Converting Non diverting DSSR20 system to diverting?

[Barry Timm]

I have a perfectly performing SBMS0 system with 3x DSSR20 (non diverting type) and 6x 315w 120 half cell PV panels.

As my system is idling much of the day after the initial daily 100% charge cycle (SOC set to 80% on my 7.7kwh 24v LFP), I would like to convert my system to enable electrical heating of my dual electric/propane hot water tank in my RV in subversion mode.

If I use the 36v 1200w dual parallel heating element, I believe that means using 2 of the new DSSR20s. Correct?

Would I then replace two of the existing three DSSR20s and add a DEXT16 (to simplify wiring)?

How would I connect them to the SBMS0? Can the two diverting DSSR20s (DEXT16) and the remaining non diverting DSSR20 share the EXTIO4 signal?

[Dacian Todea]

Yes each 600W element can be supplied by one DSSR20.

The existing DSSR20 can be controlled by the same DEXT16 so yes all 3 will be controlled by the same EXT IO4 signal. 

[Barry Timm]

Excellent!

Thanks, Dacien.

[Casey & Gina]

Dacian,

Instead of attaching each element to a different DSSR20, could you parallel the diversion output of any number of DSSR's together, and then attach one or more elements in parallel to that to simplify things?

I ask because in my case, I usually have 3 DSSR20s with panels connected, but I also have additional ones that only have panels together sometimes, and one of them may only have a single 300W panel connected.  You had mentioned before that 2 DSSR20s could be connected in parallel to one element, to get a longer time window each day of effective heating.  I would like to heat 2 600W elements (or could add another element if appropriate), whether 3, 4, 5, or 6 DSSR20s are providing power.

On Tuesday, September 1, 2020 at 11:48:07 AM UTC-6 Barry Timm wrote:

Excellent!

Thanks, Dacien.

[Dacian Todea]

Casey & Gina,

You can only parallel max two DSSR20's as in that case if there is a short circuit on the PV wires on any of the two DSSR20 the max current will still be limited to 20A but even so this will already require individual 15A fuses for each panel as if a panel for some reason develops an internal short circuit then the other parallel panel will push 10A (that is fine) but if there is another parallel DSSR20 that can add another 20A to already existing 10A so 30A pushed in to the defective panel more than rating for those panels that are just 15 or 20A rated.

[Casey & Gina]

Okay, great info, thank you for the details!  Instead of paralleling, is there another way to safely combine the power from the multiple DSSR20s to then use for whatever purpose without having to connect things to individual DSSR20s?

[Dacian Todea]

What is the disadvantage of connecting to individual DSSR20 ? Each DSSR20 will output a max of 600 to 700W depending on connected panels and amount of sun / panel temperature.

The water heating elements are already conveniently made of  2x 600W heating elements and 600W / 36V = 16.6A or so thus great with 12AWG wire (or 2.5mm2) fairly low cost and standard.

If you make custom heating elements or use PV panels or diodes for space heating you are again limited to around 600W (when using a PV panel as a radiator/heater).

Not sure what is your application and why will you want to parallel the outputs ? what will be the davantage ? 

[Casey & Gina]

I'm trying to think of how to use a variable amount of diversion power based on the number of panels connected.  You had mentioned that connecting a single element to 2 DSSR20's would provide a longer heating time each day, since even in lower light conditions, the 600W would be coming in for a longer period from 1200W of panels than 600W.  I have 6 panels permanently mounted on my trailer, and the ability to externally connect another 6 to it.  I intend to have 3 panels mounted on a van in the future that I can connect, and may set up a stationary 6 panels (or some lower number) on some property.  So my thought was that I could have the shorter heating window when no external panels are connected, and a longer one when they are, or perhaps faster heating of the water when they are than when they're not.

[Dacian Todea]

A fixed element like 600W connected to say 2x300W of panels can be up to 80% efficient in a sunny day so say that those panels can produce best case 3kWh in a sunny day about 2.4kWh can end up as heat.

Now say you have two DSSR20 each with 2x 300W and each is connected to an individual 600W heating element you can then put 4.8kWh in to water.

But if you connect two DSSR20 each with 2x300W of panels to a single 600W heating element you may get maybe 3kWh in to water heating so yes it is more than a single DSSR20 but much less than if you have two 600W heating elements one to each DSSR20

 

[Casey & Gina]

Would I be better off just using 6 elements, each one connected to an individual DSSR20?  My concern with that is that maybe my heating time window would be shorter, since the temperature would be too low to effectively heat the water without enough amperage coming in.

Basically I'm just wanting to figure out how to put as much of the diversion energy to use as possible.  Another reason I was thinking about combining the DSSR20 outputs was for more flexibility in where the power could be directed, and it seems that this would be a lot easier if not having to be done on a per-DSSR basis.  For example, maybe the water is hot enough, and a thermostat disconnects the elements.  Then it would be nice to send that power somewhere else (though I have no idea where), which may not be as easy to divide into 600W pieces.

[Barry Timm]

Has anyone here managed to utilize the SBMS0/DSSR20 diversion system for powering their RV Propane/electric refrigerator in electric mode during periods of excess solar power, and then having the fridge switch to propane mode at other times?

While I can run my RV fridge permanently on 120vAC from my SBMS0 solar system, it is quite a drain on my system if there are any clouds etc, so I've been leaving the fridge in propane mode. The fridge uses 320w-340w on 120VAC. 

I am thinking that I can use my 3rd DSSR20 with diversion to run a separate, small, 400w Inverter during excess solar production, if I can figure a way to open/close the switch on the fridge that would switch it from propane to electric mode, and have the SBMS/DEXT16/DSSR20 send the switching signal to the fridge.

Anyone have experience with this, or ideas on how to do it?

[Casey & Gina]

Hi Barry,

Absorption fridges are terribly inefficient.  I don’t have exact figures handy and it’s been a while since I had an absorption fridge, but based on my experience and memory I believe the amount of power drawn by an absorption fridge is something like 4 times that of an equivalent compressor fridge.

If you don’t really need the ability to run on propane, you’ll be better off replacing the fridge with a compressor model, which also do better at staying cold, don’t need to be level to run, gives you more interior space as there doesn’t need to be as much bulk on the exterior of the rear of the fridge, and won’t set your RV on fire.  You can find 12V/120V models designed to be fit into RV fridge locations replacing an absorption fridge - Dometic DMC4101 and Furrion Arctic are a couple examples.  Absorption fridges are the best option if you run primarily on propane, but IMHO anyone with even a fairly small solar setup doesn’t need to.  The compressor fridge will both use less power when running, and cycle on/off, whereas the absorption fridge will draw more power constantly without cycling.

Your current fridge might have the ability to run on 12V DC as well, which would save inverter loss and allow you to turn the inverter off without interrupting refrigeration.

The diversion power is not stable - a cloud passes over and the amperage drops.  If the amperage drops below what the fridge needs (or 400W inverter if you don’t have a DC option), it will shut down.  If you managed to devise a way to switch it back and forth to propane based on amperage coming in, this would likely result in a lot of switching back and forth which may be hard on the fridge and lead to premature failure - to be fair this is speculative on my part though.  You could possibly use the diversion current to both run the fridge and keep a small dedicated battery topped off.

Absorption fridges work off of any source of heat, which is what allows them to run on propane.  When run on electric, the electric simply heats it up as the propane otherwise would.  So it may work to use diversion power to run a heating element in the same way, which would be tolerant of amperage drop, and use your main stable power source for the electronics.  I don’t know how complicated that might turn out to be, but it seems feasible.

I don’t think that there’s any remote switching ability built in to the SBMS system, but it seems that you could devise a separate switch based on the amount of current coming in.  Or if you use a 400W inverter, you could have a switch based on loss of AC power.

I definitely advise going the compressor fridge route, but it would be a very interesting use of diversion power to offset propane use on an absorption fridge.

[Barry Timm]

Thanks for the advice. It is appreciated.Yes, I know that the Absorption RV fridges are very inefficient. I've been considering replacing it with a very efficient Danfos/Secop compressor 24v DC fridge (Unique makes a number of suitable choices, sold by Home Depot which are extremely efficient, but expensive). 

My current concerns are that I like the idea of utilizing the surplus solar power that is currently being wasted, by using the RV fridge in Auto mode. The fridge senses if there is any incoming AC and then switches to electric automatically, else it reverts to propane. I suppose I could simply power the 400w inverter via the DSSR20 diversion option, without even worrying about the fridge Auto switch, but like you mention, I am not sure how consistently the 2 x 315w panels will provide sufficient juice to the 400w inverter. My visual monitoring of the solar harvest indicates that it will likely generate enough power for much of the time as my harvest seems to be 1200w+ whenever the DSSR20s are switched on (unless my PV are REALLY dirty :) ).

I think this option is a halfway step to a more efficient, more expensive compressor fridge replacement, and I like the backup of the propane as I am full time RVing with no backup generator, and would prefer to not lug one around for very occasional needs.

Thanks for the thoughts!

Barry

[Casey & Gina]

Oh, an automatic switch already existing makes things simpler - the absorption fridges I’ve had haven’t had this feature but instead have required manual switching.  As you suggest, diversion powering a small inverter should probably make this pretty simple.  The inverter wouldn’t need the full 400W anyways - it will only need what the fridge needs plus losses.  So maybe only 150W.  With 600W of panel input that much should be available most of the time, and passing clouds etc. shouldn’t affect it too much.  You could also make the input to the inverter switchable between diversion and your battery, in case you should want to have the ability to run it on battery overnight to avoid propane cost when you have ample battery.

I owned two of the Unique fridges earlier this year - I first bought a 10cf model but it arrived damaged, and after looking it over in person I replaced it with a 13cf model which was a better use of space.  These are both actually 12V compressors - the fridge has an internal 24V-12V converter of some sort to allow them to run on 24V.  I couldn’t detect any difference in efficiency between using my own Victron Orion converter and letting the fridge do it.  If you want a true 24V compressor, the only model from Unique that offers one is the 470L1, a 16cf model.  I don’t know of anyone else using the 24V compressor.  If it accepts 12/24V, the compressor is 12V.

None of Unique’s fridges are designed for mobile use though, and I ended up with some water damage to my floor as water collects into an open container on the bottom which a heating coil runs through to evaporate it off passively - apparently some water had collected in there but not evaporated prior to me moving the trailer, which then spilled out and soaked into the floor.  The 370L1 was otherwise a nice fridge with a good design, but mine had a microleak in the coolant and stopped cooling effectively and would run nonstop as a result.

Also, for whatever reason, Home Depot (who I ordered through) doesn’t consider Unique fridges to be “major appliances”, so if you need to exchange it, you have to pay for another one in advance, wait 3-4 weeks for it to get to the store, then return your old one, unlike the usual case with AC fridges where they will get a replacement in stock without any additional charge and allow you to make one trip bringing in the old one and picking up the new one. After mine developed the cooling problem within the warranty period, I did buy another in advance anyways, but the person I spoke with ordered it to be delivered to my mailbox rather than for in-store pickup as I had wanted.  I can’t receive freight at my mailbox, so the carrier ended up unable to deliver it and called me about it saying that if Home Depot would authorize changing delivery to the store they could do so.  Home Depot however claimed they weren’t able to do this, even after speaking to a manager about it.  I was unable to get it rerouted to the store before the carrier took it back to Canada.

Hours of hold time trying to sort this out were involved, and I lost patience and decided I wasn’t willing to wait another 3-4 weeks for another one to ship, which I’d then need to modify to drain into my grey water to avoid damaging my floor.  This was a series of unfortunate events that probably doesn’t happen that often, but it made for quite a bad experience.  I ended up ordering a Dometic DMC4101 instead.  That all said, if I were to build a cabin or other stationary place that needed a fridge, I would definitely consider a Unique again.

[Barry Timm]

Wow, that's good info/feedback on the Unique. I was not aware of the open drain arrangement. That may be the deciding factor for me to stick with the existing fridge or get the less efficient (than Unique) Dometic DMC4101.

BTW, my existing Dometic RV fridge does use 320w to 340w on electric, not 150w! I switched it on and off multiple times while monitoring my SBMS0 load screen, to confirm the fridge load. It does have a "Climate Control" switch in addition to the "Auto" switch, which I think allows the fridge to cycle when the temps are down (just a guess), as I do see the electric load cycling at night when it is cool, but the duty cycle appears to be around 60% On 40% off on a time basis, with that constant 320w-340w load. During the day, it stays on all the time.

Thanks again!

Oh, I meant to ask, does the Unique have latching doors or did you have to make some locking arrangement for the RV travel?

[Casey & Gina]

Here is the relevant section in the Unique manual about this...

No latching doors on the Unique.  You’d have to make something yourself for that.

I am in the process of building my trailer so have only moved it a few times when needed.  I’m mostly stationary for now and probably will be for the rest of this year.

My last absorption fridge drew 11A @ 12V = 131W.  But it was a smaller counter-height model.  Perhaps yours is larger.  320-340W is a lot - the Unique 370L1 uses 62W, and at 13cu is probably larger than what you have unless you have one of those extra wide RV fridges with 4 doors.

The DMC4101 is rated at 156W, slightly higher than the significantly larger 470L1.  However, a more powerful compressor should equate to shorter runtimes.  The Secop compressors may still have the edge in efficiency - I’m not certain.  What ultimately matters is total Wh used in a given time period with the same ambient temperature, and fridge/freezer temperature settings.  It would be nice if that were standardized and we could see a more accurate efficiency comparison between brands.  Even then though, the installation considerations can make a difference.  RV-style compressor fridges are designed to ventilate air through the front bottom - if you were to box the Unique fridge in, it may build up heat and run less efficiently than advertised.