1、Energy Simulation Results for Indirect Evaporative-Assisted DX Cooling Systems James V. Dirkes II, P.E Ryan J. Hoffman Member ASHRAE ABSTRACT Evaporative cooling systems have been used for millennia and represent an effective, low energy solution for many applications in dry climates. Recently, hybr
2、id systems have been developed which couple the extremely high energy efficiency of indirect evaporative cooling with DX cooling. This combination enables the additional cooling and dehumidification capability of a mechanical refrigeration system while dramatically improving overall peak and annual
3、energy use. This paper will review results of Energy Plus simulations of InDirect Evaporative-Assisted DX systems (IDEA-DX) applied in dry climates as they compare to a conventional solution. Peak power use, annual energy consumption and indoor environmental quality differences will be compared. In
4、addition, field results from equipment which uses this design will be reviewed for a “reality check” of the simulation results. INTRODUCTION Arid climates, such as are found in many parts of the world, including the Western USA, provide an attractive setting for HVAC systems to use evaporative cooli
5、ng. Indirect evaporative cooling can provide EERs which can reach 20 - 80 and also have low peak power compared to refrgeration-based systems. Since evaporative cooling effectiveness varies with the sites humidity, overall performance is not as consistent as mechanical refrigeration alternatives. In
6、 addition, when indoor humidity control is an important design criteria, evaporative cooling systems cannot always meet those criteria. Manufacturers of packaged rooftop HVAC (RTU) equipment have tended to design and rate RTUs identically for both humid and arid climates. An opportunity seems to exi
7、st for coupling indirectBecause there are many variables at work in any HVAC application, and it is difficult to evaluate the impact of each over an entire year, an evaluation of the the IDEA-DX approach and some of its limitations was conducted using Energy Plus, a full-featured energy analysis pro
8、gram developed by the US Department of Energy. evaporative cooling with DX cooling to gain the efficiency and peak power advantages of indirect evaporative cooling and the enhanced performance available from DX cooling. ASSUMPTIONS AND PRESUPPOSITIONS FOR THIS STUDY This study was undertaken with ce
9、rtain assumptions and presuppositions and are stated below. The project HVAC design requirements include humidity or temperature control beyond that available from an indirect evaporative-only system. Although other climates may prove favorable for use of an IDEA-DX system, only arid climates were s
10、elected for study. LV-11-C016 2011 ASHRAE 1312011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or
11、digital form is not permitted without ASHRAES prior written permission. Anticipating reduced operating hours for the DX system, it was assumed that fan energy would become a much larger fraction of overall energy use. It is beyond the scope of this paper to study other combinations of components suc
12、h as direct evaporative cooling with indirect evaporative cooling and DX. Commercially available indirect evaporative cooling has a range of wet bulb effectiveness between 50% and 120%. This range of performance may affect magnitude of benefits for IDEA-DX systems. The “working air” that is normally
13、 exhausted from an indirect evaporative cooler is itself evaporatively cooled and, if directed across the DX condenser, will be useful to improve overall COP. Power use during peak demand periods is of increasing concern, and particularly so in the Western (dry) US. SIMULATION DESCRIPTION A simple,
14、hypothetical building (Figure 1) with an energy efficient envelope and lighting, and a total area of 3,000 sq. ft. (280 sq. m.) was selected to represent a typical application and size of facility on which an RTU might be applied. Figure 1. Simulation Building Walls and roof are lightweight, with in
15、sulation value consistent with ASHRAE 90.1 requirements for Climate zone 5. (Walls: U = .062; Roof: U = .048) Windows occupy 20% of the wall area and their performance is also consistent with ASHRAE 90.1 for climate zone 5. (Windows: U = .55; SHGC = 0.4) Lighting and plug loads total 1.75 W/sq.ft. (
16、20W/m2) Three HVAC system approaches were modeled: o “Traditional” RTU using DX cooling with COP = 3.0 (EER = 10.2) and differential dry bulb economizer. o Same as above, except using an indirect evaporative cooler with wet bulb effectiveness of 50%. The evap cooler is located upstream of the DX coi
17、l and all working fluid air is directed across the DX condenser coil. o Same as above, except using an indirect evap cooler with 100% wet bulb effectiveness. The arrangement of components for each IDEA-DX AHU is shown in Figure 2. Each of the above three HVAC approaches was modeled using cycling (on
18、 / off) fans and variable speed fans. Performance was determined for four cities: Sacramento, Denver, Phoenix, and Abu Dhabi using TMY3 weather data. o Because an indirect evap system may be de-energized and drained during cold weather, the indirect evap component was scheduled to operate for differ
19、ent months depending upon the weather pattern of each city. Sacramento and Denver each used a 6 month cooling season, Phoenix used 9 months and Abu Dhabi used 12 months. Because RTUs are often applied in multiples on larger facilities, it is thought that the results from these simulation criteria sh
20、ould scale well. 132 ASHRAE TransactionsLegend: 1. Outdoor Air 2. Filters (Mixed Air) 3. Indirect Evap Media 4. Downstream of Indirect Evap media 5. DX Evaporator coil 6. Supply Air fan the IDE does most of the cooling work. Almost independent of city and IDE effectiveness, however, an IDEA-DX syste
21、m with cycling (on/off operation) fans used almost the same total electricity as the DX-only system. Using Figure 3, notice that the total cooling energy, represented by the top of each bar is essentially the same as a DX-only system for all on/off fan systems with 50% effectiveness. Denver and Phoe
22、nix show slight improvement for on/off 100% effectiveness systems, but it represents no more than 4% of the total For the on/off cases, greater fan energy due to the secondary / exhaust fan and increased supply fan pressure drop equalled or exceeded the reduction of DX energy and eliminated any sign
23、ificant benefit. Independent of city, an IDEA-DX system with VSD fans and 50% effectiveness also saved little or no energy compared to the DX-only VSD system. Once again, the reduction in DX energy attributable to the IDE did not exceed the additional fan energy and eliminated any significant benefi
24、t. 100% effectiveness IDEA-DX systems with VSD fans and showed significant annual cooling energy savings for all cities, with savings varying between 11 -27%. In this case, the greater IDE effectiveness overcomes the fan energy penalty handily. IDEA-DX systems show improved performance compared to D
25、X-only based on a combination of reduced cooling coil entering air temperature, extended economizer hours and optimized fan energy. Because the IDE reduces the temperature entering the cooling coil, the hours for which economizer outdoor air is available increase. As a result, more outdoor air is us
26、ed during more hours each year, reducing cooling energy and increasing ventilation (and potentially the indoor environmental quality). This is shown in Table 4. Based on Design Day loads and the assumptions of this paper, the compressor selection for an IDEA-DX system can be 25% smaller than for a D
27、X-only system. For other design requirements, especially those requiring more outdoor air, the compressor might be more than 50% smaller. Finally, performance improvement of an IDEA-DX system compared to a DX-only system varies significantly by climate and is not necessarily intuitive. Because the h
28、undreds of variables in HVAC system design affect overall performance in ways that may not be intuitive, a detailed annual simulation such as were conducted for this paper are important in assessing overall performance. Table 4. IDE impact on Economizer Availability Table 4Notes: 1. Economizer is as
29、sumed to be Differential Dry Bulb type. FIELD EXPERIENCE WITH 100% IDEA-DX The Western Cooling Efficiency Center at the University of California - Davis created the “Western Cooling 136 ASHRAE TransactionsChallenge” in 2008 to test their hypothesis that the energy performance of RTUs might benefit s
30、ignificantly if designed specifically for an arid climate (instead of AHRI test conditions). The Challenge invited HVAC manufacturers to design RTU equipment for arid climates, anticipating that they would significantly outperform typical RTUs which were designed to meet AHRI test conditions. The WC
31、C test criteria include two representative arid design conditions (105DB/73 WB, 90DB/64 WB), laboratory verification of performance and a 12 month field test at a “real” site. The first design submitted to the WCC was tested by NREL and found to have an EER of 42 for the first test condition and 22
32、for the second. It should be noted that, contrary to conventional EER calculations, NREL considered ALL power: controls, secondary fan, DX compressor and the supply fan delivering air against 0.75” external pressure. This is remarkable performance! That equipment is now installed in the Sacramento,
33、CA area and has been operating there since late summer of 2009. Figure 4. Actual Performance of 100% IDEA-DX in Sacramento, CA Figure 4Notes: 1. The summer (almost) ended before data collection was reliable, so the data set for this chart is very limited. FURTHER STUDY Aspects of IDEA-DX systems whi
34、ch were not addressed by this study, but merit further attention, include: Improvements in small system air transport efficiency (primarily fans and motors) merit further study due to their large impact on overall energy use, especially when the DX power is significantly reduced. Determination of th
35、e merits of single vs. multi-stage compressors. The smaller compressors often used by RTU equipment become even smaller when applied in an IDEA-DX configuration. The first stage efficiency of these compressors can be less than that at full load and may compromise overall performance. The relative me
36、rit of reduced peak power versus reduced annual energy use in utility areas which are demand-challenged. 2011 ASHRAE 137 Opportunities for applying IDEA-DX systems in displacement ventilation designs. The higher leaving air temperatures typical for displacement systems seems to match nicely with IDE
37、A-DXs concept of using indirect evaporative cooling as the first cooling stage. CONCLUSION The use of IDEA-DX systems in arid climates proves to have very sizeable annual savings potential. This savings potential is indeed very sensitive to fan power and may be effective only when used with variable
38、 speed fans. This potential is also sensitive to the wet bulb effectiveness of the indirect evaporative cooling component, with indications that 50% effectiveness is the lower border for viability. Although the climates studied appear to be favorable for the application of the IDEA-DX concept, there
39、 is significant variation in energy performance. This variation, coupled with varying utility cost structures and rates makes it advisable to study each application individually. Nonetheless, IDEA-DX systems demonstrate cooling energy savings approaching 30%, peak power reduction, improved ventilati
40、on, as well as apparent suitability for displacement ventilation designs and those which require large amounts of outdoor air. All of these make IDEA-DX systems a very viable design approach for arid climates. ACKNOWLEDGMENTS The authors gratefully acknowledge the following groups for their assistan
41、ce in prepsaration of this paper: The Energy Plus Support Center team was invaluable in helping create an accurate model of this novel HVAC system. American River College provided real-time data from their installation if an IDEA-DX system. Western Cooling Efficiency Center and the National Renewabl
42、e Energy Laboratory for results of laboratory testing of an IDEA-DX system. Coolerado Corporation and Munters Corporation for technical information regarding IDEA-DX system components. NOMENCLATURE IDE = InDirect Evaporative cooler / cooling IDEA-DX = InDirect Evaporative Assisted DXWB = Wet Bulb te
43、mperature cooling system Wet Bulb effectiveness = (Entering Dry Bulb Leaving Dry Bulb)/ (Entering Dry Bulb Entering Wet Bulb) REFERENCES “Energy Standard for Buildings Except Low-Rise Residential Buildings”: ASHRAE Standard 90.1 - 2007 American Society of Heating Refrigeration and Air Conditioning E
44、ngineers, Inc. Energy Plus v5: http:/apps1.eere.energy.gov/buildings/energyplus/ “Evap Cooling Loves High and Dry”: Seminar 39, ASHRAE Annual Meeting 2010, Albuquerque, NM “Coolerado 5 Ton RTU Performance: Western Cooling Challenge Results”: National Renewable Energy Laboratory Technical Report NREL/TP-550-46524 September 2009 The Energy Plus input data files used for the simulations described here are available from the author by e-mailing . eQuest input files are also available. 138 ASHRAE Transactions
