Hot water with electricity from solar panels

[alctel]

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?

There is a reason why he specified 'lead acid batteries'.

This page has a pretty good explanation LiFePO4 Batteries - Thoughts & Musings Photo Gallery by Compass Marine How To at pbase.com

'Conversely a 400Ah lead acid battery with a Peukert of 1.27 will only deliver 215Ah's at a 200A load and just 178 Ah's into a 1C load. '

[MerlinBrasil]

Originally Posted by Stumble

Seaward still makes them, but they are pretty small. 3gallon 300w units.

They now have a 23 ltr (6 Gal) offering:

Whale Marine - Products

[baycruiser]

I use very easily with 1500w inverter and 2truck style batteries which never go below 50% A duoetto 10lit water heater This heats up very qiuckly and stays hot easily and is enuf for a shower I usually turn it on late afternoon or as I need it .I dont need gas or to waste water I have the thermostat turned down so I use only hot when I need it My mixer just stays on hot

[Ironman162]

I have a 12v 500W element that serves as a load dumping for the wind turbine.
This does not normally use the battery at all although there is a switch that would allow the water to be heated by the house batteries and effectively the solar panels or boat engine alternators.

Added to that the capacity of the water cylinder is 10 liters only.

It works reasonably well on a good day and provides for a reasonable shower before it needs to be refilled.

Wierd old system I know but using an 240V inverter is inefficient to my mind.

Ive noticed 12v elements available on ebay at a reasonable rate and it will be good to have a spare at hand.

[BjarneK]

Thanks to everyone who gave constructive input so far. It is great to hear the different approaches being used.

Originally Posted by Seriously Slow

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.

I think this is a really interesting idea. Could you let us know which panels you have and how many? I would like to do a few calculations on your setup.

Originally Posted by Seriously Slow

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).

I think proper switching is the key here as you say. I don't want to think about what happens if you feed 110 V AC into your solar panels

[Rapanui]

We have 480W of solar and regularly boost our 1200W immersion heater using the inverter fed from our 330Ahr bank. A 10-15 minute boost in the morning reduces SOC by 10-15% and takes lukwarm water to hot for showers and washing up. We rarely go below 75% SOC and return to 100% virtually every day. It may not be the most efficient method of heating water, but it is very convenient and simple. If you have real estate to fit enough solar, solar makes a quiet, cheap low carbon source of electrical energy for boats. Far better than running generators or propulsion engines to heat water and charge batteries.

[Seriously Slow]

Originally Posted by BjarneK

Thanks to everyone who gave constructive input so far. It is great to hear the different approaches being used.



I think this is a really interesting idea. Could you let us know which panels you have and how many? I would like to do a few calculations on your setup.



I think proper switching is the key here as you say. I don't want to think about what happens if you feed 110 V AC into your solar panels

I have 3 sunpower panels, 2 250 w on my bimini wired in paralell and one 345 on my davits. (I rounded up to 850 w) They use 2 MPPT controllers for battery charging.

[Ironman162]

One don't get get 850 Watt from 850 Watt solar panels

In reality as an example I have 300 Watt solar and get max 15 amp.
12 X 15 = 180 Watt at best in ideal conditions.

Against that experience you would be better off banking on half the power on a good day.

Overcast days are much worse and and it can drop to 2 amps or nothing..In my case the wind turbine then has to fill in or I have to start the engines or the generator. Simple as that.

Inverters are grossely inefficient and one loose at least 20% there too.

Sorry to appear to be a Profit of Doom but it is a reality I live with daily. Forever juggling things when the sun shines.

You would almost be better off fitting a hot water solar panel if you have the room.

[mitiempo]

Originally Posted by MerlinBrasil

They now have a 23 ltr (6 Gal) offering:

Whale Marine - Products

Whale only has one 12 volt water heater - 3 gallons. The ones you linked to are 120 and 240 volt water heaters. They were re-branded as Whale after Seaward was purchased by Whale.

[Stumble]

The tanks are pretty expensive,but I did some looking, and a 12v 600w heating element itself is only about $30 from Amazon. https://www.amazon.com/Dernord-600wa...8966G94AMSVXBH

This would be a pretty easy addition to an existing hot water tank since it matches a 1" NPT flange.

[PaulinOz]

2 kW of Sun power panels feeding to 1000 amp/hr AGM's via 2 x Outback Flexmax 80 controllers. Victron Quatro 3 kW inverter / charger.

Water heater Quick 1200 watt 220V AC / 60 liters.

Usually once Battery bank above 90% SOC I switch water heater on inverter via a 1 hour electronic timer switch. Solar panel supplies most of power and draw aprox 35 to 45 amp for 1 hour out of batteries.

Which is recovered usually over the next two hour being middle of day and Solar outputting up to 130 amps if required.

Timer switch only required to stop some idiot (myself) leaving water heater turned on and totally flattening the batteries.

Supplies enough water for 2 x showers and dish washing up each day on anchor without running engines or generator. Worked fine for past 24 months.

[Dockhead]

Originally Posted by rwidman

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

Not if you define "power" as % capacity of your batteries. Use it faster, and you will use more for the same result. That's how lead acid batteries work. If you don't know about Peukerts Equation, I suggest you have a look.

Sent from my D6633 using Cruisers Sailing Forum mobile app

[Dockhead]

Originally Posted by wiekeith

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.

And this is the other reason -- you lose energy running it through the batteries, so it is more efficient if you reduce the load, like the OP did, so that more of it is covered by power produced directly by the panels.

[BjarneK]

Originally Posted by Ironman162

One don't get get 850 Watt from 850 Watt solar panels

In reality as an example I have 300 Watt solar and get max 15 amp.
12 X 15 = 180 Watt at best in ideal conditions.

You must be further north than me with flat mounted panels or you might have a shading issue (or not an MPPT regulator?). At 11 AM yesterday I got 27 Amps from 400 W of flat mounted panels in Guadeloupe.

Originally Posted by Ironman162

Inverters are grossely inefficient and one loose at least 20% there too.

I think you are being a little harsh on inverters here. The one I use has a rated max efficiency of 89% and since it runs in the higher end of its rated Watts, I think that the efficiency will be pretty close to that.

[BjarneK]

Originally Posted by Seriously Slow

I have 3 sunpower panels, 2 250 w on my bimini wired in paralell and one 345 on my davits. (I rounded up to 850 w) They use 2 MPPT controllers for battery charging.

To figure out how it would work if you connect your two 250 W panels (in parallel) directly to the heater, I think you should proceed like this (use at your own risk).

The heater is 1500 W at 110 V which is a resistance of 110*110/1500 = 8.1 Ohm since P = U * I and U = R * I, so R = U*U / P.

When you connect the solar panels to the heater, the resistor is going to keep a constant ratio between voltage and current (U = R * I).

Since you have two panels in parallel, each is going to see a resistance of 16 Ohm.

I found a graph for the current as a function of voltage for the SPR-X20-250-BLK panels. If you plot I = U / 16 on that, it looks like the attached image (the red diagonal line is mine):

So with 800 W / m2 (80 % solar radiation), the current of each panel will be 2.9 A and the voltage will be 47 V (large red rectangle). This is 136 W per panel or 272 W between the two.

Interestingly, at 500 W / m2, the power is not much less: 2.7 A and 43 V for 116 W per panel or 232 W between the two. This is actually 93 % of the rated output. The reason is that the 500 W / m2 intersects the heater element current graph near the maximum power point of the solar panel at 500 W / m2.

So in conclusion, I think your panels are well matched to the heater and I would think that you will get around 230 - 270 W of heating from your two 250 W panels wired in parallel directly to your heater. This should be enough to make a significant difference, since I get decent hot water using 175 W.