How to design standby power systems to withstand storms

Learning Objectives

Hurricanes and other severe weather events pose a substantial risk to maintaining mission critical facility operations. Loss of power for critical water treatment and wastewater processing infrastructure can result in loss of both production and revenue and can threaten public safety and the environment. A reliable standby power system incorporating severe weather protection provisions will mitigate these risks and help maintain facility operations and continuity of service.  

Standby generators are not designed to be exposed to the elements, so they are typically protected by either a manufactured outdoor enclosure or a dedicated generator room in a building.  

An outdoor enclosure can either be a skin-over enclosure (slightly larger than the generator) with access panels to maintain the generator or a walk-in type enclosure allowing staff to enter the enclosure for maintenance. Outdoor enclosures offer minimal customization for storm protection.  

Designers should avoid using the terms “stormproof” or “weatherproof” construction when specifying generator enclosures. The proper terms are “storm resistant” or “weather resistant.” Regardless of how well-maintained the generator is, there is still a potential need to service generators during severe weather. An outdoor enclosure may subject maintenance staff to adverse weather conditions as staff may need to work outdoors or inside the small metal enclosure.  

If an outdoor enclosure is used, the design team should consider and clearly define in the project manual the following to minimize potential damage: 

• Structural engineer to determine the required wind pressures that the enclosure must resist. 
• Mechanical engineer to determine the required air inlet and outlets to mitigate water penetration into the enclosure. 

• Where custom hoods and/or Air Movement and Control Association International Inc. certified louvers are not available, screen walls should be considered to provide added protection. 

Advantages of an outdoor enclosure include smaller site footprint and lower design and construction costs. Disadvantages include limited ability to customize and limited space for maintenance, especially during severe weather.  

Buildings housing generator rooms are typically pre-engineered metal buildings, made of reinforced concrete frame with masonry infill or reinforced masonry. However, it is also important for the building construction to resist damage from windborne debris. Reinforced masonry construction is a typical method for constructing generator rooms to protect the facility during severe weather. Generator rooms offer more flexibility and customization to accommodate client preferences and aesthetics.  

Regardless of how well-maintained the generator is, there is still a potential need to service generators during severe weather. Generator rooms in buildings provide shelter and protection for maintenance personnel, enabling them to work on equipment without being subjected to adverse weather conditions. Disadvantages include increased design and construction costs. 

In coastal areas or areas prone to flooding, facility construction should include mitigation methods for rising water. For mission critical facilities, generators should be located above the 100–year flood level or other local design condition. This may require placing outdoor enclosures on an elevated platform or providing an elevated building structure. Flood barriers could be considered for noncritical facilities.  

Cooling systems

Generators produce a significant amount of heat when operating. For the generator to function properly, this heat must be removed to maintain the engine block temperature and ambient space temperature within the operational range of the generator.  

Heat within the engine is removed by circulating coolant through the engine block. The closed-loop coolant system is typically air-cooled and consists of a radiator, piping and pumps. Radiators can either be skid– or remote–mounted outdoors. Skid-mounted radiator fans are the simplest and most reliable form of generator cooling because radiator fans are connected right to the crankshaft of the engine. All components are located within the generator enclosure or room, which minimizes potential downtime during severe weather. Skid-mounted fans are sized by the generator manufacturer to provide cooling air for the radiator core and to remove heat dissipated by the engine. 

Remote-mounted radiators consist of a radiator core and a cooling fan. If the skid-mounted pumps are not capable of circulating the liquid through the radiator cooling loop, a separate heat exchanger and auxiliary pumping system will be required. When remote-mounted radiators are used, separate exhaust fans are required to dissipate heat radiated off the surface of the generator. These fans are nearly as large (50% to 75%) as a skid-mounted radiator fan.  

Remote-mounted radiators are less reliable because of the additional complexity and equipment noted above. If remote radiators are used, the design team should consider the following to minimize potential damage: 

• Secure the remote-mounted radiator, piping systems and exterior exhaust fans to meet wind loads.  
• Provide screen walls to protect remote radiators from windborne debris.  
• Use exhaust fans located inside the generator room instead of outside on the roof or walls, protecting them from potential damage from windborne debris.  

Air inlets and outlets

Air inlets and outlets required for generator cooling and combustion airflows are the most critical aspect of protecting a standby power system. Table 1 summarizes the required airflows from two generator manufacturers, as well as the gross inlet and outlet free area opening sizes for a large 2,250–kilowatt generator. Free area is the total minimum area of the opening in the supply outlet or return inlet through which air can pass. Free area is derived by taking the total open area of a louver (after subtracting all obstructions such as blades and frame) and dividing by the overall wall opening.  

The gross free area required for the air intake is roughly 150 square feet and the gross free area required for the air exhaust is roughly 100 square feet. Depending on the type of air inlet/outlet used, the net free area (or actual opening size) required can be up to four times the gross free area. Protecting these openings from the effects of wind, rain and windborne debris is imperative to maintain generator operation during severe weather.  

Louvers—Louvers are the most common form of air inlets/outlets used for generators. There are several test protocols for evaluating louver performance in simulated severe weather events. AMCA 500-L, 511 and 550 include testing protocols and ratings for the beginning point of water penetration and wind–driven rain resistance. The beginning point of water penetration test simulates a nonwind-driven (vertical) rainfall and uses a fan drawing air through the louver to determine the intake air velocity through the louver opening at which 0.01 oz. water penetration per square foot of louver free area begins to penetrate the louver. The wind–driven rain resistance test simulates a wind–driven (horizontal) rainfall and uses a fan and water nozzles to push simulated rainfall through the louver to determine the amount of water penetration. AMCA 540 includes testing protocols and ratings for impact resistance of louvers. The impact resistance test involves launching 2×4-foot timber projectiles at the louver sample from specific locations and at specific velocities to determine if the louver can resist penetration of windborne debris.  

The amount of water penetration from a nonwind-driven rainfall is typically a mist of water that could collect on electronic components of the generator and cause generator failure. The amount of water penetration from a wind–driven rainfall can range from a similar mist of water through a wind-driven rain–resistant louver, to significant amounts of water penetration through nonwind-driven rain–resistant louvers that could be drawn through the generator and cause generator failure.  

Acoustic louvers, drainable blade louvers and wind-driven rain louvers are the most common types of louvers used for generators. Acoustical louvers are commonly used for generator applications to provide sound attenuation. Acoustical louvers provide roughly 20% to 30% free area, so using our sample generator gross free area of 150 square feet of intake area, the total net free area required is 500 to 750 square feet.  

Acoustical louvers have a beginning point of water penetration ranging from roughly 800 to 1,000 feet per minute. Drainable blade louvers are designed to mitigate nonwind-driven water penetration. This type of louver provides roughly 50% to 60% free area with a beginning point of water penetration ranging from roughly 1,000 to 1,250 fpm. There are some models available for both acoustical and drainable blade that are impact rated, but neither is effective at mitigating wind-driven water penetration. Wind-driven rain-resistant louvers are designed to mitigate wind-driven rain, provide roughly 40% to 50% free area and have a beginning point of water penetration of 1,250 fpm or higher. Impact–rated models are available.  

While louvers have varying ability to mitigate water penetration, they provide no protection from the impacts of positive/negative pressurization related to direct wind. For instance, the radiator fan may not be able to overcome the positive pressure of high winds directly flowing at the louver. This could cause the generator to overheat. Additionally, high–performance louvers are typically limited to smaller sizes than other louvers. If multisection louvers must be provided, additional support framing will be required.  

Wall hoods and roof hoods—The main advantage of hoods is that they provide nearly 100% free area. Wall hoods are typically custom fabrications, while roof hoods are typically premanufactured by fan manufacturers. Wall hoods can be used for both intake and exhaust applications and provide some impact resistance for the wall openings depending on the construction of the hood. Wall hood supports must be designed for code required wind loads. Roof hoods are mostly used for intake applications.  

Exhaust applications require significant ductwork to connect the radiator fan to the roof hood. Radiator fans have limited external static pressure to accommodate ductwork. Most larger hoods will require additional tie–down supports. For the example generator air flows, using roof hoods would still require +/–150 square feet of roof opening area.  

There are no testing protocols for hoods. Both wall and roof hoods can be effective at mitigating nonwind–driven water penetration, but are ineffective at mitigating wind-driven water penetration based on empirical data. Penthouse type roof hoods using AMCA wind-driven rain louvers are effective at mitigating wind-driven rain; however, the addition of the louvers reduces the free area and increases the pressure drop across the intake.  

The performance of wall hoods can be improved by extending the bottom of the hood down below the interior wall opening. Roof hoods should not be located directly above generators. In addition to the increased potential for roof leaks, in some cases, a mist of water drawn through the hood can damage an electronic component and cause generator failure.  

Plenums—Plenums are a highly effective method of protecting air inlets and outlets for generator rooms. The plenums are typically constructed at both ends of the generator room and consist of enclosed outer walls with an open top covered by grating. The outer plenum walls should be constructed of masonry or other durable construction to protect the air inlets/outlets. The inner wall of the building will still include intake and exhaust louvers, typically drainable blade.  

Air enters at the top of the plenum on the intake side, passes through the intake louver and across the generator. The air then passes through the radiator fan and across the exhaust louver into the plenum and discharges out of the top of the exhaust plenum. The outer plenum wall blocks wind-driven rain from entering the building, mitigates the potential for wind pressure to cause back pressure on the radiator fan and protects against windborne debris. The drainable blade louver mitigates potential water penetration from vertical rainfall entering the top of the plenum. Figure 5 shows the air flow path through the plenum, as well as a representation of the plenum wall blocking wind-driven rain and windborne debris.  

Although not directly tied to severe weather protection, there are other benefits of a plenum design. The plenum design has been shown to greatly reduce sound transmission by redirecting noise, so it dissipates before reaching the site boundary. For example, a standby power system at a wastewater treatment plant in Tennessee that incorporated intake and exhaust plenums was shown to keep decibel levels relatively the same whether the generators were running. 

The plenum design has advantages when trying to aesthetically blend these standby power system facilities into the surrounding area. The plenums put the large industrial–looking louvers on the interior side of the plenum space and provides a blank canvas on the outside of the plenum for blending the facility with either the aesthetics of the adjoining buildings on-site or the vernacular of the community. 

Fueling systems 

For a generator to operate during severe weather, the fueling system must be designed to mitigate the impacts of severe weather. Fuel sources for generators include diesel, natural gas or propane. All three fuel sources are considered reliable. Both diesel storage and propane are readily available on-site fuel sources with fixed storage capacity.  

Several factors should be considered when determining the amount of on-site fuel storage, including the critical nature of the facility and the ability to refill the storage tank during an emergency. Seventy-two hours is a common storage capacity for standby generators serving critical facilities, but storage capacity should not be less than 24 hours. Natural gas is readily available with no storage required; however, there is a possibility of service interruption in the supply of natural gas, primarily caused by severe weather or natural disasters. 

Diesel fuel can be stored in sub-base fuel tanks (typically for outdoor enclosures only), exterior above–grade fuel tanks or exterior below–grade fuel tanks. Above–grade fuel tanks should be a minimum UL 142 double–wall tank to eliminate the need for installing the fuel tank in a separate containment area that can collect stormwater during severe weather. UL 2085 protected storage tanks provide fire resistance, impact resistance from vehicles and projectiles and ballistic resistance from firearms. This type of tank should be considered to provide additional protection for the fuel system during severe weather. 

For outdoor enclosures with sub-base fuel tanks, UL 2085 construction will likely require a custom enclosure and tank that will increase cost. This customization may not be available with smaller generator sets. The top of the fuel storage tank should be located above the 100-year flood level or other local design condition. This will mitigate potential water intrusion into the tank and allow the tank to be refilled even if partially submerged.  

The tanks should be secured to meet the code required wind loads and buoyancy forces. Fuel piping systems for separate above–grade fuel tanks should be secured to meet the project wind loads and piping should be laid out to minimize the length of exterior piping. Screen walls and protective pipe covers can be used to minimize potential for damage from windborne debris. Below–grade piping systems are another potential option, and will require double–wall piping and leak monitoring. Diesel fuel has a limited storage life and fuel quality will degrade with time and temperature. Fuel polishing systems should be considered for larger storage systems 

Below–grade fuel tanks are not as common for standby generator applications due to additional regulatory requirements, but may be considered on a case-by-case basis if there are space constraints or aesthetic concerns, for example. Tanks should be double–wall and secured to resist buoyancy forces. Ground monitoring stations may be required by local codes. The normal fill port may be submerged during severe weather, so an above–grade remote fill station should be considered.  

Natural gas and propane generators produce fewer emissions than diesel generators. Natural gas generators require a feed from the local utility. The gas meter and pressure regulator(s) should be located above the 100-year flood level or other local design condition. Propane gas generators require above grade or below–grade storage tanks and interconnecting gas piping between the tank and generator. Installation requirements are similar to diesel fuel storage tanks, but there is not a similar “protected” storage tank construction standard available.  

The concepts of weather hardening generator installations can be applied to other mission critical facilities like hospitals, fire stations and polices stations; noncritical commercial and industrial facilities; and even residential construction. The design consultant, building owner and end user should work together to determine the best method to provide reliable standby power for each application.  

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