AC PV heater minimal setup

fabriced

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Mar 16, 2021
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Hello,

First at all hello to everybody ! I am new here and new in PV. Writing from Belgium. I need some advises !

First at all, the objective:
I have no solar panels, a heat pump (air-water), and a 300L water tank heated by the heat pump and by a 3kW resistance to make the bridge when heat pump is not enough.
This additional heating element is 240V AC and managed by the heat pump motherboard, with just an on/off relay.
This tank is my main electricity consumer. Weather is quite cold in Belgium but the isolation is good. Also regulation makes PV installation expensive if I plug it on the network. Prosumer taxes are going higher, and not suitable for a very small installation. So I have to stay off-grid.
My idea would be to inject some PV power in this boiler and only this. I expect the boiler capacity to be higher than panels. I do not need any battery storage, tie-grid, ... I cannot change the AC heating element. I cannot add a second element. So I will plug my system on it, and share it with the heat pump controller.

My current understanding:
- I would need 2x 300W panels, a MPPT charger, a 12V or 24v battery, a 1000W inverter
- Since I will never reach 3kW, my idea is to use a SSR to limit current in the heating element and bring it down to 500W (or another power to chose). With some Arduino, and electronic, I will manage to disconnect my injection when the heat pump power on the 3kW circuit. When this circuit is off, my regulated circuit would be on.

Now I have lots of questions/doubts:
- I need battery with this setup for the MPPT but in fact I do not need battery storage. Is there a more efficient way without battery ? Maybe without MPPT but I have doubt since the solar panel management with variable impedance seems needed to me.
- If I cannot make it without battery, I feel I can just take 2x 12V AGM battery plugged in series. Since I do not need storage, they can be quite small, just be sure it can deliver the injected power in the heater. As soon as it discharge, my Arduino would cut off the load. Or maybe MPPT can be setup to not really use the battery ?
- I could play with DC current into the AC heating element, but I am not very confortable with this.
- I think I will try to put the SRR at test and check how much heating I get with 500W power for testing before buying solar system
- About rentability, if I inject all 2 panels power in the heating tank, I guess it should be ok, loss would be limited. Should be paid in < 4 years (guts feeling).

So this is a first description of my DIY project with PV. I am very motivated :). Please ask anything and share your thoughts. I did not find similar project on forums (no battery storage, AC heating element, no usage outside of sunny period, water heating only, off-grid).
 

svetz

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Welcome to the forums!

...My idea would be to inject some PV power in this boiler and only this....

- 2x 300W panels, a MPPT charger, a 12V or 24v battery, a 1000W inverter
- Since I will never reach 3kW, my idea is to use a SSR to limit current in the heating element and bring it down to 500W (or another power to chose). With some Arduino, and electronic, I will manage to disconnect my injection when the heat pump power on the 3kW circuit. When this circuit is off, my regulated circuit would be on.
Before properly answering your questions there's a number of things in your post that have me confused that hopefully can be straightened out.
Let me echo back my understanding, then we can go from there.

It sounds like you have a heat pump for heating the residence and you're not interested in that part directly.
What you're interested in is secondary auxiliary 230V resistive heat element that consumes 3000 watts that you'd like to power from solar.


If I have this wrong, a picture would help.


Powering a single device is perfectly doable. Something like this would get you there:

panels->SCC->battery->Low-Voltage-Disconnect->inverter->device​
Or, with an all in one:

panels->all-in-one w/battery->device​
You talk about using a 1000 watt inverter, that won't provide enough power for a 3000 watt resistive heating element. Can you clarify how many watts the device consumes and how many hours per day it runs?


Panel Sizing Calculations
Let's say you're in Antwerp, then setting the tilt to 24° for best winter performance, from an insolation calculator your worst insolation is 1.1.
Let's say the heater consumes 3000 watts and lets say it needs to run 2 hours a day (adjust for your actual numbers). Then you'd need 3000x2 = 6,000 wh/d.

With an insolation on 1.1, you'd need an array size of 6,000 / 1.1 = 5500 watts. Using 350W panels, you'd need 16 panels. This is far larger than the two panels.

So, if you can clarify what it is you're trying to do hoping someone on the forums can answer those questions.
 

fabriced

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Mar 16, 2021
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Hello !

It sounds like you have a heat pump for heating the residence and you're not interested in that part directly.
What you're interested in is secondary auxiliary 230V resistive heat element that consumes 3000 watts that you'd like to power from solar.

Yes ! Exactly.

You talk about using a 1000 watt inverter, that won't provide enough power for a 3000 watt resistive heating element. Can you clarify how many watts the device consumes and how many hours per day it runs?

My idea is to limit it around 500W, I could even limit it to 300-400W. I know 3kW is too high for this system. So I will use a SSR to limit the current and permit the use of a lower power. On the other hand it can run whole day, at least when the heat pump does not decide to inject 3kW. I will managed a controlled relay to automatically switch on/off the solar injection when the heat pump decide to turn on the auxiliary heating element. But as soon as temperature is maintained high enough by the solar system, the auxiliary will never be powered on by the heat pump controller.

I did ordered a SSR to test if the heating resistance works with a limited power... Because this would put the project to an end if this does not.
 

newbostonconst

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Not sure how you are going to limit current with a ssr....it is a high speed relay that is solid state...you will need more then just a ssr.

Welcome to the forum....
 

fabriced

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Thanks. The SSR is used for this I think. It is not just a relay for AC application. There is a DC input to set the % of phase you want to switch off, resulting by a real power limitation. See https://datasheetspdf.com/pdf-file/789332/Fotek/SSR-40DA/1 we some clear chart showing how it works. But yes I will test this first with a SSR, a temperature sensor and a wattmeter.

But I have no idea how to select the battery capacity. Could I take a very small one ? How much small ?
 

newbostonconst

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They are giving examples that this SSR can used in. They do not work by themselves. You will need a high level circuit design to do what you are wanting.
 

Supervstech

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I would recommend installing a lower watt heater if you want to power it with an 1100W inverter.
It is just a Nichrome wire string.
Many supply houses sell lower wattage elements.
Alternatively, you could reduce the voltage feeding the element to lower the wattage.
Dropping the output to 96V would do it.
 

svetz

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It sounds like a 3kW heater element, at 230V, that's (3000/230=) 12.5 amps.

A heating element get's hot because it's "resisting" the flow of electrons... if you reduce the amps, you may exponentially reduce the heat. That is cutting it to 6 amps might not be 1500 W.

Can you swap the element? Something like: https://www.homedepot.com/p/7-1-2-i...er-Heater-Element-Normal-Duty-60054/205364914

I've also seen this:
Many battery voltage water heating elements have two elements connected together with parallel shorting bars. Use only one heater for twice the resistance. Two in series for four times the resistance. These could be switched in and out with a control to closer match the panel to the heater as light level varies. Or it could save you having to replace the element as the number of panels changes as the system grows.
So that might be another route.
 

efficientPV

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Are you serious about an arduino? I've done extensive heating with PV efficiently at power point not using any charge controllers or batteries. I think it is fairly simple and cheap, but you need to have experience in electronics. Contact me if interested.
 
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