I’m fortunate to live in a sunny climate that averages 5.8 peak sun-hours per day. But I also live at 5,600 feet above sea level, where the heating season is much more significant than the cooling season (5,700 heating degree-days versus 700 cooling degree-days). Despite this, and that my 620-square-foot shop’s south face receives morning shade from a neighbor’s house and tree, my two solar systems—passive solar heating and a solar hydronic floor system—provide enough heating to make the building comfortable in all but January, the coldest month of the year. Even then, the relatively warm surface temperatures and warm floor allow me to work without gloves in air temperatures as low as 45°F (the lowest recorded interior temperature). During the coldest times of the year, solar heating typically keeps temperatures between 50°F and 65°F. At first light, due to the previous day’s solar heat collection and storage, temperatures are generally 40°F above ambient, a testament to the power of the sun and a little RE technology. Paired with passive solar, my drainback hydronic heating system offers two main benefits:
From the two 4- by 10-foot, flat-plate collectors mounted on the shop’s south-facing wall, water flows through a small drainback tank and then directly into the radiant tubing embedded about 3 inches deep in the 4-inch-thick concrete floor. As it circulates through the tubing, the solar-warmed water gives up its heat to the slab and then returns to the collector for reheating. In turn, the slab transfers heat to the shop’s interior by convection and radiation.
This straightforward system has only two control mechanisms: the sun shining on the PV module that drives the circulator pump, and a manual switch used to turn off the pump (and therefore, the system) during the summer months when additional heating is not needed.
Designing any energy system is all about the load. With a solar-electric system, it’s your electrical loads you’re concerned with. With solar heating, either passive or hydronic, that load is what it takes to heat an interior space. It’s a little less straightforward than sizing a PV system. Knowing the size of the heating load requires intimate knowledge of your climate and your building envelope, including the thermal resistance of all the surfaces.
All heat transfer is driven by temperature difference (delta T or DT). The rate of that heat transfer depends on the DT and the amount of thermal resistance (R-value) at the locations of those temperatures. Lower R-values will result in higher heat transfer rates.