By-Passive Solar

(like many pages on my fledgling website… this one is still a work in progress.  Illustrations to come).

Introduction to By-Passive Solar


You can read a lot about Passive Solar on the web or in books.   The benefit of this arrangement is that it is economical…  nothing “extra” to build.  Just rearrange your home’s windows a little so they are mostly on the south side, add some overhangs to keep out the summer sun and tada! you have passive solar.  Adding some mass to soak up those rays (such as a cement slab floor) would be a good idea.  In the winter, the sun’s heat comes in the windows and is already in the living space where you need it.  In many cases, the air just circulates around by natural convection and floors radiate warmth back into the rooms at night.

I am sure I will use some aspects of direct-gain passive solar in my home (all solar homes build on passive solar concepts)…  perhaps even “augmented” with a fan or in-floor pex pipe to move the heat around…

but what is “By-Passive Solar”…?

It starts with the need to fix the main problem with direct-gain passive solar…  The solar collector IS the home.  This requires certain compromises.  For instance, you need to leave the curtains open to let the sunlight in.  You need too keep your floors bare (and many recommend darker colors unless you have masonry walls to capture the reflected energy).  Also, because the home is the collector, the home is often uncomfortably warm during the solar heating hours.

But what really pushed me down this path is that I am in Michigan where I can’t rely on a daily solar charge.  I can only expect one sunny day a week in January or February.   I needed my solar charge to last longer.  Therefore, I needed to heat more mass; not just my home, but a huge mass of earth around my home (PAHS Umbrella style).  Collecting that much energy requires letting the heat in during the summer, at the same time that I would like to enjoy a cool earth sheltered home…  I don’t want the heat to be building up in the home and surrounding earth all summer,   I want the heat of summer to enter my home 6 months later, when I need it…  So the key is to flip things upside down and heat the earth 10ft away/below my home all summer.  By the time that energy reaches my floors, it will have saturated the earth around my home and be ready to carry me thru the cloudy SE Michigan winter.

This sort of by-passive solar heating system would need two extra components.

  • Solar Air Heater to absorb the sunlight energy and transfer it to air
  • Earth Tubes to carry the heat under the earth by convection or perhaps a small solar powered fan


Solar Hot Air Collector


It is called a collector, but really it is a generator.  It takes in air at some temperature and then heats it up with solar energy.  You can find many different designs on the internet…  But the best ones seem to be a shallow box with a glazed front and an insulated back.  The glazed front lets in sunlight and heats up air, the high temps drive the air thru by convection (or augmented by a small fan on a thermostat switch).  There is an inlet and an outlet and some means of snaking the air thru the box to gather as much heat as possible, but without creating too much back pressure… that’s it 😉

I plan to make my boxes somewhat modular so I can experiment with different configurations and swap them out later.  Each module should start with a 4×8 sheet of plywood.  You could nail a 2X6 frame around the plywood such that the plywood is approximately 1/3rd of the way thru the frame.  On the back of the plywood, you glue a 2 inch thick sheet of poly iso rigid foam insulation, such as Super Tuff-R that won’t melt under the heat.

On the front of the plywood, you build the “solar absorber”.  This is where the innovation happens.  The solar heat should be transferred to internal airflow and not lost thru the frame or glazing.  Air should not be moving near the glazing as this will result in more heat lost.  The absorber should not reflect any light, so paint it flat black (not glossy or metallic).   It should have lots of surface area to exchange heat with the air. If you design with ducts across the absorber, the spaces between the ducts should be a material that can conduct heat to the back of the ducts.

You need a way for the air to enter and exit the absorber.  In some designs you also need to turn the air within the absorber.  The performance of the Solar heater is very sensitive to the plenum or manifold design.  You want to distribute the air evenly and with a minimum of back pressure or leakage.  Adding simple turning vanes can make a huge difference.  A back flow damper may also be a good idea.

The Glazing is attached to the front of the frame… Ideally, it is something cheap and easy to work with such as Suntuff or Tuff Tecs or some other UV stabilized plexiglass.  The experts warn against using double pane glass because its seals can’t handle the heat.

Controls can be as simple as letting the sun drive the flow thru buoyancy, or you could augment the flow by adding a fan on a thermostat switch ($10 snap switch).  Some fancy designs even use 2 fans with the second one kicking in at a higher temperature.  You can buy some pretty high efficiency duct fans or use something as simple as a PC fan.  Many attic fan assemblies come with solar panels so the system can operate off the grid while the sun is shining, which is the only time you will need to pump the air anyway.

For materials, I think I will start with an “aluminum downspout design”.  This design uses rows of Aluminum downspouts glued to an aluminum back plate, all painted flat black.  The air comes in at the bottom of the left side and turns around in a plenum on the right side before returning and exiting on the top left side.  I may also try a vented soffit or double layer fiberglass screen design, but I suspect they are not as efficient because they don’t keep the air in the channel and away from the glazing.

One other interesting aspect of my design (images or sketches to come) is that it will be adjustable.  I plan to pivot my assembly by the lower intake.  The output will be able to pivot. between two positions.  The lower position (~30°) is for summer use (when the sun is higher in the sky) and will pump the heated air under the house for thermal storage.  This closed earthtube loops back to the solar heater inlet where the air is reheated.   Above this earth tube is a second earth tube, an inlet opening that takes fresh air directly into the home, moderated thru the cool earth.   In winter, I tilt the heater up to the winter angle (~60°).  This connects the outlet of the heater to the inlet earthtube and pumps the hot air directly into the home.  Meanwhile the closed circuit earth tube is now open to the winter air, since this is actually the inlet for the heater, air is first drawn under the home and tempered before running thru the solar heater and then up into the house.  Get it?  Maybe not.  I made a 2D sketch, but it doesn’t do it justice, I will cad it up some time…


A Freebee Earth Tube!


I would avoid the use of corrugated drainage pipe as a fresh air Earth Tube because of the water that gets trapped in the corrugations and the potential for mold entering your home…  It is highly advisable that all earth sheltered homes be equipped with as many waterproofing measures as possible, including foundation perimeter drains made with these corrugated drainage pipes.    I have heard of earth sheltered home owners who are both glad and a little sad that their drainage pipes never get any water in them.  What if we used them for a dual use as by-passive solar earth tubes.  The loop would never enter the home, so there would be no risk of mold or radon from the corrugated pipes.   The modifications could be small, instead of branching your drain pipes, you would need to make a continuous loop that would start and end at the solar heater.   All summer these pipes would be warming the foundation under the umbrella and helping to increase the temperature of the earth under the home…   I might take it a bit farther and use the more expensive 6 inch corrugated drain pipe for easier air flow.


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