Solar thermal powered air to air heat exchanger.

Quick explanation I live in a passive solar home built in 1984 that still needed a bit of auxiliary heat.  So last year I added a thermal collector on the west end of the south wall.  One of the results is learning without the propane heater the house needs more air interchange.  So that has me looking at air to air heat exchangers both commercial and DYI.  Now I also find I still need to about triple the amount of active collector.  So it has me asking if I can do both at once?

solar collector write up.

So if I move to the south wall near the other end of the house I have another upper clearstory  opening window to come in through and bare wall for the collector.  Here is the photo of it with a few lines overlaid onto it.  The red rectangle is where I want to put the collector to heat the incoming air.  The orange triangle is an empty cavity under the stairs where I would like to bring my exhaust air out.  The green rectangles are on 2 of the 4 large clearstory windows that are the primary sun entry for passive solar.  Now having learned how much heat is lost at night while playing collector one of the long term goals is to put insulated sliding shutters over them so picture the green rectangles eventually sliding up and down into the empty area below the window.  And the blue rectangle is a potential suction by solar thermal.  Will get to it last.

Starting next thread to keep the drawing with discussion

So here is my thinking.  One of the things that concerned me about the main collector was possibly creating a moisture problem in the wall that it covered.  So that lead me to wanting an air gap here.  In the process I realized it might solve several problems.  If I put the air intake at the top and ran it down the wall behind the collector I get up into fresher drier air than I would have at the bottom.  I might get 15 feet of fall head pressure of my cold column to maybe increase static pressure and or flow.  By having the inlet screen facing sort of down it should be self cleaning.  So here is a drawing of my thinking.  I will have a bit of a constriction problem getting by the window but I think it can be solved with a trapezoidal bump out in the collector backing as it goes by the window..  So the thinking is take the air in 15 feet and let it fall clear to ground level.  Then go back up thru the heat exchanger gathering air from the basement to vent.  Then solar heat it.  If the collector was glazed with a double wall poly-carbonate greenhouse panel I would have roughly 4' x 14" of glazing for collector area which nearly doubles the area of the heating collector.  If I pull the air from the basement floor level and bring it out thru a heat exchanger horizontally and go in high at the inlet and go out low at the outlet.  So at zero outdoors I would potentially have nearly 60 degrees to give up to heat the incoming air.  Of course it won't do that much.  But even 10% is 6 degrees to immediately start lifting the air for convection.

Since the collector does roughly 120 cfm with a 50 degree temperature rise by doubling the collector size and taking the static air lift out of basement off I should be able to roughly either double the flow or double the lift.  If it was 40 below out and I wanted 70 degrees indoors that would mean a 110 degree rise needed to break even.  100 of it should be able to come from the collector and if I can get even 10 degrees from the heat exchanger I should have 120 cfm pushing into the house hot.  At warmer outdoor temps I should actually heat the house and not just break even.  The system should drive completely passively.  The air out of the basement is rising 8 ft while going from roughly say 50 to 60 degree to outdoor so that should drive the convective loop too.  Now there will be some losses too.  The air is turning 4 to 5 90 degree corners to get in the house.  Will that eat all the gains from the vertical column out outdoor temperature air?  Or maybe even eat more and come out with a net loss in volume for doing it?

Now in the photo the blue rectangle is where I might get a little more.  If I spill the exhaust air that is already still warmer out and heat it with another nearly 4 foot wide collector panel before exhasting it I could have another nearly 8 feet of rise sucking on the outlet.  Now this is completely passive but expensive.  I could put a fan and solar panel in cheaper.  But this should be maintaince free till the glazing needs replacement.  So food for thought.  Which is the better answer?  Fan and solar with replacement at say 20 years or polycarbonate with replacement at unknown age.

And that brings us to the final thing.  How to build the actual heat exchanger?  Currently I am thinking doing 3 or 4 long oval pipes by wrapping thing aluminum flashing in tall flat ovals.  If its volume is the equivalent of 1 foot thick by 3 foot high by 4 feet across that is 12 cubic feet.  So the outbound air would have just 6 seconds of exposure to the inbound air with the conductivity of thin aluminum in between.  If each oval blade was 24 square feet would  either 72 or 96 square feet of aluminum be enough?  Does someone have a better answer that is cheap and easy?

Hi there,
What do you mean by heat exchanger?

I appreciate the concepts, but is that a term or a device?

Joyce Harris wrote:Hi there,
What do you mean by heat exchanger?

I appreciate the concepts, but is that a term or a device?

It is a device.  Originally I was intending to just do a bunch of aluminum tubes and run the household air thru them and the outside air around the outside of them.  Then I got to pricing tubing and couldn't find something I could afford in quantity.  

Then I found the HRV information.(heat recovery ventilator)  roughly 80% heat recovery because it is partially reverse flow.  But the cores were expensive.  $600 range.   Thought the cores were really high tech because of the cost.

Next tripped over this video for building your own.  For this core they are using a plastic cardboard.  NOT high tech.  Still expensive if you have to buy it plus with 2 layers of plastic between each layer will be less efficient.



So that left me asking if I could successfully make aluminum parts shaped like card board?  So that is what I am working on for this one right now.  Going to be tedious but I think I can keep it under $100 using heavy weight aluminum foil and just enough JB weld to hold it all together.  Will see if I can make it work.

Here is a quick explanation.  There are lots of them on youtube so google it for more.

Okay thanks.
I will have a watch and see.

So You are building a solar heated HRV essentially? Using the sun to warm the incoming air instead of the conventional way? What is the role of the heat exchanger in the design?

I have a super-insulated home I built so I am familiar with most of this.

I have been thinking more and more about adopting something like this  wall panel for my home, or at least in the front.  The length from the rear to the front of my home is @14meters (x3 for feet) and 7.5m wide.

(YES - incase folks have read previous entries in 'how to heat my home', I am still seeking insulation, and still seeking what is the best method.  Presently, my city is in our 6th lockdown since April 2020, and this one has just been extend another 2 weeks, with night curfews between 9pm-?5am, and we need permits to go to work. So......  insulation quotes cannot happen in a hurry when such C.V.calamity is current. But I intend to get insulation on all exterior walls then re-assess my 'feel' temperature. But in between times I am thinking I will still need some comfort, (only getting older! as we all are!!) so if there is some way to de-roost the chill factor and boost the warmth naturally, then, why not?

I have thought though of approaching the thermal harvesting from two sites the south and the north of my home. The rear and front of my home respectively.  

Single story, flat roof, no crawl space in the subfloor. (What other details might help to generate some strategy here?)

Potential Rear site:
South (is the cold of the southern hemisphere) I have been thinking of the notion of sourcing a water bed type bladder e.g. double bed size and laying it across the top of my closed water tank with is round e.g. ?2meters diameter, and the same tall.
My roof is flat, so the top of the tank is exposed virtually all day every day.
But HOW to use that to heat up my southern rooms (bath, toilet, kitchen (currently, yet I want to turn it into a mud room and storage walk way since it is an alley kitchen etc etc)). There is the bathroom window one meter from the water tank. It is lower than the top of the water tank. Yet hot air rises.  Which makes me think, can I create (How?) a hot water radiator type system, where the water from the bladder heats it  (especially if the bladder is painted or enveloped  or encased in black absorbent material for greater potential of getting warmest).
That radiator can be secured to the wall inside, to promote and keep the chill off the air via warming surround objects.

Potential Front Site:
Then for the northern (sunny side), I have brick either side of an expansive window frame.  The eastern side of the window, there is maybe 1meter (3ft) of brickwork.  This corner of the front gets 90% of the days sunlight. There is no shade in winter, after 9am until it sets.  The two trees in its path are prune-able or deciduous during winter.
I am thinking the thermal heat can be exchanged inside via the sliding window beside the brickwork.  (HOW?)

I have looked and looked for various ways to use natural heating and I am still unsure what is the most effective and simple means that will support this.

Anyone have any hints?

Supplies for things I dont have will be available, after this lockdown, (God willing- NO MORE lockdowns please), e.g computer fan to draw the thermal air collection inside or push the cold air out or both!!  I am not so savvy YET with such simple technology, through lack of experience.  But willing to learn.

Anyone tried to use a waterbed bladder for such purposes? it would be weighty, what impact would that have on my 13,600 litre poly tank? I would need to protect the bladder from direct light as I am sure it is not UV protected. If it adversely, busts what would be the worst scenario - mmmm my garden soaks up the stale water that the bladder held? plastic bag bits to recover?

Questions?
Tips?
Videos links?
other posts for me to read and consider?
Or warnings???

Patrick Harvey wrote:So You are building a solar heated HRV essentially? Using the sun to warm the incoming air instead of the conventional way? What is the role of the heat exchanger in the design?

I have a super-insulated home I built so I am familiar with most of this.

Originally the thinking was simply to preheat the air coming outside air in the bottom of the solar collector with household air, then heat it with the collector increasing the total BTUs incoming while drying/replacing the air in the house to get rid of humidity problems of a sealed house.  Basically I was trying to combine 3 functions.  Solar collector pulling outside air in, heat exchanger for short of hrv and finally convection replacing fans for simplicity.  Before I started researching this I thought an hrv was a refrigeration/heat pump based device given the cost combined with greater than 50% efficiency.  I did NOT understand that they were basically a passive device and that the greater efficiency was coming from partial reverse flow energy path gains.

So originally I was looking at aluminum irrigation tubes as the heat exchanger because I had just been at an auction where they went really cheap.  the alternate answer was simply to build a round oval duct to exchange heat.  Expectation of at best 50% efficiency with it being more likely to be down around 10%-20%.   That was the point this original post was written at.  Had hoped for some feedback that would improve the thinking and it drew nothing.   Needless to say once I started shopping for cheap tubes they were bringing real money and when I priced thin aluminum sheeting to build ducts in big pieces I was in sticker shock there too.  The $1000 price tag of an hrv was looking better.

Then I learned how the HRV core was actually made and was left asking can I build one far cheaper than I could buy one?  There are gains to be made here.  If the HRV core becomes a bit of a collector too then it should melt itself out if it ices up eliminating any fancy control system for deicing.  So the current hope is solar collector + HRV + air circulation without fans in a single assembly.  If I reroute the drier duct thru the bottom of it too I may be able to get a bit of heat recovery there too.  Will want it automated but that will amount to 2 simple window/door opener closer systems.  I had gotten all the glass for this free by last fall.  But when started to look at purchasing the plywood it was a bit expensive and the lumberyard guy said it would come down after the first of the year.  Needless to say I waited he was way wrong and I held off on this last winter.  Currently trying to get a bunch of pallet topper sheets from a local company to use instead of buying plywood to keep costs down.  At any rate the solar collector I am building either way simply because I need to increase BTU's in if I am going to keep the house comfortable on just solar.  The rest if I can make it work is just gravy.

I was in the same situation with the core for the HRV. I didn't want to spend the money on purchasing a pre-made unit but I was unable to find a viable option to build the core myself that performed well and was less expensive.

The closest option I found were intercooler cores from turbocharged cars. They flow well, all aluminum construction and are some of the lowest costing heat exchangers I could find for the size. I am a machinist by trade so modifying was no big deal... In the end.. I bought a used HRV unit of appropriate size.

Patrick Harvey wrote:I was in the same situation with the core for the HRV. I didn't want to spend the money on purchasing a pre-made unit but I was unable to find a viable option to build the core myself that performed well and was less expensive.

The closest option I found were intercooler cores from turbocharged cars. They flow well, all aluminum construction and are some of the lowest costing heat exchangers I could find for the size. I am a machinist by trade so modifying was no big deal... In the end.. I bought a used HRV unit of appropriate size.

Well I am looking at the 18" heavy Costco aluminum foil as my build material with the goal of doing a 12" x 12" x X" core.   Don't know yet if I can make it work.  It will be tedious as all get out and I will be making something like  1 1/8" of core per evening if it works.  Will add a write up on it if I get it working.