Energy Efficiency And HVAC Technology
The following overview offers a quick reference to key considerations with some of the most effective technologies. As with lighting, trial installations are a good idea; so is working with manufacturers and distributors.
Getting the most from HVAC controls
Because a building’s performance can be dramatically improved by installing and fully using HVAC controls, it is essential to understand and correctly use those controls. The place to start is with a close look at what is really transpiring in your building, 24 hours a day, seven days a week.
What is happening with each piece of equipment? On holidays? Weekends? As the seasons change, do your operations change? It is important to understand where and how energy is being consumed in order to identify where waste is occurring and where improvements can be implemented. Then it is imperative to ask, “What exactly do I want these controls to do?”
Energy management systems (EMS) are designed to run individual pieces of equipment more efficiently and to permit integration of equipment, enhancing performance of the system. In a typical EMS, sensors monitor parameters such as air and water temperatures, pressures, humidity levels, flow rates, and power consumption. From those performance points, electrical and mechanical equipment run times and setpoints are controlled.
Seven-day scheduling provides hour-to-hour and day-to-day control of HVAC and lighting systems and can account for holidays and seasonal changes. As the name implies, night temperature setback allows for less cooling in summer and less heating in winter during unoccupied hours.
Optimal start/stop enables the entire system to look ahead several hours and, relative to current conditions, make decisions about how to proceed; this allows the system to ramp up slowly, avoiding morning demand spikes or unnecessary run times.
Peak electrical demand can be controlled by sequencing fans and pumps to start up one by one rather than all at once and by shutting off pieces of HVAC equipment for short periods (up to 30 minutes), which should only minimally affect space temperature. Economizers reduce cooling costs by taking advantage of cool outdoor air. Supply-air temperature-reset can prevent excessive reheat and help reduce chiller load.
An EMS can provide an abundance of information about building performance, but someone has to figure out what they want the EMS to do and then give it directions. Calibrating controls, testing and balancing are key to any well-maintained HVAC system, but are especially critical to optimize control efforts.
Variable speed drives and energy-efficient motors
Variable speed drives (VSDs) are nearly always recommended as a reliable and cost-effective upgrade.
VSDs are profitable where equipment is oversized or frequently operates at part-load conditions. Savings of up to 70 percent can be achieved by installing VSDs on fan motors operating at part-load conditions. They may be applied to compressor or pump motors and are generally used in variable air volume (VAV) systems. They are also cost effective in water-side applications. Backward-inclined and airfoiled fans are the best VSD candidates.
Air-handler configurations controlled by variable inlet vanes or outlet dampers squander energy at part-load conditions. Using throttle valves to reduce flow for smaller pumping loads is also inefficient. The efficiency of motors begins to drop off steeply when they run at less than 75 percent of full load; they can consume over twice as much power as the load requires. VSDs operate electronically and continually adjust motor speed to match load.
The power to run the VSD is proportional to the cube of the speed (or flow), which is why this technology is so efficient. If the speed is reduced by just 10 percent, a 27 percent drop in power consumption should result. A VSD pilot study performed by EPA found that VSD retrofits realized an annual average energy savings of 52 percent, an average demand savings of 27 percent and a 2.5-year simple payback.
Perform harmonic, power factor, electric load, and torsional analyses before selecting a VSD. Though harmonic and power factor problems are not common in VSD applications, VSDs should generally be equipped with integral harmonic filters (or a three-phase AC line reactor) and internal power factor correction capacitors (or a single capacitor on the VSDs’ main power line). In general, this equipment is not standard and must be specified.
Improved design and better materials enhance the performance of energy-efficient motors, which use 3 to 8 percent less energy than standard motors; units with efficiencies of 95 percent are available.
To achieve maximum savings, the motor must also be properly matched with its load, increasing run time at peak efficiency. Motors operate best when running at 75 to 100 percent of their fully rated load; motors routinely operating below 60 percent of rated capacity are prime candidates for retrofit. For motors whose loads fluctuate, VSDs should also be considered.
Smaller, more efficient motors are integral to a system downsizing stratagem; downsizing a 75 horsepower standard motor to a 40 horsepower energy-efficient model will result in energy savings of 15 percent.
Some energy-efficient motors have less “slip” than standard-efficiency motors, causing energy-efficient motors to run at slightly higher speeds; consider a larger pulley to compensate for the higher speed and to maximize energy savings. Installing a new pulley or adjusting the existing one can also be an alternative to a VSD when the cost for the VSD is prohibitive or the load has been reduced.
Improving fan system performance
A common way to improve the efficiency of the air distribution system is to convert constant air volume (CAV) systems to VAV. One authority on energy issues, E-Source, reports that “typical (VAV) air flow requirements are only about 60 percent of full CAV flow.”
VAVs respond to load requirements by varying the volume of the air through a combination of pressure controls and dampers rather than by varying the air’s temperature. According to the air pressure, fan power and volume of conditioned air are reduced, thus increasing energy efficiency. Of course, it is crucial to maintain indoor air quality (IAQ) when altering air handling systems.
To maximize savings, VAV components such as VSDs, variable-pitch fan blades, diffusers, mixers, and VAV boxes must be operating properly; careful zoning is also required to achieve VAV optimization.
E-Source recommends considering the following VAV retrofit procedures:
• complete load reduction measures and calculate the maximum and minimum air flow requirements,
• measure existing fan performance; examine duct system for possible improvements,
• stage fans that are in parallel configurations,
• commission the system thoroughly,
• optimize static pressure setpoint and implement reset control, and
• possibly remove return air fans.
Energy-efficient and properly sized motors are also recommended along with careful control strategies. Installing a self-contained, thermally powered device to each diffuser can add greater control to VAV systems by controlling individual spaces, rather than entire zones, and eliminate the need for VAV boxes. Such a device also offers VAV-style capabilities to CAV systems.
VAV retrofit costs and paybacks can vary widely. Installation problems related to fan control, reduced supply air distribution, location of pressure sensors and their reliability, in addition to deficient design, can diminish a VAV retrofit’s performance. Because VAV boxes are relatively expensive and one is required for each zone, it is generally not cost effective to partition the space into many zones. Careful zone designation — according to occupancy, internal loads and solar gain — will maximize efficiency, increase comfort and reduce reheat.
When reheat cannot be eliminated, consider these steps to minimize it: ensuring thermostat calibration; increasing supply air temperatures during the cooling season; and monitoring reheat year round and possibly employing reheat only during winter months. Where reheat is used primarily to control humidity, a desiccant wheel or a heat pipe might be considered.
Downsizing existing VAV fan systems is a relatively low-cost way to save energy when loads have been reduced or when the air distribution system was oversized to begin with. The following are means to downsize fans or airflow requirements:
• Reduce static pressure setpoint to meet actual temperature and airflow requirements.
• Rightsize motors and upgrade to energy-efficient models; install larger pulleys.
• Replace the existing fan pulley with a larger one; that will reduce the fan’s power requirements by reducing its speed.
• Make sure the fan’s speed corresponds to the load. Reducing a fan’s speed by 20 percent reduces its energy consumption by approximately 50 percent.
There are several ways to determine if VAV fan systems are oversized. If a motor’s measured amperage is 25 percent less than its nameplate rating, it is oversized. If a fan’s inlet vanes or outlet dampers are closed more than 20 percent, it is oversized. If the static pressure reading is less than the static pressure setpoint when inlets or dampers are open and VAV boxes open 100 percent, as on a hot summer day, the system is oversized. Again, be sure to consider IAQ requirements when downsizing air handling systems.
Chillers and thermal storage
No one wants to replace a perfectly good chiller just because of the CFC phaseout. But once load-reducing efficiency upgrades have been completed, it may actually be profitable to replace an oversized chiller. That’s especially true given rising prices and tightening supplies of CFC refrigerants.
Oversized units 10 years or older are good candidates for replacement. A high-efficiency chiller reduces energy costs throughout its lifetime; initial costs are reduced because the replacement chiller is smaller than the old one. Depending on the old unit’s efficiency and load, a high-efficiency chiller’s energy consumption can be.15 to.30 kW/ton less, decreasing energy consumption by as much as 85 percent if combined with downsizing.
An alternative to replacement is to retrofit chillers to accommodate a new refrigerant and to match reduced loads. That may involve orifice plate replacement, impeller replacement and possibly compressor replacement, depending on the chiller’s specifics.
Retrofitting may entail gasket and seal replacement and motor rewinding. Depending on the refrigerant and the way the retrofit is performed, the chiller may lose either efficiency or capacity. To determine whether replacement or retrofit is a better option, consider both initial and life-cycle costs.
Retubing the condenser and evaporator yields sizable energy savings but whether it makes sense, given its high cost, depends on the condition of the chiller. Water-cooled condensers are generally more efficient than air-cooled units. Because condenser water flows through an open loop, it is susceptible to fouling. Scale build-up will inhibit heat transfer efficiency; maintenance is therefore required to keep the surfaces clean.
Absorption chillers are an alternative to centrifugal models. Absorption chillers cost up to $150 per ton more than vapor compression chillers like centrifugal units, but can be profitable in areas of high electrical demand charges or where steam or gas is available, depending on the local utility rate structures. Using a combination of the two chiller types can reduce electrical demand charges.
Thermal energy storage (TES) uses conventional chiller equipment to produce conditioned water or ice (or occasionally another phase-change material) in off-peak periods. Water is withdrawn from storage during the day or at peak hours and circulated through the cooling system.
TES systems can be incorporated into new and existing systems and can provide partial load leveling or full load shifting. TES helps decrease operating and maintenance costs; in some cases, a smaller chiller can be specified. Some systems provide lower supply air and water temperatures, so air and water flow requirements can be cut.
Fill material, size and fan configurations affect cooling tower efficiency. Cellular fill (aka film packing) increases efficiency over other fill types. Oversizing the tower to allow for closer approach to ambient wetbulb temperature can improve its efficiency. Generously sizing the tower and increasing its share of the chiller load can make economic sense because a cooling tower’s initial cost and energy use per ton are less than a chiller’s.
At part-load conditions, applying a VSD to the fan (or pump) will improve the tower’s efficiency. Systems with VSDs and several fans are more efficient when all tower cells are operating at reduced speed as opposed to one or two cells at full speed.
Because cooling towers contain large heat exchange surfaces, fouling — scale or slime build-up — can be a problem. The efficiency of improperly treated systems can be improved with effective water treatment. High-efficiency towers are available; induced-draft types are more popular and efficient than forced-draft towers. Performance can also be improved by increasing cooling surface area.
In traditional pumping systems, flow is generally constant volume; a throttle valve reduces flow at part-load conditions, inhibiting efficiency.
Installing VSDs on secondary pumps in variable flow systems, rightsizing pumps and motors to meet load requirements, and upgrading single loop systems to primary/secondary loop configurations can increase the performance and reliability of pumping systems. In upgrading chilled water pumps, it is important to meet maximum and minimum flow rates through the chiller.
Other cooling options
Desiccants are dehumidification materials which can be integrated into HVAC systems to reduce cooling loads and increase chiller efficiency while improving indoor air quality and comfort. Formerly found only in niche and industrial applications, desiccant cooling is extending throughout commercial markets.
Desiccants make sense when the cost to regenerate them is low compared to the cost to dehumidify below dewpoint and can reduce HVAC energy and peak demand by more than 50 percent in some cases.
Evaporative coolers provide one of the most economical and efficient means of cooling, using up to 75 percent less energy than vapor-compression systems. Though initial cost is typically higher, paybacks for evaporative coolers range between six months and five years. Though evaporative coolers are particularly prevalent in the arid West and Southwest, they can service most U.S. climates. E-Source states that, in combination with evaporative cooling, desiccant cooling can eliminate refrigerative air conditioning in many climates.
Hybrid systems that integrate evaporative cooling with conventional HVAC technologies offer additional opportunities. To improve performance consider lower air velocity; better fill materials; higher fan, pump and motor efficiencies, including VSDs; better belts or direct drive; improved housing; improved controls; and duct sealing. Proper maintenance is key to energy-efficiency.
Packaged air-conditioning units are typically found in buildings or building zones where the cooling load is less than 75 tons. Running these units at part load can severely reduce efficiency. They are generally not as efficient as chiller systems but can be upgraded and rightsized when replaced. Existing systems can be improved by using higher efficiency compressors, larger condensers and evaporators, and VSDs, though life expectancies of 10 to 12 years for these technologies may mean that retrofits are not cost-effective.
Heat pumps are among the most energy-efficient heating and cooling technologies available today. Low operating costs, increased reliability and long life expectancies improve their viability. They function best in moderate climates and proper sizing is critical.
Multi-unit configurations can service larger loads and provide zoning; large, modernized central units offering capacities of up to 1000 horsepower or 750 kilowatts are gaining popularity. Air-to-air type heat pumps are the most common because of low up-front costs; ground supply heat pumps are the most efficient but tend to have higher initial costs.
Especially in colder climates, improved boiler performance — with improved fuel and airflow controls over a range of load conditions and increased heat transfer surface areas — can contribute substantially to energy savings. Smaller units arranged in modular systems increase efficiency up to 85 percent while small units replacing those with open-loop condensing systems shoot combustion efficiency up to 95 percent.
Boiler retrofits, combined with improved maintenance measures, can also increase efficiency — up to 90 percent. New burners, baffle inserts, combustion controls, warm-weather controls, economizers, blowdown heat recovery and condensate return conversions provide increased efficiency opportunities. A smaller “summer” boiler might be a good option when a boiler is required year round though at reduced capacities in warmer conditions. The much smaller summer boiler is sized for reduced loads; the main boiler is shut down.
HVAC upgrades can provide tremendous economic benefits, improve occupant comfort and system reliability, and reduce operating costs. But to maximize benefits and minimize capital investment, load-reducing measures, such as lighting upgrades, should precede HVAC system upgrades.