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diy solar

DC Electric Hot Water Pre-Heater

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Dec 30, 2019
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Hello friends, I have completed my Grid-Tie, Cheap and Simple project. Overall a success and makes me want to move on to something with storage. With that in mind I present my newest project. DC electric solar hot water PRE-heating. I will not be off grid as I like grid stability but I will attempt to reduce grid reliance. I was thinking of a battery system but with limited funds I decided my "storage" will be an electric hot water heater placed in series before my main hot water heater. I will replace the elements with DC elements found inexpensively on ebay and amazon, attempt to regulate temp somehow... still thinking on that one. The goal is to have something that still produces in light sun or cloudy weather where a traditional water-on-your-roof system fails. Also my goal is for this to produce a meaningful amount of hot water AND be cheaper than a heat pump style hot water heater. I will source used panels again for as little money as possible and attempt to get the largest return on im investment as I did with my Grid-Tie system.
If anyone has any advice or even questions I would appreciate it as the more thinkers we get on a project the better the outcome.

Thanks!!
 
I like it!
Presumably you would have some kind of feedback system to turn off the DC immersion pre-heater once the water was up to temp so it doesn't overheat (you should be so lucky!) In my case, my municipal water supply comes from a large northern river which never really warms up, even in the heat of the summer.
Your idea nicely matches solar power production to a relatively constant demand for residential hot water.

Another nice natural match I've been thinking about is Air Conditioning. Certainly in temperate regions (in my case northern), the demand for AC is seasonal and coincidentally matches peak solar power production. The thing that works against this is Shade; you would obviously prefer to have a big deciduous tree shading the roof if you could.
 
yeah so i have been seeing inexpensive 140° thermal switches for a few dollars. Im thinking something along the lines of this on the tank running off a voltage regulator from the panels and controlling a contactor ? or relay? that is between the panels and the heating element. Not sure of my options but trying to stay cheap. Golf cart contactor? solenoid from an old ford starter system? The hunt has begun.
 
i never used a mini split before but can they handle 40amps or more? Right now im thinking 2 x 600 watt elements one upper and one lower but each on its own system to keep Amp handling under control. $12 on amazon for Dc-Dc relays that handle 40 amps and 60 volts for the circuit and 3-32v for the trigger circuit. I could possibly just run the trigger with the same panel voltage or a dc wall adapter. The 100 amp relays start costing more than the heating element, the wire and the 40 amp relays combined. LOL I can make half the system and if i need double the power easily add on. tell me more about these mini splits though im unfamiliar with them. can they run continuous at their rated output for hours at a time 365 days a year?
 
Dunno anything about em other than they can be DC powered, run ac, and heat water.
Amazon?
anybody else?
 
Ahh ok well this will be a 100% Dc standalone system. I ordered a 12v heating element for a spare 10 gal hot water heater i have sitting around. I will start with a hot water hose bib outside with no safety other than the tank blow off. work up from there.
 
it is difficult t have a DC (if you mean DC=low voltage) pre-heater inline with water because heating water means a lot of power, and low DC voltage then suggest very high current.
What you can do is to get an isolated tank (like 50 gallons) with water you can slowly heat. You do not even need electricity for that, solar heater are very efficient.
Better , you can cool something and transfer the generated heat to water.
 
I understand what you are saying but i will address the reasons in order that you stated.

Low power from solar can be used over time to heat the same as high power heating but with more duration. example: 4,000 watt heats 40 gal in 30 min. 2000 watt in 1 hour. 1,000 watt in 2 hour and finally 500 watt in 4 hours. i plan on using 2 elements @ 300-600 watts each.

I could do as you say and get a solar tank of water, with or without a panel and put it on my roof. When it freezes (every winter) i would have to abandon and drain it or use expensive pumps, heat exchangers and glycol to make it work. Also what about when its cold outside or cloudy or cold and cloudy? That solution might even cool my water more than heat it. Lastly the units made for this specifically are $2000+

The heat pump idea of cooling something to heat something else is something I'm interested in as well. Heat pump hot water heaters have been around for a while now. Read the reviews though, not quite ready for prime time and again a $2000 solution, worthless in the winter and seem to fail often of the reviews are accurate.

Lastly, I really want this to be seamless. if my proposed system under produces, my regular 4,000 watt hot water heater works exactly the same as it always did. That means hot showers with full pressure, dishwasher, laundry ect like normal. I would like this system to not inconvenience me after install. More importantly if my wife has to do ANYTHING differently to use it I will be told NO. Happy wife... happy life

I appreciate the suggestion though. I literally have a Hot tub heated by a circulation pump and black panels and my above ground pool. I'll post pictures
 
Here we go, just snapped them for ya! :) like o said i appreciate the feedback but im pretty set on DC solar heating, just trying to make it reliable and safe. This is nothing new for livestock tanks. Im no visionary Im doing what other people have already done with my own twist. David Poz inspired me.20200505_150904.jpg15887166253293095465480082818077.jpg1588716662926865285517472893708.jpg
 
A friend who has a heat pump water heater and uses the innards of a water heater in his garage to preheat his water. The theory is that the ambient temperature of his garage is usually 10 to 15 degrees warmer than the water that comes out of the pipes in the ground. I am not suggesting this as an alternative to what you are thinking about, but rather an enhancement that could make your idea more efficient.

I do think it is an intriguing idea, especially after the two thermal solar hot water systems that I installed in the past usually wore out out over time and I have given up on that medium for domestic hot water. It is apparently still viable for pool heating because the temperature differentials are a lot less than for domestic hot water. Keep us informed on your progress.
 
Ok so i spoke with an HVAC engineer and he was loving the idea. I can use what is called a snap disk thermostat. i would need a temperature rise open snap disk thermostat ($6) that is normally closed to control a 40 amp relay($12). The snap disk thermostats are AC powered. I found an AC powered DC relay that can handle 40 amps but probably couldn't do more than 25amps continuous. That relay is powered by ac wall current when it has power to it. I have seen some other temperature probe adjustable digital switches that are inexpensive as well ($12). we are talking in the realm of $60 for everything including heating element not including the tank and panels. I ordered a 12v dc heating element that i will hook directly to my 12v 100watt tent trailer panel and into a 10 gal hot water heater. I guess we will see together if it fries the element with panel voltage hitting possibly 22v ? should be less voltage with resistance though... I may still use a golf cart contactor in place of that relay (300 Amps)
 
I am waiting for a 12v 600w element to get here just to pre-heat like yourself. Plan to install it in a 300mm length of 40mm galv pipe with threaded fittings to take the 1inch bsp element. In series to the water inlet and sloping down allowing for heat transfer through convention. No other modification to the 240v system other than the minimum 60 C re-setting of the thermostat. Now from October to February power to the 240v system is turned off since there is 50m of polypipe under glass/pre heater- Brisbane summers are rather warm. 600watts may extend this period of no grid and reduce it outside the summer window.
I am following this thread and others so plans may change while the element is on the trip from China.

" my "storage" will be an electric hot water heater placed in series before my main hot water heater. " - that was my idea as well and could be if one turns up or a future upgrade.

"I can use what is called a snap disk thermostat" --would a 600watt element really need a thermostat?

"DC relay that can handle 40 amps but probably couldn't do more than 25amps continuous" - my original plan was to use one of these 12V 140Amp Dual Battery Isolator (Voltage Sensitive Relay) wired into one of the elements(300w) as a dump load from my current 12v system plus direct from 2 extra 300w panels. Maybe able to alter the busbars on the dual element 12v element and wire the panels in series with the vsr only connected to one half of the system?? knowing there wont be much capacity for taking any dump load under full sunlight - the panels Voc are 12v and the vsr cuts in at 13.2v.
 
I like that battery isolator/relay. I plan to use the native panel voltage (whatever it ends up being) I'm not a fan of making heat to convert something when I am trying to make heat. As for the dual 600 watt elements not being able to overheat a tank you are probably correct under normal usage. I just want the peace of mind to know that if i go on vacation for a week I'm not thinking... "did i turn off that heater?" The tank blow off valve should do the trick if it ever happens but mechanical blow off valves fail closed. (ex. chernobyl) here are the products I am leaning towards. I am not an amazon affiliate so I get nothing from these links.

thermostat.
This isnt the snap disk but then I can log temps and see whats going on. Just stick the probe against the metal tank between insulation and use some thermal grease or arctic silver for thermal convection.

Relay
This is pretty robust for not a lot of money. Im afraid of welding a 12v relay shut at 35ish volts.

Another interesting note... you could make a battery or solar disconnect for low temp cutoff with this same system.
 
32 Heating hours to boiling (when suns up and producing) with dual 600 watt elements, 100 gallon tank and 55 degree inlet temps. Caveat, that's to boiling temp at ambient pressure. My well water system is always at 40psi or higher so there is some head room since water under pressure boils at higher temps.

With that being said I also don't want water or over 120 degrees coming out of my hot water system. Total heating hours 13 hours with zero usage.
 
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Here we go, just snapped them for ya! :) like o said i appreciate the feedback but im pretty set on DC solar heating, just trying to make it reliable and safe. This is nothing new for livestock tanks. Im no visionary Im doing what other people have already done with my own twist. David Poz inspired me.View attachment 12664View attachment 12665View attachment 12666
Those pool collectors are powerful! Use the one we have for a camp shower. It will drive 150°f water, 50 gallons from ground temp to operating temp by 11am in good weather.

A heat pump would yield 4:1 sometimes 5 or 6:1 units of heat movement to one unit of electricity. At least 2.5-3 times the heat as resistance elements.

They can be dc driven.
They are available solar ready.
They can heat space, via air or liquid, heat domestic water, chill water, and provide space cooling amd dehumidification.

There is not much to not like if you have or are going to get electricity amd especially in the case of solar grid ties and double for some off grid homes.

Hotspotsolar.com
Cyboenergy.com
 
With a heat pump you can heat the pool and domestic water with air conditioning waste heat from the home!*
 
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