Hot water with electricity from solar panels

[TMT2]

Agree with StuM...use excess energy (diversion load) after batts are full to heat water using D.C. Heating elements. I have the gear but am struggling with the physics of setting up the diversion load circuit? I have 450W (max) of wind thru a Marine Kenetix controller (12V) and 560W of solar through a Morningstar/Tristan TS-45 controller and 2 x 600W DC water heating elements (12/24V). Anyone out there able to advise me on how to set up the diversion load to turn all excess (heat) into the water tank D.C. Element?

TMT2

[offpist]

My suggestion is you buy yourself the Morningstar MPPT 45 or 60 for your solar. Then you can use your TS-45 for diversion dump load controll. You might strugle using 2x600w elements.. one option is to connect them in series.

[Dockhead]

Originally Posted by valhalla360

Fundamental problem with your analysis. To heat the same amount of water to the same temperature uses the same amount of power. Changing the voltage doesn't solve that problem.

Most people using solar to heat water put in a voltage switch so when the batteries reach a high state of charge, it diverts the excess power to the water heater (or you can do it manually if you like to watch your battery state). At this point, however many watts your solar panels put out is how many you can put into heating water. For best efficiency, it would probably be better to use a DC heating element and power it directly off the panels DC output.

Heating water off the batteries is just silly.

No, his analysis is sound. His approach is clever because using the very same heating element (expensive or impossible to mount dual heating elements in most calorifiers), he has a 4 times smaller one using the 110v inverter. This is small enough that it can be primarily driven by his solar panels without drawing power from the batteries. If he drove it at 230v, it would be sucking power from the batts like crazy.

Furthermore, it is simply not true, when using lead-acid batteries, that "To heat the same amount of water to the same temperature uses the same amount of power". Ever heard of Peukert?

His purpose is to divert excess solar power into the calorifier. His solution is elegant, avoiding a second immersion element, and avoiding power loads which would flatten bis batteries.

[rwidman]

Originally Posted by Dockhead

...................... Furthermore, it is simply not true, when using lead-acid batteries, that "To heat the same amount of water to the same temperature uses the same amount of power". .........

You are wrong. The source of the power is irrelevant.

[wiekeith]

Originally Posted by rwidman

You are wrong. The source of the power is irrelevant.

Really? IMHO, to utilise the solar power to charge batteries for future use is less efficient than utilising the solar power directly via an inverter.

[rwidman]

It takes "X" amount of energy to raise the temperature of "X" amount of water "X" degrees.

Did nobody take physics in high school?

[rwidman]

Originally Posted by senormechanico

We actually did that on our previous boat.
I made a 2'x4' (approx) hot water panel with copper tubing covered with plexiglass and integrated it in the center of our stern arch solar panel array.
A food grade march pump recirculated the water from the output and input connections on our hot water tank after going through a non return valve.
The pump was controlled by a couple of thermisters and a differential controller.
One thermistor was on the tank and the other was on the array.
The system's pump ran only when the panel was hotter than the tank and it was silent.
We always had hot water for showers and dishes.
The only down side of the system was that I needed to periodically burp the air at the panel on the arch.

And you had free and simple hot water.

[wiekeith]

Originally Posted by rwidman

It takes "X" amount of energy to raise the temperature of "X" amount of water "X" degrees.

Did nobody take physics in high school?

You are correct, of course, but I didn't think anyone was questioning that aspect.

[offpist]

You can loose as much as 20% from the loss in inverter and battery.
Then.. you can loos as much as 20% one more time charging the battery trying to put in the energy that you took out.

With that in mind, it makes sense using low power heating element and using excess power from solar/wind when available instead of draining the battery.

[chris95040]

Originally Posted by rwidman

You are wrong. The source of the power is irrelevant.

...

Did nobody take physics in high school?

No. You should have googled Peukert, that name Dockhead mentioned in his post.

We all understand that a watt hr is a watt hr. What you don't understand is that the useable capacity of a lead acid battery is, confusingly, affected by the rate of discharge.

Take a moment to consider this next time you feel like rocking a condescending tone.

[wiekeith]

Originally Posted by offpist

You can loose as much as 20% from the loss in inverter and battery.
Then.. you can loos as much as 20% one more time charging the battery trying to put in the energy that you took out.

With that in mind, it makes sense using low power heating element and using excess power from solar/wind when available instead of draining the battery.

I agree. The beauty of the OP's solution is that it utilises solar power available after the batteries have been charged, hence bypassing the inefficiencies of the the charge/drain cycle. There will, of course, be an inefficiency associated with inverter, but considering that the excess solar power would have otherwise effectively been dumped, as batteries are charged, it is a good use of it.

[rwidman]

Originally Posted by chris95040

No. You should have googled Peukert, that name Dockhead mentioned in his post.

We all understand that a watt hr is a watt hr. What you don't understand is that the useable capacity of a lead acid battery is, confusingly, affected by the rate of discharge.

Take a moment to consider this next time you feel like rocking a condescending tone.

Whatever, dude.


BTW: I understand how a battery works..

[High Pointer]

Like sennormechanico,I built a solar panel 20X66 using black 3/4" PBC pipe,I had cut 2" PBC in half to create reflectors buy sanding and painting the up curved half with chrome paint, screwed them inside black painted box under were each of the 3/4"pipes were then suspended with elbows a the ends to create a continous flowthrew to the tank with the aid of a low amp pump.The box was 66" x 20" x3"with a clearplex cover it supplied all the hot water we ever needed and even more,to reduce the excess we would just cover the low cost panel.Ed

[Seriously Slow]

Hello all, my first post.
I have just been giving this some thought since I just installed 850 W of solar. The panel voltage is 50 volts and it seems I could divert 450 W of it from my MPPT for that panel to a 1500W 110 v water heater element when my batts are full. Why use an inverter at all? It seems it will heat the water in a little more than double the time as if it was running at 120 v.

The desired benefit is that I will have hot water on the hook and still be able to use shore power (with proper switching for both options).

According to what I read as long as the load is resistance only (like an element) there is no difference between a/c and d/c. therefore dc elements are likely only re-marked a/c elements of an appropriate resistance.

[senormechanico]

Seriously Slow,

Your plan will work, but half the voltage will also produce half the current.
Therefore, you will only get a quarter of the power (assuming full sun on the panel).
I = E/R and all that.