Solar Hydronic Heating.

Calculations which might be done to size system.

I'm trying to put in a hydronic loop on the north side of my house, also.

The power from the sun maxes out at about 1Kw/sq.meter. If we were to start with one 4'x 8' collector, we would have about 3 sq.meters, so at 50% efficiency we would have 1.5Kw of energy in full sun.

One would have to find the average hours of sun for a specific location to see how much average energy one can expect to collect.

For my area with 2.5 hours of sun, I might expect to capture 3.75Kw of energy a day. In S.Ca with 6 hours of sun a day, one might expect to get 9Kw of energy per day. Arizona might be more.

Looking it up,I get 3412BTU/Kw-hr, so I'd be collecting 3412*3sq m=around 5,000BTU/hr. at 50% efficiency with the panel. For 2.5 hours, I'd get a total of an average of 12,500 BTU per day with this system, in my region.

I could, for example, use a drainback system from a hot water heater in which the panels are located on the roof and a pump starts pumping water from the tank to the panel whenever the temperature in the panel is greater than that in the tank. When the temperature of the panel falls lower than that of the tank or the temperature in the tank gets near boiling, the pump will stop and the water will drain back into the tank. In this way no antifreeze would be required in freezing climates. I would capture all useable energy.

Whenever the thermostat calls for heat, the hydronic system will pump hot water from the solar storage tank whenever its temp is greater than a certain temperature. If its temperature drops below this setpoint, alternative heating sources could be employed.

I could use one tank as a solar tank and a second tank as a alternate fuel source heated water tank, for example. I would draw off the solar tank as long as the temperature was great enough. Then the solar tank pump or valve would be switched off and the water would come from the alternate tank.

To figure the amount of water required is not too hard. The greater the amount of water, the more efficiency from the solar panels. This is because the greater the delta t (difference in temperature between the panels and the water) the faster the heat transfer will occur and the less energy will be lost from the panels into the outside air.

If the water is stored in the area to be heated, then all heat lost will actually not be lost. The more thermal mass in the area to be heated, the less the temperature will fluctuate with large inputs and withdrawals of energy.

If you want to calculate the amount of thermal mass, you can do this also, based on the amount required to absorb the maximum daily heat input with only a couple degree change in temperature.

For example, the heat capacity of pure water, by definition states that it takes 1 calorie to raise 1 gram of water 1 degree celsius. 1 calorie is 0.00397BTU, so we can make up a chart or graph and plot the number of grams of water against the temperatures at a given BTU input. We can plot this at the maximum and minimum expected energy inputs to get the possible temperature ranges for a given amount of water. 1 gram of water takes up a volume of 1 cubic centimeter (more or less, depending on the temperature). We can figure from this the number of gallons, knowing there are 0.0002642 gallons in one cc.

Hope this was not too complicated. With only 3 sq meters of collector area it shouldn't require a whole lot of water to keep it from boiling.

People I know tell me the simplest systems are the best. They say passive solar requires zero maintenance. It can also be quite efficient if it is designed properly.

For example, I could use a hot air collector of the same size and a simple fan and baffle to circulate the air when the temperature of the panel is greater than that of the house. I could employ enough thermal mass in the house to absorb the heat from the panel with minimal temperature fluctuation.

One friend of mine built a thermosiphon system which collected hot water on the roof thermosiphoning to a blower/water to air heat exchanger in the dwelling unit below. When the temperature got above a certain point the thermostat on the blower came on, releasing the heat into the house. If anyone is interested in patenting his design, and making a production version, let me know.

To simplify this further, I could employ glass on the south facing side of the habitation which is superinsulated and contains a substantial amount of stone within. An overhang could shade the windows in the summer and possibly insulating blinds could be employed at night in the winter.

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