Is waste heat recovery right for your project?

How to evaluate and specify waste heat recovery systems in various nonresidential building markets.




Learning objectives

  • 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.

This article has been peer-reviewed. 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.

Figure 1: Recommended steps for determining opportunities and getting buy-in for energy recovery. All graphics courtesy: Environmental Systems DesignFour 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:

  1. 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.

  2. 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.

  3. 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.

  4. 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.

Figure 2: The adjacent figure indicates a sample daily process load profile. The different colors represent different loads; notice the peaks when certain components run. This energy generation can be aligned with loads to find opportunity for energy recovery. Waste heat recovery systems

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.

Building examples

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.

Figure 3: Analyzing the building processes and loads is a critical step in determining feasibility for energy recovery. Some processes produce energy, generally in the form of heat, and can be viewed as a source (faucet). Other processes require energy that can be converted and transported from energy sources; these can be viewed as sinks. 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.

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