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Theoretical Analysis of Passive Lateral Ventilation Evaporative Cooling Based on the Capillary Action
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Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
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Energy Procedia 158 Energy Procedia 00(2019) (2017)3177–3183 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Theoretical Analysis of Passive Lateral Ventilation Evaporative Theoretical of Passive Lateral Ventilation Evaporative TheAnalysis 15th International Symposium on District Heating and Cooling Cooling Based on the Capillary Action Cooling Based on the Capillary Action AssessingRuixin the feasibility of using the heat demand-outdoor Li*, Nannan Hao, Changhai Liu, Jiayin Zhu Ruixin Li*, for Nannan Hao, Changhai Liu, Jiayin temperature function a long-term district heatZhu demand forecast School of Civil Engineering, Zhengzhou University, Zhengzhou, 450001, China School of Civil Engineering, Zhengzhou University, Zhengzhou, 450001, China
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b Recherche & Innovation, 291passive Avenue ventilation Dreyfous Daniel, 78520 Limay, evaporative France In contemporary architecture,Veolia the combination of utilizing and spray-based cooling technology c Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France In architecture, the combination of utilizing passive ventilation spray-based evaporative cooling hascontemporary been proven to be very effective in improving the interior thermal climate and of traditional construction, which hastechnology played an has been proven be very effective in improving the buildings. interior thermal of traditional construction, which has playedhas an important role in to advancing the development of green But theclimate application of this technology in modern buildings important role in advancing the development green buildings. application of thismethod technology in modern buildings has always been limited at best. With the advent ofofcapillary action, a But new the evaporative cooling combined with liquid window always beenpassive limitedlateral at best. With the proposed advent of in capillary action, a new evaporative combined with liquid window screen and ventilation this paper, it becomes feasible to cooling cool themethod air through a liquid capillary Abstract screen passive lateral ventilation proposed in this paper, it becomes to cool air through a liquid capillaryproviding window screen,and which was developed theoretically by drawing enthalpy diagram,feasible calculating air the enthalpy and CFD simulation, screen, which was theoretically drawing enthalpy of diagram, calculating air enthalpy and CFD simulation, providing the theoretical basisdeveloped and data to support theby future advancement this technology. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the the theoretical basis and data to support the future advancement of this technology. greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat Copyright © 2018 Elsevier Ltd. All rights reserved. sales. to the changed climate conditions © 2019 Due The Published by Elsevier Ltd. and building renovation policies, heatth demand in the future could decrease, Copyright ©Authors. 2018 Elsevier Ltd. Allresponsibility rights reserved. Selection and peer-review under of the scientific committee of the 10 International Conference on Applied prolonging theaccess investment This is an open articlereturn underperiod. the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). Peer-review underofresponsibility scientific committee of ICAE2018 The 10th International Conference on Applied The main scope this paper is of to the assess the feasibility of using the heat –demand – outdoor temperature function for heatEnergy. demand Energy (ICAE2018). forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Keywords: capillary action; passive ventilation; evaporative cooling; liquid window screen; heat and mass transfer buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords: capillary action; passive ventilation; evaporative cooling; liquid window screen; heat and mass transfer renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. 1.The Introduction results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1. Introduction (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Evaporation is a value phenomenon through the absorption energy will convert a state ofconsidered). liquid to scenarios, the error increasedwhich up to 59.5% (depending on the of weather andmatter renovation scenariosfrom combination Evaporation is a phenomenon which through the absorption of energy matter will convert from a state of liquid to that of a gas. The main factors of this mechanism are temperature, humidity, surface area of liquid, air to flow The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds the that of a gas. The main factors of this mechanism are temperature, humidity, surface area of liquid, air flow characteristics of liquid surface and other minor influences. The evaporative cooling technology proposed will cause decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and characteristics of liquid and other influences. Thewill evaporative cooling technology cause renovation considered). Onform the other hand, function intercept increased 7.8-12.7% per (depending on the the liquid toscenarios contact thesurface gaseous ofminor the matter, which acceleratefor the process of decade theproposed heat andwill moisture the liquidscenarios). to contact thevalues gaseous form ofdecrease thebematter, the process of the also heat increasing and moisture coupled The suggested used modifywill the accelerate function for scenarios considered, and exchange during evaporation. This willcould thetowhich temperature of the parameters incoming air the while its improve the accuracy of heatmakes demand estimations. exchange during evaporation. This will decrease thevery temperature ofregions the incoming airindoor while climate also increasing its humidity. This interaction evaporative cooling practical in where the is generally
humidity. This interaction makes evaporative cooling very practical in regions where the indoor climate is generally © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +86-157-367-02207.
address:author. [email protected] * E-mail Corresponding Tel.: +86-157-367-02207. Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility the scientific 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.1021
Ruixin Li et al. / Energy Procedia 158 (2019) 3177–3183 Ruixin Li et al. / Energy Procedia 00 (2018) 000–000
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hot and dry, such as desert regions [1]. With the continuing advancements of energy saving and emission reduction policies in China, passive evaporative cooling technology alongside phase shift materials with better thermal properties have become more widely used in buildings developed in recent years. Structures such as water wall, Oyster-shell wall, water storage houses, planted roofs etc are becoming more commonplace. As more research is done, scholars have surmised that the climate of Southern China has provided defining results in this field [2-5]. However, application of this technology in more commonplace construction has been constraint by elevated building costs as well as the later required increased maintenance [6]. In addition to evaporative cooling, the application of passive ventilation technology in the cooling and dehumidification of green buildings has become more trending year after year. In recent years, relevant researchers have made a breakthrough in the field of traditional passive downward ventilation cooling (PDC, Passive Downdraught Cooling) [7]. On that basis, it is proposed that the evaporative cooling technology and passive ventilation technology are combined to contact the cold air and to further cool the indoor air by taking advantage of the characteristics of air density increasing and sinking in the room, which produces much more effective results (see Fig.1). What sets traditional and modern buildings apart is the reliance of the wind tower, or the well structure in traditional construction [8], whereas modern buildings rely more upon the doors and windows. Therefore, there needs to be a window structure that is much more effective at facilitating and regulating the state of incoming air. In this paper, a passive lateral ventilation evaporative cooling technique (PLVEC, Passive Lateral Ventilation Evaporative Cooling), which is based on the capillary action, is proposed. The method making passive evaporative cooling possible in modern high-rise buildings under zero energy consumption on account of utilizes the capillary action, natural ventilation technology and direct evaporative cooling principle to rapidly cool the entering air, without using the height difference of the wind tower and the airflow cooling by the sprayer. The feasibility of cooling in prefabricated house is analyzed and verified by the heat and mass transfer theory in this paper, which provides theoretical basis and data support for the follow-up research of the passive technology. Nomenclature t tb dz db h hmd r A Qs Ql
temperature of the mainstream air, ℃； temperature of the saturated air in the boundary layer, ℃； mainstream air humidity ratio, kg/kg； humidity ratios of the saturated air in the boundary layer, kg/kg； heat transfer coefficient, W/(m2·℃)； coefficient of mass transfer； latent heat of vaporization of water, 2538 kj/kg. surface area of all water columns, m2 sensible heat transfer of the wet air containing 1kg dry air through the capillary window screen in τ time, flowing from air to water, kj/kg; latent heat transfer of the wet air containing 1kg dry air through the capillary window screen in τ time, flowing from water to air, kj/kg.
2. Principle of the PLVEC Evaporation of water is very effective for cooling air due to the large latent heat transfer. For example, liquid water at 25 ℃ is under 100 kPa pressure, the heat needed to raise the water temperature by 1 ℃ is about 4.18 kJ/kg, while the latent heat of evaporation in the process is up to 2257 kJ/kg [9]. PLVEC technology transforms the traditional outer window screen into a capillary screen with water source on one side or both sides, and uses the capillary action to keep the screen window moist, which helps keep the air cool. In the passive ventilation technology, the indoor and outdoor air exchange is increased so that the water evaporates and absorbs heat when passing through the capillary screen window (see Fig.2). The process of air-water contact evaporative cooling is an adiabatic process, and the temperature of air decreases along the wet bulb temperature line. But the direct evaporative cooling of liquid capillary screen is a complicated heat and mass transfer process, which is subjected to
Ruixin Li et al. / Energy Procedia 158 (2019) 3177–3183 Ruixin Li et al. / Energy Procedia 00 (2018) 000–000
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indoor and outdoor air temperature, air humidity, air velocity, solar radiation conditions, screen moisture, capillary column temperature and capillary column roughness and other factors.
Fig.1. The PDC system
Fig.2. Capillary action and the liquid capillary window screen
3. Theoretical analysis of heat and mass process of liquid capillary window screen Based on the Typical Meteorological Year Data, the data of 13:00 on June 21 in Zhengzhou were selected to carry out the relevant calculation and analysis. Taking into account the heat and mass process characteristics of evaporative cooling, from McKell equation [10]:
Q h(t tb ) hmd r ( d z db )
(1)
From formula (1), we can tell that there is not only sensible heat transfer, but also latent heat transfers in the direct contact between water and air, both of which can be in the same direction or opposite direction. Assuming that the water temperature of the capillary window screen is 20 ℃ and the outdoor air temperature is 34.3 ℃, get that the qualitative temperature of saturated air in the boundary layer is 27.15 ℃, the humidity ratio is 22.6g/kgdry air, and humidity ratio of the incoming air is 16.31 kg/kgdry air, where the air transfers sensible heat to the water and the latent heat from the water to the air. According from psychrometric chart, we can see that the air state is enthalpy increasing, humidification, cooling process for air, as shown in Fig.3. When the saturated air temperature is 27.15℃, consistent with it, λ=2.646‧10-2 w/(m‧K), ν=15.718‧10-6 m2/s, Cp=1.005 kj/(kg‧k), Pr=0.703, a=22.9‧10-6 m2/s, the diffusion coefficient can be obtained from the formula (2):
D D0 (
P0 T 0.5 )( ) P T0
(2)
In formula (2): D0=0.22‧10-4 m2/s, P0=101325 pa, T0=273 K
Sc
0.643 D
(3)
According to the condition: 0.6
hm
h cp
(4)
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Based on the theory of convection heat transfer over holes circular tube [11]:
Re
ud
(5)
Nu P Pr r Pw
0.25
CRen
(6)
In formula (5), (6): d=0.4mm (design value), u=0.15 m/s, Re=3.817, C=0.75, n=0.4, for mainstream Pr=Pw=0.703, Nu=1.125.
h Nu
hmd
d
h cp
(7)
(8)
3.1. The drop in air temperature through liquid capillary window screen based on psychrometric chart The increment of mainstream humidity ratio can be obtained by formula (9).
w hmd db d z A
(9)
The A is the surface area of all water columns, because the diameter of the capillary column in this design is 0.4 mm, the size of window by 0.8 m height and 1.2 m width, the corresponding capillary column number is 400 and 600, and the total area of air flow around the S can be obtained by formula (10).
S nd l
(10)
The τ is the time for dry air flowing through the capillary window to reach 1kg. When the mass flow rate of inlet air is 0.1645 kg/s, τ = 6.097 s, the above conditions can be substituted into the formula (9) to get: w=3.41 g/kg, dz=16.31 g/kg, and d'z=19.72 g/kg. When the relative humidity reaches 64% (comfortable relative humidity for personnel 60%~70%), mainstream through over surface of the capillary column, after that the saturated air in the boundary layer is blown away and mixed, which attached to the surface of the capillary column, so the state of the mixed air must be on the intersection point of the psychrometric chart (see Fig. 4). By comparison from Fig.4 we can see that, the state point C of air is nearly 2.2 ℃ lower than point A, that means the temperature of air decreases by 2.2 ℃ after passing over through the capillary window, but this value does not exclude the possible errors caused by drawing, marking and reading. 3.2. The drop in air temperature through liquid capillary window screen based on calculation of air enthalpy When the humidity ratios were known, enthalpy of wet air is a monotone function of air temperature, so the premise of calculating air temperature is to determine the enthalpy and moisture content. Therefore, the enthalpy of wet air can be determined by formula (11).
Ruixin et /al. / Energy Procedia 158 (2019) 3177–3183 Ruixin Li etLial. Energy Procedia 00 (2018) 000–000
i 1.005t d 2501 1.85t
31815
(11)
According to the above conditions, enthalpy of mainstream is known as 76.298 kj/kg and the enthalpy of saturated air in boundary layer is 84.94 kj/kg, so enthalpy difference of air crossing over through the capillary window screen can be determined by formula (12).
Q hmd ib i A
Fig.3. Change of mainstream state on the psychrometric chart
(12)
Fig.4. The air state of point A and C
By formula (12) ∆Q=4.809 kj/kg, therefore, the enthalpy of the air after the evaporative cooling, which crossing over capillary window is calculated to be 80.979 kj/kg. As we know that the humidity ratio of end state of the air is 19.72 g/kg, so we can get temperature of end state of the air (t=32.097 ℃). By comparison, we found that the temperature of inlet air was reduced by 2.4 ℃. In addition, the enthalpy of wet air is calculated on the basis of 1 kg dry air, but the dry air flow is less than 1 kg per unit time, so the time factor should be taken into account in the calculation. Since the heat transfer coefficient and coefficient of mass transfer have been obtained in the above formula (see formula 7 and 8). The sensible heat transfer and latent heat transfer can be obtained respectively by following formulas based on this condition:
Q h(t t0 ) A s Ql h(2501 1.85t ) w
(13) (14)
Acquired energy of the air entering the room can be calculated to be 4.8 kj/kg by formula (15). The enthalpy of end state of the air is 81.110 kj/kg when the enthalpy of initial state of the air is known, and the temperature of end state of the air is 32.145 ℃ by formula (11). So we can clearly see that the air flow temperature can be reduced by 2.15 ℃.
Q Ql Qs
(15)
Ruixin Li et al. / Energy Procedia 158 (2019) 3177–3183 Ruixin Li et al. / Energy Procedia 00 (2018) 000–000
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3.3. The drop in air temperature through liquid capillary window screen based on CFD simulation For the convenience of calculation, the original design size of the capillary window screen was zooming down 2000 times to create 2D model with ICEM. The intersecting surface and mesh of the physical model are shown in Fig.5. Species-transport model is first opened and the component is set to an air mixture in the discretization model calculation. After iterative calculation, the Discrete-phase model is opened. The evaporation rate involved in the calculation can be obtained because the coefficient of mass transfer and the contact area of the capillary window screen as we know (model size 2cm×3cm, short edge 12 capillary columns with a diameter 0.4 mm per column). Evaporation rate is obtained to be 3.34·10-5 kg/s accordingly. The results of simulation are shown in Fig.6.
Fig.5. 2D model of the liquid capillary window screen
Fig.6. Temperature distribution of through air flow
From Fig.6, temperature drop is obvious when the mainstream air through the capillary window screen, part of the high – temperature air flow permeation into the room from flow separation zone near the window frame side, but the temperature stratification attenuates rapidly and reaches uniformity quickly. Because of the tight spacing between the capillary columns, there is no obvious high temperature air infiltration, and all mainstream can be evenly cooled. The heat transfer is more intense in the cavity zone of the capillary columns due to back-flow, and there concentration low temperature flow in the wake zone, but temperature gradient can be restored in the short watershed, and the overall airflow organization and temperature drop are reasonable. By using the report in fluent, the outlet boundary air flow temperature of the model is 31.7056 ℃, which is 2.59 ℃ lower than the inlet boundary temperature of 34.3 ℃. Therefore, combined with the above three theoretical analysis methods and calculation results, air at 34.3 ℃ can be cooled not less than 2.2 ℃ after crossing through the liquid capillary window screen. This technology is helpful to improve indoor thermal environment and reduce energy consumption of equipments. 4. Conclusion PLVEC technology was first proposed in this paper, the application and research of PLVEC technology in modern high-rise buildings is still a blank. Under the combined effects of urban heat island effect and urban gradient wind, the use of PLVEC technology in dry and non-high-humidity environment has great potential for cooling the building space. From a series of theory analysis, some conclusions are drawn as follows: The technology PLVEC can effectively abate indoor solar radiation heat, reduce indoor heat load in summer and cooling air flow temperature in order to improve indoor thermal environment and cut down energy consumption of air conditioners. Liquid capillary window screen can be integrated with the traditional screen window. Based on the climatic characteristics of Zhengzhou area, the unilateral or bilateral water charging system can be used to achieve direct evaporative cooling of the inlet airflow and the relative humidity of the air increases in accordance with
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the comfort requirements. It is necessary to increase the contact area between the inlet airflow and the liquid screen window to humidify indoor air in arid regions. Based on the typical meteorological data of 13:00 on June 21 in Zhengzhou city, the effect of direct evaporation and cooling of liquid capillary window screen in the prefabricated house is analysis theoretically by methods of the psychrometric chart, calculation of air enthalpy and CFD simulation. The results show that, considering the different methods and calculation errors, the three verification conclusions are different, but based on the same conditions, air at 34.3 ℃ can be cooled not less than 2.2 ℃ after crossing through the liquid capillary window screen.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51808506), the grants from the scientific research project of Henan province (No.16A560038) and Henan postdoctoral sustentation fund (No. 2015022) of China. References [1] Bogdan Porumb, Paula Ungureşan, Lucian Fechete Tutunaru, Alexandru Şerban, Mugur Bălan. A Review of Indirect Evaporative Cooling Technology [J]. Energy Procedia, 2016, 85:461-471. [2] LI Ning, MENG Qinglin, WANG Yanfei, LIU Fengjun, YE Ping. Study on passive evaporation of porous building materials in hot summer and warm winter area [J]. Building Science, 2016, 32(8):52-55. [3] Jiang Yi, Xie Xiaoyun, Yu Xiangyang. Indirect evaporative cooling technology: high-performance application of renewable dry air energy in northwest China [J]. Heating Ventilating & Air Conditioning, 2009, 39(9):1-4. [4] LIU Xikang, ZHANG Lei, MENG Qinglin, LI Chengshu. Heat transfer coefficient of the oyster- shell wall by in- situ measurement [J], Building Energy Efficiency, 2012, 40(262): 31-33. [5] K.J. Lomas,D. Fiala,M.J. Cook,P.C. Cropper. Building bioclimatic charts for non-domestic buildings and passive downdraught evaporative cooling [J]. Building and Environment, 2004 (6):342-346. [6] YAN Fengying, WANG Xinhua, WU Youcong. Simulation of interior natural ventilation and thermal comfort based on CFD [J]. Journal of Tianjin University, 2009, 42(5): 407-412. [7] Qiu Jing. The feasibility research on passive hybrid downdraught cooling for public buildings-a case study of teaching buildings in university in Wuhan [D]. Huazhong University of Science and Technology, Wuhan, 2012.5. [8] Qiu Jing, Li Baofeng, Qiu Yu. Reflections on the application of passive downdraught evaporative cooling in atriums [J]. Architectural Practice, 2011, 11: 60-63. [9] ASHRAE. ASHRAE Handbook: Fundamentals [M]. ASHRAE, 2005. [10] Lian Zhiwei. Principle and Equipment of Heat and Mass Transfer [M]. China Architecture & Building Press, Beijing, 2011. [11] Yang Shiming, Tao Wenquan. Heat Transfer [M]. Higher Education Press, Beijing, 2007.
ScienceDirect ScienceDirect
Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)3177–3183 000–000 www.elsevier.com/locate/procedia
10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
Theoretical Analysis of Passive Lateral Ventilation Evaporative Theoretical of Passive Lateral Ventilation Evaporative TheAnalysis 15th International Symposium on District Heating and Cooling Cooling Based on the Capillary Action Cooling Based on the Capillary Action AssessingRuixin the feasibility of using the heat demand-outdoor Li*, Nannan Hao, Changhai Liu, Jiayin Zhu Ruixin Li*, for Nannan Hao, Changhai Liu, Jiayin temperature function a long-term district heatZhu demand forecast School of Civil Engineering, Zhengzhou University, Zhengzhou, 450001, China School of Civil Engineering, Zhengzhou University, Zhengzhou, 450001, China
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b Recherche & Innovation, 291passive Avenue ventilation Dreyfous Daniel, 78520 Limay, evaporative France In contemporary architecture,Veolia the combination of utilizing and spray-based cooling technology c Département Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France In architecture, the combination of utilizing passive ventilation spray-based evaporative cooling hascontemporary been proven to be very effective in improving the interior thermal climate and of traditional construction, which hastechnology played an has been proven be very effective in improving the buildings. interior thermal of traditional construction, which has playedhas an important role in to advancing the development of green But theclimate application of this technology in modern buildings important role in advancing the development green buildings. application of thismethod technology in modern buildings has always been limited at best. With the advent ofofcapillary action, a But new the evaporative cooling combined with liquid window always beenpassive limitedlateral at best. With the proposed advent of in capillary action, a new evaporative combined with liquid window screen and ventilation this paper, it becomes feasible to cooling cool themethod air through a liquid capillary Abstract screen passive lateral ventilation proposed in this paper, it becomes to cool air through a liquid capillaryproviding window screen,and which was developed theoretically by drawing enthalpy diagram,feasible calculating air the enthalpy and CFD simulation, screen, which was theoretically drawing enthalpy of diagram, calculating air enthalpy and CFD simulation, providing the theoretical basisdeveloped and data to support theby future advancement this technology. District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the the theoretical basis and data to support the future advancement of this technology. greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat Copyright © 2018 Elsevier Ltd. All rights reserved. sales. to the changed climate conditions © 2019 Due The Published by Elsevier Ltd. and building renovation policies, heatth demand in the future could decrease, Copyright ©Authors. 2018 Elsevier Ltd. Allresponsibility rights reserved. Selection and peer-review under of the scientific committee of the 10 International Conference on Applied prolonging theaccess investment This is an open articlereturn underperiod. the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). Peer-review underofresponsibility scientific committee of ICAE2018 The 10th International Conference on Applied The main scope this paper is of to the assess the feasibility of using the heat –demand – outdoor temperature function for heatEnergy. demand Energy (ICAE2018). forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Keywords: capillary action; passive ventilation; evaporative cooling; liquid window screen; heat and mass transfer buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district Keywords: capillary action; passive ventilation; evaporative cooling; liquid window screen; heat and mass transfer renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. 1.The Introduction results showed that when only weather change is considered, the margin of error could be acceptable for some applications 1. Introduction (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Evaporation is a value phenomenon through the absorption energy will convert a state ofconsidered). liquid to scenarios, the error increasedwhich up to 59.5% (depending on the of weather andmatter renovation scenariosfrom combination Evaporation is a phenomenon which through the absorption of energy matter will convert from a state of liquid to that of a gas. The main factors of this mechanism are temperature, humidity, surface area of liquid, air to flow The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds the that of a gas. The main factors of this mechanism are temperature, humidity, surface area of liquid, air flow characteristics of liquid surface and other minor influences. The evaporative cooling technology proposed will cause decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and characteristics of liquid and other influences. Thewill evaporative cooling technology cause renovation considered). Onform the other hand, function intercept increased 7.8-12.7% per (depending on the the liquid toscenarios contact thesurface gaseous ofminor the matter, which acceleratefor the process of decade theproposed heat andwill moisture the liquidscenarios). to contact thevalues gaseous form ofdecrease thebematter, the process of the also heat increasing and moisture coupled The suggested used modifywill the accelerate function for scenarios considered, and exchange during evaporation. This willcould thetowhich temperature of the parameters incoming air the while its improve the accuracy of heatmakes demand estimations. exchange during evaporation. This will decrease thevery temperature ofregions the incoming airindoor while climate also increasing its humidity. This interaction evaporative cooling practical in where the is generally
humidity. This interaction makes evaporative cooling very practical in regions where the indoor climate is generally © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +86-157-367-02207.
address:author. [email protected] * E-mail Corresponding Tel.: +86-157-367-02207. Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility the scientific 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.1021
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hot and dry, such as desert regions [1]. With the continuing advancements of energy saving and emission reduction policies in China, passive evaporative cooling technology alongside phase shift materials with better thermal properties have become more widely used in buildings developed in recent years. Structures such as water wall, Oyster-shell wall, water storage houses, planted roofs etc are becoming more commonplace. As more research is done, scholars have surmised that the climate of Southern China has provided defining results in this field [2-5]. However, application of this technology in more commonplace construction has been constraint by elevated building costs as well as the later required increased maintenance [6]. In addition to evaporative cooling, the application of passive ventilation technology in the cooling and dehumidification of green buildings has become more trending year after year. In recent years, relevant researchers have made a breakthrough in the field of traditional passive downward ventilation cooling (PDC, Passive Downdraught Cooling) [7]. On that basis, it is proposed that the evaporative cooling technology and passive ventilation technology are combined to contact the cold air and to further cool the indoor air by taking advantage of the characteristics of air density increasing and sinking in the room, which produces much more effective results (see Fig.1). What sets traditional and modern buildings apart is the reliance of the wind tower, or the well structure in traditional construction [8], whereas modern buildings rely more upon the doors and windows. Therefore, there needs to be a window structure that is much more effective at facilitating and regulating the state of incoming air. In this paper, a passive lateral ventilation evaporative cooling technique (PLVEC, Passive Lateral Ventilation Evaporative Cooling), which is based on the capillary action, is proposed. The method making passive evaporative cooling possible in modern high-rise buildings under zero energy consumption on account of utilizes the capillary action, natural ventilation technology and direct evaporative cooling principle to rapidly cool the entering air, without using the height difference of the wind tower and the airflow cooling by the sprayer. The feasibility of cooling in prefabricated house is analyzed and verified by the heat and mass transfer theory in this paper, which provides theoretical basis and data support for the follow-up research of the passive technology. Nomenclature t tb dz db h hmd r A Qs Ql
temperature of the mainstream air, ℃； temperature of the saturated air in the boundary layer, ℃； mainstream air humidity ratio, kg/kg； humidity ratios of the saturated air in the boundary layer, kg/kg； heat transfer coefficient, W/(m2·℃)； coefficient of mass transfer； latent heat of vaporization of water, 2538 kj/kg. surface area of all water columns, m2 sensible heat transfer of the wet air containing 1kg dry air through the capillary window screen in τ time, flowing from air to water, kj/kg; latent heat transfer of the wet air containing 1kg dry air through the capillary window screen in τ time, flowing from water to air, kj/kg.
2. Principle of the PLVEC Evaporation of water is very effective for cooling air due to the large latent heat transfer. For example, liquid water at 25 ℃ is under 100 kPa pressure, the heat needed to raise the water temperature by 1 ℃ is about 4.18 kJ/kg, while the latent heat of evaporation in the process is up to 2257 kJ/kg [9]. PLVEC technology transforms the traditional outer window screen into a capillary screen with water source on one side or both sides, and uses the capillary action to keep the screen window moist, which helps keep the air cool. In the passive ventilation technology, the indoor and outdoor air exchange is increased so that the water evaporates and absorbs heat when passing through the capillary screen window (see Fig.2). The process of air-water contact evaporative cooling is an adiabatic process, and the temperature of air decreases along the wet bulb temperature line. But the direct evaporative cooling of liquid capillary screen is a complicated heat and mass transfer process, which is subjected to
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indoor and outdoor air temperature, air humidity, air velocity, solar radiation conditions, screen moisture, capillary column temperature and capillary column roughness and other factors.
Fig.1. The PDC system
Fig.2. Capillary action and the liquid capillary window screen
3. Theoretical analysis of heat and mass process of liquid capillary window screen Based on the Typical Meteorological Year Data, the data of 13:00 on June 21 in Zhengzhou were selected to carry out the relevant calculation and analysis. Taking into account the heat and mass process characteristics of evaporative cooling, from McKell equation [10]:
Q h(t tb ) hmd r ( d z db )
(1)
From formula (1), we can tell that there is not only sensible heat transfer, but also latent heat transfers in the direct contact between water and air, both of which can be in the same direction or opposite direction. Assuming that the water temperature of the capillary window screen is 20 ℃ and the outdoor air temperature is 34.3 ℃, get that the qualitative temperature of saturated air in the boundary layer is 27.15 ℃, the humidity ratio is 22.6g/kgdry air, and humidity ratio of the incoming air is 16.31 kg/kgdry air, where the air transfers sensible heat to the water and the latent heat from the water to the air. According from psychrometric chart, we can see that the air state is enthalpy increasing, humidification, cooling process for air, as shown in Fig.3. When the saturated air temperature is 27.15℃, consistent with it, λ=2.646‧10-2 w/(m‧K), ν=15.718‧10-6 m2/s, Cp=1.005 kj/(kg‧k), Pr=0.703, a=22.9‧10-6 m2/s, the diffusion coefficient can be obtained from the formula (2):
D D0 (
P0 T 0.5 )( ) P T0
(2)
In formula (2): D0=0.22‧10-4 m2/s, P0=101325 pa, T0=273 K
Sc
0.643 D
(3)
According to the condition: 0.6
hm
h cp
(4)
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Based on the theory of convection heat transfer over holes circular tube [11]:
Re
ud
(5)
Nu P Pr r Pw
0.25
CRen
(6)
In formula (5), (6): d=0.4mm (design value), u=0.15 m/s, Re=3.817, C=0.75, n=0.4, for mainstream Pr=Pw=0.703, Nu=1.125.
h Nu
hmd
d
h cp
(7)
(8)
3.1. The drop in air temperature through liquid capillary window screen based on psychrometric chart The increment of mainstream humidity ratio can be obtained by formula (9).
w hmd db d z A
(9)
The A is the surface area of all water columns, because the diameter of the capillary column in this design is 0.4 mm, the size of window by 0.8 m height and 1.2 m width, the corresponding capillary column number is 400 and 600, and the total area of air flow around the S can be obtained by formula (10).
S nd l
(10)
The τ is the time for dry air flowing through the capillary window to reach 1kg. When the mass flow rate of inlet air is 0.1645 kg/s, τ = 6.097 s, the above conditions can be substituted into the formula (9) to get: w=3.41 g/kg, dz=16.31 g/kg, and d'z=19.72 g/kg. When the relative humidity reaches 64% (comfortable relative humidity for personnel 60%~70%), mainstream through over surface of the capillary column, after that the saturated air in the boundary layer is blown away and mixed, which attached to the surface of the capillary column, so the state of the mixed air must be on the intersection point of the psychrometric chart (see Fig. 4). By comparison from Fig.4 we can see that, the state point C of air is nearly 2.2 ℃ lower than point A, that means the temperature of air decreases by 2.2 ℃ after passing over through the capillary window, but this value does not exclude the possible errors caused by drawing, marking and reading. 3.2. The drop in air temperature through liquid capillary window screen based on calculation of air enthalpy When the humidity ratios were known, enthalpy of wet air is a monotone function of air temperature, so the premise of calculating air temperature is to determine the enthalpy and moisture content. Therefore, the enthalpy of wet air can be determined by formula (11).
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i 1.005t d 2501 1.85t
31815
(11)
According to the above conditions, enthalpy of mainstream is known as 76.298 kj/kg and the enthalpy of saturated air in boundary layer is 84.94 kj/kg, so enthalpy difference of air crossing over through the capillary window screen can be determined by formula (12).
Q hmd ib i A
Fig.3. Change of mainstream state on the psychrometric chart
(12)
Fig.4. The air state of point A and C
By formula (12) ∆Q=4.809 kj/kg, therefore, the enthalpy of the air after the evaporative cooling, which crossing over capillary window is calculated to be 80.979 kj/kg. As we know that the humidity ratio of end state of the air is 19.72 g/kg, so we can get temperature of end state of the air (t=32.097 ℃). By comparison, we found that the temperature of inlet air was reduced by 2.4 ℃. In addition, the enthalpy of wet air is calculated on the basis of 1 kg dry air, but the dry air flow is less than 1 kg per unit time, so the time factor should be taken into account in the calculation. Since the heat transfer coefficient and coefficient of mass transfer have been obtained in the above formula (see formula 7 and 8). The sensible heat transfer and latent heat transfer can be obtained respectively by following formulas based on this condition:
Q h(t t0 ) A s Ql h(2501 1.85t ) w
(13) (14)
Acquired energy of the air entering the room can be calculated to be 4.8 kj/kg by formula (15). The enthalpy of end state of the air is 81.110 kj/kg when the enthalpy of initial state of the air is known, and the temperature of end state of the air is 32.145 ℃ by formula (11). So we can clearly see that the air flow temperature can be reduced by 2.15 ℃.
Q Ql Qs
(15)
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3.3. The drop in air temperature through liquid capillary window screen based on CFD simulation For the convenience of calculation, the original design size of the capillary window screen was zooming down 2000 times to create 2D model with ICEM. The intersecting surface and mesh of the physical model are shown in Fig.5. Species-transport model is first opened and the component is set to an air mixture in the discretization model calculation. After iterative calculation, the Discrete-phase model is opened. The evaporation rate involved in the calculation can be obtained because the coefficient of mass transfer and the contact area of the capillary window screen as we know (model size 2cm×3cm, short edge 12 capillary columns with a diameter 0.4 mm per column). Evaporation rate is obtained to be 3.34·10-5 kg/s accordingly. The results of simulation are shown in Fig.6.
Fig.5. 2D model of the liquid capillary window screen
Fig.6. Temperature distribution of through air flow
From Fig.6, temperature drop is obvious when the mainstream air through the capillary window screen, part of the high – temperature air flow permeation into the room from flow separation zone near the window frame side, but the temperature stratification attenuates rapidly and reaches uniformity quickly. Because of the tight spacing between the capillary columns, there is no obvious high temperature air infiltration, and all mainstream can be evenly cooled. The heat transfer is more intense in the cavity zone of the capillary columns due to back-flow, and there concentration low temperature flow in the wake zone, but temperature gradient can be restored in the short watershed, and the overall airflow organization and temperature drop are reasonable. By using the report in fluent, the outlet boundary air flow temperature of the model is 31.7056 ℃, which is 2.59 ℃ lower than the inlet boundary temperature of 34.3 ℃. Therefore, combined with the above three theoretical analysis methods and calculation results, air at 34.3 ℃ can be cooled not less than 2.2 ℃ after crossing through the liquid capillary window screen. This technology is helpful to improve indoor thermal environment and reduce energy consumption of equipments. 4. Conclusion PLVEC technology was first proposed in this paper, the application and research of PLVEC technology in modern high-rise buildings is still a blank. Under the combined effects of urban heat island effect and urban gradient wind, the use of PLVEC technology in dry and non-high-humidity environment has great potential for cooling the building space. From a series of theory analysis, some conclusions are drawn as follows: The technology PLVEC can effectively abate indoor solar radiation heat, reduce indoor heat load in summer and cooling air flow temperature in order to improve indoor thermal environment and cut down energy consumption of air conditioners. Liquid capillary window screen can be integrated with the traditional screen window. Based on the climatic characteristics of Zhengzhou area, the unilateral or bilateral water charging system can be used to achieve direct evaporative cooling of the inlet airflow and the relative humidity of the air increases in accordance with
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the comfort requirements. It is necessary to increase the contact area between the inlet airflow and the liquid screen window to humidify indoor air in arid regions. Based on the typical meteorological data of 13:00 on June 21 in Zhengzhou city, the effect of direct evaporation and cooling of liquid capillary window screen in the prefabricated house is analysis theoretically by methods of the psychrometric chart, calculation of air enthalpy and CFD simulation. The results show that, considering the different methods and calculation errors, the three verification conclusions are different, but based on the same conditions, air at 34.3 ℃ can be cooled not less than 2.2 ℃ after crossing through the liquid capillary window screen.
Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 51808506), the grants from the scientific research project of Henan province (No.16A560038) and Henan postdoctoral sustentation fund (No. 2015022) of China. References [1] Bogdan Porumb, Paula Ungureşan, Lucian Fechete Tutunaru, Alexandru Şerban, Mugur Bălan. A Review of Indirect Evaporative Cooling Technology [J]. Energy Procedia, 2016, 85:461-471. [2] LI Ning, MENG Qinglin, WANG Yanfei, LIU Fengjun, YE Ping. Study on passive evaporation of porous building materials in hot summer and warm winter area [J]. Building Science, 2016, 32(8):52-55. [3] Jiang Yi, Xie Xiaoyun, Yu Xiangyang. Indirect evaporative cooling technology: high-performance application of renewable dry air energy in northwest China [J]. Heating Ventilating & Air Conditioning, 2009, 39(9):1-4. [4] LIU Xikang, ZHANG Lei, MENG Qinglin, LI Chengshu. Heat transfer coefficient of the oyster- shell wall by in- situ measurement [J], Building Energy Efficiency, 2012, 40(262): 31-33. [5] K.J. Lomas,D. Fiala,M.J. Cook,P.C. Cropper. Building bioclimatic charts for non-domestic buildings and passive downdraught evaporative cooling [J]. Building and Environment, 2004 (6):342-346. [6] YAN Fengying, WANG Xinhua, WU Youcong. Simulation of interior natural ventilation and thermal comfort based on CFD [J]. Journal of Tianjin University, 2009, 42(5): 407-412. [7] Qiu Jing. The feasibility research on passive hybrid downdraught cooling for public buildings-a case study of teaching buildings in university in Wuhan [D]. Huazhong University of Science and Technology, Wuhan, 2012.5. [8] Qiu Jing, Li Baofeng, Qiu Yu. Reflections on the application of passive downdraught evaporative cooling in atriums [J]. Architectural Practice, 2011, 11: 60-63. [9] ASHRAE. ASHRAE Handbook: Fundamentals [M]. ASHRAE, 2005. [10] Lian Zhiwei. Principle and Equipment of Heat and Mass Transfer [M]. China Architecture & Building Press, Beijing, 2011. [11] Yang Shiming, Tao Wenquan. Heat Transfer [M]. Higher Education Press, Beijing, 2007.