When the rate of heat loss through a structure equals the rate of heat gain (from the sun, in this case), a balance is reached and the interior temperature is stable. With solar gain, the strategy is to put heat in during the day at a much higher rate than it’s lost, increasing the temperature of the mass while the sun is shining and storing heat in that thermal mass. After the sun has set and no more gain is available, interior air and surface temperatures will begin to drop. As the temperature difference is created, heat will begin to be released from warmer surfaces—in this case, the floor slab.
Solar space-heating is not magic, but it is a delicate balance between heat losses and gains. For building design, following a few simple principles will allow you to reap the benefits of passive or active systems:
A solar thermal collector’s performance is crucial to an active system’s efficiency. This performance can be expressed in graphical form as efficiency (fluid power out divided by solar power in) versus a combination of DT and solar irradiance. The Solar Rating and Certification Corporation (SRCC) tests collectors for performance efficiency and for average daily energy output. I didn’t have this data for my collectors, which were reclaimed units with new absorbers and insulation. So besides getting a handle on the collector’s unknown efficiency, measuring the actual performance of the entire system would allow me to get a much better idea of how well it was operating so I could improve the system’s output.
To get reliable data, the system must operate in steady conditions of consistent flow rate and irradiance, so I only used data from sunny days during the heating season, and disregarded data during start-up and shutdown conditions. Fluid temperatures were measured going into (supply) and coming back from (return) the collectors. Solar irradiance was measured and then converted to total power, in watts, landing on the collectors. Fluid power in the loop was calculated with the flow rate, and DT (return minus supply temperatures).
The data shown in the January 18 graphs was collected on one of the year’s coldest days; the ambient temperature was -3°F at sunrise and the average ambient temperature during the test was 25°F. The DT (at a constant flow rate of 1.6 gallons per minute) is proportional to the amount of heat going into the floor. It includes the drainback tank losses and two short runs of uninsulated pipe inside the building. The difference between supply and ambient temperatures (Ti – Ta) represents the potential for heat loss from the collectors. SRCC uses this value, and the irradiance (I) to characterize the “harshness” of the collector operating conditions. Higher Ti – Ta translates into lower collector efficiency, and higher “I” values mean higher efficiency.