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Forum Index : Solar : Deciding if I should build this solar heater
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Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 12:41am 12 Apr 2021
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A 30 tube heat-pipe system heating hot water will warm up approx 200L water to > 60c in a day, so that would give you some indication of max heating possible.
Bit difficult to couple porous fly screen into a water container, I have a 2M^2 x 10cm height flat timber box lined inside with shiny duct insulation and a copper pipe grid with copper foil soldered to it painted black, 1.5mm polycarbonate glazing. small magnetic drive dc water pump, that arrangement heats up a 180L HWC that supplies the kitchen tap and dishwasher. Water heats up > 60c most days, so flat plate collectors are easy to make and cost effective. A larger system on the roof 8M^2 using alloy flat boxes heat the main HWC, power is switched off to the cylinder 95% of the time.
fly screen type collectors heat the air passing through with a delta input\output of 10 to 30 degrees and can work with natural convection ie no pumps, the low temperature means losses are also very low; its all about volume of air being warmed, we don't want a little bit of high temperature air as the losses get too high in the ducting etc. But if you have access to cheap tubes, then its worth giving your idea a try.
Cheers Mike
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 04:42am 12 Apr 2021
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Thanks for that Mike, real world examples are a lot easier for me to get my head around.
And that output would be very useful for us too. We could do with proper hot water in the kitchen ... we have one of those instantaneous gas systems on the house ... but it takes so long to get through to the kitchen and we waste so much precious rain water in the process.
I did fit an instantaneous electric heater under the sink which helps, but still poor compared to the system on our previous 70 year old house.
Again, very helpful. Thanks.
Both collectors made by you?
They don't struggle in winter too much?
Ok, that makes sense. I've seen those set up on end walls in houses and workshops ... but most of them are in really cold places ... snow and ice ... so a bit hard to know how they would work in our more temperate climates.
I would expect much better of course ... but a bit surprised I haven't seen any examples over here.
Yep, probably time for me to put up or shut up.
I am slowly swaying over to fitting it on the tiled roof because of the tight fit on the side wall and shading issues ... so will probably need a complete new manifold design.
Thanks for your input.Cheers, Roger
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 05:44am 12 Apr 2021
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Yep made them all except for the alloy cases, the first timber one mounted on posts on the ground was the prototype, it works so well I haven't removed it, been there > 15 years, distance from the HWC hot outlet to the tap is 1M.
System on the roof, got a mate with a tig welder and a huge workshop to bend and weld the edges of the 2mm alloy cases. That system has been there > 10 years, no problems. Polycarbonate glazing will require replacing at some point, its an easy job to replace, cost approx $200 for the 6 panels.
Winter, no where as much sun input, have to occasionally turn on the electric element, its off 95% of the time.
I think if you can get cheap second hand PV panels, direct PV to the HWC element would be the best way to do it now, much simpler to do and less potential problems.
Cheers Mike
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 11:29am 12 Apr 2021
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That would have to give you a sense of satisfaction having them work so well and so long.
That makes sense, though I don't have enough experience with PV Panels to know just what to expect of them during winter.
We can certainly get second hand panels much more cheaply than even a year ago ... so really just a matter of adding more I suppose. And if I can make my Warpinverter work as well as I hope, I should be able to run quite a lot of gear.Cheers, Roger
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 11:31am 13 Apr 2021
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Mk
Many variables but expectaround 2.4 kwh a day per kw of panels for a due north facing array at 30o tilt.
5kw of panels is going to give you around 12kwh day in june in melb which is the worst month. Its not a lot of power to heat with even x 2 ....or 3.
Have a look at the Pvwatts calculator for variations.
Little off track but looking at the pics of the fly screens, I'm wondering how adapting solar panels would go?
Im thinking something like wood ( for insulation properties) 1x4" planks affixed to the sides of the panels and centered. Base under panel insulated like the screen heater with air gap, perspex/roof sheet over the top. Panel is the collector, air is drawn from one end over and below panel. Should heat the air and give some power.
Far as I can see much the same as the screen collector only with power as a bonus much like a co gen engine.
Thoughts on effectiveness anyone?
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 12:18pm 13 Apr 2021
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I think that idea would work well, pv panels are not very efficient, they heat up a lot, the more they heat up the less power they put out. Placing them inside a sealed case will allow the air from your house to move past them and effectively cool them down, you don't require much insulation as the moving air shouldn't get over 30-40c so the re-radiated losses are low. Just having a second skin sealed air layer on the bottom of the case below the PV, by stapling a sheet of shiny tin foil (or shiny Mylar green house film) stapled to 20mm bits of timber will work here. Still a bit of work to build it!
Cheers Mike
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 01:47pm 13 Apr 2021
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Yes. Would be difficult to handle more than a couple of panels put together as well.
Is there a way to roughly determine heat in watts generated if taken the same as power generated @ 1000 sqm?
I was looking at 12v car type heatersto run off the power generated. I doubt any of them would last long enough to repay their cost against the cost of just buying grid power for the same heat value.
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 01:09am 14 Apr 2021
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The energy out = In - losses, if the energy hitting surface of your panel = 1Kw M^2 and the efficiency of an air panel is approx 55% then that's what you will get, 550 watts odd. That being a combination of DC from the PV panel inside the box and warmed air. If all you are going to do is heat the air, then having a PV panel in there isn't going to do anything, however if you want some DC to charge a battery and get some warm air then that setup may be useful.
Cheers Mike
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 02:30am 14 Apr 2021
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Its actually a very interesting question.
Some practical testing may be called for. Edited 2021-04-14 12:36 by WarpspeedCheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 07:15am 14 Apr 2021
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Iwas thinking something like co generation with a engine, use the heat for warming the house or hot water and get the power as a bonus.
From what you have said, the heat from th panels would be the primary gain and the power would be the bonus.
I believe a 250w panel is 1.6 sqm. If one could get from your numbers, 880w worth of thermal energy plus the 200 (in real world numbers) worth of power, would seem to me one would be getting near reverse cycle Ac type efficency.
Allowing for winter,I'd still expect something like 600w+ peak total energy. 4 250w panels, 2.4 kw of energy.... quite worthwhile I'd think.
What do the clever and knowledgeable people suggest for heating? I was thinking some powe could be tapped off for running a Dc fan, could a cheap element be made from nichrome wire? I'm thinking a controller like the water heater types would be needed or ohm match the element to the panel array.
With the current weather here around Syd and the mid north coast with 25 o days and 5 o nights, I was thinking of just having a fan on a thermostat so once the outside air temp hits say 20, the fan blows warm air into the home to heat sink it... unless your house is kept above that of course. Mine has been dropping to around 17 so 20-25 would be a useful gain.
Low value heat but with my favourite, a car radiator fan, the volume would be very substantial. If I could warm up me ensuite a bit, that would be very worthwhile .
My parents had an evacuated tube water heater on the house for a number of years. A friend of my Dad was bringing them in from China so they ended up with it for cheap or free, I'm not sure.
The interesting thing is that it never had any heat tubes. The evacuated tubes were simply inserted in holes along the tank, and filled with water as the tank was filled. I guess it relied on a thermo siphon of sorts, as the water in the tube was heated it would rise out of the tube and draw cooler water in from the tank.
Although it did work, it wasn't as good as I thought it should be, given the reputation of evacuated tubes. I always figured having hot water rising out of the tube and cold water trying to get in at the same time couldn't be very effective. That being said, it was only pulled off the roof after a nasty hail storm smashed it up a bit, and insurance coughed up for a new one.
I'd never heard of heat tubes before reading this thread, so it makes me think that maybe it was supposed to have them but they were missing. I always though it would be better with a pipe run to the bottom of each tube, to force cooler water right to the bottom, without interfering with hotter water coming out of the top.
Its still tucked away here somewhere so I think maybe I should do some experimenting with some of the tubes.
All this is to say that maybe there's a way to heat water with the evacuated tubes, without the hassle of building 30 or more heat tubes.
Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138
Posted: 02:19am 17 Apr 2021
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You are correct, some very low pressure systems run the water direct into the inside of the glass tube. On these the tank usually lies on its side with a large connection manifold containing silicon rubber sleeves, the evac tubes push into the sleeves making a water tight seal. I have seen them used at dairy cow sheds sitting on a steel frame ground mounted as its not really recommended placing 300Kg odd of water tank on the roof. They don't last long, haven't seen one of those in years, suspect lying in sheds somewhere broken and unused.
That's pretty much the one I'm talking about in a nutshell. The tank on its side. It was on the roof though, as is the replacement, which is a more traditional style of collector. I was skeptical about putting the tank on the roof at first, although it does seem to be very common practice, at least here in QLD, evacuated tubes or otherwise.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 02:56am 17 Apr 2021
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Very helpful post thanks Jacob, I did wonder if they might work without heat tubes/heatpipes.
That was my thoughts too.
Yes, agreed 100%. And whether that is to heat water or air ... I think it would still have to work better with a pipe down the centre to deliver the colder fluid and allow it to collect heat on the way back out.
Yes, great idea if you have any ideas for getting some free heat. At least being free your initial experiments don't have to cost too much.
Having said that, if I do put plain 1 inch copper pipes down to the bottom of 30 tubes ... it's likely to cost me $700+ just for the copper ... so I have to be certain that is what's needed.
Of course I can use plain steel tubes and can get the price right down ... but not quite sure yet. Need more experiments first.
Agreed again. I do have a gut feeling that heat tubes help provide the highest output for an evacuated tube system ... but in my research I've seen them built as you described ... and also with a continuous copper pipe that travels to the bottom of a tube ... does a 180 (very tight bend) and returns to the mouth and on to the next tube.
Disregarding the efficiency of heat tubes, they also have the advantage that you don't need to worry about sealing each evac tube against water leaks ... though obviously that has worked ok for your parent's heater.
In fact, being able to avoid dealing with seals between evac tubes and the tank may be the main advantage of heat tubes ... and not their efficiency
... plus with heat tubes, the ends can plug into the mainfold without any risk of leaks, because the heat is transferred metal to metal and not metal to water. Makes building a heater so much easier and hopefully leak freeCheers, Roger
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 03:06am 17 Apr 2021
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Thanks Mike, that makes sense that it could work with low pressure otherwise with high pressure you could have leaks everywhere.
I also had considered having the evac tubes on their side but in a vertical stack ... all plugged into a manifold ... allow the cold inlet at the bottom and draw hot air off the top ...
... though as Jacob already pointed out, I think it needs a tube to the bottom of the tubes to allow cold air to travel in and gain heat on the way out.
I guess I should try that with a couple tubes.Cheers, Roger
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 03:42am 17 Apr 2021
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To conduct the heat out of the collector and into something else may only require a solid metal bar. Not as efficient as a heat pipe, but it should work. Aluminium is cheaper than copper, but it might be worth a try. Solid metal bar is rather expensive though...Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 07:02am 17 Apr 2021
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Perhaps a tube would be better and give more surface area? I think a hex is best of all.
From memory, the heat conduction of ally and copper are very close anyway.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 07:20am 17 Apr 2021
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Yeah, still trying to work all that out.
I know if I stick my finger in the open end of the tube it can feel nice and warm ... but if I touch the inner surface my finger comes off real quick. There is certainly a lot of heat in that inner surface. I've seen 200 degrees C on a warm day using a traditional glass cooking thermometer.
Of course if there was a lump of metal in contact with it, it probably won't be able to maintain that temperature ... depends on it's mass ... and how quickly the heat can be bled away.
Just to change the subject ... what are the best sort of temp sensors to use for any testing.
In all my years in electronics, it's not something I've ever really had much to do with.
I could do with some thermistors I suppose, on the end of a lead so I can monitor temperatures down inside the tubes and also the airflow temp in various places.
I have a basic infra red sensor, but that can be very vague at times. Ok for measuring a surface temperature up close ... but can be all over the place.
Being able to record and log a number of temperatures at the same time would be very handy. Better than the guess work I've been doing so far. Cheers, Roger
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 10:13am 17 Apr 2021
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Doing a bit of mental arithmetic wrt the tubes on their own...
If they were filled with water and nothing else in them then i see the water heating up. It will have to rise, convey the heat to cooler water or it will heat till it turns to steam... which I dont think is at all likley.
Given the assumption of one of the other 2 scenarios, once the heat is in its going to be difficult to get back out through the tube which is the reason its vacced. Like the window scenario Mike explained, semms oncenthe heat is in the tubes,its in and the water is warmed one way or the other.
It the water is stagnant, the only way it can dissapte its energy i can see is one of those 3 ways. In that, the water can only get to 100c and then the energy is passed on. The tubes may run hot but the temp is limited and the losses as such can only be the amount of energy it takes to heat the waterbin the tubes to a max of 100c.
Other than that, the energy has to go somewhere and if there was colder water circutating past the end of the tubes I feel it would end up there one way or the other.
Thats my theroy anyway.
Thinking it through with air, a quandry. On one hand i see the working medium being little different as i believe thats the scientific principal. Air could get much hotter but is also a lot less dense so the " retained" energy in the tubes would probably be similar. At some point I see the air reaching a critical temp where it either thermosyphons or conducts the heat away. I cant see it sitting stagnant indefinantly.
Using tubes within the tubes might be as simple as bevel cutting the ends on opposite sides. If the top of the insert tube was faced toward the incoming airstream, ti sgould cause a positive pressure inducing the air down the centre of the tube to the bottom. The heated air at the bottom would want to rise creating a negative pressure like a chiminey pulling cool air down.
The area at the top of the tube behind the bevel cut would also be a negative pressure zone further aiding the warm air rise from the bottom of the tube.
The bevel cut at the bottom end of the insert would keep it off the bottom of the gas tube so the air could flow and may help seperate the airstreams to each side. The angles may want to be at 90o so the top facesvthe incoming air flow and the bottom bevel is to the top of the tube where gteatest heating and natural rise will be.
Hope that is understandable.
Both the water and air heat paths seem Would seem viable in my tiny brain. I have setup Themosyphoning systems on engines and it is far more powerful than i would have ever given credit for previous to doing it. With a bit of aerodynamics thrown in to aid the gas flow scenario, I see it as a creditable working soloution.
Interested to hear what the knowledgeable people think.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 11:59am 17 Apr 2021
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I'm sure air and water would have similar flow patterns, just don't know what they'd be.
I hadn't thought of the beveled top, that's a good idea.
I started off thinking that maybe the venturi effect might come into play, but that seems quite different to what we have here ... especially seeing we cant open up the bottom end and let air in from there.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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