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Forum Index : Solar : Deciding if I should build this solar heater
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rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 04:49am 13 May 2021
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Forecast for today was for 17 and raining, and that is spot on.
There was a bit of sun while I was doing the school run but by the time I got back it was getting grey.
The air outlet was up around 27, but with the rain, then 24, 20, 18 ...
I decided to slow the motor more from its already low setting, so used the variac on 140 volts.
That's allowed the output to hang between 20 and 27 degrees, but obviously with a much slower airflow.
Just for some context ... on the normal low speed at 42 watts ... airflow is 3.0 cubic meters per minute and velocity 3.15 meters per second.
Run at 140 volts ... airflow is 1.5 cubic meters per minute and velocity 1.46 m/s ... so driven with 140 volts looks pretty much half speed for the fan.
It is so quiet now that even outside you can barely hear it ... and inside you only hear a bit of wind noise as it goes in the duct.
Of course, todays results are going to be nothing like yesterdays ... so will be interesting to look at the temps tonight.
Certainly it looks like controlling the fan speed in relation to the outside light will be the best way to keep the output temperature from dropping too low.
I did a power test mid morning on the pv panel and was getting 9.16 watts = 18.32 watts for two.
At 1pm it was 87 watts x 2 = 174 watts. Yesterday it was 289 watts.Cheers, Roger
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 11:38am 13 May 2021
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Well I'm back with todays update.
A bit disappointing though definitely not unexpected considering the lower temperature and the rain ... along will full cloud most of the day.
Probably stating the obvious but it's not the outside temperature so much that affects the performance ... rather the amount of sunlight available.
Once the sun appears between the clouds, temperatures rise quickly ... but go away just as quick.
Tomorrows forecast is for the same, 17 and rain ... but the next few days are 17 and 18 ... but with a bit of sun. That should supply results similar to yesterday so am looking forward to that.
In the chart below you can see the result of the lower fan speed from about 11:30 ... where there is a higher difference between the outside sun temperature and the output from the heater.
I'm guessing I can put that down to the lower airspeed and volume, allowing the air more time inside the tubes gaining more heat.
During the jump in output around 13:00 from ~27 up to ~34 there was only a one degree rise in the ambient temperature. No question it was just the sunlight through the clouds that had the affect.
Edit: The software has swapped the black and green line colours and I didn't realise it ... so it is the green line that shows the temperature of the hot outlet ... and the black line shows temp in the sun next to the array.
Edited 2021-05-14 15:53 by rogerdwCheers, Roger
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 08:31am 14 May 2021
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Update for Friday 14/05/21
First off, I weakened and lit the fire last night.
Made a big difference and the starting temp in the passage this morning was 19.0 (yesterday was 14.8)
Forecast was for 17 and rain so I expected similar results to yesterday ... but there was less rain and a few more periods of sunshine.
Coz there was a little sunshine when I switched on the fan this morning, I went back to 240 volts and used the low setting.
The graphs show the story.
Cheers, Roger
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 09:55am 14 May 2021
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I fired up the Diesel heater! :0)
The effect of the sun on the tubes is exponential. The more sun they get, the greater the temp difference they produce. Does not seem linear.
If the fan was faster than yesterday, the temps also seemed higher so that is good. Maintaing temp isn't a bad thing. The fan heater has been doing that well here and keeping it around 19-20. It hasn't had to offset real cold yet. As long as it does not get below 19-20,I'm happy.
Supposed to get colder here over the next few days, 1o on Sunday, 3 for the rest of the week but also supposed to be sunny. Hopefully the sun will be out for you too.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 12:35pm 14 May 2021
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Yes, I saw that, great news. I'll come over and ask some questions in your thread.
Yeah, almost seems like an amplifier.
And if the fan is off, they act like a capacitor or battery ... building up and storing heat energy. Then take a number of minutes to discharge once the fan is turned on.
Yes, it was quite noticeable that the sun was out a lot more than yesterday, even though the forecast was much the same as the day before.
Certainly looks like varying the fan speed can control the temperature ... but it's only the sunlight that can increase the power out.
If the assy was to stay on the ground, I wonder if a large parabolic reflector wing could be raised to reflect light back across the surface of the tubes from either side ... oposite the direction of the sun.
On the west side in the morning and the east side in the afternoon ... and both sides in the middle of the day.
That's good to hear. I've never really taken note of actual temperatures until the last week ... now I'm getting out of bed in the morning to check the thermometer ... walking around with a clipboard noting down the time and temps from a whole swag of thermometers.
Could certainly feel the difference between 14.8 yesterday morning and 19.0 this morning ... because of the wood fire in this case.
Haha, I never used to pay attention ... but ours is supposed to be 17, 18, 18, 19 over the next four days, so looking forward to that to see more output (I hope).Cheers, Roger
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 07:50am 15 May 2021
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Well today's results were pretty much as expected due to the low temp and clouds virtually all day. Only a couple bursts of sun in the afternoon.
Once again you can see the substantial difference between the temperature outside in the sun ... and the heat coming out of the outlet tube. A peak of 22.5 equates to a top of about 37.5 ... but again, because the fan is on slow the overall power is not great.
Tomorrow's forecast is for 18 and bit of sun with clouds... no it's not!!! They've changed the forecast to cloudy again.
Haha, never cared before, now I'm ticked off with poor weather.
Cheers, Roger
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 08:42am 16 May 2021
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At last!!! Today was pretty respectable in regards to heat output. The highest I've seen so far.
Had a high of 45.6 out of the collector late morning.
I had also been looking for the formula for determining power output in watts, from air velocity, difference in temperature between inlet and output etc ...
... but most that I had seen were using imperial units and farenheit ... and it was a pain trying to convert everything without introducing mistakes.
I finally found one that seems to make sense ...
Airflow in meters per sec x
Outlet Cross Section Area (in sq meters) x
Specific heat of air (~1000 J/kg-C) x
(Outlet temperature minus Inlet temp)
= Output Power in Watts
Assuming this formula is legit ...
... then the best I saw today was 1,181.96 Watts.
Not sure if this is good or not really. Have watched lots of videos on flat plate solar collectors now ... and when they do quote figures, they are quite a lot higher than mine ...
... and they are usually for simple boxes with either soft drink cans with the ends opened up and in stacks ... or with black metal flyscreen material draped inside to act as the collector.
I don't know if they are not being honest, being careless with their measurements ... or whether those collectors really are a better method than evacuated tubes.
If it is the latter ... then a big panel with flyscreen stretched across the works has to be so much simpler that several sets of evacuated tubes all assembled and hoisted into place. Mmmm!!
Cheers, Roger
InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178
Posted: 12:29pm 16 May 2021
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The dimensions of that formula don't add up to watts. Watts are kg*m^2/s^3. What you have there comes out to m^5/s^3...
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 12:40pm 16 May 2021
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I can think of one very easy way to sway the numbers legitimately... Latitude. People In brisvegas could probably do 50% better than you are getting straight up with the exact same equipment.
I would also tend to think that the influence of snow on the ground could help Tubes. I know with a bit of smarts it can help panels do well OVER rated output.
Yet another question that immediately comes to mind is the volts/ kwh equation. Maybe the flat plates can outdo the PEAK output of a tube collector BUT, what is the TOTAL output of each one over the course of the day? I can see where a flat plate could outdo the tubes at Midday but what about the 4 hours before and after? As I see it, sun angle is going to be far less important to a tube than a flat plate unless the flat plate is tracking... which would be a huge pain in the arse and an expensive pain in the hip pocket.
My thought here is what I have harped on all along :0) Surface area. The way I see it, the tubes are good at not loosing heat but still need surface area to gather it. I also see they are probably getting 50-60% Coverage of the area as you have set them up.
The enclosed designs would also probably be good at minimising heat loss and if they can say get another 30% surface area with only 10 % loss....
The other thing is if you are comparing the same collector area. I'm not sure if you are going by total width or width of tubes or even any similar measurement. I doubt you have the same dimensions as the flat collectors so maybe you have to work it out on a M2 Comparison to see how you are going?
I have found repeatedly many do exaggerate performance rather than what I perceive you do ( as do I) and tend to err on the side of understating rather than overstating. I have also found that some people don't deliberately lie but they do tend to forget or overlook outside influences that either add or subtract from the real numbers and it always tends to be in their favour.
With my vast and total non experience with these things, I find it a bit hard to imagine that these DIY boxes could outdo something like an evac tube when correctly compared. There is some science and precision put into those tubes, far more than I would perceive the same level COULD be put into home made devices.
The one thing I do know is real efficiency tends to suck. You may think you are getting a good result but good efficiency is a very evasive thing. I remember making Biodiesel and heating oil. I made a contraption that boiled it in 1/3rd of the time we were doing it before. My very learned friend sat down and calculated we increased efficiency from about 4% to about 14. Made a difference but still woeful in the real numbers.
Anything home built tends to have a lot lower efficiency that what people think and if you look at the modern car engine with the endless Billions that have been sunk into them over 100 years, they still suck in efficiency as well even though they have improved dramatically.
I don't see where your design could be loosing a lot of heat so would tend to think That there are some apples coming into the equation somewhere along the counting of oranges.
Again, double your collectors and you are getting 2.2 KW, that's going to make a difference. What you need might be different but I would suggest it's a brute force game and there is little to be gained in trying to increase efficiency, it's sq M that's going to be the factor.
Again, whether that should be calculated as overall width of the equipment or measured as combined tube width I have no clue but, either way I would suggest you need to be sure you are comparing the same thing even after all the other pervasive errors may have leaked in.
I was thinking of solar panels. Rather than duct the air around the back of the panel, would be easier just to insulate it to stop the heat loss. Bit of insulation batt held in by some plywood and just have the air blowing over the top. Yes, the panel -MAY- run hotter and loose some electrical efficiency but if there is enough air blowing over the top, what is the loss through increased temp going to be and is it worth the extra effort and cost in chasing it? Having measured panel temp laying flat on my corrugated roof and that of panels with a meter or more free air space behind them and the close ( 1 o or so ) difference , I'd bet a lot of money on the answer being NO.
Would be damned interesting to build one of these collectors out of a panel and test the combined output that's for sure.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 02:44pm 16 May 2021
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The thought did cross my mind today about all the snow and the extra reflection, though I never considered the latitude. That could make a big difference.
Yeah, good point.
I have no idea how to do it, but I probably could feed my datalogger data into a program where it spits out the instantaneous power level for each minute of the day ... at least when it's producing ... and put a figure on it.
I really need a couple extra loggers to monitor other variables to see if I can see anything worthwhile. Even just having one on the inlet manifold the whole time could be interesting.
Yeah I'm convinced of that now, and some of their panels are quite large ... so their actual total area is going to be much, much bigger than mine.
I think it was just a couple of the can ones that overall were not that big ... and yet their figures looked really good ... and that guy was very careful with his figures and explained very well what he came up with.
Having said that, his columns of cans were all touching each other and filled the whole area.
I wonder if the evac tube setups are spaced reasonably far apart because to have them touching would be a no-no ... and they have to have a reasonable distance because a water tank with a row of 60mm holes all adjoining each other would have no room for seals and would cause the tank to be seriously weak all the way along there.
There's always that temptation to brag about a result or enhance it, but it always show somewhere ... plus we know, so we can't fool ourselves
There are so many who quote really high temperatures ... but without mentioning an airflow volume, or air velocity and duct size ... you can not tell if it's good or not. Like your example of a match burning at high temp but with zero power.
That was my thoughts too, but I think you've probably hit the nail on the head with the actual area of collection surface ... and it's only the inner tube that has the absorbsion coating on it, so really very small area despite the overall area the array covers.
Yes, that's true. Really should be happy with what I've got
Yeah agreed again. I'm pretty sure I'll go three times the width. That's still only 54 tubes but potentially 3.3kW on a good day ... but only .75kW on a bad day.
Today has shown me that this thing is worthwhile. I stoked the fire last night before bed and where I would normally have stoked it up again in the morning, then throttle it back for the day ... today I let it go out and only got it going again this evening.
That's twice I've done that now since I lit it a few days ago ... so that's 30% less wood used already ... and the house is cooking.
Yeah, seems it's the only way to get some of these questions answered, try it and see. There'll always be someone who will tell you it won't work or you got the formula wrong. Cheers, Roger
InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178
Posted: 04:08pm 16 May 2021
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If you're referring to my comment on how the formula you found couldn't equal watts, well, too bad. It doesn't have the units to be equal to watts. Would you rather I lie and say "Good job! Spot on!"?
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:37pm 16 May 2021
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Measuring power output by doubling the temperature rise is fairly intuitive and easy to do, but it does not have to be double.
You could just as easily add one tenth of the solar power to get a ten percent increase in total temperature rise. Or feed in a constant heating power and work it out from the relative rises.
One Kw does not sound like a lot, and over a few minutes it is not a lot of energy. But over many hours the effect is cumulative. It only takes about five to eight extra degrees to make a significant difference to the apparent comfort level.Cheers, Tony.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 11:35pm 16 May 2021
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Exactly what I have seen with running the little fan heater in my large back room. It's not much at any given time but it does add up as at the end of the day you have put X ( 10-15 Kwh of energy into the space and if nothing else, it stops the temp dropping.
I would suggest it takes a lot less than that for the temp to drop to make a Huge difference to the UNcomfort level. :0)
Even with the diesel heater pushing out much more than a KW, the night temps I am seeing are still only 19-21 at best. Not exactly warm but, far from cold and I'm thinking the consistency and evenness of the temps it is providing add a lot of comfort as well. Had it running on low again last night and it just eliminates that cold morning feeling when one gets up in the early hours or in the early morning.
I can tell by the fuel consumption it's probably not even doing a KW on that lowest setting but it's certainly sustaining a very nice comfort level. Sun is out now and hitting the place so I can go turn it off till about 5 Pm then I'll run it flat out till about midnight and put it into slow mode again.
There are many Hundreds Of Kg of Cupboards, granite, appliances, tiles and cement Furniture and other things out there and in the surrounding rooms and they would require a LOT of energy to bring them up in temp. Keeping them consistent and not letting them cool even if not increasing their temp seems to be working very well for the comfort level we are experiencing.
I think the total amount of energy being put in there is significant at the end of the day and certainly the month BUT, it is very gradual and certainly from a solar perspective, easy to generate.
Having a kilo of solar heat going in there on the less sunny days would still be an asset in keeping the place warm ( or not letting it cool) even if it did not warm it to an increased temp per se.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 11:47pm 16 May 2021
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There will also be a drop in relative humidity that comes with the temperature rise, which also makes the room "feel" more comfortable.Cheers, Tony.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 11:58pm 16 May 2021
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Thanks, I'd never thought of that. Though I think I just need to rebuild the output mixing chamber and use a physically larger element, then it should work just as well as the original did on the smaller level.
Once I had the process worked out before, I was surprised at how consistant the results were.
The fact that the output can change drastically in just seconds means that the test has to be done quickly, or at least a number of times to be sure of an accurate result.
Yes, agreed. And this is really only to suppliment the wood heater and get more warmth to the cold spots further away from the heater, which it is certainly doing whenever there is a little sun ...
... so a full size one on the roof should be very effective and may possibly need to be throttled back on sunny winter days, but should still provide some useful warmth on less sunny days.
I found where to download historical data for our local weather stations and that includes the daily solar exposure ... so it's interesting to see those numbers and how they relate to my outputs. It's clear there are going to be 25-30% of days where I get very little ... but the rest of the time will definitely be worthwhile.Cheers, Roger
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 01:17am 17 May 2021
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Why so aggro so often?
Pretty sure Roger was referring to what I was being internet mentality with claims and measurement of things people have built ( or want to build) in general,not trying to have crack at you. Seems pretty clear to me that Roger is interested in getting the most accurate numbers he can and was not trying to dismiss your comment.
I will say it would have been better to point out in simple terms the measurements and calculations that would be required to get the power output. Personally, not being an engineer and struggling with basic maths at the best of times, Formulas are not much good to me. You have to spell it out in simple terms the dilettantes can understand and go away and apply.
I and I am sure roger are more than happy for any input and help given but no use getting too technical with things we can't understand.
I would like to have a crack at the solar panel air/ power collector but the thing holding me back the most is motivation. If I don't know how to measure the thermal energy coming out the thing ( which I don't), then the whole point of the exercise is futile if I can't get a remotely accurate number. Does not satisfy my own agenda to only be able to say " it feels nice and warm" without knowing what I'm actually getting and being able to relate that to numbers I do understand.
rogerdw Guru Joined: 22/10/2019 Location: AustraliaPosts: 852
Posted: 02:14am 17 May 2021
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Yes, I have been pleasantly surprised at how well the temperature is being maintained even with a fairly low input.
For us, the difference between getting up in the morning to 19 or 20 degrees instead of 14 or 15 is massive, though that improvement is because of the wood heater ... but it's great to see this air heater take over for the day and bring temperatures even higher ... especially knowing it's only costing the running of a 40-50 watt fan.
Yes, there's a lot to be said for a consistant temperature. At least you can dress for it and not have to go in and out of extremes of temperature, which I am sure cause a lot of the winter ailments for people. If it's too comfortable in the house, it's hard to get motivated to get back out to the workshop or go do some work in the yard.
It sounds really useful and obviously with a bit of thought you can get the best results without just running it flat out and burning through all your diesel.
Sounds a bit like what I'm trying to do here with adding heat during the day ... and then burning wood during the night when my air heater can't work. I'm confident I can save 30% of the wood I burn with my experience already.
I'd never really thought much about the furniture and cupboards etc ... really just the tiled floor, but that does make sense. I'll keep taking temps of the tiles in various places, but it's fascinating to see how much warmer they are already and the gradient across the floor.
Yep, agreed. And even if the temperature does go down, it's still going to go lower at a slower rate ... but then on the days when it does produce, look out!
I wonder the same thing.
Yes, correct. When I talk about what I'm doing to anyone, friends, family or technical people, I want to be able to put real and honest numbers on things, that I can back up ... not some exaggerated, made up BS that makes me look totally dishonest.
Yes, that would have been helpful.
I've listed my figures for one example. What is the formula needed for watts ... and do you need any other measurements to get to that figure?
Yep. I'm comfortable enough with electrical stuff ... but this is simply out of my experience level. To see a formula and then see it with real life numbers applied, gives me the start I need.
I'd certainly be interested to see the results too. Seems like it's the testing of the setup that needs plenty of thought beforehand so you can work out which way to go. A bit similar to what I'm looking at except the added part of seeing what changes with the PV output as it heats up or cools down.
I don't know if you have any temperature dataloggers, but they are certainly handy to have so you can sit back and look at the whole picture at the end of an experiment, rather than rely on notes taken during the event. I need more of them myself. Cheers, Roger
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 04:34am 17 May 2021
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How does this work Tony?
I thought about that with the fan heater Vs the better comfort levels perceived with the diesel heater brining outside air in but I could not see how passing the air over an element to change the humidity, just the temperature. The water far as I can see doesn't go anywhere, it just raises temperature probably briefly and then would reduce again.
I know from Drying oil, elevating the temp over atmosphere makes the water evaporate more readily but, if you don't replace the moisture laden air there is no drying effect. In a home very little of the air would be replaced and what was is likley to have the same or more humidity.
I was thinking the fan heater may be dropping the humidity which was why it was feeling stuffy, but again, could not see how that would occur?
InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178
Posted: 05:00am 17 May 2021
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Aggressive? Me? Not at all, you've read it wrong. Happens all the time in text, I know. And so often? I can't recall any time on this forum I have been aggressive.
But I don't know how to calculate the power output based on the parameters given. All I know is that the formula he found couldn't possibly give an answer in watts. I can't be helpful in giving the right answer, but I can at least inform when the given answer is wrong, which I think is also quite useful. It might help cut the time wasted barking up the wrong tree.
I thought I was being helpful by letting him know that whatever number that formula was spitting out wasn't in watts. In my life, knowing I'm not right about something has been at least as useful, and maybe moreso, than times I might have been correct. I apologize.
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 05:03am 17 May 2021
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The warmer the air, the more moisture it COULD hold. So if you want to dry something out, you heat up the air.
If you cool the air, the same amount of moisture is still there, but it approaches 100% humidity at some lower temperature, and the moisture condenses out as dew or as rain.
So if you heat the air in a cold room, clammy damp cold, then becomes dry and warm, even though the same amount moisture content is still in the air.
The heating and air conditioning guys are really into this stuff, as are meteorologists. Here is a psychometric chart that ties it all together:
At the top is the 100% humidity, or wet bulb temperature, or dew point. Along the bottom (dry bulb) temperature. Vertical scale is the actual physical water content. Curved lines are RELATIVE humidity.
The red lines show "typical" ambient conditions, 20 Celsius and 60% humidity. If the room cools to 12 Celsius humidity reaches 100% and everything feels very cold and damp.
If you fire up your oil heater to reach a room temperature of 35 Celsius, humidity falls to about 25% which is very dry.Cheers, Tony.
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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