| What about situations in which remote or non-grid-connected power is needed, but that power must always be available - for example, to keep vaccines cold, or a rural clinic's lights on, or communications equipment running continuously? Or the times when users know they'll occasionally need a larger amount of power than a PV system can supply alone - say, in a national park camping ground, where it's difficult to predict how much power will be needed? In those cases, PV is still a practical choice. We just need to add an electric generator that can work effectively with a PV system to supply the load.
During the day, the PV modules quietly supply daytime energy needs and charge batteries. If the batteries run low, the engine generator runs at full power - its most cost-and fuel-efficient mode of operation - until the batteries are charged. And, in some systems, the generator makes up the difference when electrical demand exceeds the combined output of the PV modules and the batteries.
Systems that use several types of power generation have the advantages of each one. Engine generators can produce electricity any time. So, they provide an excellent backup at night or on cloudy days for the PV modules, which produce power only during daylight hours. The advantages of a PV system are that it operates quietly and does not pollute. As to the rather high initial cost, we can select a smaller PV system if we operate it with a generator.
Where no other form of power generation is available, the PV array and the battery storage have to be large enough to supply nighttime electrical needs. However, having an engine generator as a backup means fewer PV modules and batteries will be necessary to supply power whenever it's needed.
Including a generator makes designing a PV system more complex, but it's still easy to operate. In fact, modern electronic controllers allow these kinds of systems to operate automatically. Controllers can be set to automatically switch generators, to supply AC or DC loads, or to do some of each. Wind generators, small hydro plants, and any other source of electrical energy could also be added to make an even larger hybrid power system.
A PV-Powered Telephone Signal Booster in Nevada
The place: a remote signal station in the mountains of northern Nevada. The situation: Sprint Communications decides to reduce the amount of time the company runs its propane-powered electric generators by adding PV modules.
When Sprint Communications built its east-west fiber-optic cable line in the late 1980s, it needed a regenerator station every 22 miles (35 kilometers) to boost the signal. These regenerator stations use electricity 24 hours a day, 7 days a week, to power the transmission equipment and control shelter temperatures. Utility power serves most of the stations, which have 2 hours of backup battery capacity should the power line go down.
But at Sand Pass, Nevada, bringing in utility power was prohibitively expensive. When Sprint built a regenerator station there in 1986, it installed two propane-powered electric generators. But to maintain the generators, a mechanic had to drive out to the pass every month, and to provide fuel, a teamster hauled a heavy propane tank over rough dirt roads to the site every 3 months.
The solution was to install a PV system at the Sand Pass station. The PV array and propane generators at Sand Pass complement each other well. The PV array charges batteries that power the dc transmission equipment. The generators are used to power the ac motor in the air-conditioning unit only when the shelter thermostat calls for air conditioning. As added backup, the generators can charge the batteries, if necessary. Because relieving the generators of their everyday battery-charging duties has reduced the amount of time they run, the PV system has significantly lowered the station's fuel and maintenance costs.
Original Source: Energy Efficiency and Renewable Energy (EERC)
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