Siemens Solar in telecom: six installations

A repeater on a mountaintop needs a few hundred watts, no fuel deliveries and no power line, which is the case Siemens’ telecom page makes for solar. The Siemens Solar telecom page lists the modules it recommended (SM6, SM10, SM50, SM55, SP18, SP36 and SP65/70/75) and links six installation notes. Here they are with the component lists as published and a few calculations of our own.

El Negrito repeater station, Tucumán, Argentina

System components as published:

• 188 Siemens M75 solar modules, 48 W each
• 48 V system with 4,000 Ah of battery
• 3 Solartec controllers
• Diesel generators as backup in prolonged bad weather
• Load: 800 W constant; array stated as 8,400 W peak

Our arithmetic:

• 188 × 48 W = 9,024 W. The note’s 8,400 W is 7 % lower; the note does not say why.
• 800 W continuous is 19.2 kWh per day. At 5 peak-sun-hours and 0.75 system efficiency the 8.4 kW array yields about 31.5 kWh, 1.6 times the load. At 3 peak-sun-hours it yields 18.9 kWh, about equal to the load. The note gives the diesel as backup for prolonged bad weather; the thin margin at low sun is consistent with that, but the note does not state the sun-hours.
• 4,000 Ah at 48 V is 192 kWh nominal: ten days of load at total discharge, or about five days if the lead-acid bank is limited to 50 %.

Emergency mobile communications and lighting, Los Angeles earthquake, summer 1991

System components as published:

• 4 × M75 modules
• 8 × 220 Ah batteries
• 8 × 30 W DC fluorescent floodlights
• 1 × 12 V, 700 W inverter

Our arithmetic:

• Four 48 W modules give about 192 W at peak. The eight floodlights alone draw 240 W, so the lighting must draw on the batteries; the array can only top them up.

Rural radio-telephony, Mexico

System components as published:

• Cellular repeater stations: 12 to 20 M75 modules with 1,200 to 2,000 Ah of batteries
• Subscriber systems: 3 to 5 M75 modules with 200 to 500 Ah

Our arithmetic:

• At 48 W per module the repeaters use roughly 580 to 960 W of array and the subscriber stations 145 to 240 W. The note says PV was chosen for all radio and repeater stations because it was the most economical and fastest to deploy.

Rural communications plan, Nepal

System components as published:

• M55 modules from 250 to 800 peak watts per site
• Siemens Showa Solar SPS-series regulators controlling battery charging and load supply

Our arithmetic:

• At 55 W each, 250 to 800 W is about 5 to 15 modules per site.

Two installations described only briefly in Siemens’ notes

• North America’s first solar-powered digital telephone switch, installed by GTE near San Bernardino, California, using a Siemens Stromberg-Carlson MLC 24-line digital loop carrier. The note gives no module count.
• An amateur-radio repeater on Sulfur Mountain, Southern California. After the 1994 Northridge earthquakes drained backup power, operators connected solar modules from their personal emergency systems to keep a two-metre repeater available to emergency agencies. Siemens Solar and the Sulfur Mountain Repeater Association then configured a permanent independent system for the inaccessible, fire-prone site.

What the telecom notes show

Where the notes give details, the systems are stand-alone layouts (see the system diagrams): modules, a regulator, a battery bank and, at El Negrito, a diesel backup. Only El Negrito states a load, so it is the only note where the sizing can be checked: its 192 kWh nominal bank holds about ten days of the 19.2 kWh daily load. The notes do not state a sizing rule and do not name the battery chemistry. These sums describe how the systems were built; they are not a design basis for a rebuild.

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