The passive solar furnace is an efficient source of renewable energy but it is rarely considered as a viable heat source. Truth is it is one of the easiest devices to create by the average handyman and rarely discussed.
The heart of the passive solar furnace is the absorber. This is where most folks get creative by incorporating a wide range of material to include aluminum cans, aluminum gutters, copper tubing, and corrugated roofing panels. I have personally used all of them with the exception of the aluminum cans. All of them worked well but there are other factors to consider in your design.
Designing Your Furnace
To plan a solar furnace you need to determine a few things in advance.
- How much area are you trying to heat?
- Will you need to run any ducting?
- Will your solar furnace have the ability to be adjusted?
- How large can you make it?
- Will your wife let you.
These are only a few things to consider when designing your furnace but they will impact your design.
Anatomy of a Solar Furnace
The Solar Furnace, or solar collector/heater as it is sometimes referred to, is one of the few solar related devices that can be built, modified and enhanced by nearly anyone. The absorber can be modified as well as the baffles, insulation and even the medium (air). Water is a wonderful medium and my personal favorite but if I wanted to heat with water I would invest in a large evacuated vacuum tube collector. This is still considered a Solar Furnace and is quite possibly the best means of heating a living space. That subject is worthy of it’s own post.
Here are the major components of a traditional Solar Furnace/Collector:
The outer frame of my garage door serves as a mounting surface for the glazing. A professionally manufactured solar collector typically would use an aluminum frame with rivets. The average handyman might use wood but the walls will need to be lined with foil-faced insulation and the exposed wood must be sealed.
All Solar Furnaces have a metal absorber; the type of metal, thickness and color will determine the efficiency. If the metal is aluminum, or too thin then heat will dissipate too quickly, too thick and it may radiate too much heat back through the glazing.
A well designed Solar Furnace will have diverters, or baffles and channels, to redirect the air and slow it down so that it can pick-up heat. The longer the path the more heat can be collected. A progressive path has worked best for me.
The final component is the glazing; this is mounted on the front face of the Solar Furnace. A manufactured version will typically use tempered glass for the glazing because it has a high transmittance of light but most handypersons will use polycarbonate because it is resistant to breakage.
Principle of Operation
As the Sun’s rays pass through the glazing they strike the absorber. As the absorber (a flat black colored metal) heats up the air that enters the furnace also heats up. The light that passes through the glazing strikes the dark surface of the absorber and a portion of that light is reflected back through the glazing, this is why the distance between the glazing and the absorber is important. If the absorber is too close than too much heat will be radiated back through the glazing.
The air enters the furnace via ducting or an air plenum via a solar-powered fan, or ideally through natural convection. This process of heat exchange is referred to as a thermo-siphon. Cold air enters at floor level, is heated and then exits near the ceiling for this particular application.
Baffles & Channels
Baffles and channels slow the airflow down to allow the air to pick up heat as it traverses over the absorber and through the channels. Forcing air through the collector should be controlled, too much air and there will be a cooling effect. If the air is constricted too much heat can accumulate and it will radiate back through the glazing.
Thermostats & PLCs
To maximize the output of your Solar Furnace you need to establish operating parameters. This device is no different than any other heat-producing device. You set the operating temperature with a thermostat and when that temperature is reached you want to maintain that set point. I use thermostats and a PLC controller to maintain my set point.
Reverse Thermosiphon Effect
One very overlooked phenomenon is the “reverse siphon effect” where the system allows the outside cold air to travel back inside via the inlet duct after sunset. There are a couple of techniques that can be employed to combat this effect. One technique is to not draw the air indirectly at the floor level. The air can be routed vertically through a riser that will create a slight temperature difference. This will help but it can impact the airflow through a naturally convected collector. I use motor operated dampers that close when the PLC tells it to.
This post is not to instruct anyone on how to build a passive Solar Furnace there are plenty of people out there that have far more experience than me. My system works for me and I am constantly modifying it. The purpose of this post is to make you aware that passive solar does not receive the attention it deserves. If you have southern exposure, you live in an area where the solar days are many and you want to heat a space without fear of the neighbors snubbing you than consider building a Passive Solar Furnace.