Chart 1
Most of the electrical energy
from the house meter went
to heat water.
Chart 2
• The solar pump turns on and the tank temperature rises. [B]
• Even when no water is drawn, the tank’s
temperature drops slowly, indicating a low and
constant rate of heat loss. [C]
• Other hot water use, such as late-night washing, also draws down the temperature. [D]
After measurement, the following issues were
noted and corrected when feasible:
• The solar storage tanks lose heat (and energy) even when no water is drawn. These mechanisms may be at work:
- Conductive loss from the storage tanks. To
reduce this loss we added insulation blankets.
These recovered their cost in less than one year.
- Nighttime thermosiphon circulation
may draw heat-transfer fluid from the hot
storage tanks to the cooler collectors. Further
measurements will determine if this is occurring.
If so, the addition of a one-way valve may resolve
the issue.
• The factory set the controller to shut off
the solar pump when the storage tank temperature exceeded 140˚F ( 62˚C). This set point was
raised to prevent much higher stagnation temperatures in the collector plates, which might
lead to degradation of the glycol mixture.
In addition:
• I reinforced the lightning protection by
grounding the glycol pipes directly to the cold
water supply line with heavy copper wire.
• Pipes and collector racks should be raised
from the roof surface to permit rainwater and
debris to flow freely to the gutters. We missed
that issue, but it would be critical in a colder climate to prevent formation of ice dams.
The solar water heater, with the later improvements, has reduced the building’s electric draw
from about 800 k Wh to less than 300 k Wh per
month. After a state grant and the 30 percent
federal tax credit, this system will pay for itself via
energy savings in four years (see “Solar Water-Heating Economics” on page 35). ST
The author installed an OnSet data logger (onset
comp.com) to evaluate performance. He uses the
data to help plan system improvements. The device
records when the solar pump turns on and off, and
it graphs both the temperature near the top of the
storage tank and the electric current used by the
backup heater.
controller and pump also draw power, but it’s a
negligible amount compared to what’s needed by
the backup heater.
Chart 1, above, shows the performance of the
system over several days in April 2008. On the
days when the sun was shining, April 15 to 18,
the solar pump ran, the storage tank temperature rose and the backup heater demanded little
current. Chart 2 shows energy demand was lowest on those days. By contrast, April 13 and 20
were overcast. The pump ran less frequently, the
storage tank temperature stayed lower, and the
backup heater current and energy demand were
higher. Energy use was high on April 14 because
the two previous days were mostly overcast. Residents took their morning showers before sunrise,
so the backup heater did the work.
Chart 1 shows that the storage tank temperature follows a consistent pattern on sunny days:
• Morning showers lower the tank temperature. [A]
