Resistive heating element sizing tool for DMPPT

[Noel Barlau]

Good morning Dacian,

I've been going over the DMPPT manual and I think I was able to correctly come up with the required lengths of 18ga wire to suit my 39x 245w used panels, but as always I like to "measure twice, cut once" whenever possible.

In the manual you mention a tool you would be providing to easily size resistive loads based on our personal needs and circumstances.  Is this tool available right now?  I'm ready to order some wire and water heating elements.  For this winter at least it will be used strictly for bulk water heating in 55-gallon drums, but eventually we'll be laying in wire under tile in the basement as you did on your slab.

Thanks very much,

Noel Barlau

[Dacian Todea]

Noel,

I had not time for the DMPPT450 since last winter to busy with the new Kickstarter and SBMS0

There is a newer firmware for the DMPPT450 that I will need to add to DMPPT450 manual as there where a few things I changed last winter. I noticed that even with my 33x 260W panels and the 8 heating loops connected each heating loop is made with 18 AWG wire the thick but small surface aluminium plate will not keep the SBMS internal temperature below 50C where it should be so I use a 120mm fan in front of the DMPPT450 at 10V (is a 12V fan) and that  keeps the temperature at around +45C that is acceptable.

The thing is that temperature measurement is made inside the microcontroller that is in the middle of the board but all power mosfets are next to power connectors at the outside perimeter so while the microcontroller will only see 45 or 50C the mosfets may be closer to 80 or 90C that is already to close to limit.

With just the diversion to battery the current is smaller so there is not that much heat so in summer time (non heating season) I do not need any cooling fan. I guess that is how you use the DMPPT450 until now.

The 18 AWG should have 21mOhm/m  (mine had that) but you should measure to make sure it is that and not something else depending on where you got the cable from.  In my case loops where around 2Ohm so about 95m long loops (can have a bit of tolerance).

For the 55 gallon drum I will say just one of this heating elements should be used max will be 36V/2Ohm = 18A x 36V = 650W (and in some cases when is cold and sunny here I seen 650W trough one of this heating elements).

The 55 gallon drum if is plastic can handle max 55C so there is not much temp range 20C min to say 55C is 35C delta x 1.16 x 200 liters =8.1kWh

Having one heating element and drums not isolated sort of make sure you almost never get to the +55C

For bathroom floor it will depend on the size in my case one loop covered about 5m^2 but 7 to 8m^2 may be better not to heat the concrete to much mine may get close to 30C according to thermal camera and mine is relatively deep under about 4cm of concrete the tiles are still not installed those should add another 2cm.

And you do not need to install a max number of this heating loops just as many as you need will work just fine.

you have 39 x 245W = 9555W and say about 3000W may be diverted to your battery then you still have 6555W that is about 10 of those heating loops but if you only need say 3 of those loops there is no problem. The max power point voltage may be higher maybe 34 or 35V with less heating loops but the loops as mentioned are already designed to handle 36V so it will be fine.

I will also add a large diode where you have the loop end as is a fairly large inductor with those 95m or so of wire. I should also add that to the user manual update.

Edit:

I will add two photos to illustrate what I have connected now.

At this time 4 of the panels are used to test the SBMS0 so I have just 29x 260W connected to DMPPT450

You can see the fan in first photo blowing some air on the DMPPT450 but also all the breakers and cables.

In second photo you can see there are 8 of those loops I mentioned channel 1,3,4,5 have each two of those loops in parallel and then on channel 2 there is a single smaller power loop used to heat some water.

The PV4 is diverted to battery charging 53.4A at the time I took the photo and 155A going to heating 32.3V at that time for the array. The DMPPT450 internal temperature is +47C and ambient temperature is around +24C

You can maybe see the firmware version is 1.2 for the DMPPT450

[Noel Barlau]

Thanks very much for the detailed reply.

I probably should have given my full intentions with the DMPPT450 and the SMBS units beforehand.  As of right now I haven't received any of your products yet, and that's quite ok because I'm up to my armpits in other projects at the moment.  So all my assumptions about the operation of both the SBMS and the DMPPT are all guesses based on reading the manuals, and I have no hands-on time with them.  But I'm absolutely waiting with bated breath for them to arrive!  It's like waiting for Christmas...

The intention is to use the DMPPT450 strictly for heating purposes.  I won't have a battery connected to it at all.  Initially I'm planning on having 10x 55-gallon drums of water piped in series via a thermosiphon setup, or possibly a small DC pump if the thermosiphon idea doesn't pan out.  I'd also be very happy using metal drums so the Delta T could be much higher than it would be with the plastic drums.  After learning more about the system I'm hoping I'd then be comfortable taking on the massive project of laying wire and tiling over it.

The 245 watt panels are rated 37.5 VOC, 30.2 VMP, 8.68 ISC, and 8.13 IMP.  Considering there would be 39 of those, if you could shoot me a recommendation for the water heater elements you'd select for this purpose I'd really appreciate the input.

As with everything I do, I was planning on overkilling the heck out of the heat sinks.  I'm either going to roll my own out of huge chunks of aluminum and my metal shaper to create thick fins, or I found some pretty skookum USSR-made giant finned heat sinks on eBay.  Either way would be suitable to keep the heat within limits, I'd wager.

If I'm completely off base with my usage intentions strictly for heating, please let me know.  I'm covering the 48v 520ah Lifepo4 batteries and electrical supply side with 60x 340w panels coupled to 3x 5000w LV5048 inverter/chargers from MPP Solar.

Thanks again for your time, I know you're a very busy man right now!

[Dacian Todea]

Sorry Noel, I had confused you with someone that had the DMPPT450 from last Kickstarter two years or so ago.

The 245W panels are still 60 cells so not much different from my 260W panels. Those ratings are for +25C panels but as you need this for heating there is winter there and likely temperatures well below freezing where PV panels open circuit voltage will slightly exceed 40V as noticed on mine.

If you go with wire try to get around 2Ohm per loop for 18AWG wire. In case of water heating elements the 36V 1200W rated ones should be fine those are usually two 600W 36V elements  so they should have around 2.16Ohm each.

The issue is that aluminium is not that great of a thermal conductor and so as I discovered at this power levels small air flow is the best option. My aluminium plate is 3/8" thick (about 1cm) and while that seems much it will not be able to transport the heat much as temperature under the DMPPT450 will be quite a few degree higher than under SBMS120 that is just next to it.  Yes fins and more space in the back will have helped but not sure it will still have been enough for passive cooling. The low speed fan in the front is super effective as it directs air over the power connectors that are aluminium and in direct contact with power mosfets. It also helps with the circuit breakers that while a bit over sized 63A breakers for about 40A max they still get relatively hot and as all off them see that current and are next to each other it add's up without any airflow.

The 10x 55 gallon seems about the right amount of thermal storage and have one 650W heating loop in each. You will need some small water pump to circulate the water a bit else there will be a lot of heat stratification.

25kWh battery is fairly large but they will not like charging with 20kW of PV panels even 15kW based on chargers. Ideal will be about 5 to 6kW max charge rate for that battery both for battery longevity and practicality.

The SBMS0 has a new function where 3x larger PV array can be connected 18kW where charging will be maintained at say 6kW max. Will explain this function in the user manual as this is a new software function. Hope to have the manual made today.

[Noel Barlau]

Great info, thanks!

I'll plan on adding a fan to the setup as well.  The infrared camera will be the final word on the situation, and I'll follow your temperature recommendations.  Also I just ordered the water heater elements.  Just to verify, those elements are two resistive loops each and I'll separate them by removing the bus bars joining them, and one of the elements per unit will each be run off of a separate output on the DMPPT450, correct?

The LV5048 has quite a good range of options for charging rates, it's user-selectable from 20a to 220a.  I'll target .25C initially and see how the batteries react thermally.

Looking forward to seeing the manual!

[Dacian Todea]

Noel,

You can use the heating element with both elements connected in parallel if you will be using those in metal barrels as those can store more energy so it will be fine. Not sure how the metal barrels will deal with water over time unless they are the expensive stainless steel.

Also max current should be 17A per element at 36V so for the two in parallel is just 34A thus OK on any of the DMPPT450 outputs.

The SBMS0 manual can be seen here http://electrodacus.com/SBMS0/SBMS0.pdf

[WG Doug]

Epoxy lined barrels are available. New epoxy lined barrels are not much higher cost. I think most used food products barrels are epoxy coated.

On Thursday, October 17, 2019 at 2:17:20 PM UTC-7, Dacian Todea wrote:

Noel,

[Noel Barlau]

Thanks Doug,

I already have the steel drums, that's why I'm angling that direction.  This should be a relatively temporary setup, as I'm planning on using the DMPPT mostly as a slab heater per Dacian's arrangement in the manual.  But - and here's the thing - I need to fully understand its operation and capabilities before I can properly plan the layout pattern for the 18 awg wire.  I'm still very new to the solar game, and I'm the weirdo who has a very hard time learning through reading, and I need to experience something first hand and then I quickly get a grasp on its operation.  Using the barrels will give me a perfect playground to learn how to work with it.

Hopefully I can get through one winter on the non-lined drums I already have, but if not it's really not a big deal since our basement floods at least once a year.  That's part of the reason we're building a new house - this one wasn't graded properly back in 1979 and it's been problematic ever since.

I'll definitely keep your recommendations in mind for a "permanent" solution in the new house for domestic water heating.

Thanks again!

Noel

[WG Doug]

Hi Noel, I don't think you will have a problem with unlined steel barrels for a number of years. I've seen plain steel barrels lie open to harsh elements for 20+ years and still hold water (and other nasty things...). They are rather thick material. The lined barrels would have far less electrolysis and resulting corrosion which may be the larger problem with a bare barrel.