Fonte (Source): Consulting-Specifying Engineer

Por (By): Robert Thompson, PE, SmithGroupJJR Inc., Phoenix

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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.

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 6.5.7.2 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.