Sizing Examples for
PV Systems for Security and Health Applications
Navigation Beacon
Lighting Security
Vaccine Refrigerator
In this section you will find filled-in worksheets to illustrate the calculations required in determining the size of power system needed to meet the load. Blank worksheets and instructions may be accessed and copied by going here.
This example is a flashing beacon mounted on a navigation buoy in the shipping channel near Port Arthur, Texas. The load consists of a single lamp operating 1.0 second on and 3.6 seconds off during hours of darkness. The lamp draws 2 amperes when lighted. The flasher controls the lamp and draws 0.22 amperes when the lamp is lighted. There is a surge current of 0.39 amperes each time the flasher turns on. This current flows approximately 1/10 of the time the flasher is on. A photocell controls the hours of operation. The design has 14 days of battery capacity (required by regulation) and does not include a charge controller. Provision is made to disconnect the load if the battery voltage drops below 11 volts.
Site
Offshore Texas
Location/Elevation
31°N - 110° W - 5 meters
Environment
Salt Spray
Temperature Range (°C)
-15 to 42
Maximum Wind Speed (m/s)
60
Availability Required
99 percent (critical)
Days of Storage
14
Load Profile
Hours of darkness - 10 in July to 13 in December
Installation
The photovoltaic array is mounted horizontally above the beacon using an anodized aluminum framework mechanically and electrically attached to the metal body of the buoy. The module is wired to the battery using a thermoplastic jacketed TW cable designed to be both sunlight resistant and weatherproof in a marine environment. No fuses are used but a blocking diode is installed in the positive lead between the battery and the module. The diode is located inside the battery compartment for protection from the environment. The conductors for the beacon are of the same type as the array conductors. Both array and load conductors are run in conduit attached to the buoy structure to prevent them from becoming abraded or damaged. The batteries are installed inside a vault on the buoy with wire access through grommet-protected holes. All battery terminals are coated with a corrosion inhibitor. A light sensing photodiode switch used to control the hours of operation is installed inside the standard beacon fixture. The fixture is sealed to protect both lamp and photodiode from the salt-spray environment. All wiring connections are made with crimped and soldered connectors. Strain relief is provided.
Worksheet 1-Calculate the Loads
Worksheet 2-Design Current and Array Tilt
Worksheet 3-Calculate system Battery Size
Worksheet 4-Calculate System Array Size
Switches and Protection Components
DC Wire Sizing Specifications
Security Lighting
An officer at a military base near Yuma, Arizona, was required to install security lighting on a one acre area. Overhead lines were prohibited and underground service was expensive. Her options were PV or a diesel generator and after she investigated pole mounted PV lights the choice was obvious. She could provide enough light to guard against intrusion with three 18-watt pole lamps. They were independent, no interconnect wires to run, and they could be located at any convenient place within the designated area. She decided to put one pole light near the guard shack, double the power system size on that unit, and provide an 18-watt lamp inside the shack. All lights were to operate all night every night so she decided to use a current sensing relay in the PV array circuit to control the lamps. Any time current was flowing from the array the lamps would remain off. With this control method, all three independent lights would turn on and off at approximately the same time.
Yuma, Arizona
32°N - 114° W - 300 meters
Desert
-5 to 45
40
95 percent (critical)
7
Dusk to Dawn
The base engineers designed the system and hired a local contractor to build a pole kit with PV modules mounted above the lamp and the battery enclosure mounted to the pole near ground level. The modules were prewired and assembled in an aluminum frame that was attached to the pole at the proper tilt angle. The array conductors were run down the inside of the metal pole to the control box mounted to the pole behind the battery enclosure. The battery box was shaded with a metal overhang to maintain temperatures near ambient. The metal pole was used as the ground and the negative conductor was bonded to it in the controller box. A lightning rod was bonded to the pole and extended above and on the north side of the modules. A design and operations manual was generated and a maintenance logbook was placed in the guardshack. Personnel were instructed to keep a record of all maintenance actions.
A total of 4 lamps were used, three pole lamps plus a guard shack light. The 13 hour usage is winter-time estimates for dark hours in Yuma, AZ. Since the batteries will not experience cold weather they were derated only 10 percent.
The array tilt angle was set to 50 degrees - latitude plus 15° - to maximize wintertime energy production.
Worksheet 3-Calculate System Battery Size
Sealed batteries were selected but the designer plans to keep them at greater than 50% state of charge at all times. The sealed batteries will require less maintenance since no water will be added during the hot season. However, the batteries may not last as long as flooded batteries in the high temperature months.
It is important to select a module that will have sufficient voltage to charge the batteries during the hottest temperatures. A PV module that generates 14.9 Volts at 75°C was specified.
Worksheet #5 was not used for this application since a hybrid system is clearly not appropriate.
The following worksheets were used to specify characteristics of the BOS required for the system.
Controller Specifications
Vaccine Refrigeration/Freezer
A PV powered refrigerator/freezer was needed for medical vaccine storage on the remote tropical island of Roatan, Honduras. A dual compressor R/F unit was chosen. Each compressor operates independently and draws 5 amperes when operating. The sealed lead-acid battery bank is enclosed beneath the R/F unit. This arrangement reduces the chance of accidental contact with the battery terminals and the room is well ventilated so no danger of gas build up exists. The R/F is used every day with the active compressor time estimated at 9 hours per day for the refrigerator and 5 hours for the freezer in the summer time with corresponding wintertime numbers of 7 hours and 4 hours. The operators of the clinic were briefed on the operation and maintenance of the installed R/F and told how to conserve energy by keeping the R/F closed as much as possible.
Roatan, Honduras
16°N - 86° W - 30 meters
Island
15 to 35
Variable
This refrigeration system uses a roof-mounted array that supplies power to the internal batteries in the R/F. The array was placed on a portion of the building roof that was free from shadows caused by vent stacks, chimneys, trees, and overhead wires. The array conductors were routed in conduit around the base of the building and entered the building at the back of the R/F. The system switches were mounted on the wall adjacent to the refrigerator/freezer unit. After the batteries were installed in the R/F, it was carefully leveled to provide optimum operation. The location chosen for the refrigerator/freezer was in a room that provided good ventilation for the compressor while keeping the unit out of direct sunlight.
Worksheet 1-Calculate the loads
Worksheet #1 was filled in for both summer and winter seasons since the summer time use was expected to be higher. However, the solar asset was expected to be higher in summer so a comparison of load vs. solar availability had to be made for both summer and winter. See worksheet #2 below.
The refrigeration unit gave separate access to freezer and refrigerator. Each had its own compressor.
The solar data for this location was estimated using worldwide solar maps. The array was installed horizontal.
The modules were derated only 5% because the operators were instructed to clean them weekly.
Worksheet 5-Hybrid Design Determination
Controller Specification
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Acknowledgment and Disclaimer
