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Development of solid desiccant dehumidification using electro-osmosis regeneration method for HVAC application
Building and Environment 48 (2012) 128e134
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Development of solid desiccant dehumidification using electro-osmosis regeneration method for HVAC application B. Li, Q.Y. Lin, Y.Y. Yan* Energy & Sustainability Research Division, Faculty of Engineering, University of Nottingham, Lenton Firs Building, 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 9 May 2011 Received in revised form 5 September 2011 Accepted 7 September 2011
In this paper a novel method of electro-osmosis (EO) based regeneration for solid desiccant is reported, and the potential of the method for application in HVAC particularly for dehumidification process in air conditioning system is discussed. The results of the measurement of water removal rate inside solid desiccant array show that the mass flow rate of EO driven mass flow is achievable at 0.953 g m2 s1 with 20 V DC voltage on the installed electrodes. By comparing the theoretical and experimental results, the current limitations of theory and experimental apparatus were explored. The experimental results show that the energy consumption in EO integrated air conditioning system is averagely 23.3% lower than the conventional air conditioning system in respect of the different configurations in air handling process. In the case study, among the three potential locations for EO device, the position in Point C is the best choice with an ideal COP of 12. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Solid desiccant dehumidification Electro-osmosis HVAC Microporous media Latent heat recovery
1. Introduction Among the energy consumptions in the buildings, HVAC systems currently cover about 30e50% of total amount consumption. Due to higher demand of thermal comfort inside building and larger energy consumptions within air conditioning system, the improvements of the whole system using less energy and being more efficient is crucial to achieve sustainability in building service sector. Since the 1990s, the air conditioning industry has been faced with a number of challenges including the increased energy efficiencies, the improved indoor air quality, the growing concern for improved comfort and environmental control, the increased ventilation requirements, the reduction of chlorofluorocarbons (CFCs), and the rising peak demand charges [1,2]. New approaches to space conditioning will be required urgently to resolve these economic, environmental, and regulatory issues [3e5]. To achieve cooling purpose, the cooling coil is supplied by vapour compression cycle using refrigerant. In conventional air conditioning system, the cooling capacity needs to match the cooling load, which consists of sensible heat and latent heat generated by occupants and some facilities. A common commercial air conditioning system mainly consists of a cooling coil and heating coil with supply/extract fans as shown in Fig. 1.
* Corresponding author. Tel.: þ44 0115 846 7263. E-mail address: [email protected] (Y.Y. Yan). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.09.008
Although latent heat counts less ratio of total load from ambient normally, but it is relatively difficult to remove them effectively in the process due to the phase change and heat exhausting from it. Moreover, considering the indoor air quality and energy use issues, which significantly influence the application of HVAC system, the problems from high humidity need to be re-considered as around 80% of the dehumidification load comes from outside fresh air and it can affect the performance of the air conditioning system dominantly [6]. Hence, dealing with the latent cooling load, which can be commonly achieved by dehumidification process, is a critical and decisive stage in air conditioning process that can largely influence the whole energy consumption. In commercial buildings, the traditional way to control the humidity is to cool the air down to dew point temperature till condensation and then reheat them to the design temperature. It is obviously that the reheating process in the cycle costs large amount of energy inevitably. The industry is, however, quickly learning that by controlling the humidity to 50% RH or less, and independent of temperature, dry bulb temperature can be raised to 25e26 C in consideration of thermal comfort, yielding great energy savings in the meantime. For every degree of the set points in temperature is raised, a 5% reduction in air conditioning energy is realized [7]. However, both solid desiccant and liquid desiccant dehumidification techniques in existing industrial application require considerable heat and electricity to operate the regeneration process which is less competitive in domestic dwellings and office buildings [8]. Hence, the development of dehumidification techniques, in retrospect, is urgently to
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Nomenclature D L veo 3
z h Df E
mion meo j _ eo m
rwater
Diameter of the cylindrical container The length of EO process Average velocity of EO flow Electric permittivity Zeta potential Dynamic viscosity Electrical potential drop Electric field Ionic mobility Electro-osmosis mobility Porosity Mass flow rate for EO driven flow per m2 Density of water
be expended in domestic application as replacement of current products in the market [9]. Particularly, in some extremely hot and humid climate area, the current existing technology cannot meet the comfort requirements due to the humidity removal capacity of the system, and the crystallization, corrosion problems of haloids on the wheel or desiccant pool [10]. Therefore, the study on EO dehumidification method will be the first of its kind in domestic application and definitely it will keep the industry sustainable. Based on our reviews, with respect to this novel application, the system design is a dominant factor for validating the electrochemical phenomenon. Qi et al. designed an EO system and used desiccant powders in a humid environmental chamber to achieve dehumidification [11,12]. On their system, the flow rate of water removal was measured at 0.0021 g s1. Although DC pulse was applied into the application to reduce the Joule effect and erosions on the electrodes, there are still some obstacles to obtain higher capacity and longer durability which were dominant by the system design and configuration. Hence, a more dedicated system design is needed to precisely and simultaneously measure the actual EO performance. The electro-osmotic flow (EOF) theory with a historical retrospect based on the electrokinetic effects is observed by F.F. Reuss in 1809 when doing an experimental investigation on porous clay, and it has been currently widely studied and analyzed in areas such as electrochemistry, physics and vascular plant biology [13e16]. Due to its importance, EOF theory has currently been applied in various fields such as chemical engineering [17,18] and civil engineering including mine tailing water prevention and waste sludge treatment [19e21]. Basically, EOF is the motion of polar liquid passing through porous structure, micro-channel or other membrane under
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applied electric field [22]. It has also been used to characterize and design the salt rejection properties of reverse osmosis membranes and to help understand the behaviour of biological membranes. In this paper, the EOF theory has been used to guide the regenerate solid desiccant by regarding desiccant structure as micro-channel and applying electric field between the desiccants membrane. The moisture adsorbed by desiccant can be pumped out in a certain rate. Generally, electro-osmosis flow is a flow in porous media or micro-channel structure, driven by so-called EO force with applied electrical field. The EOF dynamics in porous media has been studied much both theoretically and numerically. Most of papers are based on the theory of double layer model which was developed by Stern and Guoy-Chapman, respectively. For most EO flow, an electric double layer (EDL) will be formed spontaneously at the liquid/solid interface due to the electrochemical reaction. As depicted in Fig. 2, the charged surface will attract positive ions present in the water and repel negative ions owing to the polarization effect of porous media and this region at liquid/solid interface is the so called EDL. In the inner layer, the counter ions near the wall are called the Stern layer while the outer diffuse layer is called Gouy-Chapman layer [23]. The positive ions will therefore predominate in the Debye sheath next to the charged surface so application of an external electric field results in a net migration towards the cathode of ions in the surface water layer. Due to viscous drag, the water in pores is drawn by ions and therefore flows through the porous media. The schematic diagram of EOF in porous media and its velocity profile has been shown in Fig. 3. In EO flow, the process will gradually become weaker due to several reasons. One of the most significant reasons is the corrosion in electrodes and the process will be interrupted when electrodes are fully corroded and the pH of the flow will also change. This occurs due to the oxidation and reduction reactions at the anode and cathode [21]. The anode will become acidic as carbon acid is formed by the reaction due to carbon dioxide with water and cathode becomes basic. Due to the movement of ions from anode to cathode, as the pH become more basic, some composites like complex calcium precipitates will be formed and these composites will fill the voids along the porous material, which will significantly reduce the EO flow and finally interrupt the flow. For most EO flow, it is influenced by several forces such as gravity, hydraulic pressure, viscosity, temperature driven force and EO but the hydrostatic pressure and electro-osmotic force are dominant components. Therefore, the total flow rate becomes the sum of EO flow and hydrodynamic flow [24]. As one of the popular alternatives dealing with separate humidity control, the desiccant system which has a large potential market including applications in schools, restaurants, hotels and
Fig. 1. Schematic diagram for a conventional air conditioning system.
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Fig. 2. Charge distribution and water affinity form at interface with solid desiccants.
food stores as it can significantly reduce operational costs, improve indoor air quality and achieve precise humidity control [25]. However, the current desiccant cooling systems have not been used widely as it should because its efficiency is still below break-even. The energy invested in running the system was more than the traditional air conditioning system. And it is this reason that may lead to a narrow market and less popular than existing techniques. Unlike traditional vapour compression cooling systems which are driven by electrical power to cool and dehumidify air, desiccant systems, instead, use thermal energy to accomplish the same effect. Generally, a desiccant cooling system may use either solid or liquid desiccant to dry the air by adsorbing moisture. And then this air is cooled down by evaporative cooling method and sent to the airconditioned space finally. The saturated desiccant is usually reheated by gas or electric heating to release moisture, which is defined as the conventional regeneration method and prevailing in the current HVAC market [26]. The rapid development of desiccant air conditioning technology, which can handle sensible and latent heat loads independently, has expanded desiccant applications into hospitals, supermarkets, restaurants, theatres, schools, office buildings, stores or even agricultural related but it still consumes a large amount of energy for reactivation of desiccants [27e31]. It should be noted that whereas the reactivation heat in the original desiccant systems was produced by burning natural gas, all-electric desiccant systems are
Fig. 3. Principle of EOF.
now available. These existing desiccant systems are not very efficient and quite simple in terms of function and operational range in that they use rejected heat from the condenser to regenerate the desiccant wheel [1]. Although the heat source can be supplied using renewable energy such as solar thermal, the energy requirement still makes the whole system less competitive. Therefore, the novel idea of using EO theory to regenerate solid desiccant has been technically addressed here. Based on this new idea, the EO phenomena in nature and the applications in Microelectromechanical systems (MEMS) fields have been conclusively reviewed. It is also promising that applies this technique into a larger scale like dehumidification in buildings as it consumes low DC voltage which can be easily fetched through various means. Hence, this can be potentially improved by using a new regenerating method: Electro-osmosis regeneration method instead of conventional heat source mentioned above. The general principle of EOF in air conditioning application is that the moisture can be absorbed by a membrane composed of a desiccant, being removed across the membrane and rejected on the other side by electroosmotic pumping. Based on electro-osmotic counter pressure concept, the liquid flow occurs in the direction of the electric field. Positive ions move towards cathode, dragging water molecules. 2. Experimental setup and method In order to evaluate the performance of EO process on regenerating solid desiccant and optimise our design and configuration, an experiment is carried out using Zeolite as desiccant forming the micro-channel structure. To achieve precise measurement and avoid vapour fluctuating from testing chamber as some investigators carried in the past, the separation of generated bubble and the actual contact area are reinvestigated [14]. Fig. 4 shows the improved schematic diagram of the EO regeneration system. The apparatus includes the EO device, the electronic scale and a data logger. Zeolite has been filled into the EO device, a simple cylindrical container (D 16 mm L 12 mm) between two electrodes rings. Two pieces of filter paper are used between the electrodes in both sides to block leakage from the container. Considering the thickness of the electrodes and filter paper, the actual length for EO process is set as 10 mm. The water used in the experiment is de-
Fig. 4. Schematic diagram of the EO regeneration system.
B. Li et al. / Building and Environment 48 (2012) 128e134
ionized water to make the ions concentration at low level and keep the control volume unaffected by the outside changes of concentration especially for the charges from solution. To begin with the experiment, water was continuously adsorbed by the Zeolite before applying the electric field and the mass of droplet at the bottom was collected and measured by the electric weight scale to evaluate the effect caused by gravity. When mass flow reached stable and Zeolite was saturated, 20 V DC voltage is applied and the mass of droplet is recorded by a data logger. Theoretically, the mass flow rate can be calculated by multiplying the theoretical total volume flow rate driven by EO process with water density, where the total volume flow rate is the sum of flow rate in all the micro-channels formed by Zeolite particles. Due to the complexity of the actual micro-channel in porous medium formed by the Zeolite particles and the irregular shape of each desiccant, an average EO velocity is calculated macroscopically [32]; and the velocity driven by EO force, veo, is defined by the HelmholteSmoluchowski relation as:
veo ¼
z Df E ¼ meo E ¼ meo h L
3
(1)
Therefore, considering the porosity of Zeolite, the velocity equation could be adjusted as:
z h
3
veo ¼ j E ¼ jmeo
Df
(2)
L
Thus, the theoretical mass flow rate for EO driven flow per m2 becomes:
_ eo ¼ rwater veo ¼ jrwater meo m
Df L
(3)
3. Results and discussion Following the experimental procedure using the apparatus, the results for both cumulative collected mass of water and pure EO regeneration rate have been shown in Fig. 5. As depicted in Fig. 6, at the beginning of the experiment, the mass of collected water is relatively slow, approximately 0.038 g/min. This may be contributed by the adsorption ability of Zeolite. When the Zeolite is saturated, the water vapour is condensed to small droplets which are collected by a container. According to the results, the mass of the collected water is up to 0.0038 g/min. After
Fig. 5. Change of mass of collected water.
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applying 20 V DC, simultaneously extra droplets are created and the mass of collected water is ramped up to a rate of 0.0154 g/min over a period of 70 min. As the flow rate created by gravitational force is 0.038 g/min, then the result due to electro-osmosis process is calculated as 0.0115 g/min by deducting flow rate value caused by gravitational force factor from the total flow rate recorded here. By specifying the flow density of electro-osmosis, it is easily calculated as 0.953 g m2 s1 which is divided by the actual testing volume from electro-osmosis flow rate in the pump. In fact, the effect of EO pumping force is significant from the results and it shows a great potential to regenerate desiccant. Theoretically, the mass flow rate value can be calculated by multiplying the theoretical total volume flow rate driven by EO process with water density, where the total volume flow rate is the sum of flow rate in all the micro-channels formed by Zeolite particles. Due to the complexity of the actual micro-channel in porous medium formed by the Zeolite particles and irregular shape of each desiccant, an average EO velocity is calculated macroscopically. In the experiment, although the de-ionized water has been used, there could still be a low ion concentration due to the ions in solid particles are partly soluble in liquid. At room temperature, the mobility, mion, has been shown in Table 1 [32] and meo in the experiment is cited as 6.6 108 m2 (Vs)1. Therefore, the theoretical mass flow rate for EO driven flow per m2 is calculated as:
_ eo ¼ rwater veo ¼ jrwater meo m
Df L
¼ 19:8 g m2 s1
(4)
It is evident that the actual mass recorded in testing is much less than the theoretical value as calculated above. Some reasons may contribute to the discrepancy: The main reason is the bubbling effect at both interfaces of the electrodes which continuously generates small amount of hydrogen at cathode and oxygen at anode during the testing. Not only these gases may block the inlet and outlet of functional EO pump but also create random voids inside the solid particle channel which leads to the reduction of contact area. Consequently, the actual DC voltage, which applied on the solid desiccant, would vary significantly that affects the performance of solid desiccant regeneration. The solution to avoid this problem or relieve the erosion of the electrodes is that change the active metallic electrodes to the carbon fibre or graphite electrodes which exerts less ions during the testing. Alternatively, it can be modified by releasing a fixed volume above the electrode, which is controllable by valves, in order to give enough space to the bubble floating up. Another critical reason is the interaction of configuration inside the EO pump. It is found that the best performance of regeneration happens when the filter is installed at outside surface of the electrodes which means the solid particles are wrapped by the attached electrodes and Perspex container. The filters are acted as the membranes that keep the desiccant particles inside the container, and also the contained water could be freely pumped outside. Currently, the optimal design is undergoing that identifies the interactions between membrane and electrodes. The last possible reason is that the filter meshing size. As the Zeolite is synthetic (nonpolarised) and purified (dealumination) at size below 45 nm which could challenge our and membrane in use, it is suggested that the more dedicated filter should be applied to maximally avoid the contents loss during operation. However, in our testing, it is still encouraging that the actual removal rate by our prototype is acceptable for the threshold of solid desiccant dehumidification. Hence, a specific evaluation is carried out in next chapter for assessing the feasibility of this testing result in real air conditioning application.
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Fig. 6. Psychrometric cooling processes for (a) Conventional system. (b) EO (L1). (c) EO (L2). (d) EO (L3).
4. EO integrated air conditioning system 4.1. Analysis on EO process in air conditioning application As stated above, the energy consumption in air conditioning system is to overcome the cooling load involving two major factors. One is the sensible cooling load which affects the change of dry bulb temperature; the other is the latent cooling load which fluctuates with the change of moisture content generated by metabolic activity of human body and ambient inherent humid air. Taking the advantage of the specific characteristic of EO phenomena, it already has been approved that the great potential application in Table 1 Ionic mobility for small ions at small concentration.
Mobility 108 m2(Vs)1
Agþ
Kþ
Liþ
Naþ
Br
Cl
F
I
6.42
7.62
4.01
5.19
8.09
7.91
5.7
7.96
regenerating solid desiccant continuously which replace the heat source used for regeneration in conventional solid desiccant cooling system. Besides this, the simple configuration without moving parts makes it better to fit for various requirements. Different from the reheating process of the desiccant wheel, the desiccant powders in the EO unit is continuously retreated by the DC power which means there is a maintained mass gradient between the circumstance and inside the EO unit. Hence, the regeneration method needs no rotary wheel and motor. In EO integrated air conditioning system, the air can be dehumidified directly and the process can be assumed as isenthalpic change. Due to the EO dehumidification effect, the moisture can be conditioned to a certain level before being supplied into the room. Generally, there are three possible locations for the EO dehumidification device, namely EO (L1) located in Point C, EO (L2) located in Point W and EO (L3) in Point L. At stage of EO (L1), the mixed air passes through the dehumidification device and reaches to Point C1 where the moisture content is the same as Point O (L). It means that the mixed air at
B. Li et al. / Building and Environment 48 (2012) 128e134
Point C1 could be cooled directly to Point O (L) without super cooling to dew point to obtain the moisture by conventional condensation process using chilled water. As well known, it costs large amount of energy consumption in super-cooling process resulting from the occurrence of phase change heat. However, scenes are different in EO (L2) process that the fresh air is conditioned separately before being mixed with returned air while the following processes are the same as EO (L1). If EO process is set at point L3, the mixed air is designed to be conditioned to the dew point temperature and then it is dehumidified within the EO dehumidification device. At this stage, most of moisture in terms of the latent load in supply air is removed that the condensate is collected and repelled to the outside system. All these processes have been shown in Fig. 6 Based on above analysis, the energy consumptions for each configuration including conventional air conditioning system and modified EO systems in different locations were concluded. Summarily, it mainly consists of cooling process and reheating process to achieve accurate temperature control and thermal comfort condition. The energy consumption for EO integrated systems is mainly the combination results of adsorption process in solid desiccant, regeneration process regarding to power consumption of EO electric supply and sensible cooling process in terms of conventional air conditioning cycle. The total energy consumption can be calculated using equations shown in Table 2. Theoretically, the system using EO should be more energy efficient. However, the actual location of the EO device can also be reviewed in respect that complex configurations existing in reality. At EO (L2) stage, there is a noticeable change in moisture content during the process W W2 although the flow rate of the air is quite small. As the air velocity in air handling unit is relatively high and the dehumidification capacity using EO is not relatively low, the huge change regarding to the moisture may not be achieved completely in actual situation. Moreover, at stage of EO (L2), the dry bulb temperature of the air may rise significantly when the moisture is supplied into desiccant arrays. It is noticed from the Psychrometric chart that the increase of temperature is much higher than the location at EO (L1). Therefore, EO (L1) location seems more suitable choice for configuration of EO dehumidification device but actual performance needs to be validated in future. Last but not least, the comparison between EO (L1) and EO (L3) can be carried out by analysing the working conditions for compressors which could be different in terms of COP using conventional vapour compression refrigerant cycle with different refrigerant through case study. 4.2. Case study 4.2.1. Design condition In this section, an air conditioning system for a commercial building is designed to meet thermal comfort condition in summer incorporating two systems virtually: conventional air conditioning Table 2 Energy consumption for different systems. Conventional air conditioning system Process Energy input _ c hL Þ Cooling C L QC ¼ mðh _ o hL Þ Reheat L O QR ¼ mðh Total QAC ¼ QC þ QR
EO (L1) Process Energy input EO C C1 Qelec1 _ c1 ho Þ Cooling C1 O (L) QC1 ¼ mðh Total QEO1 ¼ Qelec1 þ QC2
EO (L2) Process EO W W2 Cooling C2 O (L) Total
EO (L3) Process Cooling C L3 EO L3 O Total
Energy input Qelec2 _ c2 ho Þ QC2 ¼ mðh QEO2 ¼ Qelec2 þ QC2
Energy input _ c hL3 Þ QC3 ¼ mðh Qelec3 QEO3 ¼ Qelec3 þ QC3
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system and modified system using EO dehumidification device in three locations. The outside conditions are set as: 35 C dry bulb temperature (tw), 60 % relatively humidity (RHw%) and 101,325 Pa for pressure. Then the sensible cooling load (Qs) is 11.3 kW and the total number of occupants is 60 with latent heat rate 45 W/person. Based on the functions of the building, the design condition can be determined based on CIBSE Guide A [33]. Hence, the design room temperature (TN) is 24 0.5 C and design relative humidity (RHN%) is 55%. The required fresh air flow rate is set as 10 L/s per person. As the allowed error in the room is 0.5 C, the temperature difference between the supply air temperature and room design temperature is set as DTo ¼ 5 C. Therefore, the supply air temperature is To ¼ 24 5 ¼ 19 C. The fresh air ratio is set at 15% from the total volume of air supply at mixing point C. 4.2.2. Energy performance evaluation According to Table 2 the detailed calculation for different situations can be carried out. Different systems integrated with EO regeneration method with different locations have been compared with conventional air conditioning system using vapour compression cycle for cooling. It is distinctive that air conditioning system with EO dehumidification device is more efficient based on our calculations, which means that EO dehumidification method has brighter application in air conditioning technology. In this particular case, the percentage of energy saving is achieved as (54 41.4)/54¼23.3%. It can be also seen that the electricity power consumption to regenerate desiccant in EO (L2) process is slightly lower than EO (L1) process. However it could still be better due to the limit of regeneration rate using EO and operational temperature. As the air velocity in air handling unit is relatively high, it may be difficult to achieve large moisture change in actual situation. In EO (L2) process, there is a large change of moisture content of 0.0122 kg/kg while the moisture content change in EO (L1) process is around 0.0034 kg/kg. Previous experimental result shows that the moisture removal rate is around 0.953 g m2 s1. The required EO device contact area is nearly the same for EO (L2): 7.68 m2 and EO (L1): 8.03 m2 in respectively. However, larger change of moisture content means higher moisture removal rate for EO process with similar contact area. Hence, EO (L1) should be better than EO (L2) because it requires a smaller regeneration rate and can be easily achieved by EO regeneration process. In addition, in EO (L2) process, the dry bulb temperature of the air may rise to around as high as 65 C due to phase change heat from condensation when the moisture in the air has been absorbed into desiccant, whereas the temperature for EO (L1) may only rise from 27 C to 35.5 C. Conclusively, EO (L1) is more competitive with lower requirement of regeneration rate and operational temperature. Comparing EO (L1) with EO (L3), the EO (L3) process still stands out although the calculated energy inputs are the same. Through the calculation of COP on R 134a and ammonia, the ideal COP for compressor in EO (L3) is higher than EO (L1) and this finally leads to the reduction of energy input for the compressor in that evaporative temperature is increased slightly due to EO dehumidification effect. However, further calculation and analysis needs to be done to evaluate the actual performance and difference between EO (L1) and EO (L3). In addition, the application of EO regeneration method has fairly positive effects on the basis of conventional desiccant cooling system. EO integrated air conditioning systems can reduce the energy consumption (Table 3) of fossil fuels due to energy saving with small electricity consumption for EO regeneration process using simple low voltage DC supply rather than high voltage AC supply which means various energy sources can be alternatively used instead of heat source from the low grade energy form. Moreover, all the process can be operated at normal temperature,
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Table 3 Energy consumption for different configuration by EO integration.
Air temperature Refrigerant temperature Cooling (QC) Reheat/EO (QR/Qelec) Total COP R 134a Ammonia
Conventional
EO (L1)
EO (L2)
EO (L3)
27.0e16.5 C 32.0e11.5 C QC ¼ 46.125kW QR ¼ 7.875kW QAC ¼ 54kW 12.19 12.30
35.5e19.0 C 40.5e14.0 C QC1 ¼ 38.25kW Qelec1 ¼ 3.24kW QEO1 ¼ 41.49kW 9.25 9.48
35.5e19.0 C 40.5e14.0 C QC2 ¼ 38.25kW Qelec2 ¼ 3.13kW QEO2 ¼ 41.38kW 9.25 9.48
27.5e16.5 C 32.5e11.5 C QC3 ¼ 38.25kW Qelec3 ¼ 3.24kW QEO3 ¼ 41.49kW 12.03 12.04
EO regeneration method can also partly avoid using Chlorinated Fluorocarbon Compounds (CFCs) which has removed the latent cooling load and the system only needs to overcome the sensible cooling load. This could further reduce the size of compressors, fans, ductworks, etc. and finally reduce the capital cost of the system. In future, the modified standalone system can control the humidity of air independently of its temperature to achieve better comfort level especially for those high humid climate zones where normal cooling system may fail to tackle with large amount of latent heat from outside environment. 5. Conclusions Conclusively, this paper has introduced the development of a novel EO based regeneration method for solid desiccant which has shown a great potential for application in HVAC particularly for dehumidification process. Based on the present studying, it has been found that the EO mass flow rate is achievable at 0.953 g m2 s1. By comparing the theoretical and experimental results, the current limitations of theory and experimental apparatus have been explored, and the reasons for constraining the development of EO performance, such as bubbling effect and electrode erosion, have been identified. It is predictable that the introduction of graphite electrodes or carbon fibre electrodes will make the operation more durable. Furthermore, the progress in fined meshing of filter will promote the EO pump system efficiently. In the last part, comparing different configuration feasibilities in air handling unit, the energy consumption for EO integrated air conditioning system has been evaluated with reference to the testing result. A case study with detailed designs of using different systems involved in both conventional air conditioning systems and the EO integrated systems (with different locations of the EO device) has been carried out. The case study shows that the adoption of the EO regeneration method can offer higher energy efficiency by reducing 23% energy consumption. References [1] Pesaran AA, Penney TT, Czanderna AW. Desiccant cooling state-of-the-art assessment. National Renewable Energy Laboratory, A Division of Midwest Research Institute; 1992. [2] Ma Z, Wang S. Building energy research in Hong Kong: a review. Renewable and Sustainable Energy Reviews 2009;13:1870e83. [3] Dorer V, Weber A. Energy and CO2 emissions performance assessment of residential micro-cogeneration systems with dynamic whole-building simulation programs. Energy Conversion and Management 2009;50:648e57. [4] Diaconu BM. Thermal energy savings in buildings with PCM-enhanced envelope: influence of occupancy pattern and ventilation. Energy and Buildings 2011;43:101e7. [5] Beerepoot M, Beerepoot N. Government regulation as an impetus for innovation: evidence from energy performance regulation in the Dutch residential building sector. Energy Policy 2007;35:4812e25. [6] La D, Dai YJ, Li Y, Wang RZ, Ge TS. Technical development of rotary desiccant dehumidification and air conditioning: a review. Renewable and Sustainable Energy Reviews 2010;14:130e47. [7] Daou K, Wang R, Xia Z. Desiccant cooling air conditioning: a review. Renewable and Sustainable Energy Reviews 2006;10:55e77.
[8] Yu BF, Hu ZB, Liu M, Yang HL, Kong QX, Liu YH. Review of research on airconditioning systems and indoor air quality control for human health. International Journal of Refrigeration 2009;32:3e20. [9] Ghesmat K, Hassanzadeh H, Abedi J. The impact of geochemistry on convective mixing in a gravitationally unstable diffusive boundary layer in porous media: CO2 storage in saline aquifers. Journal of Fluid Mechanics 2011;673: 480e512. [10] Hao X, Zhang G, Chen Y, Zou S, Moschandreas DJ. A combined system of chilled ceiling, displacement ventilation and desiccant dehumidification. Building and Environment 2007;42:3298e308. [11] Qi R, Tian C, Shao S. Experimental investigation on possibility of electroosmotic regeneration for solid desiccant. Applied Energy 2010;87: 2266e72. [12] Qi R, Tian C, Shao S, Tang M, Lu L. Experimental investigation on performance improvement of electro-osmotic regeneration for solid desiccant. Applied Energy 2011;88:2816e23. [13] Yan YY, Hall JB. The concept of electroosmotically driven flow and its application to biomimetics. Journal of Bionic Engineering 2004;1(1):46e52. [14] Yan YY, Zu YQ, Ren LQ, Li JQ. Numerical modelling of electroosmotically driven flow within the micro thin liquid layer near an earthworm surface e a biomimetic approachProceedings of the Institution of Mechanical Engineers, Part C. Journal of Mechanical Engineering Science; 2007:1201e10. [15] Wang MR, Chen SY. Electroosmosis in homogeneously charged micro- and nanoscale random porous media. Journal of Colloid and Interface Science 2007;314:264e73. [16] Zu YQ, Yan Y. Numerical simulation of electroosmotic flow near earthworm surface. Journal of Bionic Engineering 2006;3:179e86. [17] Bertolini L, Coppola L, Gastaldi M, Redaelli E. Electroosmotic transport in porous construction materials and dehumidification of masonry. Construction and Building Materials 2009;23:254e63. [18] Neve RS, Yan YY. Enhancement of heat exchanger performance using combined electrohydrodynamic and passive methods. International Journal of Heat and Fluid Flow 1996;17(4):403e9. [19] Mahmoud A, Olivier J, Vaxelaire J, Hoadley AFA. Electrical field: A historical review of its application and contributions in wastewater sludge dewatering. Water Research 2010;44:2381e407. [20] Raats MHM, Van Diemen AJG, Lavèn J, Stein HN. Full scale electrokinetic dewatering of waste sludge. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2002;210:231e41. [21] Morefield SW, McInerney MK, Hock VF, Marshall OS. Interface modeling for electro-osmosis in subgrade structures; 2004. [22] David WG. Electro-osmosis for dehumidification. School of Mechanical Engineering, Georgia Institute of Technology; 2006. [23] Zeng SL, Chen CH, Mikkelsen JC, Santiago JG. Fabrication and characterization of electroosmotic micropumps. Sensors and Actuators B: Chemical 2001;79: 107e14. [24] Li Yan. Solid desiccant dehumidification techniques inspired from natural electroosmosis phenomena. Journal of Bionic Engineering 2011;8:90e7. [25] Mei VC, Chen FC, Lavan Z, Collier RK, Meckler G. An assessment of desiccant cooling and dehumidification technoloy; 1992. [26] Harriman L. In: The ASHRAE guide for buildings in hot and humid climates. 2nd ed. ASHRAE; 2009. [27] Calton DS. Application of a desiccant cooling system to supermarkets. ASHRAE Transactions 1985;91(1B):441e6. [28] Burns PR, Mitchell RB, Bechman WA. Hybrid desiccant cooling systems in supermarket applications. ASHRAE Transactions 1985;91(1B):457e68. [29] Mazzei P, Minichiello F, Palma D. HVAC dehumidification systems for thermal comfort: a critical review. Applied Thermal Engineering 2005;25: 677e707. [30] Bellia L, Mazzei P, Minichiello F, Palma D. Air conditioning systems with desiccant wheel for Italian climates. International Journal on Architectural Science 2000;4:193e213. [31] Mei L, Dai Y. A technical review on use of liquid-desiccant dehumidification for air-conditioning application. Renewable and Sustainable Energy Reviews 2008;12:662e89. [32] Bruus H. Theoretical microfluidics. Oxford University Press; 2008. [33] CIBSE. CIBSE guide A, environmental design. Norwich, Norfolk NR6 6SA, Great Britain/London: Page Bros. (Norwich) Ltd./The Chartered Institution of Building Services Engineers; 2006.
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Development of solid desiccant dehumidification using electro-osmosis regeneration method for HVAC application B. Li, Q.Y. Lin, Y.Y. Yan* Energy & Sustainability Research Division, Faculty of Engineering, University of Nottingham, Lenton Firs Building, 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 9 May 2011 Received in revised form 5 September 2011 Accepted 7 September 2011
In this paper a novel method of electro-osmosis (EO) based regeneration for solid desiccant is reported, and the potential of the method for application in HVAC particularly for dehumidification process in air conditioning system is discussed. The results of the measurement of water removal rate inside solid desiccant array show that the mass flow rate of EO driven mass flow is achievable at 0.953 g m2 s1 with 20 V DC voltage on the installed electrodes. By comparing the theoretical and experimental results, the current limitations of theory and experimental apparatus were explored. The experimental results show that the energy consumption in EO integrated air conditioning system is averagely 23.3% lower than the conventional air conditioning system in respect of the different configurations in air handling process. In the case study, among the three potential locations for EO device, the position in Point C is the best choice with an ideal COP of 12. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Solid desiccant dehumidification Electro-osmosis HVAC Microporous media Latent heat recovery
1. Introduction Among the energy consumptions in the buildings, HVAC systems currently cover about 30e50% of total amount consumption. Due to higher demand of thermal comfort inside building and larger energy consumptions within air conditioning system, the improvements of the whole system using less energy and being more efficient is crucial to achieve sustainability in building service sector. Since the 1990s, the air conditioning industry has been faced with a number of challenges including the increased energy efficiencies, the improved indoor air quality, the growing concern for improved comfort and environmental control, the increased ventilation requirements, the reduction of chlorofluorocarbons (CFCs), and the rising peak demand charges [1,2]. New approaches to space conditioning will be required urgently to resolve these economic, environmental, and regulatory issues [3e5]. To achieve cooling purpose, the cooling coil is supplied by vapour compression cycle using refrigerant. In conventional air conditioning system, the cooling capacity needs to match the cooling load, which consists of sensible heat and latent heat generated by occupants and some facilities. A common commercial air conditioning system mainly consists of a cooling coil and heating coil with supply/extract fans as shown in Fig. 1.
* Corresponding author. Tel.: þ44 0115 846 7263. E-mail address: [email protected] (Y.Y. Yan). 0360-1323/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2011.09.008
Although latent heat counts less ratio of total load from ambient normally, but it is relatively difficult to remove them effectively in the process due to the phase change and heat exhausting from it. Moreover, considering the indoor air quality and energy use issues, which significantly influence the application of HVAC system, the problems from high humidity need to be re-considered as around 80% of the dehumidification load comes from outside fresh air and it can affect the performance of the air conditioning system dominantly [6]. Hence, dealing with the latent cooling load, which can be commonly achieved by dehumidification process, is a critical and decisive stage in air conditioning process that can largely influence the whole energy consumption. In commercial buildings, the traditional way to control the humidity is to cool the air down to dew point temperature till condensation and then reheat them to the design temperature. It is obviously that the reheating process in the cycle costs large amount of energy inevitably. The industry is, however, quickly learning that by controlling the humidity to 50% RH or less, and independent of temperature, dry bulb temperature can be raised to 25e26 C in consideration of thermal comfort, yielding great energy savings in the meantime. For every degree of the set points in temperature is raised, a 5% reduction in air conditioning energy is realized [7]. However, both solid desiccant and liquid desiccant dehumidification techniques in existing industrial application require considerable heat and electricity to operate the regeneration process which is less competitive in domestic dwellings and office buildings [8]. Hence, the development of dehumidification techniques, in retrospect, is urgently to
B. Li et al. / Building and Environment 48 (2012) 128e134
Nomenclature D L veo 3
z h Df E
mion meo j _ eo m
rwater
Diameter of the cylindrical container The length of EO process Average velocity of EO flow Electric permittivity Zeta potential Dynamic viscosity Electrical potential drop Electric field Ionic mobility Electro-osmosis mobility Porosity Mass flow rate for EO driven flow per m2 Density of water
be expended in domestic application as replacement of current products in the market [9]. Particularly, in some extremely hot and humid climate area, the current existing technology cannot meet the comfort requirements due to the humidity removal capacity of the system, and the crystallization, corrosion problems of haloids on the wheel or desiccant pool [10]. Therefore, the study on EO dehumidification method will be the first of its kind in domestic application and definitely it will keep the industry sustainable. Based on our reviews, with respect to this novel application, the system design is a dominant factor for validating the electrochemical phenomenon. Qi et al. designed an EO system and used desiccant powders in a humid environmental chamber to achieve dehumidification [11,12]. On their system, the flow rate of water removal was measured at 0.0021 g s1. Although DC pulse was applied into the application to reduce the Joule effect and erosions on the electrodes, there are still some obstacles to obtain higher capacity and longer durability which were dominant by the system design and configuration. Hence, a more dedicated system design is needed to precisely and simultaneously measure the actual EO performance. The electro-osmotic flow (EOF) theory with a historical retrospect based on the electrokinetic effects is observed by F.F. Reuss in 1809 when doing an experimental investigation on porous clay, and it has been currently widely studied and analyzed in areas such as electrochemistry, physics and vascular plant biology [13e16]. Due to its importance, EOF theory has currently been applied in various fields such as chemical engineering [17,18] and civil engineering including mine tailing water prevention and waste sludge treatment [19e21]. Basically, EOF is the motion of polar liquid passing through porous structure, micro-channel or other membrane under
129
applied electric field [22]. It has also been used to characterize and design the salt rejection properties of reverse osmosis membranes and to help understand the behaviour of biological membranes. In this paper, the EOF theory has been used to guide the regenerate solid desiccant by regarding desiccant structure as micro-channel and applying electric field between the desiccants membrane. The moisture adsorbed by desiccant can be pumped out in a certain rate. Generally, electro-osmosis flow is a flow in porous media or micro-channel structure, driven by so-called EO force with applied electrical field. The EOF dynamics in porous media has been studied much both theoretically and numerically. Most of papers are based on the theory of double layer model which was developed by Stern and Guoy-Chapman, respectively. For most EO flow, an electric double layer (EDL) will be formed spontaneously at the liquid/solid interface due to the electrochemical reaction. As depicted in Fig. 2, the charged surface will attract positive ions present in the water and repel negative ions owing to the polarization effect of porous media and this region at liquid/solid interface is the so called EDL. In the inner layer, the counter ions near the wall are called the Stern layer while the outer diffuse layer is called Gouy-Chapman layer [23]. The positive ions will therefore predominate in the Debye sheath next to the charged surface so application of an external electric field results in a net migration towards the cathode of ions in the surface water layer. Due to viscous drag, the water in pores is drawn by ions and therefore flows through the porous media. The schematic diagram of EOF in porous media and its velocity profile has been shown in Fig. 3. In EO flow, the process will gradually become weaker due to several reasons. One of the most significant reasons is the corrosion in electrodes and the process will be interrupted when electrodes are fully corroded and the pH of the flow will also change. This occurs due to the oxidation and reduction reactions at the anode and cathode [21]. The anode will become acidic as carbon acid is formed by the reaction due to carbon dioxide with water and cathode becomes basic. Due to the movement of ions from anode to cathode, as the pH become more basic, some composites like complex calcium precipitates will be formed and these composites will fill the voids along the porous material, which will significantly reduce the EO flow and finally interrupt the flow. For most EO flow, it is influenced by several forces such as gravity, hydraulic pressure, viscosity, temperature driven force and EO but the hydrostatic pressure and electro-osmotic force are dominant components. Therefore, the total flow rate becomes the sum of EO flow and hydrodynamic flow [24]. As one of the popular alternatives dealing with separate humidity control, the desiccant system which has a large potential market including applications in schools, restaurants, hotels and
Fig. 1. Schematic diagram for a conventional air conditioning system.
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B. Li et al. / Building and Environment 48 (2012) 128e134
Fig. 2. Charge distribution and water affinity form at interface with solid desiccants.
food stores as it can significantly reduce operational costs, improve indoor air quality and achieve precise humidity control [25]. However, the current desiccant cooling systems have not been used widely as it should because its efficiency is still below break-even. The energy invested in running the system was more than the traditional air conditioning system. And it is this reason that may lead to a narrow market and less popular than existing techniques. Unlike traditional vapour compression cooling systems which are driven by electrical power to cool and dehumidify air, desiccant systems, instead, use thermal energy to accomplish the same effect. Generally, a desiccant cooling system may use either solid or liquid desiccant to dry the air by adsorbing moisture. And then this air is cooled down by evaporative cooling method and sent to the airconditioned space finally. The saturated desiccant is usually reheated by gas or electric heating to release moisture, which is defined as the conventional regeneration method and prevailing in the current HVAC market [26]. The rapid development of desiccant air conditioning technology, which can handle sensible and latent heat loads independently, has expanded desiccant applications into hospitals, supermarkets, restaurants, theatres, schools, office buildings, stores or even agricultural related but it still consumes a large amount of energy for reactivation of desiccants [27e31]. It should be noted that whereas the reactivation heat in the original desiccant systems was produced by burning natural gas, all-electric desiccant systems are
Fig. 3. Principle of EOF.
now available. These existing desiccant systems are not very efficient and quite simple in terms of function and operational range in that they use rejected heat from the condenser to regenerate the desiccant wheel [1]. Although the heat source can be supplied using renewable energy such as solar thermal, the energy requirement still makes the whole system less competitive. Therefore, the novel idea of using EO theory to regenerate solid desiccant has been technically addressed here. Based on this new idea, the EO phenomena in nature and the applications in Microelectromechanical systems (MEMS) fields have been conclusively reviewed. It is also promising that applies this technique into a larger scale like dehumidification in buildings as it consumes low DC voltage which can be easily fetched through various means. Hence, this can be potentially improved by using a new regenerating method: Electro-osmosis regeneration method instead of conventional heat source mentioned above. The general principle of EOF in air conditioning application is that the moisture can be absorbed by a membrane composed of a desiccant, being removed across the membrane and rejected on the other side by electroosmotic pumping. Based on electro-osmotic counter pressure concept, the liquid flow occurs in the direction of the electric field. Positive ions move towards cathode, dragging water molecules. 2. Experimental setup and method In order to evaluate the performance of EO process on regenerating solid desiccant and optimise our design and configuration, an experiment is carried out using Zeolite as desiccant forming the micro-channel structure. To achieve precise measurement and avoid vapour fluctuating from testing chamber as some investigators carried in the past, the separation of generated bubble and the actual contact area are reinvestigated [14]. Fig. 4 shows the improved schematic diagram of the EO regeneration system. The apparatus includes the EO device, the electronic scale and a data logger. Zeolite has been filled into the EO device, a simple cylindrical container (D 16 mm L 12 mm) between two electrodes rings. Two pieces of filter paper are used between the electrodes in both sides to block leakage from the container. Considering the thickness of the electrodes and filter paper, the actual length for EO process is set as 10 mm. The water used in the experiment is de-
Fig. 4. Schematic diagram of the EO regeneration system.
B. Li et al. / Building and Environment 48 (2012) 128e134
ionized water to make the ions concentration at low level and keep the control volume unaffected by the outside changes of concentration especially for the charges from solution. To begin with the experiment, water was continuously adsorbed by the Zeolite before applying the electric field and the mass of droplet at the bottom was collected and measured by the electric weight scale to evaluate the effect caused by gravity. When mass flow reached stable and Zeolite was saturated, 20 V DC voltage is applied and the mass of droplet is recorded by a data logger. Theoretically, the mass flow rate can be calculated by multiplying the theoretical total volume flow rate driven by EO process with water density, where the total volume flow rate is the sum of flow rate in all the micro-channels formed by Zeolite particles. Due to the complexity of the actual micro-channel in porous medium formed by the Zeolite particles and the irregular shape of each desiccant, an average EO velocity is calculated macroscopically [32]; and the velocity driven by EO force, veo, is defined by the HelmholteSmoluchowski relation as:
veo ¼
z Df E ¼ meo E ¼ meo h L
3
(1)
Therefore, considering the porosity of Zeolite, the velocity equation could be adjusted as:
z h
3
veo ¼ j E ¼ jmeo
Df
(2)
L
Thus, the theoretical mass flow rate for EO driven flow per m2 becomes:
_ eo ¼ rwater veo ¼ jrwater meo m
Df L
(3)
3. Results and discussion Following the experimental procedure using the apparatus, the results for both cumulative collected mass of water and pure EO regeneration rate have been shown in Fig. 5. As depicted in Fig. 6, at the beginning of the experiment, the mass of collected water is relatively slow, approximately 0.038 g/min. This may be contributed by the adsorption ability of Zeolite. When the Zeolite is saturated, the water vapour is condensed to small droplets which are collected by a container. According to the results, the mass of the collected water is up to 0.0038 g/min. After
Fig. 5. Change of mass of collected water.
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applying 20 V DC, simultaneously extra droplets are created and the mass of collected water is ramped up to a rate of 0.0154 g/min over a period of 70 min. As the flow rate created by gravitational force is 0.038 g/min, then the result due to electro-osmosis process is calculated as 0.0115 g/min by deducting flow rate value caused by gravitational force factor from the total flow rate recorded here. By specifying the flow density of electro-osmosis, it is easily calculated as 0.953 g m2 s1 which is divided by the actual testing volume from electro-osmosis flow rate in the pump. In fact, the effect of EO pumping force is significant from the results and it shows a great potential to regenerate desiccant. Theoretically, the mass flow rate value can be calculated by multiplying the theoretical total volume flow rate driven by EO process with water density, where the total volume flow rate is the sum of flow rate in all the micro-channels formed by Zeolite particles. Due to the complexity of the actual micro-channel in porous medium formed by the Zeolite particles and irregular shape of each desiccant, an average EO velocity is calculated macroscopically. In the experiment, although the de-ionized water has been used, there could still be a low ion concentration due to the ions in solid particles are partly soluble in liquid. At room temperature, the mobility, mion, has been shown in Table 1 [32] and meo in the experiment is cited as 6.6 108 m2 (Vs)1. Therefore, the theoretical mass flow rate for EO driven flow per m2 is calculated as:
_ eo ¼ rwater veo ¼ jrwater meo m
Df L
¼ 19:8 g m2 s1
(4)
It is evident that the actual mass recorded in testing is much less than the theoretical value as calculated above. Some reasons may contribute to the discrepancy: The main reason is the bubbling effect at both interfaces of the electrodes which continuously generates small amount of hydrogen at cathode and oxygen at anode during the testing. Not only these gases may block the inlet and outlet of functional EO pump but also create random voids inside the solid particle channel which leads to the reduction of contact area. Consequently, the actual DC voltage, which applied on the solid desiccant, would vary significantly that affects the performance of solid desiccant regeneration. The solution to avoid this problem or relieve the erosion of the electrodes is that change the active metallic electrodes to the carbon fibre or graphite electrodes which exerts less ions during the testing. Alternatively, it can be modified by releasing a fixed volume above the electrode, which is controllable by valves, in order to give enough space to the bubble floating up. Another critical reason is the interaction of configuration inside the EO pump. It is found that the best performance of regeneration happens when the filter is installed at outside surface of the electrodes which means the solid particles are wrapped by the attached electrodes and Perspex container. The filters are acted as the membranes that keep the desiccant particles inside the container, and also the contained water could be freely pumped outside. Currently, the optimal design is undergoing that identifies the interactions between membrane and electrodes. The last possible reason is that the filter meshing size. As the Zeolite is synthetic (nonpolarised) and purified (dealumination) at size below 45 nm which could challenge our and membrane in use, it is suggested that the more dedicated filter should be applied to maximally avoid the contents loss during operation. However, in our testing, it is still encouraging that the actual removal rate by our prototype is acceptable for the threshold of solid desiccant dehumidification. Hence, a specific evaluation is carried out in next chapter for assessing the feasibility of this testing result in real air conditioning application.
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Fig. 6. Psychrometric cooling processes for (a) Conventional system. (b) EO (L1). (c) EO (L2). (d) EO (L3).
4. EO integrated air conditioning system 4.1. Analysis on EO process in air conditioning application As stated above, the energy consumption in air conditioning system is to overcome the cooling load involving two major factors. One is the sensible cooling load which affects the change of dry bulb temperature; the other is the latent cooling load which fluctuates with the change of moisture content generated by metabolic activity of human body and ambient inherent humid air. Taking the advantage of the specific characteristic of EO phenomena, it already has been approved that the great potential application in Table 1 Ionic mobility for small ions at small concentration.
Mobility 108 m2(Vs)1
Agþ
Kþ
Liþ
Naþ
Br
Cl
F
I
6.42
7.62
4.01
5.19
8.09
7.91
5.7
7.96
regenerating solid desiccant continuously which replace the heat source used for regeneration in conventional solid desiccant cooling system. Besides this, the simple configuration without moving parts makes it better to fit for various requirements. Different from the reheating process of the desiccant wheel, the desiccant powders in the EO unit is continuously retreated by the DC power which means there is a maintained mass gradient between the circumstance and inside the EO unit. Hence, the regeneration method needs no rotary wheel and motor. In EO integrated air conditioning system, the air can be dehumidified directly and the process can be assumed as isenthalpic change. Due to the EO dehumidification effect, the moisture can be conditioned to a certain level before being supplied into the room. Generally, there are three possible locations for the EO dehumidification device, namely EO (L1) located in Point C, EO (L2) located in Point W and EO (L3) in Point L. At stage of EO (L1), the mixed air passes through the dehumidification device and reaches to Point C1 where the moisture content is the same as Point O (L). It means that the mixed air at
B. Li et al. / Building and Environment 48 (2012) 128e134
Point C1 could be cooled directly to Point O (L) without super cooling to dew point to obtain the moisture by conventional condensation process using chilled water. As well known, it costs large amount of energy consumption in super-cooling process resulting from the occurrence of phase change heat. However, scenes are different in EO (L2) process that the fresh air is conditioned separately before being mixed with returned air while the following processes are the same as EO (L1). If EO process is set at point L3, the mixed air is designed to be conditioned to the dew point temperature and then it is dehumidified within the EO dehumidification device. At this stage, most of moisture in terms of the latent load in supply air is removed that the condensate is collected and repelled to the outside system. All these processes have been shown in Fig. 6 Based on above analysis, the energy consumptions for each configuration including conventional air conditioning system and modified EO systems in different locations were concluded. Summarily, it mainly consists of cooling process and reheating process to achieve accurate temperature control and thermal comfort condition. The energy consumption for EO integrated systems is mainly the combination results of adsorption process in solid desiccant, regeneration process regarding to power consumption of EO electric supply and sensible cooling process in terms of conventional air conditioning cycle. The total energy consumption can be calculated using equations shown in Table 2. Theoretically, the system using EO should be more energy efficient. However, the actual location of the EO device can also be reviewed in respect that complex configurations existing in reality. At EO (L2) stage, there is a noticeable change in moisture content during the process W W2 although the flow rate of the air is quite small. As the air velocity in air handling unit is relatively high and the dehumidification capacity using EO is not relatively low, the huge change regarding to the moisture may not be achieved completely in actual situation. Moreover, at stage of EO (L2), the dry bulb temperature of the air may rise significantly when the moisture is supplied into desiccant arrays. It is noticed from the Psychrometric chart that the increase of temperature is much higher than the location at EO (L1). Therefore, EO (L1) location seems more suitable choice for configuration of EO dehumidification device but actual performance needs to be validated in future. Last but not least, the comparison between EO (L1) and EO (L3) can be carried out by analysing the working conditions for compressors which could be different in terms of COP using conventional vapour compression refrigerant cycle with different refrigerant through case study. 4.2. Case study 4.2.1. Design condition In this section, an air conditioning system for a commercial building is designed to meet thermal comfort condition in summer incorporating two systems virtually: conventional air conditioning Table 2 Energy consumption for different systems. Conventional air conditioning system Process Energy input _ c hL Þ Cooling C L QC ¼ mðh _ o hL Þ Reheat L O QR ¼ mðh Total QAC ¼ QC þ QR
EO (L1) Process Energy input EO C C1 Qelec1 _ c1 ho Þ Cooling C1 O (L) QC1 ¼ mðh Total QEO1 ¼ Qelec1 þ QC2
EO (L2) Process EO W W2 Cooling C2 O (L) Total
EO (L3) Process Cooling C L3 EO L3 O Total
Energy input Qelec2 _ c2 ho Þ QC2 ¼ mðh QEO2 ¼ Qelec2 þ QC2
Energy input _ c hL3 Þ QC3 ¼ mðh Qelec3 QEO3 ¼ Qelec3 þ QC3
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system and modified system using EO dehumidification device in three locations. The outside conditions are set as: 35 C dry bulb temperature (tw), 60 % relatively humidity (RHw%) and 101,325 Pa for pressure. Then the sensible cooling load (Qs) is 11.3 kW and the total number of occupants is 60 with latent heat rate 45 W/person. Based on the functions of the building, the design condition can be determined based on CIBSE Guide A [33]. Hence, the design room temperature (TN) is 24 0.5 C and design relative humidity (RHN%) is 55%. The required fresh air flow rate is set as 10 L/s per person. As the allowed error in the room is 0.5 C, the temperature difference between the supply air temperature and room design temperature is set as DTo ¼ 5 C. Therefore, the supply air temperature is To ¼ 24 5 ¼ 19 C. The fresh air ratio is set at 15% from the total volume of air supply at mixing point C. 4.2.2. Energy performance evaluation According to Table 2 the detailed calculation for different situations can be carried out. Different systems integrated with EO regeneration method with different locations have been compared with conventional air conditioning system using vapour compression cycle for cooling. It is distinctive that air conditioning system with EO dehumidification device is more efficient based on our calculations, which means that EO dehumidification method has brighter application in air conditioning technology. In this particular case, the percentage of energy saving is achieved as (54 41.4)/54¼23.3%. It can be also seen that the electricity power consumption to regenerate desiccant in EO (L2) process is slightly lower than EO (L1) process. However it could still be better due to the limit of regeneration rate using EO and operational temperature. As the air velocity in air handling unit is relatively high, it may be difficult to achieve large moisture change in actual situation. In EO (L2) process, there is a large change of moisture content of 0.0122 kg/kg while the moisture content change in EO (L1) process is around 0.0034 kg/kg. Previous experimental result shows that the moisture removal rate is around 0.953 g m2 s1. The required EO device contact area is nearly the same for EO (L2): 7.68 m2 and EO (L1): 8.03 m2 in respectively. However, larger change of moisture content means higher moisture removal rate for EO process with similar contact area. Hence, EO (L1) should be better than EO (L2) because it requires a smaller regeneration rate and can be easily achieved by EO regeneration process. In addition, in EO (L2) process, the dry bulb temperature of the air may rise to around as high as 65 C due to phase change heat from condensation when the moisture in the air has been absorbed into desiccant, whereas the temperature for EO (L1) may only rise from 27 C to 35.5 C. Conclusively, EO (L1) is more competitive with lower requirement of regeneration rate and operational temperature. Comparing EO (L1) with EO (L3), the EO (L3) process still stands out although the calculated energy inputs are the same. Through the calculation of COP on R 134a and ammonia, the ideal COP for compressor in EO (L3) is higher than EO (L1) and this finally leads to the reduction of energy input for the compressor in that evaporative temperature is increased slightly due to EO dehumidification effect. However, further calculation and analysis needs to be done to evaluate the actual performance and difference between EO (L1) and EO (L3). In addition, the application of EO regeneration method has fairly positive effects on the basis of conventional desiccant cooling system. EO integrated air conditioning systems can reduce the energy consumption (Table 3) of fossil fuels due to energy saving with small electricity consumption for EO regeneration process using simple low voltage DC supply rather than high voltage AC supply which means various energy sources can be alternatively used instead of heat source from the low grade energy form. Moreover, all the process can be operated at normal temperature,
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Table 3 Energy consumption for different configuration by EO integration.
Air temperature Refrigerant temperature Cooling (QC) Reheat/EO (QR/Qelec) Total COP R 134a Ammonia
Conventional
EO (L1)
EO (L2)
EO (L3)
27.0e16.5 C 32.0e11.5 C QC ¼ 46.125kW QR ¼ 7.875kW QAC ¼ 54kW 12.19 12.30
35.5e19.0 C 40.5e14.0 C QC1 ¼ 38.25kW Qelec1 ¼ 3.24kW QEO1 ¼ 41.49kW 9.25 9.48
35.5e19.0 C 40.5e14.0 C QC2 ¼ 38.25kW Qelec2 ¼ 3.13kW QEO2 ¼ 41.38kW 9.25 9.48
27.5e16.5 C 32.5e11.5 C QC3 ¼ 38.25kW Qelec3 ¼ 3.24kW QEO3 ¼ 41.49kW 12.03 12.04
EO regeneration method can also partly avoid using Chlorinated Fluorocarbon Compounds (CFCs) which has removed the latent cooling load and the system only needs to overcome the sensible cooling load. This could further reduce the size of compressors, fans, ductworks, etc. and finally reduce the capital cost of the system. In future, the modified standalone system can control the humidity of air independently of its temperature to achieve better comfort level especially for those high humid climate zones where normal cooling system may fail to tackle with large amount of latent heat from outside environment. 5. Conclusions Conclusively, this paper has introduced the development of a novel EO based regeneration method for solid desiccant which has shown a great potential for application in HVAC particularly for dehumidification process. Based on the present studying, it has been found that the EO mass flow rate is achievable at 0.953 g m2 s1. By comparing the theoretical and experimental results, the current limitations of theory and experimental apparatus have been explored, and the reasons for constraining the development of EO performance, such as bubbling effect and electrode erosion, have been identified. It is predictable that the introduction of graphite electrodes or carbon fibre electrodes will make the operation more durable. Furthermore, the progress in fined meshing of filter will promote the EO pump system efficiently. In the last part, comparing different configuration feasibilities in air handling unit, the energy consumption for EO integrated air conditioning system has been evaluated with reference to the testing result. A case study with detailed designs of using different systems involved in both conventional air conditioning systems and the EO integrated systems (with different locations of the EO device) has been carried out. The case study shows that the adoption of the EO regeneration method can offer higher energy efficiency by reducing 23% energy consumption. References [1] Pesaran AA, Penney TT, Czanderna AW. Desiccant cooling state-of-the-art assessment. National Renewable Energy Laboratory, A Division of Midwest Research Institute; 1992. [2] Ma Z, Wang S. Building energy research in Hong Kong: a review. Renewable and Sustainable Energy Reviews 2009;13:1870e83. [3] Dorer V, Weber A. Energy and CO2 emissions performance assessment of residential micro-cogeneration systems with dynamic whole-building simulation programs. Energy Conversion and Management 2009;50:648e57. [4] Diaconu BM. Thermal energy savings in buildings with PCM-enhanced envelope: influence of occupancy pattern and ventilation. Energy and Buildings 2011;43:101e7. [5] Beerepoot M, Beerepoot N. Government regulation as an impetus for innovation: evidence from energy performance regulation in the Dutch residential building sector. Energy Policy 2007;35:4812e25. [6] La D, Dai YJ, Li Y, Wang RZ, Ge TS. Technical development of rotary desiccant dehumidification and air conditioning: a review. Renewable and Sustainable Energy Reviews 2010;14:130e47. [7] Daou K, Wang R, Xia Z. Desiccant cooling air conditioning: a review. Renewable and Sustainable Energy Reviews 2006;10:55e77.
[8] Yu BF, Hu ZB, Liu M, Yang HL, Kong QX, Liu YH. Review of research on airconditioning systems and indoor air quality control for human health. International Journal of Refrigeration 2009;32:3e20. [9] Ghesmat K, Hassanzadeh H, Abedi J. The impact of geochemistry on convective mixing in a gravitationally unstable diffusive boundary layer in porous media: CO2 storage in saline aquifers. Journal of Fluid Mechanics 2011;673: 480e512. [10] Hao X, Zhang G, Chen Y, Zou S, Moschandreas DJ. A combined system of chilled ceiling, displacement ventilation and desiccant dehumidification. Building and Environment 2007;42:3298e308. [11] Qi R, Tian C, Shao S. Experimental investigation on possibility of electroosmotic regeneration for solid desiccant. Applied Energy 2010;87: 2266e72. [12] Qi R, Tian C, Shao S, Tang M, Lu L. Experimental investigation on performance improvement of electro-osmotic regeneration for solid desiccant. Applied Energy 2011;88:2816e23. [13] Yan YY, Hall JB. The concept of electroosmotically driven flow and its application to biomimetics. Journal of Bionic Engineering 2004;1(1):46e52. [14] Yan YY, Zu YQ, Ren LQ, Li JQ. Numerical modelling of electroosmotically driven flow within the micro thin liquid layer near an earthworm surface e a biomimetic approachProceedings of the Institution of Mechanical Engineers, Part C. Journal of Mechanical Engineering Science; 2007:1201e10. [15] Wang MR, Chen SY. Electroosmosis in homogeneously charged micro- and nanoscale random porous media. Journal of Colloid and Interface Science 2007;314:264e73. [16] Zu YQ, Yan Y. Numerical simulation of electroosmotic flow near earthworm surface. Journal of Bionic Engineering 2006;3:179e86. [17] Bertolini L, Coppola L, Gastaldi M, Redaelli E. Electroosmotic transport in porous construction materials and dehumidification of masonry. Construction and Building Materials 2009;23:254e63. [18] Neve RS, Yan YY. Enhancement of heat exchanger performance using combined electrohydrodynamic and passive methods. International Journal of Heat and Fluid Flow 1996;17(4):403e9. [19] Mahmoud A, Olivier J, Vaxelaire J, Hoadley AFA. Electrical field: A historical review of its application and contributions in wastewater sludge dewatering. Water Research 2010;44:2381e407. [20] Raats MHM, Van Diemen AJG, Lavèn J, Stein HN. Full scale electrokinetic dewatering of waste sludge. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2002;210:231e41. [21] Morefield SW, McInerney MK, Hock VF, Marshall OS. Interface modeling for electro-osmosis in subgrade structures; 2004. [22] David WG. Electro-osmosis for dehumidification. School of Mechanical Engineering, Georgia Institute of Technology; 2006. [23] Zeng SL, Chen CH, Mikkelsen JC, Santiago JG. Fabrication and characterization of electroosmotic micropumps. Sensors and Actuators B: Chemical 2001;79: 107e14. [24] Li Yan. Solid desiccant dehumidification techniques inspired from natural electroosmosis phenomena. Journal of Bionic Engineering 2011;8:90e7. [25] Mei VC, Chen FC, Lavan Z, Collier RK, Meckler G. An assessment of desiccant cooling and dehumidification technoloy; 1992. [26] Harriman L. In: The ASHRAE guide for buildings in hot and humid climates. 2nd ed. ASHRAE; 2009. [27] Calton DS. Application of a desiccant cooling system to supermarkets. ASHRAE Transactions 1985;91(1B):441e6. [28] Burns PR, Mitchell RB, Bechman WA. Hybrid desiccant cooling systems in supermarket applications. ASHRAE Transactions 1985;91(1B):457e68. [29] Mazzei P, Minichiello F, Palma D. HVAC dehumidification systems for thermal comfort: a critical review. Applied Thermal Engineering 2005;25: 677e707. [30] Bellia L, Mazzei P, Minichiello F, Palma D. Air conditioning systems with desiccant wheel for Italian climates. International Journal on Architectural Science 2000;4:193e213. [31] Mei L, Dai Y. A technical review on use of liquid-desiccant dehumidification for air-conditioning application. Renewable and Sustainable Energy Reviews 2008;12:662e89. [32] Bruus H. Theoretical microfluidics. Oxford University Press; 2008. [33] CIBSE. CIBSE guide A, environmental design. Norwich, Norfolk NR6 6SA, Great Britain/London: Page Bros. (Norwich) Ltd./The Chartered Institution of Building Services Engineers; 2006.