Cover Photo

“Home of architect Richard Crowther, that is heated with solar energy collected by flat plate collectors. A flat black aluminum plate behind the glass absorbs sunlight and heats air forced between the glass and the black plate. The air is then forced through a bin containing several tons of two inch sized rocks, that store the heat. Because of extra special insulating features of this home, the solar heating system provides almost 90 percent of the heat needed,” May, 1975.

 

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Note: I am not an expert on this stuff.  Assume the vocabulary was made up as I went along.

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My wife and I bought a tiny house in a tiny town in Iowa to rent to my brother.  The house dates back to the 60s.  It has meager insulation and electric baseboard heating.  Even though it is only 400 square feet in size, it cost my brother about $300 a month in electricity to heat it last winter.

So, we were looking at options to provide supplemental heat to the house with minimal installation cost.  And I began to think back to the glorious 1970s and the energy crises.  Many people experimented with using solar energy to heat their homes or to heat water.

While the modern culture is fixated on photovoltaic panels to produce electricity, back in the 1970s these were just not commercially available for do-it-yourselfers to play with.  So, the majority of solar collectors were designed to heat air (and transmit that air directly into the home or to a thermal mass for long term storage) or the collectors were designed to heat water.

As it turns out, there is still a lot of interest in making small-scale solar thermal collectors to provide heated air for rooms, garages, or other small spaces.  There are many websites and youtube videos available to show how to build these.  Most of these small-scale thermal collectors are based on one of two designs: 1) using pop cans as the medium to absorb sunlight; 2) using black screening material as the medium to absorb sunlight.  See the image below for two nice examples.

 

Either way, the solar absorber is contained in a box with transparent cover (glass or plastic) with inflow and outflow channels on the back.   Click here for more details.

Many of these styles of thermal collectors are designed to be installed vertically on an exterior wall, and then holes are cut into the wall for the inflow and outflow channels of the collector.  PC fans are frequently used for forcing air through the collector.

Another popular option is to route the inflow and outflow channels through an existing window.  This allows for a semi-permanent installation that does no damage to the existing structure.

 

This is the design option that I choose to follow.  Note that this type of solar collector may be active (having a fan to push or pull air through the collector) or passive (relying only on convection to move air through the collector).

Passive collectors work because the sun heats the solar absorber.  The absorber then conducts heat to the air surrounding the absorber.  The heated air rises up the front of the collector and exits the collector through the outflow vent.  Cooler air from inside the house is drawn in through the inflow vent and then descends down the back of the collector below the solar absorber.

When the sun heats up the solar absorber, much of that heat is conducted to the in the collector.  But some that energy is radiated back out through the transparent glass/plastic cover. Thus, the efficiency of the passive solar collector is limited by how much air will flow based upon convection alone.  An active system with a fan can transfer more heat from the solar absorber to the air that is drawn/forced through the collector.

An active system with a fan gets more complicated and more expensive to build.   A simple switch can be used to turn the fan on and off, but his requires someone to be available to turn the system on and off.  This becomes impractical when the occupant of the house needs to leave before the sun comes up and gets home after the sun goes down (e.g., pretty much anyone with a day job in Iowa during winter).

I spent a lot of time trying to figure out how to build some kind smart switch using a photo sensor to tell when the sun was out.  But this idea didn’t go very far.  I also considered using a thermocouple inside the collector to tell when the solar absorber was getting warm, but abandoned this idea as well.

Then, one day I stumbled across register booster fans on Amazon.

 

 

These booster fans are designed to work with for both heating and cooling operations.  For heating purposes, when the furnace kicks on, warm air starts moving through ductwork.   When the air coming through the ducts exceeds a user-set temperature, the boost fan turns on.  When the furnace stops and the air in the ducts cools down, the booster fan turns off.

This is an elegant solution for the solar collector.  When the sun comes out and the solar absorber gets hot, then convection will start to move warm air through the outflow vent.  The booster fan turns on and dramatically increases the air flow through the solar collector.  When the sun goes away and the solar absorber cools off, the booster fan turns itself off.

Seventy bucks from Amazon.  No invention or labor required.  It’s perfect.

This is the design for the collector.  It is intended to go through a small window on the south side of the house.  The bottom of the storm window is 59 inches off the ground.  I intended the collector to be short enough that it would not sit on the ground directly.  I want to avoid snow drifts from covering the lower part of the collector.  And I want to minimize rot and insect damage to the main body of the collector.

 

 

The collector is tilted at 30 degrees from perpendicular.  The sun is very low in the sky during the middle of winter in Iowa.  This angle puts the solar absorber nearly perpendicular to the sun from October through February while still providing good angles to the sun during the rest of the heating season from September through April.

Air flows in from the inside of the house through the back panel of the collector.  This air descends to the bottom of the collector, and then flows up the front of the collector. The solar absorber is roughly centered in the front/top chamber of the collector.  This allows air to flow past the front and the back of the solar absorber doubling the surface area exposed to the air.

The outer dimensions of the solar thermal collector are 77 inches long, 46 inches wide, and 9 ¼ inches deep.

The materials used for the main body of the solar collector are:

  • 2 each of 2 by 10 by 10 ft construction grade lumber
  • 2 each of 4 ft by 8 ft exterior grade plywood (1/2 thick)
  • 2 each of 4 ft by 8 ft rigid insulation (1 inch thick) {Polystyrene Board Insulation at Lowes}
  • 2 each of 2 ft by 8 ft Corrugated Silver Steel Roof Panel {Union Corrugating at Lowes}
  • 1 each of 4 ft by 8 ft Polycarbonate Sheet (1/4 inch thick, hollow) {PLASKOLITE at Lowes}
  • Miscellaneous scraps of 3/4 inch pine; 2x4s; and 3/8 inch plywood.

The solar absorber is made from corrugated, galvanized steel roof panels which are painted flat black.  These roof panels are far more durable and have much greater thermal mass than pop cans or screen mesh.

This is the solar collector as it is installed.  It faces due south (thanks to tendency for all small towns in in Iowa to be laid out on east-west/north-south grid).

 

Note that I installed the collector with temporary legs this fall.  I will need to replace them with pressure-treated lumber next spring.

The vent box for the collector was designed to be 1/2 inch narrower than the exterior storm window.  When installed, there is roughly a 1/4 gap on both sides of the vent box.  The main body of the collector barely touches the exterior storm window above the vent box.  This does a decent job of shielding the vent box and the open storm window from the weather.  We should have sealed the 1/4 inch gap with duct tape or something, but didn’t bother at this point.  Some swirling snow might get blown in during winter, but it should be fairly well protected until spring when it starts to rain.   The storm window is lowered to rest directly on the vent box.  So, there is no gap above the vent box.

Note, those “holes” in the side of the collector (the exposed dadoes) will need to get filled in the spring before it starts raining (hopefully).   The exposed dadoes at the bottom end of the collector are covered by the temporary legs.

This is the vent box extending through the double hung window into the room.  It ain’t pretty, and it doesn’t need to be.  We stuffed a cheap bath towel into the window to seal the gaps between the sides/bottom of the vent box and the window frame.  The window is dropped down directly onto the vent box.  Long term, we need to do a better job of sealing this up.  But, it’s good enough for now.

The register booster fan drops into the hole in the top of the vent box.  It is plugged into a standard electrical outlet on the wall just below the window.

And the big question is: Does it work?

Initial Results: 18 December 2022.  First full day of sunshine since the collector was installed.   The outside air temperature is 13 degrees F.  Midday, the collector is putting out heated air at 101 degrees F.  The register booster fan kicked on automatically as expected.

So yes, it works and works pretty damn well.

Will it make a difference?  We’ll have to see what his electric bill is.  But, we know this.  The register booster fan has a maximum flow rate of 120 cubic feet per minute.  The house is nominally 400 square feet with 9 foot ceilings in most of the house.  So 3600 cubic feet.  The booster fan is capable of moving the whole volume of the house through the collector in 30 minutes.  We have the fan set to 60%, so 72 CFM.   Thus, the collector will move the whole volume of the house through the collector in 50 minutes.  Air goes in at room temperature and comes out at 100 degrees.   The system will turn over the whole volume of the house 7 times from 10 am till 4 pm (when the sun is doing a good job of heating up the collector) heating up the air 20 to 30 degrees each time it goes through the collector.   I expect that will add up to significant savings this winter.