DSSR20 question

[Tim D.]

I had a few questions on the SBMS0 and the DSSR20's

I'm planning on powering a remote cabin in Texas with Solar, I'm looking at using 30 260 watt 60 cell solar panels, and paring that with a 24V 840Ah LiFePO4 battery pack (using 24 of these 280ah batteries https://www.aliexpress.com/item/1005001566815335.html?spm=a2g0o.productlist.0.0.76f277833hR8q9&algo_pvid=f8fcdc28-b18e-4253-8ace-34c6a3c7f9a9&algo_expid=f8fcdc28-b18e-4253-8ace-34c6a3c7f9a9-6&btsid=0b0a555b16122869835312785e785c&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_)

So I'll need 15 DSSR20 for this project

My question is would there be any cost benefit to ordering the DSSR20's with Diversion? and if so any ideas or suggestions on how to best implement the diversion power for hot water thermal storage

In Texas we deal with long hot summers and our winters are generally mild, but I was thinking it may be possible to use a two 300 Gallon IBC tanks for hot water thermal storage, probably have a storage temperature goal of around 140F (60C), If I use 2 of the 300 gallon tanks and store water from 110F up to 140F  I should have 44 kWh of thermal storage with a 30F delta down to 110F up to 140F. 

At a bare minimum solar hot water storage would provide battery free hot water to the cabin with the unused solar power and I thought it might be possible to rig up a few heat exchangers, a small pump, and use the stored hot water in winter for heating the cabin. I'm unsure if radiant floor heating or if forced air thru a heat exchanger would be a better solution??

I was reading that Dacian recommends using diodes for thermal heating with eh DSSR20's because they naturally operate at their MPPT, but I'm a bit lost on how I would implement diodes into heating water, using restive heating elements seems pretty straight forward, any examples of using diodes for heating water would be greatly appreciated

Also had a question on how I would set a thermostat to limit the water temperature to 140F, how would it be wired into the Dex16, or is there a way to control the water temp with the SBMS0?

Thanks, appreciate the help

Tim

[Tim D.]

I've been doing a bit more google searching for using Diodes as a water heater with the diversion DSSR20's and found this article

https://www.sciencedirect.com/science/article/pii/S2352728519300508

If I'm reading this right I could use a chain of 1N5402 diodes and match the number of diodes to the panel voltage , these diodes are only $0.04 each when purchased in bulk https://www.futureelectronics.com/p/semiconductors--discretes--diodes--switching/1n5402-bp-micro-commercial-components-5157144?loc=1&fbclid=IwAR2aWP5ZZUGKjmOp255S2gr33zOXzaBbgvLv-BQ0EpQ3C9CmXrwkHFy6ZCg

they could fit inside 3/8" aluminum fuel tubing, https://www.amazon.com/Allstar-ALL40180-Diameter-Aluminum-Coiled/dp/B006K8JJ1C/ref=pd_bxgy_2/131-2616873-8028245?_encoding=UTF8&pd_rd_i=B006K8JJ1C&pd_rd_r=8b41e626-69dd-409d-891f-d0e084c0d5cb&pd_rd_w=ohxgB&pd_rd_wg=NCItP&pf_rd_p=f325d01c-4658-4593-be83-3e12ca663f0e&pf_rd_r=XP65WGSFRQAA1AJSSAZ5&psc=1&refRID=XP65WGSFRQAA1AJSSAZ5 and I could fill the tubing with a thermal dielectric grease to increase the heat transfer into the aluminum tubing and this tubing would be placed in the hot water tank

Any other suggestions would be welcome

[Dacian Todea]

Tim,

Have you purchased the 260W panels ?  That is the same type of panels I have but I live in a colder climate and I do not even need much of the energy in summer (I only use 10% of that my PV array generates). The 260W panels when hot will have lower max power point voltage so you may get just 6 to 7A compared to using newer 290 to 330W panels that will provide you with 10 to 11A even in hot summer days.

30 panels may provide over 300A and that is a bit high for 840Ah battery but should be fine tho a bit high charge rate.

In that experiment they used just a small 5A panel and even so they abused those small diodes.

You need larger diodes for 20A output from a DSSR20 connected to two large panels. You may want to read this post I made related to diode heating https://groups.google.com/g/electrodacus/c/UqJte2_FRzQ/m/OgeMLKJeBwAJ

[Tim D.]

Dacian,

No, I have not purchased the panels yet, but I have found some REC brand locally that are used (2016 manufacture date) for $50 a panel if I buy 30 of them. Of course I wouldn't have to use all 30, but i was thinking it would be better for cloudy/rainy days to have more solar rather than less. Texas summers are very hot, with many days over 105F (40C) , I figure this heat will reduce panel efficiency a lot, A/C is a must so I need a lot of power to run a mini-split heat pump

I do plan on ground mounting the panels to get good airflow around them to help combat the heat buildup and make cleaning them easier, I have 30 acres so space isn't an issue.

I'm still in the planning stage for this project and looking for the best way to move forward, but I'm leaning toward the DSSR20's with diversion, may as well get battery free hot water for a little additional cost upfront

With the 50A Diodes you mention, how would you waterproof them for immersion in a 600 gallon water tank? I'm sure I can find large aluminum heat sinks to mount them to, but I'm still not sure how I'd waterproof them. 

Do you have any suggestions for a radiant floor heating vs a hot water based forced air system? From what I'm reading if I go with a radiant floor I can use lower water temps, say 85F to 120F and can store approximately 50KWh with that temperature delta

Thanks again

Tim

[Dacian Todea]

Tim,

It only gets around 30C here in summer (may be a few days above 30 even one or two 37C days) but sure not as hot as at your location and I say as low as just 6A in to battery from my 260W panels in to my 8s battery. While for me is not an issue at all as I need just 10% of what the array produces in summer and in Winter I get 9 to 10A in to battery from same panels and that is when I need most of the energy.

Seems like you will need most of the energy in summer so in that case I will look for other panels even if a bit more expensive.

You may get as much as 2x the energy from a 300W+ panel compared to the older 260W and that is very significant. Also my panels are fairly close to the house about 15m (less than 50ft) in average for the PV cables lengths (that is also important as there will be some small voltage drop on cables on top of the lower voltage due to hot panels).

So if you have maybe 10C hotter panels than mine plus longer cable it may make the panels almost unusable in some days. And yes in colder cloudy days you will maybe get more from 30x 260W panels than you will get from 15 or 20x 300W+ panels but I will still suggest finding a bit newer panels for your conditions.

I have no decided how I will be using (or if I will be using) those diodes but likely I will have them outside the water mounted on some water block type construction.  There is the advantage of no moving parts if they can be inserted in to water but then they need to be perfectly waterproof. Maybe something like a large surface disk at the bottom of the tank made of two sandwiched aluminium parts having the diodes on the inside (likely a bit complex and expensive). Or a rectangular aluminium pipe caped and submerged with diodes again on the inside.

I do not think your winters requires fancy hating like radiant in floor heating. But large floor thermal mass will be useful in most setups.

[Tim D.]

My panels will also be close to the cabin, probably within 50 to 75 feet, and I will size the wire accordingly for the run that I end up with. 

What type of panels would you recommend? I've seen the new half cell split panels claim to offer more efficiency. , ones like this https://ressupply.com/solar-panels/hanwha-q-cells-q.peak-duo-g7-320-solar-panel ..... but being 10 amp panels would I be limited to one panel per DSSR20? 2 of them might go over 20 Amps in the right conditions 

And would you still recommend 60 cell panels?

[Dacian Todea]

The DSSR20 can handle any two 60 cell parallel panels and is designed to handle 22 to 24A without any problems.

The half cells help with shading mostly but I will not pay that much for 320W panels assuming that $193 is USD (that is the price I see in Canada no not sure).

Price is just to high for that panel (anything above $0.4/Watt will be high). Maybe you can find used panels with 72 cells if price is better then they will be a better deal than new 60 cell panels as space is not an issue for you.

10AWG PV cable will be more than good enough for two parallel panels at that distance (cables are also expensive so you will not want to overdo things there).