Types of HVAC Systems
Cooling towers are one of the most overlooked opportunities for saving energy in cooling plants. Cooling tower technology has improved over the last 10 years. The average tower today operates at about .12 kw/ton, while new, efficient, plastic-filled, counterflow towers can perform at .011 kw/ton (10 times better). These efficient towers use less fan horsepower per unit of cooling but, more importantly, they deliver cooler water to the chiller(s). For centrifugal chillers, each degree F the condenser water temperature is depressed, the chiller efficiency will increase about 1 percent.
To increase the efficiency of a boiler by 2 to 3 percent, install an economizer. This device preheats the feedwater (returned condensate). In the case of a 250 horsepower boiler, an initial investment of $6,000 provides an annual savings of more than $3,000.
Some packaged HVAC systems support economizers that use dampers and related controls to allow outside air to be used for cooling when outdoor temperatures are moderate. This “free cooling” can provide significant energy savings in some climates.
Add Enthalpy Control for “Free Cooling”: Provide all air handling units with an economizer with enthalpy control. Applications include all areas and buildings that can use outside air for “free cooling” during a significant portion of the year. This reduces the energy required to operate a chilled water plant.
Water-Side vs. Air-Side Economizer System
Use a water-side economizer system instead of the more common air-side economizer when the air supplied to the space must be kept within tight humidity limits. Using an air-side economizer would introduce low humidity air to the space that would then have to be humidified. A water-side economizer means that chilled water is cooled by the cooling tower without mechanical refrigeration when outdoor temperatures are low enough. Minimum outdoor air is introduced to the space by using this method. Applications include laboratories, hospitals, data processing centers and other areas where specific minimum humidity levels are required. Savings will be highest when electric humidification must be used and when the ambient conditions are very dry. Savings from this measure will vary based on local conditions, but the cost of humidification must be considered when making system selections.
Electronic Air Cleaners
Electronic air cleaners are a great energy-saving tool. They have less air resistance than standard bag or cartridge filters, and the resistance remains constant. A standard filter’s air resistance increases as the filter gets dirty. Air resistance increases fan static pressure, which in turn means motor energy use increases. Electronic air cleaners can be used in any type of commercial or industrial building. Energy savings will occur because of the decrease in static pressure resulting from the use of electronic air cleaners. The amount of the decrease is dependent on static pressure created by other system components.
Enhanced Temperature & Time Controls
Highly programmable controls are available that can operate HVAC systems based on building occupancy schedules, including nighttime temperature setbacks. This can significantly reduce energy costs.
Evaporative Rooftop Cooling
For some areas of the country, evaporative rooftop cooling is reducing summer cooling loads by as much as 25 percent. Prior to installation of such a system, a 124,000-square-foot software development facility in Dallas calculated that it needed to add 41 tons of additional air conditioning to adequately cool the building. What building owners discovered was the water-spray rooftop cooling cut the cooling load, allowing the company to recover the costs of its installation in one year.
For process heat, install a heat exchanger rather than an economizer in the boiler stack. The amount of heat transferred by the exchanger can be considerably greater than by an economizer, which makes it a more energy- and cost-efficient option. Please note: the stack temperature must exceed 350°F to safely use either a heat exchanger or economizer.
Indirect Evaporative Cooling
Some packaged HVAC systems support modules that circulate water over a heat exchanger to use the cooling effect of evaporating water to pre-cool air entering the system’s cooling sections. This is called indirect evaporative cooling and can provide significant energy savings in some climates.
Powered Exhaust Fans
Some packaged HVAC systems support powered exhaust fans that help maintain desired pressurization of the building (or of selected areas in the building).
Use Steam Turbines to Capture Pressure Reducing Valve Energy Losses
Steam turbines can be used to recover valuable energy from pressure reducing valves (PRVs). PRVs can be bypassed by back-pressure turbines, which exhaust steam at the same pressure as the PRV yet produce useful work. For example, they can be used to drive a generator, pump, chiller or compressor. A back pressure turbine operating at 250 psi supply and 15 psi exhaust, expanding 20,000 Btu of steam per hour, can drive a generator producing as much as 129 kw. If this operates for 6,000 hours per year, the electricity produced is worth more than $38,000 per year if you are paying 5 cents per kwh.
Variable Air Volume Systems
Changing from a terminal reheat system–which provides good thermal comfort but is known for its poor energy efficiency–to a variable air volume system can have significant savings. For example, the conversion at a 10,000-square-foot dental clinic in Texas reduced chiller energy use by at least 50 percent and heating fuel by nearly 70 percent.