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Renewable Energy Questions/Discussion => Solar (heating or electric) => Topic started by: joemtl on May 15, 2017, 07:49:49 pm

Title: Passive solar air heater - for home
Post by: joemtl on May 15, 2017, 07:49:49 pm
Hello,

I have been looking over a few ideas for the house.

I like a variety of solar ides have crossed my way, but one that seems most interesting is:

Making a air heater, and pumping the heated air into the home.

I have seen a variety of heating systems based on tin cans, to just paining the inside of a box black, that has plexiglass allowing sun to enter and heat the air.

The idea seems rather good.  I have been looking at ways to get it into the house, and I know how I could accomplish this.

My last remaining question is something that is not easily measured, or calculated.  I want to be able to get an idea of how much heat can be had out of a passive solar air heater, so that I do not make it too small, large.

Or, might I consider making one, and that find a way to add one in series or parallel?

I am not looking for an answer, as there likely is not a simple back and white answer.  But, feedback on the idea, and what to look at/for.

Thanks

Joseph
Title: Re: Passive solar air heater - for home
Post by: eidolon on May 16, 2017, 03:05:48 pm
People are attracted to these things because of their simplicity. Done wrong they can loose as much heat as they gain. Very hard to measure. Probably the reason many think they are wonderful. Any solar project is quite site dependent.  Here is a nifty cheap controller you can use to operate a set of fans.  Good luck with your project.   http://www.ebay.com/itm/DC-12V-Thermostats-Temperature-heat-sensor-switch-50-110-C-AD/112176134648?_trksid=p2481888.c100678.m3607&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908131621%26meid%3D97847831ac204f0bbf9685870b2c6848%26pid%3D100678%26rk%3D3%26rkt%3D15%26sd%3D410991690870&_trkparms=pageci%253Abd4abf77-3a70-11e7-9034-74dbd1807d1c%257Cparentrq%253A12ceaff315c0ab6bace20539fffdb0dc%257Ciid%253A1
Title: Re: Passive solar air heater - for home
Post by: joemtl on May 16, 2017, 09:50:09 pm
Thanks for your reply.

The thermostat, this is to do what, exactly?

Might it be to measure the temperature with in the passive solar heater, so one knows when to use it or not?

This idea had crossed my mind, but I was not sure how to go about it, yet.

The idea is to possibly have a pipe up to the roof of our house to feed it with cold air (but warmer than the outside air).  Then, reheat the air, and pipe it in to 2-3 rooms in the house.

I had thought of having a fan at each exit.  It could be controlled by the thermostat that you proposed.  But then the idea of having all three open at once, and each fan might just draw air out of the other two, in a worse case scenario.

Then, an idea of have 1 main fan before the duct splits to each of the rooms. The exit could be opened by the thermostat, which would allow the air to flow.  But, how do you make sure that each duct gets some heat?  Meaning, what if one duct gets preferential air flow?

There also needs to be a main controller that makes sure that thee is actually warm air in the passive solar heater.  If it is too cold, it woudl nto work and all ducts are closed.

Getting back to your comment about if done wrong, all the het made could be lost.  Could you explain what you mean?   What can go wrong?  What are things to avoid or look for?

Thanks

Joe
Title: Re: Passive solar air heater - for home
Post by: eidolon on May 17, 2017, 03:03:27 am
That controller is to turn on the fan when the air is hot. It has differential and delay setting so the doesn't rapid cycle when the temp drops a couple degrees.  All these systems provide an opening to the outside world.  Just a little flap door will let heat out.  In a small system the heat loss when no sun can be nearly as much as that gained with sun. People have used this principle since the dawn of time to some degree.  Yet, it hasn't made it into standard building practice. As much as you would like to think, it isn't because people are just stupid.  Myself, I'd do PV heating if I had the logistics for it.  Just a cleaner install.
Title: Re: Passive solar air heater - for home
Post by: joemtl on May 17, 2017, 08:44:28 am
Hello again,

When you say PV heating, you mean what exactly?

Generate electricity with pholtovoltaics, and that use that electrity to power the heat making system in the house?

I could agree with the losing heat easily if the system is ill designed, or poorly implemented.  If the ducts leading to and from the systems are leaky, this wold guarantee a net loss, and wasted time and effort.

Thanks

Joseph
Title: Re: Passive solar air heater - for home
Post by: eidolon on May 17, 2017, 09:22:45 am
I'm at a disadvantage of not knowing your location and other circumstances.  A relative bought a solar home and the first thing they did was tear all that stuff out. The great thing about the world is everyone has their own ideas and criteria for success. An interesting read on PV heating is electrodacus.com.  Dacian is a strong proponent of it. One advantage is it can at least do something in the summer.  Of course his approach is almost impossible to duplicate unless starting from the beginning with new home construction.  Not trying to discourage you.  Implementation of any system comes down to the details.  I heat water with PV and almost everyone thinks I am wrong.
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 17, 2017, 06:12:03 pm
Wrong is only wrong if it tries to circumvent the laws of physics, or doesn't work. Everything else is somewhere on the practicality line.

I use peltier to do thermal management for my batteries in the truck, I can only imagine the sneering and snickering that took place over that, before I proved it to be viable for my situation.

Truth of the matter is, in my experience, it's never been so much the "what" as it has been the "how".

Implemented wrong, ideal concepts can prove totally useless if not even (as mentioned) have a negative impact.

With some experimenting and careful planning, concepts that are blindly shot down by the masses because of perceived and propagated issues with them, can prove not only viable, but even exceed expectations.

It's all in how it's done. The key is to figure out what is what before you either blow a bunch of cash up front on a concept that isn't going to work, or find yourself having to blow a bunch of cash undoing damage.

All that said, PV is cheap now. And surplus PV is the best kind of PV. That's not to say that it's the end all solution, but sure is a lot more viable than it was once upon a time...

Just my 2 cents...

Steve
Title: Re: Passive solar air heater - for home
Post by: welshman on May 19, 2017, 04:31:01 pm
Maybe this?
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 19, 2017, 09:55:06 pm
Welshman -

On paper, that's pretty much where it begins... There's a few gotchas associated with that tho, and really just with passive of this nature in general.

The biggest has already been mentioned, and is very easy to overlook... What happens when it's cold out and the sun isn't shining... Convection is a two way street...

Ultimately, I've found that one way or another, fans have to be involved with anything like this because even if the surface is insulated to reduce conduction losses to the outside ambient, the energy will just "shape shift" and radiate as IR, still cooling the air in the plumbing, inducing the reverse flow, still causing a draft.

Flaps of whatever sort also seem minimally effective at preventing the reverse flow, and also provide some opposition to the desired direction of flow when the sun is shining.

Several revisions of the airflow and thermal management concepts have come and gone in the truck (and I'm sure I'm not completely done, as long as I'm still occupying the thing full time)... and just so happens winter is the "most conquered" of the big seasons... The scheme in play now, while not explicitly designed for direct heating via solar-to-air, IS designed to minimize the negative impact of night losses when it's cold. It's probably best described as "passive self inhibiting convection with active management"... A diagram would help clarify this, tho I don't have one on tap as of the moment, but I'll try to describe it...

Basically, my intake and exhaust ports leading to the outside are at more or less the same level, and up high on the outer box, forming a loop that naturally tends to cancel convection. Both ports have fans associated with them, but the important ones for the concept at hand are in what was originally considered to be the "intake", which forms the self canceling part of the big loop. There's also an additional port between the inner and outer boxes, but largely, it doesn't apply much to this.

I have plans to experiment more with the fine tuning, but will need another round of winter to do so... However as it sits, it's very effective at being able to confine heat inside. To accomplish this, a small fan opposes convection, trying to pull a negative pressure on the inner box (living space), with its source being down low all the way at the floor. This results in 2 things: 1, allows the air trapped inside the pipe/air handling chamber(s) to get cold, without being able to form a draft that would let it into the inner box... and 2, tries to prevent the warmer air toward the ceiling from escaping into the void between the ceiling and roof (essentially what would be the "attic" in a house).

In my specific situation, I can bring small amounts of extra heat in the "attic void" into the inner box during the day when the sun is beating on the roof as well, but the effect has minimal impact because outside on the roof, the PV shades the roof skin too much to be of real use in this way.

That said, I don't think I'd run it in the direction of natural convection where an explicit solar-to-air collector scheme is involved anyway, but that begins to fringe very heavily back into theory and away from "tried and true". Someone with more solar-to-air experience would need to chime in there, as from what I can tell with what I *do* know of it based on other things I've encountered with the truck, the ability to be able to move the air in both directions, and variably, would likely be important.

I'll try to get a pic or two soon and possibly a little diagram drawn up with some annotations to clarify what's happening if it might help... but hopefully this gives you a general idea of what I'm getting at.

At the very least, I hope it illustrates that while "pure passive" sounds good at first, it's not really possible by itself (at least as far as I've been able to tell with anything I've tried with it). Fans are my best friends, and yes, I spend a little energy moving/controlling air, but it's not extreme. In fact more often than not in winter, finesse is the key word.

Steve
Title: Re: Passive solar air heater - for home
Post by: welshman on May 20, 2017, 04:03:56 pm
very interesting, esp about the negative pressure aspect of it.
Title: Re: Passive solar air heater - for home
Post by: DJ on May 22, 2017, 09:52:55 pm

I have just bought a new home with a metal roof.  A roof is a huge area for solar heat gathering and I plan to use it.
The place has ducted air but I have spoken to a fridgy and he says it's no problem to set up an extra inlet that will draw this warm ceiling air  and pump it into the house in the fan mode of the AC. I'll stick a filter on the end of the pickup to eliminate any dust being blown into the home. I already have some thermostats like linked in the previous post so I can compare the home temp and the roof air temp.  My thought is that I probably won't be able to run the thing constantly but have to wait for the air to heat up and then run that into the house. Then again, probably depends on the day and if only a small rise is required, may be ok. Otherwise, anything is a bonus and cheaper than running the compressor on the AC unit.

I'm not sure if there is insulation under the tin itself, there probably will be a form of it in a waterproof/ insulation membrane but given the area of the roof and the amount of sun falling on it, I believe worthwhile amounts of heat can still be harvested. There is insulation on the ceiling and the good part about this system is any escaped heat will be re used as it were.

I'm also looking at adding another similar pickup on the AC and putting a radiator in front of it so it can draw through and heat air from an oil fired water heater and disperse that into the house. Main thing is to work out a practical and aesthetic place to put the burner/ boiler.

Looking on the net at DIY heating ideas always does my head in.  I literally want to pound my skull against a brick wall at so many of the idiotic concepts and moronic rubbish people go on with.
Are these people mentally impaired to think that a few tea light candles are really heating a room? Are they too stupid or just so wanting to believe it works to do some simple research as to how much heat a candle generates and not see that their plasma TV generates a load more heat and does it at a fraction of the cost of buying tea candles?  Amazes me when you see these people are talking about heating a dorm at a university! So much for higher education and learning.
It beggars my belief that they just can't look at the thing and know it's going to put out no worthwhile heat at all.

But then there is the be all and universal method of DIY heating.... a bit of copper pipe coiled  and sat over a typically open fire.
Yep, if you wanted to go low efficiency, Couldn't think of anything much worse... or better if low efficiency is what you are after. Just let 99% of the heat from that fire blow away doing nothing at all but kid yourself you are putting 100Kw worth of heat into that pool and it will be warm as toast come summer.
 If you want useless, forget about the Candle in the room, try heating an above ground swimming pool with water running through a bit of 3/4 or 1" copper that's sitting  over an open fire. Now THAT is kidding yourself.

The internet is for more than watching porn and recordings of video games. There is an incredible amount of information and calculators that would let you see how useless this is through all sorts of heat rise tables and calculators, thermal capacity charts and many other related information.  Once you look a couple of these things up and even get a slight feel for it, you can then look at something and straight away have a fair idea how poor it's going to be.... unless you are one of these people that likes to kid themselves badly and cheat themselves.
   You don't need to be smart to look this up, I'm dumb as dirt but fortunately, smarter, educated people made it easy for pelicans like me to look up what is going on so we can see if something it worth while or a complete waste of time.

As I like results and loathe Kidding myself, I like to make things that WORK. Like really work.... using all that stuff most of these DIY heating clowns hate... like fact... and physics and scientific principals.... and lord forbid..... Measurements and reality.  Yes, I know, using all that stuff limits your thinking and it's much easier to make something that keeps you warm if you don't actually measure the results using something as sophisticated as a thermometer
to verify your delusio... work but it unfortunately, paying attention to fact and scientific principals and laws at the end of the day will keep you warmer.
Like as in really warmer, not just mentally warmer.

It amazes me the way people will spend so much more on their hair brained schemes to save money that what they would spend on conventional practices like electricity or gas.  Look up the actual heat energy in that packet of $4.99 tea candles and work out the cost per BTU or KW. Then compare that to what you pay for a KW of electricity.  Yes, you just spent 20x more on the candles didn't you? Amazing! Who could have ever worked that out.... except anyone with access to the net and a desire to separate fact from stupidity.

I also want to incorporate PV for heating.  I just bought another 2.5Kw of used panels for $500. I'll have a large ( and going to be made larger) shed roof that points directly north that I can put a LOT of PV on before I even get to the house which for the most part is east west. The way the property is means there is nothing to stop me just putting up a ground mounted system which I will use the panels as a roof for a shed or pergola anyway and seal them together with silicone when I mount them.
I'd like to heat water and then back feed the excess through the old analogue meter and run it backwards  so what I make during the day can be used at night.

As mentioned, PV, particularly used is so cheap now if one has the situation for it, it's a very viable and practical thing. You hang your panels and don't have to change anything else in your home or the way you live. Using the grid for a power bank and getting a 1:1 feed / return ration although a little " grey" market is very efficient and serves the green principals and objectives that we are constantly having shoved down our throats so is something able to make use of that crock as well.
Title: Re: Passive solar air heater - for home
Post by: rossw on May 23, 2017, 03:01:29 am
If you want useless, forget about the Candle in the room, try heating an above ground swimming pool with water running through a bit of 3/4 or 1" copper that's sitting  over an open fire. Now THAT is kidding yourself.

Quote
I also want to incorporate PV for heating.  I just bought another 2.5Kw of used panels for $500. I'll have a large ( and going to be made larger) shed roof that points directly north that I can put a LOT of PV on before I even get to the house which for the most part is east west.

From a purely EFFICIENCY point of view, using all that roof space for PV, at a return of around 15% (by the time you take out inter-cell space, inter-panel space etc), assuming you then use resistive heating to heat water, it's still a low yield.

Evacuated tubes (wet or heat-pipe) can give you around 4 times that, and achieve decent temperatures.

Quote
I'd like to heat water and then back feed the excess through the old analogue meter and run it backwards  so what I make during the day can be used at night.

Of course, it's more difficult to run hot water backwards into the grid :)
Don't count on your old analogue meter staying there for too much longer. As soon as you start using it to YOUR benefit (like, reducing your power bill), they'll be out to change it over. Either because they think it's gone faulty (and reading less power than you're using) or that you're running it backwards (which they don't want!).

Quote
Using the grid for a power bank and getting a 1:1 feed / return ration although a little " grey" market is very efficient

There seem to be different interpretations of "net metering". A number of authorities add up the "numbers" and then charge you at their sell rate, or pay you at THEIR buy rate, on the net. That's not "net" in my view. Still, it's better than the alternative of selling ALL your power to them at their buy rate, and buying ALL your power from them at their sell rate!

Title: Re: Passive solar air heater - for home
Post by: eidolon on May 23, 2017, 09:24:53 am
Heating with PV may not be great area wise, but it is cost wise.  PV also works in the summer which the evacuated tube people never talk about.  If you don't keep the panel voltage at MPPT, you will waste half the power over the day. I do this with elaborate electronics, but if multiple heaters (as few as 4) can be switched in and out throughout the day more than 85% efficiency can be maintained.  See electrodacus.com for an explanation, I don't approve of his product at all.  I have a small efficent system of heating water with waste PV, enough for my limited use.  More interesting is my system works with only one car battery. It is only used for motor surge current and a couple minutes of running the fridge. I store cold with large fluid mass to keep the frequent cycling.  You could easily run the refrigerator all day with PV, switching in and out of grid power during the day.  All this takes intelligent control that isn't practical at this time, maybe in 15 years it will be common.   

Wife wants a dishwasher this year. Saying I thought I married one apparently wasn't a correct answer. So, I will fill and then heat.  When up to temp I'll start the cycles.  A cloud comes over and I will stop for a while then restart.  Might take twice as long, so what.

There is a big difference between possible and probable..
Title: Re: Passive solar air heater - for home
Post by: DJ on May 23, 2017, 12:01:14 pm

From a purely EFFICIENCY point of view, using all that roof space for PV, at a return of around 15% (by the time you take out inter-cell space, inter-panel space etc), assuming you then use restive heating to heat water, it's still a low yield.

I have looked into this and done my homework about as best as I can figure.
For me personally, there are a number of factors that make PV the preferred method....

As far as roof space, I went and looked at some options for a patio roof this afternoon. I'll have 40 SqM facing north with that, another 50 Sqm  on the shed and probably about 10 SqM on the house directly facing and 150?? sqm orientated east/west.  -I- am NOT going to be short of roof area in any way shape or form!! :0)
From what I can look up, a 10Kw system of 250W panels is only going to take 65 Sqm of roof.  Thats not going to be a problem and 10Kw is probably as much as I can use in reality.

The PV won't just be used for water heating. Once the water is up to temp I can switch that power to something else. I'm looking at a controller to say kick in the clothes dryer, run the pool pump ( when I put one it) run the AC or just back wind the meter for later use.
Far as I can see, when your tube heater had the tank boiling probably by 9am here in summer when it can already be 30oC easily, WTF is the thing going to do the rest of the day? Boil off water? Not even sure how they work once up to temp and with excess solar energy going onto them.
To me that's the thing, a solar water heater heats water, that's it. Nothing else. PV can do a lot more for me.

For heating water, with PV I don't have to change a thing.  Can use my current heater and for me, a bit of wiring is a lot easier and cheaper than plumbing. I'm thinking of running a 2nd heater as well. This could be run as either a full temp heater to boost capacity or as a dump load pre heater feeding the existing tank. I'm not sure how much hot water I'm going to need as yet. Our old heater died a couple of years back and as I had in mind to move, I put in a tiny one that I got cheap to get us by. We are all looking forward  to long hot showers and something we have never had before, a bath.
No idea what the usage will be as the "facilities" for bathing in the home are different as is the current family size.

Solar water heaters here are a LOT more exy than PV, used or new.  For me putting up PV and wiring it is also far cheaper than plumbing and running pumps etc. It's also less difficult from an aesthetics POV.

One other thing for me is Oil heating. I have built all sorts of weird, wonderful and powerful oil burners and converted gas heaters. When our electric hot water blew I plumbed up the oil fired gas heater and we used that for a few days. Once up to heat the thing could keep up with the demand indefinitely.
When push comes to shove, my oil fired heaters will outdo any form of solar heating and the gap gets wider the worse the weather. :0)

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Evacuated tubes (wet or heat-pipe) can give you around 4 times that, and achieve decent temperatures.

If I have 10 or even 5 Kw of solar on teh roof, I don't think there will be many days in the year I'll have to rely on grid power for hot water if I go a PV system. I figure about 30 a year max. With the feedback I can get the rest of the time, I think I'll be far better off in the big picture. If I double my tank capacity, I can probably go 5 days with water of sufficient temp before I have to boost from outside sources. That said, I also have my own generatorS I can and will hook into the grid power to backfeed or if I want direct feed. I'm going to set my lister genset up in the garage and put on a transfer switch so I will be self sufficient if need be and can generate power during rainy periods if I want to still offset my grid consumption.

Quote

Don't count on your old analogue meter staying there for too much longer. As soon as you start using it to YOUR benefit (like, reducing your power bill), they'll be out to change it over. Either because they think it's gone faulty (and reading less power than you're using) or that you're running it backwards (which they don't want!).


No, I'm sweating that's what will happen when the owners of the new place disconnect and I re connect. I have already said to the Mrs that I will want it in writing the meters won't be interfered with as a condition of who we go with.  They have wanted to change them here where I am and do the bare minimum to comply with the law as far as notice and compliance. The letter I got said they needed to do safety checks and Maintenance. A call and some very specific questions which they skirted around ( telling me they KNOW people don't want these things) uncovered that their real intention was to take away my spinny meter and replace it with a smartarse meter.
One " Supervisor" I spoke to whom thought he had all the pat answers, went on about saving me money and checking my usage at any time and all the rest of their predictable and parroted crap they use world wide.

I said to him, you really expect me to believe your company is spending tens of millions on equipment and installation that is going to loose them tens to hundreds of millions in revenue by reducing your customers bills when they could spend nothing and make more? Do you really believe that or is that what you have been brainwashed to believe or are you just hoping I'm stupid enough to fall for it?
Then I got the bit about being able to monitor my homes power use even from my phone. By this time I'm asking the guy if he's for real? I said I have better things to do in my day than check what time is the cheapest to do a load of washing.  And as for phones, if they don't have a 30" screen I'm not going to bother looking at them, I'll use my computer but again, WTF makes you think I want to be checking my power bill all day and have nothing more important to do with my time?
Good job this was over the phone and not in person or I may have been tempted to smack the guy for insulting the little intelligence I have.

I plan to cut a hole in the meter box, put a perspex panel over the meters so they can be read and then lock the box securely. Technically illegal here to deny them access however the amount of cases where they have illegally swapped meters with no notice would make any any action on their part a very unwise move in negative media coverage I would create let alone anything else. 


Quote
There seem to be different interpretations of "net metering". A number of authorities add up the "numbers" and then charge you at their sell rate, or pay you at THEIR buy rate, on the net. That's not "net" in my view. Still, it's better than the alternative of selling ALL your power to them at their buy rate, and buying ALL your power from them at their sell rate!

Yeah, net to them means net profit and nothing else.

A big flaw I see in this net metering is Hot water.
Here off peak around the country is 11PM to 5am.  where I am off peak is about 15C kwh all up. Where my father is it's about 20.
Almost universally, bar remote locations, the feed in tariff is 6-8C Kwh.

Therefor what happens is, you sell your solar at 8C kwh through the day but then even the cheapest buy back is 15C.  Great deal for the power companies.  NEVER seen them ever mention this little cash cow they have. Big Ra Ra about saving with solar but they have it sussed as always. They never make mistakes in the revenue improvement department.

Here I have the off peak connected and the heater coupled to the normal circuit. My current solar system is back feeding and winding the meter backwards so I get back what I give at the same rate which to me is only fair. Their malarkey about maintenance cost and distribution is just that, crap. It costs NOTHING to send the power I make next door or 10 houses up where it will be consumed because not everyone has solar and those that don't are more than putting in enough for maintenance.  Of course that's just domestic, what about the megawatts used by factories and office blocks and people in units?

The power companies post tens of millions in net profit per QUARTER here. Can't tell me they need the money for maintenance and I'm doing them out of income when they are making all that profit. They want to jump on teh green bandwagon ( again purely for profit) then you stick to it. I'm making green power, saving loads of Co2 and all that garbage and also saving the company a heap of losses in transmission by giving them something they are selling every bit of at a LOWER cost than their own wholesale.  If' its all about saving the planet and they charge so much more for " green " power, then why the heck aren't they paying me MORE for what I'm putting in not less!!

No, I have no compunctions about rewinding my meter what so ever.
The new place actually has one smart meter now on one of the 3 phases the home is wired into . Currently does the hot water peak and off peak, one leg of the AC and either the lighting or septic system.
 First thing will be to remove everything from that phase including the water heater  leaving only the AC which I need that leg being a 3 phase unit.  That's OK, I'm happy to pay what I am here, about 150  a quarter just to keep from having to go off grid and not to arouse too much suspicion. The rest I'll move onto the other 2 phases which I'll identify to back feed. What I have looked up tells me that the smartarse meter on the place will register any back feed as a cost so I'll ignore that leg of the supply although If I am running the AC I can measure the draw on that leg and feed a limited supply to offset the grid cost.

Don't think I'll need to though. Should have the consumption low enough and I don't want a zero bill in fact. 
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 23, 2017, 06:10:03 pm
Quote from: DJ
Are these people mentally impaired to think that a few tea light candles are really heating a room? Are they too stupid or just so wanting to believe it works to do some simple research as to how much heat a candle generates and not see that their plasma TV generates a load more heat and does it at a fraction of the cost of buying tea candles?

I doubt I'll convince you otherwise, but while I haven't gone along with the whole "tea light" thing in whatever cultish forms it takes, I can attest that the pilot light of a propane heater is a viable source of heat under the right circumstances. I'll add to this by saying I didn't set out to exploit it from the beginning, it was noticed almost as an "irritation" of sorts, until I found the conditions that it is most useful in, and set out to exploit *those*.

As for the TV mention, indeed this is just as true. And while mine is almost never on these days (probably going to remove it soon even), I exploit numerous other "waste" sources of heat to augment space heating in winter. Namely, the fridge, inverter, charge controller, and laptop.

The fact is, heat is heat. It's the "final form" of energy that it all ultimately becomes, and aside from a bunch of magic tricks to keep it under control... in places we want it and out of places we don't, it's there, and adds up. Don't just make blind assumptions based on some theories that don't add up in your head.

There's a lot of crap out there, I completely agree. There's also a lot of goodies wedged in the cracks here and there. I've played with countless ideas in the thermal department, some my own, some borrowed from elsewhere, and everything in between. To the same end, the results have been everywhere from worse than useless to beyond what one would think is possible.

All that said, I'll leave it up to you to figure out how a whole 25W of battery power at night can let me cut the main propane burner off at night and let the main box cool down to save energy (and by extension in that case, *money*), all while making the difference that allows me to remain cozy in the process. Try and bust it all you want and say it doesn't work, won't bother me, I not only know *that* it works, but *how* it works, and *why* it works, despite the fact that at a glance, it appears "delusional". The word I used in a previous post was "finesse", I meant and maintain it.
Title: Re: Passive solar air heater - for home
Post by: rossw on May 23, 2017, 06:22:14 pm
There may also be an element of scale at work here, Steve.
I suspect (no, I know) that your interpretation of space heating is different :)
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 23, 2017, 06:29:06 pm
Ok, yes, you're absolutely correct.

And I won't claim a propane pilot light or throttled back crock pot will heat a house... but one might be amazed at what can be done with not much.

The main thing is, "you know what they say about assumptions" ;D

Title: Re: Passive solar air heater - for home
Post by: eidolon on May 23, 2017, 06:43:41 pm
I made an apartment in a house I owned. In one room all I had was a thermostat relay, and a top burner element from a stove sitting on some bricks running on 120V.  The room was well insulated. The heat would not come on if I was watching TV.  That TV was about 150W.  It is all relative.  Your blue sky is not my blue sky.
Title: Re: Passive solar air heater - for home
Post by: DJ on May 24, 2017, 09:17:57 pm

I doubt I'll convince you otherwise, but while I haven't gone along with the whole "tea light" thing in whatever cultish forms it takes, I can attest that the pilot light of a propane heater is a viable source of heat under the right circumstances.

Actually that would take no convincing at all.  I'm not smart but I can do a bit of research on the net and what I have looked at  tells me that the pilot light on a water heater is worth a good number of KW per day.
I wondered about this when thinking about how the surface area in the heater which is used to transfer heat to the water could also radiate it back the other way and had a nice Chimney effect to do that. I pondered if the pilot light would offset that and tried to find something on it and there it was.
I forget the numbers now but I do remember it was significant and much more than one would assume.

If this was an indoor heater as my old aunt used to have and the room was small, then this could easily make a difference to the comfort level of a place.
Co levels may be an issue in a confined space depending on setup but that's another story.

And that's the thing, You don't have to be smart with this stuff, any pelican like me can look it up and find the truth even if it is different to what most people would assume.

One thing I see and strikes me a lot is with Slow Combustion wood heaters.  People often Duct the flue out through a wall as soon as possible or straight up through the ceiling.  No matter how efficient the firebox of the heater, there is still a HUGE amount of energy in the flue.  -IF- I had one of these heaters, I'd run the flue to the other end of the room and back along the wall and then go up. I'd also put a small fan blowing along the length of the pipework to make sure the max amount of heat was extracted.

I verified my belief on this last year. Went and looked at a house where the owner had ducted the flue straight up from the ground floor  through the floor in the first story and then out the roof instead of going out through a wall.  He'd put a piece of stainless ( nicely Polished) around it leaving a gap forwards and had a couple of small fans on the back of the metal.  The heat coming off that flue into the top floor was warming it beautifully.  I think a lot of people stopped and looked at that and got a bit of an education. :0)

Quote
All that said, I'll leave it up to you to figure out how a whole 25W of battery power at night can let me cut the main propane burner off at night and let the main box cool down to save energy (and by extension in that case, *money*), all while making the difference that allows me to remain cozy in the process.

I guess we may get into semantics here but on this we may have a difference of opinion or terms.
And I have not looked this up but pretty sure the numbers would prove that 25W, the  heat output of a tail light globe or around that of a candle, would not raise the temperature of a room in degrees as displayed on a thermometer on the wall,  even a single degree in an average 9x9ft room with typical and realistic airflow.

Now, If you are using that 25W in heated clothing such as they wear on Motorcycles or for hiking,  directly convecting that heat  to your person, Different matter. As you mention battery power and I don't take you to be a person that believes in any of this fantasy over reality make believe, I suspect thats what you may be talking about and it is a realistic and creditable outcome.

I have asked all these proponents of tea light candles I have engaged with to tell/ show me how many degrees they raise the temp of a room on a thermometer.  Not a single one has done that.  Warming to me is doing that, demonstrating a temp on a devise made for that purpose. A thermometer.
If the source won't do that then to me, it's not doing it, belief is. If I can stick a thermo in say electric clothing and the thermo goes up, then to me that's realistic and factual.  Electric blankets on beds are low powered as well but they too can keep you toasty warm at night and again, if you threw a thermo in the bed and measured the before and after temps you would see a difference.

Someone saying something does keep them warm but can't be demonstrated with a thermometer to me is just pure and utter crap.

I remember some years back going away for a baseball Tournament for my son. We booked this quaint little guest house just round the corner from the place that was also quite cheap. Unfortunately the place had no heating and the weather was getting down to a degree or 2 above freezing. Anymore than a single fan heater would trip the breaker and that wasn't nearly enough to heat the place to any level of comfort. After the 2nd time of resetting the breaker, I looked at the fuse layout and saw while all the lights and power were on one circuit, the Stove and bathroom heater were on separate circuits.

Came in, opened the oven door and turned that on and all the elements as well. Went in and turned on the bathroom heater and the place did start to become more comfortable.  After a couple of hours the place had come up to 20oC so I backed the stove off a bit and stabilised the place at that.
as said, heat is heat and it's cumulative whatever the source.


A friend of mine in AC was telling me about a chain of Jewelry stores he had as a client. He had done the AC in a number of their shops some years back but they were now getting complaints the places were too  hot even in winter with the AC on full tilt.  He couldn't understand it at first how multiple units must be failing all at once. 
When he went to look at the problem, he found they had remodeled said stores and were now running up to 20KW of incandescent lighting in these places completely changing the heat load he calculated when he installed the AC. What he had put in was now over taxed on the lighting let alone any other outside heat sources like people and weather.  He said he always leaves margin for the inevitable overly hot days etc but  more than doubling the lighting load was not something he took into account.
They didn't want to cut back on lighting or go to fluros so he installed another AC unit. Not cheap in a couple of the stores as new mains had to be put in to meet their power demands.

Hate to think what the power bill in these shops must have been!
Title: Re: Passive solar air heater - for home
Post by: rossw on May 25, 2017, 04:05:48 am
-IF- I had one of these heaters, I'd run the flue to the other end of the room and back along the wall and then go up. I'd also put a small fan blowing along the length of the pipework to make sure the max amount of heat was extracted.

I applaud your enthusiasm, but some of what you say is badly tainted with lack of understanding of real-world facts.
There is a reason gas-fired flues are ducted out as soon as practical, and although you may extract somewhat more energy, there is a limit.

Combustion products of many readily available fuels include sulfur and water. If you cool them too far, the water condenses from a reasonably safe vapour to liquid... and captures sulfur. Result: H2SO4. Sulphuric acid. It'll eat out your heat exchangers, flue, duct etc in no time. Oh, I suppose you could re-plumb it in something that won't be affected, but it's an interesting balancing act - withstanding heat AND acid. And cost effective. And not too high an expansion coefficient.

We need to balance all our grand ideas with reality and practicality.
Title: Re: Passive solar air heater - for home
Post by: DJ on May 25, 2017, 07:00:04 am

Yes and you can measure that Condensation point so you avoid it. Also burning good Timber with a low moisture content and burning it hot rather than having it starve for air and giving off tars etc also helps eliminate the problem. There are additives you can put on the fire also to help with the cleaning.
You don't have to have the flue stone cold but there is a heck of a lot more energy available than what most people harness.

As far as gas goes, Modern heaters to get best efficiency ratings must have a flue temp of 40o C or under. They often have PVC flues.

Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 27, 2017, 02:08:28 am
I haven't directly messed with high temp exchange for exhaust, as the truck uses a ventless system that is very efficient (tho I apologize up front, I don't reveal the details on that particular design, not to keep secrets, but because I'm breaking just about every rule for LPG, and while I have no idea the extent I could ever be liable, I'll let the "destructables dot something" work that out lol)... anyhow, not the main point...  ::)

It brings two things to mind... I'm not sure where the sulfur part comes in, but I can say "American propane" as I burn it, there's no sign of it in there. I only pick up on sulfur when I'm equalizing the batteries. There is however, definitely plenty of water... and I still need to work out exactly where it goes with the very latest scheme, but I can tell you that it no longer rains inside, at the very least.

That said, and really mostly to feed into the original physics of air to air heat transfer, I've found, much to my dismay in this case, that PVC can make a rather "efficient" air to air interface. I ended up exploiting and using it to my advantage, but as originally intended, part of its purpose was to circulate air from the floor to the ceiling, which it will do, but it's heat content is gonna change non-trivially by any stretch, if there's any kind of delta to outside.

Ok, so I told you that to ask you this... It would seem the trick would be then cooling the exhaust just enough to keep the poly safe, and then let the condensation "run wild" in a downhill slope... You're cooling the gas as it travels, if done right, I'd imagine both would flow just fine and leave the outside vent with all natural physics, so no moving parts either... The question, lol: "Yes?"

[MOD NOTE:]
I might see about forking this thread somewhere back because it's all good stuff, I'm all about deep critical thinking... but we've derailed this one pretty good, and I myself for one would actually be very interested in what might come of the solar-centric version originally posted as well.
Title: Re: Passive solar air heater - for home
Post by: DJ on May 27, 2017, 03:20:18 am

Condensation only occurs when one medium is cooler than another.
There may be some condensation at startup but that would quickly dry out as the flue in this case heats up. Same effect as a car exhaust.  Also like a car exhaust there is likely to be a layer of carbon on the metal which will stop or reduce the direct contact of any formed acids with the metal.

 I would also suggest that any moisture is in the flue and the heater and once driven out will not keep developing.  After warm up where the moisture is quickly driven out,  I see no process that is going to create a steady stream of moisture as seems to be proposed.
I know Brivis gas heaters have a flue temp of 40o c or under and don't create any substantial condensation and the heat exchange cores are made from Mild steel.  I had one which was many years old and it was in fine condition only replaced because of the better cost efficiency of reverse Cycle AC.
Any flue is going to get condensation on start up but in the as case of a car exhaust, no matter how much you cool it once running it's not going to be dribbling water like a tap.

Commercial heaters disprove the condensation acid theroy.
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 27, 2017, 04:17:54 am
You're forgetting a very key component of combustion, which very most certainly continuously produces water... the key attribute to most fuels we burn is that they are all hydrocarbons in various forms... looking up wood, specifically, I quickly found this basic core equation, via big G fu "wood combustion process":

Organics (CH2O) + O2 ?CO2 +H2O + Heat

This applies to all of them, appropriately, say, propane:

C3H8 + 5 O2 ? 3 CO2 + 4 ?H2O + heat

... you get the idea...

Yes, there is water, it is generated by the combustion process itself (in *addition* to whatever liberation from previously existing water trapped in say, wood).

You're correct in that the steam will not condense on a surface that is above a momentary sample's dew point temperature, but if you are talking about bringing superheated steam in exhaust gases thru a long pipe where it reaches room temp before it leaves the pipe, all of the moisture generated by the combustion that is taking place, will condense somewhere along the line as it loses its ability to hold the water anymore at the lower temps. Standard flue pipes, like car exhaust, don't stay cool enough to allow this condensation to form, with the one exception (as you said): when it is starting up cold.

If you took that exhaust pipe and extended out in a zigzag or even long straight line, eventually the exhaust will give up all the heat it is going to, (only limited by the outside temp and length of the pipe), and in the process, release all its water vapor because it can no longer hold it in the cooled stream.

What you then have is cool carbon dioxide + liquid water (ignoring any other additional components and contaminants that might be present as well).

That water will go *somewhere* and not if, but *when and as* the heat gets extracted, whether that's inside the pipe, or after it has left the vent and cools sufficiently while mixing with the cooler outside air.


Title: Re: Passive solar air heater - for home
Post by: frackers on May 27, 2017, 06:33:37 am
Its common here for a wood burner flue to have a clip-on heat radiator between the stove and the ceiling - mine certainly has.

I've seen older (euphemistically called character) homes with a log burner or open fire in each room that gets lit in April and burns all through winter to October without a break. In all of those, a chimney rather than a flue was the idea of the day (pre 1st war) which means there wasn't much gain from the exhaust gases (and being brick most fell down in the earthquakes!!).
Title: Re: Passive solar air heater - for home
Post by: eidolon on May 27, 2017, 08:22:59 am
Older than that. Pioneer chimneys were massive so they could heat up all day and radiate through the night as the fire waned without maintenance through the night.
Title: Re: Passive solar air heater - for home
Post by: DJ on May 27, 2017, 09:51:54 am
Older than that. Pioneer chimneys were massive so they could heat up all day and radiate through the night as the fire waned without maintenance through the night.

The russian Fireplaces being a prime example.
A ton or more of thermal mass designed to extract a lot of heat from the fire and store it.  A long criss crossing path for the exhaust gasses to traverse and give up a lot of the heat normally discharged and wasted to atmosphere.

I don't know if the emissions were room temp but they were certainly a lot cooler than in conventional designs and massively more efficient that's for sure. .
Title: Re: Passive solar air heater - for home
Post by: DJ on May 27, 2017, 09:57:47 am
but if you are talking about bringing superheated steam in exhaust gases thru a long pipe where it reaches room temp before it leaves the pipe,

No, not thinking of cooling it that much and I suspect it would be actually quite difficult to do that through a conventional flue due to the boundary layer of the gasses and other thermodynamics. I also think that would take an impractically long amount of ducting.
That said, there would certainly be a LOT of heat that could be extracted before the gasses cooled enough to generate the condensation problems.

As mentioned, commercial heaters MUST have a flue temp of 40o or below to gain the max efficiency ratings so it must be possible to get this sort of efficiency without major problems.
When I get a chance, I'll try and look this up to see what I can find. always intersting to find the facts on this sort of thing because rarely is it what you would think.
Title: Re: Passive solar air heater - for home
Post by: MadScientist267 on May 27, 2017, 02:35:52 pm
Thing is, you're talking about taking it down to 40C... and condensation happens *during* cooling.

All comes down to the amount of water produced... which for wood, again, I can't say exactly what the dynamics are, but with propane, it's non trivial. However they're dealing with it, the only thing certain in my mind is that they *are* dealing with it hehe
Title: Re: Passive solar air heater - for home
Post by: lighthunter on May 27, 2017, 03:03:07 pm
The heat exchanging isnt as difficult as building a combustion process (at least for wood) that burns very hot and clean. Clean burn is important otherwise heat xchanger will plug.  If i remember correct a company called "garn" uses a refractory ceramic tube to finish the hydrocarbon/oxygen reaction before water immersed flue in their owb. Probably the simpest design ive seen. I dont have so i cant verify. Many others use downdraft to get the combustion temps to 2500F, some of those can b a headache.