Sizing Examples for
Communications Applications
Radio Repeater
Traveler's Radio
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. To see an example of a communications application that uses a PV/engine hybrid system go here.
Radio Repeater Station
This 350-watt PV system powers a radio repeater in Oregon. The load demand peaks in the summer when use is approximately three times the winter usage. Use of two solid-state charge controllers increase reliability. The solid-state regulators have the low operating temperature range needed for this site. Low voltage disconnect capability is desired but is not possible in the current range for the regulator chosen. A separate 30 amp power relay was installed and activated by the low-voltage disconnect provided by the regulator. Movistors were placed across positive and negative conductors to ground. The negative of the battery was not grounded. This system contains large, deep cycle, maintenance free, batteries. The batteries require no electrolyte replacement and they are not susceptible to freezing. The critical availability of the system is provided by the large storage capacity of the batteries.
Site
Oregon
Location/Elevation
44°N - 120° W - 1300 meters
Environment
Mesa
Temperature Range (°C)
-20 to 35
Maximum Wind Speed (m/s)
50
Availability Required
near 100 % (critical)
Days of Storage
14
Load Profile
Variable daily
Installation
The array was ground mounted on a concrete foundation. Array conductors were run directly into a weatherproof control box located on the back of the array mounting structure. Charge regulators, lightning arrestors, fuses, switches, and load management equipment are all located inside the weatherproof control box. The batteries are contained in an insulated weatherproof enclosure which is ground mounted beneath the array and connected to the control box with watertight flexible conduit. Power leads to the nearby repeater are enclosed in PVC conduit, properly sized to contain the No. 6 copper conductors from the control box. Neither conductor of the PV system is grounded and fuses are installed in both leads. Lightning or transient suppression is provided by varistors between the conductors and a grounding electrode. Current limiting fuses were installed on the battery leads to prevent damage from catastrophic short circuits.
Worksheet 1a-Calculate the Loads
This worksheet is for winter load from October to March. The following worksheet is for the summer load from April to September.
Worksheet 1b-Calculate the Loads
Worksheet 2-Design Current and Array Tilt
Solar data were estimated using data from three locations surrounding the site of this system. Both winter and summer conditions were checked using the respective loads.
The 60° tilt angle will help dislodge snow from the array.
Worksheet 3-Calculate System Battery Size
Flooded industrial batteries were selected. The large battery size (14 days of storage) means the batteries will seldom be discharged deeply.
Worksheet 4-Calculate System Array Size
Worksheet 5-Hybrid Design Determination
Controller Specification
Two controllers were used. The array was split into two separate circuits.
Switches and Protection Components
DC Wore Sizing Specifications
Traveler's Information Radio
The State of New Mexico recently installed several short range radio systems to broadcast historical facts and folklore to passing motorists. These PV powered systems were installed along highways in the southern part of the State. The land varies from high desert to mountains with elevations greater than 12,000 feet. The pole mounted 12 volt dc systems include the PV modules, battery, controller, transmitter and tape player. The messages play continuously. The load demand is dominated by the transmitter; its current requirement depends on the effectiveness of the antenna ground plane. Initial testing showed the current demand to be about one ampere but under worst-case conditions could go as high as 1.5 ampere. This worst-case value was used for the design. State personnel plan to inspect the systems for operation every six months.
Southern New Mexico
31-36°N - 103-109° W - 1000-1500 meters
High Desert
-10 to 45
90 percent (not critical)
5
Continuous
The pole mounted array was tilted at 50° to optimize wintertime insolation capture. The batteries were buried near the foot of the pole and the transmitter antenna was located on top of the pole. Array conductors were run to a weatherproof control box located behind the modules. The negative conductor was grounded to the pole and a movistor was used from the positive conductor to ground. A controller with low voltage disconnect was used. This system uses deep cycle maintenance free batteries and the batteries are large enough to keep state of charge over 60 percent in all but the worst conditions. Operating in this manner, the batteries should provide many years of trouble free service and not be susceptible to freezing. The batteries are connected to the control box with watertight flexible conduit. Power conductors to the antenna are inside the pole. An in-line fuse was installed on the battery conductors. A single pole switch was installed to disconnect array power from the system.
Worksheet 1-Calculate the Loads
DC Wire Sizing Specification
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Acknowledgment and Disclaimer
