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Old 20-03-2012, 00:29   #1
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From 47 Degrees to 160 in One Hour on Solar

I have a 17 liter hot water tank on my Dragonfly which can be heated from the engine's cooling system.

I also have rebuilt my refrigerator with Aerogel insulation and installed a keel cooled Vitrifrigio compressor.

As I have only one bank of batteries (200 ah worth of LiFePo4) but 320 watts worth of solar panels, I have LOTS of spare electricity, and don't need to run the engine for power.

Hot water is the only thing left wanting, so I decided to do an experiment with my 1500 watt sine wave inverter.



This time of year the weather around the PNW is nasty and cloudy most of the time, but I decided to go ahead with the experiment anyway.

The water tank started out at 47 degrees fahrenheit, and the lithium phosphate 200 amp/hour battery bank was 100%.
(See the thread on LiFePo4 for house bank)
The battery temp sensor indicated 48 degrees at the start.

I hooked up the 1.5 kw sine wave inverter to the 1 kw hot water tank and took measurements every 5 minutes.

As soon as I turned on the inverter, the floating batteries (13.8v) immediately went to 12.6 volts.

The discharge current from the batteries averaged about 105 amps.
Solar input was minimal as we had heavy overcast during the whole test.

Slowly over the space of one hour, the battery voltage dropped in a linear fashion from 12.6 to 12.3 volts while the discharge current stayed constant at 105 amps.

The inverter supplied 117 VAC @ 9.5 amps AC as measured with my trusty Fluke 97 at the input to the tank through a cheesy six foot extension cord from the inverter to the tank.

At one hour and one minute, the thermostat on the tank shut off the heating element.

The battery monitor indicated 105 amp hours used, my 200 ah battery bank was at 45% and the battery temp was at 65 f.
(LiFePo4 batteries can go to 20% before any damage takes place)

The inverter never missed a beat, the fan only ran at low speed the whole time, and the water temp (measured at the galley faucet) was 157 f !

The battery voltage immediately jumped back to 12.6 when the tank thermostat turned off the load.

I figured doing this experiment with COLD water this time of year and no sun would be the absolute worst case scenario, so if it was doable now, it ought to be a piece of cake in the summer with longer days.

Kicking the hot water tank around noon, and then topping up the batteries during the rest of the day ought to keep the MPPT in bulk mode most of the day instead of floating half the time. Make the sun work for its living!!

Hot showers, here we come. WOO HOO !!
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Old 20-03-2012, 03:31   #2
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Re: From 47 degrees to 160 in one hour on solar

I only have a 6 gallon Raritan hot water tank. I'm going to give one of these panels a try. It runs on it's own very small ten watt PV panel. The collector only takes up a 2' x 2' space on deck and they make a marine version. If my test goes well I'm going to mount it in a teak frame because the corners are too sharp in my opinion and rounded teak corners will keep lines from getting snagged.

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Old 20-03-2012, 04:39   #3
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Re: From 47 degrees to 160 in one hour on solar

320w of solar will only generate 150ahrs a day on reasonably good day. If you want to 100ahrs for water heating you will only have 50 ahrs left, unless you can plug into a marina . 50ahrs is unlikely to enough for your domestic use.
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Old 20-03-2012, 09:53   #4
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Re: From 47 degrees to 160 in one hour on solar

The test was a worst case scenario.
March with COLD water, no sun to speak of, relatively angled (49 degrees North) on the date of the spring equinox.
If I use some of my left over Aerogel on the tank, it should hold over pretty well, and I'm not using a whole tank every day.
A 50 ah/day consumption might be more realistic.
I'm also going to turn down the thermostat a bit, so I can take a shower with full hot setting on the faucet.
The keel cooled refrigerator which I recently built (see Aerogel threads here on CF) is extremely efficient, and in hot weather I expect it will use only around 20 ah/day.
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Old 20-03-2012, 11:13   #5
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Re: From 47 degrees to 160 in one hour on solar

Its seems to me that 157 is a bit hot for the hot water. That is a dangerous temp for water comingout of the faucet. If you set the WH to around 120-125, that gives you hot water and should cut the AH needed to heat it.
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Old 20-03-2012, 11:24   #6
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Re: From 47 degrees to 160 in one hour on solar

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Originally Posted by senormechanico View Post
A 50 ah/day consumption might be more realistic.
I'm also going to turn down the thermostat a bit, so I can take a shower with full hot setting on the faucet.
.
50ahrs is much more realistic. If you have excess power why not use it.
The only problem is when it's cold there won't be be as much solar power.
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Old 20-03-2012, 11:35   #7
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Re: From 47 degrees to 160 in one hour on solar

I think I'd look for a 12 volt water heater element as a diversion load for when the batteries are fully charged.
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Old 20-03-2012, 11:35   #8
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Re: From 47 degrees to 160 in one hour on solar

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50ahrs is much more realistic. If you have excess power why not use it.
The only problem is when it's cold there won't be be as much solar power.
Good argument to head South again !
And yes, I will turn the thermostat down a bit.
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Old 02-07-2017, 16:08   #9
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Re: From 47 degrees to 160 in one hour on solar

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I think I'd look for a 12 volt water heater element as a diversion load for when the batteries are fully charged.
They are available. I looked at some a while back. This is one of my very low priority projects too.
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Old 02-07-2017, 16:39   #10
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Re: From 47 Degrees to 160 in One Hour on Solar

Please adjust your heating element set point to a safe setting. At temperatures you are achieving severe burns can occur in seconds. This is dangerous.

Typical setpoints for domestic hot water systems are 105 - 110 degrees f. You can never be certain that someone unaware of the temperature would not have access to the hot water.

Under no circumstances should 120 degrees be exceeded. At temperatures you are reaching you are only a few degrees from having the t&p relief valve opening.

I can refer you to statistics on burn injuries in the USA if you need more convincing.
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Old 02-07-2017, 17:47   #11
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Re: From 47 Degrees to 160 in One Hour on Solar

I turned down the thermostat to 120 f. five + years ago.
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Old 02-07-2017, 18:01   #12
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Re: From 47 Degrees to 160 in One Hour on Solar

If you are truly set at 120, and you are actually measuring 157, it is time to replace the temperature controls.

Not to.be a nag, but at 157 f burning is instantaneous. Plus, if drawing off small quantities of heated water, there is a tendency for stratification in the tank, resulting in even higher temps. Be safe. There is enough to worry about on the water.
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Old 02-07-2017, 18:06   #13
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Re: From 47 Degrees to 160 in One Hour on Solar

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Originally Posted by redsky49 View Post
If you are truly set at 120, and you are actually measuring 157, it is time to replace the temperature controls.

Not to.be a nag, but at 157 f burning is instantaneous. Plus, if drawing off small quantities of heated water, there is a tendency for stratification in the tank, resulting in even higher temps. Be safe. There is enough to worry about on the water.
You might like to take a look at the dates of the posts you are responding too.

He was getting 157 back in 2012 when the thread started. Now, 5 years later, he is getting 120 (According to his earlier post , as a result of heeding that advice 5 years ago).
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Old 02-07-2017, 18:53   #14
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Re: From 47 Degrees to 160 in One Hour on Solar

Ha, ha, ha. Now that is funny.

This was the first post I have made from my phone, and I didn't see the date. I'm gonna blame Samsung Galaxy.

Anyway, never mind!

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Old 02-07-2017, 19:54   #15
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Re: From 47 Degrees to 160 in One Hour on Solar

To be honest, I eyeballed the temp setting. After letting it heat for a half day, I turned on the galley faucet. It's nice and hot, but I can still leave my hand in it.
I can take water up to about 125 before it's too much, so I'm in the ballpark.

Most of the time if I want hot water, I don't wait for it to heat to shutoff, I just give it about 15 or 20 minutes and it's plenty warm enough to do dishes or take a shower.
Our cruising is mostly in the summer anyway, so the starting temp of the water is more like 70 degrees.
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