Backup power during an outage maintains comfort while it keeps home and family safe, prevents business losses, and may allow a business to remain open during an extended outage. An Automatic Standby Generator, sometimes called a backup generator, is an automatic generator that provides backup power during an outage. They “stand by” waiting for a power outage. When it happens, the generators start automatically and switch the home or business to generator power. No human intervention necessary.
Standby Generators for homes and businesses fall into two main classes: air cooled and liquid cooled. The cooling refers to how the engine cools itself. Air Cooled Generators are the choice of many homeowners, but also fit the needs of some small businesses. Commercial Standby Generators offer the reliability and longevity required by operations from retail to farming.
A generator engine converts fuel to mechanical energy which turns an alternator to generate electricity.
Power capacity is determined by the alternator capability and engine size. As the power drawn from the alternator increases, the load on the engine also increases and it uses more fuel. Generator manufacturers provide ratings in running watts and starting watts. Running watts equals the maximum continuous capacity of the generator. Starting watts, also called Surge Watts, is the maximum power the generator can produce for a few seconds to start motors or power large inductive loads like transformers.
For example, the air-cooled Generac 7043 22 kW Generator has a rated capacity of 22,000 running watts. It can provide 22kW all day. The engine in the 7043 has the power to turn the alternator while the generator supplies the maximum rated load. It must also have the power to provide the high starting current when it starts a heavy load like an air conditioner motor. For a few seconds, the starting watts of a 5-ton air conditioner could be 20,000 watts or more.
If the home or business is already drawing 14,000 watts from the generator, starting a 5-ton air conditioner adds a momentary 20,000 watts to the load for one or two seconds as the motor starts. The air conditioner load then falls to 7000 watts.
What is an Air Cooled Generator?
An air cooled generator uses the surrounding air to cool the engine. It could be passive—the surrounding air absorbs heat from the engine. As it rises or the wind blows it away, more air moves in and removes more heat. Passive cooling provides enough heat removal for an open frame generator like a portable.
Forced air cooling uses a fan to blow air over the generator engine to remove more heat. Manufacturers equip air cooled generator models with a one or two-cylinder engine less than one liter (1000cc) in size. The engine turns the cooling fan in addition to the alternator. It is efficient and suitable for moderate climates where temperatures rarely rise above 100° F.
However, in hot climates, an air cooled generator enclosed in a metal cabinet cools less efficiently during hot weather. Overheating can occur during extended use under a moderate to heavy load. The generator controller senses the engine temperature and if it exceeds the safety limit, the controller shuts the engine down to prevent damage. The generator won’t restart until the engine cools and may require the owner to intervene.
The Liquid Cooled Generator Difference
A liquid-cooled engine pumps a coolant through the engine block where it absorbs heat from the engine before it enters a radiator. A fan blows air through the radiator to remove heat from the coolant, which then returns to the engine and the cycle repeats. Air blowing through the radiator may blow over the engine to remove additional heat.
Not only does a liquid cooled generator engine turn the alternator, it must also pump the coolant and turn the fan. These functions require additional power from the engine.
Some refer to a liquid-cooled generator as a Water Cooled Generator—another name for a generator that uses a liquid coolant that is part water. Water Cooled Generators use a mix of coolant and water to raise the boiling point of the coolant above 212 degrees and to prevent it from freezing in cold weather.
The liquid cooled generator has two advantages over air-cooled models. Liquid cooling is more efficient. The coolant can still remove heat even at the extreme temperatures inside the engine. It is not unusual for engine coolant to exceed the boiling point of water. This efficiency allows the generator to operate in hot climates where temperatures often exceed 100 degrees and may climb much higher.
A liquid cooled generator can use a larger engine than an air cooled, which allows the generator to produce more power. The largest air-cooled standby available is 24kW, a big step ahead of 20kW competitors, but doesn’t compare to a 150kW generator for very large estates, agricultural operations, or businesses.
Air Cooled Generator Advantages vs Liquid Cooled
An air-cooled generator is less complex, usually has a smaller engine, and weighs much less than a comparable liquid-cooled system. The smaller engine and lower overall capacity (in general) lower the purchase price. Easy to move with a cart. Two people can install the generator in one day. It sits directly on the soil or a concrete Generator Pad at additional cost. In either case, the installer levels the site and places the pad directly on the ground with the generator on top. Installation costs less and takes less time. Air-cooled generators typically use less fuel than liquid-cooled counterparts. Most air-cooled generators are certified for installation at 18 inches from the building.
Liquid cooled generators can weigh three or four times the weight of an air-cooled model. They require power equipment to move and place the generator. The additional weight requires a prepared bed or pedestal to place the generator on.
The overall size, placement, and installation costs of liquid-cooled generators give the air-cooled an advantage. Some liquid-cooled models require five feet of clearance to the building. With a 50 percent larger footprint than air-cooled models, the larger generators use up valuable space in a backyard instead of fitting into the landscape. Distance from the building becomes additional landscaping repairs and higher costs for plumbing and electrical work.
Liquid Cooled Generator Advantages
Power requirements that exceed 20-24kW require a liquid cooled generator. Liquid-cooled power capacity starts at 15kW for Diesel and 22kW for natural gas or propane. Air-cooled generators start at 7.5kW and max out at *20-24kW.
Manufacturers may rate air-cooled generators at a lower capacity for natural gas than propane, in part due to the limitations of the smaller engines. The larger engines found in liquid cooled models make up the difference and provide the same performance on either natural gas or propane. For example, the air-cooled Generac 22kW Generator has a 19.5kW rating on natural gas and 22kW on propane. The Generac 22 kW Liquid Cooled Protector is rated at 22kW on natural gas and on propane.
Liquid Cooled Generators have a clear advantage in hot climates, especially the desert Southwest. When temperatures climb over 120 degrees, air-cooled generators won’t run long before they overheat and shutdown. This additional reliability makes them more suitable for business applications or critical systems that must run during a power outage.
*Generac is currently the only manufacturer that offers 22kW and 24kW air-cooled generators.
Both types of generators have their place. Air-cooled generators are fuel efficient and economically priced. They fit the average home in a moderate or cool climate where temperatures rarely rise over 100 degrees Fahrenheit. Businesses with power needs similar to a moderate to large home will also find the air-cooled models a good choice, especially where installation space is a consideration. Several air-cooled 3-phase generators are available from Generac or Briggs and Stratton. With few exceptions, 3-phase power is limited to business applications or large residential buildings.
Larger homes and businesses that need more power or anyone in a hot desert climate should consider a liquid cooled generator. Superior cooling and power capacity that reaches far beyond the limits of air-cooled generators make the larger generators a natural choice for these applications.
Natural gas, diesel, and propane liquid-cooled generators provide fuel choices for anyone who needs backup power during an outage.
Guide to Choosing a Liquid-Cooled or Air-Cooled Generator
Homes and Businesses in Hot Climates Benefit from More Power and Fewer Heat-Related Shutdowns
A generator uses an internal combustion engine to turn an alternator. The alternator (generator) converts mechanical energy into electrical energy. The rapid expansion of hot gases inside the engine drive the pistons and turn the engine. Uncontrolled, the heat created by combustion and friction could damage the engine or the alternator. Generators must remove excess heat to prevent permanent engine damage.
This article examines the advantages of cooling methods employed to remove heat from a liquid-cooled or air-cooled generator.
Liquid Cooled Generator
- Liquid Cooled Generator Advantage
- Air Cooled Generator vs Liquid Cooled
- Liquid Cooled Portable Generator
- Liquid-Cooled Portable or Air-Cooled?
- Liquid Cooled Standby Generator
Small engines typically allow heat to escape through cooling fins on the engine block and cylinder heads. The fins increase surface area which creates more contact with the air around the engine and allows the air to absorb some of the heat. This passive cooling works quite well for many small engines with enough difference in temperature between ambient air and the engine block.
Larger engines take longer to heat up which adds unburned fuel to the exhaust. An engine-driven fan pulls in outside air, blows it over the engine and alternator, and returns it outside. The cooling system is always functioning, even when the engine is cold. Many air-cooled standby generators with single or twin cylinder engines less than 1-liter (1000 cubic centimeters or cc) displacement employ this active cooling method.
Generator engines with a displacement larger than 1 liter employ a cooling system similar to that found in modern automobiles. The engine pumps liquid coolant through the engine and into a radiator. The coolant absorbs heat from the engine. A fan blows air through the radiator and transfers the heat to the air. This active method of cooling is very efficient and is employed in engines for cars, trucks, equipment, and liquid-cooled generators. A thermostat regulates the flow of coolant to help the engine warm up faster and then provides greater control over temperature while it runs.
The Liquid Cooled Generator Advantage
An air-cooled engine depends on the transfer of heat from the engine to the air. A big difference in temperature allows the engine to transfer more heat to the surrounding air. Adding a fan allows faster heat transfer, but the transfer is limited by the temperature difference between the engine and the air.
Liquid coolant can absorb much more heat than the surrounding air. In the radiator, the liquid is given many times the surface than is possible on the engine surface. Large amounts of air blowing through the radiator remove heat fast and cool the liquid again on its path back to the engine. This advantage allows a liquid-cooled generator to operate in hot environments and still cool itself efficiently.
Engines work best at specific temperatures, especially diesel engines. A thermostat located in the cooling system opens and closes to allow or prevent coolant from circulating through the engine. As the engine nears optimum operating temperature, the thermostat opens to prevent the temperature from rising too high. If the coolant temperature falls, the thermostat begins to close.
A well-regulated operating temperature provides better combustion and lowers emissions.
Air Cooled Generator vs Liquid Cooled Generator
A liquid-cooled design is great for applications that require a larger, multi-cylinder engine or in locations where temperatures often exceed 100 degrees Fahrenheit. Someone living in the desert southwest may only need a 15kW generator. An air-cooled unit costs less but may experience frequent shutdowns due to overheating. A liquid-cooled generator solves the cooling problem.
In hot climates, the passive cooling of a hard-working portable generator may be inadequate. Overheating will shorten the generator life and cause premature failure. A portable liquid cooled generator may prove a better and more reliable choice.
Moderate climates like the Midwest, Northeast, or East Coast are well suited to air-cooled generators. They rarely have temperatures over 100 degrees. The average to large home in these regions generally requires less than 20 kilowatts of power, even with multiple air-conditioners. A smaller, fuel efficient air-cooled generator with a power management option meets the requirements of the home and family.
Multiple heavy loads may add up to more power than an air-cooled generator can provide. Luxury class homes with three or more air conditioners often need the power offered by liquid-cooled generators.
The frequency of AC current is directly related to engine speed. An alternator with a single pair of N and S poles runs at 3600 RPM for a frequency 60 hertz. This is the typical speed of many generators that run on natural gas, gasoline, or propane. Many larger generators and diesel generators operate at 1800 RPM and have an alternator with two pairs of N and S poles. Lower noise and longer engine life results.
Liquid Cooled Portable Generator
Portable with regard generators usually means it is not permanently installed in a single location. Generators on a jobsite provide power for tools, equipment, and buildings without access to an electric utility service. This may include remote locations, mines, or sites where utility service is not yet available. A new commercial building site or highway construction project, for example. Some air-cooled portables can do the job, but extended use over long period requires a generator rated for a prime power application.
An air-cooled portable might be light enough to lift off the back of a pickup truck. The small inverter generators are easily carried by one person. Generators over 5000 watts can weigh over 150 pounds and are less portable for a single person. They come with wheel kits and lift hooks.
A 7000-watt air-cooled portable might weigh 175 to 250 pounds with an empty fuel tank. By comparison, a comparable liquid-cooled portable generator like the Kubota GL7000 Lowboy II weighs more than 500 pounds.
Towable Trailer Generator and Commercial Mobile Generator Models designed for vehicle or trailer mounting make up another type of liquid-cooled portable generator. Diesel fuel is a common choice for liquid-cooled portable generator engines. It’s easy to get and deliver directly to the generator fuel tank.
Liquid-Cooled Portable or Air-Cooled?
If you’re running power tools like saws and a small air compressor, or charging batteries for power tools, a portable generator is all you really need, even in climates where it gets hot. Do the maintenance on schedule, especially oil changes, and the generator will last for years.
Power for bigger tools or equipment, jobsite buildings, and heavy-duty motors probably needs the power from a liquid-cooled portable generator rated for prime power. They still require on-time maintenance and will perform better under heavy-load in a hot climate for days at a time.
Liquid Cooled Standby Generator
Any application that requires more than 18kW to 20kW generator capacity will probably require a liquid-cooled generator. Kohler manufactures air-cooled generators that can operate in parallel for specialized applications.
Commercial applications including office buildings, restaurants, retail, light manufacturing, agriculture, and warehouses can all benefit from a liquid-cooled standby generator. The short story is that losses from a power outage lasting more than an hour or two make the investment in a standby generator worth the cost. Businesses have power requirements far above that of the average home.
Reliability and full power are important. A business can’t shut down on refrigeration unit to let another run or turn off power to the registers while the gas pumps stay on. Outdoor temperatures in the triple digits means the air conditioners run constantly. Some compromises may be acceptable, but for close to normal operation the business must have the power it needs.
A liquid-cooled standby generator can handle the heat and provide the necessary power for the business to stay open.