energies Article
Performance Characteristics of Solid-Desiccant Evaporative Cooling Systems Ramadas Narayanan 1, *, Edward Halawa 1,2 and Sanjeev Jain 3 1 2 3
*
Central Queensland University, University Drive, Bundaberg, QLD 4670, Australia;
[email protected] Barbara Hardy Institute, School of Engineering, University of South Australia, Mawson Lakes, SA 5095, Australia;
[email protected] Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India;
[email protected] Correspondence:
[email protected] or
[email protected]; Tel.: +61-74150-7107
Received: 28 August 2018; Accepted: 18 September 2018; Published: 27 September 2018
Abstract: Air conditioning accounts for up to 50% of energy use in buildings. Increased air-conditioning-system installations not only increase total energy consumption but also raise peak load demand. Desiccant evaporative cooling systems use low-grade thermal energy, such as solar energy and waste heat, instead of electricity to provide thermal comfort. This system can potentially lead to significant energy saving, reduction in carbon emissions, and it has a low dew-point operation and large capacity range. Their light weight, simplicity of design, and close-to-atmospheric operation make them easy to maintain. This paper evaluates the applicability of this technology to the climatic conditions of Brisbane, Queensland, Australia, specifically for the residential sector. Given the subtropical climate of Brisbane, where humidity levels are not excessively high during cooling periods, the numerical study shows that such a system can be a potential alternative to conventional compression-based air-conditioning systems. Nevertheless, the installation of such a system in Brisbane’s climate zone requires careful design, proper selection of components, and a cheap heat source for regeneration. The paper also discusses the economy-cycle options for this system in such a climate and compares its effectiveness to natural ventilation. Keywords: air conditioning; desiccant wheel; evaporative cooling
1. Introduction The air-conditioning industry faces many challenges as the demand for comfortable indoor environments continues to increase [1]. Air conditioning accounts for a major part, up to 40%, of energy use in buildings [2]. Widespread air-conditioning-system installation not only increases total energy consumption but also raises peak electricity demand. Peak demand in many countries and Australian cities is growing at about 50% more than base demand growth and, in some areas, up to four times the average, which, in turn, requires higher generation capacity [3]. Power generation accounts for the majority of greenhouse-gas emissions and associated climate change. These challenges lead to the need for green and sustainable and innovative systems [4] with minimum impact on the environment, materials, resources, and processes that prevail in nature [5]. Heat-driven air-conditioning systems are systems that use low-grade thermal energy such as solar energy and waste heat rather than electricity. These systems can produce significant energy savings, have low global-warming and zero ozone-depletion potential, while at the same time being able to deliver better indoor-air quality. Studies have indicated that such a system is applicable to a wide range of climatic conditions and offers a feasible alternative to conventional air-conditioning systems [6–11]. Among these systems, desiccant evaporative cooling systems are worthy of consideration due to their low dew-point operation and large capacity range. Their light weight, simplicity of design, Energies 2018, 11, 2574; doi:10.3390/en11102574
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and close-to-atmospheric operation make them easy to maintain. In terms of providing thermal comfort, the system combines the dehumidifying capability of a solid desiccant system with the sensible cooling capability of an evaporative cooling system. For hot dry summers as experienced in Adelaide and Melbourne, evaporative cooling achieves coefficient of performance (COP) values of more than 20 [12]. Furthermore, since these systems provide 100% ventilation, they reduce the amount of heat entering the home through walls and roofs, as cooled air absorbs this heat as it exits the building. This characteristic, known as displacement ventilation, results in less cooling being required compared to a refrigeration-based system that has to remove all the heat that enters the building to achieve the desired conditions [13]. However, evaporative cooling is ineffective in humid climates, including in Brisbane, where thermal comfort is unachievable even using two-stage cooling systems [14]. In Australia, the energy efficiency of cooling systems is regulated through the Minimum Energy Performance Scheme (MEPS) and the star rating of air-conditioning systems, which is based on the COP measured at an indoor temperature of 27 ◦ C and an outdoor temperature of 35 ◦ C operating at 100% output (AS/NZS 3823.2:2011). The rating only considers the unit itself and does not consider the energy-efficiency impact of ducting or using multiple heads, as found in whole-of-house systems. In addition, inverter air conditioning involves the compressor within the system adjusting its rotational speed to match the load requirement. Usually, these systems have the ability to reach a cooling capacity of more than 150% of the rated capacity, which is a ‘hidden’ capacity. This will increase power demand beyond the rated capacity. There is no regulation limiting this additional capacity [12]. The COP of a refrigerated air conditioner is a function of the outdoor air temperature. The COP value is reduced by half of its rated value during hot conditions, which are more likely to occur frequently in future [12]. In a refrigerated air conditioner, air is dehumidified by cooling air below dew-point temperature to condense water vapor in the air using a cooling coil that uses a vapor-compression system. Besides being energy-intensive, these systems are not particularly effective in dealing with higher latent loads [13]. The cooling and dehumidifying process in a refrigerative air conditioner may necessitate heating after reducing moisture content to achieve comfort conditions, particularly for commercial systems. Water condensation during this process can also cause mildew, leading to material degradation and potential health problems. This paper presents an analysis of the technical suitability of a residential desiccant evaporative cooling system for Brisbane, Queensland, Australia. According to the Köppen Climate Classification, Brisbane (and Queensland in general), has subtype Cfa (humid subtropical) climate with “relatively high temperatures and evenly distributed precipitation throughout the year” [15]. Recent studies have indicated that a desiccant evaporative cooling system is technically viable for office buildings in warm and humid climates such as Darwin and Brisbane [16,17]. The present work focuses on residential applications and shows how the values of certain system parameters affect indoor thermal-comfort conditions, as well as the system’s overall performance. The TRNSYS computer modeling package was used to analyze the performance of a system hypothetically installed in a residential building. While the paper presents the specific case for Brisbane, the outcomes of the study should be applicable to regions with climates similar to Brisbane. In addition, this study should trigger further investigations on the technical and economic viability of such a system in more challenging (humid and warm to hot) climates. The paper also presents conclusions and recommendations arising from the study. 2. Solid-Desiccant Evaporative Cooling System—Overview A typical ventilation-mode solid-desiccant evaporative system consists of a desiccant wheel, an energy-recovery wheel, evaporative coolers (at supply and exhaust streams), and a regeneration (reactivation) source, as shown in Figure 1. The desiccant wheel dehumidifies the air, but substantially raises its temperature (Processes 1→2). The first (sensible) cooling of this supply air takes place in the energy-recovery wheel (2→3). Before entering the conditioned space, further cooling of supply air occurs in a direct evaporative cooler (3→4). Air humidity is slightly raised (3→4), but is still well
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cooling of supply air occurs in a direct evaporative cooler (34). Air humidity is slightly raised (34), but is still well below comfort requirements. As the supply air enters the conditioned space, below comfort requirements. As theincrease supply air conditionedand space, itsheat temperature and its temperature and humidity dueenters to the the space-sensible latent gains (45). humidity increase due to the space-sensible and latent heat gains (4→5).
Figure 1. Solid-desiccant evaporative cooling system. A: desiccant wheel, B: energy-recovery wheel, C and D:Figure evaporative cooler, E: regeneration 1. Solid-desiccant evaporativesource. cooling system. A: desiccant wheel, B: energy-recovery wheel,
C and D: evaporative cooler, E: regeneration source.
The air coming from the air-conditioned space is directed to the exhaust, where it undergoes several psychrometric processes with cooling in an evaporative →6). This brings the The air coming fromstarting the air-conditioned space is directed cooler to the (5 exhaust, where it undergoes air close to the saturation line with humidification. In the energy-recovery wheel, the air is preheated several psychrometric processes starting with cooling in an evaporative cooler (56). This brings the (6→7) before it passes through the heater →8), where its In temperature is further raised that air close to the saturation line with(7 humidification. the energy-recovery wheel,so the airitiswill preheated be able(67) to desorb the water from the desiccant on the regeneration side of the desiccant wheel (8 → 9). it will before it passes through the heater (78), where its temperature is further raised so that The regeneration heat source (E) can be either a solar thermal system or waste heat. be able to desorb the water from the desiccant on the regeneration side of the desiccant wheel (89). Typical air temperature and humidity levels at athe inlet and outlet of each component The regeneration heat source (E) can be either solar thermal system or waste heat. of this configuration, extracted from Reference are given in Table 1. inlet and outlet of each component of this Typical air temperature and[6], humidity levels at the
configuration, extracted from Reference [6], are given in Table 1.
Table 1. Typical air-temperature and humidity levels at the inlet and outlet of each component of solid-desiccant evaporative air-cooling (SDEAC) system whose configuration is shown in Figure 1. Table 1. Typical air-temperature and humidity levels at the inlet and outlet of each component of DW: Desiccant wheel, ERW: Energy-recovery wheel, DEC: Direct evaporative cooler, REG: Regenerator. solid-desiccant evaporative air-cooling (SDEAC) system whose configuration is shown in Figure 1. ◦ C DW: 35 Desiccant wheel, wheel, evaporative cooler, REG: 53 ERW: Energy-recovery 27 20 DEC: Direct Temp Regenerator. → DW → ERW → DEC → Sup. Side g/kg 11 627 8.5 Humidity Ratio Temp °C 35 536 20 ◦C
g/kg °C
g/kg
g/kg
48 11 ← 48 19 19
DW
DW DW
75 6 75← 13 13
ERW
REG REG
45
6 ←45 13 13
DEC
20
← ERW 13
ERW
8.5 20 DE 13
27
← DE11
Sup. Side Temp Humidity Ratio Exhaust 27 Temp Hum Exhaust 11 Hum
Recent numerical studies have indicated that a desiccant evaporative cooling system with solar Recent numerical studies have indicatedfor that a desiccant evaporative cooling system with solar thermal collectors, as the heat source is suitable warm and humid climates such as Darwin thermal[16,17]. collectors, heat source is suitable warm andcooling humiddemand climatesoccurs such asduring Darwin and and Brisbane Thisas is the particularly relevant in thefor case where Brisbane [16,17]. This is particularly relevant in the case where cooling demand occurs during a a period of maximum solar gains. The system’s dehumidification performance, however, needs to period of maximum solar gains. The system’s dehumidification performance, however, needs be improved to enable it to address a high latent load. Such improvement in dehumidification to be improved to enable it achieved to address a The highso-called latent load. Such improvement in dehumidification performance has recently been [18]. ‘nonadiabatic desiccant wheel’ has been performance has recently been achieved [18]. The so-called ‘nonadiabatic desiccant wheel’ has been found to improve dehumidification capability by around 45%–53% under identical supply-air and found to improve dehumidification capability by around 45%–53% under identical supply-air regeneration-air conditions [18]. In this study, however, the standard desiccant wheel model available and regeneration-air conditions thisthestudy, however, the standard desiccant wheel model in the TRNSYS was employed. This [18]. meansInthat benefits of the improved desiccant wheel system available in the TRNSYS was employed. This means that the benefits of the improved desiccant were not considered. wheel system were not considered. 3. Computer Modeling Analysis of System Performance 3. Computer Modeling Analysis of System Performance 3.1. Computer Modelling Setup 3.1. Computer Modelling Setup The TRNSYS modeling package [19] was employed to analyze system performance. The main TRNSYS packageweather [19] wasdata, employed to analyze performance. The main componentsThe of the systemmodeling include: Brisbane a reference house system modelled using TYPE56, thedesiccant system include: Brisbane data, a reference house modelled and thecomponents components of the evaporative coolingweather system (available in TRNSYS/TESS library), using TYPE56, the wheel components of energy-recovery the desiccant wheel evaporative cooling system (available in which consist of: and desiccant TYPE716, TYPE760, one direct evaporative library), whichone consist desiccant cooler wheelatTYPE716, cooler TRNSYS/TESS at the supply side (TYPE506), directof: evaporative the returnenergy-recovery side (TYPE506), wheel
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Energies 2018, 11, 2574 4 of 14 TYPE760, one direct evaporative cooler at the supply side (TYPE506), one direct evaporative cooler at the return side (TYPE506), TYPE930 to model the regeneration heat source, and several printers (TYPE25) theregeneration relevant outputs. At this and stage, the main purpose of this to modeling is to TYPE930totorecord modelallthe heat source, several printers (TYPE25) record all the assess the outputs. capabilityAtofthis thestage, solid-desiccant evaporative system to deliver thermal relevant the main purpose of this modeling is to assess the comfort, capabilityhence of the choosing electricevaporative heater TYPE920 asto the heat source. research will explore the heater most suitable solid-desiccant system deliver thermalFuture comfort, hence choosing electric TYPE920 regeneration heat source(s). as the heat source. Future research will explore the most suitable regeneration heat source(s).
3.2. 3.2.Reference ReferenceHouse House ToTohave haveananidea ideaasastotohow howa adesiccant desiccantevaporative evaporativecooling coolingsystem systemserves servesa ahouse housebuilt builtininthe the Brisbane zone, a house, of which the the floorfloor planplan is shown in Figure 2, has2,been The Brisbaneclimate climate zone, a house, of which is shown in Figure has modeled. been modeled. floor theofhouse is adapted fromfrom the the “Example” house in Cheetah program [20], used The plan floor of plan the house is adapted “Example” house in Cheetah program [20], usedinin References References[21,22]. [21,22].The Thehouse houseconsists consistsofofthree threebed bedrooms, rooms,one onelounge, lounge,and andone onedining diningroom/kitchen. room/kitchen. AApassage passageconnects connectsthe thebedrooms bedroomsfrom fromthe thelounge loungeand anddining diningspace spaceasasshown. shown.The Thelaundry laundryroom room and bathroom (called service section) are located close to each other at the right side of the building and bathroom (called service section) are located close to each other at the right side of the building space. height ofof the building was setset toto 2.42.4 mm and the two roof sections, facing north and south, space.The The height the building was and the two roof sections, facing north and south, respectively, respectively,have have25° 25◦pitch. pitch.
Figure 2. A reference house based on Reference [20]. Figure 2. A reference house based on Reference [20].
The thermal zoning of the house was set so that only the living room and bedrooms were The thermal zoning of the house was set so thatwere onlynot. the Internal living room bedrooms were conditioned, while the passage and service sections heatand gains came from five conditioned, while the passage aand serviceasections were not.inInternal heat and gains came from occupants, kitchen appliances, computer, TV set, and lights every room passage. Heat five gains occupants, kitchenaccording appliances, a computer, TV set, and lights in every room and passage. Heat were estimated to the scheduleda conditioning of rooms: 07.00–23.00 (living room) and gains were estimated according to the scheduled conditioning of rooms: 07.00–23.00 (living room) 23.00–07.00 (bedrooms). and 23.00–07.00 The house(bedrooms). has single-glass external windows in the external walls of the living room and bedrooms. housewall has consists single-glass externalfoil, windows in and the external wallsThe of the room TheThe external of gypsum, airspace, brick layers. totalliving area of the and glass bedrooms. external wallwalls consists gypsum, foil,ofairspace, brick layers. Thewalls total area windows The on living-room wasof6.3 m2 , and those onand bedroom external was of 4.5the m2 . glass windows on living-room walls wasof6.3 m210 , and ofgypsum those onlayers bedroom external m2. Internal and adjacent walls were made two mm separated by walls a 100 was mm 4.5 airspace Internal andceiling adjacent walls were twoand 10 mm layers separated by gypsum a 100 mmand airspace layer. The separating themade roof of space the gypsum rooms was made of 10 mm a layer layer. The ceiling separating roof space andconsisted the rooms made of 10 mm steel, gypsum and airspace, a layer of insulation having an R1.3the value. The roof ofwas 0.8 mm corrugated 20 mm ofand insulation having value. consisted of 0.8 mm corrugated steel, 20 mm airspace, a 0.6 mm thickan GIR1.3 sheet layer.The Theroof floor was made of stone, concrete, silencer, and insulation. and 0.6 mm thick GI sheet layer. floor made of stone, silencer, and shading insulation. All Allaother main dimensions of the The house arewas shown in Figure 2. concrete, In addition to internal devices other maincurtains), dimensions the house are protected shown in from Figure 2. Insunlight additionby to the internal devices (internal theof windows were direct eavesshading surrounding the (internal curtains), the windows were protected from direct sunlight by the eaves surrounding the upper part of the roof. upper part of the roof.
3.3. Room Set Point Temperature-Design Indoor Temperature
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3.3. Room Set Point Temperature-Design Indoor Temperature
In aa residential home conditioned is In residential home conditioned by by aa refrigerative refrigerative cooling cooling system, system, the the set-point set-point temperature temperature is based on the ASHARE/Fanger’s comfort chart [23]. For Australian homes, the Australian Institute of based on the ASHARE/Fanger’s comfort chart [23]. For Australian homes, the Australian Institute Refrigeration, AirAir Conditioning, and indoor-design of Refrigeration, Conditioning, andHeating Heating(AIRAH) (AIRAH)specifies specifies the the acceptable acceptable indoor-design ◦ ◦ temperature range of 22.5–26 °C while maintaining dew point at or below 13 °C [24]. Realising the the temperature range of 22.5–26 C while maintaining dew point at or below 13 C [24]. Realising different nature nature of of evaporative evaporative cooling, cooling, Australian Australian Standard Standard AS AS 2913 2913 Evaporative Evaporative air-conditioning air-conditioning different ◦ equipment [25] specifies 27.4 °C as the target indoor temperature. equipment [25] specifies 27.4 C as the target indoor temperature. 4. Results and Discussion 4.1. Humidity, and and Irradiance Irradiance Profiles Profiles 4.1. Temperature, Temperature, Humidity,
Ambient Temperature, °C
According According to to TRNSYS TRNSYS weather weather data data used used in in the the model, model, Brisbane Brisbane has has an an annual annual maximum maximum ◦ ◦ ◦ C, temperature C and a minimum temperature of 3.4 °C, C, with an annual mean of 20.5 °C, temperature of 33.2 °C minimum temperature Figure maximum occurs occurs during during the the four four summer summer (or (or warmer) warmer)months monthsof ofJanuary, January, Figure 3. 3. As expected, the maximum February, November, and December. Cooling can be expected to occur with less frequency February, November, and December. Cooling can be expected to occur with less frequency during during transition transition periods periods to to and from winter. The The winter winter months months of May to August are practically free from cooling In terms terms of of humidity, humidity, typical of subtropical subtropical climates, climates, cooling demand when some heating is needed. In Brisbane is relatively dry, although some dehumidification is required during the summer months. Brisbane is relatively dry, although some dehumidification is required during the summer months. The thethe TRNSYS weather data data (Figures 3–5) is3–5) veryisclose to close that reported informationprocessed processedfrom from TRNSYS weather (Figures very to that The information in Reference [15]. reported in Reference [15]. 40 30 20 Minimum Maximum Mean
10 0 J
F
M
A
M
J
J
A
S
O
N
D
Figure Figure 3. 3. Monthly Monthly ambient ambient temperature temperature profiles. profiles.
Relative Humidity, %
100 80 60 40 Minimum Maximum
20
Mean
0 J
F
M
A
M
J
J
A
S
Figure 4. Monthly relative humidity profiles.
O
N
D
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Humidity g/kg Ratio,g/kg HumidityRatio,
25 25 20 20 15 15 10 10 5 5 0 0
J J
Minimum Minimum Maximum Maximum Mean Mean F M A M J J A S O F M A M J J A S O Figure 5. Monthly humidity ratio profiles. Figure 5. Monthly humidity ratio profiles.
N N
D D
Humidity dryairair g/kgdry HumidityRatio, Ratio,g/kg
Figure 6 shows the plot of the humidity ratio against the dry bulb temperature of air in Brisbane Figure 6 shows the plot of the humidity ratio against the dry bulb temperature of air in Brisbane during the cooling months. As shown, the number of points where humidity ratios are below during the cooling As shown, shown, the the number number of of points points where where humidity humidity ratios are below cooling months. months. As ASHRAE’s upper recommended value and that lie between 20–26 °C quite significant. In such a ◦ Care ASHRAE’s upper recommended recommended value value and and that thatlie liebetween between20–26 20–26°C arequite quitesignificant. significant.InInsuch sucha are situation, mechanical cooling is not required and, as discussed in Section 4.3.4, the economy cycle is asituation, situation, mechanical cooling not required and, discussed Section 4.3.4, the economy cycle mechanical cooling is is not required and, asas discussed inin Section 4.3.4, the economy cycle is not required, either. However, the same figure also shows that mechanical cooling is required for a is not required, either. However, the same figure also shows that mechanical cooling required fora not required, either. However, the same figure also shows that mechanical cooling is is required for significant number of hours during the cooling season. asignificant significantnumber numberofofhours hoursduring duringthe thecooling coolingseason. season. 25 25 20 20 15 15 10 10 5 5 20 20
25
30
25 Ambient Temperature 30 Ambient Temperature
35 35
Figure Figure 6. 6. Air-humidity Air-humidity ratio ratio vs. vs. ambient ambient temperature temperature for for cooling cooling months. months. Figure 6. Air-humidity ratio vs. ambient temperature for cooling months.
4.2. Living-Room and Bedroom Temperature and Humidity Profiles 4.2. Living-Room and Bedroom Temperature and Humidity Profiles 4.2. Living-Room Bedroom Temperature and Humidity Living-roomand and bedroom temperature profiles Profiles are shown in Figures 7 and 8. As shown, Living-room and bedroom temperature profiles are shown in Figures 7 and 8. As shown, during the conditioning periods, temperature in both types in of Figures room stays the Living-room and bedroom temperature profiles are shown 7 andat8.around As shown, during the conditioning periods,◦ temperature in both types of room stays at around the room-temperature set point of 26 C ( ± 0.5 K). The peaks in the profile indicate the temperatures during the conditioning periods, temperature in both types of room stays at around the room-temperature set point of 26 °C (0.5 K). The peaks in the profile indicate the temperatures where the zone (bedroom) is not conditioned. room-temperature set point of 26 °C (0.5 K). The peaks in the profile indicate the temperatures where the zone (bedroom) is not conditioned. where the zone (bedroom) is not conditioned.
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Figure 7. Living room temperature profile. Figure Figure 7. 7. Living Living room room temperature temperature profile. profile.
Figure 8. Bedroom temperature profile. Figure 8. Bedroom temperature profile. Figure 8. Bedroom temperature profile profile. of the bedrooms is much more It is worth noting that the peaking of the temperature
It is worth noting peaking theistemperature profilezone, of the bedrooms much more pronounced than that ofthat the the living room. of This because the living situated in theisnorthern part, It is worth noting that the peaking of the temperature profile of the bedrooms is much more pronounced than that of the living room. This is because the living zone, situated in the northern gets less sun exposure and it is conditioned during the whole day through to near midnight (7–23 h). pronounced than that of the living room. This during is because the living zone, situated inmidnight the northern part, gets less hand, sun exposure and itsituated is conditioned whole to near On the other the bed zone, in the southernthe part, gets day morethrough sun exposure during the (7– day part, gets less sun exposure and it is conditioned during the whole day through to near midnight (7– 23 hours). On the other hand, the bed zone, situated in the southern part, gets more sun exposure when it is not conditioned. 23 hours). On the other hand, the bed zone, situated in the southern part, gets more sun exposure during the day when it is not conditioned. during the day when it is not conditioned. 4.3. Cooling-Energy Demand 4.3. Cooling-Energy Demand 4.3. Cooling-Energy Demand 4.3.1. Cooling Demand During Normal Operation 4.3.1. Cooling Demand During Normal Operation Figure 9 shows the cooling-energy demands of the living room and bedrooms as well as the total 4.3.1. Cooling Demand During Normal Operation cooling demand of the with normal operation of living the system use of economy cycle). Figure 9 shows the house cooling-energy demands of the room(i.e., and no bedrooms as well as the Figure 9 shows the cooling-energy demands of the living room and bedrooms as well as the As shown, monthly demand of the living roomof is the twice as high as the monthly cooling total coolingthe demand of cooling the house with normal operation system (i.e., no use of economy total cooling of the with normal operation of the system no use of monthly economy demand thedemand bedrooms. Thishouse is understandable theliving cooling demand of (i.e., bedrooms during cycle). Asofshown, the monthly cooling demand ofas the room is twice as high asoccurs the cycle). As shown, the monthly cooling demand of the living room is twice as high as the monthly night time, whenofheat are low. Onisthe other hand, the demand of theofliving roomoccurs is high cooling demand the gains bedrooms. This understandable ascooling the cooling demand bedrooms coolingnight demand ofwhen the bedrooms. This islow. understandable the as cooling demand of bedrooms occurs because of high heat gains during the day: radiation asaswell the cooling internal gains, such gain during time, heat gains are On thegain other hand, the demand of as thethe living during night time, operations. when heat gains areduring low. On other hand, gain the cooling demand of thegains, living from thehigh appliance room is because of high heat gains thethe day: radiation as well as the internal roomasisthe high because of high heat gains during the day: radiation gain as well as the internal gains, such gain from the appliance operations. such as the gain from the appliance operations.
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Figure 9. Monthly cooling energy for living room and bedrooms, and the total (normal operation). Figure 9. Monthly cooling energy for living room and bedrooms, and the total (normal operation).
Monthly cooling energy room and bedrooms, andboth the total (normal operation).low as ItFigure is also9.interesting to note thatfor theliving latent energy demands for zones are relatively It is interesting to note that latent energy demands for both in zones are relatively low as shown inalso Figure 10. This confirms thethe previous brief/general observation Section 4.1. It isinalso interesting note that latent energy demands for both zones are relatively low as shown Figure 10. This to confirms thethe previous brief/general observation in Section 4.1. shown in Figure 10. This confirms the previous brief/general observation in Section 4.1. 2000
MJ MJ Load, Load, Cooling Cooling
2000 1500 1500 1000 1000
TOTAL LOAD
LATENT LOAD
TOTAL LOAD
LATENT LOAD
500 500 0
J F M A M J J A S O N D 0 Figure 10. Monthly total and latent cooling demands for the house (normal operation). J F and M latent A cooling M demands J J for the A house S (normal O N D Figure 10. Monthly total operation).
4.3.2. System’s Electrical Energy Consumption Figure 10. Monthly totalConsumption and latent cooling demands for the house (normal operation). 4.3.2. System’s Electrical Energy In this research, it is assumed that the energy required for desiccant regeneration comes from nonelectrical sources, it such as solar thermal waste required heat, although, during modeling, it was modeled In this research, is assumed that theorenergy for desiccant regeneration comes from 4.3.2. System’s Electrical Energy Consumption using an electrical heater TYPE930. system’s consumption nonelectrical sources, such as solar Therefore, thermal orthe waste heat,electrical-energy although, during modeling, itcomes was In this research, it is assumed that the energy required for desiccant regeneration comes from from the operation of the supply air fan, desiccant wheel, energy-recovery wheel, and evaporative modeled using an electrical heater TYPE930. Therefore, the system's electrical-energy consumption nonelectrical sources, as the solarsupply thermal waste heat, wheel, although, during On modeling, it basis, was coolers.from Figure 11 operation showsuch the monthly electrical-energy consumption of the system. an annual comes the of air or fan, desiccant energy-recovery wheel, and modeled using ancoefficient electrical heater TYPE930. Therefore, the which system's electrical-energy consumption the performance of the system is around 4.0–4.9, is comparable the refrigerative evaporative coolers. Figure 11 show the monthly electrical-energy consumption ofto the system. On an comes the operation of coefficient the supply air system fan, desiccant wheel, energy-recovery wheel, coolingfrom system. a monthly basis, peakof energy consumption occurs during theissummer months of annual basis, theOn performance the is around 4.0–4.9, which comparable toand the evaporative coolers. Figure 11 show the monthly electrical-energy consumption of the system. On an December, January, and February. is expected, as peak high cooling occuroccurs duringduring this period refrigerative cooling system. On This a monthly basis, energy demands consumption the annual basis, thesystem performance of the system isThis around 4.0–4.9, which is comparable to the that require the to workcoefficient longer. summer months of December, January, and February. is expected, as high cooling demands refrigerative cooling system. On a monthly basis, peak energy consumption occurs during the occur during this period that require the system to work longer. summer months of December, January, and February. This is expected, as high cooling demands occur during this period that require the system to work longer.
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System Energy Consumption, MJ Energy Consumption
600
400
200
0 J
F
M
A
M
J
J
A
S
O
N
D
Figure 11. Electrical-energy consumption of the cooling system. Figure 11. Electrical-energy consumption of the cooling system.
It should be stressed that the seemingly rather low COP of the system compared with the results of It should be stressed that the seemingly rather low COP of the system compared with the the recent studies for office buildings [16,17] can be caused by several factors, two of which are briefly results of the recent studies for office buildings [16,17] can be caused by several factors, two of which discussed here, namely: the saturation efficiency of the evaporative cooler (TYPE506), and the sensible are briefly discussed here, namely: the saturation efficiency of the evaporative cooler (TYPE506), and effectiveness of the energy-recovery wheel (TYPE760) used in the model. The saturation efficiency the sensible effectiveness of the energy-recovery wheel (TYPE760) used in the model. The saturation of the evaporative cooler dictates the air outlet temperature and relative humidity (or humidity efficiency of the evaporative cooler dictates the air outlet temperature and relative humidity (or ratio). For instance, the value of 0.85 typically used in the system evaluation [14] will give the outlet humidity ratio). For instance, the value of 0.85 typically used in the system evaluation [14] will give temperature of 16.5 ◦ C from inlet temperature 27.1 ◦ C, or a ∆T of 10.6 K across the evaporative cooler, the outlet temperature of 16.5 °C from inlet temperature 27.1 °C, or a T of 10.6 K across the but with an outlet humidity ratio of 9.6 g/kg. On the other hand, the effectiveness value of 0.65 gives evaporative cooler, but with an outlet humidity ratio of 9.6 g/kg. On the other hand, the effectiveness a ∆T of 8.2 K with a lower humidity ratio of 8 g/kg. In other words, higher effectiveness results in value of 0.65 gives a T of 8.2 K with a lower humidity ratio of 8 g/kg. In other words, higher higher ∆T (a desirable factor from a system-efficiency point of view), but with a higher outlet-humidity effectiveness results in higher T (a desirable factor from a system-efficiency point of view), but with ration (an undesirable factor from a thermal-comfort standpoint). In short, higher effectiveness of the a higher outlet-humidity ration (an undesirable factor from a thermal-comfort standpoint). In short, evaporative cooler results in a more efficient system but with more hours of humidity levels being higher effectiveness of the evaporative cooler results in a more efficient system but with more hours pushed above the acceptable comfort boundary. The same can be said of the effectiveness of the of humidity levels being pushed above the acceptable comfort boundary. The same can be said of the energy-recovery wheel, which affects the inlet and outlet conditions of the evaporative cooler. effectiveness of the energy-recovery wheel, which affects the inlet and outlet conditions of the evaporative cooler. 4.3.3. Humidity Levels in Air-Conditioned Spaces Shown in Figures and 13 are plots ofSpaces humidity ratio vs dry bulb temperature for living room and 4.3.3. Humidity Levels 12 in Air-Conditioned bedrooms, respectively, during the times of cooling demand while the system is operating. As shown, in Figures and 13hours are plots of living humidity dry bulb temperature for living room onlyShown for around 50% of12 cooling for the roomratio doesvsthe humidity ratio fall within the comfort and bedrooms, during theastimes of cooling by demand while the system operating. As boundary, i.e., respectively, below 12 g/kg dry air recommended ASHRAE [23] while the is rest falls beyond shown, only for around 50% of cooling hours for the living room does the humidity ratio fall within the upper boundary. The upper value of humidity ratio is for the room expected to be conditioned by the comfort boundary, i.e., below 12where g/kg dry air velocity as recommended bym/s. ASHRAE while theby rest a conventional refrigerative system the air is below 0.1 In the[23] room cooled an falls beyond the upper boundary. The upper value of humidity ratio is for the room expected to be evaporative cooling system, the cooling effect from higher room air velocity compensates for slightly conditioned by a conventional refrigerative system where thehas airan velocity is below 0.1 m/s. In the higher humidity. However, although the system under study evaporative cooling component, room by an (i.e., evaporative cooling system, the cooling effect frombe higher room velocityto its aircooled conditioning cooling and dehumidification) capability should expected to air be similar compensates for slightly higher humidity. However, although the system under study conventional refrigerative systems. In Figure 13, the number of data plots is much less thanhas thatanin evaporative cooling component, its air conditioning (i.e., cooling and dehumidification) capability Figure 12. This merely indicates that, during bed-time cooling, the number of hours where cooling is should belower expected to be similar to conventional refrigerative systems. In Figure 13, the number of active is compared to that for the living room. data plots is much less than that in Figure 12. This merely indicates that, during bed-time cooling, the number of hours where cooling is active is lower compared to that for the living room.
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Figure 12. Temperature vs.humidity ratio—living room. Figure 12. Temperature vs.humidity ratio—living room. Figure 12. Temperature vs.humidity ratio—living room.
Figure 13. Temperature vs humidity ratio-bedrooms. Figure 13. Temperature vs humidity ratio-bedrooms.
Figure 13.on Temperature vs humidity ratio-bedrooms. 4.3.4. Impact of the Economy Cycle Cooling Energy Consumption and Comfort 4.3.4. Impact of the Economy Cycle on Cooling Energy Consumption and Comfort temperature of theCycle outside is significantly less than theand room temperature, and when 4.3.4.When Impact of the Economy on air Cooling Energy Consumption Comfort When temperature of the outside air is significantly less than the room temperature, when outside humidity levels are acceptable, then the economy cycle can be considered for and reducing outside humidity levels are acceptable, then the economy cycle can be considered for reducing the When temperature of the outside air is significantly less than the room temperature, and when the cooling energy provided to the conditioned space [26]. In this exercise, the economy-cycle cooling energy provided to the conditioned space [26]. In this exercise, the economy-cycle control outsidestrategy humidity levels solely are acceptable, then the economy cycle can be considered for reducing control is based on the temperature of the outside air. The use of humidity sensorsthe in strategy is based solely on the temperature of the outside air. The use of humidity sensors in cooling energy provided to the conditioned space [26]. In this exercise, the economy-cycle control economy-cycle strategies is possible [26], but it may be impractical. In this case, the economy cycle economy-cycle strategies is possible [26], but it may be impractical. In this case, the economy cycle ◦ strategy is based solely on the temperature of the outside air. The use of humidity sensors in is activated when the ambient temperature is 20 C or below, as in Reference [26]. Figure 14 showsis activated whenstrategies the ambient temperature is 20 °C or as inisIn Reference Figure shows economy-cycle is possible [26], but itcycle may be impractical. this case,[26]. the cycle is cooling-energy demands when the economy of below, the system activated. Aseconomy shown,14there is cooling-energy demands when the economy cycle of the system is activated. As shown, there activated when the ambient temperature is 20 °C or below, as in Reference [26]. Figure 14 shows insignificant reduction of cooling-energy demand on a monthly and annual basis compared to theis insignificant reduction cooling-energy demand monthly and annual basis compared to the the cooling-energy demands when the cycle of system is activated. As shown, there is normal operation of theof system. As economy expected, moston (inathe fact: all) economy-mode occurs during normal operation of the system. As expected, most (in fact: all) economy-mode occurs during the insignificant reduction of cooling-energy demand on a monthly and annual basis compared to the night when bedrooms are being conditioned. night when bedrooms aresystem. being conditioned. normal operation of the As expected, most (in fact: all) economy-mode occurs during the The values shown in Figures 14–16 are very much similar to those for normal cycle (Figures 9– night when bedrooms are being conditioned. 11) indicating insignificant impact14–16 of theare economy-cycle operation. The values shown in Figures very much similar to those for normal cycle (Figures 9– 11) indicating insignificant impact of the economy-cycle operation.
Energy, Cooling MJMJ Energy, Cooling Energy, Cooling MJ
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MONTHLY COOLING ENERGY, MJ MONTHLY MONTHLY COOLING COOLING ENERGY, ENERGY, MJ MJ
2000 2000 2000 1500 1500 1500 1000 1000 1000 500 500 500 0 00
J JJ
F M A M J J A S O FF M M AA M M JJ JJ AA SS OO LIVING ZONE BED ZONE TOTAL LIVING BED TOTAL LIVINGZONE ZONE BEDZONE ZONE TOTAL
N NN
D DD
Figure 14. 14.Monthly Monthlycooling cooling energy living room and bedrooms, and the(economy-cycle total (economy-cycle Figure energy forfor living room and bedrooms, and the total mode). Figure mode). Figure 14. 14. Monthly Monthly cooling cooling energy energy for for living living room room and and bedrooms, bedrooms, and and the the total total (economy-cycle (economy-cycle mode). The values shown in Figures 14–16 are very much similar to those for normal cycle (Figures 9–11) mode).
indicating insignificant impact of the economy-cycle operation.
MJMJ Load, Cooling Load, Cooling MJ Load, Cooling
2000 2000 2000 1500 1500 1500 1000 1000 1000
TOTAL LOAD TOTAL TOTALLOAD LOAD
LATENT LOAD LATENT LATENTLOAD LOAD
500 500 500 0 00
J F M A M J J A S O N D JJ FF M A M J J A S O N D M A M J J A S O N D Figure 15. Monthly total and latent cooling demands for the house (economy cycle mode). Figure 15. Monthly total and latent cooling demands for the house (economy cycle mode). Figure Figure15. 15.Monthly Monthlytotal totaland andlatent latentcooling coolingdemands demandsfor forthe thehouse house(economy (economycycle cyclemode). mode).
Consumption Energy Consumption Energy Consumption Energy
600 600 600
System Energy Consumption, MJ System System Energy Energy Consumption, Consumption, MJ MJ
400 400 400
200 200 200
0 00
J F M A M J J A S O N D JJ FF M AA M JJ JJ AA SS OO NN DD M M economy-cycle mode. mode. Figure 16. Electrical-energy consumption of the cooling system during economy-cycle Figure Figure16. 16.Electrical-energy Electrical-energyconsumption consumptionof ofthe thecooling coolingsystem systemduring duringeconomy-cycle economy-cyclemode. mode.
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Increasing the temperature when economy cycle is set from 20 to 22 ◦ C results in insignificant reduction of system energy consumption, from 1902 to 1891 MJ p.a. (for 21 ◦ C set point) and 1856 MJ p.a. (for 22 ◦ C set point), respectively. On the other hand, this rise in economy-cycle set points results in the rooms’ humidity ratio being pushed much further beyond the upper limit prescribed by ASHRAE. Furthermore, anecdotal evidence shows that people sometimes opt to rely on natural ventilation by opening windows and/or doors when the ambient temperature is below 25–26 ◦ C even if that means humidity is slightly or rather high. In such a case, economy-cycle consideration is no longer relevant. 5. Conclusions and Recommendations The performance evaluation of a residential desiccant evaporative cooling system operating in the Brisbane climate zone was carried out using the TRNSYS modelling package. The following are the initial findings of the research: •
•
•
The study based on the TRNSYS supplied weather data, which seem confirmed by other observations, found that the sensible cooling demand is the predominant load, while the monthly latent load during the cooling periods represents about 20% of the total cooling load. This, however, is still significant in terms of providing comfort to the occupants of the house. The number of hours where plots of humidity ratio vs. temperature are within the comfort zone during the cooling season are quite significant. During these hours, occupants can rely on the natural ventilation or economy cycle for thermal provision by allowing outside air entering the room though open windows or doors. Likewise, the number of hours where plots of humidity ratio vs. temperature are outside the comfort zone during the cooling season are equally quite significant. During most of these hours, the proposed desiccant evaporative cooling system can technically provide thermal comfort, and its thermal performance is comparable to refrigerative cooling systems. Specifically, the following is the summary of the findings: –
–
–
–
•
About half of the plots of temperature vs. humidity ratio during system operation fall beyond the upper acceptable boundary. The system’s capability to provide suitable temperature and humidity air levels supplied to the room is dependent on the performance of ‘exhaust components’ of the systems, namely, the evaporative cooling system, energy-recovery system, and heat-regeneration system. The heat required to regenerate the desiccant on the exhaust side can be quite significant. Therefore, the availability of a cheap or waste-heat source is essential in making this system economically viable. It is observed that the admission of some return air to the conditioned space, as normally practiced in conventional cooling systems to save energy, seems to have some, albeit limited, benefit in this system. This is because while the return air reduced the system’s burden on the supply side in terms of temperature and humidity, the exhaust side requires raising the temperature for regeneration purposes. In this particular study, the optimum ratio of return air and supply admitted to the room and that which returns to the exhaust side for treatment is 50:50. For the Brisbane climate, the introduction of an economy cycle seems to not be very prospective since periods in which it is applicable are mostly at night, when people may be expected to opt for natural ventilation.
The values of some of the parameters can significantly affect system performance, especially the humidity level of the conditioned space. These include saturation effectiveness of the evaporative cooler and the energy-recovery device. The proper selection of these values should be considered in the design stage of the system.
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The research scope presented in this paper is limited to analyzing the capability of the system to satisfy the sensible and latent demands of the house being modeled. Author Contributions: R.N. initiated the project and developed the initial TRNSYS model. E.H. assisted in the detailed development of the TRNSYS model and provided inputs and analysis based on the model outcomes. S.J. provided general inputs and comments for the manuscript. Funding: The APC was funded by Central Queensland University. Acknowledgments: Funding for this project was provided by CQ University. The authors also acknowledge the constructive comments on the manuscript of Stephen White of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). Conflicts of Interest: The authors declare no conflict of interest.
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