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.
Calibrate pneumatic thermostats every three to six months
To avoid losing efficiency, calibrate pneumatic thermostats every three to six months. The exact frequency depends on the condition of the air supply and how often occupants tamper with thermostats.
Change Filters Regularly
Maintaining cleaner heating and cooling coils by using and regularly changing filters can lead to greater efficiencies. Effective filter replacement schedules will reflect changes of use in the building. (See Electronic Air Cleaners.)
Change or Modify the Pump
After reducing system head loss, you will probably find you have more water running through the system than the chiller and pump can handle within specified limits. The next step is to consider having the manufacturer trim the pump impeller (or change the pump) to deliver the specified GPM (about 2.4 GPM per ton) at the new, lower total head loss. If you measure flow and power at the pump motor, you could see a performance of about .026- .03 kw/ton.
Change to a Variable Air Volume System
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.
Choose strategies that make the HVAC system more efficient
Although individual components of the HVAC system, such as the chiller, may be quite efficient, the overall system may contain inefficiencies that need to be remedied.
Choose the Proper Pipe Insulation
When specifying piping insulation, look at both the maximum temperature it will be exposed to and the minimum temperature. For example, the piping serving the fan coils in a building was insulated with a high R-value material, based on the maximum temperature it would be exposed to. The contractor, however, failed to take into consideration that the piping would also carry chilled water. Consequently, the insulating material had no vapor barrier. During the air conditioning season, moisture condensed within the insulation, destroying not only its insulating properties but also the pipe it was supposed to protect.
Clean and maintain the HVAC mechanical system
For greater HVAC system efficiency, clean and maintain its mechanical parts. Check for dirty coils and filters that restrict air flow, loose fan belts, outside air dampers that do not close correctly, and improperly functioning control valves.
Consider a New Cooling Tower
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.
Consider 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.
Consider Thermal Storage System
Consider a thermal storage system when designing your chiller plant. With a thermal storage system, the idea is to run chiller equipment off-peak and store cooled water or ice, then draw on this cooling during the peak times of the day. These systems take one of three forms: chilled water, ice or a salt-water hybrid of both-called a eutectic system. Specifying which system is based on the availability of space for storage media, cooling load profile, rate schedule and current equipment.
Consider Variable Speed Drives on Pumps and Fans
Pump and fan capacities can be reduced and energy saved by using variable speed drives to control their speed. However, don't forget to consider taking low-cost measures to reduce capacity, especially where the pump or fan is running at constant speed most of the time. For instance, if a fan is driven by V-belts, its capacity can be changed by altering the size of the drive pulleys. Similarly, a pump's capacity can be changed by trimming its impeller. These are low-cost alternatives to expensive electric drive modifications. Reductions in both peak and off-peak energy costs can be obtained by using variable speed drives on pumps, fans and compressors that operate at varying loads. The use of these drives will have little impact on demand because they will require the same kilowatts at peak-demand periods as fixed speed drives. They pay off better if the systems they are applied to operate at part load for relatively long hours.
Consider Water-Loop Heat Pumps
Consider installing water-loop heat pumps. Applications for these systems include schools, medical centers, hotels, offices and even small airports. Why? There are opportunities for energy recovery from core areas and from simultaneous heating/cooling situations. The energy recovered can be redistributed to space conditioning and domestic water heating. Costs are typically low for installing, operating and maintaining these systems. And they are adaptable to future energy sources, such as solar heating. What's more, individual unit control means greater flexibility and comfort.
Control Cooling Tower Fans by Sensing Ambient Wet Bulb Temperature
Control cooling tower fans by sensing ambient wet bulb (wb) temperature. Adjust the set point for an approach of about 2°F (controller will measure outside wb and adjust set point to 2°F warmer). A word of caution: Don't try this tip with old inefficient towers. The increase in fan kw will eat up chiller efficiency gains.
Control Multiple Fans with One Variable Frequency Drive
Consider controlling all the cooling tower fans with one variable frequency drive and modulate the fan speed together.
Convert a terminal reheat system to a variable air volume system
Realize significant savings by changing from a terminal reheat system to a variable air volume system. Such a conversion at a 10,000-square-foot dental clinic in Texas reduced chiller electricity by at least 50 percent and heating fuel by nearly 70 percent.
Decrease pump and fan capacities using low-cost modifications
Save energy by using low-cost measures to reduce pump and fan capacities. For example, alter the size of the drive pulleys of fans driven by V-belts. To reduce a pump's capacity, trim its impeller. These changes are much less expensive than electric drive modifications.
Free Hot Water When You're Cooling!
Reuse waste heat. Many opportunities exist to reuse thermal energy within a building. For instance, rejected waste heat from air conditioning or refrigeration equipment can often be used to serve building needs. To capture waste heat, hot gas desuperheaters, double-bundle condensers and auxiliary condensers can be used on just about every type of air conditioning and refrigeration equipment. Whether the recovery and reuse of waste equipment is practical, however, depends on the availability of and simultaneous need for that energy. The potential energy savings and cost benefits depend on how many hours-per-year of excess energy are available and also on whether that heat can be used for purposes that would otherwise require purchased energy.
Implement new design features when the air conditioning system is replaced
At present, many air conditioning systems are being replaced due to the phaseout of ozone-depleting refrigerants. This is an excellent opportunity to incorporate design features that reduce the cooling load on the system. Updating the system results in lower equipment and energy costs.
Increase the Size of Distribution Piping
During renovation or expansion, consider increasing the size of distribution piping or, in the case of retrofit, adding parallel pipes to double the cross sectional area of the flow path. Amory's theorem states: "The energy required to move water (or air) through a pipe varies with the inverse fifth power of pipe diameter."
Inspect and Repair Boilers
Before making any attempts at modifying the boiler or its controls, the boiler should be inspected and all controls recalibrated to ensure that the system is operating at its design efficiency. Scale build-up, leaky tubes, leaking flanges, damaged insulation and worn control linkages all contribute to losses in efficiency. Good maintenance practices always are a worthwhile investment, and the annual costs are recovered fully in fuel savings.
Install a Blowdown Heat Exchanger in Steam Boilers
Use a boiler blowdown heat exchanger to preheat make-up water going to the deaerator. This can be used on any larger steam boiler that has continuous blowdown. This measure reduces the energy required to preheat make-up water and partially cools blowdown water prior to discharge to the sanitary server.
Install a Boiler Economizer
The installation of an economizer, a device that preheats the feedwater (returned condensate), typically will increase the efficiency of a boiler by 2 to 3 percent. In the case of a 250 hp boiler, this can result in an annual fuel savings of more than $3,000, for an initial cost of approximately $6,000.
Install instrumentation to monitor realtime, cooling plant efficiency in kw/ton
Install instrumentation to monitor realtime, cooling plant efficiency in kw/ton.
Insulate exposed hot-water, steam and chilled-water distribution piping and valves where feasible.
Investigate using the economizer cycle to cool a building at night
To reduce costs, investigate using the economizer cycle to cool a building at night.
Look at System Efficiency Not Just Unit Efficiency
Think system efficiency when making decisions about conservation strategies. For instance, you may have the most efficient chiller available, but if parasitic loads from chilled water and cooling tower pumps are high, then the system efficiencies could be quite low.
Lower the Cooling Tower Fan Horsepower
To improve cooling tower efficiency, lower the cooling tower fan horsepower by adding surface area and free area within tower fill. These additions result in a five to 10 times lower load on the cooling tower fans. The motors can therefore be resized. The tower performance goal should be .012 kw/ton or better.
Make minor repairs and improvements to the HVAC mechanical system
To conserve energy, make inexpensive repairs and improvements to the HVAC mechanical system as required. For heating efficiency, repair or replace burners and add radiator reflectors. Install flue dampers or balance the ventilation system to reduce the exhaust rate. Relocate thermostats, install fans to keep hot air off the ceiling, and install thermostats in hot water tanks.
Make Sure Everything Is Working Properly
The effectiveness of an energy management system largely depends on the physical condition of the equipment it is intended to control. No system can achieve the best efficiency if a mechanical system is plagued with dirty coils or filters that restrict air flow, loose fan belts, outside air dampers that do not close correctly, or improperly functioning control valves.
Minimize Excess Air in Boiler Operations
Minimize "excess air" in boiler operations. In order to ensure complete combustion, most boilers are designed to operate with excess air (more air than is theoretically required for the combustion process). However, this excess air reduces the boiler's efficiency. If excess air can be minimized, efficiency gains of 1 to 2 percent can be realized. Excess air controllers (sometimes called "O2 trimmers") are available. They control excess air by sampling combustion products and adjusting the amount of air supplied to the burner accordingly. For a 250 hp boiler running 2,000 hours per year, the installation of an O2 trim system would result in an increase in efficiency of 1 to 2 percent, which is equivalent to an energy cost savings of about $1,700. Initial cost of a unit for a package boiler of this size is approximately $9,000. If the boiler ran for more than 2,000 hours per year, the savings would be correspondingly higher.
While leakage in the condensate return system is inevitable, such leaks should be kept to a minimum. It takes approximately .150 Btu/lb. to heat make-up water from 50°F to 200°F. One leak at one drop per second represents almost 3 million Btu wasted annually, or $16.50 in wasted fuel, assuming the fuel is gas costing 44 cents a therm.
Minimize leaks in the condensate return system
To avoid costly waste, keep leaks in the condensate return system to a minimum. Leaking one drop per second wastes $16.50 annually, assuming the use of gas at 44 cents per therm.
Monitor outside air temperature and humidity to control chilled water supply temperature
To improve chiller performance, monitor outside air temperature and humidity to control chilled water supply temperature. Even in hot, humid climates, you can increase the chilled water supply temperature and still maintain building comfort year round.
Monitor Your Efficiency
Consider installing instrumentation to monitor real-time, cooling plant efficiency in kw/ton. The saying goes, what gets measured, gets done. If efficiency is your goal, then you need an indicator of how well you are doing.
Multiple Tower / Chiller Installations
In multiple tower/chiller installations, run all the water over all the fill when possible.
Prevent simultaneous heating and cooling
Simultaneous heating and cooling is a senseless waste of energy and is easily preventable. You can recognize this problem by investigating unusually high utility bills and by performing energy audits. The management of a department store, when confronted with an abnormally high electric bill, solved its simultaneous heating and cooling problem by widening the thermostat deadband. During a routine energy audit of a manufacturing plant, a faulty air conditioning valve was discovered. Replacing the valve saved the plant $1,000 per month in energy costs!
Raise Chilled Water Supply Temperature and Lower Condenser Water Temperature
The coefficient of performance (energy used per ton of cooling) can be improved by raising the chilled water supply temperature and lowering the condenser water temperature. Maintain condenser water as cool as possible (with clean and efficient cooling towers), but not less than 20 degrees above chilled water supply temperature.
Raise the Cold Air Temperature Set Point at the Thermostat
Raise the cold air temperature set point at the thermostat. Most air conditioning systems are designed to maintain temperatures at 72°F to 75°F dry bulb (db) and 50 percent RH (relative humidity). Higher temperatures and slightly higher humidity levels can be maintained without noticeable discomfort to the room occupants. Increasing room conditions from 74°F db and 50 percent RH to 78°F db and 55 percent RH will save approximately 13 percent of the energy required for cooling.
Raise the temperature of the chilled water supply, and lower the temperature of the condenser water
To improve chiller performance, raise the temperature of the chilled water supply, and lower the temperature of the condenser water. Keep the condenser water as cool as possible, but not less than 20 degrees above the temperature of the chilled water supply.
Raise the thermostat setpoint
Raising the cold air temperature set point on your thermostat can result in significant energy savings without causing noticeable discomfort to room occupants. Most air conditioning systems are designed to maintain a temperature of 72oF to 75oF and 50 percent relative humidity. By raising room conditions to 78oF and 55 percent relative humidity, you can save approximately 13 percent of the energy required for cooling.
Recover energy from steam
Structures such as hospitals, universities, and industrial facilities use high- and medium-pressure steam containing valuable energy that can be recovered. Avoid venting this steam into the atmosphere where its heat is wasted. Instead, use the recovered heat to preheat domestic hot water or returned hot water from the building's heating system.
Recover Flash Steam
Whenever medium- or high-pressure steam boilers are used, there will be flash steam in the condensate system. This flash steam contains valuable energy that can be recovered. High- and medium-pressure steam is used in many types of structures, including hospitals, universities and industrial facilities. Good uses for the recovered heat are preheating domestic hot water or preheating returned hot water from the building's heating system. From 5 to 15 percent of returned condensate will flash to steam at approximately 5 psi. If this steam is vented to atmosphere, the heat is wasted. In addition, the vent will produce a visible plume of steam. which may be undesirable.
Reduce Pumping Power in Cooling Systems
Of all the things you can do in a cooling plant to save energy, reducing pump power offers the greatest savings for the least cost. The trick to the following tip is to exploit the cube law of pumps (and fans). This law says that the energy required to move water varies directly with the cube of the total head loss across the system (pump). Head loss can be decreased by systematically reducing pressure losses in the distribution system. This can be accomplished by eliminating bypass valves and three-way valves; removing auto flow valves, pressure regulating valves and other flow restrictors; and opening balancing valves at the pump.
Repair or Replace Boiler Insulation
For peak efficiency, radiation loss from a boiler should be minimized by keeping insulation in good repair. It may not be worthwhile to add to existing insulation, but old, damaged or missing insulation should be repaired or replaced.
Reset Chilled Water Supply Temperatures
Monitor outside air temperature and humidity to control chilled water supply temperature (chilled water reset strategy). For centrifugal chillers, each degree F you raise chilled water supply (CHWS) temperature above 42°F increases chiller efficiency by 1 percent. Even in hot, humid climates, you can increase CHWS temperature and still maintain comfort in the building much of the year.
Run all the water over the cooling tower fill
In plants with multiple cooling towers and chillers, run all the water over the tower fills when possible.
Should You Have 100 Percent Redundancy?
Often, critical items of equipment are provided with 100 percent redundancy. (One standby operates when the normal unit is out-of-service.) By providing three units each with a capacity equaling 50 percent of total load rather than two units able to serve 100 percent of total load, you save first cost and create the opportunity for additional energy savings. The units can be sequenced so that additional units can come on-line only when the running units are operating at full load. Applications include air compressors, vacuum pumps, heating water pumps, boilers and chilled water pumps. Benefits: One unit sized at 50 percent can handle the load 65 percent of the time, thereby saving energy associated with operating one larger unit at a less efficient part-load condition.
Shut down unnecessary auxiliary HVAC equipment
Shutting down unnecessary auxiliary HVAC equipment improves system efficiency without impacting performance. To save energy in central HVAC plants, turn off auxiliary equipment such as cooling tower fans and circulating pumps for chilled water and condenser water when not required.
Shutdown Unnecessary Auxiliary Equipment
Shut down unnecessary auxiliary equipment. Large central HVAC plants require auxiliary equipment like cooling tower fans and circulating pumps for chilled water and condenser water. This equipment should be shut down when not required. This does not result in lower chiller coefficient of performance but does improve the efficiency of the overall system.
Subcooling the Building?
Investigate using the economizer cycle to subcool a building at night, much like a whole house fan in your home. The economizer cycle should be enthalpy controlled to reduce operation when outdoor air is too humid.
Survey Your Mechanical Systems
A survey of mechanical systems can reveal a host of opportunities for conserving energy, often at a relatively small initial cost. Heating efficiency may be improved by: repairing or replacing burners; adding radiator reflectors to direct heat; installing flue dampers or balancing the ventilation system to reduce exhaust rates; relocating thermostats; installing destratification fans to keep hot air off the ceiling; or installing thermostats in hot water tanks.
Use a thermal storage system in the chiller plant
Using a thermal storage system in the chiller plant conserves energy, particularly when there is a significant difference in energy demand between on-peak and off-peak hours. Three types of thermal storage systems are available: chilled water, ice, and hybrid. With a thermal storage system in place, you run the chiller equipment during off-peak hours and store cooled water or ice. You then draw on this cooling during the day.
Use an economizer in air handling units
To reduce the energy required to operate a chilled water plant, use an economizer in the air handling units of areas that use outside air for "free cooling."
Use Cooling Towers for "Free Cooling"
Consider free cooling with cooling towers in commercial buildings and industrial plants where space conditioning or process cooling is done with mechanical refrigeration. When the cooling requirements are year-round, in cool months, they may be met with just cooling tower operation alone. How? By taking advantage of the large number of hours of low ambient wet bulb (wb) temperatures. The refrigerant compressor is turned off during this period, accounting for large energy savings. This is weather dependent, and proper analysis and design is required for both retrofit and new construction applications. Simple payback is possible in two to five years.
Use cooling towers to save energy
Use cooling towers to save energy in cooling plants by delivering cooler water to the chillers. Cooling tower technology has improved ten-fold over the last ten years.
Use Electronic Air Cleaners
Consider using electronic air cleaners in building air handling units in lieu of standard bag or cartridge filters. Electronic air cleaners have less air resistance, 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.
Use Energy Estimating Software as an Analysis Tool
Use energy estimating software as an analysis tool. Two restaurants were owned by the same person. The number of meals served per day was the same. The size of the buildings was the same. The amount of equipment was the same. But one used much more energy than the other. A detailed inventory was done and the results were fed into a building energy simulation software package. The results showed that the restaurant using more energy actually had five older, less efficient rooftop HVAC systems, compared to four newer ones at the other restaurant; had much more incandescent lighting; and had gas cooking equipment with greater standby losses and a higher ventilation rate. All of these contributed to the higher energy bills. The simulation effort made an apples-to-apples comparison possible.
Use energy estimating software to identify inefficiencies
Use energy estimating software to identify ways to increase energy efficiency and lower utility bills. For example, assume that two separate restaurants occupy the same sized buildings and serve the same number of meals daily. Oddly, the energy bills for one restaurant are significantly higher than for the other. After performing a detailed inventory of each site, the results are fed into a building energy simulation software package. The results show that the restaurant with the higher bills has an older HVAC system, uses more incandescent lighting, and has less efficient cooking equipment. The use of such software analysis makes an "apples-to-apples" comparison possible.
Use More Smaller Units Rather Than One Large
More may be better than less when it comes to saving energy. In two Ohio retirement and nursing home communities, the owner reduced gas usage by as much as 30 percent by using three smaller, high efficiency units instead of one large single, gas-fired boiler for each wing in the two complexes. Sequencing allows only those boilers required to meet the heating load to be fired, thereby reducing standby losses and increasing seasonal operating efficiency.
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.
Use variable frequency drives to control the motors of variable air volume fans
Using variable frequency drives to control the motors of variable air volume fans saves a significant amount of energy when the system is operating at low cooling loads. A study conducted by the Commonwealth Edison Company found that this modification results in an energy savings of 48 percent.
Use variable speed drives on pumps, fans, and compressors operating at varying loads
To reduce both peak and off-peak energy costs, use variable speed drives on pumps, fans, and compressors operating at varying loads. The use of these drives has little impact on demand because variable speed drives require the same kilowatts during peak-demand periods as fixed speed drives.
Variable Speed Fans
Many variable air volume fans operate at constant speed over the system's entire operating flow range. The fan flow is controlled by opening and closing dampers to provide cooled air to the conditioned spaces. A significant amount of energy is wasted when the system is operating at low cooling loads. A variable frequency drive (VFD) can be used to vary the speed of the motor, thus allowing the fan to match its output to the varying system load. A study conducted by Commonwealth Edison Company, in which inlet guide vanes equipping 200 hp supply and 50 hp return fans were replaced with a VFD, found that the VFD provided average energy savings of 48 percent. In this case, the result: annual energy savings of $15,734.
Watch Out for Simultaneous Heating and Cooling
Watch out for simultaneous heating and cooling! At one department store, electric bills were unusually high. Investigation showed that half of the 10 rooftop systems were heating and half were cooling at the same time. This was fixed by widening the thermostat deadband to prevent simultaneous operation. In the office of a manufacturing plant, the refrigerant control valve was stuck open on a large direct expansion (DX) split air conditioning system. As a result, it was cooling well into the heating season. The mixed air controller caused the heating coil to reheat the air, so the occupants never knew there was a problem. This was discovered during an energy audit. Fixing the valve saved about $1,000 per month.
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.
When practical, recover and reuse waste heat
When practical, recover and reuse waste heat from air conditioning and refrigeration equipment. Potential energy savings are based on how many hours-per-year of excess energy are available, and whether that energy can be used for purposes that would otherwise require purchase. Waste heat capturing equipment includes hot gas desuperheaters, double-bundle condensers, and auxiliary condensers.