Understanding the different forms of energy, how it is converted to different forms and how it moves through space and matter will provide further insight into how the Thermal Wall Technology works. For the non-technical audience, there are a few new scientific terms, however, relax as the new terms will quickly be related to common terms we use on a daily basis (please see the Glossary for definitions of terminology). For the purposes of the TWT passive solar demonstration home there are two principles or vehicles on the transfer of energy:

  • Electromagnetic radiation (EMR)
  • Kinetic energy

EMR is the type of energy we receive from the sun and has several different levels of intensity as illustrated below. The energy moves in oscillating waves; the more oscillations or waves per second, also referred to as frequency, the higher the intensity of the energy. Most of the electromagnetic frequencies are undetectable to humans, however, our senses can detect some ranges of EMR; visible light and infrared rays (referred to as radiant heat or just heat).

  • Visible light is light within the visual perception of humans.
  • Infrared radiation (IR) is a form of EMR humans senses perceive IR as heat.

The high energy spectrum, such as gamma rays, x-ray and ultraviolet can be damaging to our bodies as they have the ability to penetrate through and into our bodies. The damaging high energy Gamma, X and ultraviolet radiation are mostly absorbed by the earth’s atmosphere and converted to less energetic forms of EMR such as visible light and infrared rays. A sunburn is caused primarily by ultraviolet rays which escape being absorbed in the atmosphere and make it to the earth’s surface. Low energy EMR rays such as radar, FM shortwave and AM are the energy waves we use for communications; they also continuously bombard our bodies but at natural occurring intensities have no physical impact.

Spectrum of Electromagnetic Radiation

Electromagnetic radiation has the ability to travel through a vacuum. This is how the suns energy reaches the earth. The energy that most effects the earth’s surface temperatures are in the visible light and infrared spectrum of electromagnetic radiation. So we know the sun emits electromagnetic radiation because we can see it and feel it. Here’s where it gets interesting… all matter above the temperature of absolute zero (-459.67˚F) emits electromagnetic radiation. So everything around us is constantly exchanging electromagnetic energy at the speed of light, even an ice cube. You can feel the heat from a burner on a stove; that is infrared electromagnetic radiation (heat). If you turn the stove element to high it will also glow red at which point it is radiating both infrared heat and visible electromagnetic radiation (light). When electromagnetic radiation strikes a surface (including our bodies) some is absorbed and some reflected. We often use the terms hot or cold to define a temperature; hot and cold however are only terms relevant to our senses. If we stand in a corner of a room next to a operating wood stove we feel warm. If we walk away from the stove to window or door in the winter we feel cold. What is occurring is the window is radiating lower energy electromagnetic infrared heat and our bodies sense the difference, however the window is still radiating heat, just a lot less than the wood stove.

When electromagnetic energy strikes matter the energy is either absorbed or reflected. If absorbed into the matter it becomes kinetic energy and in this process atoms in a high energy state will collide with adjacent atoms of a lower energy state, transferring some of their kinetic energy (heat) in the process. Kinetic energy is the energy contained within matter (atoms). As an example: If you heat up the end of a copper pipe the heat will transfer down the pipe. The high-energy atoms at the heated end through kinetic energy transfer the energy throughout the pipe to low-energy atoms eventually warming the entire pipe. In the TWT the hot water in the tubes in the walls and floors, through kinetic energy, are transferred from the water through the polymer pipe into the concrete wall. Then when the kinetic energy (heat) reaches the interior surface of the wall or floor it then radiates electromagnetic radiation (infrared heat) to the interior. Once in the infrared energy state, if released to the interior of the home, it is either absorbed or reflected and the process continues. The kinetic energy in the concrete also migrates toward the exterior of the home. In this case as it migrates it bumps up to the thermal insulation. The thermal insulation is comprised of a polymer with microscopic air bubbles. As the kinetic energy migrates through the polymer matrix it reaches a bubble (air space) then radiates electromagnetic radiation (infrared radiation) across the air space where it strikes the other side of the bubble.  At that point the infrared radiation is either absorbed or reflected back the other way. If it is reflected it crosses back across the airspace where it just came from and is either absorbed or reflected and the process is repeated. This constant back and forth process is how thermal mass retards the heat loss from a structure. Eventually the kinetic energy makes it to the exterior skin of the house and is radiated to the exterior or carried away via kinetic energy by air molecules.

The process which occurs with a solar collector is similar but slightly different. Visible light energy has the ability to pass through certain elements such as glass. Once it passes through the glass as visible light it is either absorbed or reflected. If you look through a window into an unlit room what you see is light (electromagnetic radiation) that is being reflect back through the glass. Different elements and compounds reflect or absorb light differently; mirror, white or light colored surfaces reflect more light while dark surfaces generally absorb the light. If the light is absorbed it becomes kinetic energy (heat). As we discussed in the above paragraph one of two things will occur; the kinetic energy will continue to migrate through the substance or be radiated from the surface as infrared radiant heat. Unlike higher energy visible light, infrared energy cannot pass back through the glass thus capturing the energy. It can be absorbed on the surface of the glass as kinetic energy and migrate as kinetic energy to outside but this is a slower process. The glass can also be treated with a micro-thin metal coating which you can see through but enhances the reflectivity properties, thus reflecting the infrared radiation back towards the collector’s absorber plate. The absorber plate of the collector (generally made of copper) has a special coating designed to absorb a large portion of the visible light spectrum. The light is absorbed and converted to kinetic energy and the energy is transferred from atom to atom through the copper and comes in contact with water filled tubes. The kinetic heat is then transferred through the tubes to the water and the water is then circulated to the walls, floors, or storage for later use.

Passive solar collection operates similarly to the solar collectors. Light passes through the window and strikes a surface where part of it is absorbed heating the object and the now kinetic energy transfers energy from atom to atom in the concrete mass and can also be re-radiated to the interior. Part of the visible light energy is reflected and then absorbed by other objects including the occupants. In the case of the TWT the energy in the heated floor can be transferred via water in tubes to other floors and walls throughout the structure. The atoms on the surface of the object have no direct atoms to transfer energy to so they emit electromagnetic radiation in the form of infrared energy, which in turn travels through the room striking and heating other objects. As with the solar collectors infrared energy cannot pass back through the glass. It can, however, be absorbed by the glass. To reduce absorption a special metallic film is deposited on the interior window surface to enhance reflection of the infrared heat, effectively trapping a portion of the heat in the home.

Wood-Stove Providing Radiant Heat

From the example above imagine you are standing next to a 4ft square wood stove radiating enough heat at 250°F to heat a room 16ft by 16ft. The 4ft square wood stove has six sides making the total surface 4ft X 4ft X 6-sides = 96 square feet. The amount of heat radiated from the stove is a function of the temperature (250°F) and the amount of surface area (96 square feet) radiating heat. If the wood stove was only 1ft square and radiated infrared heat at a temperature of 250°F the room would be cold, however, if the stove were 8ft square at 250°F the room would overheat. This example illustrates that as the surface area of the radiating source increases the temperature must be reduced. Now imagine the heat is radiating from the walls and floor of the 16ft by 16ft room. This provides 768 square feet of heating surface and represents 8-times the surface area of the wood stove. Because of this principle the area of the walls and floors enables the heating of the room at a much lower temperature, as illustrated below, creating numerous efficiencies. 

Providing Comfortable Heat for Your Home

Another way to view this is if you were in a concrete room and the walls were maintained at 70⁰F and the only heat loss from the room was whatever your body produced; the temperature of the room would be 70⁰F. So the temperature of the walls in the TWT whether heating or cooling will basically feel very much the same temperature (perhaps slightly warmer or cooler) than the walls in a conventional home.

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