1、 Jonathan Woolley is an Associate Research Engineer with the Western Cooling Efficiency Center at the University of California, Davis. Mark Modera is Professor Civil and Environmental Engineering and Director Western Cooling Efficiency Center at the University of California, Davis Advancing Developm
2、ent of Hybrid Rooftop Packaged Air Conditioners: Test Protocol and Performance Criteria for the Western Cooling Challenge Jonathan Woolley Mark Modera ASHRAE Member ASHRAE Member ABSTRACT The Western Cooling Challenge is a multiple-winner competition that invites manufacturers to develop and commerc
3、ialize the next generation of rooftop packaged air conditioners appropriate for dry western United States climates. Certification centers on requirements for sensible energy efficiency at two separate test conditions that are representative of western climates. Criteria for minimum energy and water
4、use efficiency were developed based on the estimated performance of market-available retrofit solutions for conventional rooftop package units; thus it is expected that comprehensive ground-up system designs should easily achieve the performance requirements. This paper outlines and discusses the de
5、velopment of the test protocol and performance criteria for the Challenge. The choice of laboratory test conditions is discussed. The rationale for and calculation of performance metrics including nominal cooling capacity and credited cooling capacity are presented. Additionally, the assumptions und
6、erlying requirements for minimum sensible energy efficiency are summarized, and key non-performance-based criteria for the program are explained. INTRODUCTION The Western Cooling Challenge (WCC), hosted by the Western Cooling Efficiency Center (WCEC) at the University of California Davis, is a multi
7、ple-winner competition that encourages HVAC manufacturers to develop and commercialize rooftop packaged air conditioning equipment for dry climates that will reduce electrical demand and energy use by at least 40% compared to DOE 2010 standards. The units in design, testing, and demonstration are al
8、l some variation of a hybrid system that couples indirect evaporative cooling with high efficiency vapor compression. In such a configuration, each component can operate either independently or in unison based upon ambient conditions and cooling demand. In addition to a number of non-performance-bas
9、ed requirements, WCC certification requires that equipment meet stringent criteria for sensible energy efficiency and water use. Such performance must be proven through WCEC-observed laboratory tests at two outdoor air conditions that were chosen as surrogates for peak-day design and average cooling
10、-season conditions in hot-dry climates of the Western United States. The Challenge was developed in part by encouragement from large retailer affiliates of the WCEC who are aggressively pursuing energy efficiency in their buildings and who would install very high efficiency hybrid equipment en masse
11、 if the technology was well proven, commercially available, and cost effective. The Western Cooling Challenge criteria were developed in such a way that incremental improvement to a conventional vapor compression cycle could not meet energy performance requirements. However, the Challenge was design
12、ed such that conventional HVAC equipment could qualify with the addition of commercially available add-on evaporative technologies. Although the intent was to encourage manufacturers to develop and commercialize hybrid units that integrate these efficiency-improving components into a single package,
13、 partnerships between manufacturers to submit high-efficiency conventional rooftop units with add-on evaporative components was allowed and encouraged because ground-up design requirements would discourage major manufacturers. LV-11-C066 2011 ASHRAE 5332011. American Society of Heating, Refrigeratin
14、g 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 digital form is not permitted without ASHRAES prior written permission.As of August 2010, on
15、e entry had been laboratory tested for the Challenge by the National Renewable Energy Laboratory (NREL). The system, referred to herein as a WCC Type 1 Hybrid, uses a Maisotsenko cycle indirect-evaporative heat exchanger in series with a vapor compression system. The system includes a number of ener
16、gy-efficient components and control strategies, and demonstrated performance well beyond the Challenge requirements. Evaluation of the laboratory results indicate that at the WCC annual test conditions, the system can achieve a COP for sensible space cooling of more than three times that of conventi
17、onal equipment meeting DOE 2010 efficiency standards. The sensible space cooling capacity at this level of performance is much lower than the nominal capacity, but even operating at full capacity under these conditions the WCC Type 1 Hybrid has a COP for sensible space cooling that is more twice tha
18、t of standard equipment. WESTERN COOLING CHALLENGE REQUIREMENTS, TEST POINTS, AND PERFORMANCE CRITERIA Non-performance based requirements The intent of the Western Cooling Challenge is to push beyond prototype high-efficiency cooling equipment by advancing the market introduction of fully-commercial
19、ized equipment. Thus, although the Challenge focuses on energy and water-use efficiency, it also includes a number of non-performance-based requirements. Most importantly, in order to qualify, a manufacturer must demonstrate the capacity to produce a minimum of 500 units per year. In this way, parti
20、cipants are challenged to develop commercialized products. They must consider design factors such as cost-effectiveness, robustness, longevity, availability of replacement parts, accessibility for maintenance, and non-energy code compliance. Further, they must be prepared to provide marketing, docum
21、entation, warrantees and support for the products. The Challenge also specifies that equipment must self-detect and communicate performance degradation, and must respond to line-voltage droop without increasing current draw on the electrical grid. These requirements were included to address specific
22、 customer and utility problems associated with air conditioners. Many rooftop units consume more energy than they should merely because of poor maintenance. The root of degraded performance is often not easily identifiable and will generally go unnoticed by the customer, resulting in poor energy eff
23、iciency that may persist for the life of the equipment. An emerging solution to tackle this problem is the inclusion of some form of fault detection diagnostics on each rooftop unit. The appropriate method to effectively detect and report faults or poor energy performance is ambiguous, but a require
24、ment to include such capabilities in all new rooftop package units is under consideration for the California Building Energy Standards and is included as criteria for the Challenge. In response to concerns from California electric utilities about overloaded electrical grids, the Challenge requires t
25、hat systems respond to voltage droop and power outages in a way that isnt unduly aggravating to an already overdrawn grid. Fan motors and compressors should not draw additional current when line-voltage is low, and must have the ability to restart in a way that minimizes startup stress on the grid.
26、Other requirements that would improve overall energy efficiency were considered, such as criteria for fan energy use, but ultimately the performance criteria were limited in part to maintain simplicity, and to in part to allow standard packaged units to compete by focusing simply on retrofit evapora
27、tive components. Laboratory Test Conditions Western Cooling Challenge certification of a machine centers on steady-state sensible energy efficiency at full capacity operation, with 120 cfm/nominal ton (16.1 L/s-nominal kW) ventilation rate, and 0.7 Inches WC (174 Pa) external static pressure, at two
28、 different outdoor psychrometric conditions. The laboratory test protocol, described in Table 1, was designed roughly around conditions in a large retail facility. The two outdoor conditions were chosen to represent peak-day design conditions and average cooling-season conditions for cooling intensi
29、ve regions in the Western United States. For both tests, the equipment must provide a minimum outside air ventilation rate of 120 cfm/nominal ton (16.1 L/s-nominal kW). This ratio was derived from two rough metrics for air conditioner sizing: 534 ASHRAE Transactions1. An outside air flow of 0.2 cfm/
30、ft2(1.02 L/s-m2) the California Energy Code (Title 24) required ventilation rate for retail stores 2. An installed cooling capacity of 600 ft2/nominal ton an approximate design point for large retail facilities in CA Climate Zone 12 (Sacramento CA). Note that normalized cooling capacity varies signi
31、ficantly by climate zone and building type between 300 ft2/nominal ton (7.92 m2/nominal kW) and 800 ft2/nominal ton (21.1 m2/nominal kW) Requirements for the Challenge also specify a full capacity test at AHRI 340/360 standard rating conditions for units that can operate with zero percent outside ai
32、r. There are no WCC minimum energy performance criteria for this test, but it is used to determine the nominal system capacity. The protocol for evaluating the nominal capacity of systems that cannot operate at 0% outside air, such as the certified WCC Type-1 Hybrid, is described in a later section.
33、 Table 1. Western Cooling Challenge Laboratory Test Conditions AHRI 340/360 Conditions WCC Peak Conditions WCC Annual Conditions Outside Air Condition TdbF/TwbF (C) 95/75 (35/23.9) 105/73 (40.6/22.8) 90/64 (32.2/17.8) Return Air Condition TdbF/TwbF(C) 78/67 (25.6/19.4) 78/64 (25.6/17.8) 78/64 (25.6/
34、17.8) Outdoor Ventilation cfm/nominal-ton (L/s-kW) 0 120 (16.1) 120 (16.1) External Static In WC (Pa) 0.2-0.75 (50-187) 0.7 (174) 0.7 (174) Figures 13 each plot the three outdoor-air test conditions alongside a scatter-plot of the hourly psychrometric conditions from typical meteorological years in
35、three different California Climate Zones. For comparison, Figure 4 plots the same for Baton-Rouge Louisiana. It is worth noting that the WCC peak conditions are generally more demanding on evaporative equipment than the 0.4%-occurrence design conditions for most western climates. Locations with desi
36、gn conditions at or above 105 F (40.6 C) Tdb typically have mean coincident wet-bulb temperatures much lower than the WCC peak condition; locations with design conditions at or above 73 F (22.8 C) Twb have significantly lower mean coincident dry bulb temperature. Its also clear from Figures 1-4 that
37、 AHRI 340/360 standard rating conditions are generally not representative of the cooling-intensive California climates. Determining Credited Cooling Capacity The test protocol for the Challenge was designed to evaluate system performance while operating with 120 cfm/nominal ton (16.1 L/s-nominal kW)
38、 outside air. For systems that have a minimum outside air fraction that exceeds 120 cfm/nominal ton (16.1 L/s-nominal kW), the WCC calculates a credited cooling capacity that does not count the cooling and dehumidification of additional outside air to return air conditions. This is important because
39、 it allows capacity and energy efficiency to be compared fairly between units even if they operate at different ventilation rates. If the correction were not made, the sensible capacity and energy efficiency of a system operating with 100% outside air would be misrepresented since it would include c
40、ooling of excess ventilation air. The following equations describe calculation of several metrics used to characterize WCC equipment: nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnulln
41、ullnullnullnullnullnullnullnullnull nullnullnullnullnull1nullnullnullnullnullnullnullnullnullnullnullnullnullnull 2011 ASHRAE 535536 ASHRAE Transactionsnullnullnullnullnullnullnullnullnullnull nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull120nullnullnullnullnull
42、nullnull nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull1null120 nullnullnullnullnullnullnull nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull
43、nullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnullnull nullnullnullnullnullnullnullnullnullnullnullFigure 5 shows the various metrics for describing system capacity. It is important to note that nullnullnullnullis
44、used as the normalizing flow rate for both the figure and for the proceeding equations. Thus the ventilation cooling and credited ventilation cooling are calculated using specific enthalpy differences between theoretical mixed-air conditions and return-air conditions, not between outdoor air conditi
45、ons and return air conditions. Also, note that the minimum energy efficiency criteria for the Challenge are based only on the sensible component of the credited cooling capacity described here. Determining nominal capacity Nominal capacity of a system is typically determined at AHRI standard rating
46、conditions. For the Challenge this nominal value is used to determine the credited ventilation cooling, the sensible credited cooling capacity, and thus the sensible credited EER by which a unit qualifies for certification. However, since the AHRI test occurs with 0% outside air and some hybrid equi
47、pment will have a non-zero minimum outside air fraction, the Cooling Challenge includes an alternate method to determine nominal capacity that uses measured performance from full-capacity operation under WCC peak conditions. This alternate nominal capacity is determined by: nullnullnullnullnullnulln
48、ullnullnullnullnullnullnullnullnullnullnullnull31.5 nullnullnullnullnull where 31.5 is the specific enthalpy of return air for AHRI nominal capacity tests. This metric is plotted for reference purposes in Figure 5. The method uses the enthalpy difference between return air and supply air to discount
49、 the capacity for cooling ventilation air and count only the space cooling delivered. This effectively scales the capacity measured under WCC peak conditions to a value that represents operation with 0% outdoor air, as in an AHRI test scenario. However, it does not represent space cooling capacity under AHRI outdoor air conditions, nor does it represent an actual space cooling capacity that would be achieved under any particular condition since the measurements are taken during full capacity operation at WCC peak conditions and the results are mingled post
