Anybody experience with overloading heating elements? (eg 36V 400W as diversion with two panels)

[Peter]

Unfortunately I have very limited space to install heating elements in my water tank (2 spots), so I want to maximize the limited heating power diverted. So I am thinking about using a 36V400W heating element with two 120 halfcell panels. This specific heating element has 3 heating circuits instead of the more common 2 giving me 50% more heating power compared to the 48V750W ones. 

But that would lead up to 450W when its cold outside. Anybody have experience what that would mean for a 400W heating element? 

Obviously it will be hotter than intended, but than again, the 48V750W elements arent any bigger, leaving the same surface for a nearly twice the heating power. 

[Dacian Todea (electrodacus)]

Peter,

Where did you got that graph from ?

What is the spec of your PV panels ? The panel temperature will not be the same with ambient temperature so it will likely need to be -15 to -20C ambient for the panel to be at -5C and on the other side when ambient is 30C the panel could be 50C or even above.

As for overloading the heating element that will short the life of the heating element. How much faster it will fail depends on what sort of margins the manufacturer used and also what the water temperature is.

[Peter]

I want to use the heating elements for hot water so normally it will work from 30 to 55°C. I bought those cheap elements for 20-30€ from ali/amzn. What are you guys using and how are they holding up? What does failure look like? just not heating any more? Or is there any chance of water leaking out? 

 Panels 370W: 

That temperature in the graph is supposed to be air temperature, so panel temp would be plus 23K acc. to the datasheet at STC, but I have used that offset value as constant. The graph is based on some calculation of panel curves including the temperature coefficients and heating element resistance, but I havent checked that against any measurements and I dont know how accurate it is, but it is the best I came up with, even if distorts a bit for temperature off STC

[Peter]

Pictures got lost...

[Dacian Todea (electrodacus)]

The way the heating elements are build is a stainless steel pipe and inside that there is some sort  sand like Magnesium oxide powder and a resistive heating element. So failure will mean that the heating element at some point will burn like a fuse and thus no current flow thus no hot water.

The 400W 36V element will have a resistance of 400W / 36V = 11.11A   36V /11.11 = 3.24Ohm

Not sure what are you using for diversion as DSSR20 can not handle two of this panels.

-5C air temperature means panel temperature at around +20C so very close to STC but if it is windy panel temperature may be lower so even higher open circuit voltage.

Also solar output can easily get to 1200W/m^2 either from edge of clouds effect (that usually will not last long) or from snow reflection on the ground or a combination of both.

Even at 1000W/m^2 with about 39V / 3.24Ohm = 12A  you are at around 470W and you will be there for many hours assuming the thermostat will not stop the heating.

You may also have problems with water stratification meaning water will be at +55C but only at the top of the water tank.

I have no idea how long will the heating element last if used at 470W instead of 400W. Maybe you find it OK if you need to replace it every two years or so.

Depending on how large the tank is you may not even need that power level.

 

It depends on how long the heating element can be in your tank but you can find for example 3000W 48V heating elements (2x1500W) https://www.aliexpress.com/i/1005005306941002.html

or maybe 2000W 48V so (2x1000W) https://www.aliexpress.com/i/1005005390750380.html   or 1500W 48V 2x750W

1000W / 48V = 20.83A    48V/20.83A = 2.3Ohm

Then 37V / 2.3Ohm = 16A   37*16 = 592W about 1200W per heating element 2400W total about the same as you will had with those 3x400W heating elements if you where not exceeding the limits.

This 48V heating elements are used under the nominal power so they will just last forever.

[Peter]

Ok, so you are saying, that the steel pipe should not get damaged? Have you had a failure yet? In that case I might just try and see, how long they hold up. 

The 48V2000W are indeed actually a better option for my 1" spot, where I currently have the 48V1500W (the other spot is 1 1/4"), I didnt see those. But the 3 loop 36V1200W would still top them in total power. Since we have lots of clouds in winter and my orientation is less than ideal for some panels, so I am focused on efficiency in the mid section of spectrum. Perfect fit would be 3 loop 48V2250W heater with 1 1/4" :-/

Stratification is an issue I will have to live with. No space and also I am definitely not going to have a impeller shaft penetration through the cover. This is tap water, so 4-6bar...

 

 

[Dacian Todea (electrodacus)]

I do not have any failure and I do not expect any at least for the next 30 to 50 years because I use my heating elements quite a bit under the spec power.

If you did not seen my setup here are some photos https://groups.google.com/g/electrodacus/c/fx9FHoU5KBc/m/xO_CtyqVAgAJ  here I also made the description of how I use the heating elements https://groups.google.com/g/electrodacus/c/fx9FHoU5KBc/m/B14Da_0GBgAJ  and I use them at about half their rated power 1500W 48V made of 2x 750W elements each element used at just around 300W so less than half the rated power.

Yes you can not have a shaft in to a pressurized tank.  For me the tank is for space heating not for hot water. For hot water can still be done but that requires a copper or stainless coil in to a non pressurized water tank so a heat exchanger.

What I wanted to say by stratification and 55C limit is that there will not be much energy stored in the tank so in a sunny day it may get to temperature fairly fast and thus efficiency is not relevant.

In any case the stainless tube will not fail unless your water is that corrosive that can damage stainless steel so just the heating wire inside the tube will eventually fail when used for many hours above spec. But it can last 6 months or 5 years depending on what sort of margins the manufacturer added to this heating elements. It is fairly similar to incandescent lights and how they fail based on applied voltage.

[Dacian Todea (electrodacus)]

Also on the 48V 2x 1000W you can connect 4 PV modules on each element if you want so a total of 8 panels for the 48W 2000W heating element as that will give you the extra energy in cloudier days and in sunny days the elements will not get damaged.

Any number of parallel panels can be connected to the 48V elements.

[Peter]

That sounds like a great idea. I havent thought about connecting more than 2 panels on one coil! I will have to investigate this option  (eg. fusing)  for when the current ones do burn out. Thanks!