PV panels to immersion queries?

[Just David]

Relative newbie here, I have searched through the threads that seem to cover this topic but still need some assistance.

I have a grid-tied system consisting of 19 panels (7.2KW) with batteries. This I wish to be left undisturbed as it was installed by a company a few months back and don't want any issues in case of warranty.

I have 2 water tanks, one I wish to have an immersion element connected to a new set of panels solely for this purpose. It will not be grid-tied nor go through the consumer unit (so totally stand-alone).

I want to keep it simple, the immersion element in the tank is rated at 3000w/ 230volts, and my multimeter shows a resistance of 18.9ohms. It would be tricky to replace this immersion as it is a bit unusual with 2 thermostat pockets (1 for the element, 1 for the gas boiler) also it is quite recessed and of an odd design shape.

I can fit 4 of the panels below onto the roof (a total of 1820w). If connected in series it should give 169V at 10.7A, this should (I think) should power the immersion OK, albeit not at full power but I can accept that.

Is this the most efficient method of getting the most power from the panels into the element? I don't think it would be advantageous to go through an inverter as this could lead to some losses? Also, it would mean the additional cost of an invertor.

I understand I would need a suitable DC-rated relay for the thermostat switching.

What have I missed?

The data from the 4 panels I have in mind:

Nominal capacity 455 Wp

MPP current 10.77 A

MPP voltage 42.26 V

Short circuit current 11.38 A

Open circuit voltage 49.45

[Mike 134]

Interesting idea. The immersion element is nothing more than a resistor it could care less AC or DC as long as the voltage rating was not exceeded. Should work although 1800 vs 3000 watts your recover time will likely be doubled. Need to maintain any low water controls (if they even exist).

[PNPmacnab]

I heat water with PV using a MPP controller similar to the ACTii/PL. Everyone talks about the best days. My wife doesn't want to hear any crap about not shining. While most generation is around midday peak power, there is still a lot of power that can be harvested the rest of the day. And what about those times when the sun has a few clouds. You have calculated your resistance for optimal sun in direct connect. One study shows that total overall long term heating increases when that ideal resistance is almost doubled. So, if you want to save money, forget ideal resistance.

73K png or jpg file would not upolad

https://www.hindawi.com/journals/ijp/2017/7540250/ fig 2

[Just David]

Thanks for the replies.

PNPmacnab, excuse me if this is obvious to you but I'm unsure if I have understood it correctly. I can't change the immersion element unless I had one custom made which could be expensive and troublesome, the current one I can pick up easily are OEM ,seem good quality and is relatively cheap. The resistance of my immersion is 18.9ohms.So when you say increasing the resistance could be beneficial, if I rearange the panels to 3 in series/2 parallel array (at a push I could get 2 more on the roof, 6 total) this would give 126V and 21.5A but would drop resistance to 5.88ohms. Is this going to help or is there a better configuration. I'd much rather solve these issues now than have to clamber back up on the roof and redo all the wiring later!

[Mike 134]

Since the heater resistance is fixed and is rated at 230 volts you want to put your panels in series to maximize your voltage. I don't know why you'd put mpp controller what is the benefit? I'm guessing you just want to temper the water with the solar panels and the gas portion of the heater will bring it up to 125-130 degrees

[PNPmacnab]

Just making a comment on what many think is ideal resistance. A target of 83% of panel rated current should work very well for daily production. You will need some solid state relays to turn it off with your thermostat since those mechanical contacts will arc severely. Take the current nameplate ratings with a grain of salt if from China. That is with an infinite heat sink. They will get hot and when they get hot the resistance will double which increases heat four times. Two in parallel will greatly reduce heat and possible failure.

[Just David]

Since the heater resistance is fixed and is rated at 230 volts you want to put your panels in series to maximize your voltage. I don't know why you'd put mpp controller what is the benefit? I'm guessing you just want to temper the water with the solar panels and the gas portion of the heater will bring it up to 125-130 degrees

Pretty much, these panels will do what they can and gas + some electricity from the other grid tied panels will do the rest.

[Just David]

https://www.hindawi.com/journals/ijp/2017/7540250/[/url] fig 2

That link was very interesting. Cheers

[PNPmacnab]

Most take the approach of PV heating as better than nothing. I have specialized in PV power point water heating since 2016. Not a big fan of using dedicated PV panels just for hot water. A successful system turns off. And what do you do with the power after that? I've designed off grid systems that harvest excess energy from the charge controller array without the need for batteries charge controller or inverter. Typically, there is a lot of excess as systems have to supply daily needs as well as recharge from cloudy days. With lead batteries there is also excess from charge cycles which limit battery current. Li isn't as bad. I think everyone would be quite surprised watching the system work through the day, it is a beautiful thing to watch. This technology is almost unknown in the solar world. I have a detached garage where I do laundry and it has a water heater which only gets the excess of the excess. All cycles of the laundry use hot water and the clothes come out steaming.

Your area experiences clouds and the following chart demonstrates the losses of MPP vs direct connect. Power is I squared x R so dropping the current dramatically reduces power on a fixed resistance.

% Rated Increase over
Panel Amp Direct Connect with MPP

100% 0%
90% 10%
80% 25%
70% 50%
60% 67%
50% 100%
40% 250%
30% 333%
20% 500%
10% 1100%