Water Storage Heater

Which has a higher specific heat (is that the right term?) water or concrete?
Why not heat the water directly?
Del

Yes, if the intent is to heat the water - it would be better to have the heating element in direct contact with the water - with the best as an immersion heater.

I have some experience with a heat storage tank in my hot water radiation system. I agree with Del, you will find it easier and more efficient to transfer heat to the water. The hot water will transfer heat to the concrete tank, and the concrete will provide passive storage mass - slowly giving up its heat to the surroundings, or if insulated, returning its heat to the water when this has cooled down.

Are you talking about a hot water radiation system? I was thinking about you folks on the other side of the Atlantic today - heard about your bad weather. I've been to Britain, and the heating systems are not generally very good - pretty much what we'd consider an emergency backup system for power outages (short ones!).

The specific heat of water is 1 and concrete is around .2

To raise one KG of water by 1 C it takes about 13.52 Watts.

To raise one KG of concrete by 1 C it takes about 2.7 Watts.

I'm not sure I understand your figures tcmtech, or why they suggest it is easier to heat concrete than water.

The heat capacity of concrete is 31.7 btu/cu ft.

The heat capacity of water is 62.4 BTU/cu ft.

Bottom line, heat as you may, the concrete will only ever be half as hot as you can make the water. Since the temperature gradient is small, the mechanics of heat transfer are more difficult.

Water is an optimal material for heat storage and for heat transfer. It can be made very hot and then readily piped or circulated for heat transfer where it is needed - usually into your dwelling lets say. The high temperature water will readily lose heat to the cool air in your room because of the steep gradient: no further energy cost to transfer heat, except for your circulating pumps.

If the concrete tank is used as an insulated heat storage material, we assume the heat must be removed from it, to circulate into the home. How would you do that, in an energy efficient manner?

I was just stating the energy required to make a change in temperature. As far as a working medium water is far better and cheaper but over all to store a good usable amount of heat you need a good size of volume and a lot of energy to heat it up.

I have a roughly 420 gallon water capacity in my boiler system and a thermal mass equivalent to about 450 gallons of water total. My system can use anything over 90 F for workable heat transfer to the house. Still that gives me about 412 K BTU's of usable stored heat or about 12 hours of my typical house usage when the whole system is brought up to 200 F.

The point is to get usable amounts of energy stored you need a fair volume of storage and of course an energy source of considerable capacity to heat it up first!

A 1 KW wind generator would take about 5 days to heat up my system if I had a zero heat losses, which I don't of course. So as you can see where the numbers problem comes in when using certain power sources and their relative capacities.

Yeh - I see your point alright. I don't have any experience with wind generators but the difficulties with irregular energy sources are pretty clear.

This is what my system looks like: the grey item far right is a Slantfin electric boiler - just installed, as backup for the New Yorker wood boiler on the left. Storage tank in the middle, and you can see there's a big passive storage mass in the concrete block chimney. I don't say there's as many gallons in this system as in yours, but once I have heated the water in the system with the wood fire, the tank is enough to keep the house warm for 24 hours under most weather conditions.

I'm still troubleshooting the new electric add-on: I have to run a test and see what the power consumption is, if I use the electric to heat the water in the tank (about two hours) and then switch off for the day, vs. running on electric with the tank shut out of the loop.

It seems to me that passive heat storage sources make a big difference to the comfort level. With the tank and wood boiler out of the loop a day or two, the furnace room is cold and the whole chimney mass is also not contributing - I found it chilly even though the radiators were working fine.

Interesting setup and I'm why you didn't include the furnace room inside the heated envelope?

Dag blasted keyboard's missing whole words

I'm curious why you didn't place the furnace room inside the heated envelope?

You mean, why no radiators in the furnace room? It's a pretty small room: it was always a wood heat design and there is enough heat transfer from the flue to the chimney mass to keep it quite cosy under normal circumstances. It is inside the insulated envelope of the house.

Passive heat storage masses are very worthwhile in a home heat economy. They give up their heat very slowly and moderate temperature extremes without any energy consumption.

I've seen designs from old days, where a very massive chimney was central to the house. The heat stored in the masonary was enough to keep the house warm for a day after the fire was out. So they tell me...

http://www.evsource.com/articles/mind_your_units.php

I have a similar project an asked the same questions.

The suggestions given included use of a concrete tank containing salts which provide a greater mass and allow much higher heating of it (500°C - 600°C) which in turn provides a much longer time frame of heat in reserve during times the energy source is unavailable.

My issue now is how to insulate the plumbed water from the high temp mass to produce hot water but no steam while yet maintaining integrity of the thermal mass. If I can get by for two weeks without sunlight so wonderful would it be

How about separate tanks for the salt/fluid and the domestic hot water? Pump the salt/fluid through a heat exchange coil in the water tank, shutting off the pump when the water is hot enough.

What do you speculate is the cost difference of materials?

Hmm, I don't think it would be very much of a cost adder.

If I picture the original salt tank idea correctly, there would be a large salt tank with an electric element in it. You would need to have a separate domestic hot water tank anyway, with a pump to circulate water from the tank through a heat exchange coil in the salt tank, and then back to the water tank, and a thermostat to shut off the pump when the water is hot enough. (At least I think you need this, or something equivalent.) But when the pump stops, the water remaining in the coil will boil.

By switching the coil to the water tank, the boiling is avoided. The tanks, their insulaton, the pumps involved, and the controls all remain the same. The only down side I can see is that the pump must be designed to handle the hotter salt fluid. This will cost a little more, but shouldn't be a lot more.

The OP mentioned a 200°C/392°F heat exchanger made of concrete tube to heat water which can not possibly be efficient due the piping and pump required to carry water at the speed necessary to prevent steam. In contrast to using the salt tank as a thermal mass and circulating water through tubes placed on the outside of the 200°C brine tank. Placing the tubes on the outside allows a less expensive pump an piping to maintain a heated temperature of about 210°F at less than 15 PSI.

I'm considering a similar idea but using a parabolic dish or a trough to concentrate sunlight through a Fresnel lens upon a heat exchanger coil to heat a brine tank to about 550°C give or take 50°. Using the above mentioned circulatory concept enabling a thermal mass to continue providing radiant heat for about two weeks between times the sun is behind clouds, storms etc.. Wouldn't it be a sweet deal if I can manage a syphon and eliminate or reduce need of a pump too

The way I calculated my average heat needs was simply taking my known fuel usage numbers from my highest demand days and working out from there.

My house has crappy insulation and poor over all heat retention which means my old 60000 BTU output furnace ran about 22 hours a day in the worst weather and it was still miserably cold anyway. So for me that meant I needed a peak daily output of at least 1.32 million BTUs.

My boiler heat exchanger now puts out around 80K - 100K BTU's and this year that works out to being around 1.7 million BTU's per day in the worst conditions and around 1 million average per day for the whole heating season.

When you carry those numbers out over a longer time period and you start getting into swimming pool sized storage capacities to hold that much heat. Then you run into the added problem of how do you effectively heat all of that mass up in a short time. There is no point in having a 100 million BTU storage system if the heat source cant heat 10% of it in a realistic time frame.

Salt brine may be able to run far higher temps but then where do you buy tons of it and the right type and size of tanks that can effectively hold that temperature without reacting with the salts for less than you can get large capacity water tanks and water for?

The whole concept of alternative heating and energy systems is to get what you need for less overall cost and effort not more!

Salt brine may be able to run far higher temps but then where do you buy tons of it

At the Co-op

and the right type and size of tanks that can effectively hold that temperature without reacting with the salts for less than you can get large capacity water tanks and water for?

Concrete septic tanks

Care to comment on what is in the rest of that sentence? Mainly the 'cheaper than' part. Pick and choose seems to be a convenient way out of much of the theoretical AE system problem solving issues. Unfortunately in real life situations you cant just ignore the rest of the physics and dynamics of system or their related costs and working capacities because they don't fit your budget and or knowledge constraints!

I would like to see you cycle a concrete septic tank through a 50 - 1050 F temperature range a few times and still have it hold it integrity without leaking!

By the way salt has a specific heat of around .22 of water. changing 1 mass unit of water by 1 degree is the same energy as changing one mass unit of salt by 4.54 degrees.

So your tank of salt will hold about 2x the heat energy as its same volume of water even though its running 4.5 times hotter.

I am still sure water is cheaper and easier to work with!

There is expense in procuring the brine of course but I don't have space enough to use water.

The temperatures the brine tank will not quickly fluctuate over broad ranges, a two week time frame from 1000° to about 250°F would be the exception and the norm being from 1000° to 500° - 600° before brine heating resumes.

The temperature outside right now is -17°F and in order to keep water the hole would be huge, top of the tank at least 8' under so a 20' x 20' hole is out of the question.

I need or would like heat storage for about two weeks at a minimum temp of about 250°F after that the back up fuel burner would lite-off. Except for storms or heavy blowing snow it'd like to recoup the energy stored in the reservoir and it just so happens the concrete tank and salt will float the boat with a 4-5 year payback.

Water just won't work, it's inadequate to the task.

Thanks I appreciate you

Got any realistic heat demand numbers, cycle time frames, actual materials to be used and overall realistic cost expectations to go with that?

Where are you located that does not have a 20 x 20 x 8 volume available but will allow you to have a 1000 F container of salt sitting around with a heating system that can work with it and still have home owners insurance?

What method of getting all of that mass up to 1000 F in a short time frame are you planning to use?

What type of insulation are you planning to use that will keep 1000 F from cooling down over a two week time period without massive percentages of loss along the way?

As many are engineers here I think we are all open to new ideas provided the numbers add up to being beneficial or realistically adventitious over other methods. And by numbers that means peak daily heat requirements, total system costs what exact materials you will use and why.

By the way salt reacts with common concrete which is not 1000 F compatible by any stretch of the imagination. Refractory cement will work with that temp well enough but costs about 10X what common cement costs but still has the problems that it also reacts slowly with salt. Plus refractory cement has rather poor structural integrity and is very prone to cracking with any level of thickness and physical stress when hot.

A high temp and reliable seal or liner will be needed inside the tank along with special high temp insulation being common home insulation does not cut it at 1000 F! And a common steel will rapidly corrode when placed in contact with liquid salts. Plus common stainless steel tank systems will not take those temperatures as well.

The big coal fired power plants have their boilers made out of massively reinforced refractory cement with high temp steel liners but still they don't have any liquid in direct contact with cement or liner material. Plus they cost 10's of millions of dollars to build. Even at small scale I would like to know how you plan to off set the initial material costs in 4 -5 years by any realistic heating methods.

Just looking for a realistic approach!

So far my numbers suggest your off by a long shot!

A 10 x 10 x 8 foot cube of salt at 1000 F has roughly (800 Ft^3 x 89# x .22 s.h. x 900 F change) = ~14.1 million BTU. The same volume of water at a 100 F temp change has ~ 5 million BTU.

10 x10 x 8 is just under 6000 gallons and a common used but good condition tank of that size is around $1500 -$2000. Home built can be far less than that.

800 cubic feet of salt weighs about 71200 pounds and bulk salt costs around 10 cents a pound. That still puts the salt cost at around $7120 not including transport or the tank!

I am just not seeing any realistic cost savings or justifications number here and I have not yet even touched the realistic tank or possible heating system costs! 4 -5 year pay back only seems realistically plausible if your current heating costs are pushing $10K a year or more! And your heat energy source costs you $0 a year and has $0 annual up keep and maintenance!

*Got any realistic heat demand numbers, cycle time frames, actual materials to be used and overall realistic cost expectations to go with that?

There will be 360 gallons of water in the hydronic heating system. The thermostat will call for heat every hour for about 15 minutes. Salt will remain dry below phase change threshold, containment tank materials you have explained well enough.

*Where are you located that does not have a 20 x 20 x 8 volume available but will allow you to have a 1000 F container of salt sitting around with a heating system that can work with it and still have home owners insurance?

Location is on top of a thin; about 15', layer of dirt covering a large granite slab of rock and not having sifted the location there maybe boulders the size of a two car garage just one meter below. I need to place a tank below the frost line, the frost line is at approximately five foot depth and I'd prefer to place a tank five feet below the frost line equaling ten feet from the surface to top of the insulation atop the tank.

*What method of getting all of that mass up to 1000 F in a short time frame are you planning to use?

Sunlight from concentrate, a sun tracker dish, a Fresnel lens, a coil to heat exchanger and a pump or a small turbine to spin a generator and bank excess energy into the grid an withdraw during cloudy times.

*What type of insulation are you planning to use that will keep 1000 F from cooling down over a two week time period without massive percentages of loss along the way?

Salt, gypsum, rock wool all around then 8' - 10' of dirt on top.

*A high temp and reliable seal or liner will be needed inside the tank along with special high temp insulation being common home insulation does not cut it at 1000 F! And a common steel will rapidly corrode when placed in contact with liquid salts. Plus common stainless steel tank systems will not take those temperatures as well.

May use a clay liner and salt will not be liquid.

How would you then spec materials to the tank casting plant?

*That still puts the salt cost at around $7120 not including transport or the tank!

A trade-off may help, I get by with a little help from my friends...

_{+on a side note we'll be putting it to the county about investment of a "pocket reactor"}

Neat but still how do you propose to get enough heat from sunlight to be able to reheat this thing in any realistic time frame? If you also have the same 5 foot frost line requirements that we do around here you also have the less than spectacular levels of solar heat energy to work with in the annual time lines of when you need the heat the most.

My reference sources quote the solar energy that is recoverable from sunlight in the winter around here to be around 70 watts per square foot. Plus getting actual usable sunlight on good days is still limited to under 8 hours in ideal conditions and locations. To heat up the pre mentioned 14.1 million BTU volume I had mentioned earlier that would need around 7400 square feet of solar collection area working at 70 watts per square foot for 8 hours and is still assuming zero percent losses while running.

Some how this seems like an ill thought out and likely giant waste of money being placed on someone else's shoulders. No one I know would ever consider the real honest time, effort, and cost outlays to construct it anywhere near practical and justifiable.

And just out of curiosity you said you have no room for a 20 x 20 x 8 water tank yet you just said your putting this under ground in an area that 15 feet deep with boulders the size of a two stall garage.

More than just the math doesn't seem to add up here.

I have done a good deal of excavation work and even doing a 20 x 20 x 15 foot hole by myself with rented machinery will still coat over $500 for the machine rental and run time.

I am rather starting to feel like I am just being jerked around on this topic by someone who has no realistic plans or ideas of what is all involved for a project of this design and construction. It is in no way a real DIY cost justified concept that would truly save anyone money on realistic winter heating costs in their lifetime even if it never broke down let alone over 4 -5 years.

If it is a real project start a thread on it so we all can pitch in on the numbers and design. I would still like to see what something like this may actually involve and cost in realistic engineering terms!

Okay no jerky here...

Yeah the low sunshine of this latitude stymied me for some time but then the use of the sun for heat instead of electricity I found very intriguing. Even though the reduced sun exposure with the dish concentrator setup and sun tracker programing it will give hi-heat about 600°C 8hrs/day when no cloud. There is no large cost to me it is time only from late march to early November maybe in the time frame it will build to storage temp.

There is an engineer involved and so we're brain storming/bench marking what have you. We could use a small crucible of salt within a water jacket so to speak inside a concrete tank.

Curious stuff...

Sounds like a lot more realistic math and physics needs to be applied here along with some basic thermal dynamics and chemistry as well.

Taking from March to November to charge up a system that gives you at best two weeks worth of usable heat assuming near zero losses in that time frame seems pretty inefficient, poorly thought through and basically useless. Especially for the more plausibly realistic construction costs, time and effort that such a design will take.

If you know your average heating energy usage you can come up with a fairly reliable total energy needed numbers to work from. If your in an area like me and hopefully have a far better insulated and smaller house you will still be likely to be in the 200 million BTU hours range of needed heat energy after all losses are factored in. And don't be surprised if you find out that your pre end use losses are 80% or more of whats put in!

If you cant come close to 1 billion BTU hour gross storage capacity and a realistic and cost effective way to produce it your not going to be pleased with your end results. For basic reference a billion BTU's is what a water tank approximately 126 feet square by 10 feet deep changing from 200 F down to 100 F would release!

The house was as drafty as you have described yours to be, one could feel a breeze when walking through a room with the windows closed. I have spent much time and a few $$$ tightening the envelope. According to the math pertaining sizing a boiler to an energy star home a 60k BTU boiler is adequate.

*Taking from March to November to charge up a system

I wrote that it would have that much time not take that amount of time nor is it thought no losses would occur. I am will to except inefficiency of the design to reduce construction costs if the system reduces my need for fuel in the boiler; if can save me $600 to $1000 a year in heating that will be efficient in my book. I suppose it were naivety on my part to suggest anything expecting helpful replies without vilification.

*If you cant come close to 1 billion BTU hour gross storage capacity and a realistic and cost effective way to produce it your not going to be pleased with your end results. For basic reference a billion BTU's is what a water tank approximately 126 feet square by 10 feet deep changing from 200 F down to 100 F would release!

Thank you for an all encompassing reason not to use water.

A 2250 sq/ft house, with five exterior doors, 10 windows, 26R value in basement walls, 42R value in sidewalls, 25R value in angle roof, 56R value in 2nd floor ceiling and a detached garage. Original boiler 105k BTU.

Wind energy isn't an option and sunlight in a reduced area needs a concentration para dish. I will of course keep the fuel oil burner.

Liked your post, it hit the right buttons for me! Therefore also GA.

You really don't need an alternator for your system.

You are converting mechanical energy from the wind into heat. You could convert the mechanical energy directly into heat with:

1. A water cooled friction brake on the windmill shaft. Run the cooling water into the storage tank.

2. A water pump on the windmill shaft that pumps your storage water. Restrict the flow to create heat.

Both of these methods would have higher efficiency, since you are converting mechanical energy directly to heat and avoiding the losses in the alternator core.

Another point..Pipework embedded in concrete will be rather inaccessible in case of problems.
Del

But a band heater may suit the purpose...

Just an idea it's not set in concrete

Yeah, but just face it, it's a bad idea...the thermal conductivity of concrete is probably worse and the convection currents in concrete are non exixtent, this will mean that any heating element will run hotter.
I could go on, but I'm even boring myself now
Del

Yep and is why I interjected post #9, not to hi-jack but steer a little maybe...

<...Just an idea it's not set in concrete...>

Surely warming up concrete (or using concrete) is not the way to go? If I have missed something here my apologies.

A hot water tank needs to be really well insulated to conserve energy/heat.....

A good way to warm water with a wind turbine is directly, by "thashing" the water around in a special made tank, with paddles connected to the wind turbine and fixed paddles in the tank with only a small gap between both.....

Never try using electricity for this.....somewhere on CR4 is details of such a tank....

Hello, I have an idea (not original to me) we all could explore. I would like to use a wind turbine to turn two discs in opposite directions, which are close enough together to generate heat without destroying them. These discs would be immersed in water or oil which would heat my house. What material for the discs would be best? How could we best engineer this? Tim

We're not sure exactly where your going...

I have seen wind turbine designs, where the excess capacity [fully charged batteries] was shed using heaters.

Couldn't this method be used to help limit the maximum rpm?

Depending on the installation the efficiency [or lack] won't matter...

Any decent wind generator alternator is already around 90% efficient or better. Unless its one of those homemade ones, then you get what you pay for. If you need more power from wind you get bigger blades and catch more wind.

...or eat more baked beans?

My comment may be off the storage topic for now, but I want to connect turning blades mechanically thru reduction drives to the two discs which are (inside the house) in the oil filled heat storage tank.

Are you saying that electric from alternator to heating element is almost as efficient? tim

So how is mechanical linkages, gearboxes, and friction devices going to possibly be cheaper and more cost effective to build and maintain over electric? There is a reason our national power system is electric and not mechanical!

Any wind power system that is big enough to generate the needed energy for any practical level of heating is going to be way to big to ever be legally installed on, over, or near a house!

Reason out the basic physics and numbers along with the realistic costs of materials and maintenance in relation to basic electric devices.

To use your own words:-

Reason out the basic physics and numbers along with the realistic costs of materials and maintenance in relation to basic electric devices.

That is the reason!

A VAWT directly driving paddles or a special pump in say water, causes a rise in temperature whenever the wind blows, no gears, no electrics and very little, but relatively simple, mechanics.

There is someone (I have forgotten who) on CR4 who made them years ago and I am told that people in Sweden uses such items even today......The Guy on CR4 said the only problem on windy days was in stopping the water boiling!!!

Rremember that when converting energy, there are always losses.....converting wind to electrical energy, then heat has two losses......

There is little conversion when you go from mechanical wind energy directly to mechanical paddles......

Here are some YouTube videos showing one of these water heating pumps being driven here by an electric motor, but it could just as easily be a windmill.....

http://www.youtube.com/watch?v=yh_-DUKQ4Uw

http://www.youtube.com/watch?v=UBxpn6odtcA&amp;NR=1

http://www.fieldlines.com/story/2007/10/3/195821/664

This type of pump appears to be called a "hydrosonic" pump as far as I can tell....but no guarantees mind you!!

well, I went and had a look: here's a NASA page about it. It seems there were some problems with bearings not standing up to the heat in the first prototype:

http://www.sti.nasa.gov/tto/spinoff2000/ip3.htm

The company that makes these seems to be focused on applications other than heat, at least at the present, although the nasa spinoff article mentions they hope for future applications as you said, using wind energy to power the heater.

http://www.hydrodynamics.com/index.htm

http://www.hydrodynamics.com/technology_review.htm

If they develop a version of this machine that can handle the ups and downs of wind and not destroy itself, I'll be very interested in hearing about it.

Thats the physics I am referring to is to factor out how big of wind generator and storage system you would need to heat a house given a normal wind and typical winter temps for anyone that does not live near the equator and in a mountain pass.

I suspect others will be checking your math too so no over unity assumptions as well.

Stuff will get very big, very expensive, very fast! Unless you can prove me wrong!

To explain the discs, as they rotate against each other the friction generates the heat used to heat the oil. Same oil as used in oil filled electric space heaters. Oil is than circulated through the house or hot water heater. tim

I saw an article a while back in Home Power magazine about a guy that used a concrete tank (500 or 1000 gallons or so) to store his heated water. He used a solar collector array to collect heat and dumped it into the concrete insulated tank with a heat exchanger. The tank was large enough to supply heat for his domestic hot water as well as radiant heating needs, with a gas-fired system for back-up (extended number of coudy days). I don't see why you couldn't use a wind turbine to heat the water, but just as the sun doesn't always shine, I think you may find some days with not much wind. Again, some back-up may be in order. Anyway, the article had some pretty good information and some basic calculations that you may find helpful. If I can find the date of the article I'll pass it back to the forum, but just in case I can't find it it was in the Home Power magazine in the last year or maybe two at the most.

Heating the water isn't the main issue, as there are plenty of point-of-use water heaters installed in washrooms across the globe.

The issue is matching the availability of wind power to the demand for power. For that, some form of storage is needed, be it a hot water reservoir, a battery, hydrogen, a flywheel, whatever...

In the UK, the hot water storage arrangments in most homes involve the use of a well-insulated copper tank with a heat exchange coil in it as well as, usually, a 3kW electric "immersion heater" element, which is usually switched off as it is more economical to heat water via the heat exchanger. It is not beyond the wit of Man to disconnect this immersion heater from the mains supply and connect it instead to a microgenerator such as the 1kW wind turbine advocated above. Then, while the wind is blowing, some heat will be generated in the tank thereby reducing the demand on the unit connected to the heat exchanger when the unit is timed to come on. The issue arises as to what to do with the wind power when the tank is coming up to the boil (unlikely though possible in a hypothetical perfectly-thermally-insulated arrangement) as a result of the windspeed and the timing of it. That is why most wind harvesters are connected via a special inverter so as to push power back into the local grid, and "make the meter run backwards" as it were, as the most practical way to do it; if this home doesn't need it then there are others that will, as it were.

Bear in mind that, just because a 1kW-rated generator is connected to a 3kW-rated element in the simple water-heating arrangement in the paragraph above, does not mean that 1kW is going to be dumped into the water. What needs to happen is for the output impedance of the generator to be made equal to the resistance of the heating element, which (in the case of a 3kW element rated for 230V supply is 17.7Ω), for the maximum amount of power to be transferred. Otherwise, for most of the time, the wind generator will be underloaded, and valuable harvestable wind power will be lost.

The properties of water and steam are widely understood and widely published, and a set of Steam Tables is a valuable tool for any practising Engineer. These tables may be found in any number of publications, ranging from Kempe's Engineers' Yearbook, through Perry, "The Chemical Engineer's Handbook", through to Mayhew & Rogers, "Thermodynamic Properties of Fluids" (usual disclaimer), among many others.

It is far easier to heat water than concrete!

Aha! The overtemperature protection is provided already. Immersion heaters usually have a thermostat wired in series, set to 60degC.

i heat with a outside wood stove that circulates hot water. i have been considering increasing my water capacity with a underground storage tank also. i know wind and solar are nice and work well intermittently. therefore i think one needs a backup system to keep the water hot. i have seen other posters have mentioned this also. their information has helped me considerably, thanks everyone. when i install my storage tank i will also install a gasoline driven generator. it will be duel fuel natural gas and gasoline. this might not make much since. but when i couple to the exhaust from my wood stove i will burn the residual gases in my generator. once i run the engine exhaust through a heat exchanger and capture the engine coolant i think this will be fairly efficient. i added this info thinking it might help you with low wind periods or on cloudy days with solar. let us know how your system works out thanks.