CSE Pure Power - Fall 2008 - (Page 20)

❯❯ PURE POWER // FALL 2008 20 cover story is shown in Figure 3, with a reduced ﬂow rate once every day from July 6 through July 18. The switch from CHP cooling only mode to simultaneous cooling mode occurred whenever the absorber output alone could not satisfy the hotel demand for chilling. In this case, the absorber capacity could not suppress the chilled water temperature returning from the hotel (returning temperature) to the desired setpoint for the chilled water temperature required to cool the hotel. The chilled water temperature leaving the absorber (leaving temperature) could be used for mode control if a parallel chiller was not present. However, with the electric chiller, the returning temperature was a proper indicator that the absorber was not keeping up with the demand and that simultaneous mode should be initiated. The general sequence when switching from CHP mode to simultaneous mode was: ■ The absorption chiller output satisﬁed the hotel demand as indicated by stable and acceptably low returning temperature. ■ As the hotel demand grew, the diverter valve closed to deliver increasing energy to the absorber and to try to maintain absorber leaving temperature. However, both return and leaving temperatures increased. ■ When the returning temperature exceeded a “high” setpoint, the motorized valves of Figure 1 activated and the electric chiller started to achieve the simultaneous mode. ■ The absorber chilled water ﬂow rate dropped suddenly by 100 gpm. The lower demand on the absorber required the diverter valve to open to maintain the absorber leaving temperature setpoint even though the return temperature was high. ■ The hotel demand was not satisﬁed until the electric chiller output and the reduced absorber output stabilized the return temperature. ■ CHP cooling only mode was re-established only after the hotel www.purepowermagazine.com Lessons learned Combined heat and power (CHP) systems applied to the built environment have created a series of new entrants supplying predesigned and engineered energy solutions, which are likely to grow in their usage. The CHP plants are more complex and require systematic approaches to commissioning. Current commissioning practices appear to be intuitive and not systematic. General themes arising out of this review are: ■ A written commissioning report is essential to determine exactly what was tested and how the tests were accomplished ■ All essential elements must be tested to ensure functional performance ■ If timing and weather precludes performance testing of certain systems, arrangements should be made to perform these tests at a later date ■ Site engineering is a vital ingredient particularly for retroﬁt CHP installations because as-built drawings of complete building systems are often inaccurate or incomplete ■ In retroﬁt situations, series ﬂow should be used on all CHP thermal loops to balance ﬂow and eliminate pumping problems ■ Balancing valves are essential to assure ﬂows are correct. ence, which concluded that high levels of chilling were required every hour of the year. While electrical load was easily determined from utility electricity bills, it was not as easy to determine thermal loads, particularly chilling loads. Perhaps this design ﬂaw suggests the need for a new assessment approach to measuring thermal system performance prior to building retroﬁts. The second design ﬂaw stems from the integration of the absorption chiller into the existing chilled water circuit, which led to a 100 gpm absorption chilled water drop when the electric chillers were engaged. This untimely reduction in ﬂow dramatically reduces the CHP performance. This critical design/operating ﬂaw provides an important planning lesson, that if timing and/or weather precludes certain testing, then allowance must be made to perform critical testing when the timing/weather is right. ABOUT THE AUTHOR demand reduced sufﬁciently to allow the returning temperature to drop below a “low” setpoint (5 F lower than the “high” setpoint to reduce mode cycling). Sweetser is president of EXERGY Partners Group, Herndon, Va., which he founded in 1998 as a consulting ﬁrm designed to capitalize on opportunities arising out of utility deregulation and global climate change in the energy and construction industry. Sweetser has spent 36 years commercializing advanced energy, refrigeration, and HVAC technology. OBSERVATIONS The commissioning process for this cooling and power system took place in the winter, which precluded meaningful interaction between the CHP absorption chiller and the hotel’s existing electric chillers as the cooling load was low. Thus, two critical design deﬁciencies were not uncovered. The anticipated chilling level was based on both predictions and experi- ACKNOWLEDGMENTS The U.S. Dept. of Energy and Oak Ridge National Laboratory have provided continuing support for energy-efﬁcient CHP systems, including the demonstration project described in this article and support for the CHP commissioning assessment. Timothy Wagner of United Technologies Research Center provided data and review for the case study. http://www.purepowermagazine.com

Table of Contents Feed for the Digital Edition of CSE Pure Power - Fall 2008

CSE Pure Power - Fall 2008 In the News Commissioning CHP Enhancing Emergency Lighting Data Centers for Uncle Sam Handling a Nuisance Trip Sustainable Projects and Partnerships New Products Ad Index

CSE Pure Power - Fall 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.