Backup Generators for PV Systems

We typically use generators as a supplement to photovoltaic power. There are some very large applications for which continuous-use generators may prove more cost ­effective than a PV system, but in almost all of the applications with which we work, the economics of generators are maximized by restricting them to providing backup power.

Generators are used for backup in situations where seasonal variability of insolation is substantial as in a cloudy climate, or for systems where occasional very large loads are powered, as for intermittent use of large shop tools or a deep well pump in a residence. We typically design residential PV systems to provide 80 to 90 percent of the home's annual electrical power. The last 10 to 20 percent is more economically supplied by a generator.

The reason for this is simply economics. In many cases we would double the cost of the system to provide this last 10 to 20 percent of annual power. It is much more Cost effective to employ a backup source of power during the least sunny time of the year.

The cost per kilowatt hour of electricity produced by a generator used in conjunction with a battery bank and inverter is much cheaper for residential type load profiles than is power produced by a continuously running generator. This is because engine-driven generators perform poorly when under-loaded. Low-load hours on the engine, especially diesel, can actually age it more than hours under full load. Fuel costs suffer too.

A 6500 watt generator, for example, powering a 100 watt load will consume perhaps 50% as much fuel as it would consume if operating at full capacity. Therefore, work the generator near its capacity for shorter periods and then shut it down. Batteries can be charged while washing machines, pumps or other large loads are running. This maximizes efficiency while reduc­ing generator run time, wear, and fuel costs.

In a grid intertie system the grid acts as your storage system therefore when it goes down you are with out power.

Generators and Battery Charging

Battery chargers take the 120 volt AC power from the generator and convert this power to low voltage DC. They are typically the largest consumer of the generator's output.

Many of today's inverters incorporate a battery charger and transfer switch as optional or even standard equipment. These chargers are powerful, charging the batteries at a high rate and requiring a good sixed generator to power them. We recommend a generator of at least 4 to 5 kw in size for full time remote homes.

Remember that these inverter/chargers also include an automatic transfer switch. This switch selects among the two sources of AC power to be delivered to the loads - inverter or generator power. The switch is biased to inverter power which is supplied to the loads whenever the generator is off. Once the generator is started, the switch senses the presence of generator voltage, waits a predetermined "warm-up" period then switches over, allowing generator power to flow to the loads.

The generator power is now running all loads in the house as well as powering the battery charger. Therefore the generator should be sized to not only run the battery charger at a high rate, but also any AC loads that may be running at the same time.

If the generator is undersized for the loads being run, the battery charging rate will be reduced. This may mean the generator is run for a very long time to fully charge the batteries.

So how do you avoid this problem that many have experienced? Simply select a good-sized battery charger and generator when designing your system. This is not the place to cut the budget, as generator fuel can cost you dearly.

Generator placement and Sol Sheds™

Sol Sheds are a common outbuilding on the homestead. They often serve multiple functions of housing the generator, batteries and power conditioning equipment required with your PV system. Beyond this, they can also house tools and equipment as well as support the solar array if placed in a sunny, non-shaded area. It is a good idea to keep flammable fuels and gasses in a separate building rather than the building in which one sleeps.

The sound insulation qualities of a Sol Shed offer another advantage... There is nothing like driving up a beautiful mountain to a picturesque home site, then being greeted by the howling of a generator as you step out of your truck. Sol Sheds can be placed a distance from living spaces, decks and the like.

If you are thinking of a Sol Shed to house your PV components, don't forget that your batteries are most efficient between 60°F and 80°F. In cold climates, the tremendous amounts of heat given off by the generator can aid in heating the space. Propane catalytic heaters with thermostats can also help. Excess heat can also be a problem if the generator is not properly ventilated to the outside.

Another good idea is to install DC lighting in your shed. With battery direct DC power, you will still have light when servicing your AC equipment.

Next - Grid Inter-Tie Systems

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