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©2017 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 59, no. 1, January 2017

Marvin Kirshenbaum, Member ASHRAE

About the Author
Marvin Kirshenbaum is project mechanical engineer at Argonne National Laboratory in Argonne, Ill.

Heat recovery is a common approach to improving the energy efficiency for a wide spectrum of building types. For commercial and research facilities, this is often restricted to preheating and some limited precooling of outdoor air. Most buildings rely on some form of heat for temperature control throughout the year, and some tap into available waste heat sources to fulfill some or all of this need. Expanding the reach of available waste heat would provide significant enhancement to a building’s energy efficiency. Beyond this, tapping into waste heat streams to provide primary building heating in cold climates can open up new avenues for additional energy conservation.

This article addresses extending the application of waste heat recovery to both reheat and primary heating, allowing for a heat reclaim system no longer restricted to seasonal operation. It will begin with a review of work completed to date to illustrate our progression in waste heat utilization and finish with a description of a novel design that approaches the practical limits of utilization.

The overwhelming majority of electricity consumed by commercial buildings eventually turns into low-grade (temperature) waste heat at or below 90°F (32°C). While this waste heat can be recovered during cold weather for preheating outdoor air, its low temperature makes effective recovery and reuse for other purposes extremely difficult. The majority of this waste heat is rejected to the outdoors.

The Department of Energy reports, “Commercial buildings represent just under one-fifth of U.S. energy consumption.... In aggregate, commercial buildings consumed 17.9 quads of primary energy in 2009, representing 46% of building energy consumption and 18.9% of U.S. energy consumption.” Over the past 15 years, we have designed and constructed mechanical systems that have expanded the use of heat recovery, focusing on applications that go beyond outdoor air preheat, with applications for reheat and perimeter heating.

While building heating operation is seasonal, reheat systems typically operate continuously to provide space temperature control. This is especially true in the case of laboratories, hospitals, and related facilities. Many large office buildings also employ reheat for space temperature control. In mild climates, the energy used by space reheat can dominate the building heating energy use, making it a prime candidate for recovered sources of energy.

This discussion divides waste heat applications into two categories: high airflow demand (i.e., laboratory) and low airflow demand (commercial offices). High flow facilities have greater preheat and reheat demands, and shifting this load to waste heat sources magnifies the effectiveness of the energy savings. Low flow facilities, while less stressed by outdoor air preheat and reheat, can still benefit. Efforts to expand the waste heat source into the primary heating system can tip the balance of the economics.

Rapid improvement in lighting and office equipment energy efficiency has reduced internal loads, while the need to maintain indoor air quality has pushed increases in ventilation rates in many cases. This results in greater dependency on reheat to avoid space subcooling. Before presenting the final version of the proposed waste heat recovery system design, I’ll summarize the evolution of increased use of waste heat.

 

Design Evolution

As part of our program for sustainability, a portion of the work at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL) has focused on the recycling of low-grade waste heat generated by the scientific tool set of that facility. The first efforts at APS focused on winter outdoor air preheat. A 4,000 cfm (1,888 L/s) dedicated outdoor air (DOA) unit supplying ventilation air to the APS electron linear accelerator tunnel (LINAC) was retrofitted with a conventional multi-row heating coil to establish the efficacy of using conventional, cost effective heating coils with very low-grade waste heat.

 

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