Tankless water heaters and solar

[tornado6]

Was wondering if anyone has tried to power a tankless water heater system with solar panels.Thinking about a few tankless systems but dont like the amps they pull even if it is on demand.Seems with this system it would be perfect for solar-small time delayed discharges would be nothing for even the smallest of 6V trojan or surrette banks....

[veewee77]

Well, those things use MASSIVE amounts of electricity when they are on.

if you are planning a 12V system, and one of those water heaters (I'm assuming 4KW draw when running. . .)that is 333AMPS at 12V into an inverter. . .

Even if your system was 48V, you'd still need 83AMPS.

And these figures don't take into account losses in conversion. . .

So, you have a Tankless water heater that uses 333 amps and we'll say 220 amp-hour 6V batteries on a 24volt system.

Those batteries can do 220AH, but you need 333 so to start with you need 8 batteries. Then you never want to drain them below 80% charge so you have to multiply the batteries by 5. that is 40 batteries. So, now you take a shower. Say it was 15 minutes long. You pulled roughly 1/3 of your usable battery capacity, about 120 amp-hours.

Now, get your solar panels out. suppose thay are 150 watt panels at 24Volts.

150 Watts at 24V is 6.29 amps.

Using the sun-hours for my area, 5 hours, times 6.29 amps, is 31 amp-hours.

You used 120 amp-hours on one shower. Now suppose you take one shower a day.

120 amp hours for that shower, divided by the 31 amp-hours from the panels, you need 4 panels just to take that one shower per day.  And this depends on sun, FULL SUN every day. And this does not take into account losses associated with charging and discharging the batteries and in the inverter.

So, lets do some other math here. . .

4. KW inverter (guessing this here and the price) $3000
   
4. 150W solar panels at $5 per watt is $3000
   
   

Batteries, 40 of them and we're going to be cheap here and use Wal-Mart golf car batteries at $50 each is $2000.

Add this up and then buy the wiring and all other associated stuff and it is going to be in the neighborhood of $8000-$9000 to equip yourself to run that tankless, on demand water heater on solar energy.

Spend about $1000 and make a solar thermal water heater with a tank and be done with it. You could ptobably build a great one for less than $1000. . .

I am sure my numbers here are off, but this hypothetical.  Those who are more into the math thing can probably get better numbers, but these are probably usable numbers. . .

JMHO - YMMV

Doug

[Nando]

Tankless water heater have a range from 4 to about 35 KW operating power depending on the liter/minute or gallon/minute flow.

I have several catalogs with water flow and power usage.

Heck a lot of power for battery fed converters.

Nando

[richhagen]

Bosch makes, or made a unit to work in conjunction with a solar hot water system.  The water flows through the solar water heating panels, and then through the on-demand system.  The unit I saw used natural gas, but a similar system may be made for propane or electric.  The amount of gas was adjusted based upon the input temperature of the water to the unit to keep the output temperature relatively constant.  Rich

[Tom in NH]

We've been running a Rinnai tankless propane heater and gas space heaters for two years. They have electric blowers and ignition systems that we power with our solar electric system. The panels can easily keep up with the demand, even on cloudy days. The nice thing about it is, if the utility power goes out, we still have all the heat and hot water we need because the water is on a gravity feed and the solar runs the water heater and space heater blowers and ignition. Very important in January and February when it's 20 below outside. Rinnai is a top quality product. --tom

[Phil Timmons]

I tend to agree that this is not such a good mix -- if I understand that you are talking about directly using electric tankless water heaters driven by PV panels?

My quick and dirty math is a $20,000 to $50,000 solar PV installation to drive a water heater for a hour or two a day?

OTOH, you could simply grid-tie a small PV system and use it to off-set the cost of power to run the tankless heater from the grid.  At least takes the battery expense out of the system -- but the money is still not real favorable.

But all of this misses the entire point of heating water with the sun . . .  first, best, cheapest option is to take out the electric "middle man" and directly heat the water with a solar collector.  Is there something about your location/application that makes that impractical?  It is hot enough here (Texas) that we can create a solar heat collector with just a string of garden hoses in the yard, but with a concentrator/collector, you can use a solar collector water heater into the far North, even in Winter.  

[tornado6]

Yeah sorry guys work got in the way of time so didnt have time to reply but let me be more specific(guess i should have at first)....

The system now is a really old(15 years) natural gas 50 gal. water heater.Ill just sum up the bills for it one word,insane.Just unbelievable but there is a problem with using the progressive tube or another collector on the roof,the plumbing runs would cost twice of the system itsself.This is a brick colonial and even if you run the lines straight down from the roof you would have to drill through two brick walls to make the plumbing even come remotely close to an acceptable distance from the tank.Wouldnt even guess at the footage it would be ...too far.A mile and a half is pretty close.So the options really are tankless and solar electric but ive heard or read rather that the tankless work better with gas anyway--more effecient per therm.(should have said therms ,not amps before,my bad!)

Standby is what i have to get rid of,it is burning half of the therms just sitting there warming 50 gals of water i dont use but i think i can drastically cut the gas usage with tankless?.I think rich? said rinnai was a good one,i think takagi and bosch specialize in gas id have to look at it again but to offset the therm usage i thought that solar(panels and a bank-i guess from the numbers people have been quoting it would have to be larger for faucets)could be used for small things like toilets and faucets and then when it hits a preset amp/therm barrier? or maybe time use barrier it would switch back to gas>not sure if thats possible but i am really interested in talking with companies that do make solar/gas tandem systems.Guess it would be electric/gas tandem.

If not though i think tankless is the way to go still though because if i went solar i would have to replace with a new electric WH(650$ new) and here in virginia,winter is a 35% sun loss and my existing array really pushes the batteries dec-mid march.I know what i would need for it and its doable ......dunno really just looking to REDUCE the gas usage,its really all i can do in this situation if i look at it in short term dollars.Cant drop 4k right now for a total off grid system.Would if i could but ive GOT to cut the gas down these dirtbags are just killin  me on a/c/WH and i am definitely retrofitting back to electric burners that will be 100% solar.That one is simple as it would just be a short run from my existing controller and a 2 panel add on to my brackets as is.Workin on that right now but this is really a reduce situation,not offgrid.

So yeah any ideas or companies that yall know of or any ideas i havent considered....im gonna call rinnai and bosch,takagi today but thanks in advance for taking the time to respond.

[tornado6]

Forgot to add,is the federal rebate 300$ off your total taxes at end of year for FY 2007?Is it still valid?Not sure if the 2005 energy bill ultra scam giveaway to coal and oil killed the renewable table scraps we were thrown.Still worked last year....

[helowrench]

What about changing over to an electric water heater and replacing one of the two 120V elements with a 12/24/48V elemant powered by solar?

That is what I am contemplating, but my water heater is already electric.

Rob