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d. Control Sub-System. The control sub-system must first determine when
enough energy is available for collection. It must then activate the entire system to
collect this energy until it is no longer available as a net energy gain. The control sub-
system thus consists of electronic temperature sensors, a main controlling unit that
analyzes the data available from the temperature sensors, and the particular control
strategy used by the controller.
SOLAR ENERGY APPLICATIONS
a. Types of Loads. Due to the intermittent and varying amounts of solar
radiation available, solar systems used to heat service water are usually not intended to
meet the full thermal energy demands of the process being served. For any given
thermal load, an integrated system should be designed which consists of both a solar
energy collection system and a backup system that can meet the full load requirements.
The solar system size and configuration will be a function of the annual or monthly
energy loads. It is up to the designer to specify a system that will be expected to
provide a given fraction of this load. This is in contrast to the design of a conventional
heating, ventilation, and air-conditioning (HVAC) system, which is typically sized to
meet an anticipated maximum or design load with no provision to be augmented by
another source. For this reason, solar systems are often sized to meet the average
expected load. Important characteristics of a load include the amount of energy
required, the time of the demand (load schedule), and the temperature range required.
Each of these factors is discussed below solar service water applications.
b. Service Water Heating. Heating domestic hot water and low-temperature
process water (both referred to as service water heating) will normally be the most
thermally efficient means of using solar energy. The reason is that the demand for
thermal energy for these applications is approximately constant during the entire year,
with the result that auxiliary fuel savings can be realized over the year. In the preheat
configuration, solar heated water is useful at any temperature above that of the
incoming water. An additional benefit is that, when preheating process hot water,
thermal energy may be delivered at a relatively low temperature, which increases the
efficiency of the solar collection process.
F-3. BASIC MATERIAL CONSIDERATIONS IN SOLAR ENERGY SYSTEMS. The
designer should be alert to fundamental materials problems that can occur with solar
energy systems, and careful attention must be given to the materials and fluids used.
Large temperature fluctuations, severe ambient weather conditions, and the variety of
possible fluids and metals that can come in contact with each other are often a cause of
system failure. Some of the basic issues that must be addressed are discussed briefly
a. Metallic Corrosion and Erosion. Common causes of corrosion include the
presence of dissimilar metals (galvanic corrosion), the presence of dissolved oxygen, or
fluids with a chemical composition that adversely affects the wetted metal surface.
Corrosion may be minimized in solar systems by avoiding dissimilar metals, decreasing
the amount of available dissolved oxygen, and treating particularly corrosive fluids with