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Oak Ridge National Laboratory (ORNL) initiated an investigation into the effectiveness and efficiency of using waste heat like that from a localized electrical generator source for desiccant regeneration in newly developed active desiccant HVAC products like the SEMCO, Revolution™ integrated active desiccant/vapor-compression hybrid rooftop air conditioner (IADR). Thermally activated technologies (TATs) like desiccant dehumidification and absorption cooling are key components in the U.S. Department of Energy’s plans for efficient/optimal use of national resource energy. Not only can on-site TATs alleviate peak electrical grid loads because they are directly fired with fossil fuel, renewable biomass, and/or hydrogen, but also they can use waste heat from a variety of sources to provide the heating, cooling, and humidity control required to maintain comfortable, healthy buildings. Thus, TATs constitute the necessary “bottoming cycle” waste heat utilization methodologies for distributed generation (DE), integrated energy system (IES), and combined heating and power (CHP) systems. To test the effectiveness and efficiency of low-quality waste heat as a substitute for a natural gas burner typically used for desiccant regeneration in this application, ORNL conducted laboratory tests in which the regeneration air stream in a SEMCO IADR was heated by a hot water loop circulated through a coil integrated within the SEMCO IADR and installed upstream of the desiccant wheel in the regeneration section. The hot water was produced by a heat recovery unit processing the exhaust gas from a microturbine, Figure 1.
Data presented in Figures 2 and 3 illustrate the viability of using waste heat for desiccant regeneration in this integrated energy system application. Red bars in Figure 2 show desiccant wheel leaving humidity ratios during natural gas (NG)-based and waste-heat (WH) regeneration of the desiccant wheel component in the SEMCO unit at comparable indoor coil leaving temperature, regeneration air temperature, and flow rate conditions. Temperatures of regeneration air for both the NG and WH mode were approximately 170°F, similar to that available from an IC engine water-cooled jacket with water temperature of 185-188°F.
Figure 3 compares results at the higher temperature typically used when desiccant wheel regeneration is accomplished with gas to the more moderate regeneration temperatures associated with CHP system integration. These results clearly show that even at reduced regeneration temperatures (176.2°F vs. 199.4°F) it is possible to produce the same level of dehumidification by increasing the regeneration air flow rate, in this case from 593 cfm to 840 cfm. This is possible with CHP integration since ample waste heat is almost always available, the problem is the temperature limitation associated with the resulting hot water.
Integration of highly versatile and efficient rooftop package air conditioners like the SEMCO unit with CHP systems is an important step toward making on site power generation viable for all building types, not just larger building complexes where chilled water is often used. These laboratory test results also indicate that multiple IADR desiccant units may often be served by a single generator source since the thermal regeneration energy required by these desiccant/vapor-compression hybrid systems is far less than that associated with previous, active desiccant-based dedicated outdoor air systems. Results from the testing performed in ORNL’s CHP Integration Laboratory are significant since they confirm that low-grade waste heat like that available from a gas-powered internal combustion engine (IC) jacket cooling water can effectively be used with the SEMCO hybrid desiccant product to achieve optimal performance. |
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