Q & A: Solar Assisted Radiant Heating Systems

My company is beginning to get calls for radiant solar thermal space heating systems, which is a new application for us. Any related design advice for collector, storage and system control approaches would be appreciated.

Radiant floor heating systems are a good fit with solar energy for two reasons. First, the temperature needed for radiant heating is relatively low, and solar collectors work more efficiently for lower temperature applications. Second, if the tubing that circulates the heated fluid from the collectors is embedded in a floor with high thermal mass, such as a concrete slab, the storage for nighttime heating is already built in.

System sizing basics. Every solar thermal space heating system is unique, and sizing is extremely dependent on the project’s location and the building’s construction. In space heating applications, collector surface area in square feet will typically range from 8% to 15% of the building’s square footage of conditioned space. The collector area of systems installed in milder climates with sunny winters will fall on the lower end of the approximation. Where required, storage tanks in locations with higher winter irradiance are sized at approximately two gallons for every square foot of collector surface area. In climates with lower irradiance, the tank should be downsized to about one and half gallons per square foot of collector.

Solar assisted radiant floor systems can provide up to 80% of a building’s heating load, but more often they are designed to offset 50% to 60% of the fuel usage. The solar thermal system displaces some of the heating load by circulating the floor loop fluid through the solar collectors.

System design options. Solar thermal space heating systems can be configured as either drainback or antifreeze designs. Some system designers prefer a closed loop design that uses an antifreeze solution of propylene glycol and water, since the pumps required for its circulation can be smaller. Summer overheating is an issue with antifreeze designs for space heating due to seasonal load imbalance. (See "Thermal Balance" in SolarPro, Feb/Mar 2009 for overheating solutions.) Most installers prefer drainback designs because of the inherent over-temperature protection against high stagnation temperatures.

The simplest space heating design involves integrating the solar collectors directly with the cross-linked polyethylene (PEX) circulation loops. The PEX, in turn, serves as the exchange mechanism to deliver heat to the slab. This approach eliminates components—the storage tank and heat exchanger—but comes with a caveat: A buffer tank is needed to protect the PEX tubing from high collector fluid temperatures.

For heating applications, PEX is rated for service temperatures up to approximately 200°F, depending on system pressure. Any interruption of the pump, including power outages or thermostat controls, that circulates fluid through the collector array during hours of good solar irradiance will soon heat the collector fluid to stagnation temperatures (~ 350°F). When the pump or pumps come back on, the first few gallons of fluid returning to the tubing in the floor are considerably above the PEX service temperature and can cause permanent damage. Including a buffer tank in slab-direct heating systems is imperative. A good example of a buffer tank for this type of system is a drainback tank of sufficient size placed on the hot return piping. The extremely hot collector loop fluid will mix with the cooler fluid in the drainback tank prior to being introduced into the PEX loops.

Most existing radiant floor systems use a simple zone valve to control the flow to separate parts of a floor. A newer type of system gaining popularity uses very small individual pumps instead of traditional valves to control the flow in each zone.

Additional storage approaches. The decision to add a high volume storage tank to a radiant system is primarily based on the tubing installation. If the tubing is embedded in high thermal mass, an additional storage tank is not always required. The minimum thermal mass considered adequate for storage is a 4-inch concrete slab or equivalent, insulated from the ground surface with high-density rigid foam board. Solar assisted heating systems are also being integrated with PEX tubing embedded in up to 2 feet of sand topped with concrete, bricks or stone. Floors made with large amounts of thermal mass can have a heating lag time issue, however. Heat is lost through solids like concrete, brick and sand in all directions, resulting in a lag time of up to several hours from the point of calling for heat at a thermostat to the time the indoor temperature reaches the thermostat setpoint.

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