1、Dew Point Evaporative Cooling: Technology Review and Fundamentals Paul Glanville, PE Aleksandr Kozlov, PhD, ScD Valeriy Maisotsenko, PhD, ScD Associate Member ASHRAE Member ASHRAE ABSTRACT Used throughout history, evaporative cooling is an effective means of air conditioning in hot and dry climates.
2、 Despite its effectiveness, there is not substantial market penetration versus vapor compression systems in more humid climates. This is historically the case, as in its most common form, the direct evaporative cooler, evaporative cooling suffers from substantial water consumption, humidification of
3、 supply air, and limited cooling to ambient wet bulb temperatures. The recent development of several innovative evaporative cooling cycles have broken through these traditional technical barriers. Dew point evaporative cooling, using a novel heat exchanger and flow path arrangement, can deliver unhu
4、midified air below wet bulb temperatures consuming less water than direct evaporative and vapor compression coolers. Supply air temperatures approaching the dew point temperature are achieved in a single-stage unit with cooling capacity independent of the ambient air dry bulb temperature. Recently,
5、a prototype 5 ton rooftop unit delivered 80% energy savings relative to a conventional vapor compression system, demonstrating the potential for dew point evaporative cooling in Zero Energy Design. This paper describes the technology fundamentals of dew point evaporative cooling through said heat ex
6、changer and its context in the technology evolution of evaporative cooling, ranging from direct to multi-stage indirect-direct evaporative cooling, with performance comparisons under common operational conditions. INTRODUCTION The primary method of air conditioning currently is a refrigerant-based v
7、apor compression system (VCS) at over 90% of the market (Westphalen 2001). While vapor compression for refrigeration has patents dating back to the early 19thcentury, it was Willis Carrier who employed this cycle first for air temperature and humidity control in 1902 (History of the Carrier Corporat
8、ion 2006), and the fundamental air conditioning process has changed little since then. The longevity and widespread application of VCS is a testament to its effectiveness. This widespread use of VCS is not without its drawbacks. Residential and commercial VCS consume 1,304 TWh(Westphalen 2001; DOE 2
9、009)of primary energy, which results in the release of 1,357 million metric tons of carbon dioxide (CO2)from residential sector and 1,196 million metric tons of CO2from the commercial sector. In addition, the common refrigerants currently used in VCS for space cooling are a growing source of concern
10、 for their contribution to the climate change. Typical VCS refrigerants include hydrochloroflourocarbons (HCFC) and hydroflourocarbons (HFC), which have global warming potentials (GWP) several orders of magnitude above that of CO2. For example, two common refrigerants in air-conditioning R-410aand R
11、-134a have a GWP of 2,088 and 1,430 respectively (Leck 2010), where the GWP of a compound is its impact toward climate change scaled relative to CO2. An alternative to VCS is evaporative cooling, which has been used throughout history as an effective means of air LV-11-C014 2011 ASHRAE 1112011. Amer
12、ican 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 digital form is not permitted without ASHRAES prior wri
13、tten permission.conditioning in hot and dry climates. Evaporative coolers generally have substantially reduced power consumption and installed cost, as both the compressor and copper heat exchanger are eliminated from the system. Additionally the use of high-GWP refrigerants is also eliminated, as t
14、he phase change of water is what drives cooling1. Despite its effectiveness and relative simplicity, there is not substantial market penetration versus VCS in more humid climates. This is historically the case as in its most common form, the Direct Evaporative Cooler, evaporative cooling suffers fro
15、m substantial water consumption, humidification of supply air, and limited cooling to ambient wet bulb temperatures. These systems facilitate a direct swap of latent for sensible cooling, thus operate ideally under adiabatic conditions. More complex and non-adiabatic evaporative cooling systems miti
16、gate the drawbacks of the Direct Evaporative Cooler, achieving reduced supply air humidification, enhanced cooling, and more efficient operation. This comes at a cost of increased materials, water consumption, and a greater proportion of rejected “working” air, the fraction of ambient air that is hu
17、midified and rejected to drive sensible cooling, requiring increased fan energy. These advanced evaporative cooling systems achieve these improvements with a flow arrangement combining some or all of the following three processes: Direct evaporation a humidifying channel that cools through a swap of
18、 latent for sensible heat. Indirect evaporation two channels, exchanging heat through a common wall, where the channel that contains the product fluid (e.g. supply air) is cooled sensibly by the other channel, a direct evaporator channel. Dehumidification increasing the evaporative cooling potential
19、 of the working air upstream of the direct and/or indirect evaporator, using a desiccant and/or selective membrane. Recently new products have reversed this trend and broken through these traditional technical barriers, such as supply air at temperatures below the ambient wet bulb. This sub-wet bulb
20、 cooling, hereafter called “dew point evaporative cooling”, can be achieved using a novel heat exchanger and flow path arrangement, delivering unhumidified air below wet bulb temperatures while consuming less water than other evaporative coolers. In some cases, supply air temperatures approaching th
21、e dew point temperature are achieved in a single-stage unit with cooling capacity independent of the ambient air dry bulb temperature. Recently, a prototype 5 ton rooftop unit delivered 80% energy savings relative to a conventional vapor compression system, demonstrating the potential for dew point
22、evaporative cooling in Zero Energy Design (Kozubal 2009). This paper describes the technology fundamentals of dew point evaporative cooling through said heat exchanger and its context in the technology evolution of evaporative cooling, ranging from direct to multi-stage indirect-direct evaporative c
23、ooling, with performance comparisons under common operational conditions. EVAPORATIVE COOLING TECHNOLOGY SURVEY Direct Evaporative Cooling technology is a simpler and cheaper alternative to VCS. Used in “swamp coolers” for air conditioning, warm and dry air is passed by a wetted surface, and the lat
24、ent heat of water vaporization cools and humidifies the air. As defined, the theoretical cooling limit is the wet bulb temperature (tWB) of the incoming air stream. Ideally, this is an adiabatic process in which there is a direct energy swap of the latent heat of vaporization of water to sensible co
25、oling of the supply air and as such there is no net cooling capacity2. While a simple process, often cooling and humidifying the air is not desirable. A diagram of Direct Evaporative Cooling with example psychrometric conditions is shown in Figure 1. 1 As employed in current evaporative cooling syst
26、ems, water is not technically a refrigerant as the cycles are open loops and water comes into contact with the cooled fluid (air). At the time of writing, the UN Framework Convention on Climate Change does not recognize water vapor as a greenhouse gas (GHG), thus a Global Warming Potential (GWP) has
27、 not been agreed upon. 2 In other words sensible cooling and latent heating are coincident, thus Direct Evaporative Cooling is enthalpy neutral. 112 ASHRAE Transactions86 F(30 C)68(30 C)12t1DP=54.7F(12.6 C)t1WB=66F(18.9 C)Psychrometric Chart(direct evaporation)Dry Bulb TemperatureSaturationlineNo co
28、oling capacity(dh=0, where h is enthalpy)t1DB = 86 F (30 C)t1WB = 66 F (18.9 C)t2DB = 68 F (20 C)t2WB = 66 F (18.9 C)12t1DP = 54.7 F (12.6 C)AirCooled humidified airWet channelFigure 1 Diagram and example psychrometric chart for Direct Evaporative Cooling Slightly more complex systems that prevent h
29、umidification of the supply air employ Indirect Evaporative Cooling. This improvement upon Direct Evaporative Coolers comes at a cost of increased water consumption, decreased efficiency, and increased installed and operating cost. Indirect Evaporative Coolers avoid humidifying the product air throu
30、gh use of a dual-channel sensible heat exchanger. This is also a direct swap of latent for sensible energy, however, the heat exchanger prevents the product air from being humidified. Using the example diagram in Figure 2, the outside air, at points 1 and 3, is split into the following two channels:
31、 The wet channel is effectively a Direct Evaporative Cooler, which ideally humidifies the air to its wet bulb temperature t1WB= 66F (18.9C). On the psychrometric chart in Figure 2, this is the dashed line of constant wet bulb temperature from Point 3 to 66F (18.9C) on the saturation line. In reality
32、, the air is humidified to higher wet bulb temperature t4WBat Point 4 because of sensible heat transferred from the dry channel. The dry channel is cooled through the heat exchanger to 72F (22.2C) at Point 2. On the chart this moves horizontally at constant absolute humidity from a dry bulb of 86F (
33、30C) to 72F (22.2C) Figure 2 Diagram and example psychrometric chart for Indirect Evaporative Cooling. 2011 ASHRAE 113Like Direct Evaporative Cooling, Indirect Evaporative Cooling is limited by the wet bulb temperature of the incoming fluid. As such, they often are commercialized as “hybrid” systems
34、 that employ a secondary VCS stage to address this capacity limitation, at an additional increased operating and installed cost. While an efficient design, wholly indirect evaporative systems are more energy efficient in providing significant energy savings with sub wet-bulb temperatures. Dew point
35、cooling (sub-wet bulb) is currently achieved predominantly with Indirect-Direct Evaporative Cooling, which combines the two abovementioned evaporative processes in series. With a simple arrangement, Indirect-Direct Evaporative Cooling reroutes the dry channel through a second wet channel. It takes a
36、dvantage of the fact that when a parcel of air is sensibly cooled, the saturated water vapor pressure decreases, reducing its wet bulb temperature thus increasing its evaporative cooling potential. Note that in the psychrometric chart on Figure 2 this occurred as well; as the wet bulb temperature at
37、 Point 2 was lower than that of Point 1. This process is shown in Figure 3 and by examining the psychrometric chart, it is apparent how through this staged sensible (Point 1 to 2) then latent cooling (Point 2 to 3), a supply air dry bulb temperature (t3DB) below the incoming wet bulb temperature (t1
38、WB) is achieved. The limit for cooling now is the dew point temperature (t1,2DP), which is indicated by the horizontal dotted line extending from Point 2 to the saturation line. t1DB = 86 F (30 C)t1WB = 66 F (18.9 C)t2DB = 72 F (22.2 C)t2WB = 61.3 F (16.3 C)1245t5DB = 68 F (20 C)t5WB = 67 F (19.4 C)
39、t4DB = 86 F (30 C)t4WB = 66 F (18.9 C)t1DP = 54.7 F(12.6 C)Working airProduct airCooled humidified product airDry channelWet channelHumidified working air23t3DB = 63.3 F (17.4 C)t3WB = 62.3 F (16.8 C)Wet channelt2DP = 54.7 F (12.6 C)dh = cooling capacity86 F(30 C)72 F(22.2 C)1, 42t1,2DP=54.7F(12.6 C
40、)t1WB=66F(18.9 C)Psychrometric Chart(indirect-direct evaporation)Dry Bulb Temperature5Saturationline3t2WB=61.3F(16.3 C)63.3 F(17.4 C)dhFigure 3 Diagram and example psychrometric chart for Indirect-Direct Evaporative Cooling. One could add another dry channel atop the supply air wet channel (Point 2
41、to 3) and perform a second latent for sensible energy swap, which could then facilitate dew point cooling without humidification. This process of adding additional dry-to-wet channels could theoretically repeat ad infinitum until the supply air is cooled to the ambient dew point temperature (t1,2DP)
42、. This is both inefficient with energy and space however, as each combined dry/wet channel pairing must be thermally separated to remain adiabatic, thus increasing the flow path and pressure drop. Additionally fan energy increases, as with each additional dry/wet channel pairing a greater fraction o
43、f the incoming air is humidified and rejected, as done at Point 5. Commercially available systems often employed a dehumidification step, drying the working air and/or the cooled and humidified supply air. Generally, installations are engineered systems and this dehumidification step is accomplished
44、 through use of a solid desiccant wheel. While delivering high performance, the added dehumidification step adds system cost and the increased water consumption of the Indirect-Direct Evaporative system is not reduced. These thermally activated desiccant technologies are proven to be a viable altern
45、ative to VCS, however, the desiccant regeneration process is energy intensive. 114 ASHRAE TransactionsThese three general classes of evaporative coolers: Direct, Indirect, and Indirect-Direct, represent the majority of the market. Each has their pros and cons, as summarized in Table 1. Table 1. Pros
46、 and Cons for Direct, Indirect, and Indirect-Direct Evaporative Cooling Evaporative Cooling Type Relative Cost Pros Cons Direct Low - Effective and used throughout history - No cooling capacity - Cooling limited to the ambient tWBIndirect Mid - Has cooling capacity - Benefits of Direct Evaporative C
47、ooling without supply air humidification - Cooling limited to the ambient tWB- Working air must be rejected, increasing fan work - Additional material without cooling benefit is often not justifiable Indirect-Direct High - Has cooling capacity - Cooling is not limited to incoming tWB - Supply air is
48、 humidified - Working air must be rejected, increasing fan work - Indirect to direct channels must be separated increasing material and space requirements DEW POINT EVAPORATIVE COOLING Rather than add active dehumidification to an Indirect-Direct Evaporative Cooler, an alternative is a Dew Point Eva
49、porative Cooler arrangement that takes advantage of the previously discussed wet-bulb depression in a non-adiabatic design. A compact hybrid (evaporative cooling with supplemental vapor compression) dew point evaporative cooling system was demonstrated with a sensible energy efficiency ratio (EER) in excess of 40, corresponding to a Coefficient of Performance (COP) of 12.0 (Kozubal 2009). When operating with the VCS portion disabled, it maintained a dry bulb temperature difference of 5F (2.8C) between exhaust and supply air. For reference, the mi
