Solar Heating Design
Heating System – Solar
I never realized how many options there were to heating a house. The house will be heated using radiant floor heat and solar hot water collectors. That is a fact. Where things start to get murky is how to store the heat and what options to use for backup heat when the sun is not shining.
The most important aspect in this design is KEEP IT SIMPLE. I have to sometimes remind myself of this. The more controls, valves and pumps there are, the more things can go wrong. The “keep things simple” concept also helps to eliminate some options. With that in mind here are the key points to the design:
- Keep it simple
- Keep it as self-sustaining as possible. (In other words, it must run itself without me having to mess with it.)
- Longevity (Costs might be higher up front, but design a system that will last without any major overhaul.)
- Use renewable energy whenever possible.
I started the design by first determining what our heating requirements will be for an energy efficient, cordwood home. The heating industry uses Btu’s (British thermal unit) to measure heating requirements. A Btu is the amount of heat required to heat 1 pound of water by 1 degree Fahrenheit (in case you were wondering).
The calculations were done based on an outside temperature of -20F degrees and an indoor temperature of +70F degrees. Insulation values for the house were used as follows: double cordwood wall (~R37), ceilings (~R55), windows (R3) and doors (R5). Heat loss through the footings and the heat loss of air infiltration were also taken into account. The end result was a heat loss of ~25,000 Btu/Hr. for a worst case scenario. For a 2,000 sq.ft. home, that’s pretty darn good!
In most cases, when the temperatures fall way below zero, the sun is usually available to heat the house. The solar collectors will be positioned to take advantage of the winter sun and should be able to heat the house adequately. But, the first question that needs to be addressed is how to store the heat? There are two very good ways to store the heat: water and sand.
Heat Storage using Water
Water is by far one of the best materials to store heat. Water will store ~62.4 Btu/F/cu.ft. To put things into perspective, that’s about four to six times the heat capacity of sand! Quite a few radiant systems use large tanks of water to store excess heat. The disadvantages to using a tank are that they take up space, need occasional maintenance, and sometimes leak. Water is also corrosive to certain materials.
Heat Storage using Sand
Sand does not nearly have the same capacity to store heat as water, but it can be placed under the house to store heat. The advantages are it’s a material that is easy to use and doesn’t require awkward tanks to store it. The disadvantage is that you need lots and lots of it, and it is not as efficient in transferring heat.
There is some debate as to how well sand performs especially when it is dry. The dryer the sand, the less efficient it becomes.
I had read a few articles in Home Power magazine regarding two homes that used this concept in central Wisconsin. The homes were heated using solar hot water panels and masonry stoves as backup. The sand beds were two feet thick under their homes with radiant floor tubing run throughout the sand bed.
After a bit of research, I was able to find the phone numbers to two home owners and I gave them a call to find out how they like their systems. In both cases, they seemed quite pleased with their systems and the only downside to their systems might be the fact that they lack some control over the amount of heat stored.
The most striking aspect of this system is its simplicity. Although there are valves used in this system, the only electrical needs to the system is a 12 volt DC pump that is powered by a PV panel. This means that the system runs only when the sun is out and is totally self-running.
The disadvantage to this system is somewhat the lack of control and the long heat lag time to heat or cool down the floor. If you need heat quickly, you’ll need to use your backup heat system and if you turn the system off, it might take days for it to cool down.
This is where a water tank system really does have an advantage. You can control the battery. You can pull heat out of the tank only when you need it. But in doing so, you forfeit simplicity and the ability to be self-running. It will require electronic controls, valves and more than one pump.
The next journal entry will discuss backup systems. A backup system will be required for extended cloudy periods and in case we might be away from our house for an extended period.