Solar Solutions Bristol Stickney, chief technical director, SolarLogic LLC, Santa Fe, N.M.
Bristol’s Six Principles for Good Solar Hydronic Design Alternative backup heat sources
I
n a well-designed solar heating system, the solar col- lectors provide most of the heat throughout the year, and the conventional boiler or other familiar heat
source becomes the “backup system.” Even in the sunni- est climates, there are days when the heat provided by the sun is not enough to satisfy the entire heating load. Whether this is due to extreme weather or extreme heat demand, even if the situation is rare or intermittent, the need for heat must be satisfied to keep the users satisfied. This is why no contemporary solar heating system is com- plete without a backup system that is reliable, fuel-effi- cient and properly controlled. In a solar water heater, the backup can be as simple as
an electric heating element that only turns on when the water drops below “shower temperature.” In solar com- bisystems installed in recent years, condensing boilers have become very popular backup burners because of their small size and high fuel efficiency. A typical backup boiler might burn propane, natural gas, electricity or fuel oil. The controls must prevent the burner from firing whenever solar heat is available at a useful temperature, either directly or from the solar heat storage. Figure 31-1 shows the typical piping configuration that
I have installed many times to provide these functions in a solar combisystem. Using a primary/secondary “flow cen- ter,” the solar collectors, heat storage, backup boiler, domestic hot water (DHW) and space heat zones can all be connected in a standard plumbing pattern. Notice that there are three heat sources and three heat loads. Solar combisystems always include multiple heat sources and loads. Even in this simple home heating design, the solar collectors, the boiler and the heat storage can provide heat (sources), while the heat storage, DHW and living spaces may demand heat (loads). The heat storage system can act as a source or as a load.
Unconventional backup heat One major benefit of using a flow center design is that
any source (and any load) can be “plugged-in” with two pipes. In some installations this has worked out nicely using alternative heat sources; some of them have been unconventional. When designing for energy efficiency, “waste heat” and convenient local fuel sources are hard to ignore, especially when they are expedient, plentiful and inexpensive. If you have multiple local resources, you might want to connect three or four heat sources, or even more, if the connection is rela- tively easy and if the potential for savings is real. These resources generally
come in two flavors: On demand and Intermittent. Figure 31-2 shows how other sources of heat can be easily added to a primary loop using two pipe connections at closely spaced tees. In a primary loop, the heat sources are best placed in order of lowest temperatures first and higher temperatures last, around the loop.
On demand Alternative heat sources that
Figure 31-1 Page 22/Plumbing Engineer
can be started and stopped on demand (only when heat is needed) can be installed, simi- lar to a “boiler” and controlled similar to multiple boilers. This is done most often to take advantage of alternate fuels, as the price of fuel changes over time or the performance of the equipment changes seasonally. So, for example, a ground- Continued on page 24
February 2011
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