Experimental evaluation and performance enhancement prediction of desiccant assisted separate sensible and latent cooling air-conditioning systemEvaluation expérimentale et prévision de l'amélioration de la performance d'un système de conditionnement d'air à chaleur sensible et latente assisté d'un déshydratant séparé

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Abstract

CO2 and R410A desiccant wheel (DW)-assisted separate sensible and latent cooling (SSLC) air-conditioning systems were tested under the AHRI standard. At a 50 °C regeneration temperature, the coefficient of performance (COP) of the vapor compression cycles improved only 7% from the respective baseline systems for both refrigerants. This paper proposed the idea of applying divided condensers (or gas coolers) to the R410A (or CO2) SSLC system to enhance its performance. It was found that the application of divided heat exchangers to the SSLC system provided sufficiently hot airflow for regenerating the desiccant wheel at both a reduced high side pressure (from 10.4 MPa to 9.7 MPa for CO2, from 3.46 MPa to 3.45 MPa for R410A) and a reduced discharge temperature from the condenser (gas cooler) (4 K lower for both refrigerants). The COP improvement is 36% and 61% to R410A and CO2 baseline systems, respectively.

Introduction

Improving energy efficiency of air-conditioning systems is an everlasting research topic attracting the attention of numerous scientists and engineers. Air-conditioning (AC) systems account for over 15% of total household electricity consumption, a statistic that amplifies the significance of such studies (EIA, 2005). The technology of separate sensible and latent cooling (SSLC) is among those capable of reducing the power input of compressors. In an SSLC AC system, the total amount of cooling is delivered by two sub-systems: a sensible cooling sub-system, which provides only sensible cooling, and a latent cooling sub-system, whose major function is to dehumidify incoming air. In the sensible sub-system, it is not necessary to cool air below its dew point, so the evaporating pressure of the vapor compression cycle can be higher than in a conventional system. The increased evaporating pressure results in a reduced pressure ratio across the compressor. The reduced pressure ratio provides the benefits of both better compressor efficiencies (isentropic, volumetric and compressor efficiencies) and a reduced compressor power input. Different techniques can be applied to achieve the separation of total cooling. Ling et al. (2009) proposed an idea of using two vapor compression systems for the separation, the first of which dealt with sensible load only, while the second system dealt with latent load and a small amount of sensible load. The energy consumption of such an SSLC system was reduced by 30% compared with that of the baseline system’s compressor under the standard condition (35 °C, 44% RH), and up to 50% under the hot and dry condition (37 °C, 15% RH). More studies were focused on the application of using a vapor compression cycle for sensible load removal and solid/liquid desiccant materials for latent load removal. Dai et al. (2001) studied the application of integrating liquid desiccant equipment and a vapor compression cycle. The test was conducted under the AHRI standard 210/240 condition (35 °C, 44% RH, AHRI, 2008) and the cooling capacity was 5 kW. The coefficient of performance (COP) of the vapor compression cycle improved from 2.2 to 3.39 because of the assistance from liquid desiccant. Dhar and Singh (2001) simulated a hybrid system of a solid desiccant wheel (DW) and a vapor compression cycle. They demonstrated that the hybrid system had maximum energy savings under hot and dry weather. In hot and humid region, energy savings was still possible but the latent load should be high. Depending on different desiccant materials, the temperatures of regeneration can vary between 50 °C and above 100 °C. Jia et al. (2006) studied the performance of a solid DW which used lithium chloride as the adsorbent. The temperature required to regenerate the wheel was set to be 100 °C, and one regeneration heater was used as the heat source. Casas and Schmitz (2005) studied the integration of a DW and a cooling, heating, and power (CHP) unit. In their study, the waste heat from the CHP unit could be utilized for the lithium chloride regeneration. However, the regeneration temperature was only in the range between 50 °C and 60 °C. The difference in regeneration temperatures in these two works may be caused by different dehumidification requirements. Silica gel is another widely accepted candidate for desiccant material, and its regeneration temperature is usually higher than 70 °C (Neti and Wolfe, 2000). As new technology emerges, low-temperature regenerated desiccant materials have attracted the attention of many researchers studying the integration of low-temperature driven DWs and vapor compression cycle, because it enables use of the waste heat directly from the condenser or the gas cooler.

With the increasing concerns about global warming, people have become more and more cautious over the selection of refrigerants. The phase-out of CFCs and the potential elimination of HCFCs make researchers shift their research focus back to the usage of natural refrigerants. CO2 is gradually regaining its popularity. The traditionally held opinion of the poor thermo-efficiency of CO2 (Pearson, 2005) has been challenged by current research. Tamura et al. (2005) constructed a prototype of a CO2 automotive cooling and heating air-conditioning system, which recovered waste heat from the heat pump cycle during dehumidification of incoming air. The waste heat was used as an auxiliary heat source instead of an electric heater. It was reported that the performance of the system was equal to, or exceeded that of R134a systems. In the residential application, Stene (2005) investigated a brine-to-water CO2 heat pump system for combined space heating and service water heating. The performance of such system was compared with a high-efficiency brine-to-water R410A (or R407C) system in terms of seasonal performance factor (SPF). It was found that the CO2 system outperformed its HCFC opponent when the ratio of domestic hot water (DHW) to the total heating capacity was higher than 30%.

After reviewing the above literature, we would like to link two areas of research together: the SSLC-DW-assisted system and the CO2 vapor compression cycle. To the best of our knowledge, such linked research is still missing. Given the opportunity of achieving an improved COP, it can potentially become a new application for natural refrigerant CO2.

This paper focuses on the integration of a low-temperature regenerated DW and a vapor compression system. The DW removes the required amount of latent load and is regenerated by discharge air from the condenser (or the gas cooler). The vapor compression cycle only removes sensible load. An experimental setup was constructed to evaluate the performance of such DW-assisted SSLC systems. Two refrigerants, R410A and CO2, were tested and the results were compared to each other. The experimental data was then analyzed and used to develop an SSLC system model by using an Engineering Equation Solver (EES F-chart, 2009). Then the computer model was used to predict the performance of a different configuration of DW-assisted SSLC system. The objectives of the modeling work are to search for the best possible configuration for DW-assisted SSLC systems and to minimize the performance gap, in particular, the COP difference between R410A and CO2.

Section snippets

Experimental setup

Fig. 1 shows the schematic diagram of the experimental setup. The experimental setup consisted of a vapor compression system and two wind tunnels. In the vapor compression system, two pairs of compressors, evaporators, a condenser and a gas cooler were prepared for the R410A and CO2 tests. The two variable-speed rotary compressors provided the same cooling capacity, which was set to be 3.5 kW. The two evaporators and condenser (gas cooler) were almost identical to each other, except that the

Test results

Fig. 2 depicts the psychrometric process of the SSLC system where point 1 through point 4 represent the psychrometric process of indoor air loop and point 5 through point 7 represent the psychrometric process in the outdoor air duct. All the condition points are corresponding to the locations numbered in Fig. 1. Fig. 3, Fig. 4 show two refrigeration cycles in the pressure–enthalpy (P–h) diagram. The refrigerants’ pressure and temperature profiles in the tests were plotted from Fig. 5, Fig. 6.

Exploration of better SSLC configurations

The previous chapter brought out the issue that the current SSLC system could not provide as much COP improvement over the baseline system as expected. Two factors have been identified to have a negative effect on the COP of SSLC systems. First, operation of a DW generates heat of adsorption (>2500 kJ kg−1, Gao et al., 2005), which varies according to desiccant material, but is usually higher than the heat of evaporation of water vapor (∼2500 kJ kg−1). The difference of these two forms of heat

Conclusions

In this paper, the technology of DW-assisted SSLC system was discussed in detail. The experimental results of the SSLC system with a single condenser (gas cooler) demonstrated a limited COP improvement, which was 7% for both R410A and CO2 systems, when the DW regeneration temperature was set at 50 °C. Although bigger improvement was recorded at the 45 °C regeneration temperature case, which were 32% for CO2 and 34% for R410A, such improvement could not be provided by the DW. Two problems, heat of

Acknowledgment

The support of this research through both the Alternative Cooling Technologies and Applications Consortium of the Center for Environmental Energy Engineering at the University of Maryland and Sanyo Electric Co. Ltd. is gratefully acknowledged.

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    However, the electrical COP value was similar to previously reported ones in literature for this configuration [59]. Ling et al. [60] experimentally studied CO2 and R410A hybrid VCC-DW systems under the AHRI [61] standard rating conditions (35 °C, 44% RH). The electrical COP of the VCC increased by 7% compared with the conventional system for both working fluids.

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