Solar Thermal Information

44
STORAGE TANK SIZE IN COMBISYSTEMS:
The size of the combisystems’s storage tank affects its
annual performance. A rule of thumb is to size storage
tanks in combisystems within the range of 1.25 to 2.5
gallons per square foot of collector area. Tanks larger
than the upper end of this range usually show little return
on the extra investment. Research has even shown that
tanks larger than about 3.7 gallons per square foot of
collector can decrease annual system performance due
to increased heat losses.
Figure 6-5 shows the affect of storage tank size on the
annual solar heating fraction for the 8-collector system
on the larger house in Colorado Springs, as discussed
earlier in this section. There is an increase in annual solar
heating fraction as tank volume increases, but the rate of
increase goes down as the size of the tank increases.
SUMMARY:
Solar thermal combisystems are a natural extension of
solar domestic water heating. Their objective is to provide
a high percentage of the building’s domestic hot water
and some percentage of its space heating energy.
There are countless variations in how solar combisystems
can be designed. A system that’s “perfect” for one situation
may not be suitable for the next. However, all combisystems
should address the following considerations.
• For best performance, solar subsystems should be
interfaced with space heating delivery systems that operate
at low water temperatures. Do not simply assume that
a solar subsystem can interface directly to any existing
space heat distribution system. A suggested maximum
operating temperature for a space heating distribution
system that will interface to solar collectors is 120ºF.
• All combisystems must include mixing assemblies that
prevent potentially high-temperature water generated by
the solar subsystem from reaching low-temperature heat
emitters. This mixing device can be operated based on
outdoor reset control logic to allow the lowest possible
water temperatures to satisfy the load.
• All systems should include a simple and automatic means
of switching the distribution system from using the storage
tank as the heat source to some form of auxiliary heating
when necessary. Building occupants should not experience
any loss of comfort as the system transitions between these
heat sources. An outdoor reset controller that calculates the
lowest possible temperature at which the solar storage tank
can supply the load is ideal for selecting which heat source
supplies the distribution system.
• All solar combisystems benefit from the same state-of-
the-art hydronics technology used in non-solar hydronic
systems. These technologies include pressure-regulated
circulators in combination with valve-based zoning,
hydraulic separation, manifold-based distribution systems
and high-performance air separation.
• All combisystems should provide year-round preheating
of domestic water, and thus utilize solar-supplied heat at
its lowest possible temperature.
• For good performance, the effectiveness of the collector-
to-storage heat exchangers in antifreeze-based systems
should be at least 0.55 (see appendix B for information on
calculating heat exchanger effectiveness).
• Systems should be designed with the assistance
of software. The latter can be used to estimate the
combined effects of many variables that vary from one
system to the next.
• All piping components should be well insulated to
minimize extraneous heat loss and optimize the delivery
of heat precisely where and when it’s needed.
• Optimum collector orientation in combisystem
applications is generally due south, with a slope angle of
latitude plus 15 degrees.
• Storage tanks in combisystem applications should be
sized between 1.25 and 2.5 gallons per square foot of
collector area. Tanks larger than this add very little to
annual system performance.
25
25.5
26
26.5
27
27.5
1 1.5 2 2.5 3
Annual solar heating fraction (%)
storage tank size
(gallons/sq ft of collector)
figure 6-5