Waste Heat Recovery
By Gerald Robinson
In a campus setting with many sources of waste heat, a large percentage of the heating load normally carried by conventional heating equipment can be directly offset. Waste heat recovery should be implemented as part of a comprehensive and integrated campus energy and water plan. Waste heat capture schemes directly offset hydrocarbon use, reduce water consumption (cooling tower use) and increase energy efficiency, reliability and service life of compressor-based equipment (chillers, food service equipment) and data center servers. Waste heat recovery technologies, ground source heat pumps (or air source heat pumps) and district energy systems are often highly complimenting and interdependent technologies.
Applications
District heating loops are critical in making the link between waste heat (sources) and end uses (loads) physically separated in disparate buildings (Figure 1). Examples of sources are shown in Table 1. Thermal storage is important in breaking through the limitation of timing waste heat source generation with when it is needed by end uses. District heat loops can be established between a smaller cluster of facilities or campus-wide depending on density of buildings and the sources and uses for waste heat. Through the use of hot water heating loops and or thermal storage, waste heat system are highly flexible in meeting space heating and domestic hot water needs.
“Quick payback” present-day technology
Data centers using water-cooled CPUs represent an existing cooling technology that captures higher temperature waste heat (up to 80C) that can be directly injected into heating loops to provide very cost effective space and domestic hot water (Coles et al., 2014). Water cooled servers not only make use of a high quality waste heat source but also increase reliability as the critical componentry is kept well below manufacturer's recommended operating temperatures. There are other data center cooling schemes in use today that capture large volumes of lower temperature waste heat that can also be very valuable (see Google case study for an example).
Boiler stack retrofit recuperators and heat recovery ventilators are two very cost effective heat recovery technologies for retrofit of existing equipment. These technologies when applied correctly do not interfere with normal operations of the host equipment. Heat recovery ventilators, which exchange thermal energy from exhaust and outside air are particularly effective in facilities (labs and clean rooms) that must have a very high percentage of exhaust and outside air with several full building air exchanges per hour. Boiler stack recuperators are applied to non-condensing equipment as a means of using stack waste heat.
Lifecycle-cost-effective
Heat recovery chillers can be installed when the infrastructure (low temperature heating loop) is in place to capture and transport that heat to end uses. This type of chiller is designed to develop higher temperatures on the condenser side of the machine and are often cost effective when electricity prices are low. Instead of a single condenser bundle connected to a cooling tower, a heat recovery chiller has a second bundle for capturing waste heat - waste heat is taken first and then what remains is dissipated by a conventional cooling tower. A small amount of additional space is needed for the extra pumps and piping to make the connection to a district heating loop (Trane 2015).
Designing a low temperature hot water loop is not only important for efficiency but also for effective waste heat capture and for keeping the capital and operating expenses low with the associated equipment. Designing the hot loop operating temperature low enough so that waste heat can be taken directly off a heat recovery chiller is ideal as the capital and operating expense of installing and using a heat pump to raise the temperature is avoided altogether. If the hot water loop design temperature is too high, a heat pump will be needed to raise the temperature of the waste heat. McQuay is one manufacturer that offers an appliance designed specifically for this purpose (McQuay 2015).
Figure 1. Common Waste Heat Opportunities - Commercial Sector (Gerald Robinson LBNL 2015)
Table 1. Waste Heat Sources Availability and Hardware Needed. (Gerald Robinson, LBNL 2015)
Emerging technologies and practices
Using double-walled brazed heat exchangers to move heat between dissimilar substances such as refrigerants and water or hot gases under pressure and water has proven to be a very effective technology in industrial settings and could easily be adapted in commercial buildings. This is an available but highly underutilized technology that can very cost effectively (simply, safely and by code) bridge between waste heat sources that in the past could not be associated such as hot refrigerant gases under pressure and domestic hot water.
Extracting heat from waste water streams should be considered where there are large outflows. The City of Vancouver is extracting heat from waste water for injection into a rapidly grown hot water district plant. Large waste water outflows should be examined for the potential for waste heat capture (Bula 2014).
Campus-scale and other considerations
Low temperature hot water district energy loops with thermal energy storage is needed to allow for substantial waste heat capture and to associate sources and uses that are rarely co-located. With the underlying infrastructure in place, the campus design team is freed from the constraints of co-locating sources and uses of waste heat which in turn allows important architectural and planning features to be realized.
Waste heat capture technologies in a commercial setting represent an improvement in safety and noise pollution from a reduction in the use of combustion appliances and cooling towers. It also can reasonably be assumed that waste heat capture would also likely improve the service life of compressor-based equipment as the work load is greatly reduced.
References
Bula, F., 2014 " Vancouver: Heating the city, one neighborhood at a time" Citiscape
Cole, H., S. Greenberg 2014. "Direct Liquid Cooling for Electronic Equipment" 22-25pp.
McQuay. 2015. http://www.georgia.daikinmcquay.com/Templifier
Trane CenTraVac Heat Recovery Chiller. 2015. http://www.trane.com/commercial/north-america/us/en/products-systems/equipment/chillers/water-cooled-chiller/centrifugal-liquid-cooled-chillers/earthwise-centravac/HeatRecovery.html