how feasible is a solar powered on demand electric water heater

WillyP

Would running an on demand water heater use so much electricity, that it would require a huge array? I see they make them that use 50 watts, can I run something like that or am I dreaming? I plan on twenty LG Neon R 375 watt panels.

Jamie Hall

You're dreaming. 50 Watts is about 170 BTU/h. If we assume that your incoming water is at 50, and you want to shave with water at 110, that's a 70 degree rise. That much power will raise 2.8 pounds of water that much in an hour. Or somewhere around a quart. Per HOUR.

They tell me that patience is a virtue, but that's a bit much.

To raise 2 gallons per minute -- 16 pounds of water, more or less -- a reasonable faucet or miserable shower -- that much will take around 1100 BTU per minute, or almost 70,000 BTUh. or around 20 kilowatts.

At current efficiencies, that will take an array -- when the sun is shining squarely on it -- about 100 square meters -- or about 30 feet on a side.

Your 20 panels will provide, when the sun is shining squarely on them, about 7.5 kilowatts. You'd need around 60 of them to power your water heater.

iced98lx

WillyP said:

Would running an on demand water heater use so much electricity, that it would require a huge array? I see they make them that use 50 watts, can I run something like that or am I dreaming? I plan on twenty LG Neon R 375 watt panels.

You sure can use an on-demand water heater with that array. It'll be a propane powered one that requires a plug in and a few watts for the electronics.

As noted, electric resistive on demand units require vast amounts of power.

JUGHNE

Some have purchased an on demand electric water heater, only to find out that they need to upgrade their 100 amp electric service to 200 amps......just like adding an electric furnace to the house.

WillyP

Iced98lx I love that answer. It is kind of what I thought. I just hate the idea of breaking my thermal (insulation) barrier for gas vents.

WillyP

So obviously I wouldn't be able to power the hot water heater with just solar. But it seems if I am on a grid tied system it should replace the power used at about twice the time I use the water. Is that right? In other words if I take a half hour shower, I should restore to the grid, that much power, in an hour.

Jamie Hall

WillyP said:

So obviously I wouldn't be able to power the hot water heater with just solar. But it seems if I am on a grid tied system it should replace the power used at about twice the time I use the water. Is that right? In other words if I take a half hour shower, I should restore to the grid, that much power, in an hour.

Hour and a half.

The other problem is the sheer amount of power involved -- if you have 240 volt service it's still around 100 amperes to run the thing.

If you really want to go to solar hot water, may I humbly suggest that solar hot water heating systems are much more efficient that PV -- so a much smaller array is required. For that matter, Cabela's sells a nice little portable thing which is ample for a quick shower and fits in your backpack...

iced98lx

WillyP said:

Iced98lx I love that answer. It is kind of what I thought. I just hate the idea of breaking my thermal (insulation) barrier for gas vents.

I understand if you're going tight and controlling the air barrier to very exact standards it can be frustrating to look at MORE big holes in your envelope. Modern sealed combustion units have a feed air, and an exhaust air pipe so at least you're not trying to handle make up air. You just need to seal around the PVC, which can be accomplished. Depending on your location an outdoor unit or semi-outdoor (think, non-conditioned extension to avoid weather) or putting it in an attached garage if that's a thing could be an option to lessen the size of holes.

WillyP said:

So obviously I wouldn't be able to power the hot water heater with just solar. But it seems if I am on a grid tied system it should replace the power used at about twice the time I use the water. Is that right? In other words if I take a half hour shower, I should restore to the grid, that much power, in an hour.

Attempting to be power neutral instead of actually power a water heater makes more sense. I'd suggest that this is a part of your larger load/array calculation.

JUGHNE

Tank type electric waters use 4.5KW elements, your 7.5 KW could power 2 tanks of 50 gallon water as long as the sun shines. That 100 gallons would get you thru easily one day of use.

Jamie Hall

I hope, by the way, that if you really are going so tight that openings for makeup air and a a flue for a gas water heater are a significant concern that you are using a heat recovery ventilation system for the building -- otherwise your indoor air quality is going to be horrible.

WillyP

Jamie Hall said:

I hope, by the way, that if you really are going so tight that openings for makeup air and a a flue for a gas water heater are a significant concern that you are using a heat recovery ventilation system for the building -- otherwise your indoor air quality is going to be horrible.

Yes. This is a big concern. I am still learning about potential air exchangers. I am building in an extremely cold area (central Maine). SO any outside air would have to be heated. I've thought about using a long 4 inch duct pipe that would run lengthwise in a ceiling cavity. I wonder if anybody has done this, with a pex pipe wrapped around it, using hot water from the radiant floor heat.

Jamie Hall

WillyP said:

Jamie Hall said:

I hope, by the way, that if you really are going so tight that openings for makeup air and a a flue for a gas water heater are a significant concern that you are using a heat recovery ventilation system for the building -- otherwise your indoor air quality is going to be horrible.

Yes. This is a big concern. I am still learning about potential air exchangers. I am building in an extremely cold area (central Maine). SO any outside air would have to be heated. I've thought about using a long 4 inch duct pipe that would run lengthwise in a ceiling cavity. I wonder if anybody has done this, with a pex pipe wrapped around it, using hot water from the radiant floor heat.

What you are looking for is a heat recovery ventilator. This is a unit -- usually with two fans -- which exhausts air from the building and brings in fresh air. The trick is the heat exchanger, which -- in the better units -- will recover most of the sensible heat from the outgoing air. Even Amazon has them. It should be sized to give you between two and four air changes per hour. Much more satisfactory than a home-brew.

WillyP

iced98lx said:

WillyP said:

Iced98lx I love that answer. It is kind of what I thought. I just hate the idea of breaking my thermal (insulation) barrier for gas vents.

I understand if you're going tight and controlling the air barrier to very exact standards it can be frustrating to look at MORE big holes in your envelope. Modern sealed combustion units have a feed air, and an exhaust air pipe so at least you're not trying to handle make up air. You just need to seal around the PVC, which can be accomplished. Depending on your location an outdoor unit or semi-outdoor (think, non-conditioned extension to avoid weather) or putting it in an attached garage if that's a thing could be an option to lessen the size of holes.

WillyP said:

So obviously I wouldn't be able to power the hot water heater with just solar. But it seems if I am on a grid tied system it should replace the power used at about twice the time I use the water. Is that right? In other words if I take a half hour shower, I should restore to the grid, that much power, in an hour.

Attempting to be power neutral instead of actually power a water heater makes more sense. I'd suggest that this is a part of your larger load/array calculation.

I have thought of mounting a gas fired tankless heater in the garage. The whole house (including the garage) will be heated by a solar powered radiant floor heating system. You may have seen my post about the system I deigned with input from professor Philip Brainard. But that is a different thread.
But a big part of everything working is a very efficient wall system, that will block out the cold northern air.

JUGHNE

Air in must equal air out. A 4" pipe is pretty small for fresh air IMO.
Your plan is to pre heat the air using boiler energy.
As Jamie said you should consider heat recovery ventilation, pull in outside air that gets preheated somewhat by exhaust air of equal CFM. Most of these have a 6" duct for in and out.
I have built an "air tight" house in an area that has over 7000 HDD, about like your location. I have a 6" static inlet, run exhaust fans in every bath on 15 minute timers as needed and then crack some windows unless it is below 10 degrees.

The point is that there is no free lunch.....you need fresh outside air....and you have to sacrifice some energy BTU's to bring it in. We still keep most of the house at 74 degrees. But the bedroom has a window open just about year around, has gotten down to 58 in there...insulated walls throughout the house BTW.
As a child I needed an ice scraper to look out of my bedroom window....decades later I will pay to have the house warm (but the bedroom cool) with fresh air available. I know the "green" police will find me someday....I might also have guns and a freezer full of real beef also.

Maybe I am digressing to the "vitiated" fresh air theory of 1918...

WillyP

What you are looking for is a heat recovery ventilator. This is a unit -- usually with two fans -- which exhausts air from the building and brings in fresh air. The trick is the heat exchanger, which -- in the better units -- will recover most of the sensible heat from the outgoing air. Even Amazon has them. It should be sized to give you between two and four air changes per hour. Much more satisfactory than a home-brew.
Okay. I just googled that. I see how it works. far more efficient than what I cooked up in my mind. But they aren't cheap are they.

WillyP

JUGHNE said:

Air in must equal air out. A 4" pipe is pretty small for fresh air IMO.
Your plan is to pre heat the air using boiler energy.
As Jamie said you should consider heat recovery ventilation, pull in outside air that gets preheated somewhat by exhaust air of equal CFM. Most of these have a 6" duct for in and out.
I have built an "air tight" house in an area that has over 7000 HDD, about like your location. I have a 6" static inlet, run exhaust fans in every bath on 15 minute timers as needed and then crack some windows unless it is below 10 degrees.

The point is that there is no free lunch.....you need fresh outside air....and you have to sacrifice some energy BTU's to bring it in. We still keep most of the house at 74 degrees. But the bedroom has a window open just about year around, has gotten down to 58 in there...insulated walls throughout the house BTW.
As a child I needed an ice scraper to look out of my bedroom window....decades later I will pay to have the house warm (but the bedroom cool) with fresh air available. I know the "green" police will find me someday....I might also have guns and a freezer full of real beef also.

Maybe I am digressing to the "vitiated" fresh air theory of 1918...

Stuff like this (and what Jamie wrote) is why I love this board. The air exchange problem is outside my area of expertise, but I feel like I just conquered it, as a side note, to a different conversation. By the way, I am also the kind of guy with a well stocked supply of food and enough ammo to take on a small army.

hot_rod

A solar thermal panel or two would get you maybe 45% of the DHW an average family uses, maybe 50 gallons a day. Gas, Lp back up maybe.

hot_rod

Sun Bandit is a DIY, PV DHW kit.

Jamie Hall

Quite true, @hot_rod -- with, of course, adequate storage. What a solar panel or panels won't do is produce on-demand hot water...

GW

It takes a gigantic amount of energy to make water hot instantly. Oh wait, fossil fuel does that😀

Jamie Hall

I didn't fully read your title... "on-demand electric solar water heater". Do you mean by that an on-demand water heater powered by PV arrays? A more or less typical on-demand electric water heater for one bathroom and one kitchen will take around 100,000 BTUh input, or on the order 40 kilowatts. Which, with current PV cell efficiencies will be an array oriented exactly to the sun (that is to say, a sun-tracking array) with an area of 200 square meters, or about 2,000 square feet.

Is this feasible? I would say ... not really. There is an old saying in engineering circles that if you give me enough money I can design and build anything you want, but such an array -- with the tracking -- is going to be much money indeed. We don't quote money here, but just let me say that in this case I would estimate in the high five figures or low six figures.

ethicalpaul

I read through this thread and nowhere does anyone say "hybrid". Get a "hybrid" heat-pump tank water heater and you'll have the most efficient hot water possible. It's quite amazing.