Electric panel resistive element water heating

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Electric panel resistive element water heating

Postby APR » Tue Jul 19, 2016 10:13 am

34by151 in another thread... wrote:I also have 2Kw of panels connected to a DC controller that powers the hot water, long story but uses what was spare panels. The controller for this is built specifically for resistive loads and drives the second element directly via DC


34by151, can you tell me more about your DC water heating controller, string voltage, element, temperature controller, and storage tank? Is the tank mains pressure?

I am on grid with batteries and heat a 250 litre 240v 3,600w mains pressure immersion element HWS using a power diverter that diverts excess solar, that would otherwise go to the grid, to the HWS. Through winter I am struggling to keep water hot as all solar goes to running the house and charging the batteries. Currently, because of cloud and insufficient solar generation, I am partially charging the battery pack overnight at off peak TOU rates and relying on any excess solar generation to heat the HWS.

I have acquired 10 X 250 watt panels with the view of using their generation solely to heat hot water, and am interested to hear how your system is configured.
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Re: Electric panel resistive element water heating

Postby 34by151 » Tue Jul 19, 2016 2:45 pm

You can find the controler here http://techluck.com/

My Tank is a regular Rheem dual element electric unit although any water heater with an electric element will work.

I have 9 * 230W panels left over from an insurance claim
They replaced all 13 in the string and I kept the good ones

The Max watts for the controller is 1250 but you can fin all the specs on the page

Its not an inverter as it supplies DC directly to the element
To connect you would need to remove the AC wiring to the HWS and connect the DC supply
IE leave the thermostat in place

If you have a dual element HWS you will have 2 elements and 2 thermostats
In this case you would leave one thermostat/element in place and isolate the second one

Let me say that the 2Kw I have is overkill. 800W is all that is needed.
I installed them because I can but I also used the panels to create an awning along the back of the house

Ive just checked my logs and the second element has not run for over 8 months.
This means every day for 8 months has been free from the sun, no grid use
You can probably guess, Its been installed for 8 months

Its taken the second biggest load off my energy use, the biggest being the pool
Before adding this I had setup load shedding for the HWS preventing it running when im exporting <3kw

Im going to get rid of this and use the contactor to control the washer and dryer.
I must confess though I haven't had time to look into how I can suspend the cycle of either machine

I must say Ive had all types of WHS over the years (Electric, Gas, Solar tube) and this has by far been the best and probably the cheapest to setup

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Re: Electric panel resistive element water heating

Postby APR » Thu Jul 21, 2016 9:55 am

Hi 34by151,

One of the concerns I have had with this concept of running DC through the typical HWS thermostat is the possibility of arcing across the thermostat contacts. I have considered a snubber circuit to reduce the potential for arcing of the contacts, as well as minimising the potential voltage across the contacts by limiting the number of panels in series. I am wondering if the device you are using is PWM output to the element, as if so the DC is not continuous, and any arcing that may be likely would be terminated by the off period of PWM.
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Re: Electric panel resistive element water heating

Postby 34by151 » Thu Jul 21, 2016 12:56 pm

Power is drawn from the panels using MPPT
The DC is sent to the Element via an IGBT Gate Drive, so yes it PWM

As the voltages are much lower than for AC arcing across the thermostat contacts is not an issue, it built to handle this anyway.

IF your really concerned about the contacts you and add a solid state relay very cheaply to drive the element.

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Re: Electric panel resistive element water heating

Postby davidg » Thu Jul 21, 2016 4:45 pm

I'd question how well this works without optimising the array voltage after after-all Ohms law applies. I'm sure it works just no where as efficiently as it could / should without ensuring the array voltage is optimal for the element.

The "resistance" of the element will control how much "power" in watts you it will or can expect the heating element to absorb.

If we take an STD hot water heating of say 3500W heating element and we say that the voltage the hot unit is rated for a voltage of 230V then to produce 3500watts of heat energy the element will need to have resistance of 15.11 ohms (for 3kW's 17.62ohms) using "V2 / P = R" generally a heating element will have a higher resistance than this when measured so the power absorbed and turned into heat will be lower.

So if you have a voltage of, lets say 90V under load from a 3 x panel (250Watt ea) in series array and using the resistance of the element in this case 15.11 ohms then using V2 / R = P We get an actual power consumption of 536 watts, or approx 2/3rd's of the arrays peak, invariably it will be somewhat less.

So unless the voltage is sufficiently high there will never be a time when the array will ever produce even close to it's full potential. PWM or MPPT might improve it slightly, however, to optimise the output of an array the output voltage should be about the RMS voltage the heating element was designed for, so it can work efficiently.

#just saying.
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Re: Electric panel resistive element water heating

Postby Gordon-Loomberah » Thu Jul 21, 2016 5:01 pm

Another point to consider if you want the maximum amount of heating from the sun is that PV panels are at best 20% efficient (and in practice, significantly less as David points out above), but evacuated tubes are better than 80% when the water is starting to be heated, so you need 5 or 6 times the area of PV panel vs evacuated tube to get the same amount of heating. If you want to use electrical power, I think a heat pump is the way to go (if finances allow), to bring the efficiency of heating via PV up to that of evacuated tubes. 500W of PV output can give you ~2kW of water heating with a modern HP.
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Re: Electric panel resistive element water heating

Postby 34by151 » Thu Jul 21, 2016 6:37 pm

If you check the specs of the controller the Max PV you can connect is 2Kw which is less than the rated wattage of the element. Most are 3kw or 3.5 kw

The MPPT in the controller will give you the highest output in watts for your given amount of panels and sunshine. Whatever that is in watts 97% is passed to the element

That fact that the PV is less watts then the element is not an issue as it is powering the element for longer

If your 3500W element is on for 10 mins per hour you roughly need 1/6th of the element watts in panels
1/5th would be better to allow for clouds. This is because the PV will be heating the water almost all the time not in big bursts like running from the grid.

As for Area the smallest is evacuated tubes but I can say from having them its the most problematic setup as compared to Rheem type Solar, Gas and electric. Using PV panels is a set and forget setup where your only chance of failure is a broken panel.

As for my setup. I had damage to the Evac tubes (not the first time). The same storm damaged a PV panel. Insurance paid for the whole PV string, and the damaged evac tubes to be replaced as a cash payout.
The payout allowed me to expand my PV on the inverter maxing out the panels, use 2kw the good old panels for HWS (taking it off grid) and add load sheeding to the sub breaker panel.

I still have 8 spare (good) panels if any are damaged on the HWS

As part of expanding the PV on the inverter, I removed the evac tubes all together, caped off the plumbing and removed the pump. I kept the resol controller hooked up to the tank but its only used as a thermostat for the element.

The net results are, lots more available PV solar for longer, loads that dont create imported power and a HWS that hasn't needed the grid for 8 months.

Given a choice (not going to happen with energex!!!) I would have gone to 10 or 15KW 3 phase solar (adding 1 or 2 more inverters) and just run the HWS from the excess PV but we do what we can

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Re: Electric panel resistive element water heating

Postby davidg » Thu Jul 21, 2016 11:43 pm

34by151 wrote:The MPPT in the controller will give you the highest output in watts for your given amount of panels and sunshine. Whatever that is in watts 97% is passed to the element

Ah thats not what it says if you read it properly. Whats actually says is " It's 97% efficient! (pass through)" which means that the device is 97% efficient in pass through mode not that you get 97% of the energy from the PV in output. My math is effectively correct, if you don't believe it then put a DC clamp meter on one the leads to the element and see what it says, then measure the voltage being input into the element, then you can use the very simple math of ohms law again V x I = P to verify it.

34by151 wrote:If your 3500W element is on for 10 mins per hour you roughly need 1/6th of the element watts in panels
1/5th would be better to allow for clouds. This is because the PV will be heating the water almost all the time not in big bursts like running from the grid.

When a STD element in a hot water system is connected to the grid or suitable power source to heat water it will be on continuously until its up to the preset temp, only then the thermostat turn on/off to maintain the temp of the water.

I'm not canning the idea, just correcting inaccurate information and providing the basic math to be able to prove/verify it. Unless the mppt controller boosts the voltage considerably and it clearly doesn't, even the specs confirm that. It simply cannot ever maximise all the available output of the panels for peak efficiency of possible available power.

If want to get close to best output you'd need a 4800W element in the Hot water service for a 90V approx under load array of 3 x 250w panels in series that in theory could possibly absorb upto 736 watts assuming the element had a resistance of 11ohms, if it's 12 ohms then it would be approx 675 watts peak. However as the temperature of the panels goes up so the voltage output of the panels goes down and therefore the output will also drop off.
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Re: Electric panel resistive element water heating

Postby 34by151 » Fri Jul 22, 2016 7:18 am

The voltage you are getting at the element is dependent on 2 things
How many panels you have in series and the volts they are run at for MPPT

After the MPPT the power from the panels is run though the IGBT and which creates pulsed DC to prevent arcing.

I haven't had a need to put my CRO on the output to look at the waveform as I never had a need to do so but may be worth doing.

The losses in the controller are 3% but you need to add in some for the cables, a good guide is about 5%
The closer you get the voltage to 200V the lower your losses in the cables

The only time you need to heat all the water in the tank is on the first fill. You only need to heat the amount of water you use plus some for overnight heat loss. The more water you use the more power you need to bring it up to temp.

For my tank I was using 6-6.5 KWh a day keeping it hot.
I know this because every RCD on my sub panel has a CT that logs the usage.
If I was to drain and refill, granted that would be three or more time higher for the first day.

If you want to not boost the system you will need 1/6th of the consumption from PV each day as you will be putting power in continuously. In my case that's about 1kw
If you provide more PV you will be reaching the temp quicker and the thermostat will cut off the PV
More PV gives you a buffer for bad whether

As you want to use your PV as much as possible and have as much residual heat in the system in the morning its a good idea to turn up the thermostat as well

When I was running the system from the grid the Element ran for 10-15mins. I haven't logged the PV setup in terms of time but im guessing it running for about 2-3 times that but the PV ive dedicated to it overkill.

That comes back to what I said before. Id much rather just have the HWS entirely on the grid and dedicate the PV to grid production (eventually battery storage). However Im never going to get approval for more PV. With spare panels this is the next best thing.
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Re: Electric panel resistive element water heating

Postby offgridQLD » Sun Jul 24, 2016 8:50 am

I get what your saying David and agree with the maths.

as for the negatives mentioned regarding this system. I think in the case of this particular system the space for lots of panels isn't a concern and only is if you don't have the space for panels vs a vac tube system. The cost of the panels wasn't a concern as they where free.

Efficiency is only a concern when if starts to effect you some how like cost. You don't see to many people in the US worrying about the efficiency of there 7.6lt v8 trucks as fuel is inexpensive. If your installing a hot water system in a caravan you might be concerned with space and efficacy.

There seems to be a endless supply of brand name panels available 2nd hand from upgraded systems or insurance jobs where only a few panels where damaged but hard to obtain replacement so the remaining panels get sold off as mentioned. I have a friend who has been picking up everything he can get his hands on as he has a large farm with several off grid homes. Typical price is about $50 for a 250w panels. I was shocked at some of the brands and condition of them.

I think if your in the city where it's noisy (background noise) anyhow and space it tight. Gordon's idea of the heat pump would be my pick.

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