GREEN SYSTEMS SOLAR SOLUTIONS
Alternative backup heat sources I
BY BRISTOL STICKNEY CONTRIBUTING WRITER
n a well-designed solar heating system, the solar collectors provide most of the heat throughout the year, and the conventional boiler or other familiar heat
source becomes the “backup system.” Even in the sunniest 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 complete without a backup system that is reliable, fuel- efficient and properly controlled. In a solar water heater, the backup can be as simple as
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an electric heating element that only turns on when the water drops below “shower temperature.” In solar combisystems 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 center,” 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.
Figure 31-1
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 relatively easy and if the potential for savings is real.
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 relatively 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 can be started
and stopped on demand (only when heat is needed) can be installed, similar 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
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phc february 2011
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