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Feasibility study of a novel dew point air conditioning system for China building application
Building and Environment 44 (2009) 1990–1999
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Feasibility study of a novel dew point air conditioning system for China building application Xudong Zhao*, Shuang Yang, Zhiyin Duan, Saffa B. Riffat School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2008 Received in revised form 7 February 2009 Accepted 9 February 2009
The paper investigated the feasibility of a novel dew point evaporative cooling for air conditioning of buildings in China regions. The issues involved include analyses of China weather conditions, investigation of availability of water for dew point cooling, and assessment of cooling capacity of the system within various regions of China. It is concluded that the dew point system is suitable for most regions of China, particularly northern and west regions of China where the climate is hot and dry during the summer season. It is less suitable for Guangzhou and Shanghai where climates are hot and humid. However, an air pre-treatment process involving a silica-gel dehumidification will enable the technology to be used for these humid areas. Lower humidity results in a higher difference between the dry bulb and dew point of the air, which benefits the system in terms of enhancing its cooling performance. Tap water has adequate temperature to feed the system for cooling and its consumption rate is in the range 2.6– 3 litres per kWh cooling output. The cooling output of the system ranges from 1.1 to 4.3 W per m3/h air flow rate in China, depending on the region where the system applies. For a unit with 2 kW of cooling output, the required air volume flow rate varies with its application location and is in the range 570– 1800 m3/h. For a 50 m2 building with 60 W/m2 cooling load, if the system operates at working hours, i.e., 09:00 to 17:00 h, its daily water consumption would be in the range of 60–70 litres. Compared with mild or humid climates, the dry and hot climates need less air volume flow rate and less water. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Dew point Evaporative cooling Air conditioning Building Effectiveness China
1. Introduction In China, energy use in buildings accounts for about 30% of its total primary energy consumption. It contributes to similar proportions of the national total carbon emissions. Of this, HVAC (heating, ventilation and air conditioning) systems consume approximately 50% of the building energy [1]. Reducing energy consumption of HVAC systems is therefore important in terms of controlling national carbon emissions. In recent years, frequent summer warm spells, improved insulation of buildings, and growth of indoor facilities have led to an increased requirement for air conditioning of the indoor environment. The conventional mechanical compression air conditioning systems consume huge amount of electrical energy that is largely dependent upon fossil fuel. This mode of air conditioning is, therefore, neither sustainable nor environment-friendly. Evaporative cooling utilises the latent heat of water evaporation to perform air conditioning of buildings, and is therefore a potential
* Corresponding author. Tel.: þ44 115 846 6140; fax: þ44 115 951 3159. E-mail address: [email protected] (X. Zhao). 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.02.003
replacement of existing systems. However, evaporative cooling has encountered several technical difficulties that impede its wide application. Direct evaporative cooling adds moisture to room air which causes thermal discomfort [2]. Indirect evaporative cooling lowers air temperature and avoids adding moisture to the air, but it limits the temperature of supply air to some degrees above the wet bulb of the outdoor air, which is too high to perform air conditioning of buildings [2,3]. The dew point (evaporative) cooling breaks the limit of wet bulb, and allows the supply air to be cooled to a level below the wet bulb and above the dew point of the outdoor air [4,5]. A new type of polygonal exchanger for dew point cooling has been recently developed as the result of the authors’ research, which allows an enhanced dew point effectiveness of up to 85% to be achieved [6]. Advance on dew point cooling technology opens up the opportunity for wide application of evaporative cooling for air conditioning of the buildings in China. Although a great deal of initial research work has been carried out on the dew point cooling technology, a gap still exists between the research results and practical application, which mainly lies in the study of the suitability of the novel dew point technology for China climate and building construction. To narrow this gap, this
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Nomenclature Cp d h M Mdaily Q Qp Qp1 Qp2 t
V Vtarget
specific heat of air, 1.01 kJ/kg C moisture content of air, kg/kg dry air enthalpy of air, kJ/kg dry air water consumption rate per kWh cooling output, litres/kWh daily water consumption for the selected building, litres/day Cooling capacity of the dew point system, W cooling output of the supply air, W internal sensible load taken by the supply air, W fresh air load taken by the supply air, W temperature of air, C
paper investigated the most critical issues related to the dew point cooling application [7], including (1) analysing China weather data to identify the suitability of dew point technology in China buildings; (2) investigating the availability of water resource of China regions and estimating the water consumption rate of the dew point air conditioners; and (3) working out the cooling output potential of the system and air flow rate needed for a typical building in China. 2. Description of the dew point system and its application in buildings 2.1. Dew point heat and mass exchanger A polygonal-sheets-stacked heat and mass exchanger was developed by the authors recently to perform dew point cooling of the buildings, which allows an enhanced dew point effectiveness of up to 85% to be achieved [6]. The polygonal sheets can be stacked together using guides of the same material, as shown in Fig. 1, and one side of each sheet is coated with a waterproof material to avoid water penetration. The intake air is brought into the dry channels from the lower part of the right-hand side of the stack. Operation is as follows: The air flows through the channels and is divided into two parts at the other end of the channels: One part of the air stream keeps moving in the same direction and is finally delivered to the space where cooling is required, and the other part of the air stream is diverted into the adjacent wet channels where the surfaces are wetted by water. The wet channels allow heat to be absorbed through the channel walls by vaporising the water on the surfaces. The air in the wet channels flows in a reverse direction and is finally discharged to atmosphere from the upper part of the right side of the stack. In this design, the dry channels contain both
r rw hdp
1991
air volume flow rate, m3/h air volume flow rate for the target building, m3/h air density, 1.2 kg/m3 density of water, 1 kg/litre dew point effectiveness
Subscripts O outdoor air 0 fresh air after the dehumidifier i indoor air 1 intake air 2 supply air 3 discharging air dp dew point product and working air, and the wet channels take only working air, a division of the intake air. Because of the heat transfer between the dry channels and their adjacent wet channels, the product air in the dry channels will be cooled and the working air in the wet channels will be humidified and heated. Optimisation to the geometrical sizes and operating conditions of the heat exchanger was carried out and the results were presented in Ref. [6]. Detailed design parameters, including channel length and height, discharge-to-intake-air ratio, velocities, pressure loss, exchanger plate materials and water distribution pattern were addressed in an international patent, which is currently under processing. The air treatment process can be illustrated in a Psychrometric Chart, as shown in Fig. 2. Outdoor air with state ‘O’ is initially pretreated using a moisture/temperature controller which is structured by a ventilated silica-gel wheel and allow its moisture content to be lowered to the same level as the indoor air, thus reaching a state ‘0’. The air is then mixed with indoor air with state ‘i’, creating a new air state ‘1’ which is the state of the intake air of the exchanger. The intake air is delivered into the dry channels, where it transfers heat to the adjacent wet channels, and is cooled from state ‘1’ to ‘2’, with no moisture added into the air. Part of the air is delivered to room space for cooling of the space. The remaining air flows into the adjacent wet channel, where it firstly becomes saturated due to absorbing moisture reserved on the channel surface, and then continues to absorb sensible heat and moisture due to heat transfer between the dry and wet channels, which contributes to evaporation of water on the wet surface. The air is finally discharged to the atmosphere at the saturated and hot air streams, defined as state ‘3’. For comfort air conditioning, the moisture level of indoor air could vary in a wide range, with the associated relative humidity
Fig. 1. The polygonal stack exchanger configuration.
1992
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 2. Psychrometric indication of the heat and moisture transfer in a dew point evaporative cooling system.
ranging from 30% to 70%. This would allow the smallest possible moisture removal from the fresh air and minimum energy consumption used for air dehumidification.
Fig. 4. Layout of the stand-alone dew point unit in an office environment.
2.2. Mathematical analyses of the cooling performance of the dew point system The following assumptions have been made to simplify the feasibility analyses, which should be acceptable due to the less strict accuracy requirement of engineering problem. First, as the temperature of air during the evaporative cooling process experiences a relatively small variation, which is usually in the range 10– 40 C, its density has very little change and therefore could be approximately treated as a constant, i.e., 1.2 kg/m3. And secondly, as the evaporative cooling is usually designed for comfort air conditioning which mainly targetspublic (office) and residential buildings where latent heat is very small, the analysis below ignores the existence of this part of the load for the sake of simplicity. This treatment may cause a certain level of error in room air relative humidity, but it may be acceptable as humidity control in these types of buildings is less strict. The cooling capacity of the dew point system can be calculated as follows:
Q ¼ rV2 ðh1 h2 Þ=3:6
(1)
The relationship between states 1, 2 and 3 can be expressed as follows:
h1 h2 ¼
V3 ðh h2 Þ V1 3
(2)
Where the energy brought in by the added water is neglected due to its tiny weighing in comparison with the main air flow.
h2 ¼ h1 hdp h1 hdp
(3)
t2 ¼ t1 hdp t1 tdp
(4)
d2 ¼ d1
(5)
h3 ¼ h2 þ
V1 ðh h2 Þ V3 1
(6)
Obtaining the value of h3, d3 and t3 can be acquired from the psychrometric correlation equations of air accordingly. Water consumption per kWh cooling capacity will be
M ¼ 3600
d3 d1
V3
rw ðh1 h2 Þ V2
(7)
To keep room air distribution in balance, fresh air flow volume should be same as the exhaust air flow volume, i.e., V0 ¼ V3, and the return air flow volume would be same as the supply air flow volume, i.e., Vi ¼ V2. In that case, the cooling capacity of the system can be calculated using the following equation:
Q ¼ Cp rV2 ðt1 t2 Þ=3:6
(8)
If room temperature is ti, the cooling energy used for removing internal sensible load can be written as
Qp1 ¼ Cp rV2 ðti t2 Þ=3:6
(9)
This part of cooling energy is defined as the effective cooling output as it is used to remove internal load. The cooling energy used for removing fresh air load can be written as,
Qp2 ¼ Cp rV2 ðt1 ti Þ=3:6
(10)
For a 2 kW effective cooling output, the required volume flow rate can be calculated as follows:
V2kW ¼
Fig. 3. Schematic of the layout of the stand-alone dew point units in a residential house.
2000V2 Qp1
(11)
Taking a 50 m2 office building space with 60 W/m2 cooling load as an example, if the system operation is limited to daytime, i.e., 09:00 to 17:00 h, then the total cooling energy required would be
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 5. Schematic of the central air handle unit using dew point exchanger and its application in buildings.
Fig. 6. Beijing summer temperature profile: 24-h operation.
1993
1994
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 7. Beijing summer temperature profile: daytime (09:00–17:00 h) operation.
50 60 8/1000 ¼ 24 kWh. The water consumption for the daytime operation would be:
Mdaily ¼ 24M
(12)
2.3. Configuration of the dew point air conditioner and its application in buildings This type of exchanger could be made as stand-alone units which would be positioned at the individual rooms of a house, or spaces in an office building, as shown in Figs. 3 and 4. Alternatively, the exchanger may be integrated into a central air handling unit. This application will allow air to be treated centrally by water evaporation and delivered to the individual room spaces through the pre-set ducting system. If an accurate control of the room air temperature and humidity is required, the process air may need a pre-dehumidification treatment prior entering the dew point exchanger, which could be made using a ventilated silica-gel wheel to perform regeneration and subsequent cooling. This treatment enables a constant dew point for supply air to be obtained and subsequently, a constant cooling capacity to be achieved. The schematic of the system layout is shown in Fig. 5. 3. Analyses of China weather data Weather data relevant to various locations of China were analysed. Seven locations including Harbin, Beijing, Shijiazhuang, Xi’an, Shanghai, Kunming and Guangzhou were selected, and these
present all parts of climate conditions occurred in China regions. Hourly based weather data in the summer season (June to September) of a typical year, including dry bulb, wet bulb and dew point of ambient air, were examined. Temperature differences between dry bulb and dew point as well as dry bulb and wet bulb were calculated. This allowed the average, maximum and minimum values of those temperature items to be obtained, and frequency of temperature occurring in different bands recorded. Three operation schemes, i.e., 24 h, daytime and night-time, were considered and their relevant temperature profiles were generated correspondingly. Figs. 6, 7 and 8 show Beijing temperature profiles in summer season at three operation schemes, i.e., 24-h, daytime and nighttime, respectively. In Beijing summer duration, ambient air temperature is in the range 11–37.4 C, and its average is 24.3 C. In 90% of summer time, the air temperature fell into the temperature band of 15–35 C. The average temperature difference between dry bulb and dew point is 6.1 C, which is 2 C higher than the difference of dry bulb and wet-bulb. In daytime, this temperature difference is as high as 8.3 C, which means that a higher cooling capacity can be achieved in day time than in night. Higher value of temperature difference between the dry-bulb and dew point temperatures results in higher cooling capacity of the dew point system. If the temperature difference were 6 C, and the cooling effectiveness of the dew point system were 0.85, a 5.1 C temperature difference between supply air and intake air would be achieved. This temperature difference would generate a significant amount of cooling energy to conduct air conditioning of the target space.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
1995
Fig. 8. Beijing summer temperature profile: night-time (17:00–09:00 h) operation.
Weather data relevant to other locations of China were investigated but not presented in the form of diagrams due to limitation of the pages. Instead, a summary of weather data is given in Table 1. These form the database used for the design of a dew point system for use in China climate conditions. For most regions of China, the air relative humidity is below 70%, which allows air to be treated by the dew point system directly without the need for prior dehumidification. The drier the air in the region, the better the performance of the dew point cooling system will be. The ideal regions for this application are Harbin, Xi’an, Shijiazhuang and Beijing where the climate remains dry and hot in the summer season. In some regions of China, such as Shanghai and Guangzhou, dew point cooling is unsuitable for use, as the outdoor air relative humidities in these regions exceed 70%, which leaves very little room for utilising the benefit of the dew point cooling, i.e., temperature difference between the dry bulb and dew point. However, implementing a pre-dehumidification device will allow the dew point system to be used in these humid areas. The predehumidification could also be used in other areas in order to obtain the controlled inlet fresh air dew point, enhanced cooling output and controlled room temperature and humidity. 4. Availability of water source, temperature level and volume consumption Owing to its instantaneous supply and easy connection, tap water is the most convenient medium used for the dew point
system. To assess its availability, tap water temperature and volume consumption rate are the greatest concerns which need to be taken into careful consideration. Ideally, tap water temperature should be lower than dew point of the atmosphere, which allows an effective cooling to be achieved when system is in operation. Since tap water is delivered from the water source through the pipe services, and the pipes are embedded at the level of 50–100 cm below the ground, the water temperature will eventually reach the soil temperature at the same depth level. The monthly average water temperature in different locations of China is shown in Table 2 [8]. It was found that tap water temperature is about the same or slightly lower than the dew point of the atmosphere above the
Table 1 Statistical data of dry bulb and difference between dry bulb and dew point for the selected China cities (24 h operation). Location
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming
Dry bulb (DB), C
Difference between DB and DP (dew point), C
Max.
Min.
Ave.
Max.
Min.
Ave.
31.1 37.4 40.0 37.9 38 27.2
0.9 11.0 11.1 13.3 14 13.6
19.7 24.3 24.7 24.6 26 19.6
28.5 37.4 31.5 26.9 19.4 17.9
0 0 0.2 0 0 0
5.8 6.1 6.8 6.5 3.8 3.9
1996
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Table 2 Monthly average water temperature in the selected cities of China ( C). Month
June
July
August
September
5.58 20.16 19.49 14.71 19.49 15.72 22.58
10.09 24.64 21.23 17.68 21.23 16.98 24.41
13.22 26.01 21.66 19.74 21.66 17.86 25.35
13.95 23.68 20.57 20.22 20.57 18.06 25.61
Location Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
Table 3 Average water consumption of the dew point system in different locations of China. Country City
Water consumption Tap water Daily water consumption rate (average), temperature for the targeted building litre/kWh (average), % (average), litre
China
2.82 2.67 2.92 3 2.75 2.96 2.67
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
10.71 23.62 20.74 18.09 20.74 17.16 24.45
68 64 70 72 66 71 64.1
earth. This allows dew point cooling to be carried out in an effective way. In terms of water volume consumption, a calculation can be made based on the following assumptions (1) discharged-to-totalair ratio is 0.5; and (2) dew point effectiveness is 0.85. The calculation yields the water consumption rates in various regions of the whole range of China, as shown in Table 3. In all regions of China, water consumption rates are in the range 2.6–3 litres/kWh. It should be addressed that the calculation is only for the case of 50% supply and 50% discharging. If this ratio changes, the results will be changed accordingly. Taking a 50 m2 office building space with average cooling load of 60 W/m2 as an example, if the system is running only during the day, i.e., 09:00 to 17:00 h, a calculation was made to work out its daily water consumption, which was found in the range of 64– 72 litres per day. Dry and hot climate regions usually consume more water than that of mild and humid regions.
5. Cooling capacity and air flow rate scale of the dew point cooling system Cooling capacity of the dew point system were calculated using Eqs. (1)–(9). Based on 1 m3/h of air supply/discharge flow rate, the calculations yielded the system’s total cooling capacity, as well as
Fig. 9. Cooling capacity per unit air volume flow rate: Beijing 24-h operation.
Fig. 10. Cooling capacity per unit air volume flow rate: Beijing daytime operation.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
1997
Fig. 11. Cooling capacity per unit air volume flow rate: Beijing night-time operation.
the ventilation load associated with the system operation, which is the energy used for bringing temperature of fresh air from outdoor down to indoor level. As a result, the net cooling output, known as the effective cooling capacity, is a figure of total cooling capacity subtracted by the ventilation load. The effective cooling capacity is dependent on the weather condition, particularly dry bulb, wet bulb and dew point of the ambient air, and therefore varies from location to location. The results for Beijing are shown in Figs. 9–11. It was found that in Beijing summer duration, the effective cooling capacity is in the range 0–9.5 W per m3/h air flow rate, and its average is 2.97. In 90% of summer time, the effective cooling capacity fell into the cooling band of 1–5 W per m3/h air flow. Night cooling capacity is slightly higher than that in the daytime, as the ventilation load at night is lower than that in daytime. A summary of average cooling capacity of China cities over the summer duration is given in Table 4. Comparison among the selected China cities indicated that the effective unit cooling capacity of the dew point system varies from city to city. Higher ambient temperature leads to a lower effective cooling capacity as a larger part of cooling energy generated from the system is used for removing the ventilation load. Higher ambient humidity also reduces the system’s cooling capacity due to the smaller temperature difference between its dry bulb and dew point. In China, Harbin has the highest effective cooling capacity, which is as high as
Table 4 Cooling capacity, air flow rate and moisture removal issues in the selected cities. Country
City
Moisture removal frequency, %
Average moisture removal, g/kg dry air
Cooling capacity, W per m3/h
Air flow rate for 2 kW effective cooling output
China China China China China China China
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
0 0 0 0 55 0 87
0 0 0 0 1.24 0 1.91
4.23 2.98 3.02 2.96 1.72 3.74 1.14
540 840 802 771 1400 568 1870
4.23 W per m3/h air flow, and therefore is the place most suitable for application of the dew point system. Kunming, Shijiazhuang and Beijing have cooling capacities of 3.74, 3.02 and 3 W per m3/h respectively, which are slightly less than in Harbin. Shanghai and Guangzhou get the lowest cooling capacities, i.e., 1.72 and 1.14 W per m3/h, which means that the system is less effective in these two cities. For a fixed effective cooling output of 2 kW, and the required volume flow rate can be calculated using Eq. (11). The results for the selected China cities are also presented in Table 4. The cities with
Fig. 12. Air flow rate variation with time: Beijing 24-h operation.
1998
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 13. Air flow rate variation with time: Beijing daytime operation.
Fig. 14. Air flow rate variation with time: Beijing night-time operation.
lower cooling capacity require higher volume flow rate in order to meet the required cooling requirement, i.e., 2 kW fixed effective cooling. In China, volume flow rate for 2 kW cooling varies from city to city, and is in the range 540–1900 m3/h; Harbin is the lowest, followed by Kunming, Xi’an, Shijiazhuang, Beijing and Shanghai, and Guangzhou gets the highest level of air volume flow requirement. The air volume flow rate variation with time in Beijing area is presented in Figs. 12–14, representing three different operation schemes, i.e., 24-h, daytime and night-time operation respectively. An investigation was also made into the moisture removal issue. To retain a comfortable indoor air condition, the intake air should be kept at a humidity level of 70% and below. This is unachievable in Guangzhou and Shanghai in China, and a moisture removal practice should be taken into consideration for this particular application. The time frequency and average volume for moisture removal were calculated and these results are also shown in Table 4. In Guangzhou, 87% of summer time needs moisture removal and its average volume is 1.91 g/kg dry air, while in Shanghai, time frequency for moisture removal is 55% and its average volume is 1.24 g/kg dry air. 6. Conclusions The dew point air conditioning system is suitable for most China regions, particularly Harbin, Kunming, Shijiazhuang, Beijing and Xian, where hot and dry climate conditions are the common situations. The system is unsuitable for some regions in China, such as Shanghai and Guangzhou, where the air is too humid to be dealt with. However, implementing a pre-dehumidification device will allow the dew point system to be used in these humid areas.
Lower relative humidity results in higher temperature differences between the dry bulb and dew point temperatures, and higher cooling capacity of the dew point system. If air is at a relative humidity of 70% or below, the dew point system could be used for cooling of buildings. Tap water can be easily used to support cooling of the dew point system. Its temperature is about the same and slightly lower than the dew point of the ambient air, which ensures the effectiveness of cooling. The water consumption rate varies with the region where the system applies, but is usually in the range 2.6–3 litres/kWh output. The cooling output of the dew point system varies with the region where the system applies, but is usually in the range 1–5 W per m3/h air flow. Higher ambient temperature leads to a lower effective cooling capacity as a larger part of cooling energy generated from the system is used for removing the ventilation load. Higher ambient humidity also reduces the system’s cooling capacity due to the smaller temperature difference between its dry bulb and dew point. For a fixed effective cooling output, e.g., 2 kW, the required volume flow rate varies from location to location. The cities with lower cooling capacity require higher volume flow rates. In China, volume flow rate for 2 kW cooling varies from 540 to 1900 m3/h. To retain a comfortable indoor air condition, the intake air should be kept at a humidity level of 70% and below. This is unachievable in Guangzhou and Shanghai in China, and a moisture removal practice should be taken into consideration for this particular application. In Guangzhou, the time frequency and average volume for moisture removal are 87% and 1.91 g/kg dry air, respectively. In Shanghai, the associated figures would be 55% and 1.24 g/kg dry air.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Taking a 50 m2 building with 60 W/m2 cooling load as an example, if the dew point system is in operation in the daytime, i.e., 09:00 to 17:00 h, its daily water consumption would be in the range 64–72 litres per day. The system consumes more water in dry and hot climate regions than in mild and humid regions. To allow a controlled intake air dew point, increased cooling capacity, and controlled room temperature and humidity, an air pre-dehumidification treatment would be needed. This could be accomplished by using a ventilated silica-gel wheel which allows simultaneous dehumidification and regeneration. Acknowledgements The authors would like to acknowledge thanks for the support of ICUK (Innovation China UK) to the project.
1999
References [1] Ortiz J, Pout C. Go figure. Building Services Journals (UK) November 2006:38–40. [2] http://www.idalex.com/technology/how_it_works_-_engineering_perspective. htm. [3] Stoitchkov NJ, Dimitrov GI. Effectiveness of crossflow plate heat exchanger for indirect evaporative cooling. International Journal of Refrigeration 1998;21(6):463–71. [4] Maisotsenko V, Gillian LE, Heaton TL, Gillan AD. Method and plate apparatus for dew point evaporative cooler, United State Patent 6,581,402, June 24, 2003. [5] Coolerado. CooleradoHMX (heat and mass exchanger) brochure. Arvada, CO: Coolerado Corporation; 2006. [6] Zhao X, Li JM, Riffat SB. Numerical study of a novel counter-flow heat and mass exchanger for dew point evaporative cooling. Applied Thermal Engineering 2008;28:1942–51. [7] Zhao X. Feasibility study of novel dew point air conditioning for the UK and China buildings. Research Proposal to ICUK Partnership Grant; June 2008. [8] http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data3.cfm/ region¼6_europe_wmo_region_6/country¼GBR/cname¼United%20Kingdom.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Feasibility study of a novel dew point air conditioning system for China building application Xudong Zhao*, Shuang Yang, Zhiyin Duan, Saffa B. Riffat School of the Built Environment, University of Nottingham, University Park, Nottingham NG7 2RD, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2008 Received in revised form 7 February 2009 Accepted 9 February 2009
The paper investigated the feasibility of a novel dew point evaporative cooling for air conditioning of buildings in China regions. The issues involved include analyses of China weather conditions, investigation of availability of water for dew point cooling, and assessment of cooling capacity of the system within various regions of China. It is concluded that the dew point system is suitable for most regions of China, particularly northern and west regions of China where the climate is hot and dry during the summer season. It is less suitable for Guangzhou and Shanghai where climates are hot and humid. However, an air pre-treatment process involving a silica-gel dehumidification will enable the technology to be used for these humid areas. Lower humidity results in a higher difference between the dry bulb and dew point of the air, which benefits the system in terms of enhancing its cooling performance. Tap water has adequate temperature to feed the system for cooling and its consumption rate is in the range 2.6– 3 litres per kWh cooling output. The cooling output of the system ranges from 1.1 to 4.3 W per m3/h air flow rate in China, depending on the region where the system applies. For a unit with 2 kW of cooling output, the required air volume flow rate varies with its application location and is in the range 570– 1800 m3/h. For a 50 m2 building with 60 W/m2 cooling load, if the system operates at working hours, i.e., 09:00 to 17:00 h, its daily water consumption would be in the range of 60–70 litres. Compared with mild or humid climates, the dry and hot climates need less air volume flow rate and less water. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Dew point Evaporative cooling Air conditioning Building Effectiveness China
1. Introduction In China, energy use in buildings accounts for about 30% of its total primary energy consumption. It contributes to similar proportions of the national total carbon emissions. Of this, HVAC (heating, ventilation and air conditioning) systems consume approximately 50% of the building energy [1]. Reducing energy consumption of HVAC systems is therefore important in terms of controlling national carbon emissions. In recent years, frequent summer warm spells, improved insulation of buildings, and growth of indoor facilities have led to an increased requirement for air conditioning of the indoor environment. The conventional mechanical compression air conditioning systems consume huge amount of electrical energy that is largely dependent upon fossil fuel. This mode of air conditioning is, therefore, neither sustainable nor environment-friendly. Evaporative cooling utilises the latent heat of water evaporation to perform air conditioning of buildings, and is therefore a potential
* Corresponding author. Tel.: þ44 115 846 6140; fax: þ44 115 951 3159. E-mail address: [email protected] (X. Zhao). 0360-1323/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2009.02.003
replacement of existing systems. However, evaporative cooling has encountered several technical difficulties that impede its wide application. Direct evaporative cooling adds moisture to room air which causes thermal discomfort [2]. Indirect evaporative cooling lowers air temperature and avoids adding moisture to the air, but it limits the temperature of supply air to some degrees above the wet bulb of the outdoor air, which is too high to perform air conditioning of buildings [2,3]. The dew point (evaporative) cooling breaks the limit of wet bulb, and allows the supply air to be cooled to a level below the wet bulb and above the dew point of the outdoor air [4,5]. A new type of polygonal exchanger for dew point cooling has been recently developed as the result of the authors’ research, which allows an enhanced dew point effectiveness of up to 85% to be achieved [6]. Advance on dew point cooling technology opens up the opportunity for wide application of evaporative cooling for air conditioning of the buildings in China. Although a great deal of initial research work has been carried out on the dew point cooling technology, a gap still exists between the research results and practical application, which mainly lies in the study of the suitability of the novel dew point technology for China climate and building construction. To narrow this gap, this
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Nomenclature Cp d h M Mdaily Q Qp Qp1 Qp2 t
V Vtarget
specific heat of air, 1.01 kJ/kg C moisture content of air, kg/kg dry air enthalpy of air, kJ/kg dry air water consumption rate per kWh cooling output, litres/kWh daily water consumption for the selected building, litres/day Cooling capacity of the dew point system, W cooling output of the supply air, W internal sensible load taken by the supply air, W fresh air load taken by the supply air, W temperature of air, C
paper investigated the most critical issues related to the dew point cooling application [7], including (1) analysing China weather data to identify the suitability of dew point technology in China buildings; (2) investigating the availability of water resource of China regions and estimating the water consumption rate of the dew point air conditioners; and (3) working out the cooling output potential of the system and air flow rate needed for a typical building in China. 2. Description of the dew point system and its application in buildings 2.1. Dew point heat and mass exchanger A polygonal-sheets-stacked heat and mass exchanger was developed by the authors recently to perform dew point cooling of the buildings, which allows an enhanced dew point effectiveness of up to 85% to be achieved [6]. The polygonal sheets can be stacked together using guides of the same material, as shown in Fig. 1, and one side of each sheet is coated with a waterproof material to avoid water penetration. The intake air is brought into the dry channels from the lower part of the right-hand side of the stack. Operation is as follows: The air flows through the channels and is divided into two parts at the other end of the channels: One part of the air stream keeps moving in the same direction and is finally delivered to the space where cooling is required, and the other part of the air stream is diverted into the adjacent wet channels where the surfaces are wetted by water. The wet channels allow heat to be absorbed through the channel walls by vaporising the water on the surfaces. The air in the wet channels flows in a reverse direction and is finally discharged to atmosphere from the upper part of the right side of the stack. In this design, the dry channels contain both
r rw hdp
1991
air volume flow rate, m3/h air volume flow rate for the target building, m3/h air density, 1.2 kg/m3 density of water, 1 kg/litre dew point effectiveness
Subscripts O outdoor air 0 fresh air after the dehumidifier i indoor air 1 intake air 2 supply air 3 discharging air dp dew point product and working air, and the wet channels take only working air, a division of the intake air. Because of the heat transfer between the dry channels and their adjacent wet channels, the product air in the dry channels will be cooled and the working air in the wet channels will be humidified and heated. Optimisation to the geometrical sizes and operating conditions of the heat exchanger was carried out and the results were presented in Ref. [6]. Detailed design parameters, including channel length and height, discharge-to-intake-air ratio, velocities, pressure loss, exchanger plate materials and water distribution pattern were addressed in an international patent, which is currently under processing. The air treatment process can be illustrated in a Psychrometric Chart, as shown in Fig. 2. Outdoor air with state ‘O’ is initially pretreated using a moisture/temperature controller which is structured by a ventilated silica-gel wheel and allow its moisture content to be lowered to the same level as the indoor air, thus reaching a state ‘0’. The air is then mixed with indoor air with state ‘i’, creating a new air state ‘1’ which is the state of the intake air of the exchanger. The intake air is delivered into the dry channels, where it transfers heat to the adjacent wet channels, and is cooled from state ‘1’ to ‘2’, with no moisture added into the air. Part of the air is delivered to room space for cooling of the space. The remaining air flows into the adjacent wet channel, where it firstly becomes saturated due to absorbing moisture reserved on the channel surface, and then continues to absorb sensible heat and moisture due to heat transfer between the dry and wet channels, which contributes to evaporation of water on the wet surface. The air is finally discharged to the atmosphere at the saturated and hot air streams, defined as state ‘3’. For comfort air conditioning, the moisture level of indoor air could vary in a wide range, with the associated relative humidity
Fig. 1. The polygonal stack exchanger configuration.
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X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 2. Psychrometric indication of the heat and moisture transfer in a dew point evaporative cooling system.
ranging from 30% to 70%. This would allow the smallest possible moisture removal from the fresh air and minimum energy consumption used for air dehumidification.
Fig. 4. Layout of the stand-alone dew point unit in an office environment.
2.2. Mathematical analyses of the cooling performance of the dew point system The following assumptions have been made to simplify the feasibility analyses, which should be acceptable due to the less strict accuracy requirement of engineering problem. First, as the temperature of air during the evaporative cooling process experiences a relatively small variation, which is usually in the range 10– 40 C, its density has very little change and therefore could be approximately treated as a constant, i.e., 1.2 kg/m3. And secondly, as the evaporative cooling is usually designed for comfort air conditioning which mainly targetspublic (office) and residential buildings where latent heat is very small, the analysis below ignores the existence of this part of the load for the sake of simplicity. This treatment may cause a certain level of error in room air relative humidity, but it may be acceptable as humidity control in these types of buildings is less strict. The cooling capacity of the dew point system can be calculated as follows:
Q ¼ rV2 ðh1 h2 Þ=3:6
(1)
The relationship between states 1, 2 and 3 can be expressed as follows:
h1 h2 ¼
V3 ðh h2 Þ V1 3
(2)
Where the energy brought in by the added water is neglected due to its tiny weighing in comparison with the main air flow.
h2 ¼ h1 hdp h1 hdp
(3)
t2 ¼ t1 hdp t1 tdp
(4)
d2 ¼ d1
(5)
h3 ¼ h2 þ
V1 ðh h2 Þ V3 1
(6)
Obtaining the value of h3, d3 and t3 can be acquired from the psychrometric correlation equations of air accordingly. Water consumption per kWh cooling capacity will be
M ¼ 3600
d3 d1
V3
rw ðh1 h2 Þ V2
(7)
To keep room air distribution in balance, fresh air flow volume should be same as the exhaust air flow volume, i.e., V0 ¼ V3, and the return air flow volume would be same as the supply air flow volume, i.e., Vi ¼ V2. In that case, the cooling capacity of the system can be calculated using the following equation:
Q ¼ Cp rV2 ðt1 t2 Þ=3:6
(8)
If room temperature is ti, the cooling energy used for removing internal sensible load can be written as
Qp1 ¼ Cp rV2 ðti t2 Þ=3:6
(9)
This part of cooling energy is defined as the effective cooling output as it is used to remove internal load. The cooling energy used for removing fresh air load can be written as,
Qp2 ¼ Cp rV2 ðt1 ti Þ=3:6
(10)
For a 2 kW effective cooling output, the required volume flow rate can be calculated as follows:
V2kW ¼
Fig. 3. Schematic of the layout of the stand-alone dew point units in a residential house.
2000V2 Qp1
(11)
Taking a 50 m2 office building space with 60 W/m2 cooling load as an example, if the system operation is limited to daytime, i.e., 09:00 to 17:00 h, then the total cooling energy required would be
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 5. Schematic of the central air handle unit using dew point exchanger and its application in buildings.
Fig. 6. Beijing summer temperature profile: 24-h operation.
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X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 7. Beijing summer temperature profile: daytime (09:00–17:00 h) operation.
50 60 8/1000 ¼ 24 kWh. The water consumption for the daytime operation would be:
Mdaily ¼ 24M
(12)
2.3. Configuration of the dew point air conditioner and its application in buildings This type of exchanger could be made as stand-alone units which would be positioned at the individual rooms of a house, or spaces in an office building, as shown in Figs. 3 and 4. Alternatively, the exchanger may be integrated into a central air handling unit. This application will allow air to be treated centrally by water evaporation and delivered to the individual room spaces through the pre-set ducting system. If an accurate control of the room air temperature and humidity is required, the process air may need a pre-dehumidification treatment prior entering the dew point exchanger, which could be made using a ventilated silica-gel wheel to perform regeneration and subsequent cooling. This treatment enables a constant dew point for supply air to be obtained and subsequently, a constant cooling capacity to be achieved. The schematic of the system layout is shown in Fig. 5. 3. Analyses of China weather data Weather data relevant to various locations of China were analysed. Seven locations including Harbin, Beijing, Shijiazhuang, Xi’an, Shanghai, Kunming and Guangzhou were selected, and these
present all parts of climate conditions occurred in China regions. Hourly based weather data in the summer season (June to September) of a typical year, including dry bulb, wet bulb and dew point of ambient air, were examined. Temperature differences between dry bulb and dew point as well as dry bulb and wet bulb were calculated. This allowed the average, maximum and minimum values of those temperature items to be obtained, and frequency of temperature occurring in different bands recorded. Three operation schemes, i.e., 24 h, daytime and night-time, were considered and their relevant temperature profiles were generated correspondingly. Figs. 6, 7 and 8 show Beijing temperature profiles in summer season at three operation schemes, i.e., 24-h, daytime and nighttime, respectively. In Beijing summer duration, ambient air temperature is in the range 11–37.4 C, and its average is 24.3 C. In 90% of summer time, the air temperature fell into the temperature band of 15–35 C. The average temperature difference between dry bulb and dew point is 6.1 C, which is 2 C higher than the difference of dry bulb and wet-bulb. In daytime, this temperature difference is as high as 8.3 C, which means that a higher cooling capacity can be achieved in day time than in night. Higher value of temperature difference between the dry-bulb and dew point temperatures results in higher cooling capacity of the dew point system. If the temperature difference were 6 C, and the cooling effectiveness of the dew point system were 0.85, a 5.1 C temperature difference between supply air and intake air would be achieved. This temperature difference would generate a significant amount of cooling energy to conduct air conditioning of the target space.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
1995
Fig. 8. Beijing summer temperature profile: night-time (17:00–09:00 h) operation.
Weather data relevant to other locations of China were investigated but not presented in the form of diagrams due to limitation of the pages. Instead, a summary of weather data is given in Table 1. These form the database used for the design of a dew point system for use in China climate conditions. For most regions of China, the air relative humidity is below 70%, which allows air to be treated by the dew point system directly without the need for prior dehumidification. The drier the air in the region, the better the performance of the dew point cooling system will be. The ideal regions for this application are Harbin, Xi’an, Shijiazhuang and Beijing where the climate remains dry and hot in the summer season. In some regions of China, such as Shanghai and Guangzhou, dew point cooling is unsuitable for use, as the outdoor air relative humidities in these regions exceed 70%, which leaves very little room for utilising the benefit of the dew point cooling, i.e., temperature difference between the dry bulb and dew point. However, implementing a pre-dehumidification device will allow the dew point system to be used in these humid areas. The predehumidification could also be used in other areas in order to obtain the controlled inlet fresh air dew point, enhanced cooling output and controlled room temperature and humidity. 4. Availability of water source, temperature level and volume consumption Owing to its instantaneous supply and easy connection, tap water is the most convenient medium used for the dew point
system. To assess its availability, tap water temperature and volume consumption rate are the greatest concerns which need to be taken into careful consideration. Ideally, tap water temperature should be lower than dew point of the atmosphere, which allows an effective cooling to be achieved when system is in operation. Since tap water is delivered from the water source through the pipe services, and the pipes are embedded at the level of 50–100 cm below the ground, the water temperature will eventually reach the soil temperature at the same depth level. The monthly average water temperature in different locations of China is shown in Table 2 [8]. It was found that tap water temperature is about the same or slightly lower than the dew point of the atmosphere above the
Table 1 Statistical data of dry bulb and difference between dry bulb and dew point for the selected China cities (24 h operation). Location
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming
Dry bulb (DB), C
Difference between DB and DP (dew point), C
Max.
Min.
Ave.
Max.
Min.
Ave.
31.1 37.4 40.0 37.9 38 27.2
0.9 11.0 11.1 13.3 14 13.6
19.7 24.3 24.7 24.6 26 19.6
28.5 37.4 31.5 26.9 19.4 17.9
0 0 0.2 0 0 0
5.8 6.1 6.8 6.5 3.8 3.9
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Table 2 Monthly average water temperature in the selected cities of China ( C). Month
June
July
August
September
5.58 20.16 19.49 14.71 19.49 15.72 22.58
10.09 24.64 21.23 17.68 21.23 16.98 24.41
13.22 26.01 21.66 19.74 21.66 17.86 25.35
13.95 23.68 20.57 20.22 20.57 18.06 25.61
Location Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
Table 3 Average water consumption of the dew point system in different locations of China. Country City
Water consumption Tap water Daily water consumption rate (average), temperature for the targeted building litre/kWh (average), % (average), litre
China
2.82 2.67 2.92 3 2.75 2.96 2.67
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
10.71 23.62 20.74 18.09 20.74 17.16 24.45
68 64 70 72 66 71 64.1
earth. This allows dew point cooling to be carried out in an effective way. In terms of water volume consumption, a calculation can be made based on the following assumptions (1) discharged-to-totalair ratio is 0.5; and (2) dew point effectiveness is 0.85. The calculation yields the water consumption rates in various regions of the whole range of China, as shown in Table 3. In all regions of China, water consumption rates are in the range 2.6–3 litres/kWh. It should be addressed that the calculation is only for the case of 50% supply and 50% discharging. If this ratio changes, the results will be changed accordingly. Taking a 50 m2 office building space with average cooling load of 60 W/m2 as an example, if the system is running only during the day, i.e., 09:00 to 17:00 h, a calculation was made to work out its daily water consumption, which was found in the range of 64– 72 litres per day. Dry and hot climate regions usually consume more water than that of mild and humid regions.
5. Cooling capacity and air flow rate scale of the dew point cooling system Cooling capacity of the dew point system were calculated using Eqs. (1)–(9). Based on 1 m3/h of air supply/discharge flow rate, the calculations yielded the system’s total cooling capacity, as well as
Fig. 9. Cooling capacity per unit air volume flow rate: Beijing 24-h operation.
Fig. 10. Cooling capacity per unit air volume flow rate: Beijing daytime operation.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
1997
Fig. 11. Cooling capacity per unit air volume flow rate: Beijing night-time operation.
the ventilation load associated with the system operation, which is the energy used for bringing temperature of fresh air from outdoor down to indoor level. As a result, the net cooling output, known as the effective cooling capacity, is a figure of total cooling capacity subtracted by the ventilation load. The effective cooling capacity is dependent on the weather condition, particularly dry bulb, wet bulb and dew point of the ambient air, and therefore varies from location to location. The results for Beijing are shown in Figs. 9–11. It was found that in Beijing summer duration, the effective cooling capacity is in the range 0–9.5 W per m3/h air flow rate, and its average is 2.97. In 90% of summer time, the effective cooling capacity fell into the cooling band of 1–5 W per m3/h air flow. Night cooling capacity is slightly higher than that in the daytime, as the ventilation load at night is lower than that in daytime. A summary of average cooling capacity of China cities over the summer duration is given in Table 4. Comparison among the selected China cities indicated that the effective unit cooling capacity of the dew point system varies from city to city. Higher ambient temperature leads to a lower effective cooling capacity as a larger part of cooling energy generated from the system is used for removing the ventilation load. Higher ambient humidity also reduces the system’s cooling capacity due to the smaller temperature difference between its dry bulb and dew point. In China, Harbin has the highest effective cooling capacity, which is as high as
Table 4 Cooling capacity, air flow rate and moisture removal issues in the selected cities. Country
City
Moisture removal frequency, %
Average moisture removal, g/kg dry air
Cooling capacity, W per m3/h
Air flow rate for 2 kW effective cooling output
China China China China China China China
Harbin Beijing Shijiazhuang Xi’an Shanghai Kunming Guangzhou
0 0 0 0 55 0 87
0 0 0 0 1.24 0 1.91
4.23 2.98 3.02 2.96 1.72 3.74 1.14
540 840 802 771 1400 568 1870
4.23 W per m3/h air flow, and therefore is the place most suitable for application of the dew point system. Kunming, Shijiazhuang and Beijing have cooling capacities of 3.74, 3.02 and 3 W per m3/h respectively, which are slightly less than in Harbin. Shanghai and Guangzhou get the lowest cooling capacities, i.e., 1.72 and 1.14 W per m3/h, which means that the system is less effective in these two cities. For a fixed effective cooling output of 2 kW, and the required volume flow rate can be calculated using Eq. (11). The results for the selected China cities are also presented in Table 4. The cities with
Fig. 12. Air flow rate variation with time: Beijing 24-h operation.
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X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Fig. 13. Air flow rate variation with time: Beijing daytime operation.
Fig. 14. Air flow rate variation with time: Beijing night-time operation.
lower cooling capacity require higher volume flow rate in order to meet the required cooling requirement, i.e., 2 kW fixed effective cooling. In China, volume flow rate for 2 kW cooling varies from city to city, and is in the range 540–1900 m3/h; Harbin is the lowest, followed by Kunming, Xi’an, Shijiazhuang, Beijing and Shanghai, and Guangzhou gets the highest level of air volume flow requirement. The air volume flow rate variation with time in Beijing area is presented in Figs. 12–14, representing three different operation schemes, i.e., 24-h, daytime and night-time operation respectively. An investigation was also made into the moisture removal issue. To retain a comfortable indoor air condition, the intake air should be kept at a humidity level of 70% and below. This is unachievable in Guangzhou and Shanghai in China, and a moisture removal practice should be taken into consideration for this particular application. The time frequency and average volume for moisture removal were calculated and these results are also shown in Table 4. In Guangzhou, 87% of summer time needs moisture removal and its average volume is 1.91 g/kg dry air, while in Shanghai, time frequency for moisture removal is 55% and its average volume is 1.24 g/kg dry air. 6. Conclusions The dew point air conditioning system is suitable for most China regions, particularly Harbin, Kunming, Shijiazhuang, Beijing and Xian, where hot and dry climate conditions are the common situations. The system is unsuitable for some regions in China, such as Shanghai and Guangzhou, where the air is too humid to be dealt with. However, implementing a pre-dehumidification device will allow the dew point system to be used in these humid areas.
Lower relative humidity results in higher temperature differences between the dry bulb and dew point temperatures, and higher cooling capacity of the dew point system. If air is at a relative humidity of 70% or below, the dew point system could be used for cooling of buildings. Tap water can be easily used to support cooling of the dew point system. Its temperature is about the same and slightly lower than the dew point of the ambient air, which ensures the effectiveness of cooling. The water consumption rate varies with the region where the system applies, but is usually in the range 2.6–3 litres/kWh output. The cooling output of the dew point system varies with the region where the system applies, but is usually in the range 1–5 W per m3/h air flow. Higher ambient temperature leads to a lower effective cooling capacity as a larger part of cooling energy generated from the system is used for removing the ventilation load. Higher ambient humidity also reduces the system’s cooling capacity due to the smaller temperature difference between its dry bulb and dew point. For a fixed effective cooling output, e.g., 2 kW, the required volume flow rate varies from location to location. The cities with lower cooling capacity require higher volume flow rates. In China, volume flow rate for 2 kW cooling varies from 540 to 1900 m3/h. To retain a comfortable indoor air condition, the intake air should be kept at a humidity level of 70% and below. This is unachievable in Guangzhou and Shanghai in China, and a moisture removal practice should be taken into consideration for this particular application. In Guangzhou, the time frequency and average volume for moisture removal are 87% and 1.91 g/kg dry air, respectively. In Shanghai, the associated figures would be 55% and 1.24 g/kg dry air.
X. Zhao et al. / Building and Environment 44 (2009) 1990–1999
Taking a 50 m2 building with 60 W/m2 cooling load as an example, if the dew point system is in operation in the daytime, i.e., 09:00 to 17:00 h, its daily water consumption would be in the range 64–72 litres per day. The system consumes more water in dry and hot climate regions than in mild and humid regions. To allow a controlled intake air dew point, increased cooling capacity, and controlled room temperature and humidity, an air pre-dehumidification treatment would be needed. This could be accomplished by using a ventilated silica-gel wheel which allows simultaneous dehumidification and regeneration. Acknowledgements The authors would like to acknowledge thanks for the support of ICUK (Innovation China UK) to the project.
1999
References [1] Ortiz J, Pout C. Go figure. Building Services Journals (UK) November 2006:38–40. [2] http://www.idalex.com/technology/how_it_works_-_engineering_perspective. htm. [3] Stoitchkov NJ, Dimitrov GI. Effectiveness of crossflow plate heat exchanger for indirect evaporative cooling. International Journal of Refrigeration 1998;21(6):463–71. [4] Maisotsenko V, Gillian LE, Heaton TL, Gillan AD. Method and plate apparatus for dew point evaporative cooler, United State Patent 6,581,402, June 24, 2003. [5] Coolerado. CooleradoHMX (heat and mass exchanger) brochure. Arvada, CO: Coolerado Corporation; 2006. [6] Zhao X, Li JM, Riffat SB. Numerical study of a novel counter-flow heat and mass exchanger for dew point evaporative cooling. Applied Thermal Engineering 2008;28:1942–51. [7] Zhao X. Feasibility study of novel dew point air conditioning for the UK and China buildings. Research Proposal to ICUK Partnership Grant; June 2008. [8] http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data3.cfm/ region¼6_europe_wmo_region_6/country¼GBR/cname¼United%20Kingdom.