14 June 2002
Storage Tank Location
Indoor Versus Outdoor. As with conventional energy systems, a solar
system requires an equipment room to contain the heat exchanger, pumps, control
system, and associated plumbing. If possible, the equipment room should be designed
to house the solar storage tank. For retrofit situations where existing space does not
permit the required tank volume, an outdoor location may be chosen. However, many
factors discourage the location of storage tanks outside the building, such as a higher
annual standby energy loss (in most climates) and adverse environmental effects on
the tank (including ultraviolet and moisture-based degradation). Solar storage tanks are
not to be located underground. Underground tanks have had numerous problems,
including leakage due to tank and ground shifting and thermal stresses; corrosion due
to the lack of cathodic protection; tanks surfacing due to buoyant forces while empty;
and difficulty in retrieving and repairing sensors and instruments.
Tank Support and Floor Loads. Reinforced concrete pads and footings
are often required to ensure that the weight of the tank does not endanger the structural
integrity of the building. The design load calculation should take into account the
estimated weight of the empty tank, the water to be stored in the tank, the insulation,
and the tank support structure. The design load for the footing is also dependent on
the type of tank support used.
Legionnaire's Disease. If a direct circulating system is supplying water for
domestic use, ensure that water in the storage tank is heated to a minimum of 140
degrees F (60 degrees C) in order to avoid any potential source of Legionnaire's
disease. For additional information on Legionnaire's disease refer to
Transport Sub-System. To ensure that the transport sub-system is properly
accounted for in the building design, space must be provided in the equipment room for
the heat exchanger, expansion tank, pumps, and system plumbing, in addition to the
storage tank and control system. Pipe chases are also required between the
equipment room and the space on the roof where the system will be located.
Control Strategy. For the control strategy, the designer must specify
operating modes and freeze/over-temperature protection methods. It should be noted
that the control strategy presented for the standard closed-loop system is intended to
be simple, reliable, and built with off-the-shelf components.
Pump Activation. Using the differential temperature controller, the
collector and storage loop pumps should be energized whenever the difference
between the absorber plate and storage tank temperatures is greater than some high
setpoint differential temperature TH, typically 15 to 25 degrees F (8 to 14 degrees C).
The pumps should stay on until that temperature difference is less than some low
setpoint differential temperature TL, usually between 5 to 8 degrees F (3 to 4 degrees