Using a Hybrid Solar & Wind System: Tips for Living Off-the-Grid

The following article is excerpted from Wind Power by Paul Gipe.

Prior to the development of interconnected wind turbines, wind generators had historically been used for powering remote sites where utility power was nonexistent (see figure 1 1-1, Off-the-grid wind systems). These home light plants used wind machines and banks of batteries sized to carry the household through winter winds and summer calms. Occasionally the dealer would throw a backup generator into the mix to charge the batteries during extended calms. The high cost, poor reliability, and maintenance requirements of these early systems discouraged all but the hardiest souls from living beyond the end of the utility’s lines.

That’s no longer true. Home power systems have become so mainstream, says Wes Edwards, that homes using them now qualify for mortgages. Edwards, a licensed electrician who has lived off the grid in northern California since 1974, says that today’s improved inverters, low-power appliances, and widespread availability of photovoltaic panels have revolutionized stand-alone power systems.

Data compiled by Pacific Gas & Electric Co. (PG&E) confirms Edwards’s observation. In an early-1990 study of its service area, PG&E found the number of stand-alone power systems mushrooming at a rate of 29 percent per year. The firm expects the market to continue expanding as urbanites increasingly move to rural areas not currently served by the California utility. The business prospects looked so enticing that PG&E even toyed with the idea of providing the stand-alone home power systems itself, instead of building additional power lines.

New technology has made this possible. Until photovoltaic’s entry into the market, remote power systems were solely dependent on wind machines (in some special cases, small hydro systems) and backup generators. The modularity of photovoltaics (PVs) has transformed the remote power system market by enabling homeowners to more closely tailor power systems to their needs-and their budgets.

Despite wind’s advantages, wind turbines are less modular than PVs. Although the output of some micro turbines is no more than that of most PV modules (50 to 100 watts), each additional turbine requires a separate tower and controls. When scaling up the output from a remote power system, it’s easier to add more modules to a PV array than it is to add more wind turbines.

Wind is also far more site-specific than solar. It’s safe to assume that nearly everywhere on earth the sun will rise and set every day. Not so with wind. Wind follows daily and seasonal patterns that are less predictable. In the mid-latitudes of the Northern Hemisphere, the winds are strongest in winter and spring and weakest during summer. Fortunately this pattern happens to coincide with the attributes of solar energy. Winds are generally strongest when the sun’s rays are weakest, and winds are weakest when solar radiation reaches its peak. For this reason wind and solar are ideally suited for hybrid systems that capitalize on the advantages offered by each technology.

Hybrids

With advances in solar and wind technology, it just doesn’t make sense today to design an off-the-grid system using only wind or only solar. Hybrids offer greater reliability than either technology alone, because the remote power system isn’t dependent on any one source (see figure 11-2, Hybrid stand-alone power system). For example, on an overcast winter day outside Pittsburgh, Pennsylvania, when PV generation is low, there’s more than likely sufficient wind to make up for the loss of solar-generated electricity.

Wind and solar hybrids also permit the use of smaller, less costly components than would be needed if the system depended on only one power source. This can substantially lower the cost of a remote power system. In a hybrid system, the designer need not size the components for worst-case conditions by specifying a larger wind turbine and battery bank than necessary.

Hybrids often include a fossil-fuel backup generator for similar reasons. In effect, stand-alone systems substitute the fuel and the maintenance of the backup generator for a larger wind turbine or more solar panels. Depending on the size of the backup generator and the power consumption at the time, the generator can top up discharged batteries and meet loads not being met by the combined wind and solar generation.

Despite the improving cost-effectiveness of PVs and small wind turbines, the initial cost of a hybrid system remains high. To keep costs down, it behooves users to reduce demand as much as possible. Fortunately the development of compact fluorescent lights and energy-efficient appliances now makes this possible with little sacrifice. Today’s energy-efficient appliances permit homeowners to meet their energy needs with smaller, less expensive power systems than were once necessary.

Reducing Demand

The first place to start is by reducing demand. Most North Americans can easily halve their electricity consumption. If The Company Store’s advertisement for down comforters “to take the chill off an air-conditioned room” doesn’t strike you as absurd, then it’s time to ask yourself the question of how much you’re willing to pay to generate your own electricity. Reducing your consumption by conserving and increasing efficiency improves the services that a renewable power system can provide by stretching each kilowatt-hour to do as much work as possible.

To reduce demand, perform an energy audit of your lifestyle. Knowing how, where, and when you use energy is even more important for an off-the-grid system than for an interconnected wind turbine. Determine what appliances you plan to use at your remote site, and estimate how much electricity they will consume (see table 11-1, Residential Energy Consumption).

Conserve as much as possible. It’s always cheaper to save energy than to generate it with a hybrid power system. In other words, the return on investment for conserving energy is higher than that for producing it in an off-the- grid power system (see table 11-2, Return on Investment of Conservation Measures in an Off-the-Grid Power System). To maximize the value of your renewable power system and minimize its cost, carefully pare your electricity consumption to the minimum needed for the services you require.

Turn off all unneeded loads. This should be obvious, but like the example of grabbing a quilt because the air-conditioning has made the room too cold, it isn’t. The National Renewable Energy Laboratory found that at one hybrid system in Chile, villagers left lights on 24 hours per day—despite NREL’s plea to turn them off!

Decide if there are any electric appliances, such as electric hot-water heaters or electric stoves, that can be switched to gas or other fuels. It makes no sense to squander your hard-earned electricity on inefficient appliances or on uses to which electricity isn’t well suited. Heating is one of them. Heating with gas, oil, propane, or wood is far more economical at a remote site than heating with electricity.

Though cooking consumes little energy overall, electric stoves have high peak power demands that will affect the size of inverters and other hybrid components. Cook with gas or propane, or use a microwave oven instead.

PG&E found, in its study of off-the-grid systems, that most remote generation is used for lighting, refrigeration, and water pumping. (Remote sites are seldom served by municipal water sources.) Lighting is the easiest to tackle. Compact fluorescent lamps can reduce lighting demand significantly. Task lighting—lighting only those areas where light is needed—daylighting, and simply turning off lights when they’re not needed can all cut lighting consumption by two-thirds.

Similar savings can be achieved with refrigeration. Modern refrigerators use as little as 300 kilowatt-hours per year, a fraction of what they used in the 1970s. Sunfrost refrigerators, the efficiency champions, use even less. Generally, if your refrigerator is more than 10 years old, it should be replaced-and whatever you do, don’t put that old refrigerator in the garage. Drive a stake through its heart.

Depending on the size of the house and on the climate, air-conditioning can double the consumption of an otherwise energy-efficient home. If you must have air-conditioning, ask yourself whether an evaporative cooler will suffice. Swamp coolers use far less electricity and work well in arid climates, such as the southwestern United States.

For energy-efficiency ratings of appliances from washing machines to refrigerators, and for tips on reducing your energy consumption without reducing your comfort, read Consumer Guide to Home Energy Savings by the editors of Home Energy magazine (see the appendix of Wind Power for details).

The key is to remain flexible. Sacrificing the lifestyle you desire isn’t necessary, but some modification of behavior often proves beneficial. For example, cutting back on energy-intensive discretionary loads on days when the power supply is reduced extends battery life and leaves a little extra in storage available, should you need it for more important loads such as pumping water. Not unlike our ancestors, learn to synchronize your behavior with the weather. Do the laundry when it’s windy or on a bright sunny day. In this way, you take full advantage of the fuel when it’s available.

Turning off unneeded appliances isn’t much of a burden for those who are energy-conscious; it’s already become second nature. But for those going “cold turkey” from a highly consumptive lifestyle where energy’s undervalued, it can be a rude awakening. In such cases it might be wise to gradually reduce your consumption until you’re ready to make the transition to producing your own power.

The average North American household should be able to reduce its consumption to about 3,600 kilowatt-hours per year, or about 10 kilowatt-hours per day. This isn’t spartan living. Most Europeans live comfortably on this amount or less. How much you’re able to reduce your consumption will determine not only what size system you need, but also whether you should wire for DC or AC, and at what voltage you should operate your power system.

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