An amazing article that we saw in "Home Power Magazine", Enjoy it.
I recently had a solar hot water (SHW) system installed on my house. It is a closed-loop, evacuated-tube system with a heat exchanger in the storage tank. The differential control (a Caleffi Solar Plus) provides variable pump speed control. Currently, it is using factory defaults for the conditions that determine pump speed. The system has four temperature sensors and a data logger, which help me keep tabs on its function.
I like the idea of a variable-speed pump because it seems like it can add efficiency to an SHW system—just like an MPPT controller does to solar-electric systems. That said, I am not sure what conditions drive the efficiency of the SHW system. Should the control be set for lower flows and higher temperatures, or higher flows and lower temperatures? I know there is more heat transfer at the exchanger with a higher temperature difference between the transfer fluid and tank water, but I am not sure how that balances with the collectors’ lower efficiencies at higher temperatures.
Is there a reference I can use to figure this out? I am not looking for specific numbers—more like basic explanations of relationships between the parts of the system and what to look for as a sign of how well an SHW system is performing. Perhaps it’s something like checking transfer-fluid temperature drop across the heat exchanger versus the temperature rise at the collector.
Jack Herndon • Seattle, Washington
An ideal collector loop of any SHW system would operate at a difference in temperature of just a few degrees between the inlet and outlet temperatures of the collectors. The higher this differential, the more heat is lost to the outside atmosphere.The Answer:
This loss is dependent on the outside temperature. Although evacuated tubes are more resistant to heat loss, they are not immune to it. If you’re seeing a temperature difference of 50°F or greater, your system is suffering from a low flow rate problem. A system with a 20°F difference is much closer to operating at an “ideal” temperature.
The ideal is a compromise between the lower inlet/outlet differential to minimize heat loss, and a high-enough differential to prevent the control from short-cycling. Short-cycling will occur with too high of a flow rate and will be noticeable—the system will turning on and off excessively. Turning on and off is normal in the early morning and late afternoon and in cloudy weather, but shouldn’t happen in mid- day bright sun.
Chuck Marken • Home Power solar thermal editor
Is it more energy efficient to turn off your home’s heat when you’re going to be gone all day, or to leave it at a slightly lowered set point? I realize that this is likely a complex calculation involving volume of space, outside temperatures, building envelope and insulation, number of degrees in drop and recovery, elapsed time, type and cost of heating fuel, etc. But perhaps there are some general rules or simplified formulas that can direct a homeowner on the best approach.
Temperature Differentials, With & Without Heat Exchangers
The short answer is that leaving your thermostat at a very low set point will almost always result in lower energy consumption. The long answer follows.
For most residential heating systems, the thermostat controls the heating system to maintain the set point (the temperature you set). It does this by turning the heating system on and off. As you would expect, the room temperature will fluctuate from the set point, unless you allow the heating system to cycle on and off very quickly, which will prematurely age your equipment.
During cold weather, your house is continually losing heat to the outdoors. It does this in several ways. Heat is lost by conduction through the surfaces of the house; warm air exits the house while cold air enters (infiltration); and to a lesser extent, your house radiates heat outward. Of course, it gets more complicated, since your house has a great many parts, each of which have different thermal conductivities, thermal capacities, and radiative properties.
The net effect of all this complicated heat transfer is that a typical house will (almost always) lose more heat when the inside temperature becomes higher relative to outside. I say “almost always” because it’s possible to have net heat gain on a cold day if it’s very sunny, and your house is well-insulated and sealed.