14 June 2002
Although system performance suffers by only about 10 percent for heat exchangers
with effectiveness values as low as 0.3, the popularity of compact plate-type heat
exchangers and their low add-on costs allow the designer to achieve high effectiveness
levels with only a slight increase in equipment cost.
f. Pumps. Heat transfer fluids are circulated by pumps. Two circulation pumps
are required in the system shown in Figure F-1. For the majority of liquid-based solar
energy systems, centrifugal pumps with fractional horsepower requirements are used
for heat transfer fluid circulation.
g. Transport Sub-System (Lessons Learned)
(1) Heat Transfer Fluids. To eliminate past problems with fluid
maintenance, freeze protection, and corrosion control, a USP/food-grade uninhibited
propylene glycol/distilled water mixture is required for systems that need freeze
protection and pure water is recommended for systems that do not
(2) Piping and Transport Sub-System Materials. Materials problems with
piping include corrosion, erosion, and scaling. Corrosion can be avoided by using flow
passages of copper, bronze, brass or other non-ferrous alloys. Pipe erosion and
excessive hydraulic noise can be avoided by ensuring that fluid velocities in closed
piping systems are kept below 5 ft/s (1.5 m/s).
a. Purpose and Experiences. The control sub-system consists of an
electronic control unit, temperature sensors, and interfaces to pumps. A Btu meter may
also be installed for system diagnostics and monitoring purposes. Experience with past
systems has shown that a major cause of system failure has been control systems that
were too complicated and unreliable. Control strategies for solar energy systems
should be as simple and reliable as possible.
b. Control Strategy. Most solar systems use a control strategy known as
differential temperature control. Temperature sensors are located on the collectors and
at the coolest part (the bottom) of the storage tank. Circulating pumps in the collector
and storage loop are simultaneously activated whenever the temperature of the solar
collector is a specified level greater than that of the storage tank (typically 15 to 25
degrees F (-9 to -4 degrees C). The pumps are then shut off when the temperature
difference falls below another limit (typically 5 to 8 degrees F (-15 to 13 degrees C)).
This built in hysteresis helps prevent short cycling of the pumps during start-up as the
colder water from the storage tank comes in contact with the hot collector plate.
c. Diagnostics. The control system can contribute to the system's longevity
and ease of maintenance by providing remote readings of system parameters such as
component temperatures, pump status, and maximum/minimum temperatures. If
installed, a Btu meter can measure the flow rate and temperature of fluid delivered to
storage in order to calculate the total energy contributed by a system. It is possible for
a solar system to be inoperative and yet show no symptoms due to the existence of an