Portable Solar Trackers

Passive vs. Active Solar – Why Not Both?

You will find a lot of information on the internet about active solar systems, people are fascinated with the concept of converting sunlight to useable energy. Unfortunately, active solar systems are not all that efficient, A typical solar panel is only 15-18% efficient, with some in the 24-25% range. Another component of solar that needs to be improved on are storage batteries. Lead-acid storage batteries have a relatively short life cycle and are very heavy. Also, lead-acid batteries can only be discharged to within 50% of capacity.

Passive Solar is a lot more efficient than active solar. It is amazing that more people don’t incorporate passive solar into the design of their homes.

I have built passive and active solar systems for my Barn. I wrote an article on how I converted the side door of my Barn into a solar furnace. If you would like to read about it click here. It actually heated a 200 sq. ft. space with a “bad” 100-watt solar panel behind a storm door.


Window Mounted Solar Furnace

Mounting a solar furnace in the window of a house is not new, you will see countless versions online. Pictured below is an example of what I am referring to.

Solar Window Furnace

I mounted a 4’ X 8’ solar furnace on the front of one of my homes, it worked very well. In fact, it cut our electric heating bill in half. If you have a clear view of the southern sky it will produce an incredible amount of heat.


How Solar Furnace Works

Everyone that has gotten into a car with a dark interior on a hot day knows the power of the Sun. Sunlight passes through the windows and heats the dark surfaces. A solar furnace simply distributes this heat into space.


Components of Solar Furnace

The basic components that make up any solar furnace are glazing, absorber, air channel and insulation.


The glazing should be single pane to allow sunlight to pass. I have used double-walled poly-carbonate with great results. You do not want to use double-pane glass or UV resistant glass. The spacing of the glass from the absorber is critical, too close and the light will bounce of the absorber and back through the glass.


The absorber should be a “flat” black surface preferably made of copper or steel. The idea is to absorb heat and release it slowly. Aluminum dissipates heat too quickly. Steel corrugated roof panels work great.


Insulation is installed beneath the absorber. It is used to create a temperature difference between the “trapped” and heated air between the glass and the absorber. The greater the temperature difference the better the flow of air. In fact, if you create enough of a temperature difference you will get natural convection and will not need a fan to move the air.

My system had the air channel extend down to the floor, just a few inches off the floor. The heated air was directed upward with a deflector. I did not need a fan.

Air Channel

The air channel directs the air from the space we want to heat and funnels it over the absorber. As it passes over the absorber it picks up heat. To improve efficiency and allow enough time to increase the temperature, a series of baffles are used. The best design will create a serpentine path over the surface of the absorber.


The sides of the furnace should be lined with a reflective material that directs sunlight toward the center. This concentrates the sunlight toward the center.

A New Approach

You may remember me talking about how I used a “bad” 100-watt solar panel for my solar furnace, it was mounted on the side door just behind a storm door. The temperature in this void was incredible, so hot in fact on a good solar day you had to open the door quickly. yes, I made my door into a walk-in solar furnace. The expression “from the frying pan into the fire” comes to mind.

So what if we design a window mounted solar furnace with a good 100-watt solar panel. This would be incredibly easy because a rigid solar panel has channels and a space behind the panel for us to insert insulation. We have just created a passive solar furnace that produces heat and power.


Power & Heat

The power from this 100-watt solar panel would allow us to power a fan to circulate the air, charge electronics or run LED lights using just the DC power. If we add a pure-sinewave inverter, a charge controller and a storage battery we can produce AC power to power sensitive electronics.

We would not want to try and power too much with such a small system, but it is conceivable that you could provide enough electrical power for a small room at night and provide heat during the day.

It is not realistic to think that we could heat the space at night, the best we can hope for is to collect as much heat as possible through the day by heating all the dark surfaces. A lot of passive designed homes use rock, water columns, low-temperature paraffin, and dark surfaces to retain heat. The more mass our dark surfaces have the more heat they will store and release.



Imagine what you could do with a good solar panel and a decent power pack.

I will be creating a set of build plans for this system and I will post it when I am done.