Obviously I just started here… I have a bunch of rough notes and sketches and will be back 😉
Every heating system needs some way to distribute the heat, solar heating is no exception. The heat can be distributed thru conduction, convection, or radiation The heat storage medium can also be physically transported. Various materials have various rates of conduction and capacities for heat storage. Surface finishes can reflect or re-radiate heat. Various configurations can lead to passive convection currents, or the head distribution could be augmented with an active system. Here, I will try to sort it all out.
Note: since I am not very interested in forced air heating, I have separated heat distribution from ventilation…
Direct Solar Gain
Photons are tiny packets of energy emitted from the Sun. Some make it thru the atmosphere. Some of these are headed toward homes. Some of those may actually strike windows. Of the photons of sunlight that do interact with the window glass, some are reflected and lost. Other photos are filtered out by the low-E coatings (1/SHGC) and, if the glass is well oriented, the remainder defract (are bent slightly) and enter the home. If the floor is not reflective, the photons strike the atoms of the floor and energize them. The additional energy causes the atoms to vibrate faster. If the floor is conductive, these atoms vibrate neighboring atoms and the energy is distributed. If the floor is not very conductive, the vibration remains more concentrated at the surface. Some of that vibration energy is always wasted (atomic scale friction) and becomes heat. As the energy is distributed thru the floor, waste energy slowly raises the temperature of the floor. If the energy is trapped at the surface, the resulting heat is radiated, conducted and convected into the room very quickly.
Low-E coatings have many tiny holes that allow the majority of sunlight to enter because the higher frequency waves literally “fit” thru the holes. After heating up internal surfaces, some of this energy is re-emitted as lower frequency radiation. These have larger wavelengths that do not “fit” thru the holes in the low-E window coatings, so they are trapped in the home.
In passive solar design terms, the mass of a cement floor directly in the path of the sunlight is called “primary storage”. The solar energy to heat conversion takes place in the mass its self. Ideally, the mass can absorb the energy at the same rate it is arriving, but this is not always the case. “Energy backups” at the surface of the primary storage mass increase the rate of energy conversion to heat and can lead to overheating the room. Keeping the primary storage area clear of furniture (shadows) or carpets (insulation) helps ensure the energy reaches the storage medium. Choosing a dark surface for the cement reduces reflection and is generally considered a good idea with concrete floors, but a little reflection to secondary storage is not all bad.
One Btu (British thermal unit of energy) is defined as the amount of energy required to raise a lb of water 1F°. Concrete has about 1/5th the thermal capacy of water, so 1Btu absorbed into 1 lb of concrete will raise the temperature of the concrete by 5F°, or conversely, 1 lb of concrete can store 1 Btu for every 5F° of temperature rise. I personally prefer SI units. BTU’s are energy, 1 Btu = 1.055 J (Joules), but most SI solar gain tables give power in Watts, the rate at which energy is transfered (energy over time, J/s). To convert to SI, you must use 1 Btu/hr = 0.29307 Watts.
Solar intensity tables published by organizations like ASHRAE often provide “isolation” in Btu/(hr·ft^2) for each latitude.
Since primary storage is only the mass that the sun strikes directly, the rest of the mass (typically the majority) is known as secondary storage. Primary mass is ideal because it has fewer energy conversions between the incoming solar energy and the resulting thermal storage. As mentioned earlier, energy that is not absorbed by the primary storage is reflected, radiated, and/or convected to the secondary storage mass. For instance, losses on the primary surface may heat air which then drifts over to another wall slowly warming it. This process is much slower than the direct radiant energy to conduction that happens on the primary storage surfaces. If energy is entering the home faster that than it can be stored, there is an energy backup and the room will overheat… Passive solar design books list secondary storage mass as having 1/10th the storage capacity of primary storage. But of course that is not quite right. The storage capacity is actually the same, lb for lb or kg per kg… It is the energy backup that is the problem. Primary storage, with its closer coupling, is 10 times faster at storing the incoming energy than secondary storage. If your solar home is running on a very short 24 hour charge-to-charge cycle, then this difference is very important. However, if you are running on a much longer cycle, such as a seasonal cycle, then that secondary mass becomes much more useful. It may not absorb the energy as quickly, but it can still store it and release it as necessary. You may even want to use lighter colors on the primary mass in order to reflect more energy to the secondary mass and distribute that solar charge around the room (radiant reflected to secondary mass is faster/more efficient than radiant to primary to conduction and convection to secondary).
I don’t plan on having a forced air system in my home, but I can’t ignore my ventilation needs or the fact that, like gravity, “convection happens”. This is certainly something I will want to work with, rather than against, and I have several designs in this direction.
Traditional radiant floor heating is the modern equivalent of the ancient roman hypocaust. Modern radiant air floors are still available (popular with early passive solar builders, but not recommended by me), as are electric radiant floors (ideal for renovations), but I am most interested in “hydronic radiant floors”, which is the most efficient and effective method. Water is heated and pumped thru PEX tubing embedded in the cement slab floors (wet installation). Instead of traveling from the source by the three main methods of thermal distribution (radiation, conduction or convection), the thermal storage medium its self is transported. Along the way, heat from the water is conducted thru the PEX and cement and comes up thru the floor. It leaves the floor by radiant and (mostly) convective means in a way that produces a very comfortable environment (warm feet, cool head). Other advantages include improved efficiency over other heating methods (particularly forced-air heating because it eliminates duct loses), no distribution of dust an alergens, peace and quiet, and a very flexible range of heating sources. The heat can come from conventional burners, wood stoves, solar water heaters, heat pumps, etc.
I am very interested in the ability of a hydronic system to carry heat from one area to another. Instead of just carrying heated water from the hotwater tank out to the floor, what if we could use it to redistribute heat from the primary storage areas to secondary storage areas?
More to come…