Is waste heat recovery right for your project?
How to evaluate and specify waste heat recovery systems in various nonresidential building markets.
- Understand the energy savings that waste heat recovery can provide.
- Identify the process of achieving waste heat recovery in a facility.
- Understand the maintenance and operation requirements of waste heat recovery.
As buildings in every market strive to become more energy efficient, recovering waste heat can provide a tremendous opportunity for savings. Looking inward for opportunities to capture waste heat can save energy and reduce operational costs, especially in energy-intensive facilities such as laboratories and health care institutions.
While heat recovery technology has been available for years, today it is both more cost-effective and more widely employed in light of the desire to reduce a facility’s carbon footprint and energy consumption. Additionally, buildings are more integrated and designed more holistically, affording building systems the opportunity to capitalize on internal processes enabling the sharing and exchanging of energy across sources (energy generation) and sinks (demand/consumption).
To specify the best-fit heat recovery system, it is important to understand the building heating and cooling load profiles, interior processes, heat sources, and heat sink forms. The building load profile tells the story of energy flow as consumed or produced by the building systems and internal processes. The profiles also provide insight on durations and times when energy sources are available. Matching these energy sources with energy sinks is the key for finding opportunities to recover energy.
In addition to matching a source and sink, the locations and forms where energy is produced and consumed must also be evaluated. Recovery strategies and technologies for capturing, transporting, and reusing energy have evolved significantly and can be tuned to match very specific processes. Heat recovery is best applied when the source and sink coincide in physically close proximity and can be exchanged with a minimal state conversion. Where physical proximity and energy forms do not align closely, heat recovery may not be feasible and could have significant cost implications or may require redesign of systems and processes to capitalize on energy recovery.
Four steps to specify energy recovery systems
While energy recovery has many benefits, it may not be ideal for every situation. Leading discussions with the facility owner and operator about their efficiency goals, corporate policies, and the maintenance impacts of select systems is a key step in determining the feasibility of energy recovery. In many cases the energy balance and energy forms may align; however, the maintenance of these systems can be labor intensive and costly, which will detract from the advantages. Four recommended steps in evaluating and specifying an energy recovery system are:
Code analysis: Energy codes have specific sections requiring energy recovery for systems of significant size and high outdoor air percentages, very common to laboratory makeup air systems. New code versions being adopted throughout the U.S., ASHRAE Standard 90.1-2010 and International Energy Conservation Code (IECC) 2012, require energy recovery on systems as small as 1,000 cfm depending on the climate zone and percentage of outdoor air at full design flow rate. In addition to size, the codes also stipulate a minimum of 50% total energy recovery effectiveness. Both IECC and ASHRAE have exceptions to the energy recovery requirements, which must be evaluated on a case-by-case basis. In addition to energy recovery, energy codes are also very detailed when outlining requirements for humidified spaces, dehumidification, and economizers (air and water). The aforementioned items can be large components of more technical spaces such as laboratories and industrial facilities.
Energy balance, source/sink evaluation: The first step in evaluating energy recovery is to analyze the building and process loads. Understanding the energy flow will allow the engineers to align building sources and sinks of energy, creating in essence a “mass balance” of energy that is used to match production and consumption of energy. In addition, load profiles provide insight into the availability of energy sources and sinks that will assist in developing feasibility and return on investment strategies for different technologies.
System evaluation: With potential energy sources and sinks identified, different options can then be evaluated for extracting, transporting, and discharging the energy. Physical layouts, maintainability, risk/uptime, and cost effectiveness must all be evaluated with each system.
Get buy-in from the owner and facilities department: Engaging the owner and facility managers is a critical step for successful implementation of an energy recovery program. Presenting multiple options using select key performance indicators (KPIs) is generally helpful in comparing systems. The KPIs should be developed and weighted with input from the owner based on its goals and objectives. Sample KPIs include maintenance requirements, first cost, return on investment, and any process risks depending on the facility and technology. Energy savings can be modeled using process loads and equipment efficiencies with turndown curves. Contingencies should also be developed with each energy recovery option. These contingency plans should accommodate system loads if the energy recovery has to be maintained, in scenarios where the source energy may not be available, and in scenarios where perhaps too much energy recovery is available. In addition, operators must understand the correlation between energy loads that enable the efficient operation of the energy recovery system. Getting operator buy-in and understanding is critical to prevent systems from being installed, taken down for maintenance, and left shut off and isolated. Successful implementation and maintenance of energy recovery systems can save tremendous amounts of energy; however designers must develop the correct symbiotic relationship between energy sources, sinks, and operations.
Energy recovery systems generally consist of three primary components: source energy capture system, transportation, and sink energy discharge system. The specific components will depend on the type of fluids or gases used to transfer energy. The most commonly used component to extract and discharge energy is a heat exchanger.
Heat exchangers come in many forms depending on the working fluid or gases used. The heat exchanger can be as simple as a coil, plate and frame, or shell and tube heat exchanger, or something more sophisticated, like an active desiccant wheel. Heat exchangers can also be part of a packaged energy recovery system such as the tube bundles in a heat recovery chiller.
To exchange the energy between energy sources and sinks, a transport medium must be used. The medium will depend on the type of energy being exchanged. Common fluids for energy exchange include refrigerants, water, glycol, desiccant media, and air. The physical location of the source and sink will directly affect the type of transport medium used and cost of the system. Some media, such as refrigerants, have specific physical requirements, where as other media, such as water and glycol, are more flexible.
Each type of facility will have different energy expenditure goals, needs, load demands, and energy code requirements that will be key to determining which type of energy recovery system is best. These are applied solutions.
High-density electronic R&D laboratories: A recently completed project in a process cooling dominated facility consisted of multiple high-density electronic research laboratories. The laboratories contained measured demand power densities of 15 W/sq ft. To condition these spaces, active chilled beams were used with design chilled water temperatures of 58 F/66 F. A dedicated outdoor air system (DOAS) provided 48 F low-temperature primary air to the active beams that provided minimum ventilation rates and supplemental cooling capacity, and suppressed the room dewpoint for the chilled beams.
Because the laboratories are occupied and running continuously, designers leveraged the internal process heat generation to preheat the incoming outdoor air whenever the outside temperature is below the chilled water return temperature. Side stream pumps distributed return chilled water to hydronic preheat coils in the dedicated outdoor air units. The chilled water return gets precooled in the preheat coil while the incoming fresh air is preheated. Actual operational BAS trend data indicated that the electric heat in the DOAS had extremely limited run time this past winter, resulting in significant energy savings. (Due to the location and available utilities, electric heat was required in the dedicated outdoor air units.) In addition, the chilled beams use relatively warm chilled water, which enabled the chiller plant to operate more efficiently and increase the free cooling economizer run time.
Manufacturing and industrial: Many large manufacturing and industrial facilities feature significant compressed air systems with a high number of operating hours (in many cases 24 hours a day). In critical facilities that include pharmaceuticals and petrochemical, for example, there is redundancy in almost every attribute of their critical systems—offering substantial energy recovery opportunities. As a general rule of thumb, 50,000 Btu/hour of heat is available for each 100 cfm of compressed air, depending on the type of compression technology employed. The waste heat is generally about 180 to 200 F for oil type compression and 300 to 350 F for oil-free compression technology. (Contact the equipment manufacturer to verify compression heat available for each application.) Energy recovery from this compressor waste heat can be through either a water media or air.
The first exercise the engineer should consider is the heat of compression energy available and how to effectively capture it within the plant operation including run hours and temperature of the waste heat. Where can this energy be used? Also, how can the available waste heat be balanced with the need/sink (balanced equation)? Consideration should be given to first costs of installing the system and return on investment (ROI) period. Generally speaking, the ROI (payback) must be within 36 to 60 months maximum to make this attractive to an owner.
For lower waste heat temperature (oil-type compression technology), the systems to consider for energy recovery include makeup air for HVAC, building heating water, domestic hot water, preheat of makeup water for boiler plants—essentially any system in the building that has a heating sink requirement.
For facilities that require depressed dewpoints or moisture content and/or pure air in their compressed air systems (pharmaceuticals, semiconductor, petrochemical manufacturing, and health care), oil-free compressor technology may be required. In these systems, the heat of compression temperatures can be as high as 350 F. Heat can be exchanged through air-to-water or air-to-air heat exchangers, depending on the application and energy sink/source availability and demand. An example of an air-to-air heat exchanger is a regenerative desiccant dryer system. This system uses heat of compression to regenerate the desiccant media employed to super dry the compressed air to as low as -94 F dewpoint. In these systems, if extra heat recovery capacity is available, an air-to-water heat exchanger can be used to divert heat to other loads such as boiler makeup water preheat and system reheat. The designer should determine these features during the system analysis.
Wet laboratories: In large-scale wet laboratory facilities, there is generally an attached administrative office component that houses the research scientist, technicians, and administrative staff. Because of code-required building ventilation, as much as 30% of the building supply air will be fresh ventilation air for the administrative office. The adjacent laboratory, testing, and quality analysis/quality control (QA/QC) facilities that contain fume hoods require significant amounts of outdoor air.
Generally speaking, the research laboratory space is smaller than the office space but requires higher volumes of fresh air. Conditioning this air mechanically is very expensive, especially considering these buildings often operate 24 hours a day. Air-to-air heat recovery can be an option depending on the types of processes being performed in the hoods. Where chemicals are sufficiently diluted and nontoxic, energy recovery wheels can be used to pretreat the outside air with the fume hood exhaust. In applications where there can be no chance of cross-contamination of the makeup airstream and the exhaust airstream, heat exchangers, energy recovery coils, or heat pipes can also be used for energy recovery. It is important to consider not only current but also future uses of the lab for compatibility with energy recovery devices.
In many cases, however, the air from the hood processes cannot be taken through an energy recovery device for health, safety, and maintainability reasons and must be directly exhausted outside, or in some cases treated and then exhausted. In some of these cases a cascading fresh air system can be used where the adjacent office relief air can be used as makeup air for the laboratory hoods. As long as the exhaust/relief airstream is not contaminated, this conditioned air (generally 68 to 80 F) can be reused to temper the incoming outdoor air (cold or hot) and thus reduce the total amount of energy required to achieve the supply air temperature setpoint to the space (heat reused is more efficient than heat transferred mechanically).
With the proper planning and space allocation, exhausted/relief air from administrative areas can be used as makeup air for spaces where no recirculation is allowed as well. This is accomplished simply by ducting the relief air that is normally exhausted outdoors to the fresh air intake chamber of the 100% outdoor air unit serving the non-recirculated/laboratory spaces. This creates a constant stream of air that “pretreats” the outdoor makeup to the laboratory air handling systems, reducing the total amount of energy required to condition the laboratory supply air to its referenced supply air temperature setpoint. Coupling of the office relief and makeup air systems combined with variable air volume (VAV) fume hoods can save significant amounts of energy. Considerations should be made for controlling airside economizers and air balancing. In addition, identifying the class of air as defined in ASHRAE Standard 62.1 is critical to providing safe and proper ventilation of occupied areas.
Commercial buildings: Most commercial office buildings contain technology spaces that require year-round cooling and feature a large centralized data center or main distribution frame (MDF) room and then distributed intermediate distribution frame (IDF) closets. Many designs consist of chilled water or condenser water-cooling in these spaces with waterside economizer for winter operation. Because of the technology spaces’ operational characteristics—continuous operation and constant loading—they are an ideal setting for energy recovery.
In a recent project for a 2.4-million-sq-ft, Class A owner-occupied office building, there was a constant 200-ton technology load. When evaluating options to better serve the year-round technology space, a waterside economizer was initially considered. Energy analysis of the entire building identified that during the winter, a right-sized heat recovery system performed better than free cooling. When running a waterside economizer, the building would also have to operate a boiler to produce heating hot water. The source energy from the technology spaces aligned very well with the required winter heating (sink energy) of the main building lobby that also operates continuously.
Using energy recovery chillers to cool the technology spaces and simultaneously heat, in this case the building’s lobby, resulted in tremendous energy savings. The heat recovery option performs almost 100% better than free cooling with decoupled heat, resulting in approximately $500,000/year in energy savings for the facility. Another benefit of the heat recovery chillers: modular design providing redundancy needed for the data center and 6-pipe machines, meaning they are piped to both condenser water and the hydronic heating system so the chillers can operate year round if needed. Careful consideration must be made of the interconnection between the heating hot water system and the condenser water system. Chemical treatment of these systems cannot be mixed, and system pressures may vary dramatically.
Energy recovery can have a significant impact on a building’s operational costs, a company’s carbon footprint, and systems reliability when correctly applied. Careful evaluation must be performed to align building process loads and energy sources with system demands and energy sinks. Assembling an “energy mass flow balance” helps to evaluate systems and pinpoint aligned opportunities. In addition, the energy transport medium and physical layout are critical for a successful installation, functionality, cost, and maintainability. Operationally, the facility owner and operators must be engaged early and have a deep understanding of the system to properly operate and control the energy recovery system. Controls, redundancy, and maintenance strategies must be considered in all cases. To compare multiple recovery systems, KPIs should be evaluated and presented. The KPIs should be developed together with the owner to generate a meaningful and useful comparison. Successful implementation of energy recovery systems can lead to significant improvement in facility efficiency by reducing an owner’s carbon footprint, operating costs, and even system capacities.
Daniel Cohen is a senior associate and mechanical engineer with Environmental Systems Design. He specializes in high-efficiency buildings, central plants, and analysis and building modeling. Jay F. Ramirez is a senior vice president and the practice leader for the health, science, and education market sectors at Environmental Systems Design with 28 years of experience in high-tech facilities including R&D, laboratory, manufacturing, and healthcare.