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Extremely high efficient heat pump with desiccant coated evaporator and condenser
Energy 170 (2019) 569e579
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Extremely high efficient heat pump with desiccant coated evaporator and condenser L.J. Hua, T.S. Ge, R.Z. Wang* Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2018 Received in revised form 17 December 2018 Accepted 22 December 2018
The desiccant coated heat exchanger (DCHE), utilizing inner cooling source to ease the side effect of sorption heat, is now a hot spot in the field of humidity control. Meanwhile, when incorporated in heat pumps, the function of DCHEs is expected to be further extended. Named as solid desiccant heat pump (SDHP), such system can realize weakly-coupled temperature and humidity control both in the cooling and heating modes. Promising refrigerants are near-azeotropic mixtures, and the DCHEs in SDHP then function as desiccant-coated evaporators/condensers (DCEs/DCCs). In this paper, a comprehensive physical and mathematical description of the DCEs/DCCs is first proposed to conduct the thermodynamic analysis of the component. Furthermore, the system simulation of the SDHP validates the feasibility of the vapor compression (VC) cycle with the new type of heat exchangers (HXs), and the calculated coefficient of performance (COP) is 7.19. The precision of the component model, along with the superiority of the SDHP, is then verified by an experimental setup. The experimental study also reveals that, with the outdoor air of 35 C, 21 g/kg (indoor 25%, 10 g/kg, 70%fresh air), the SDHP can provide cold and dry supply air of 20 C, 8.5 g/kg and the corresponding COP reaches to 7.14. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Solid desiccant heat pump Desiccant-coated evaporator Desiccant-coated condenser Coupled heat and mass transfer Two-phase refrigerant
1. Introduction With the rising standards of living and diminishing resources, nowadays there is an urgent need for high-efficient methods of air quality adjustment and energy recovery. In the field regarding humidity control, solid desiccant has arisen wide attention, for its advantages like feasibility, adaptability, energy conservation and environment protection [1]. The rotary wheel is the widely adopted system taking advantages of solid adsorbent. However, the device features increasing adsorption temperature and thus decaying dehumidification capacity along with high regeneration temperature, due to the inherently positive adsorption heat of the material. The recent studies on rotary wheels focus on developing isothermal dehumidification [2], which can be realized by directing air alternatively through infinite desiccant wheels and intercoolers [3]. Multistage rotary setups with intercooler(s) are then built based on the above hypothesis [2e4], more effective in moisture removal but rather complex in structure. The liquid desiccant dehumidification is also a promising alternative, since the inner cooling is much
* Corresponding author. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2018.12.169 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
easier to be realized in a liquid system [5]. However, it has the drawback of corrosion [1,5]. Further research has been done to rule out the above side effects. These endeavors accelerate the advent of DCHEs. The DCHE is processed by coating solid desiccant on the outer surface of a conventional HX [6]. Unsaturated desiccant is highly affinitive with water vapor, so mass transfer occurs when humid air flows through. The cold fluid in the tubes facilitates the process, by removing the adsorption heat and maintaining the desiccation ability. The regeneration step, conversely, is aided with hot fluid flow. In order to treat the latent load continuously, two DCHEs work alternatively between the two modes. When the cooling or heating capacity of the medium in tubes is redundant, the device can also handle sensible heat load. Single-phase fluid is the potential candidate of the heat transfer medium, and the studies regarding DCHEs facilitated by singlephase refrigerant (normally water), both numerically and experimentally, are now extensive. Utilizing solar energy (or industrial waste heat) as the heat source, DCHEs have been successfully adopted in the field of solar driven desiccant cooling system [7e11]. Typical features of such systems include periodical switchover, effective dehumidification but poor control of the temperature. Thus, a supplementary device like evaporative cooling or heat pump is needed in practical application to overcome the indoor
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Abbreviation DCC DCE DCHE EA HVAC HX MA OA RA SA SDHP VC
desiccant coated condenser desiccant coated evaporator desiccant coated heat exchanger exhausted air heating, ventilation and air conditioning heat exchanger mixed air outdoor air return (indoor) air supply air solid desiccant heat pump vapor compression
Nomenclature (usually used with subscript) c specific heat (J/(kg$K)) cp specific heat (isobaric) (J/(kg$K)) cv specific heat (isovolumetric) (J/(kg$K)) f friction (N/m3) h convective heat transfer coefficient (W/(m2$K)) a void fraction 4 relative humidity l conductivity (W/(m$K)) r density (kg/m3) i specific enthalpy (J/K) m mass flow rate (kg/s) M coating amount (kg) N rotation frequency (HZ) p pressure (Pa) P electricity consumption (W) t time (s) t thickness(m) T temperature (oC) u x component of velocity (m/s) v y component of velocity (m/s) V volume (m3) u x1 component of velocity (m/s) v y2 component of velocity (m/s) W moisture content of the material (g/g) x quality Y air humidity ratio (g/kg) Subscripts a air flowing freely outside the desiccant surface
sensible load [7,9,11]. Two-phase refrigerant, however, is also an alternative. Since a compressor is commonly adopted in the temperature regulation of two-phase refrigerants, the SDHP is then proposed to combine a VC system with DCHEs (transforming to DCEs/DCCs). When incorporated in heat pumps [8,12e17], the function of DCHEs would be further extended, that is, realizing weakly-coupled temperature and humidity control [12,13,15] both in the cooling and heating modes. Several experimental studies have been published based on it [8,12e15,17], mainly to clarify the concept, validate the practicality and show its energy saving potential. Detailed research, aiming to explain mechanism and optimize the system design, are lacked both experimentally and theoretically. To investigate the novel SDHPs, a mathematical model depicting the heat and mass transfer process in DCEs/DCCs is imperative. Noteworthy is that the fluid inside the heat exchangers is, at most of
ad C CM d da E f HX r TP rg rl t v VL
adsorption condenser compressor porous desiccant coating (including pores) air confined in the desiccant pores evaporator fin DCHE (including the tubes, fins and coated desiccant) refrigerant the two-phase refrigerant saturated gas of the two-phase refrigerant saturated liquid of the two-phase refrigerant tube water vapor (inside air & desiccant pores) throttling valve
Nomenclature (with specific definition) cr compressor clearance ratio DS surface diffusivity of the liquid water adsorbed by the desiccant (m2/s) Ky convective mass transfer coefficient (kg/(m2$s)) qst specific sorption heat of the desiccant (J/kg) Rt W *d W *d ¼ 0d Wd ðn; tÞdn=td moisture content (whose gradient along the desiccant thickness is omitted) (kg water/kg dry material) Geometric Nomenclature la la ¼ Va =Aa coefficient to transform the surface heat/ mass source into the scattered heat/mass source in the air-side (m) laHX laHX ¼ VHX =Aa coefficient to transform the surface heat source (air) into the scattered heat source in the DCHE-side (m) lr lr ¼ Vr =Ar coefficient to transform the surface heat source into the scattered heat source in the refrigerant-side (m) lrHX lrHX ¼ VHX =Ar coefficient to transform the surface heat source (refrigerant) into the scattered heat source in the DCHE-side (m) εd εd ¼ Vd =VHX volume ratio of coated desiccant corresponding to the DCHE εf εf ¼ Vf =VHX volume ratio of coated desiccant corresponding to the DCHE εt εt ¼ Vt =VHX volume ratio of coated desiccant corresponding to the DCHE
the time, in two-phase region. The two-phase refrigerant is hard to predict, but of great importance since it is a carrier transporting energy through the system components. In this paper, then, a comprehensive physical and mathematical description of the DCEs/ DCCs is proposed, taking into the consideration of the synergistic effect of the two-phase refrigerant [18e20], heat exchanger, solid desiccant [21e23] and air. Afterwards, system simulation [20] and experiment are conducted to verify the practicality and superiority of the SDHP. 2. Ideal thermodynamic process of the sensible and latent heat handling 2.1. Ideal air-side process Handling the heat and moisture load of indoor environment are
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primary tasks of the heating, ventilation and air conditioning (HVAC) equipment, especially in coastal areas which are hot and damp in summer. However, the current HVAC systems possessing dehumidification effect are usually complex and energy consuming, since they normally utilize at least two sub-systems to independently regulate indoor temperature and humidity. Among them, traditional VC system, traditional desiccant cooling system, hybrid desiccant air-conditioning system and solar desiccant cooling system are representative techniques, with their air treatment processes depicted in Fig. 1 (taking Shanghai summer condition as an example: two mixed flows of outdoor air (OA: 35 C, 60%RH) and indoor air (RA: 25 C, 50%RH), denoted as MA1 and MA2, are processed in the device and then discharged respectively as supply air (SA) and exhausted air (EA)). The most compact and widely-adopted system, traditional VC, takes advantage of the cooling dehumidification technique and condenses the excess moisture in MA1 by directly cooling it down to the dew point of the desired SA. Therefore, the outlet air of the evaporator would be with satisfying humidity but low temperature (SA dew point < SA dry-bulb temperature) and need to be reheated before exhausting. Such process of overcooling and reheating is a waste of energy. The traditional desiccant cooling [24] and the hybrid desiccant air-conditioning [25], as indicated by their names, manipulate the air humidity via desiccant. According to the former literature, the desiccation process in such system, without a cooling source, is normally isenthalpic and features high outlet temperature due to the release of adsorption heat. Auxiliary sensible handling equipment is thus imperative. Specifically, the evaporative cooling and the VC devices are adopted, respectively, for the two systems. Noteworthy is that the MA1 in the traditional desiccant cooling system has to be treated below than the desired humidity of the SA, for the humidifying nature of evaporative cooling. Air flowing through the desiccant in a solar desiccant cooling device, usually consisted of DCHEs and a sensible heat handling equipment, would at first experience a process of merely latent heat handling without temperature change. That is, the side effect of the sorption heat can be successfully suppressed by the single-phase cooling source. Afterwards, the air should be further cooled via VC or other methods [7,9,11]. The ideal process of the sensible and latent heat handling, otherwise, should be more straightforward, as indicated with the red curve in Fig. 1. And the SDHP proposed in this study, with its working principles discussed in the following, is expected to be able to handle the air in this way [15]. If so, the SDHP, compared
Fig. 1. The air treatment process of the typical HVAC systems possessing dehumidification capacity.
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with other systems, would feature higher evaporative temperature (compared to those adopting the VC technique, evaporation temperatures are marked in the short dash lines with the one for SDHP in red), lower adsorption temperature (compared to those adopting desiccant, adsorption temperatures are marked in the hollow circles with the one for SDHP in red) and thus lower desorption temperature (lower temperature of the heating source). Namely, it exhibits a compact and high-efficient process. 2.2. Ideal desiccant cycle The sorption isotherm of a typical composite adsorbent (Si Sol þ 30%LiCl þ complete lyolysis [6] þ 5-year stable operation on component) is shown in the sub-graph at the top left corner of Fig. 2. The curve is assumed insensitive to temperature variation, indicating the biunivocal correlations between the moisture content of the material and the relative humidity ratio of the air in equilibrium (thermal and concentration) with it. Therefore, the points in psychrometric chart can also present solid desiccant conditions, with its moisture content deduced from the relative humidity ratio and temperature shown directly by the abscissa value [26]. Notably, the iso-moisture content lines of the desiccant coincides with the iso-relative humidity lines of the air. Meanwhile, the desiccant and the air in the same horizontal/vertical coordinate of the psychrometric chart are in thermal/concentration equilibrium. Fig. 2 then illustrates the ideal adsorption-desorption cycle of the solid material which can realize the above-mentioned process of air handling. The desiccant at point ① is cold and dry, hence it can dehumidify and simultaneously cool down the air flowing through it (MA1). A cold source is imperative to facilitate the process by taking away the sensible and adsorption heat transferred to the desiccant timely (①-②). After a while, the adsorption ability of the adsorbent begins to decay, due to the abundant moisture inside. It is then the time to introduce a hot source to warm the desiccant up (②-③) and trigger the desorption. During the following regeneration process (③-④), the coated desiccant at a relatively high temperature desorbs heat and water vapor into the MA2, and the heat source keeps heating the desiccant to compensate the sensible and latent heat dissipation. After the material is sufficiently dried up, it should be again cooled by the cold source, retuning to point ①.
Fig. 2. The ideal thermodynamic cycle of the desiccant coated on the DCE/DCC in SDHP.
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(b)
(a) Fig. 3. The schematic of SDHP and its working principle.
Assuming infinite mass and heat transfer area, the processed air is always in equilibrium with the desiccant. Hence, to supply air of stable temperature, processes ①-② and ③-④ are expected to be isothermal. Also, temperature variation processes ②-③ and ④-① should be as quick as possible, so that there is scarcely any mass transfer to the air during the periods. That is, ②-③ and ④-① are along the iso-relative humidity lines in the psychrometric chart. When the cycle is applied to practical use, the position of the four points shown in Fig. 2 need to be specified beforehand, so as to determine the suitable operation parameters (adsorption/desorption temperature) and then supply air meeting user demand. Still, the summer condition in Shanghai is adopted as an example, and the desired temperature and humidity of SA are around 18 C, 8 g/ kg. The adsorption/evaporation temperature then can be determined from the desired SA temperature (18 C). On the other hand, the moisture content of the desiccant Wd (deduced from the 4d in psychrometric chart by the sorption isotherm and equilibrate with the SA humidity ratio YSA ) monotonously increases during the adsorption phase. The origin ① (18 C, 30%) and termination ② (18 C, 80%) of the adsorption process are then estimated according to the following equations, given a short switchover period Dtad ¼ 180s:
8 ma YMA1 YSA;set Dtad ¼ DWd rd Md > > > < , ð Y þ YSA;② > ¼ 0:008kg=kg Y b Y ðtÞdt Dtad z SA;① SA;set SA > > 2 : Dtad
(1) Where DWd and D4d are correlated by the sorption isotherm as shown in Fig. 2, and the desired value of SA humidity YSA;set, marked by blue five-point star in Fig. 2, is the time average value of YSA . However, function YSA ðtÞ is unknown up to now and the estimation in equation (1) is thus imperative. Assuming negligible pressure difference between the MA2 and the desiccant at the end of the desorption, point ④ (40 C, 30%) is the intersection of the isomoisture content line of ① and the iso-humidity ratio line of MA2. Afterwards, point ③ (40 C, 80%), with the same moisture content of ③ and the same temperature of ④, can be obtained with no difficulty.
2.3. Ideal refrigerant cycle To implement the above cycle, the desiccant is expected to fully expose to the air and easily exchange heat with a cold/heat source. Coating the adsorbent onto the surface of a traditional air-torefrigerant heat exchanger (DCHE) is one of the solutions. Refrigerant can facilitate the operation of DCHEs by removing the adsorption heat/replenishing the heat used for regeneration in time during cooling/heating phase, and single-phase and two-phase media are both potential candidates. Between them, two-phase refrigerants are assumed with stable temperature and better heat handling capacity. Since a compressor is commonly adopted in the temperature regulation of two-phase refrigerants, the solid desiccant heat pump (SDHP, as shown in Fig. 3) is then proposed to combine a VC system with DCHEs (transforming to DCEs/DCCs). Same as the conventional VC system, the refrigerant in the SDHP operates as depicted in Fig. 3: after absorbing the heat in the evaporator, the refrigerant vapor is compressed in the compressor, condenses in the condenser and expands through the throttling device, then the cold refrigerant goes to the evaporator again. The evaporator/condenser in this system, different from traditional HXs, would adsorb/desorb heat and moisture to/from the air simultaneously if facilitated by the cold/hot fluid. Then, when the evaporator/condenser is saturated/dried up, it is time to switch over the four-way reversing valve, exchanging the position between the evaporator and the condenser (Fig. 3(a)/Fig. 3(b)). Thus, the SDHP features a process of periodic switchover of the evaporator and condenser. The air duct should also be reconstructed, to persistently guide the cooling air into the condenser. As mentioned above, an ideal cycle of the desiccant under a presentative weather condition can be implemented once the refrigerant evaporates at 18 C in the evaporator and condenses at 40 C in the condenser. Such refrigerant cycles (as shown in Fig. 4), also common in traditional VC system, are believed to be easily obtained via a compressor and a throttling valve. Fig. 4 emphasizes the fact that the evaporation/condensation of refrigerant coincides with the process ④-①-②/②-③-④ of the desiccant. The COP of the ideal cycle, which is the benchmark and limit of system optimization, is estimated to be 10.89.
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Fig. 4. The ideal thermodynamic cycle of the refrigerant in SDHP compatible with air handling requirements.
3. Simulation study of the SDHP 3.1. Simulation study of the DCEs/DCCs in SDHP 3.1.1. Physical model of the DCEs/DCCs Fig. 5 tries to interpret the heat and mass transfer process of DCHEs, with Fig. 5(b) focusing on the air-side conditions. Taking the adsorption phase as an example, the unsaturated desiccant and the cold refrigerant of DCHEs altogether form an environment of low humidity and temperature at the gas-solid interface, which then induces moisture and temperature gradient in the air-side. Thereupon, the air discharges moisture and heat to the evaporator, and both of the transfer processes are accounted for by convection. Meanwhile, the regeneration, with the same mechanism as that of the adsorption, takes place when the air regains water vapor and thermal energy from the condenser. Sub-graph in the left of Fig. 5 clarifies the mass transfer in the desiccant. To be specific, desiccant normally has interconnected pores exposed to air and substrate atoms strongly affinitive to water molecules. Hence, the water molecules inside air can easily
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penetrate into the pores of unsaturated material due to molecular diffusion, then partially trapped into the potential field of adsorption [27] of the substrate, becoming adsorbed water. Also note that the water molecules, both the vapor ones and the adsorbed ones, are in most cases distributed unevenly inside the desiccant, which yield surface diffusion and bulk diffusion (including molecular diffusion and Knudsen diffusion) respectively. Namely, while molecular diffusion happens at the interface, localized adsorption, surface diffusion, molecular diffusion and Knudsen diffusion coexist inside the material. In generation phase, the direction of the concentration gradient and thus diffusion are inverted, the adsorbed water is then desorbed from the substrate atoms. Fig. 5(c) demonstrates the heat transfer of the DCHE. The DCHE is considered as a homogenous mixture of copper, aluminum, desiccant and adsorbed water (water vapor inside the adsorbent pores is negligible). Apart from the convective heat transfer due to the air and refrigerant flow, the temperature change of the DCHE is also influenced by the mass transfer. For example, when the water molecular diffuses into the adsorbent, it would exchange heat with the substrate if the temperature difference exists. Moreover, heat effect accompanies the localized sorption since the state of water changes during the process. In summary, the thermal energy variation of the component is attributed to four contributors, namely, the sensible heat accompanying the moisture transfer, the heat generation/reduction due to the sorption heat effect, the air convection and the refrigerant convection. 3.1.2. Mathematical model of the DCEs/DCCs Fig. 6 demonstrates the configuration of an example DCE/DCC and the fluids (air & refrigerant) flowing through it. The structure confined in the cuboid (length: Lx; width: Ly; height: Lz), with the repetitive unit shown in Fig. 6(c), is our object to conduct heat and mass transfer analysis. To implement governing equation derivation in the air side, a Cartesian coordinate is defined as shown in Fig. 6(a), with its origin coinciding with one of cuboid vertexes. While the refrigerant and DCE/DCC temperature calculation is conducted along the refrigerant mainstream direction, denoted as y2-axis. This axis firstly points inward the tube at the inlet of the evaporator/condenser, and then reverses its orientation once encountering a bend tube. Since the moisture content in desiccant varies throughout the desiccant coating thickness, another n-axis (Fig. 6(d) and (e)) is also imperative.
8 vðra Ya Þ vðra ua Ya Þ 1 > > þ Y ¼ K Y j > y a da n¼t > d vx1 la < vt ( Ta ; adsorption > v ra cpa þ cpv Ya Ta v ra ua cpa þ cpv Ya Ta 1 1 > > þ ¼ Ky cpv Yda jn¼td Ya Ts þ ha ðTHX Ta Þ; Ts ¼ > : la la vt vx1 THX ; desorption
(2)
vðrd Wd Þ v vðr W Þ DS d d ¼ 0 vt vn vn
(3)
" v
P j¼d;f ;t
1
r j εj c j þ
vt ¼
3
rd εd cpv W *d ATHX 5
v vT εt lt HX vy2 vy2
1 1 Ky Ya Yda jn¼dd qst þ cpv Ts þ ha ðTa THX Þ þ hr ðTr THX Þ; Ts ¼ laHX laHX lrHX 1
(
(4) Ta ; adsorption THX ; desorption
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8 vr vðrr vr Þ r > þ ¼0 > > > vy2 vt > > > > 2 ( " #) > < 2 vðrr vr Þ v rr vr vpr v x2r vðr vr Þ 2 ð1 xr Þ þ f TP þ ¼ fr ðsingle phaseÞ pr þ ðrr vr Þ ¼ r r ðtwo phaseÞ > vt vy vt vy vy r ð1 a Þ r a > r r 2 2 2 rl rg > > > > > > > : vðrr ir Þ þ vðrr vr ir Þ ¼ vpr þ 1 hr ðT Tr Þ HX vt vy2 lr vt
The governing equations (2)e(5) of the DCEs/DCCs, which respectively describe the transfer process in air-side [22,23], desiccant side [21], HX side and refrigerant side [18], are then derived to analyze the component quantitatively.
3.1.3. Practical thermodynamic processes in the DCEs/DCCs A numerical study then follows, based on the above-mentioned mathematical model, to show whether the ideal cycle can be implemented by the DCHE and how the practical cycle differs from the ideal one. During the study, equations (2)e(5), along with the initial and boundary conditions, are solved simultaneously via reliable numerical methods (implemented by Cþþ code). Table 1
(a)
(5)
lists the parameters used in the calculation, including material thermodynamic properties and operating parameters. Specifically, the DCHE is imposed by the periodic boundary conditions, that is, several minutes of cold refrigerant and then equally several minutes of hot refrigerant. The evaporation and condensation temperature are respectively around 15 C and 45 C, based on the theoretical analysis and reasonable allowance of 3e5 C. Fig. 7 shows the practical cycle of the desiccant coated on the heat exchanger facilitated by two-phase refrigerant (R410a). The calculated properties of the desiccant are marked in the psychrometric chart every 1 s, with the starting point denoted as a fivepointed star. The density of the data points reveals the speed of the process. For example, a quick temperature variation process
(b)
(c)
Fig. 5. Schematic of heat and mass transfer (a) desiccant-side (b)air-side (c) heat exchanger.
(a)
(b)
(c)
(d)
(e)
Fig. 6. Detailed configuration of the example DCHE and its corresponding dimension. (a) The schematic diagram of the example DCHE (b) the left view of the DCHE (c) the repeating unit of the DCHE (d) the left view of the repeating unit (e) the front view of the repeating unit.
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Table 1 Main parameters adopted in the numerical study of DCE/DCC. General parameters Atmospheric pressure (pa) Acceleration of gravity (m/s2)
101325 9.8
Air (dry) specific heat capacity (J/(kg$K)) Air (dry) dynamic viscosity (Pa$s) Water (vapor) specific heat capacity (J/(kg$K)) Latent heat of water vaporation (J/(kg$K))
1035 1.845e-4 1864 2260000
Material properties Critical pressure of R410a (pa) Relative molecular mass of R410a Aluminum fin density (kg/m3) Aluminum fin specific heat capacity (J/(kg$K)) Copper tube density (kg/m3) Copper tube conductivity (W/(m$K)) Copper tube specific heat capacity (J/(kg$K)) Desiccant (including pores) density (kg/m3) Desiccant (dry) specific heat capacity (J/(kg$K)) Air density (kg/m3) Air conductivity (W/(m$K))
4902000 72.58 2700 880 8900 407 390 1000 921 1.2 0.0321
Operating parameters Desiccant coating amount (kg) Air velocity of the OA (m/s) *Air conditions of the OA Air velocity of the RA (m/s) *Air conditions of the RA DCE/DCC inlet pressure (Mpa) DCE inlet quality of the refrigerant DCC inlet temperature *Refrigerant mass flux (kg/m2s) *Switchover interval of the four-way valve (s)
1.2 1 35 C,21 g/kg 1 25 C,10 g/kg 1.26/2.73 0.05 60 500 180
Fig. 7. Practical thermodynamics process of the desiccant on (a) DCE (b) DCC.
faster, since the downstream refrigerant is subcooled and would hamper the rate of the heat and mass transfer. Consequently, the downstream material (row 1e3) are not sufficiently dried up at last. Near the condenser inlet (row 11), the refrigerant is hot gas with low convective heat transfer coefficient, and the desorption curve are not isothermal. The SA conditions at the evaporator outlet are then depicted in Fig. 8(a), exhibiting great dependence on the desiccant conditions (the colorful points in the background). The averaged SA temperature and humidity are, respectively, 21.5 C, 8.19 g/kg. The unexpected high temperature of the SA are accounted for the hot downstream desiccant at the onset. The DCHE facilitated by singlephase refrigerant (water) is also studied as a reference (shown in Fig. 8(b)). Due to the finite thermal capacity of liquid water, the temperature of the water along the tube keeps increasing during the adsorption phase. Thus, there are great disparity in properties between material at different positions and the SA conditions are not satisfying (29.3 C, 15.67 g/kg). 3.2. Simulation study of the SDHP with the DCCs/DCEs
Fig. 8. Supply air conditions of the DCHE when it is facilitated by (a) Two-phase medium (b)Single-phase medium.
features sparse data along the iso-relative humidity ratio line. Different from the ideal cycle, the temperature and moisture content of the desiccant are unevenly distributed in space. The average values of each row are then calculated, and the values of odd-rows are depicted in Fig. 7. Notably, in the case of two-phase refrigerant, the inlet of the evaporator is the outlet of the condenser, and vise versa. Fig. 7(a) reveals that, the upstream tubes (row 1e5) are cooled down right after the entry of the cold refrigerant, suggested by the sparse points almost along the iso-relative humidity ratio curves. Due to the heavy heat load of the hot solid material, the downstream refrigerant is severely superheated at the onset of adsorption. Hence, the desiccant temperature of row 7e11 increases at first and then gradually declines. Afterwards, most of the desiccant (row 1-9) features an approximate isothermal adsorption process at the evaporation temperature. The desiccant near the evaporator outlet (row 11), nevertheless, is always facilitated by superheated refrigerant and performs distinguished cycles. Fig. 7(b) then illustrates the desorption phase of the desiccant, during which temperature of all the adsorbent rises up from around 17 Ce42 C, followed by the isothermal desorption. The temperature variation speed of the upstream material (row 7e11) is much
3.2.1. Mathematical model of the compressor and the throttling valve To supply indoor with comfort air and maintain high efficiency, the evaporation/condensation pressure and the mass flow rate of the refrigerant in a heat pump should be compatible with its air handling requirements. As the pressure controllers and mass flow rate regulators of the two-phase refrigerant, the compressor and the throttling valve are thus essential components in a SDHP. A simplified model based on a variable-frequency rotor compressor is applied to the system simulation. The basic governing equations (equation (6)) indicate that, given the evaporation
Fig. 9. Framework of the system simulation of the SDHP.
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pressure pr,E, the outlet refrigerant enthalpy ir,Eout of the DCE and the condensation pressure pr,C, the significant parameters of the compressor (the mass flow rate mr,CM, the electricity consumption PCM and the outlet refrigerant enthalpy ir,Cin) can then be obtained.
8 !cv;Eout =cp;Eout > > pr;C > > mr;CM ¼ NCM VCM rr;Eout hv ; hv ¼ 1 þ cr cr > > > pr;E > < ir;isoentropy ir;Eout > > þ ir;Eout ; hs ¼ 0:85 > ir;Cin ¼ > > hs > > > : PCM ¼ he mr;CM ir;Cin ir;Eout ; he ¼ 0:9 (6) While the mass flow rate mr,VL and outlet enthalpy ir,Ein of the throttling valve are governed by the pressure difference (pr,C-pr,E) and the outlet refrigerant conditions ir,Cout of the DCC.
8 < :
mr;VL ¼ K
ffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pr;C pr;E rr;Cout
(7)
ir;Ein ¼ ir;Cout
As the throttling valve and the compressor are both assumed to have very small thermal inertia and charge amount, the steady state models are applied for these component [20]. In this case, the temperature variation and the disparities of mass flow rate from inlet to outlet of the compressor and throttling valve are omitted. 3.2.2. Framework of the system simulation Fig. 9 illustrates the calculation algorithm of the numerical study on the SDHP (equations (2)e(7)) consisting of four basic components, namely, a compressor (equation (6)), a throttling valve (equation (7)), a DCE (equations (2)e(5)) and a DCC (equations (2)e(5)). Notably, unlike the compressor or throttling valve, the governing equations for the DCE/DCC are partial differential ones, with both transient and space derivative terms. In the case of SDHP, the transient term is regarded to impose considerable influence on the component and system performance because of the changing latent heat load (for the changing capacity of the desiccant with time) and the periodic switchover. That is, the SDHP undergoes unstable operation and the dynamic simulation is imperative rather than the static one. The process shown in Fig. 9 is then repeated every time step with the updated initial conditions from one time step before. With the assumed evaporation pressure p*r,E, evaporator outlet refrigerant enthalpy i*r,Eout and condensation pressure p*r,C, the refrigerant conditions flowing through the compressor, DCC, throttling valve and DCE can be successively derived. The obtained value of condensation pressure pr,C, as an output of throttling model, then can be used to supplement the assumed value of p*r,C iteratively, until they are almost equal. Similarly, the value of evaporation pressure p*r,E/refrigerant enthalpy i*r,Eout at evaporator outlet is estimated at first and amend repeatedly until the calculated enthalpy ir,Eout/charge amount equals with the assumed/practical one. When the above system encounters a sudden switchover, the
status of the DCC/DCE (the spatial distribution of the air temperature, the air humidity, the desiccant temperature, the desiccant moisture ratio and the metal temperature) right before the switchover should be logged and then inputted as initial conditions to the new DCE/DCC after the switchover. Apart from the refrigerant conditions, the transient electricity consumption, the metal temperature, the desiccant moisture content and the air output conditions can also be obtained during the process. 3.2.3Characteristics and Performance of the Practical Refrigerant Cycle. Table .2 summarizes the important parameters adopted in the system simulation. The component parameters of the compressor are empirical ones of rotor compressors commonly used in household air conditioning. While the characteristic constant of throttling valve and the system charge amount are chosen to match the compressor capacity. The DCE and DCC of the SDHP are in the same size, to facilitate the frequent role exchange between the evaporator and condenser. To dissipate the condensation heat in time and thus maintain an appropriate evaporation temperature in SDHP, the air velocity through the condenser should be larger than the evaporator. The switchover poses negative effects on SDHP system. In detail, an ideal regeneration cycle completely warms up the DCC. Then, when it is transferred into the DCE, it needs to be pre-cooled to regain the dehumidification capacity. Such process of alternative cooling and heating would waste energy. Therefore, the heat load of the DCE can then be divided into three categories: latent heat, sensible heat and transient heat, as defined in equation (8).
8 < Qlatent ¼ ma;E qst Ya;Ein Ya;Eout Q ¼ ma;E ia;Ein ia;Eout Qlatent ;ia ¼ cpa Ta þ qst þcpv Ta Ya : sensible Qtrasient ¼ mr;CM ir;Eout ir;Ein Qlatent Qsensible (8) The system COP can be calculated both in the refrigerant side and in the air side, as defined in equation (9), namely, the enthalpy change of the refrigerant/air in the DCE divided by the electricity consumption of the compressor.
8 mr;CM ir;Eout ir;Ein > > ¼ COP > r < PCM > > m ia;Eout i > : COPa ¼ a a;Ein PCM
(9)
The numerical study shows that, after several minutes of fluctuation and quick self-regulation, the SDHP features periodical performance given arbitrary initial conditions. Fig. 10(a) then depicts the outlet air conditions, along with the system evaporation and condensation temperature, of the second and third calculated cycles, showing repeatable patterns. Within one cycle, the evaporation temperature fluctuates between 16 C and 19 C, while the condensation temperature gradually increasing from 38 C to 52 C. It also reveals that the humidity removal/regeneration in DCEs/ DCCs happens together with the temperature reduction/recover, and thus validates the possibility of the simultaneous handling of
Table 2 Main parameters adopted in the numerical study of SDHP. Component Parameters Compressor volume (m3) Compressor clearance Ratio Compressor isentropic efficiency Compressor electricity efficiency Characteristic Constant of throttling valve
Operating parameters 9.8e-6 0.08 0.85 0.9 6e-7
Air velocity of the OA (m/s) *Air conditions of the OA Air velocity of the RA (m/s) *Air conditions of the RA System charge amount
0.8 35 C,21 g/kg 1.2 25 C,10 g/kg 980
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Fig. 10. The characteristics and performance of the different elements of the SDHP (a) outlet air and refrigerant conditions (b) heat load and estimated COP.
sensible and latent load in such system. Some specific features of the mass and heat transfer in DCEs/DCCs are worth mentioning here. The solid material of DCEs/DCCs is dried off/wetted out but not cold/warm enough after the sudden switchover from DCCs/ DCEs. The moisture removal/recover, promoted both by the low/ high temperature and dry/wet material, obtains a maximum during the cycle, usually quite near the onset when the DCEs/DCCs are partially cooled/heated but still almost unsaturated/saturated. The
sensible heat transfer at air-side increases monotonically and then reaches a plateau within one cycle, due to the abundant latent heat and the temperature hysteresis of DCEs/DCCs in the beginning. The averaged supply air condition is 23 C, 9 g/kg. Fig. 10(b) shows the transient load of the DCE due to the unstable operation, altogether with the sensible and the latent heat load for comparison. Definition of the above three terms are shown in equation (8). The heat waste is relatively small (8.3% for
Fig. 11. Experimental study of the SDHP system under summer condition (a) Comparison of SA conditions and system COP under different summer conditions (b) under different switchover period (c) Comparison of the SDHP to other cooling or dehumidification system.
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Fig. 12. Comparison between the simulation results and experimental data.
adsorption process), but not negligible, according to the figure. The enthalpy increase of the refrigerant in SDHP does not equal the enthalpy decrease of the air, due to the existence of the transient heat load. The air-side value is deemed more reasonable. The COP calculated at the air-side, as shown in Fig. 10(d) obtains a maximum of 7.79 at 84s with the averaged value of 7.19. 4. Extremely high efficient heat pump An experimental setup of SDHP has been built to validate the feasibility and the superiority of the system. The detailed description of the system and the construction process can be found in Ref. [17]. Fig. 11 summarizes the experimental data of the SDHP system. Fig. 11(a) shows it possesses remarkable moisture removal capacity with moderate sensible heat handling ability and high COP (4.41e6.99). The ratio of the mass to heat transfer can be regulated by changing the switchover period, as shown in Fig. 11(b). Notably, the supply air temperature seems not low enough due to the unmatched capacity of the compressor in this system. The optimization regarding the component matching and control strategies of the above system is implemented in the literature [15]. In this case, the outlet condition is improved to a satisfying level (20 C, 8.5 g/kg when the outdoor air, the indoor air and the fresh air ratio are respectively 35 C, 21 g/kg, 25%, 10 g/kg and 70%) and the system COP remains impressive (7.14). Fig. 11(c) compares the SA conditions of the SDHP with other cooling or dehumidification systems, indicating its advantages of temperature control over the solar desiccant cooling and of humidity control over the traditional VC system. COP of the SDHP varies from 4.41 to 7.14 for typical summer conditions, while that for traditional VC system is generally less than 5.0.
4.1. Comparison between the experimental data and simulation results Comparing the simulation results with the experimental data, the validation of the mathematical model could be verified. It is noteworthy that the evaporation temperature used in the calculation is approximated by the measured temperature of the DCHE inlet. Also, the measured discharge temperature and pressure of the compressor roughly equal to the condensation inlet temperature and condensation pressure, respectively. The transient mass flow rate of the refrigerant is also available, based on the compressor suction and exhaust conditions, its power consumption and a simple compressor model. Input air velocities and conditions are logged by the sensors during the experiments. Other properties and operation parameters of significance can be found in Table 1. Fig. 12(a)-(b) illustrate that, the experimental SA humidity and temperature can be well predicted expect the points right after the switchover (switchover period: 3min), where the air valves in practice are not yet in place and the unordered flows mix up with each other. However, the temperature simulation seems not convincing, since the influence of the air mixture lasts 1/3 of the whole period. Fig. 12(c) reveals, with a longer switchover period (6 min), the temperature curves from the simulation and experiment can match better with each other. The errors are evaluated in Fig. 12(d). 5. Conclusions In this paper, a mathematical model is built on the DCEs/DCCs adopting R410a as refrigerant. Afterwards, the SDHP, containing DCEs/DCCs as core components, is evaluated, constructed and
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tested. The theoretical and experimental study in this paper combine to verify the validity and superiority of the SDHP system. To sum up: 1. According to the theoretical analysis, the desiccant coated on the DCEs/DCCs can realize an ideal thermodynamic cycle, including an approximately isothermal adsorption/regeneration process. 2. Theoretically, the SDHP features periodical performance with the evaporation temperature around 16 C and the condensation temperature no larger than 52 C. The averaged supply air condition is 23 C, 9 g/kg with the averaged COP of 7.19. 3. With the outdoor air of 35 C, 21 g/kg (indoor 25%, 10 g/kg, 70% fresh air), the SDHP can provide cold and dry supply air of 20 C, 8.5 g/kg and the corresponding COP reaches to 7.14. COP of the SDHP varies from 4.41 to 7.14 for typical summer conditions, while that for traditional VC system is generally no more than 5.0. 4. The SDHP has advantages of temperature control over the solar desiccant cooling and of humidity control over the traditional VC system Acknowledgements This research work was founded by Key Program of National Natural Science Foundation of China [No. 51336004] and also the Foundation for Innovative Research Groups of the Foundation for Innovative Research Groups of the National Natural Science Foundation of China [No. 51521004]. References [1] Zheng X, Ge TS, Wang RZ. Recent progress on desiccant materials for solid desiccant cooling systems. Energy 2014;74(1):280e94. [2] Ge TS, Li Y, Wang RZ, Dai YJ. Experimental study on a two-stage rotary desiccant cooling system. Chin J Sci Instrum 2009;30(7):1530e4. [3] Zhang H, Niu J, Zhang H. A two stage desiccant cooling system using low temperature heat. Refrig J 1998;20(2):51e5. [4] Ge TS, Dai YJ, Wang RZ, Li Y. Experimental investigation on a one-rotor twostage rotary desiccant cooling system. Energy 2008;33(12):1807e15. [5] Gommed K, Grossman G. Experimental investigation of a liquid desiccant system for solar cooling and dehumidification. Sol Energy 2007;81(1):131e8. [6] Jiang Y, Ge TS, Wang RZ, Hu LM. Experimental investigation and analysis of composite silica-gel coated fin-tube heat exchangers. Int J Refrig 2015;51:
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169e79. [7] Park B, Lee S, Kato S. Technical feasibility study of a compact desiccant-based outdoor air system using cogenerated hot water. Appl Therm Eng 2016;106: 968e80. [8] Saeed A, Alalili A. A review on desiccant coated heat exchangers. 2016. 00-. [9] Simonetti M, Gentile V, Fracastoro GV, Freni A, Calabrese L, Chiesa G. Experimental testing of the buoyant functioning of a coil coated with SAPO34 zeolite, designed for solar DEC (Desiccant Evaporative Cooling) systems of buildings with natural ventilation. Appl Therm Eng 2016;103:781e9. [10] Zhao Y, Dai YJ, Ge TS, Wang HH, Wang RZ. A high performance desiccant dehumidification unit using solid desiccant coated heat exchanger with heat recovery. Energy Build 2016;116:583e92. [11] Zhao Y, Ge TS, Dai YJ, Wang RZ. Experimental investigation on a desiccant dehumidification unit using fin-tube heat exchanger with silica gel coating. Appl Therm Eng 2014;63(1):52e8. [12] Aynur TN, Hwang Y, Radermacher R. Integration of variable refrigerant flow and heat pump desiccant systems for the cooling season. Appl Therm Eng 2010;30(8):917e27. [13] Aynur TN, Hwang Y, Radermacher R. Integration of variable refrigerant flow and heat pump desiccant systems for the heating season. Energy Build 2010;42(4):468e76. [14] Aynur TN, Hwang YH, Radermacher R. Field performance measurements of a heat pump desiccant unit in dehumidification mode. Energy Build 2008;40(12):2141e7. [15] Tu YD, Wang RZ, Ge TS, Zheng X. Comfortable, high-efficiency heat pump with desiccant-coated, water-sorbing heat exchangers. Sci Rep 2017;7:40437. [16] Tu YD, Wang RZ, Ge TS. New concept of desiccant-enhanced heat pump. Energy Convers Manag 2018;156:568e74. [17] Hua L, Jiang Y, Ge T, Wang R. Experimental investigation on a novel heat pump system based on desiccant coated heat exchangers. Energy 2018;142:96e107. [18] Zhang WJ, Zhang CL, Ding GL. On three forms of momentum equation in transient modeling of residential refrigeration systems. Int J Refrig 2009;32(5):938e44. [19] Ndiaye Demba, Bernier Michel. Transient modeling of refrigerant-to-AirFinand-tube heat exchangers. HVAC R Res 2010;16(3):355e81. [20] Koury RNN, Machado L, Ismail KAR. Numerical simulation of a variable speed refrigeration system. Int J Refrig 2001;24(2):192e200. [21] Ge TS, Li Y, Wang RZ, Dai YJ. A review of the mathematical models for predicting rotary desiccant wheel. Renew Sustain Energy Rev 2008;12(6): 1485e528. [22] Hu LM, Ge TS, Jiang Y, Wang RZ. Performance study on composite desiccant material coated fin-tube heat exchangers. Int J Heat Mass Tran 2015;90: 109e20. [23] Jagirdar M, Lee PS. Mathematical modeling and performance evaluation of a desiccant coated fin-tube heat exchanger. Appl Energy 2018;212:401e15. [24] Jia CX, Dai YJ, Wu JY, Wang RZ. Use of compound desiccant to develop high performance desiccant cooling system. Int J Refrig 2007;30(2):345e53. [25] Jia CX, Dai YJ, Wu JY, Wang RZ. Analysis on a hybrid desiccant air-conditioning system. Appl Therm Eng 2006;26(17):2393e400. [26] Li Z, Liu XH, Lun Z, Jiang Y. Analysis on the ideal energy efficiency of dehumidification process from buildings. Energy Build 2010;42(11):2014e20. I, Cerný [27] Medved R. Surface diffusion in porous media: a critical review. Microporous Mesoporous Mater 2011;142(2):405e22.
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Extremely high efficient heat pump with desiccant coated evaporator and condenser L.J. Hua, T.S. Ge, R.Z. Wang* Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2018 Received in revised form 17 December 2018 Accepted 22 December 2018
The desiccant coated heat exchanger (DCHE), utilizing inner cooling source to ease the side effect of sorption heat, is now a hot spot in the field of humidity control. Meanwhile, when incorporated in heat pumps, the function of DCHEs is expected to be further extended. Named as solid desiccant heat pump (SDHP), such system can realize weakly-coupled temperature and humidity control both in the cooling and heating modes. Promising refrigerants are near-azeotropic mixtures, and the DCHEs in SDHP then function as desiccant-coated evaporators/condensers (DCEs/DCCs). In this paper, a comprehensive physical and mathematical description of the DCEs/DCCs is first proposed to conduct the thermodynamic analysis of the component. Furthermore, the system simulation of the SDHP validates the feasibility of the vapor compression (VC) cycle with the new type of heat exchangers (HXs), and the calculated coefficient of performance (COP) is 7.19. The precision of the component model, along with the superiority of the SDHP, is then verified by an experimental setup. The experimental study also reveals that, with the outdoor air of 35 C, 21 g/kg (indoor 25%, 10 g/kg, 70%fresh air), the SDHP can provide cold and dry supply air of 20 C, 8.5 g/kg and the corresponding COP reaches to 7.14. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Solid desiccant heat pump Desiccant-coated evaporator Desiccant-coated condenser Coupled heat and mass transfer Two-phase refrigerant
1. Introduction With the rising standards of living and diminishing resources, nowadays there is an urgent need for high-efficient methods of air quality adjustment and energy recovery. In the field regarding humidity control, solid desiccant has arisen wide attention, for its advantages like feasibility, adaptability, energy conservation and environment protection [1]. The rotary wheel is the widely adopted system taking advantages of solid adsorbent. However, the device features increasing adsorption temperature and thus decaying dehumidification capacity along with high regeneration temperature, due to the inherently positive adsorption heat of the material. The recent studies on rotary wheels focus on developing isothermal dehumidification [2], which can be realized by directing air alternatively through infinite desiccant wheels and intercoolers [3]. Multistage rotary setups with intercooler(s) are then built based on the above hypothesis [2e4], more effective in moisture removal but rather complex in structure. The liquid desiccant dehumidification is also a promising alternative, since the inner cooling is much
* Corresponding author. E-mail address: [email protected] (R.Z. Wang). https://doi.org/10.1016/j.energy.2018.12.169 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
easier to be realized in a liquid system [5]. However, it has the drawback of corrosion [1,5]. Further research has been done to rule out the above side effects. These endeavors accelerate the advent of DCHEs. The DCHE is processed by coating solid desiccant on the outer surface of a conventional HX [6]. Unsaturated desiccant is highly affinitive with water vapor, so mass transfer occurs when humid air flows through. The cold fluid in the tubes facilitates the process, by removing the adsorption heat and maintaining the desiccation ability. The regeneration step, conversely, is aided with hot fluid flow. In order to treat the latent load continuously, two DCHEs work alternatively between the two modes. When the cooling or heating capacity of the medium in tubes is redundant, the device can also handle sensible heat load. Single-phase fluid is the potential candidate of the heat transfer medium, and the studies regarding DCHEs facilitated by singlephase refrigerant (normally water), both numerically and experimentally, are now extensive. Utilizing solar energy (or industrial waste heat) as the heat source, DCHEs have been successfully adopted in the field of solar driven desiccant cooling system [7e11]. Typical features of such systems include periodical switchover, effective dehumidification but poor control of the temperature. Thus, a supplementary device like evaporative cooling or heat pump is needed in practical application to overcome the indoor
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Abbreviation DCC DCE DCHE EA HVAC HX MA OA RA SA SDHP VC
desiccant coated condenser desiccant coated evaporator desiccant coated heat exchanger exhausted air heating, ventilation and air conditioning heat exchanger mixed air outdoor air return (indoor) air supply air solid desiccant heat pump vapor compression
Nomenclature (usually used with subscript) c specific heat (J/(kg$K)) cp specific heat (isobaric) (J/(kg$K)) cv specific heat (isovolumetric) (J/(kg$K)) f friction (N/m3) h convective heat transfer coefficient (W/(m2$K)) a void fraction 4 relative humidity l conductivity (W/(m$K)) r density (kg/m3) i specific enthalpy (J/K) m mass flow rate (kg/s) M coating amount (kg) N rotation frequency (HZ) p pressure (Pa) P electricity consumption (W) t time (s) t thickness(m) T temperature (oC) u x component of velocity (m/s) v y component of velocity (m/s) V volume (m3) u x1 component of velocity (m/s) v y2 component of velocity (m/s) W moisture content of the material (g/g) x quality Y air humidity ratio (g/kg) Subscripts a air flowing freely outside the desiccant surface
sensible load [7,9,11]. Two-phase refrigerant, however, is also an alternative. Since a compressor is commonly adopted in the temperature regulation of two-phase refrigerants, the SDHP is then proposed to combine a VC system with DCHEs (transforming to DCEs/DCCs). When incorporated in heat pumps [8,12e17], the function of DCHEs would be further extended, that is, realizing weakly-coupled temperature and humidity control [12,13,15] both in the cooling and heating modes. Several experimental studies have been published based on it [8,12e15,17], mainly to clarify the concept, validate the practicality and show its energy saving potential. Detailed research, aiming to explain mechanism and optimize the system design, are lacked both experimentally and theoretically. To investigate the novel SDHPs, a mathematical model depicting the heat and mass transfer process in DCEs/DCCs is imperative. Noteworthy is that the fluid inside the heat exchangers is, at most of
ad C CM d da E f HX r TP rg rl t v VL
adsorption condenser compressor porous desiccant coating (including pores) air confined in the desiccant pores evaporator fin DCHE (including the tubes, fins and coated desiccant) refrigerant the two-phase refrigerant saturated gas of the two-phase refrigerant saturated liquid of the two-phase refrigerant tube water vapor (inside air & desiccant pores) throttling valve
Nomenclature (with specific definition) cr compressor clearance ratio DS surface diffusivity of the liquid water adsorbed by the desiccant (m2/s) Ky convective mass transfer coefficient (kg/(m2$s)) qst specific sorption heat of the desiccant (J/kg) Rt W *d W *d ¼ 0d Wd ðn; tÞdn=td moisture content (whose gradient along the desiccant thickness is omitted) (kg water/kg dry material) Geometric Nomenclature la la ¼ Va =Aa coefficient to transform the surface heat/ mass source into the scattered heat/mass source in the air-side (m) laHX laHX ¼ VHX =Aa coefficient to transform the surface heat source (air) into the scattered heat source in the DCHE-side (m) lr lr ¼ Vr =Ar coefficient to transform the surface heat source into the scattered heat source in the refrigerant-side (m) lrHX lrHX ¼ VHX =Ar coefficient to transform the surface heat source (refrigerant) into the scattered heat source in the DCHE-side (m) εd εd ¼ Vd =VHX volume ratio of coated desiccant corresponding to the DCHE εf εf ¼ Vf =VHX volume ratio of coated desiccant corresponding to the DCHE εt εt ¼ Vt =VHX volume ratio of coated desiccant corresponding to the DCHE
the time, in two-phase region. The two-phase refrigerant is hard to predict, but of great importance since it is a carrier transporting energy through the system components. In this paper, then, a comprehensive physical and mathematical description of the DCEs/ DCCs is proposed, taking into the consideration of the synergistic effect of the two-phase refrigerant [18e20], heat exchanger, solid desiccant [21e23] and air. Afterwards, system simulation [20] and experiment are conducted to verify the practicality and superiority of the SDHP. 2. Ideal thermodynamic process of the sensible and latent heat handling 2.1. Ideal air-side process Handling the heat and moisture load of indoor environment are
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primary tasks of the heating, ventilation and air conditioning (HVAC) equipment, especially in coastal areas which are hot and damp in summer. However, the current HVAC systems possessing dehumidification effect are usually complex and energy consuming, since they normally utilize at least two sub-systems to independently regulate indoor temperature and humidity. Among them, traditional VC system, traditional desiccant cooling system, hybrid desiccant air-conditioning system and solar desiccant cooling system are representative techniques, with their air treatment processes depicted in Fig. 1 (taking Shanghai summer condition as an example: two mixed flows of outdoor air (OA: 35 C, 60%RH) and indoor air (RA: 25 C, 50%RH), denoted as MA1 and MA2, are processed in the device and then discharged respectively as supply air (SA) and exhausted air (EA)). The most compact and widely-adopted system, traditional VC, takes advantage of the cooling dehumidification technique and condenses the excess moisture in MA1 by directly cooling it down to the dew point of the desired SA. Therefore, the outlet air of the evaporator would be with satisfying humidity but low temperature (SA dew point < SA dry-bulb temperature) and need to be reheated before exhausting. Such process of overcooling and reheating is a waste of energy. The traditional desiccant cooling [24] and the hybrid desiccant air-conditioning [25], as indicated by their names, manipulate the air humidity via desiccant. According to the former literature, the desiccation process in such system, without a cooling source, is normally isenthalpic and features high outlet temperature due to the release of adsorption heat. Auxiliary sensible handling equipment is thus imperative. Specifically, the evaporative cooling and the VC devices are adopted, respectively, for the two systems. Noteworthy is that the MA1 in the traditional desiccant cooling system has to be treated below than the desired humidity of the SA, for the humidifying nature of evaporative cooling. Air flowing through the desiccant in a solar desiccant cooling device, usually consisted of DCHEs and a sensible heat handling equipment, would at first experience a process of merely latent heat handling without temperature change. That is, the side effect of the sorption heat can be successfully suppressed by the single-phase cooling source. Afterwards, the air should be further cooled via VC or other methods [7,9,11]. The ideal process of the sensible and latent heat handling, otherwise, should be more straightforward, as indicated with the red curve in Fig. 1. And the SDHP proposed in this study, with its working principles discussed in the following, is expected to be able to handle the air in this way [15]. If so, the SDHP, compared
Fig. 1. The air treatment process of the typical HVAC systems possessing dehumidification capacity.
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with other systems, would feature higher evaporative temperature (compared to those adopting the VC technique, evaporation temperatures are marked in the short dash lines with the one for SDHP in red), lower adsorption temperature (compared to those adopting desiccant, adsorption temperatures are marked in the hollow circles with the one for SDHP in red) and thus lower desorption temperature (lower temperature of the heating source). Namely, it exhibits a compact and high-efficient process. 2.2. Ideal desiccant cycle The sorption isotherm of a typical composite adsorbent (Si Sol þ 30%LiCl þ complete lyolysis [6] þ 5-year stable operation on component) is shown in the sub-graph at the top left corner of Fig. 2. The curve is assumed insensitive to temperature variation, indicating the biunivocal correlations between the moisture content of the material and the relative humidity ratio of the air in equilibrium (thermal and concentration) with it. Therefore, the points in psychrometric chart can also present solid desiccant conditions, with its moisture content deduced from the relative humidity ratio and temperature shown directly by the abscissa value [26]. Notably, the iso-moisture content lines of the desiccant coincides with the iso-relative humidity lines of the air. Meanwhile, the desiccant and the air in the same horizontal/vertical coordinate of the psychrometric chart are in thermal/concentration equilibrium. Fig. 2 then illustrates the ideal adsorption-desorption cycle of the solid material which can realize the above-mentioned process of air handling. The desiccant at point ① is cold and dry, hence it can dehumidify and simultaneously cool down the air flowing through it (MA1). A cold source is imperative to facilitate the process by taking away the sensible and adsorption heat transferred to the desiccant timely (①-②). After a while, the adsorption ability of the adsorbent begins to decay, due to the abundant moisture inside. It is then the time to introduce a hot source to warm the desiccant up (②-③) and trigger the desorption. During the following regeneration process (③-④), the coated desiccant at a relatively high temperature desorbs heat and water vapor into the MA2, and the heat source keeps heating the desiccant to compensate the sensible and latent heat dissipation. After the material is sufficiently dried up, it should be again cooled by the cold source, retuning to point ①.
Fig. 2. The ideal thermodynamic cycle of the desiccant coated on the DCE/DCC in SDHP.
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(b)
(a) Fig. 3. The schematic of SDHP and its working principle.
Assuming infinite mass and heat transfer area, the processed air is always in equilibrium with the desiccant. Hence, to supply air of stable temperature, processes ①-② and ③-④ are expected to be isothermal. Also, temperature variation processes ②-③ and ④-① should be as quick as possible, so that there is scarcely any mass transfer to the air during the periods. That is, ②-③ and ④-① are along the iso-relative humidity lines in the psychrometric chart. When the cycle is applied to practical use, the position of the four points shown in Fig. 2 need to be specified beforehand, so as to determine the suitable operation parameters (adsorption/desorption temperature) and then supply air meeting user demand. Still, the summer condition in Shanghai is adopted as an example, and the desired temperature and humidity of SA are around 18 C, 8 g/ kg. The adsorption/evaporation temperature then can be determined from the desired SA temperature (18 C). On the other hand, the moisture content of the desiccant Wd (deduced from the 4d in psychrometric chart by the sorption isotherm and equilibrate with the SA humidity ratio YSA ) monotonously increases during the adsorption phase. The origin ① (18 C, 30%) and termination ② (18 C, 80%) of the adsorption process are then estimated according to the following equations, given a short switchover period Dtad ¼ 180s:
8 ma YMA1 YSA;set Dtad ¼ DWd rd Md > > > < , ð Y þ YSA;② > ¼ 0:008kg=kg Y b Y ðtÞdt Dtad z SA;① SA;set SA > > 2 : Dtad
(1) Where DWd and D4d are correlated by the sorption isotherm as shown in Fig. 2, and the desired value of SA humidity YSA;set, marked by blue five-point star in Fig. 2, is the time average value of YSA . However, function YSA ðtÞ is unknown up to now and the estimation in equation (1) is thus imperative. Assuming negligible pressure difference between the MA2 and the desiccant at the end of the desorption, point ④ (40 C, 30%) is the intersection of the isomoisture content line of ① and the iso-humidity ratio line of MA2. Afterwards, point ③ (40 C, 80%), with the same moisture content of ③ and the same temperature of ④, can be obtained with no difficulty.
2.3. Ideal refrigerant cycle To implement the above cycle, the desiccant is expected to fully expose to the air and easily exchange heat with a cold/heat source. Coating the adsorbent onto the surface of a traditional air-torefrigerant heat exchanger (DCHE) is one of the solutions. Refrigerant can facilitate the operation of DCHEs by removing the adsorption heat/replenishing the heat used for regeneration in time during cooling/heating phase, and single-phase and two-phase media are both potential candidates. Between them, two-phase refrigerants are assumed with stable temperature and better heat handling capacity. Since a compressor is commonly adopted in the temperature regulation of two-phase refrigerants, the solid desiccant heat pump (SDHP, as shown in Fig. 3) is then proposed to combine a VC system with DCHEs (transforming to DCEs/DCCs). Same as the conventional VC system, the refrigerant in the SDHP operates as depicted in Fig. 3: after absorbing the heat in the evaporator, the refrigerant vapor is compressed in the compressor, condenses in the condenser and expands through the throttling device, then the cold refrigerant goes to the evaporator again. The evaporator/condenser in this system, different from traditional HXs, would adsorb/desorb heat and moisture to/from the air simultaneously if facilitated by the cold/hot fluid. Then, when the evaporator/condenser is saturated/dried up, it is time to switch over the four-way reversing valve, exchanging the position between the evaporator and the condenser (Fig. 3(a)/Fig. 3(b)). Thus, the SDHP features a process of periodic switchover of the evaporator and condenser. The air duct should also be reconstructed, to persistently guide the cooling air into the condenser. As mentioned above, an ideal cycle of the desiccant under a presentative weather condition can be implemented once the refrigerant evaporates at 18 C in the evaporator and condenses at 40 C in the condenser. Such refrigerant cycles (as shown in Fig. 4), also common in traditional VC system, are believed to be easily obtained via a compressor and a throttling valve. Fig. 4 emphasizes the fact that the evaporation/condensation of refrigerant coincides with the process ④-①-②/②-③-④ of the desiccant. The COP of the ideal cycle, which is the benchmark and limit of system optimization, is estimated to be 10.89.
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Fig. 4. The ideal thermodynamic cycle of the refrigerant in SDHP compatible with air handling requirements.
3. Simulation study of the SDHP 3.1. Simulation study of the DCEs/DCCs in SDHP 3.1.1. Physical model of the DCEs/DCCs Fig. 5 tries to interpret the heat and mass transfer process of DCHEs, with Fig. 5(b) focusing on the air-side conditions. Taking the adsorption phase as an example, the unsaturated desiccant and the cold refrigerant of DCHEs altogether form an environment of low humidity and temperature at the gas-solid interface, which then induces moisture and temperature gradient in the air-side. Thereupon, the air discharges moisture and heat to the evaporator, and both of the transfer processes are accounted for by convection. Meanwhile, the regeneration, with the same mechanism as that of the adsorption, takes place when the air regains water vapor and thermal energy from the condenser. Sub-graph in the left of Fig. 5 clarifies the mass transfer in the desiccant. To be specific, desiccant normally has interconnected pores exposed to air and substrate atoms strongly affinitive to water molecules. Hence, the water molecules inside air can easily
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penetrate into the pores of unsaturated material due to molecular diffusion, then partially trapped into the potential field of adsorption [27] of the substrate, becoming adsorbed water. Also note that the water molecules, both the vapor ones and the adsorbed ones, are in most cases distributed unevenly inside the desiccant, which yield surface diffusion and bulk diffusion (including molecular diffusion and Knudsen diffusion) respectively. Namely, while molecular diffusion happens at the interface, localized adsorption, surface diffusion, molecular diffusion and Knudsen diffusion coexist inside the material. In generation phase, the direction of the concentration gradient and thus diffusion are inverted, the adsorbed water is then desorbed from the substrate atoms. Fig. 5(c) demonstrates the heat transfer of the DCHE. The DCHE is considered as a homogenous mixture of copper, aluminum, desiccant and adsorbed water (water vapor inside the adsorbent pores is negligible). Apart from the convective heat transfer due to the air and refrigerant flow, the temperature change of the DCHE is also influenced by the mass transfer. For example, when the water molecular diffuses into the adsorbent, it would exchange heat with the substrate if the temperature difference exists. Moreover, heat effect accompanies the localized sorption since the state of water changes during the process. In summary, the thermal energy variation of the component is attributed to four contributors, namely, the sensible heat accompanying the moisture transfer, the heat generation/reduction due to the sorption heat effect, the air convection and the refrigerant convection. 3.1.2. Mathematical model of the DCEs/DCCs Fig. 6 demonstrates the configuration of an example DCE/DCC and the fluids (air & refrigerant) flowing through it. The structure confined in the cuboid (length: Lx; width: Ly; height: Lz), with the repetitive unit shown in Fig. 6(c), is our object to conduct heat and mass transfer analysis. To implement governing equation derivation in the air side, a Cartesian coordinate is defined as shown in Fig. 6(a), with its origin coinciding with one of cuboid vertexes. While the refrigerant and DCE/DCC temperature calculation is conducted along the refrigerant mainstream direction, denoted as y2-axis. This axis firstly points inward the tube at the inlet of the evaporator/condenser, and then reverses its orientation once encountering a bend tube. Since the moisture content in desiccant varies throughout the desiccant coating thickness, another n-axis (Fig. 6(d) and (e)) is also imperative.
8 vðra Ya Þ vðra ua Ya Þ 1 > > þ Y ¼ K Y j > y a da n¼t > d vx1 la < vt ( Ta ; adsorption > v ra cpa þ cpv Ya Ta v ra ua cpa þ cpv Ya Ta 1 1 > > þ ¼ Ky cpv Yda jn¼td Ya Ts þ ha ðTHX Ta Þ; Ts ¼ > : la la vt vx1 THX ; desorption
(2)
vðrd Wd Þ v vðr W Þ DS d d ¼ 0 vt vn vn
(3)
" v
P j¼d;f ;t
1
r j εj c j þ
vt ¼
3
rd εd cpv W *d ATHX 5
v vT εt lt HX vy2 vy2
1 1 Ky Ya Yda jn¼dd qst þ cpv Ts þ ha ðTa THX Þ þ hr ðTr THX Þ; Ts ¼ laHX laHX lrHX 1
(
(4) Ta ; adsorption THX ; desorption
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8 vr vðrr vr Þ r > þ ¼0 > > > vy2 vt > > > > 2 ( " #) > < 2 vðrr vr Þ v rr vr vpr v x2r vðr vr Þ 2 ð1 xr Þ þ f TP þ ¼ fr ðsingle phaseÞ pr þ ðrr vr Þ ¼ r r ðtwo phaseÞ > vt vy vt vy vy r ð1 a Þ r a > r r 2 2 2 rl rg > > > > > > > : vðrr ir Þ þ vðrr vr ir Þ ¼ vpr þ 1 hr ðT Tr Þ HX vt vy2 lr vt
The governing equations (2)e(5) of the DCEs/DCCs, which respectively describe the transfer process in air-side [22,23], desiccant side [21], HX side and refrigerant side [18], are then derived to analyze the component quantitatively.
3.1.3. Practical thermodynamic processes in the DCEs/DCCs A numerical study then follows, based on the above-mentioned mathematical model, to show whether the ideal cycle can be implemented by the DCHE and how the practical cycle differs from the ideal one. During the study, equations (2)e(5), along with the initial and boundary conditions, are solved simultaneously via reliable numerical methods (implemented by Cþþ code). Table 1
(a)
(5)
lists the parameters used in the calculation, including material thermodynamic properties and operating parameters. Specifically, the DCHE is imposed by the periodic boundary conditions, that is, several minutes of cold refrigerant and then equally several minutes of hot refrigerant. The evaporation and condensation temperature are respectively around 15 C and 45 C, based on the theoretical analysis and reasonable allowance of 3e5 C. Fig. 7 shows the practical cycle of the desiccant coated on the heat exchanger facilitated by two-phase refrigerant (R410a). The calculated properties of the desiccant are marked in the psychrometric chart every 1 s, with the starting point denoted as a fivepointed star. The density of the data points reveals the speed of the process. For example, a quick temperature variation process
(b)
(c)
Fig. 5. Schematic of heat and mass transfer (a) desiccant-side (b)air-side (c) heat exchanger.
(a)
(b)
(c)
(d)
(e)
Fig. 6. Detailed configuration of the example DCHE and its corresponding dimension. (a) The schematic diagram of the example DCHE (b) the left view of the DCHE (c) the repeating unit of the DCHE (d) the left view of the repeating unit (e) the front view of the repeating unit.
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Table 1 Main parameters adopted in the numerical study of DCE/DCC. General parameters Atmospheric pressure (pa) Acceleration of gravity (m/s2)
101325 9.8
Air (dry) specific heat capacity (J/(kg$K)) Air (dry) dynamic viscosity (Pa$s) Water (vapor) specific heat capacity (J/(kg$K)) Latent heat of water vaporation (J/(kg$K))
1035 1.845e-4 1864 2260000
Material properties Critical pressure of R410a (pa) Relative molecular mass of R410a Aluminum fin density (kg/m3) Aluminum fin specific heat capacity (J/(kg$K)) Copper tube density (kg/m3) Copper tube conductivity (W/(m$K)) Copper tube specific heat capacity (J/(kg$K)) Desiccant (including pores) density (kg/m3) Desiccant (dry) specific heat capacity (J/(kg$K)) Air density (kg/m3) Air conductivity (W/(m$K))
4902000 72.58 2700 880 8900 407 390 1000 921 1.2 0.0321
Operating parameters Desiccant coating amount (kg) Air velocity of the OA (m/s) *Air conditions of the OA Air velocity of the RA (m/s) *Air conditions of the RA DCE/DCC inlet pressure (Mpa) DCE inlet quality of the refrigerant DCC inlet temperature *Refrigerant mass flux (kg/m2s) *Switchover interval of the four-way valve (s)
1.2 1 35 C,21 g/kg 1 25 C,10 g/kg 1.26/2.73 0.05 60 500 180
Fig. 7. Practical thermodynamics process of the desiccant on (a) DCE (b) DCC.
faster, since the downstream refrigerant is subcooled and would hamper the rate of the heat and mass transfer. Consequently, the downstream material (row 1e3) are not sufficiently dried up at last. Near the condenser inlet (row 11), the refrigerant is hot gas with low convective heat transfer coefficient, and the desorption curve are not isothermal. The SA conditions at the evaporator outlet are then depicted in Fig. 8(a), exhibiting great dependence on the desiccant conditions (the colorful points in the background). The averaged SA temperature and humidity are, respectively, 21.5 C, 8.19 g/kg. The unexpected high temperature of the SA are accounted for the hot downstream desiccant at the onset. The DCHE facilitated by singlephase refrigerant (water) is also studied as a reference (shown in Fig. 8(b)). Due to the finite thermal capacity of liquid water, the temperature of the water along the tube keeps increasing during the adsorption phase. Thus, there are great disparity in properties between material at different positions and the SA conditions are not satisfying (29.3 C, 15.67 g/kg). 3.2. Simulation study of the SDHP with the DCCs/DCEs
Fig. 8. Supply air conditions of the DCHE when it is facilitated by (a) Two-phase medium (b)Single-phase medium.
features sparse data along the iso-relative humidity ratio line. Different from the ideal cycle, the temperature and moisture content of the desiccant are unevenly distributed in space. The average values of each row are then calculated, and the values of odd-rows are depicted in Fig. 7. Notably, in the case of two-phase refrigerant, the inlet of the evaporator is the outlet of the condenser, and vise versa. Fig. 7(a) reveals that, the upstream tubes (row 1e5) are cooled down right after the entry of the cold refrigerant, suggested by the sparse points almost along the iso-relative humidity ratio curves. Due to the heavy heat load of the hot solid material, the downstream refrigerant is severely superheated at the onset of adsorption. Hence, the desiccant temperature of row 7e11 increases at first and then gradually declines. Afterwards, most of the desiccant (row 1-9) features an approximate isothermal adsorption process at the evaporation temperature. The desiccant near the evaporator outlet (row 11), nevertheless, is always facilitated by superheated refrigerant and performs distinguished cycles. Fig. 7(b) then illustrates the desorption phase of the desiccant, during which temperature of all the adsorbent rises up from around 17 Ce42 C, followed by the isothermal desorption. The temperature variation speed of the upstream material (row 7e11) is much
3.2.1. Mathematical model of the compressor and the throttling valve To supply indoor with comfort air and maintain high efficiency, the evaporation/condensation pressure and the mass flow rate of the refrigerant in a heat pump should be compatible with its air handling requirements. As the pressure controllers and mass flow rate regulators of the two-phase refrigerant, the compressor and the throttling valve are thus essential components in a SDHP. A simplified model based on a variable-frequency rotor compressor is applied to the system simulation. The basic governing equations (equation (6)) indicate that, given the evaporation
Fig. 9. Framework of the system simulation of the SDHP.
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pressure pr,E, the outlet refrigerant enthalpy ir,Eout of the DCE and the condensation pressure pr,C, the significant parameters of the compressor (the mass flow rate mr,CM, the electricity consumption PCM and the outlet refrigerant enthalpy ir,Cin) can then be obtained.
8 !cv;Eout =cp;Eout > > pr;C > > mr;CM ¼ NCM VCM rr;Eout hv ; hv ¼ 1 þ cr cr > > > pr;E > < ir;isoentropy ir;Eout > > þ ir;Eout ; hs ¼ 0:85 > ir;Cin ¼ > > hs > > > : PCM ¼ he mr;CM ir;Cin ir;Eout ; he ¼ 0:9 (6) While the mass flow rate mr,VL and outlet enthalpy ir,Ein of the throttling valve are governed by the pressure difference (pr,C-pr,E) and the outlet refrigerant conditions ir,Cout of the DCC.
8 < :
mr;VL ¼ K
ffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pr;C pr;E rr;Cout
(7)
ir;Ein ¼ ir;Cout
As the throttling valve and the compressor are both assumed to have very small thermal inertia and charge amount, the steady state models are applied for these component [20]. In this case, the temperature variation and the disparities of mass flow rate from inlet to outlet of the compressor and throttling valve are omitted. 3.2.2. Framework of the system simulation Fig. 9 illustrates the calculation algorithm of the numerical study on the SDHP (equations (2)e(7)) consisting of four basic components, namely, a compressor (equation (6)), a throttling valve (equation (7)), a DCE (equations (2)e(5)) and a DCC (equations (2)e(5)). Notably, unlike the compressor or throttling valve, the governing equations for the DCE/DCC are partial differential ones, with both transient and space derivative terms. In the case of SDHP, the transient term is regarded to impose considerable influence on the component and system performance because of the changing latent heat load (for the changing capacity of the desiccant with time) and the periodic switchover. That is, the SDHP undergoes unstable operation and the dynamic simulation is imperative rather than the static one. The process shown in Fig. 9 is then repeated every time step with the updated initial conditions from one time step before. With the assumed evaporation pressure p*r,E, evaporator outlet refrigerant enthalpy i*r,Eout and condensation pressure p*r,C, the refrigerant conditions flowing through the compressor, DCC, throttling valve and DCE can be successively derived. The obtained value of condensation pressure pr,C, as an output of throttling model, then can be used to supplement the assumed value of p*r,C iteratively, until they are almost equal. Similarly, the value of evaporation pressure p*r,E/refrigerant enthalpy i*r,Eout at evaporator outlet is estimated at first and amend repeatedly until the calculated enthalpy ir,Eout/charge amount equals with the assumed/practical one. When the above system encounters a sudden switchover, the
status of the DCC/DCE (the spatial distribution of the air temperature, the air humidity, the desiccant temperature, the desiccant moisture ratio and the metal temperature) right before the switchover should be logged and then inputted as initial conditions to the new DCE/DCC after the switchover. Apart from the refrigerant conditions, the transient electricity consumption, the metal temperature, the desiccant moisture content and the air output conditions can also be obtained during the process. 3.2.3Characteristics and Performance of the Practical Refrigerant Cycle. Table .2 summarizes the important parameters adopted in the system simulation. The component parameters of the compressor are empirical ones of rotor compressors commonly used in household air conditioning. While the characteristic constant of throttling valve and the system charge amount are chosen to match the compressor capacity. The DCE and DCC of the SDHP are in the same size, to facilitate the frequent role exchange between the evaporator and condenser. To dissipate the condensation heat in time and thus maintain an appropriate evaporation temperature in SDHP, the air velocity through the condenser should be larger than the evaporator. The switchover poses negative effects on SDHP system. In detail, an ideal regeneration cycle completely warms up the DCC. Then, when it is transferred into the DCE, it needs to be pre-cooled to regain the dehumidification capacity. Such process of alternative cooling and heating would waste energy. Therefore, the heat load of the DCE can then be divided into three categories: latent heat, sensible heat and transient heat, as defined in equation (8).
8 < Qlatent ¼ ma;E qst Ya;Ein Ya;Eout Q ¼ ma;E ia;Ein ia;Eout Qlatent ;ia ¼ cpa Ta þ qst þcpv Ta Ya : sensible Qtrasient ¼ mr;CM ir;Eout ir;Ein Qlatent Qsensible (8) The system COP can be calculated both in the refrigerant side and in the air side, as defined in equation (9), namely, the enthalpy change of the refrigerant/air in the DCE divided by the electricity consumption of the compressor.
8 mr;CM ir;Eout ir;Ein > > ¼ COP > r < PCM > > m ia;Eout i > : COPa ¼ a a;Ein PCM
(9)
The numerical study shows that, after several minutes of fluctuation and quick self-regulation, the SDHP features periodical performance given arbitrary initial conditions. Fig. 10(a) then depicts the outlet air conditions, along with the system evaporation and condensation temperature, of the second and third calculated cycles, showing repeatable patterns. Within one cycle, the evaporation temperature fluctuates between 16 C and 19 C, while the condensation temperature gradually increasing from 38 C to 52 C. It also reveals that the humidity removal/regeneration in DCEs/ DCCs happens together with the temperature reduction/recover, and thus validates the possibility of the simultaneous handling of
Table 2 Main parameters adopted in the numerical study of SDHP. Component Parameters Compressor volume (m3) Compressor clearance Ratio Compressor isentropic efficiency Compressor electricity efficiency Characteristic Constant of throttling valve
Operating parameters 9.8e-6 0.08 0.85 0.9 6e-7
Air velocity of the OA (m/s) *Air conditions of the OA Air velocity of the RA (m/s) *Air conditions of the RA System charge amount
0.8 35 C,21 g/kg 1.2 25 C,10 g/kg 980
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Fig. 10. The characteristics and performance of the different elements of the SDHP (a) outlet air and refrigerant conditions (b) heat load and estimated COP.
sensible and latent load in such system. Some specific features of the mass and heat transfer in DCEs/DCCs are worth mentioning here. The solid material of DCEs/DCCs is dried off/wetted out but not cold/warm enough after the sudden switchover from DCCs/ DCEs. The moisture removal/recover, promoted both by the low/ high temperature and dry/wet material, obtains a maximum during the cycle, usually quite near the onset when the DCEs/DCCs are partially cooled/heated but still almost unsaturated/saturated. The
sensible heat transfer at air-side increases monotonically and then reaches a plateau within one cycle, due to the abundant latent heat and the temperature hysteresis of DCEs/DCCs in the beginning. The averaged supply air condition is 23 C, 9 g/kg. Fig. 10(b) shows the transient load of the DCE due to the unstable operation, altogether with the sensible and the latent heat load for comparison. Definition of the above three terms are shown in equation (8). The heat waste is relatively small (8.3% for
Fig. 11. Experimental study of the SDHP system under summer condition (a) Comparison of SA conditions and system COP under different summer conditions (b) under different switchover period (c) Comparison of the SDHP to other cooling or dehumidification system.
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Fig. 12. Comparison between the simulation results and experimental data.
adsorption process), but not negligible, according to the figure. The enthalpy increase of the refrigerant in SDHP does not equal the enthalpy decrease of the air, due to the existence of the transient heat load. The air-side value is deemed more reasonable. The COP calculated at the air-side, as shown in Fig. 10(d) obtains a maximum of 7.79 at 84s with the averaged value of 7.19. 4. Extremely high efficient heat pump An experimental setup of SDHP has been built to validate the feasibility and the superiority of the system. The detailed description of the system and the construction process can be found in Ref. [17]. Fig. 11 summarizes the experimental data of the SDHP system. Fig. 11(a) shows it possesses remarkable moisture removal capacity with moderate sensible heat handling ability and high COP (4.41e6.99). The ratio of the mass to heat transfer can be regulated by changing the switchover period, as shown in Fig. 11(b). Notably, the supply air temperature seems not low enough due to the unmatched capacity of the compressor in this system. The optimization regarding the component matching and control strategies of the above system is implemented in the literature [15]. In this case, the outlet condition is improved to a satisfying level (20 C, 8.5 g/kg when the outdoor air, the indoor air and the fresh air ratio are respectively 35 C, 21 g/kg, 25%, 10 g/kg and 70%) and the system COP remains impressive (7.14). Fig. 11(c) compares the SA conditions of the SDHP with other cooling or dehumidification systems, indicating its advantages of temperature control over the solar desiccant cooling and of humidity control over the traditional VC system. COP of the SDHP varies from 4.41 to 7.14 for typical summer conditions, while that for traditional VC system is generally less than 5.0.
4.1. Comparison between the experimental data and simulation results Comparing the simulation results with the experimental data, the validation of the mathematical model could be verified. It is noteworthy that the evaporation temperature used in the calculation is approximated by the measured temperature of the DCHE inlet. Also, the measured discharge temperature and pressure of the compressor roughly equal to the condensation inlet temperature and condensation pressure, respectively. The transient mass flow rate of the refrigerant is also available, based on the compressor suction and exhaust conditions, its power consumption and a simple compressor model. Input air velocities and conditions are logged by the sensors during the experiments. Other properties and operation parameters of significance can be found in Table 1. Fig. 12(a)-(b) illustrate that, the experimental SA humidity and temperature can be well predicted expect the points right after the switchover (switchover period: 3min), where the air valves in practice are not yet in place and the unordered flows mix up with each other. However, the temperature simulation seems not convincing, since the influence of the air mixture lasts 1/3 of the whole period. Fig. 12(c) reveals, with a longer switchover period (6 min), the temperature curves from the simulation and experiment can match better with each other. The errors are evaluated in Fig. 12(d). 5. Conclusions In this paper, a mathematical model is built on the DCEs/DCCs adopting R410a as refrigerant. Afterwards, the SDHP, containing DCEs/DCCs as core components, is evaluated, constructed and
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tested. The theoretical and experimental study in this paper combine to verify the validity and superiority of the SDHP system. To sum up: 1. According to the theoretical analysis, the desiccant coated on the DCEs/DCCs can realize an ideal thermodynamic cycle, including an approximately isothermal adsorption/regeneration process. 2. Theoretically, the SDHP features periodical performance with the evaporation temperature around 16 C and the condensation temperature no larger than 52 C. The averaged supply air condition is 23 C, 9 g/kg with the averaged COP of 7.19. 3. With the outdoor air of 35 C, 21 g/kg (indoor 25%, 10 g/kg, 70% fresh air), the SDHP can provide cold and dry supply air of 20 C, 8.5 g/kg and the corresponding COP reaches to 7.14. COP of the SDHP varies from 4.41 to 7.14 for typical summer conditions, while that for traditional VC system is generally no more than 5.0. 4. The SDHP has advantages of temperature control over the solar desiccant cooling and of humidity control over the traditional VC system Acknowledgements This research work was founded by Key Program of National Natural Science Foundation of China [No. 51336004] and also the Foundation for Innovative Research Groups of the Foundation for Innovative Research Groups of the National Natural Science Foundation of China [No. 51521004]. References [1] Zheng X, Ge TS, Wang RZ. Recent progress on desiccant materials for solid desiccant cooling systems. Energy 2014;74(1):280e94. [2] Ge TS, Li Y, Wang RZ, Dai YJ. Experimental study on a two-stage rotary desiccant cooling system. Chin J Sci Instrum 2009;30(7):1530e4. [3] Zhang H, Niu J, Zhang H. A two stage desiccant cooling system using low temperature heat. Refrig J 1998;20(2):51e5. [4] Ge TS, Dai YJ, Wang RZ, Li Y. Experimental investigation on a one-rotor twostage rotary desiccant cooling system. Energy 2008;33(12):1807e15. [5] Gommed K, Grossman G. Experimental investigation of a liquid desiccant system for solar cooling and dehumidification. Sol Energy 2007;81(1):131e8. [6] Jiang Y, Ge TS, Wang RZ, Hu LM. Experimental investigation and analysis of composite silica-gel coated fin-tube heat exchangers. Int J Refrig 2015;51:
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