Solar for Telecommunications
Renewable energy systems have proven to be reliable methods of powering telecommunication systems in places where conventional electricity is unavailable or impractical. RTU, microwave repeaters, television and radio repeater sites are commonly located on mountaintops or otherwise remote sites not easily accessed during winter and inclement weather. Solar, wind and hydrogen fuel cells provide excellent sources of clean, reliable power to keep batteries charged in these locations. Even if grid power is available at these sites, a renewable energy system can provide security as a back-up in the event of grid failure.
The next task is to calculate the solar array size or wind generator size for the location of the radio site. Our data indicates that the area around Asheville, North Carolina gets 2.94 hours of full sun equivalent on an average day in January (worst case scenario). You can find the solar insolation or average wind speeds for your or any area on earth by going to the NASA web site at http://eosweb.larc.nasa.gov. If we know that we need to make a minimum of 17.8 amp hours per day 365 days of the year we can divide 17.8 amps by 2.94 and we find that our solar panel needs to make a minimum of 6.05 amps per hour of full sun equivalent. That means we would need to find a solar module with a Max. Power Current of at least 6 amps. This is approximately a 100 watt solar module. To calculate the size of a wind generator to do the job we would look at the average wind speed for the site keeping in mind that our site may or may not have the same wind speed average as that shown on the NASA web site. If the worst case is shown as less than 5 meters per second (M/S), the site is not acceptable as a wind generator site. Most wind generators in common use reach their rated power at 10 to 11 M/S and their output is usually given as a percent of their full rating at various M/S windspeeds. Our data shows that in general, the windspeed at Asheville ranges between 3.50 and 4.96 M/S throughout the year. This would not be a site that would benefit year around from a wind powered installation. However, for educational purposes, let’s say that the average daily wind speed ranges between 6.5 and 9 M/S at our particular site. The lower figure is the one we need to work with because we have to make at least 17.8 amp hours, 365 days of the year. Most wind generator manufacturers show output of their wind generators in both MPH and M/S. However, if only one figure or the other is given it is simple to make the conversion. To convert MPH to M/S divide MPH by 2.2. To convert M/S to MPH multiply M/S by 2.2. Let’s say that the wind generator we are considering is a 600 watt (full rated power at 11 M/S) unit. It is listed at 40% of rated capacity at a windspeed of 6.5 M/S. 40% of 600 watts is 240 watts (20 amps at 12 volts) so we know that it would only take about 1 hour of good wind (6.5 M/S) for the 600 watt wind generator to provide our daily power requirements.
Since many communications systems require equipment of varying voltages working in tandem, the calculations become very convoluted. We suggest that you print a copy and complete the following worksheet and fax it to us so that one of our technicians can assist you in sizing your system. Our fax number is 406-642-9768. We are always happy to assist you in your sizing efforts and provide you with a cost-effective quote for the right equipment to do the job.
SINGLE VOLTAGE SYSTEM SIZING
To size a simple (single voltage) repeater or RTU site we first need to evaluate the power consumption of the radio in both stand-by and transmit mode. We also need to determine the duty cycle (amount of time the radio is actually transmitting data during a 24 hour period) of the radio. Once we have this information, we need to know the amount of solar or wind resources available at the actual radio site. We also need to establish a period of autonomy for our system, which means that in the event of extreme overcast or calm winds, we want to know how long the system needs to be operable. This information will determine the size of the battery bank.
EXAMPLE: Let’s take the case of a simple RTU to monitor the flow of fluid in a pipeline. The equipment is 12 volt and draws .48 amps in stand-by mode and 3 amps in transmit mode. Transmit occurs for 1 second every 20 seconds around the clock. The system is located on a mountain top near Asheville, North Carolina and needs to have 10 days (minimum) of autonomy.
First, let’s calculate the standby power consumption; .48 amps X 24 hours = 11.52 amp hours per day consumed.
Next, let’s calculate the transmit power consumption; 1 second per 20 seconds is the equivalent of 3 seconds per minute or 180 seconds per hour or 4,320 seconds per day. This is equal to 1.2 hours of transmit per day at 3 amps. Multiply 1.2 hours X 3 amps and we get 3.6 amp hours of power consumed during the 4,320 transmit cycles. Next, let’s add the power consumption of the stand-by and transmit cycles and factor in 15% inefficiency for wire losses, battery inefficiency and losses in the electrical path. 11.52 amp hours + 3.6 amp hours = 15.12 (total power consumption) amp hours per day. 15.12 amp hours X 1.15 (inefficiency factor) = 17.38 amp hours per day that have to be produced to keep up with the demand of the radio as well as cover losses in the wire and batteries.
Next, we can calculate the battery size for 10 days of autonomy by multiplying the amp hour per day consumption by 10. This tells us we need a 174 amp hour battery to carry our loads for 10 days in the event of overcast or calm winds. Another factor that warrants consideration is battery temperature. Batteries loose storage capacity as they get colder so if we encounter a temperature of less than 80 degrees Fahrenheit (26.5 degrees C.) we need to increase the battery bank accordingly. For instance, a battery at 32 degrees Fahrenheit (0 degrees C.) will only have 65% of the storage capacity as the same battery at 80 degrees F. (26.5 degrees C.). If we assume the overcast or calm wind period will occur during 32 degree weather, we should increase the battery size by 35% to compensate for the cold battery. Therefore we need to multiply 174 amp hours X 1.35 which will equal 235 amp hours. We can then look for a 12 volt battery (or a combination of 2 volt or 6 volt batteries) with a storage capacity of 235 amp hours.