I've been doing a bit of thinking about solar air heaters as a way of compensating for my houses limited ability for passive solar heat gain in winter. There are some commercial models available that seem to use relatively low tech solar collectors to heat air before pumping it into the house. I was wondering if the evacuated tube collectors might be one path for improving the efficiency of solar air heaters? I was thinking that the heat pipes within the evacuated tubes could have a heat sink mounted on their inboard end inside a manifold with airflow provided by a low voltage fan. Obviously such a system has no storage capacity and can only work when the sun is shining but in situations where direct gain is difficult (ie existing building)I figure an external collector might be appropriate?
Evacuated tubes are becoming cheaper all the time (particularly those that don't qualify for recs/rebates) and offer a high temperature heat source. I haven't found anything on the web - maybe I'm using the wrong search terms. Has anyone done this before? Did it work?
Solar Air Heaters - Anyone tried evacuated tubes?(54 posts) (20 voices)
I've been doing a bit of thinking about solar air heaters as a way of compensating for my houses limited ability for passive solar heat gain in winter. There are some commercial models available that seem to use relatively low tech solar collectors to heat air before pumping it into the house. I was wondering if the evacuated tube collectors might be one path for improving the efficiency of solar air heaters? I was thinking that the heat pipes within the evacuated tubes could have a heat sink mounted on their inboard end inside a manifold with airflow provided by a low voltage fan. Obviously such a system has no storage capacity and can only work when the sun is shining but in situations where direct gain is difficult (ie existing building)I figure an external collector might be appropriate?Posted Thursday 4 Jun 2009 @ 2:57:14 am from IP #
Great idea munter.
Having seen a dismantled evacuated tube system at DesignBuild in Sydney last week I was very surprised how rudimentary the heat tube to manifold connection is - yet as many members have commented they still supply hot water in cloudy conditions.
Setting up a solid to air heat transfer system in a large insulated manifold would be simple enough, and could easily be hidden in a simple non-structural roof modification if desired. (I've used these to hide close coupled solar HWS tanks)
As the downside of most existing solar space heating technologies is limited output in dull conditions, ET systems may well be the answer, if the house wasn't too heavily massed. (Note that I'd still be hesitant about ET hydronic slab heating though, just because there would be so much 'start-up' energy required to make much difference in such a huge amount of thermal mass.)
ps Had a look at your blog the other day, as an 'ex' sailor, Cole 43 = nice ride.Posted Thursday 4 Jun 2009 @ 4:54:09 am from IP #
I posted this thought in another ATA thread, but here it is again (modified) for Munter's consideration:
Another angle is to combine evacuated tube water and air heating. We have evacuated tube hot water heating which has a small pump that pushes water through the heating chamber connected to the tubes. Im planning on putting a junction in the return hot water line and diverting through a radiator core in the living area. When the radiator tap is open the heated water will pass through and back to the storage tank - and the thermostat in the tank will keep calling on more heated water from the collector until it comes up to the pre-set temperature.Posted Monday 8 Jun 2009 @ 12:24:46 am from IP #
I've built my own solar air heater just to heat my home office and it works OK, but its a bit of an effort installing the ducting etc to push that warm air from the roof down to floor level - big holes through the roof, ceiling etc. I think the attraction of heating water and then pumping that down to floor level and through a radiator would be much easier. The piping can even come down outside the house or through a double skin wall to ground level and so be obtrusive and low power circulating pumps + controllers are all available.
I was also thinking of running an evacuated tube system just to collect hot water for space heating but it would be a bit of a waste here in Perth where we only really need heating for 3-4months. Our old flat plate solar water heater works fine in summer so the ET system would just get covered up. The "chinese" ET systems that were around on ebay for a while seem to have disappeared. I've got another project with a parabolic trough water heater half built to provide the hot water to a radiator but having trouble sourcing good quality mirror film - as other have noted - to get a radiator(really a convector) system to work well the supplied water should be about 60-70degC
Has anyone done the basic calculations on how much heating you could get ? e.e. 2msq collector, at say 50% efficiency, collects a max of about 1kW, but only maybe 4-5kwh per day - how much air could that heat from say 10 - 25degC ? ( I realise it has to heat up the house + contents as well).
I see theres another air heater on the market now - SolaMate - but as usual no mention of prices on their website.Posted Monday 8 Jun 2009 @ 1:24:12 am from IP #
Thanks for the posts. I'm actually looking to heat air directly directly off the heat pipes contained within the ETs and avoid the use of water as a transfer medium. I want to do this for simplicity and cost (both upfront and running) and because I can locate the collection panel on a north facing wall directly outside the room that I am looking to heat. The ductwork should be short and should not have excessive heat loss.
As you say John, the connection between the heat pipes and the water manifolds don't look particularly efficient. Perhaps without the need to create a water tight manifold it would be possible to have more efficient transfer of heat to the air.
If the system works then perhaps I could look at expanding it to one which uses water storage to allow a degree of decoupling between solar input and the heating of the house. It would involve a lot more equipment and more complex control mechanisms. I'm wary of jumping to that level of complexity for the first experiment.
We're looking at wall mounting both for proximity to the window through which the heat will go and because it should also facilitate mounting the panel at a steep angle to optimise heat gain in winter and hopefully reduce it through summer (though we may cover the panel anway).
JohnB - the Cole was a beautiful, but quite wet, boat. It's a pity it took 4 people to sail her because of the large headsails. I'm leaning toward smaller and lighter craft these days which can be handled by two.Posted Tuesday 9 Jun 2009 @ 3:22:57 am from IP #
I'm interested in the concept of a North wall between a cold room and the sun.
After your original post I assumed you were talking about a roof mounted system. If the wall has adequate sun to consider a wall mounted ET system, and a normal window has obviously been ruled out for whatever reason, aren't there easier options possible?
I've been looking at a Sydney project with such an issue - overlooking from next door being a problem there. I proposed a pair of long high level windows (to avoid shading from the neighbours' roof) with external louvres pointing upward at 32 degrees (local Winter sun angle). The louvres would give privacy from next door while allowing full Winter sun straight in. In Summer the angled louvres work like a vertically mounted solar pergola, combined with a lightshelf system, giving some reflected light while keeping direct sun out. (Frosted glass is not a good privacy option for large living space windows because it looks way too glary with the sun shining on it)
If the wall can stand an angled ET system, clearly it's not an appearance issue.
If not a window, what about something half way to a Trombe wall conversion. A clear 'glazed' panel external to the wall, about 200mm off the bricks. The wall locally painted a dark colour, to maximise solar gain, with the warmed air lead indoors either via the top of the existing window or through openings?? in the wall. (I understand your point about using a 100mm coring bit on a double brick wall, no wonder they charge for it!!)
Internal air could be supplied to the bottom of the collector either via floor return air grilles and ducting below floor level or openings at skirting level directly into the enclosed collector. Bridging the cavity in the wall without causing moisture issues would take some attention.
The output air wouldn't be as hot as an ET system may supply, but the collector is cheaply built so the area could be adjusted to suit.Posted Tuesday 9 Jun 2009 @ 7:14:05 am from IP #
The solar aspect of the wall is very good. The ground slopes downward to the north so the neighbour's house does not shade ours even in winter. Because our block also slopes downwards to the north the floor level on the northern side is about 1.5m off the ground so that wall is quite large. The sun hits this wall all day through winter. A photo would probably help here. The wall is double brick (no cavity insulation) and doesn't seem to transmit that heat inwards. Like the rest of the house, the wall is painted white which probably reflects a lot of the heat that could potentially be captured.
I agree that there could well be other solutions and would be interested in investigating them further. My interest in evacuated tubes was driven by a general interest in the technology and the thought that I could potentially make the solar air heater completely removable and avoid any permanent modifications to the house (which, if experimental in nature, are viewed poorly by the significant other). In keeping it a separate unit I also figures that I could complete the work myself rather than requiring a builder to make modifications to the house/wall.
From a cost perspective I'd also figure that a removable unit would be the most effective solution. If I can source the evacuated tubes and heat pipes for <$500 then, with a degree of recycling, the rest of the materials shouldn't cost more than $2-300. That sums to a lower figure than I had mentally assumed for modifications to brickwork and the installation of windows, DA's and architechtural advice etc. Does this sound reasonable? As a further benefit, if the system doesn't work and I haven't hacked it into ugly pieces some of the equipment cost might be recoverable through the sale of the components/tubes etc on ebay.Posted Thursday 11 Jun 2009 @ 1:49:55 am from IP #
I totally understand about recieving 'domestic' permission for upgrades, I have a wall here I'd like to play with but have as yet to make a successful case.
In you line of work I'm sure you could handle the drawings for a window addition, and the new NSW State Housing Code is intended to remove the need for DAs for simple additions or renovations. Perhaps a call to a local Private Certifier would clarify if a window addition was a simple thing or still a DA quagmire.
If the window option is out, your wall sounds like a beauty for a collector. There is a cheap and nasty way to trial one, in fact I was up in the loungeroom contemplating it at cuppa time this morning.
The ideal Trombe style collector takes the coolest air from the building, warms it and returns it to the building. Internal air is used where possible, because it's (hopefully) a bit warmer than external air when the sun gets up in the morning. A Trombe wall also importantly warms the masonry wall itself, so that the thermosyphoning continues after the sun has gone off the wall. In the evening, the closed off collector helps to keep the warmed wall warm for a bit longer than if it was an 'exposed' wall - in other words it functions like a mini attached greenhouse.
The glazed enclosure helps the thermal mass of the wall behave like 'internal' thermal mass, which is good, as opposed to external thermal mass which does little in Winter.
If your proposed system was to duct heated air in through the open top of an existing window, a trial collector could easily be mocked up on the wall, perhaps both sides of the window, with little more than a couple of plugged screwholes as permanent 'damage'.
If painting the area of wall a dark colour is too much like a permanent thing, tightly tape a black plastic sheet in place. Attach some sides of whatever material, 190 x 35 pine or something using either angled brackets or a batten screwed to the wall. 'Glaze' the face of the collector with clear plastic sheet or 2nd hand polycarbonate (from the tip shop) or something. If using clear plastic, the proper stuff from the greenhouse supplier lasts for ages. (we got about a year out of normal hardware shop stuff on our greenhouse) It should be fastened through timber battens to avoid point loadings. Two layers of plastic a batten thickness apart may help keep the warm air warm before it can go inside.
The big issue is going to be getting the warm air into the house. When the system is connected to indoors at top and bottom there are no pressure issues to overcome, so as soon as the collector gets warm it starts to work. (Simple flexible flap type non-return closures prevent it working in reverse overnight)
A trial unit using warmed outside air is handicapped from the start by the lower temperature of the incoming air, then it has to 'push' it's way into the house as well. Maybe it would do nothing until another small window somewhere was opened to let the exhaust air out.
I'm not sure if it will work or not - perhaps I should try one and find out. My place has the 'benefit??' of an easily access partially open crawlspace, so I could duct air from the coldest end of the house (eg. a return air grille in a linen cupboard floor) to the bottom of the collector.
Email me a picture or two if you want ([email protected]) or you can call, I discuss possible efficiency solutions with folks all the time.Posted Thursday 11 Jun 2009 @ 3:48:09 am from IP #
Given the cost of evacuated tubes, wouldn't it be more cost effective to install full blown skylights? You may have to modify your ceiling but may prove to be more effective.Posted Thursday 11 Jun 2009 @ 6:27:06 am from IP #
Hot air rises and skylights are up in the ceiling, so I don't think that skylights will provide any useful heating, as most of the heat will stay up near the skylight tube.Posted Thursday 11 Jun 2009 @ 7:45:10 am from IP #
I'd be very interested in how things progress, as I've been thinking about almost exactly the same thing on a north-east facing wall at my place. Last July I installed ET solar hot water and have been quite impressed - given how far south I am, and have been thinking it might be an option for additional space heating - boosting electric panel heating I already have in my house. Unfortunately the house is an old one with wood floors, so thermal mass is low.
I have also recently installed an HVS (roof ventilation system) which has added some heat to the house when it's sunny, so the solar wall option (Trombe) could also be a goer. Still think I prefer the ET with attached free-standing radiator option though, since we like 'sitting' on the heater!Posted Thursday 11 Jun 2009 @ 9:06:26 pm from IP #
In repect to solar air heaters, have you considered of the shelf product with proven specs and history. The Solarventi is a product now in NSW which gives fastastic heat input but also great de-humidifying properties. Completely isolated from 230v, it runs completely on solar via a pv panel and fan. E.Posted Sunday 14 Jun 2009 @ 11:40:53 am from IP #
Eamon - when you say it gives "fantastic heat input", can you quote some figures for us please ? Are there any sample installations that have been logged ?Posted Monday 15 Jun 2009 @ 12:22:54 am from IP #
There are a couple of off-the-shelf options for solar air heaters. I think they all look pretty low-tech (which isn't necessarily a bad thing!) and typically heat large volumes of air relatively modest amounts. Part of my rationale for using evacuated tubes was to attempt to make small/medium amounts of quite hot air rather than a larger quantity of mildly warmed air. This way the heat might not be accompanied by a feeling of draftiness. I might be barking up the wrong tree here.
JohnB - I've taken some photos of the side of the house. I'll grab them out of the camera tonight.
Thanks for all the suggestions.Posted Monday 15 Jun 2009 @ 3:38:58 am from IP #
Yes Benny, there have been several tests done-mainly in Denmark. The product has been around for over ten years. On the smaller units the temperature increas has been record at 15 degrees above room temp and withe bigger units 30 degrees above room temp. More information and specs can be found at http://www.solarventi.com.au.
Munter - My worry and issue you will have is the recirculation of the internal hot air through the tubes by drawing through the tubes and if it touches any cold bridges, any moisture will condensate. You then increase the probability of bacterial infections and micro-organisms growing, also you are re-heating used air. E.Posted Monday 15 Jun 2009 @ 1:04:22 pm from IP #
If you have got into a car that has been sitting in the sun even in winter this can be enough to take the chill off the air.
If a low tech solution (cheap) with a large collector or high tech (expensive) solution with a smaller collector will give a better result and better value for money could depend on a lot of things.
However the low tech solution is commercially available (and so easy to copy if you are that way inclined).Posted Tuesday 16 Jun 2009 @ 1:47:09 am from IP #
Back to the original question, evacuated tube collectors are designed to get a small volume of water to 70+ degrees C. So modifying this to heat air would mean adding a larger manifold above the collectors with heatsinks. I doubt this is the most cost effective solution. Given that you want air at a much lower temperature, I would expect that something like Sunlizard or Solarventi to be a cheaper way of heating air.
A dual collector Sunlizard costs $3500, with 3.7m collection area. In Melbourne you would gain about 11kWh per day. Each collector is effectively 1300W.
Using 40 evacuated tubes gives about the same area, and costs about $4000 (Based on 30 tube retrofit kit for solar hot water costing $3140). So similar price to a Sunlizard, but the one-off conversion to air heating will push the costs higher.Posted Tuesday 16 Jun 2009 @ 2:05:41 am from IP #
Here is another player in the Solar Air Heating market. http://www.sola-mate.com
It seems to address some of the issues raised here.
"The SolaMate panel is designed to integrate seamlessly with the HRV ventilation system. The heated air is fanned through a medical grade filter and ducted throughout your house"Posted Tuesday 16 Jun 2009 @ 3:30:27 am from IP #
Ghostgum - thanks for the pricing numbers. They make for a useful comparison. I'd go for a lesser product that the hills upgrade kite which could reduce the price a little but I think your comparison is still valid.
I've put a few pictures of the wall up on my blog (sorry for the blatant cross selling). The wall space is quite large and looks like it could allow quite a large heat collection area. I'm also considering putting a solar hot water heater in the area so that it needn't be moved if we do a second storey renovation.
Blog post & pics can be found here:Posted Tuesday 16 Jun 2009 @ 3:59:01 am from IP #
Munter, see JohnB's comments above about a Trombe wall.
I've read stories about something similar to this being used on a commercial building with a double skinned steel (corrugated?) cladding. For a retrofit, I think you would put RFL on the outside of the existing wall, then put dark coloured colourbond or similar spaced off that a short distance, such that it draws air from the bottom of the sheet, and you collect the hot air from the top. It would radically alter the look of the house. But if your building was a Bunnings warehouse you could do this to the building without anyone noticingPosted Tuesday 16 Jun 2009 @ 5:27:10 am from IP #
Your wall has 'large low tech collector' written all over it (assuming as previously that more windows are out)
The bricks would make a beaut pile of usable thermal mass in a dark colour, and the duct across the top could be lead inside via the top of both of those windows.
Another small side benefit is that any correctly sized 'duct' at the top of the window would give some hot season shade that is otherwise missing.Posted Tuesday 16 Jun 2009 @ 8:13:20 am from IP #
Thermal mass helps if you want to keep using the heat into the evening, but it reduces the peak temperature and you don't get usable heat until later in the morning.
One cheap trial would be 3 layers and some timber battens. RFL against the wall, then black builder's plastic spaced off that, then clear plastic spaced off that. Leave it open at the bottom and connect the top to a fan. Measure the temperature at the top, and when it is above 20C turn on the fan to blow the air into the house. The plastic will crack and fall apart within 2 seasons, but you'll learn whether it is worth making one with more robust materials.Posted Tuesday 16 Jun 2009 @ 11:43:24 pm from IP #
The thermal mass issue is an interesting one. I guess the mass would take a longer time to heat up and also give a longer cooling down period. It would also expect it to have lower peak temperatures than an evacuated tube system (I'm a bit hung up on these).
In cloudy weather I can see the heat output fluctuating a lot more on the evacuated tube system which would have lower thermal mass. I suppose through the addition of material it could be made to have a higher thermal mass which might balance out some of the variation.
The commercial offerings look interesting. I know that design, transport and the cost of doing business all add up but the cost for an off-the-shelf unit look high enough to push me to a DIY approach.Posted Thursday 25 Jun 2009 @ 2:59:12 am from IP #
This is a really interesting subject. I have just started some developments and I am importing the required materials.
1. Evacuated tubes should make a great air heater. However there is some limitation with the air flow because the manifolds for heat pipe connection are too small for a good air flow. But there is a cheaper and better solution: The simple evacuated tube collectors without heat pipe have a much wider manifold. Only the two end sides need to be removed and connected to an air duct. This gives an inner channel of at about 8x8 cm. These collectors are about 60% of the cost of a heat pipe system. (I sell a box with 10 spare tubes for $100.) The heat transfer should be excellent. I expect that the collector creates a thermal siphon effect inside the tubes just as it works with water.
2. Another option is to connect a solar hot water system to a circulation loop with a radiator for space heating. This is possible if the radiator is internally just a copper pipe as it will not deteriorate the quality of the hot water. I am importing such radiators and will soon begin installing the fist system at my home. To have sufficient capacity for a rainy day I have upgraded my tank volume to 1000L. A radiator space heating is especially well suited for the upgrade of an old house because in most old houses it is possible to run the heating loop to and from the collector underneath the floor. It should also make a lot of sense to use a heat pump hot water system as the hot water supply. This will give space heating for about 1/3rd of electricity cost, similar to reverse cycle air condition but with much better air quality.
If you like to co-operate in any way, let me know. I am looking for pilot projects.Posted Sunday 28 Jun 2009 @ 8:31:54 am from IP #
Ok, munter seeing as you are still seriously considering the use of the thermal mass way of heating. It is called "trombe wall heat storage". To save you all the hard work in doing product collecting, thermal calculations etc. Here is all the homework done for you ... Happy reading. Let us know how you get on, Eamon. http://nmsea.org/lib/ThermalStorageWallDesignManual.pdfPosted Tuesday 30 Jun 2009 @ 11:44:40 am from IP #
Just to add a bit of an aside to the discussion of options, I found this on the web:
It's interesting, because it has a lot of the properties of evacuated tubes (cylindrical shape creating larger surface area and better sun exposure), but is still an active air heater. Looks remarkably simple, but should be quite effective when mounted on a winter sun wall.Posted Wednesday 1 Jul 2009 @ 3:39:49 am from IP #
Sorry to be negative about this ChrisB but I think we have to be way of hype here.
It has only 1 property in common with evacuated tubes - a cylindrical shape - which is not the most important property of ET's.
It "creates a larger surface area exposed to the sun". This is a bad misconception. The cylindrical shape allows the exposed surface to be "more perpendicular" to the sun, which is better than being inclined as in a flat plate, but it does not increase the exposed surface area. The amount of energy falling on the system is the same for a flat plate or a cylindrical collector array - its just the flat area of the system multiplied by sine of incident angle.
This system sounds like a commercial form of the many "beer can" heaters out there as DIY projects.
I'm astounded by the claimed peak output of >2600W. It has an area of 2.6sqm, so thats 1kW/sqm - in Newfoundland, in October ! The max solar insolation is only about 1kW/sqm in summer, so this unit seems to be operating at >100% efficiency.
OK its a nicely built option to the usual DIY projects, but at what a price !
I'm not knocking solar air heaters - I'm sitting here now enjoying 30degC air blowing around my feet from my DIY system, with the outside air at only 16C and the sun way off axis at about 70deg to the plate collector. All built for about $150.Posted Wednesday 1 Jul 2009 @ 5:27:15 am from IP #
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