Natural disasters can disrupt the power grid for days, weeks, or months at a time. Most of us aren’t prepared for a long-term power blackout. We might think that the probability of such an event is so low that full preparedness doesn’t justify the cost (until the worst-case scenario occurs). But what about a three-hour blackout in winter that leaves your house at 50 degrees Fahrenheit because your furnace fan couldn’t circulate the air without electricity? Suppose that you want to get off the utility grid altogether, or generate enough of your own electricity so that the power company pays you for the surplus?
You can find compact, portable combustion generators for use in homes and small businesses. Some combustion generators are also suitable for use by campers. For people living in remote areas, a combustion generator might constitute the primary, if not the only, source of AC electricity for common appliances.
A small combustion generator provides 117 V AC in the United States (234 V in many other countries). Larger generators in the United States also supply 234 V AC for heavy appliances, such as electric ranges and laundry machines. The generator’s internal combustion engine can range in size from a few horsepower (comparable to the one in a lawn mower or snow blower) to hundreds of horsepower (comparable to the engines in trucks, tractors, and construction equipment). Most small generator engines burn gasoline. Larger ones burn diesel fuel, propane, or methane.
In a mechanical AC generator, a coil of wire, attached to the shaft of the combustion engine, rotates inside a pair of powerful magnets. If you connect a load (such as an appliance) to this coil, an AC voltage appears across that load as each point in the wire coil moves past the lines of flux produced by the magnets, first in one direction and then in the other direction, over and over. In an alternative arrangement, the magnetic poles revolve around the wire coil, which remains fixed.
The AC voltage that a generator can produce depends on the strength of the magnets, the number of turns in the wire coil, and the speed of rotation. The AC frequency in a simple generator depends only on the speed of rotation. In the United States, the speed is 3600 revolutions per minute (3600 r/min) or 60 revolutions per second (60 r/s), resulting in an output frequency of 60 cycles per second (60 Hz). In many other countries, the rotational speed is 3000 r/min, producing an AC frequency of 50 Hz. In order to maintain a constant rotational speed for the generator under conditions of variable engine speed, mechanical regulating devices are required.
When you connect a load to the output of a simple generator, the engine has a harder time turning the generator shaft, as compared with the situation when no load exists. As the amount of electrical power demanded from a generator increases, so does the mechanical power required to drive it, and therefore, the amount of fuel consumed per unit of time. The electrical power that comes out of a generator is always less than the mechanical power required to drive it. The lost energy shows up as heat in the generator components. To maintain the proper AC frequency, a simple generator’s engine must run at a constant speed under conditions of variable load. This state of affairs can prove difficult to attain, but there’s a way around it!
Advanced small-scale generators circumvent the need for constant motor speed by converting the generated AC to regulated DC, and then using a power inverter to generate AC from that DC. If the motor speed changes, the DC voltage stays the same because the regulator circuit holds it constant, so the output AC voltage stays constant too. In the best commercially manufactured generators, the inverter produces a near-perfect sine wave to ensure that the machine can properly operate sensitive electronic devices, such as computers, printers, scanners, modems, and routers. A “raw” generator will produce a facsimile of a sine wave, but not of the quality needed by microcomputer- and microcontroller-based devices in common use today.
Figure 4-1 shows a popular portable gasoline generator with a power inverter that can provide up to 2 kW of clean sine-wave AC electricity when needed. This machine can run any of my computers, microcomputer-controlled furnace, and microcomputer-controlled amateur (“ham”) radio transceivers perfectly well. It has a tank that holds 1.1 gallons (4 liters) of high-octane gasoline. With a load of a few hundred watts, that amount of gasoline provides several hours of continuous, reliable AC electricity. This generator has proven itself worthy as a backup power source in winter storms when utility failures would otherwise have meant no heat for my house, as the furnace electronics and fan require 117 V AC to function!
FIGURE 4-1 A portable gasoline-fueled generator, capable of providing up to 2 kW of clean sine-wave AC power at 117 V RMS. The tied-up cord is the ground wire.
A Honda EU-2000i portable gasoline-fueled generator (Fig. 4-1) forms the heart of my emergency backup power ensemble. In addition to the generator, I use several extension cords and power strips to distribute electricity to the points where I need it the most during a utility outage. I always keep in mind the maximum power that the generator can provide; I never let it come close to “maxing out” at the full 2-kW limit. You can use this general configuration as a template for your own system, if you want to install one, tailoring the specifics to meet your needs.
Figure 4-2 shows the two outlets of my generator. The cord on the left goes to the furnace fan and electronics. The cord on the right goes to my computer workstations, by way of a power strip in the garage. Figure 4-3 shows that power strip, which includes a light bulb that tells me when the generator is running, and also illuminates the garage at night. This power strip does not have a transient suppressor (or “surge protector”) because the computer workstations both have uninterruptible power supplies (UPSs) with their own transient suppressors.
FIGURE 4-2 My portable generator has two AC outlets. The cord on the left goes to the furnace electronics and fan; the cord on the right goes to the computer workstations.
FIGURE 4-3 Power strip for the cords leading from the generator to the computer workstations. The cords lead to uninterruptible power supplies (UPSs).
You should never connect devices with transient suppressors in cascade (one after another) in the same circuit. For example, you shouldn’t use a UPS along with a power strip if both devices have transient suppressors. You’ll need a power strip without a transient suppressor (they’re cheaper that way, anyhow). Transient suppressors in cascade will sometimes interfere with each other’s operation, a conflict that can produce bizarre malfunctions! I’ve seen a UPS “go crazy” with a transient suppressor connected to one of its outputs.
In the event of a utility power failure, I follow a rigorous procedure to disconnect the appliances I want to use from the utility lines before I activate the generator. For example, during a winter storm, the power went out, and I needed to keep the furnace running. I switched off the breaker that controls the furnace, and then followed a step-by-step procedure that I have provided here as Table 4-1. You should devise a similar procedure for your own home situation, with the help of an electrician, to ensure that you stay absolutely safe. Write the procedure down in detail, and tape a copy to your furnace. Then, when an outage actually occurs, follow those instructions to the letter.
TABLE 4-1 Procedure for Generator Use with My Furnace in Case of a Utility Failure
As a final precaution to keep the generator operating at peak efficiency and safety, you should connect the generator’s ground terminal to a known electrical ground, which you have tested for continuity with the main ground for your whole house. Figure 4-4 shows my arrangement, which comprises a single heavy length of wire and a clamp going to a cold water pipe. By performing the ground test described earlier in this book, I’ve satisfied myself that the cold water pipe connects directly to the main electrical ground for the house.
FIGURE 4-4 Ground clamp for the generator, in this case to a cold water pipe that has been tested to ensure continuity with the main electrical ground for the house.
Warning! Always locate a generator so that its exhaust can vent freely to the outside. The best way to make that happen is to keep the generator outdoors when running it. Never run your generator in a garage (even an open one) or partially enclosed space of any kind. Buy a carbon-monoxide (CO) detector if you don’t already have one, and place it in your house near the rooms where you sleep. Keep its batteries fresh. That way, you’ll know if generator exhaust “blows” into the house, a situation that can arise with amazing ease, as I discovered when I ran my little Honda generator in the woodshed under my dining room. My CO detector sounded its alarm after only a few minutes of generator time!
Warning! An on-site standby generator must run only when your house wiring is completely separated from the electric utility wiring with a double-pole, double-throw (DPDT) isolation switch installed and tested by a competent, certified electrician. Alternatively, you can plug individual appliances into the generator through dedicated cords that have nothing whatsoever to do with your house wiring. If you don’t follow these rules strictly, backfeed can occur, in which electricity from the generator gets into the utility lines near the home or business where the generator operates. Backfeed can endanger utility workers and damage electrical system components.
A photovoltaic (PV) cell is a specialized form of semiconductor diode that converts visible light rays, infrared (IR) rays, or ultraviolet (UV) rays directly into electricity. When used to obtain electricity from sunlight, this type of device is known as a solar cell. One of the most common types, the silicon PV cell, is made of specially treated silicon.
Figure 4-5 shows the “innards” of a silicon PV cell. It’s made with two types of silicon, called P type and N type. The functional part is the surface at which these two types of materials come together, known as the P-N junction. The top of the assembly is transparent so that rays can strike the junction. The positive electrode is made of metal strips or tiny bars called ribbing interconnected by fine wires. The negative electrode comprises a metal base called the substrate, placed in contact with the N type silicon. When energy rays (usually in the form of sunlight) strike the P-N junction, a voltage or potential difference develops between the P type and the N type materials.
FIGURE 4-5 Functional diagram of the construction of a silicon photovoltaic (PV) cell, also called a solar cell.
Most silicon solar cells provide about 0.5 V DC with no load connected. If you don’t demand that the thing deliver very much current, even moderate light, such as you get on a dreary, overcast day, can generate the full output voltage. As you demand more current, you’ll need to have better illumination to produce the full output voltage. An upper limit exists to the current that you can obtain from a particular PV cell, no matter how intense the incident light gets. This limit, called the maximum deliverable current, depends on the surface area of the P-N junction, and also on the technology involved in the manufacture of the device.
In a battery consisting of two or more identical PV cells connected in series (negative-to-positive, like the links in a chain), the total voltage increases in proportion to the number of cells, but the maximum deliverable current remains the same as that of any individual cell by itself. In a battery consisting of two or more identical PV cells connected in parallel (negative-to-negative and positive-to-positive, like the rungs in a ladder), the total voltage equals that of any cell alone, but the maximum deliverable current increases in proportion to the number of cells.
When you combine series PV cells in parallel, or parallel PV cells in series, you can get more voltage and more current than you can get from any cell all by itself: the best of both worlds! Engineers call such a set a series-parallel array of PV cells.
The maximum output power for a silicon PV cell (in watts) equals the product of the output voltage (in volts) and the maximum deliverable current (in amperes). The maximum power that you can get from a series-parallel combination of identical PV cells equals the maximum power from each cell times the total number of cells. When you connect a load to a PV system, and thereby draw current from it, the actual maximum power always turns out slightly lower than the theoretical maximum power.
In low-voltage, low-current PV systems, individual cells are normally connected in series to obtain the desired output voltage. For charging a 12-V battery, a common PV output level is 16 V, requiring 32 PV cells in series. Such a series-connected set is called a PV module. In order to get more maximum deliverable current, multiple modules can be connected in parallel to form a PV panel. Finally, if even higher levels are necessary, multiple panels can be connected in series or parallel to obtain a PV array.
Although gigantic voltages can theoretically be obtained by connecting hundreds or even thousands of PV cells in series, this approach presents problems because the internal resistances of cells in series add up, just as ordinary electrical resistors in series add up. That effect reduces the maximum deliverable current, and it also causes the output voltage to drop under load. High-power PV arrays can be constructed by connecting a large number of cells or low-voltage modules in parallel, making many identical such sets, and then connecting all the parallel sets in series.
If you want to get a medium voltage (say, the nominal voltage for a household utility circuit) from a low-voltage solar panel, you can use a power inverter along with a high-capacity rechargeable battery called a deep-cycle battery. The solar panel keeps the battery charged; the battery delivers high current on demand to the power inverter. Such a system provides common 117-V AC electricity from a 12-V DC or 24-V DC source.
In theory, a solar panel works best when it lies broadside to the incident sunlight, so that the sun’s rays shine straight down on the surface. However, this orientation is not critical. Even at a slant of 45 degrees (45°) with respect to the sun’s rays, a solar panel receives 71 percent as much energy per unit of surface area as it does when optimally aligned. Misalignment of up to 15° makes almost no difference.
You should locate your solar panels where they will receive as much sunlight as possible, averaged out during the course of the day and the course of the year. Mountings should be sturdy enough so the panels will not rip loose or wiggle out of alignment in strong winds, heavy snow storms, or ice storms. One of the most popular arrangements involves mounting a solar panel, or a set of panels, directly on a steeply pitched roof that faces toward the equator.
The ideal bearing arrangement for a solar panel would be a motor-driven equatorial mount, similar to the ones used with astronomical telescopes. This system would allow the panel to follow the sun all day, every day of the year. However, such a sophisticated mechanical device is impractical for most people, and the cost is prohibitive for large panels or multi-panel arrays. The next best thing is a mount with a single bearing that allows for the panel to be manually tilted, always facing generally south in the northern hemisphere or generally north in the southern hemisphere. Figures 4-6 and 4-7 show examples of this type of system.
FIGURE 4-6 Optimal placement of fixed, south-facing solar arrays for locations in northern temperate latitudes for year-round operation (A), low-solar-angle-season operation (B), and high-solar-angle-season operation (C). The variables x, y, and z represent angles in degrees with respect to the zenith. In each case the panel is viewed edge-on, looking west.
FIGURE 4-7 Optimal placement of fixed, north-facing solar arrays for locations in southern temperate latitudes for year-round operation (A), low-solar-angle-season operation (B), and high-solar-angle-season operation (C). The variables x, y, and z represent angles in degrees with respect to the zenith. In each case the panel is viewed edge-on, looking west.
The arrangements that you see in Fig. 4-6 will work between approximately 20° north latitude (20° N) and 60° north latitude (60° N). Cities in such locations include:
• Las Vegas, USA
• Chicago, USA
• Miami, USA
• Paris, France
• Berlin, Germany
• Moscow, Russia
• Beijing, China
• Osaka, Japan
Figure 4-6A shows a year-round panel position. You should set the angle x to 90° minus the north latitude at which your system is located. If an adjustable bearing is provided, you can use two tilt settings, as shown in Figs. 4-6B and 4-6C. From late September through late March (autumn and winter), the arrangement shown at B will work the best, and the angle y should be set to approximately 78° minus the latitude. From late March through late September (spring and summer), the arrangement shown at C will work the best, and the angle z should be set to approximately 102° minus the latitude.
The arrangements in Fig. 4-7 will work between approximately 20° south latitude (20° S) and 60° south latitude (60° S). Cities in such locations include:
• Santiago, Chile
• Buenos Aires, Argentina
• Rio de Janeiro, Brazil
• Cape Town, South Africa
• Durban, South Africa
• Perth, Australia
• Sydney, Australia
• Auckland, New Zealand
Figure 4-7A shows a year-round panel position. You should set the angle x to 90° minus the south latitude at which your system is located. If an adjustable bearing is provided, two tilt settings can be used, as shown in Figs. 4-7B and 4-7C. From late March through late September (autumn and winter), the arrangement shown at B is optimal, and the angle y should be set to approximately 78° minus the latitude. From late September through late March (spring and summer), the arrangement shown at C is optimal, and the angle z should be set to approximately 102° minus the latitude.
Before you invest thousands of dollars in a solar power system for your house, consult a competent engineer who can assess your situation. Here are some things to think about.
• You can’t expect a photovoltaic system to provide as much power as your electric utility company does.
• Photovoltaics only provide power to a system when the sun shines brightly enough. Small-scale PV systems rarely justify the cost in locations that don’t receive much sunlight.
• If the solar panels get covered with snow or debris, you’ll have to manually remove the obstruction if you want the system to keep working.
• Problems with load imbalance can occur if part of a solar array lies in bright sunlight while another part lies in shadow. You’ll have to find a location that gets plenty of continuous, total sun exposure during much of the day.
• In a PV system that uses lead-acid storage batteries, the batteries can produce dangerous fumes. All types of rechargeable batteries require maintenance, and you’ll have to replace them altogether every few years. That can be quite expensive for a large solar power plant.
A stand-alone small-scale PV system uses rechargeable batteries to store the electric energy supplied by a PV panel or array. The batteries provide power to an inverter that produces 117 V AC (in the United States). In some systems, the battery power can be used directly, but this method will work only with home appliances designed for low-voltage DC. Figure 4-8 is a functional block diagram of a stand-alone small-scale PV system that can provide 117 V AC for the operation of small appliances in a typical American household.
FIGURE 4-8 A stand-alone small-scale PV system.
The batteries allow the system to produce usable electricity even if there’s not enough sunlight for the PV cells to operate, for as long as the batteries retain some charge. A stand-alone PV system of this type offers independence from the utility companies. However, you’ll have a power blackout if the system goes down for so long that the batteries discharge and you don’t have a backup power source, such as a generator.
An interactive small-scale PV system with batteries resembles a stand-alone system, but with one significant addition. If you get a prolonged spell without enough light for the PV cells to function, the electric utility can take over, keeping the batteries charged and preventing a blackout. A switch, along with a battery-charge detection circuit, connects the batteries to the utility through a charger if insufficient power, or no power at all, comes from the PV panel or array. When conditions become favorable and the PV cells can work again, the switch disconnects the batteries from the utility charger and reconnects them to the PV panel or array.
Figure 4-9 is a functional block diagram of an interactive small-scale PV power system with batteries. In this arrangement, you don’t sell any power to the electric utility company, even when the PV panel or array generates more power than your home needs. When the utility is involved, the electrical energy only flows one way, from the utility line to the batteries through a charging circuit and switch. That situation occurs only when the batteries require charging and the PV panel or array does not provide enough power to charge them.
FIGURE 4-9 An interactive small-scale PV system with batteries.
An interactive small-scale PV system without batteries operates in conjunction with the utility company, just as the system with batteries does. You can sell energy to the company during times of minimum demand, and buy it from the company at times of heavy demand. With this type of system, you can keep using electricity (by buying it directly from the utility) if there’s a long period of dark weather, and you don’t have to care for a set of batteries. Another advantage is that, because no batteries are used, this type of system can have greater peak-power-delivering capability than a stand-alone arrangement or an interactive system with batteries. Figure 4-10 is a functional block diagram of an interactive small-scale PV system without batteries.
FIGURE 4-10 An interactive small-scale PV system without batteries.
Figure 4-11 is a simplified block diagram of a direct PV system for indoor environment modification. Remember that in bright sunshine, a single silicon PV cell produces approximately 0.5 V DC. You can connect numerous silicon PV cells in a series-parallel array that provides 12 V DC or 24 V DC output at fairly high current in direct sunlight.
FIGURE 4-11 A supplemental indoor environment control system that uses a solar panel, power inverter, voltage regulator, and switch connected to a small appliance, such as a fan or humidifier.
Figure 4-11 shows only four PV cells (for simplicity), but it illustrates the basic series-parallel principle. A real-world system can contain hundreds of individual PV cells. A solar module of 53 silicon PV cells connected in series, each rated at 0.5 V DC, theoretically yields 26.5 V DC with the same maximum current output as a single cell. When you call upon the system to produce power, this figure drops to around 24 V DC because of the internal resistance of the PV cells. By connecting multiple 53-cell series modules in parallel to form a solar panel, you can obtain high current levels at the same voltage (in this case 24 V DC).
The output of the solar panel goes to a power inverter that changes the low-voltage DC output of the solar panel into 117 V AC that can operate electric heaters. The system includes a voltage regulator to ensure that the voltage remains fairly constant under conditions of varying solar intensity. This type of system needs a high-current power inverter, and that thing can cost a lot of money. But you do have an alternative, if all you want to do is run a small electric heater from your PV array. You can do away with the inverter altogether, design your PV array to produce 117 V DC, and then supply the heater with 117 V DC instead of 117 V AC, remembering that you can’t run most other household appliances from DC. You’ll also have to keep in mind the fact that you can’t expect to run a whole household full of electric heating elements with a system like this (unless you have the money and real estate to build a massive “solar ranch”).
A well-designed direct PV system has an automatic shutdown switch that disconnects the solar panel if the daylight becomes too dim to properly operate appliances connected to it. If the system runs near peak capacity and the delivered current suddenly drops (a storm cloud moves in, for example), the switch will power down the system until sufficient daylight returns.
Fact or Myth?
• People have said that you can’t run electric baseboard heating systems, central air conditioners, or lots of heavy appliances (in general) all at the same time with solar power. Is that true?
• If you run a lot of heavy-duty appliances simultaneously, they’ll draw too much current for a solar panel of reasonable size to contend with. Theoretically, you can power up such appliances with solar panels, but the panels would have to be so large that the benefit would not justify the cost.
The term small-scale applies to wind turbines that can generate up to about 20 kW of electricity under ideal conditions, enough to power most households. All wind turbines generate power on an intermittent basis. In order to obtain a continuous supply of electricity with a small-scale wind-power plant, you’ll have to use storage batteries or an interconnection to the electric utility, or both.
Before you decide to install a wind turbine for your house, think about the following potential pitfalls. As with a solar electric system, you should consult a reputable engineer who specializes in alternative energy.
• Some places have a lot more wind than others. Ask yourself how your locale “rates” in this department, and answer yourself honestly! You can find wind maps on the Internet. Their addresses keep changing all the time, so you’ll probably want to enter a phrase like “wind power map” into your favorite search engine.
• Even in the windiest places, such as Wyoming or South Dakota or Nebraska, the wind doesn’t blow all the time.
• Small-scale wind turbines will not work properly if the wind gets too strong.
• A small wind turbine can be wrecked by a powerful thunderstorm, hurricane, or ice storm.
• It will take a long time to recoup the up-front installation cost, even if you are using a small-scale wind-power system.
• Your neighbors may dislike having a wind turbine nearby.
• Small-scale wind turbines can create significant noise at close range.
Most small-scale wind turbines are steered by a wind vane attached to the generator housing (called the nacelle). The vane works in the same way that an old-fashioned weather vane does. When the wind blows hard enough to operate the turbine, the vane orients itself to point away from the wind. Under normal operating conditions, the plane defined by the blade rotation lies perpendicular (broadside) to the wind direction.
In a small-scale wind-power system, the speed of the blade rotation vares with the wind speed, resulting in variable-frequency AC from the generator inside the nacelle. This generator resembles the alternator in a motor vehicle. (Some manufacturers call it an alternator for that reason.) The AC from the generator is converted to DC by a rectifier circuit, and the DC charges a set of storage batteries. The electricity for household appliances comes from these batteries either directly, in which case special DC appliances must be used, or by means of a power inverter that converts the low-voltage DC electricity from the batteries to 117 V AC at 60 Hz (in the United States) or 50 Hz (in Europe and some other parts of the world).
The plane defined by the blades is normally perpendicular to the axis of the vane, so that the wind blows straight at the blades. However, in a strong wind, the plane of the blades changes, so it no longer lies perpendicular to the vane axis. This adjustment reduces the wind load on the blades but allows the turbine to keep on working. As the wind speed grows stronger yet, the angle between the plane of the blades and the vane axis decreases until, at a certain speed, it becomes zero. Then the blades rotate in a plane that contains the axis of wind flow. The variation in the angle between the plane of the blades and the wind direction is called furling. It can be done in the horizontal plane (so the blades swing, or yaw, toward the left or right) or in the vertical plane (so the blades tilt up or down).
A wind turbine can also regulate its wind load by varying the blade pitch. When the blade pitch is small (the plane of each blade’s surface is nearly the same as the plane defined by the blades), the wind produces less torque in the system, and consequently less power, than when the blade pitch is large (the plane of each blade’s surface differs greatly from the plane defined by the blades). At low wind speeds, the blade pitch is at the maximum. As the wind speed increases, the blade pitch decreases. If the wind speed becomes great enough, the blade pitch becomes zero.
A stand-alone small-scale wind-power system uses rechargeable batteries to store the electric energy supplied by the rectified output of the generator. The batteries provide power to an inverter that produces a “clean” AC wave at 117 V. The very best inverters produce “true sine waves.” The second-best ones produce “modified sine waves.”
Some stand-alone systems use the battery power directly without any inverter at all, but this arrangement will work only with appliances and devices designed to run from low-voltage DC. Figure 4-12 is a functional block diagram of a stand-alone small-scale wind-power system that can provide 117 V AC.
FIGURE 4-12 A stand-alone small-scale wind-electric system.
The use of batteries allows the system to produce usable power even if there’s not enough, or too much, wind for the turbine to operate. A stand-alone system offers independence from the utility company. However, a blackout will occur if the system goes down for so long that the batteries discharge and no backup power source exists.
An interactive small-scale wind-power system with batteries resembles a stand-alone system, but with one significant addition. If you suffer through a prolonged spell in which wind conditions are unfavorable for turbine operation, the electric utility can take over to keep the batteries charged and prevent a blackout. A switch, along with a battery-charge detection circuit, connects the batteries to the utility through a charger if no power issues from the turbine. When wind conditions become favorable and the turbine supplies power again, the switch disconnects the batteries from the utility charger and reconnects them to the turbine generator and rectifier.
Most interactive small-scale wind-power systems with batteries never sell any power to the electric utility, even if the wind turbine generates an excess. Power only flows one way, from the electric power line to the batteries through a charging circuit and switch, and even that happens only when the batteries require charging and the wind turbine does not provide enough power to charge them. Figure 4-13 is a functional block diagram of this type of wind-power system.
FIGURE 4-13 An interactive small-scale wind-electric system with batteries.
An interactive small-scale wind-power system without batteries also operates in conjunction with the utility company. You sell excess energy to the company during times of minimum demand, and buy energy from the company during times of heavy demand. You can keep using electricity (by buying it directly from the utilities) if wind conditions remain unfavorable for a prolonged period. Because this type of system has no batteries, it can be larger, in terms of peak power-delivering capability, than a stand-alone arrangement or an interactive system with batteries.
An interactive system without batteries, like the type with batteries, is designed to function with the help of the utility company, and does not offer the independence that a purist might desire. This factor does not represent a technical drawback, but it can pose a philosophical problem for anyone who desires to live completely off the grid. Figure 4-14 is a functional block diagram of an interactive small-scale wind-power system without batteries.
FIGURE 4-14 An interactive small-scale wind-electric system without batteries.
Figure 4-15 illustrates a wind turbine, equipped with an electric generator and connected into a zone electric baseboard heating system. A voltage-regulation circuit maintains the system at or near 117 V AC, so the heating elements can operate as they normally would with the electric utility.
FIGURE 4-15 A supplemental residential heating system that uses a wind turbine and voltage regulator connected into a conventional electric zone heating circuit.
A medium-sized wind turbine designed for residential use can produce about 12 kW of power on a day with moderate wind. That’s the equivalent of eight electric space heaters, each rated at 1500 W. As things work out, 1 kW of electrical power equals 3410 British thermal units per hour (Btu/h) of “heating power.” Therefore, the wind turbine system of Fig. 4-15 can provide approximately 3410 × 12 = 40,920 Btu/h. A gas furnace for a typical residential home produces 80,000 to 100,000 Btu/h when running full blast. So in theory, the system shown in Fig. 4-15 can supply about half of the energy necessary to keep your house warm.
A system like the one shown in Fig. 4-15 depends on wind for its operation. Batteries of reasonable cost can’t store the large amounts of energy required for home heating. In a location where the wind does not blow often or hard enough, this scheme won’t prove cost effective. However, in some places, winters remain cold and windy for weeks or months at a time. Such places make good “proving grounds” for a system such as the one diagrammed in Fig. 4-15.
A water turbine designed for a home or small business, installed in a fast-moving stream or small river with sufficient vertical drop, can produce roughly 20 kW of electricity, more than enough for a typical household under conditions of peak demand.
Residential hydro power systems won’t work for as many people as small-scale wind or solar power systems will, and for good reasons. If you’ve contemplated a hydro system for your own home, you should consider the following factors before you proceed.
• Only a few people live on properties with streams that provide enough flow to provide hydroelectric power. Be honest with yourself: Are you among them?
• You’ll have to verify your water rights before you modify the water resources on your property, so that you know what you can legally do (or not do).
• A small stream might periodically completely freeze or dry up, shutting a small-scale hydropower system down. How cold does it get in the winter where you live? Have you ever checked out your stream at the nadir of the winter season to see if any water flows?
• A water turbine requires considerable water mass, along with a significant vertical drop, to provide enough power to effectively serve a residence. You might have to install a small dam or artificial waterfall to build a workable system, and these arrangements could give rise to environmental and regulatory issues.
• The up-front cost of a small-scale hydropower system is considerable. It takes a long time to pay for itself, and the resulting economic benefit may be outstripped by the initial cost.
A small-scale hydropower system can be configured in three ways: stand-alone, interactive with batteries, and interactive without batteries. These three types of systems work in the same way for small-scale hydro-power systems as they do for small-scale residential wind-power systems.
Most small-scale hydroelectric systems use diversion technology, in which a portion of a river or fast-moving stream is channeled through a canal or pipeline, and the current through this medium drives a water turbine. You don’t need a dam. This type of system works best in locations where a river drops considerably per unit horizontal distance. Small-scale and medium-scale diversion systems can be used next to mountain streams or fast-moving, small rivers for the purpose of providing energy to homes.
An impoundment hydroelectric power plant consists of a dam and reservoir. This type of facility works best in mountainous places where high dams can be built and deep reservoirs can be maintained. Figure 4-16 is a simplified functional diagram of an impoundment facility. The water from the reservoir passes through a large pipe called a penstock, and then through one or more water turbines that drive one or more electric generators.
FIGURE 4-16 A hydro power system that derives its energy from water impoundment.
A pumped-storage hydroelectric power plant has two or more reservoirs at different elevations. When there’s little demand for electricity, the excess available power is used to pump water from the lower reservoir into the upper one(s). When demand increases, the potential energy stored in the upper reservoir(s) is released. Water flows out of the upper reservoir(s) in a controlled manner, passing through penstocks and turbines to generate electricity.
Suppose that you’d like to install a stand-alone, small-scale, impoundment-type hydro power system for your ranch, but you’re concerned about the effect it will have on wildlife. Imagine that a fairly good-sized stream runs through your property, that an engineer has checked everything out, that the vertical drop is sufficient, and that there’s more than enough water flow all year round. You’ll need to build a small dam and back up some water to form a pond (or even a small lake). You’ve checked everything out with the local, state, and federal officials, and they’re okay with your plans. Naturalists from a nearby college or university can offer some insight as to what effects your system will have on wildlife (and you can rest assured that some effects will occur). A pond can be expected to attract birds, fish, and other wildlife. It might even serve as a watering spot for your cattle! However, the same pond will displace other wildlife, particularly mammals that dwell beneath the surface. Are all small-scale stand-alone hydro power systems bad for the planet? No. Will yours harm the environment in general? You’ll have to figure that out for yourself, with the help of objective advisors.
You can connect a water turbine to an electric generator, which can drive electric heating and cooling systems in much the same way as a wind turbine can do. With sufficient water flow and proper voltage regulation, an arrangement of this kind can provide some of the power for climate control in a typical household.
Figure 4-17 is a block diagram of a small water-driven energy system adapted for use with electric baseboard heating. This assembly resembles the direct wind-powered system, except that you’ll replace the wind turbine with a water turbine. As with the wind system, a regulator circuit keeps the voltage near 117 V AC.
FIGURE 4-17 A supplemental residential heating system that uses a water turbine and voltage regulator connected into a conventional electric zone heating circuit.
An efficient water turbine, installed in a fast-moving stream or small river with sufficient vertical drop, can produce 20 kW of power on a reliable basis. Again, recall that 1 kW = 3410 Btu/h. Therefore, a substantial water turbine system can provide approximately 3410 × 20 = 68,200 Btu/h. (Let’s round this off to 70,000 Btu/h). That amount of power can keep a small home comfortable in almost all types of weather, as long as the stream or river doesn’t dry up or freeze solid.
You’ll need deep-cycle batteries to operate any type of stand-alone alternative-electric system. Deep-cycle batteries are usually of the lead-acid type, and in that respect, they resemble automotive batteries. But there are a couple of big differences between automotive batteries and deep-cycle batteries.
Automotive batteries can provide lots of current for a very short time; you need that kind of current to start your car or truck. A typical automotive battery can produce around 750 A of cold-cranking current. That’s the current the vehicle demands from the battery when you start it up after it hasn’t run for a while. At 12.6 V (the typical voltage of an automotive battery), 750 A give you a little less than 10 kW, roughly the amount of power that your house will likely need with most of your appliances running at once! Obviously that battery, contained in a chamber about the size of a bread box, can’t deliver 10 kW for very long. But that current “surge” will be there when you need it, as long as you keep the battery charged up. A deep-cycle battery, in contrast, is not designed to produce anywhere near that much current, even for a brief moment. A deep-cycle battery is intended to produce moderate current for extended periods of time.
Here’s the other difference between automotive and deep-cycle batteries: Automotive batteries are meant to be kept at a full charge, or nearly a full charge, all the time. They’re not intended to provide power all by themselves for hours on end. You don’t need to discharge your automotive battery while you’re driving down the highway; the vehicle alternator keeps it charged up. Have you ever found out what happens when your vehicle’s alternator fails? The vehicle will run okay as you drive between two towns several hours apart, but when you stop and switch off the engine, you won’t be able to start it up again without a “jump” from another vehicle. On the other hand, a deep-cycle battery is designed to produce current for a long time on its own, losing much of its charge in the process. Most deep-cycle batteries work best if you let them discharge about halfway with each charge-discharge cycle. Sometimes you can let a deep-cycle battery discharge 75% or 80% of the way down, but you should never let it lose all of its charge. Solar- and wind-powered stand-alone systems need a battery that can “carry the load” for extended periods. A deep-cycle battery can do a good job of that. An automotive battery can’t.
You can expect a deep-cycle battery to last several years, although some batteries perform better than others in this respect. The typical deep-cycle battery might wear out after five to seven years; the very best ones (Crown and Rolls, for example) can last upwards of 15 years. No matter what the quality level of your batteries (and no matter how much money you spend on them), you’ll have to treat them properly if you want them to endure for their rated life spans. Here are some recommendations.
• The further “down the cycle” you discharge your batteries, on the average, the sooner they’ll wear out.
• Ideally, you should not let your batteries discharge below the 50% level on any cycle; or, if you have to do it, you should not do it very often.
• If possible, avoid allowing your batteries to discharge below the 20% level at any time.
• Always use a charge controller with your batteries. This precaution will prevent overcharging, which can ruin a set of batteries in a hurry. See the section about charge controllers below.
• Have a professional choose the optimum storage capacity, in ampere-hours or watt-hours, for your battery bank. That capacity will depend on the size of your PV array, wind turbine, or water turbine. It will also depend on how much electricity you expect to get from your system during the course of your everyday living.
• Try to keep your batteries at or near room temperature. Avoid letting them sit in an environment where the temperature drops below freezing.
• Battery capacity goes down as the temperature goes down, even at temperatures above freezing.
• Never try to quick-charge your batteries. Always trickle-charge (slow-charge) them.
• When you buy a set of batteries, start using them right away. You can’t store batteries for a long time, and then expect them to last as long as they should when you finally get around to using them.
• When you combine batteries in series or parallel, make sure that all the batteries are identical. Don’t combine batteries with different ampere-hour capacities or different voltages.
• When you clean the outside casing of a battery, use only distilled water. Don’t use any of those high-tech cleaning concoctions.
• Read, heed, and save all of the instructions provided with your batteries when you buy them! Pay special attention to the directions for adding fluid to batteries that need to be periodically “watered.”
If you plan to charge deep-cycle batteries with the intent of using them in an alternative-electric system of any kind, you’ll need a charge controller to limit the rate at which the current goes into, or comes out of, your batteries. The charge controller prevents the batteries from overcharging or getting too charge-depleted. Either of those conditions will shorten the working life of your battery bank considerably, and might even wreck it straightaway. Charge controllers come in two basic types.
1. A series charge controller keeps your batteries from overcharging by shutting down the charging current (disconnecting it) when the batteries reach a state of 100% charge.
2. A shunt charge controller diverts the current from the PV panel, wind turbine, or water turbine to an auxiliary load, such as a set of electric lights, when the batteries have reached full charge.
The best charge controllers have meters that will tell you how much current you’re using at any given time, and how much voltage your batteries are producing at that time. You should consult a professional who will recommend the best type of charge controller for your system when you select or build it.
In the late part of the twentieth century, a new type of electrochemical power device emerged that holds promise as an alternative energy source: the fuel cell. This device converts combustible gaseous or liquid fuel into usable electricity, but at a lower temperature than normal combustion does. In practice, a fuel cell behaves like a battery that you can recharge by filling a fuel tank, or if the fuel is piped in, by a continuous external supply.
The most talked-about fuel cell during the early years of research and development became known as the hydrogen fuel cell. As its name implies, it derives electricity from hydrogen. The hydrogen combines with oxygen (it oxidizes) to form energy and water, along with a small amount of nitrous oxide if air serves as the oxidizer. When a hydrogen fuel cell “runs out of juice,” a new supply of hydrogen will get it working again.
Instead of literally burning, the hydrogen in a fuel cell oxidizes in a controlled fashion, and at a much lower temperature. Several schemes exist for making this process go smoothly. The proton exchange membrane (PEM) fuel cell represents one of the most widely used technologies. A PEM hydrogen fuel cell generates approximately 0.7 V DC, a little less than half the voltage of a typical electrochemical dry cell. To get higher voltages, individual cells are connected in series, so that their voltages add up. For example, to obtain 14 V DC, we would connect 20 hydrogen fuel cells in series because 20 × 0.7 V = 14 V. A series-connected set of fuel cells technically forms a battery, but engineers and technicians more often use the term stack.
Increased current-delivering capacity can be obtained by connecting cells or stacks in parallel, so that the current-delivering capacities of the individual cells or stacks add up. (The voltage of a parallel-connected set of identical cells or stacks equals the voltage of any single cell or stack all by itself.) For example, if you connect five stacks in parallel, each rated at 14 V DC and capable of delivering up to 10 A, the resulting combination will provide 14 V DC at up to 50 A because 5 × 10 A = 50 A.
Fuel-cell stacks can be obtained in various sizes from commercial vendors. A stack about the size and weight of a suitcase full of paperbound books can power a subcompact electric car. Smaller cells, called micro fuel cells, can provide electricity for portable radios, lanterns, notebook computers, and other devices that have historically operated from conventional cells and batteries.
A fuel cell can get its “juice” from energy sources other than hydrogen. Almost any liquid or gas that will combine with oxygen to generate energy has aroused interest among engineers. Methanol, a form of alcohol, is easier to transport and store than hydrogen because it exists as a liquid at room temperature. Propane and methane have been used to provide the energy for fuel cells. Even gasoline, petroleum diesel fuel, and biodiesel fuel can do the job!
Figure 4-18 is a functional block diagram of a small-scale fuel-cell power plant suitable for a home or small business. This system can also work for recreational vehicles (RVs) and boats. In the case of a fixed land location, the fuel can be stored on site or piped in. Engineers have suggested conventional methane as an ideal fuel source for home power plants of this type because the delivery infrastructure exists right now, and on-site storage is not necessary. However, in rural areas, or in any location not served by methane pipelines, other fuels might prove to be more cost-effective.
FIGURE 4-18 A small-scale fuel-cell-based electric power plant.
A typical fuel-cell stack delivers several volts DC, comparable to the voltage produced by a solar array or automotive battery. Under normal conditions, the DC from the fuel cell goes to a power inverter that produces usable 117 V AC output from the low-voltage DC input. If desired, a backup battery bank can keep the electric current flowing when the fuel tank is refilled. A power control system switches the electrical appliances between the fuel cell and the battery bank as necessary.