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Trends in Technology: Alternative Power
Take a look at Figure 1, which is a basic PV system. The solar array is configured to charge large batteries; dc to ac conversion (by way of an inverter) is then used to generate the actual 60 Hz/120Vac to energize the load. Figure 2 shows an auxiliary energy source applied in parallel with the PV array (thus making it a hybrid system).
So, let’s say you want to build a remote mountain-top transmitter site, which is completely off of the grid, with no practical access to utility power. The following items will be in constant use:
> New FM transmitter, with 1kW of TPO
> Main and backup STL receivers
> Main and backup audio processors
> ISM-band radio (provides duplex IP connectivity)
> One Layer-2 (Ethernet) switch
> Main and backup remote controls
> Four 26W CFL lamps
The very first part of the process is determining what the total ac power load will be. In order of usage, we have the transmitter, for which I will use an ac to RF efficiency of 60 percent, thus requiring 1667W; the four CFLs require 104W; the audio processors, each 50W; the STL receivers, each 30W; the remote controls, each 30W; the Layer-2 switch, and the ISM-band radio, each 10W. The grand total is just over 2kW (which we’ll use for our calculations).
Through every aspect of the following calculations, you will note there are always efficiency factors to consider. The first one is called “round-trip” efficiency, which is basically the amount of energy recovered divided by the amount of energy stored in a device, such as a battery. A typical round-trip efficiency is 80 percent (we will only recover 80 percent of the energy stored in our batteries in this particular case). So, before calculating the size of the PV system or batteries necessary, we take our 2kW constant load and divide by 0.8, giving us 2,500 going forward.
Rule-of-thumb is that for power levels between 1kW and 3kW, the battery systems operate at 24Vdc. (Below 1kW, 12Vdc is used; above 3kW, 48Vdc is used.) The PV array must match that, so we’ll be looking at 24V solar arrays.
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