VRA Enhancement of Two Stage LiBr Chiller Performance Improves Sustainability

Multi-effect LiBr absorption chiller must take advantage of higher temperature heat sources to achieve higher COP so as to be competitive with lower first cost comparable commercially available, efficient electric chillers under current market pricing conditions. Yet a nominal conventional double-effect absorption chiller operating at a COP of 1.0 versus a comparable efficient motor driven centrifugal chiller operating at a COP of 7.0 will consume slightly less than twice the amount of prime natural gas (NG) source energy assuming a local 28% NG fired electric utility plant’s annual average efficiency and a 10% gas distribution leakage and 10% electric transmission loss to user’s meter. However if the COP of the above referenced double-effect LiBr absorption chiller were doubled, it would consume approximately the same amount of prime NG source energy and equally sustainable from an environmental impact standpoint. Consequently research to further improve double-effect LiBr absorption chillers beyond the VRA benefits reported to date was investigated in this study. Former simulation studies of a low differential pressure-vapor recompression absorber (VRA) reported in 2001 indicated a 7% COP efficiency gain, while additional simulation studies reported in 2006 indicated a 38% COP efficiency gain with the VRA operating at elevated differential pressures at the same upper stage concentrator temperature previously considered. Double-effect LiBr absorption chillers are limited by corrosion effects, which have been shown to accelerate significantly above 160 °C. In this paper, a reverse series flow, double-effect LiBr absorption chiller employing a VRA is investigated over a wider range of upper stage concentrator and absorber cooling temperatures but operating at the same low and elevated pressure differential levels reported earlier showed significant improvement in COP efficiency, capacity performance and projected hybrid operational cost.