DC Electric Hot Water Pre-Heater

[TheDroidsYouAreLookingFor]

That would be the ultimate in efficiency but ultimately not the most efficient use of my dollars. Solar is about 15 cents a watt used where i live. I have acreage so space is no concern. My pool is not near my freezer, my air conditioner is not near my pool. I can just keep up the solar sprawl nearly indefinitely. I can afford to waste solar production in my particular situation because power from my provider is 40 cents a KWH in the summer. Payback is quick as long as i keep initial cost down. The large black pool panels make STUPID amounts of heat when the sun is up and shining and nearly nothing when cold or cloudy. (non insulated) That being said 2 panels raise my pool temp real world 1 degree an hour (10,000 gal) and the hot tub will increase 6 degrees an hour and would be 120 degrees easily if i don't turn off the circulation pump by noon. I like heat pumps but simple is better from my own experience. Heat pumps fail more often than resisters and they cost more. So 10x the cost or more for 5x the efficiency but 1/2 the expected life and or maintenance interval. Dont get me wrong, i am a heat pump fanboy but the amount of work involved and complexity to make a system that cools my air, heats my domestic water and pool, heats my house and floors ect is an epic task. For now I will keep it simple and inefficient just because its Dirt Cheap and Just as capable. I also don't have net metering so anything i over produce from grid tied solar i lose forever. I will put a Kw meter on my hot water heater to see the difference it makes.

 

[kernel]

That would be the ultimate in efficiency but ultimately not the most efficient use of my dollars. Solar is about 15 cents a watt used where i live. I have acreage so space is no concern. My pool is not near my freezer, my air conditioner is not near my pool. I can just keep up the solar sprawl nearly indefinitely. I can afford to waste solar production in my particular situation because power from my provider is 40 cents a KWH in the summer. Payback is quick as long as i keep initial cost down. The large black pool panels make STUPID amounts of heat when the sun is up and shining and nearly nothing when cold or cloudy. (non insulated) That being said 2 panels raise my pool temp real world 1 degree an hour (10,000 gal) and the hot tub will increase 6 degrees an hour and would be 120 degrees easily if i don't turn off the circulation pump by noon. I like heat pumps but simple is better from my own experience. Heat pumps fail more often than resisters and they cost more. So 10x the cost or more for 5x the efficiency but 1/2 the expected life and or maintenance interval. Dont get me wrong, i am a heat pump fanboy but the amount of work involved and complexity to make a system that cools my air, heats my domestic water and pool, heats my house and floors ect is an epic task. For now I will keep it simple and inefficient just because its Dirt Cheap and Just as capable. I also don't have net metering so anything i over produce from grid tied solar i lose forever. I will put a Kw meter on my hot water heater to see the difference it makes.

Absolutely. I dont have one either! The one thing to not like is the upfront cost. A machine is1500$ to start and 3k$ for the one i want. It needs integration of connected heat exchangers and antifreze...... $175 a bucket.

I do though have an all aluminum 3× 4x12' collector hydronic system. With only one collector already mounted and having all components aside from some plumbing tube and antifreze.....

it still costs less to employ some of the unused pv we have in storage and resistance diversion to dhw.

 

[Ampster]

So 10x the cost or more for 5x the efficiency but 1/2 the expected life and or maintenance interval.

The recent cost of a Rheem HPWH with a 10 years warranty is about three times that of the same size water heater at Home Depot.
Most of the negativity about maintenance and expected life came from the old GE Geospring HPWH of which I have one. I have two of the Rheems and they are holding up better and have better reviews. GE sold that part of the business.

I don't disagree with your design philosophy for your property. I just wanted to get more current facts out there about price and warranty.

 

[SmartElectric]

Direct to water heating.

Is there any off-the-shelf stuff you guys know about that one could set up a mess of solar panels and magic box 1 and maybe a magic box 2 and directly drive a 240V 4,800W standard water heater element? Any ideas welcome.

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^ member here already using solar panels feeding the lower element of a water heater, works, and looks to be a lot less work than what has been talked about in this thread

 

[sunshine]

I plan to use the native panel voltage (whatever it ends up being)

I rewire all my panels. Normally they get halved to get 20voc to use with a pwm. The panels are 315w 40voc 32vmax 9.6amps. New, but came out of the container shattered. Testing shows they are close to capacity. Will retest when I've got a covering on them to work out actual configuration. Most likely will be just halved as usual and go direct to one side of the element and use the other half for the original dump load idea.

Would be great if the system does need a thermostat but no need to add extra initial complexity. Extra elements can be simply daisy chained on until a thermostat becomes necessary and we copy your solution!!

 

[TheDroidsYouAreLookingFor]

yeah awesome to see that other people are doing the same thing. you know what they say. Smart people think alike but i always joke "Smart people think like me"

 

[TheDroidsYouAreLookingFor]

I know its been a while. I picked up a 100 gallon rheem commercial natural gas heater for cheap that had shipping damage. Retail is well over $3k for this particular unit. I just finished testing it for holding pressure and all is well. It uses 2" water inlets and outlets 2 on each side. Ace hardware had reducers and i was able to fit the heating element in the reducer. One side for heat one side for water. More updates and photos to come.

 

[TheDroidsYouAreLookingFor]

Ok plans evolve over time. I am leaning towards the boost converter seen in another thread since i cant seem to easily match panels to elements. As this evolves i will update. Looking to add approx 2750 watts of used solar. This will heat water during daylight hours and then i will have a timer activate grid tie inverters during time of use price increase at 4pm each day ($.38 kwh). I dont know how the inverters will do in parralel with the boost converter but its worth a try to see. I included a picture of the hot water heater (natural gas) i purchased. It is 100 gal, with 2" inlet and output on each side damaged during shipping never operated still bolted to factory pallet. This should be the equivelent of a water battery that also reduces peak power usage and the solar array will be facing westward and produce peak power

.

Cost should be around $1200, should have an ROI of 10 months or so? Questions and comments welcomed its nice to have other peoples thoughts.

 

[efficientPV]

Boost converter bad bad bad idea.

 

[TheDroidsYouAreLookingFor]

I would really like to know the downsides to the boost converter. So far the only thing i see wrong is it uses some power and may fail over the years. The up sides are the element ohms are perfectly matched throughout the daily cycle of the panel. Also in cloudy weather i still produce the panel wattage even if voltage drops below vmp. Please elaborate because i really do want to know the other side of things.

 

[TheDroidsYouAreLookingFor]

I will just add that i have thought about going both ways. Victron makes some nice dc-dc converters for lowering voltage as well and they can do crazy amperage.

 

[efficientPV]

I've heated water for years with PV and have tried just about every off the shelf module and method that could be used or modified. No one knows more about heating water. Standard boost and buck converters are controlled by the output and will do anything possible to achieve that output. As they try to provide the output required, input current increases. That drops panel voltage even lower causing the panel to go into a death spiral. Data sheets don't lie that much, but you easily overlook max input current and that 98% efficiency is only obtained when the input voltage matches the output voltage. Count on about 82% at desired power. I modify everything and have made these keep the panel at power point voltage using a rather minor boost from 60V to 80V. The losses were horrible gaining only a small fraction of power. Everything got really hot and even had a capacitor blow up from excessive ripple currents. Believe me or not, you don't stand a chance of doing this. What is your array voltage?

 

[TheDroidsYouAreLookingFor]

Hello sorry for the slow reply. Array voltage will be 29.13 volts. I have 12 panels 228w each approx 9 uears old. I think I am going to go with a 36v element rated at 1200 watts and run at panel voltage. I want to be able to use this for actual power production after 4pm as well. I think i will put the 2nd half of my micro inverters through a pool type timer and have it kick on at 4:00 pm. Each micro inverter will be over paneled with 3 x 228w panels. This will at some times also be shared with thebheating water circuit at the same time but hopefully the water is heated at this point and i can get some good west facing late evening power production for the most expensive power. I wonder how a grid tie inverter will do sharing a load with an element... Still working out the details but the golf cart contactors look like the way to go. I bought a snap disk from a site that said it was dc rated and it wasbjust a straight lie. It cant stop dc 30v at any amperage. The cart contactors are good for up to 400 amps continuous at up to 48v... just what the dr ordered.

 

[GLC]

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 in 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!!

Ok, so I just finished testing my hot water dc element in my hot water heater. I replaced my lower ac heating element with a dc element 36 volt, 1200 watt. It is a dual 1200 element meaning it actually has hookups for two dc feeds 600 watts each. I hooked up 1/2 of the element straight to 3, 305 watt panels wired in parallel. You could do the same with the other side but I went ahead and connected it to my solar controller where it feeds energy to it when my batteries go into float mode.

 

[learnincurve]

I made a 27 gallon water heater for my camper. It uses two 300 watt tank heaters with two 120 degree cut-offs (one for each element). These were purchased from Missouri Wind and Solar for under $100 total cost (including shipping). I made my water heater tank out of 4 inch SCH 40 PVC. I used this during the four days without power following Hurricane Laura. Six people used that water heater with a 45 minute wait between showers. The PVC tank was in direct sun light which helped to warm the water but we never ran out of hot water in my camper during the four days we were without power. I would like to add that all showers were taken after sunset for three of the four days we were without power in my area. It ran off two 100 watt panels running through a 20A PWM charge controller to a single 12V 170AH battery. You could add a small 12V water pump to circulate the water and still be able to warm water about 18 to 20 hours a day using that same power setup. Changing to a 400 watt PV going to a 40A charge controller with two 12V batteries and you could have hot water 24/7. As an added note, after power was restored I turned off my water heater and connected a small pump and now I heat my water outside of my house and circulate it through my water heater and PVC heater. My washing machine, dish washer and shower has shown no loss of hot water since that day. Not to mention I can now take and run hot water in my shower for about 40 minutes before the water begins to get cold. 120 degree water is always ready when I need it and my electric bill is now lowered by around $35 dollars each month since I installed my circulating system and flipped the breaker to my water heater. I can't say it will work the same for you but in south Louisiana it works perfect for me. I am now looking for other ways to lower my electric bill. OH! My total cost for everything to do the conversion came in around the same cost as the cheapest 30 gallon water heater at Lowe's last month. That's includes the pump/wiring, PVC, heater elements/thermostats and all piping to and from the homemade tank and my installed water heater. I should have a 100% return to my investment in around 10 to 12 months.

 

[turbo6bar]

I made a 27 gallon water heater for my camper. It uses two 300 watt tank heaters with two 120 degree cut-offs (one for each element). These were purchased from Missouri Wind and Solar for under $100 total cost (including shipping). I made my water heater tank out of 4 inch SCH 40 PVC.

Is the PVC water heater tank 27 gallons in size?

Are you using your original water heater to store hot water?

 

[TheDroidsYouAreLookingFor]

Ok I am back at it. Picked up 10(ten) 4 year old never used panels from an installer for very little money. 30 volts 9 amps 255ish watts. I picked up a 36v heating element and plan to hook up directly while i work on a plan to get my panel voltage to a steady 12ish volts for the 600 watt element.

I have an idea but unsure how well it will work. I was thinking i could get an mppt controller and a 12v wall recepticle. Attach the wall recepticle to the battery leads in parralel with the heater. I know the heating element would try to use all the wattage but im wondering if the mppt wouldnt then stay running and think that the battery needed to charge at all times. When it provides amperage it could be set at whatever voltage matches the element best. Is this crazy or would it give me mppt tracking, monitoring (victron ect) and efficiency at the expense of pulling 12v @ 1 amp 24/7... seems it would be worth it in total investment.

 

[GLC]

You are making it hard. Just hook up your panels directly to to your element. If you use a battery to feed your element, it will not last very long. Think of it as a direct short or resistance heating. Worst use of battery life is resistance heating. If you did use a MPPT controller with batteries and a inverter, you could limit the time of day that you want it to be on like the peak solar gain time. You could also tell the controller to shut down and not send current when the batteries reach a specific voltage so they would not drain 100 percent. But I still think that the easiest way to heat your water is direct hook up to your panels. It has worked for me.

 

[TheDroidsYouAreLookingFor]

OK I think that I wasn't being clear so I will try to make more sense now that I'm on a desktop.
With my 30v panels and a 36v element it will be hard to match the resistance (ohms) to actually get full power. The math is pretty simple. The element is rated for 36 volts and has a resistance of 1.25 Ohms.
36x36/1.25 (ohms resistance) =1036 Watts delivered power with infinity amps available.
Now my panels are only 30 Volts so that means.
30x30/1.25 (ohms resistance)= 720 Watts Delivered with infinity amps available.
This math is not linear it is exponential
25vx25v/1.25=500 watts
20v*20v/1.25=320 watts
15v*15v/1.25=180 watts
I could throw more panels at them or I could just keep the voltage at 36volts.

With my 12v 600 watt (.6 Ohm) element I have the exact opposite issue my voltage is dropped 2 volts with 2x 255 watt panels @ 30v. The math isn't looking good into our favor now.

The idea is if I can get an MPPT controller to to produce 12v from fluctuating panel voltage (0-30v) , then the entire system will actually be efficient.
I understand that I CAN hook up my panels to a random dump element that's close enough and get some heat out of it. In fact that's what I plan to do initially. Simply put the 12v 600 watt element working properly with 600 watts of solar would out perform the 36v element running at 30v only when the sun is perfect on less panels too. I learned a lot from Opera on YouTube and he makes sense now that I understand the math. I was thinking the same as you when I started this project.

Introducing PV Solar Diversion Board To Heat Water Efficiently At Power Point Available Soon - YouTube

So like my actual question stated. Can I hook a power source in parallel with a load on a MPPT charge controller to trick a controller into thinking a battery is there and needs to be charged? Would the element stop this from working? would the MPPT controller let all the pixies out? That's the real question as I may change to 24v just to make things cheaper to purchase if needed. Just to be more clear I am talking about running a controller with NO BATTERY but a 12v power source.

 

[GLC]

To answer your question, NO. You have to have a battery hooked up to your MPPT Solar Controller to make it work. You only have two options: Direct wires from panels to your element or through a 12 volt MPPT controller with at least one 12 volt battery. Through a MPPT controller you can hook up, if designed that way, wires that may feed like a light . Depending on the controller it will dictate the voltage and amps that feed your element. I tried everything from 1 to 4 panels. I wound up figuring out the most efficient way which was 3 panels wired in parallel, 305 watts -40 volts -15 amps. Open circuit, 915 watts-40 volts-45 amps. I tried different ac and dc heating elements. Dc elements worked the best because the OHMS were between .6 and 2.1. Best element was one that was 36 volt and 1200 watts, dual element. Connected to the element readings were 30 volts at 20 amps.

One last thought, yes the 600 watt element will work great, but it wont last long when you over amp or watt it. I started out with a400 watt, worked great for a while.

here is my build thread:

DC water heater element?

Has anyone tried these? Just wondering if they actually work. I was wondering what to do with extra energy from the panels I have.

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