Energy Efficiency Measures

HVAC

High-efficiency packaged units and high-efficiency heat pumps provide the same cooling and heating capabilities as their standard counterparts while consuming less energy by integrating high-efficiency components and controls within their systems. The higher-quality components and controls found in premium-efficiency motors on fans and pumps and high-efficiency, water-cooled chillers allow these components of a central plant to perform more efficiently than standard equipment.

The Consortium for Energy Efficiency (CEE) designates specifications for high-efficiency commercial packaged air conditioning equipment and maintains a database of qualifying products at www.cee1.org.

Indirect/direct evaporative cooling uses the physics of water evaporation to cool with reduced levels of compressor cooling, which is an energy intensive process. Evaporative coolers can save 60-80% of the cooling energy for spaces such as classrooms. In addition to saving energy, direct evaporative coolers also add needed moisture to the conditioned air.

When a pound of water evaporates, almost 1000 Btu’s of cooling is associated with the process. If warm dry air is blown across a medium thoroughly wetted with water, the air is cooled and its humidity is raised. If the process were 100% efficient, the temperature drop of the air would be the difference between dry bulb and wet bulb temperatures. In practical systems suitable for commercial, industrial, and agricultural use, efficiencies of 80-85% are routinely achieved. In Colorado’s climate zones, wet bulb temperatures are lower than dry bulb temperatures by an average of over 30 degrees Fahrenheit for 99% of the cooling season, so evaporative cooling is quite feasible. Since the only energy it consumes is that associated with fan power for moving air and pump power for moving relative small amounts of water, evaporative coolers save both energy and demand by a factor of almost three versus conventional compressor-based cooling.

Although direct systems like the one shown below work well with small commercial structures and most agricultural buildings, direct/indirect systems are more prevalent with larger buildings like offices and retail establishments. Air that is cooled evaporatively from the exhaust stream from the building (or from outside air, depending on circumstances) is used to cool incoming air via an air-to-air heat exchanger. Thus, incoming air is cooled, but not humidified with a direct/indirect system. A variation, known as a hybrid system, includes a modicum of compressor-based cooling for periods of high humidity in which evaporative cooling systems become less effective. However, the conventional chillers with such hybrid systems may be downsized by a factor of five or more from a chiller used to supply all of a building’s cooling needs.

Direct/indirect and hybrid systems are more costly than simple direct evaporative cooling systems, but they routinely cost less than conventional compressor-based systems and have substantially lower energy and demand costs over their lifetimes. The best units utilize high-efficiency fans driven by variable speed drives on premium-efficiency motors. Varying fan speed with load not only raises overall system efficiency, but also extends the life of the wetted pad and other key components. Lifetime is also extended by the selection of pads that use treated glass fiber sheets (instead of conventional excelsior made of aspen wood), and housing of fiberglass or stainless steel. Routine maintenance is not costly but necessary to ensure good air quality and long life.

Direct Evaporative Cooling (Source: Platts/E SOURCE)

Direct/Indirect Evaporative Cooling (Source: Platts/E SOURCE)

Specifying high-efficiency, water-cooled chillers whenever appropriate saves energy and demand charges. In the case of facilities with cooling loads of more than 200 tons, it is generally cost-effective to install a water-cooled chiller, particularly in Colorado’s dry climate. In all events, a high-efficiency electric chiller can reduce energy consumption by 20% or more compared with a standard-efficiency chiller. Consider evaporative coolers in hot, dry climates. Specify small direct or indirect evaporative coolers instead of vapor-compression units.

Cooling towers are especially helpful in lowering cooling costs in Colorado’s dry climate. (Source: Delta Cooling Towers, Inc.)

Two-speed cooling tower fans and variable-speed drives on fans and pumps and energy management systems (or direct digital control systems) will allow for the modulating of HVAC equipment. This controls the system so that it works only to meet the space conditioning and ventilation requirements of the building spaces, and not just at full output capacity at all times. For example, demand-controlled ventilation modulates ventilated air to keep CO2 levels below a set point (for example, 1000 parts per million), thereby allowing ventilation rates to be adjusted to the number of people occupying the space and other variables. This strategy for reducing building ventilation saves energy without compromising indoor air quality, and modern CO2 sensors are both reliable and inexpensive.

The cost-effectiveness of installing direct digital controls (DDCs) on an HVAC system varies widely with the specific site and application. DDC systems save energy if they are used to turn building systems off when they are not needed. In office buildings, DDC systems can modulate HVAC and lighting equipment to achieve energy savings as well as to trim demand during peak periods, thereby lowering energy bills for months to come. This is particularly important in Colorado where commercial energy rates are only a weak function of energy charges (kWh), but a strong function of demand charges (kW). A DDC system should permit programming changes to be easily accomplished in order to alter controls for tenant turnover, to respond to new utility rate structures, or to change control sequences.

Using occupancy sensors in conjunction with digital controls can limit energy waste in unoccupied hotel and motel rooms and similar spaces, including offices. Guest room occupancy sensors or central control systems can reduce energy requirements without inconveniencing guests. For example, a central switching system at the front desk can turn on heating or air conditioning as the guest checks in or manually adjust thermostat settings if the room is unoccupied. Heat sensing (infrared) detectors can activate HVAC and lighting systems based on human presence in the room. Turn-off time delays of 10 to 30 minutes can accommodate a guest’s departure from the room for short periods of time.

Variable air volume (VAV) air-handling systems with variable-speed drives (VSDs) can save considerable fan energy over constant volume systems. Incorporating a VSD on a VAV fan allows it to slow down as load decreases. Because reducing fan speed by one-half will reduce power consumption by seven-eighths, a VSD on a VAV fan system offers compound energy savings that can provide a payback of three to five years. Typical VSD installation costs are $200 to $250 per horsepower of the motor driven. VAV can save energy cost-effectively in systems whose fans are 20 hp or more.

Variable-speed drives are also useful in a number of other commercial and industrial applications, from moving water from boilers or chillers to local heat exchangers to adjusting patterns of irrigation to optimize crop growth while minimizing water use. Motors used in pumping fluids like water or high-pressure air can match pumping rates to instantaneous demands, thereby saving both energy and demand costs.

VSD’s are also useful in adjusting ventilation rates to ensure good indoor air quality while controlling fan energy use. Instead of operating at fixed fan rates on a predetermined schedule, ventilation rates can be varied to maintain CO2 levels below a given threshold, for example, 1100 parts per million. Inserting a CO2 sensor in the return air stream to give feedback to a simple control algorithm can optimize fan use while safeguarding air quality.

The best new energy-efficient boilers are called condensing boilers because their stainless steel heat exchangers wring so much heat out of the exhaust gases that they cool to the point of condensation, thereby releasing extra useful heat. Modern condensing boilers are quite efficient at their peak ratings—90% or so—and even more efficient when they are turned down, reaching 96% or sometimes even better when operating at 10% or so of their peak rating. It is best to run these boilers at lower temperatures than conventional boilers. This minimizes heat loss from the boiler to heat exchangers (coils or radiators) and maximizes overall system efficiency.

To take advantage of high efficiency when demand for heating is moderate, it is usually cost-effective to employ a pair of condensing boilers and operate them both at a fraction of their rated outputs. Such systems can lower heating bills by 30% or more.

Condensing boilers with high turn-down ratios can also be used to supply domestic hot water (DHW) through local heat exchangers. Applications like hotels and motels, where demand is heavy but sporadic, are particularly attractive since the large turn-down ratios can meet a range of demands while maintaining good efficiency.

This pair of condensing boilers can be fired at a million Btu’s per hour or be throttled back to 67,000 Btu’s per hour at steady state efficiencies that range from 92-96%.
(Source: Jeff Bowers, Veazey Parrot Durkin & Shoulders.)

Gas-fired infrared heaters are substantially more efficient than are forced-air systems in spaces like warehouses and high-ceilinged industrial facilities. Instead of heating the entire volume of a space, radiant heating systems produce infrared waves to directly heat objects (and people) in their line of sight. Because infrared heaters are over 80 percent efficient, use no fans, and heat objects instead of air, they can have a simple payback as short as one year compared with forced-air convection heating systems.

Unit Ventilators with Face and Bypass Controls

Modern unit ventilators are optimized for use in school classrooms and other areas. They are quiet, energy-efficient, controllable by modern digital energy management systems, and are able to vary ventilation rates via local CO2 sensors to maintain good air quality while minimizing energy waste. With a face and bypass system, the flow of warm or cool water from a boiler or chiller through the heat exchanger is constant, but the amount of return and outside air that passes through the exchanger is varied with the bypass damper.

Unit ventilators that regulate space-conditioning energy via face and bypass dampers enjoy a number of advantages over units that do this job via control valves. By their nature, control valves introduce resistance to the flow of conditioned water that is reflected in a higher total pressure head that the pumping motors must overcome. Therefore, motors must be larger and electricity consumption is higher. There is, however, an even more serious concern. Valve-control units are at risk of freezing under winter conditions when ventilation air is brought in at times when the thermostat is satisfied, because water flow through the coils is quite low during such periods. Designs to avoid the problem exist, but they add costs and complications that can result in maintenance problems.

Two-pipe HVAC systems went out of style several decades ago, but thanks to the advent of digital controls, efficient boilers and chillers, and sound engineering practices, such systems have made a comeback. Instead of having a pair of pipes for supply and return conditioned cool water from the chiller and another pair from the boiler, each set going to separate heat exchangers, a single pair of pipes is used to supply cooling water or heating water to a single set of heat exchangers. Combined with digital controls responding to strategically-placed sensors, condensing boilers, and high-efficiency chillers, the resulting simplification yields excellent overall energy performance with very low maintenance costs. Modern two-pipe HVAC systems are quite cost-effective retrofits in schools wishing to upgrade older heating-only systems to include cooling. Piping costs are very moderate and modern unit ventilators which use face and bypass controls ensure high comfort and good indoor air quality, while giving each teacher an adequate measure of control of classroom temperature. A two-pipe retrofit of 20 schools in a southern Indiana school system achieved an overall savings of 17% in spite of adding air conditioning to previously un-air conditioned schools.

Ventilation in parking structures can be modulated as a function of information from carbon monoxide sensors, thereby saving energy while ensuring safe air quality. Demand-controlled ventilation in parking garages turns on ventilation fans only when levels of carbon monoxide approach unacceptable levels. A demand-controlled ventilation system reduces fan energy consumption during many hours of the day. Alternately, naturally-ventilated parking structures eliminate the need for any mechanical ventilation. In naturally-ventilated parking structures, it’s frequently possible to take advantage of daylighting by installing photocells and lighting controls as well.

Air-side economizer systems use dampers with the HVAC’s fan system to draw in substantial quantities of exterior air when its temperature drops below indoor air temperatures. Given Colorado’s moderate climate, economizer systems can significantly reduce energy use for cooling commercial and industrial buildings during much of the year. Since Colorado evenings are frequently cool even during the summer, the mass of a building may be cooled over night and thereby substantially lessen the need for cooling energy the following day. Economizer systems can work well, but fail when dampers are not controlled appropriately or stick in the open or closed positions. Choosing good quality parts that weather well, maintaining the system, and double-checking control routines is thus essential.

When weather conditions and loads permit, water-side economizers use water that is cooled evaporatively in a cooling tower to substitute for water cooled by a compressor cycle. Such systems can be very effective, but in most cases should be operated via a water-to-water heat exchanger to avoid mixing water that flows through the compressor with that which flows through the cooling tower.

© 2006 Southwest Energy Efficiency Project
2260 Baseline Road, Suite 212, Boulder, CO 80302
(303) 447-0078 fax: (303) 786-8054 info@swenergy.org