Recycling 101: waste heat recovery

Waste heat recovery systems are increasingly used in mixed-use buildings to move waste heat from laboratories, data centers, or industrial activities to provide beneficial heating in other parts of the building. Recovering waste heat becomes an attractive option for facilities working to achieve low energy use (such as net zero energy or high-performance buildings) and to reduce emissions.

By Robert Thompson, PE, SmithGroupJJR Inc., Phoenix August 13, 2015

Learning Objectives

  • Visualize the flow of energy within buildings and learn the importance of energy benchmarking.
  • Outline the codes and standards that define energy efficiency and waste heat recovery in buildings.
  • Understand the thought process behind the integration of waste heat recovery into building cooling and heating systems.

In 2012, buildings accounted for 74% of all the electrical energy use within the United States. As buildings are large consumers of electricity, there has been a concerted effort to improve their energy efficiency (and reduce wasted energy). ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings is the energy benchmark for buildings, which is updated every few years with more stringent energy guidelines. Buildings also are designed based on the U.S. Green Building Council’s LEED guideline, which encourages increased energy performance. Energy Star is another program that encourages reduced energy usage for equipment and appliances within buildings.

As a result of these programs, the energy efficiency and relative energy use within buildings is coming down. The U.S. Energy Information Administration’s (EIA) Commercial Building Energy Consumption Survey (CBECS) provides energy benchmarks for a variety of building types. The energy use intensity (EUI) is a measure of a building’s energy use per square foot per year (kBtu/sq ft/yr). An average university within the CBECS database in 2014 had a site EUI of 130 due to the large number of classrooms, laboratories, and technology-driven spaces. The average site EUI for an office within the same CBECS database for 2014 was 67, which is a significant improvement from 1995 when the average site EUI for an office was slightly higher than 100. While these programs have made and continue to make significant reductions in building energy use, there is still room for improvement by taking some lessons in recycling and waste heat recovery.

A large facility containing laboratories and offices built today likely has central chilled and heating water systems. The use of water-based systems for space conditioning is preferred as it allows for precise temperature and humidity control while reducing the total amount of heating and cooling capacity installed at the facility. Water-based cooling systems are either air-cooled, ground-source water-cooled, evaporative-based water-cooled, or a combination of these approaches. Air-cooled systems reject heat to the atmosphere while ground-source systems reject heat to the earth (through geothermal wells or deep lake-based cooling). Both approaches reduce system complexity and local water consumption, with ground-source systems being more efficient due to reduced fan energy (required to reject heat to the air).

Evaporative-based water systems evaporate water via a cooling tower or fluid cooler to offset the heat generated within the building. The heat of evaporation for water is high, which significantly improves energy performance relative to air-based cooling systems. While the local water consumption increases with this approach, the improvements in energy efficiency can result in a net water savings by reducing water consumption at utility power plants.

Water-based heating systems generally use electricity, natural gas, or solar as the heating source. The use of electricity for building heating generally is limited to small systems, as it has a higher operating cost (due to the inefficiencies in its production and transportation losses to the building). Solar has a higher first cost and, unless it is stored, is limited to production during the daytime hours. Natural gas has the potential to be extremely efficient, especially when the boiler is designed to be condensing. Condensing boilers allow the products of combustion to condense outside the heating coil, extracting heat that is normally lost through the boiler stack.

Exhaust air waste heat recovery

Exhaust air waste heat recovery takes energy from the exhaust air and uses it to precondition outside air. This heat recovery may be based on the relative difference of sensible energy (temperature), latent energy (moisture content), or enthalpy (temperature and moisture content). Laboratories are generally limited to sensible heat recovery only (due to the potential to transfer chemicals with latent recovery) while office areas may take advantage of enthalpy-type energy recovery. 

ASHRAE 90.1, Section 6.5.6 Energy Recovery, requires that such a system be installed based on airflow, climate region, percent outside air, and other factors. It provides exceptions, however, based on laboratory operation and components (according to ASHRAE 90.1, Section Laboratory Exhaust Systems). One such exception is related to capacity control and the ability to turn down the ventilation airflow in the laboratory to 50% or less. ASHRAE 90.1 recognizes the benefits of being able to reduce ventilation rates to save energy.

Building on the concept of reducing ventilation rates, excess ventilation air for offices potentially can be recycled as single-pass air for laboratories, lowering the total ventilation for the building. Oftentimes the ventilation air to office areas can also be increased (beyond minimum levels) to provide a healthier environment. As long as the ventilation provided to the offices remains less than the minimum ventilation rates in laboratory areas (minus the laboratory’s required ventilation for people), there will be a net reduction in ventilation (and energy savings) for the building. Note that this approach requires special attention to ensure laboratory pressurization and directional airflow to protect non-laboratory spaces. Refer to ASHRAE Standard 62.1: Standard for Ventilation and Indoor Air Quality, Section 5.17 Air Classification and Recirculation, for additional information and limitations with this approach.

Condenser water waste heat recovery

For water-based cooling systems, heat from the building is rejected to the chilled water system. The water-cooled chillers in the chilled water system then transfer heat (by cooling chilled water), rejecting it to cooling towers (or fluid coolers) via a condenser water system. Cooling towers then use airflow and direct evaporation to reject the building waste heat to the outdoors.

Water-cooled chillers, however, can add as much as 25% more heating energy in the process of cooling the building depending on efficiency. A heat-recovery chiller (or water-to-water heat pump) takes the heat normally rejected to the cooling tower, together with this additional heating energy from the chiller, to provide beneficial heating at the building. Waste heat recovered in this manner not only reduces the heating energy required in the building, but it also reduces cooling tower energy and water consumption.

This approach provides a significant improvement over electric heating systems (with no waste heat recovery), but can fall short of the combined energy savings of high-efficiency chillers and natural gas condensing boilers.

Process waste heat recovery

Waste heat also can be extracted from equipment or processes within the building. Hot boiler or microturbine exhaust may be used as a source of waste heat. Waste heat from steam boilers can be used to preheat boiler feed water. This same heating energy, if of sufficient quality, quantity, and regular operation, can be used for building heating as well as building cooling via absorption chillers. In an absorption chiller, the waste heat drives a refrigeration cycle that provides chilled water for building cooling. Note this approach is more commonly associated with natural gas-fired microturbines that regularly provide electrical load-shedding during periods of peak electrical demand.

Process waste heat (in the form of hot air) also can be used to provide for dehumidification within the building. Dehumidification units generally use desiccant media to absorb moisture from the air (latent energy recovery). Removing moisture from the desiccant media, however, requires a hot and relatively dry air stream to draw out the moisture from the desiccant. If the building has systems in place that generate significant amounts of hot air, this approach may be a viable alternative to cooling-based dehumidification systems. Refer to ASHRAE 62.1, Section 5.17, for limitations.

Common elements of waste heat recovery

A common thread in waste heat recovery is this concept of recycling. Energy that would otherwise be thrown away is instead converted into beneficial use within the building. Unless you intentionally focus on waste heat recovery at the beginning of design, however, your options for implementation can be very limited. The building systems selected will either enhance or limit the application of these approaches. For example, water-based heating systems designed for lower water temperatures will increase the opportunities to recover waste heat.

Too often we fail to see the wasted energy use. We instead concentrate on implementing more efficient heating and cooling systems, not considering potential energy reuse. When we do see potential applications for waste heat recovery, many times we will limit our vision to what can be done for a particular project or building. When we take a step back, though, we can then begin to envision larger applications for waste heat recovery beyond the needs of our building. When realized, these larger applications can lead to significant energy savings, not only to the project but also to the campus.

Making waste heat a priority from day one

The Dept. of Energy’s (DOE) National Renewable Energy Laboratory’s (NREL) Energy Systems Integration Facility (ESIF) was built on NREL’s Golden, Colo., campus to study the interactions of renewable energy sources with the national electric grid. The facility consists of high-bay laboratories for performing megawatt-scale testing, a high-performance computing data center (HPCDC) for studying renewable energy interactions that cannot readily be tested, and office/conferencing spaces for the scientists conducting research.

NREL recognized early on that there was tremendous potential for waste heat recovery within this facility. The HPCDC started operation at less than 1 MW, and has the capacity to grow to as large as 10 MW. One characteristic of the HPCDC, and data centers in general, is that they always require cooling (and are always rejecting waste heat). Golden, Colo. is located in a predominantly heating environment, and laboratories use large amounts of outside air that must be heated. In winter, the office and conferencing centers require heating as well.

With this knowledge, NREL issued a request for proposal (RFP) for this project based on energy efficiency and waste heat recovery. Not only did the competing teams have to design an extremely energy-efficient building, but they also needed to incorporate waste heat recovery from the HPCDC to other portions of the building. While NREL did not know the best way to implement this waste heat recovery, they did not want to miss the opportunity to take advantage of this tremendous resource. The design-build team needed to analyze these systems and, in partnership with NREL, implement a solution that not only met the day one energy-reuse (waste heat recovery) goals within the building, but also made provisions for future energy recovery to the campus.

Characteristics that promote waste heat recovery

Power usage effectiveness (PUE) is a measure of the efficiency of the data center, and in its simplest form is the ratio of all the energy (mechanical, electrical, and information technology, or IT, servers) needed to operate the data center divided by the energy needed to operate the servers. The ESIF HPCDC was designed to operate with a PUE of 1.06. Achieving this goal from a mechanical systems approach required large reductions in cooling energy, reduced fan energy (primarily liquid-based cooling), and waste heat recovery. The high desert location allows for compressor-less cooling of the HPCDC. At the same time, liquid-cooling (water) allows the data center to capture waste heat closer to its source (at higher temperatures), and easily transport from one area to another. Given 75 F incoming water, the HPCDC servers are able to reject better than 95 F water to the building for waste heat recovery.

High-bay laboratories can operate effectively with 95 F heating water in the summer. In the winter months, however, 95 F heating is not sufficient. During peak winter conditions, the laboratories require heating water temperatures as high as 130 F to maintain space temperature requirements due to large roll-up doors and the high space volumes. Make-up air-handling units (AHUs) provide conditioned outside air (ventilation) to the laboratories. They do not need heating in summer, but do require large amounts of heating in the winter. Because the make-up AHUs also provide cooling to interior zones, they will not operate with elevated supply air temperatures (temperatures of 55 to 65 F are sufficient). As a result, heating water temperatures closer to 100 F are sufficient to provide for the heating requirements of the make-up AHUs.

Office and conferencing spaces have minimal ventilation air and primarily require heating in winter months. With an efficient building envelope, 95 F heating water can effectively provide heating year-round. Special attention, though, is needed when evaluating methods of building heating. Of primary concern is to ensure that the heating approach can effectively make use of the low-grade heating water.

As the heating load of the building does not change based on the temperature of the heating water, low-grade heating water systems require either more flow, increased surface area for heat transfer, or a combination of both to provide the equivalent amount of heating to the building. Accurate heating coil selections become even more important to ensure space conditions can be maintained. Apart from a radiant slab, low-grade heating solutions on their own are not the most energy-efficient solutions for building heating. When building heating is provided through waste heat recovery, however, the net result is a significant energy reduction (particularly in colder climates). This is due to little or no energy required for heating, and a minimal increase in transport energy for water-based waste heat recovery systems.

Implementing a cost-effective solution

With an ample waste heat recovery source, and spaces that can take advantage of low-temperature heating, the next step is to design efficient systems and pathways to move that waste heat from its source to where it can be beneficially used. In its simplest form, there are three major mechanical hydronic systems within the NREL ESIF building: the cooling water system, the heating water system, and the energy-recovery system. The cooling and heating water systems both support the conditioning of the building while the energy recovery system supports the HPCDC. The successful implementation of waste heat recovery required an in-depth understanding of these systems and their interactions.

The energy recovery water system evaporatively cools the supply-water temperatures to 65 to 68 F before sending to the data center AHUs and HPCDC servers. Return water from the AHUs and HPCDC servers is then combined to provide roughly 95 F water for heat recovery. This water then passes through a heat exchanger to provide heating to the heating water system. Any waste heat that remains after the heat exchanger is then cooled (via cooling towers) and sent back through the energy-recovery water system.

The heating water system uses recovered waste heat from the data center to form the basis of a low-temperature heating loop. The low-temperature heating loop then provides heating directly for offices, conferencing center, and lobby radiant-floor systems. A second high-temperature loop serves the laboratory zones, building entry points, and snowmelt system. The high-temperature loop operates at the same temperature as the low-temperature loop in summertime, but in winter months can be reset to as high as 130 F by supplementing with campus heating water.

One concern with this approach is that, if not applied correctly, supplementing with campus heating water can slow down or halt waste heat recovery in the winter months (when waste heat recovery is the most beneficial). The solution came after studying the space characteristics of the laboratory zones. Instead of sending as high as 130 F heating water in parallel to laboratory zones and make-up AHUs (which results in raising return-water temperatures and potentially stopping waste heat recovery), the decision was made to serve these areas in series.

The hottest water goes first to the laboratory zones. Then the warm water passes by the make-up AHUs. During peak winter design conditions, this approach generated combined water-temperature differentials of nearly 80 F. When blended with return water from the low-temperature heating loop, the return-water temperatures were around 75 F. The 75 F return heating water could then be heated via waste heat recovery back up to near 95 F. Even though supplemental heating is required at certain times of the year, this approach ensures that waste heat recovery never stops.

The office areas use waste heat recovery exclusively for space heating. An underfloor air-distribution system conditions interior zones, while active chilled beams provide heating and cooling along the building perimeter. The active chilled beams use a two-pipe changeover system to maximize the coil surface area. This increased surface area coupled with minimal return air through the beams allows low-temperature heating water to offset envelope losses through the building perimeter.

Future waste heat recovery enhancements

As the HPCDC grows, its heat capacity will eventually exceed the demands of the building. NREL recognized the potential to take excess waste heat from the data center and distribute it to the campus. By bringing this observation to the design-build team, NREL was able to focus design efforts to include pathways for future distribution of recovered waste heat to other buildings on its campus.

The Science and Technology Facility adjacent to the ESIF has a large amount of ventilation, as well as heating systems designed around low-temperature heating. It was also the first federal LEED Platinum building. The Research Support Facility, also located nearby, uses radiant slabs with the ability to use water at temperatures lower than 95 F for building heating. Low-grade heating can be used directly in these applications in the future. NREL’s vision, however, extends beyond simply low-grade waste heat recovery in winter months.

NREL’s campus heating water system, which supplies supplemental heating to the ESIF in winter months, operates at 95 F during the summer months to support laboratory heating demands. The long-term vision for the campus is that the ESIF HPCDC becomes the primary source of campus heating in the summer months, so campus boilers can be shut down for the season.

During the optimization phase of the ESIF project (after initial occupancy), the design team was informed that the HPCDC servers provided could accept entering water temperatures above the minimum criteria established by the design team. In fact, the allowable entering water temperatures for the HPCDC exceeded that of the return-water temperatures from the data center AHUs. Armed with this knowledge, the design team came up with an approach to redirect return water from the AHUs back toward the HPCDC servers (instead of blending with HPCDC return downstream and lowering the effective waste heat recovery temperature). This approach is in the process of being implemented, and will be operational in the near future. Once online, this system will boost the waste heat recovery temperatures by as much as 10 F, allowing even more waste heat to be recovered within the building and beyond.

Complementary programs

Program elements that on the surface seem unrelated may actually complement each other from an energy-reuse standpoint. The NREL office area EUI (with no consideration for waste heat recovery) operates at slightly more than 35 kBtu/sq ft/yr. When waste heat recovery is applied to the office, however, the EUI drops to less than 25 kBtu/sq ft/yr, which translates into a 30% energy savings over an already efficient base case. Put another way, the percent savings through the use of waste heat recovery, in this case, is equivalent to the energy efficiency improvements in offices from 1995 to 2014. To the extent that waste heat recovery can be increased in the building and on the campus, the data center PUE also will be reduced.

The NREL ESIF facility represents the future of waste heat recovery. It is important to note that the technology applied in these systems, aside from the HPCDC servers themselves, are not exotic. They are widely used throughout the world. Pumps and heat exchangers transfer heat from one system to the next. In its basic form, these systems simply recycle heat that is otherwise thrown away, and put it to beneficial use in other areas of the building. In an age of energy conservation and innovation, engineers should not forget to also look for ways to implement an energy-recycling program within their projects.

Robert Thompson is chief mechanical engineer at SmithGroupJJR. He was a mechanical engineer on the design-build team of the NREL ESIF project, and led in the efforts to recover waste heat from the data center and put it to beneficial use, both within the building and ultimately to the campus.