When conversing with Xantrex support, the Product Manager for this product stated that this product probably never have this (useless) function.

    How will you use the Aux output ? Does your XW SCC have an Aux function active when the CC is in Float ?

    Or will you try to infer that the CC is in float based on PV Voltage, Battery Voltage ? Or ?

    Just checked the only on-line manual that I could find, and do not see Aux output for Float mode.

    Thanks, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff
    Vic wrote: »
    Am a bit late to this thread. And, have scanned every post, but may have missed a reference to the following ...

    Hi Joe B, I have an XW SCC, it has older XW which has been impossible to update without the implater thingie, so the functions of Aux may have changed. But my XW does not have a direct Aux function corresponding to CC in Float.

    When conversing with Xantrex support, the Product Manager for this product stated that this product probably never have this (useless) function.

    How will you use the Aux output ? Does your XW SCC have an Aux function active when the CC is in Float ?

    Or will you try to infer that the CC is in float based on PV Voltage, Battery Voltage ? Or ?

    Just checked the only on-line manual that I could find, and do not see Aux output for Float mode.

    Thanks, Vic

    Vic,

    My XW 4024 as well as both of my XW SCC's have one aux output. They have many trigger options but I am using the high battery voltage trigger. Set to the same voltage as the XW SCC float voltage. I have tested it by watching the green light on the SSR and it drives it perfectly. You can also program the polarity, voltage, trigger on duration and off conditions which allow you to effectively program in hysteresis.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
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    Re: More diversion load stuff

    OK, Joe_B,

    Well, I've used the MX-60's Aux output, but have decomissioned the XW SCC, as it has the FW Bugs of Rev 4, and am unwilling to buy the Surprise Dongle to implant the new FW.

    I did not test any of the Aux functions on the XW CC, but I had wondered if the SCC's Aux was programmed to the Float Voltage, if the output would become active during Bulk, as the Bat voltage rose to that value, and perhaps stall Bulking the bank or something similar.

    Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Joe_B
    Joe_B Solar Expert Posts: 318 ✭✭✭✭✭✭
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    Re: More diversion load stuff

    Vic,

    From what I understand, you can update firmware with a gateway if you have one of those.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
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    Re: More diversion load stuff

    Hi Joe,

    Thanks for the tip. If I had more of the current gen X stuff getting the SCP, or whatever the gizmo is would not be a big deal. Will check into a gateway or similar.

    Thanks again, Good Luck, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.