Elsevier

Applied Thermal Engineering

Volume 107, 25 August 2016, Pages 227-252
Applied Thermal Engineering

Research Paper
A novel approach using predictive models for performance analysis of desiccant enhanced evaporative cooling systems

https://doi.org/10.1016/j.applthermaleng.2016.06.121Get rights and content

Highlights

  • DEVAP system was modeled using the five SCST tools.

  • Models were developed to predict Tout and ωout of DEH.

  • The outlet temperature of the M-cycle stage was also predicted.

  • Comprehensive modeling and sensitivity analysis were conducted on the DEH.

  • Suggestion for enhancing the operation of the DEH was presented.

Abstract

A thorough investigation on parameters that having the potential impact on performance of the desiccant enhanced evaporative air conditioning, DEVAP, system was conducted. Five soft computing and statistical tools, SCST, including the artificial neural network, ANN, group method of data handling, GMDH, genetic programming, GP, multiple linear regression, MLR, and stepwise regression method, SRM, were used to predict the overall performance of DEVAP system. These SCST models were trained and tested using numerical and experimental data. The dehumidifier stage was assumed to be incorporated separately into two different types of counter flow indirect dew point evaporative coolers as the second stage. For each stage, the best SCST models have been determined through comparing with experimental data via error criteria, including the mean square error (MSE), and coefficient of determination (R2). It was found that the GMDH and SRM methods propose the foremost models for evaluating the performance of the second stage. Furthermore, SRM approach was found to be the best model describing the performance of the dehumidifier. Then a comprehensive sensitivity analysis was conducted for dehumidifier part. It was concluded that an effective strategy for improving dehumidifier is the implementing a part of its product air as the working air.

Introduction

As the liquid or solid hygroscopic materials, desiccants due to having the lower equilibrium vapor pressure than the air have a strong potential capability to dehumidify the air streams. By incorporating them into the cooling systems, the combined system can be used in built environment applications, especially small scales. Indirect evaporative cooling systems, IEC, vapor compression systems and absorption refrigeration systems are the alternative ones having the capability to combine with desiccants. However, due to lower operating cost, lower environmental issues and consuming lower electricity, IEC is the best choice to integrate with desiccants. Hasani Balyani et al. [1] showed that, for cooling of a 97.1 m2 benchmark building in Zahedan as a hot and dry city of Iran, IEC had lower annual primary energy consumption and carbon dioxide emission so that these parameters were respectively 4.5 and 4.6 times less than a direct expansion system.

In recent years, some integrations of the desiccant with IECs have been developed, one of which, and also best of them, is the desiccant enhanced evaporative, DEVAP, air-conditioning developed by the national renewable-energy laboratory, NREL [2]. This system is not only as a highly promising system but also it has lower energy consumption, lower environmental issues, and lower related operating costs. This system consists of two major parts: the first part is a liquid desiccant device for removing moisture in the dehumidifier, and the second part is the indirect evaporative air cooling system for removing sensible heat. Both parts have been enhanced in recent years by the usage of the fully efficient liquid desiccants, lowering energy consumption, regenerating the desiccant with low quality energy sources (waste heat or solar-energy) [3], and also employing a portion of cooled air, or return air, as the working fluid for the second part [4], [5], [6].

During five years ago, some investigations have been conducted to design a fully efficient integrated desiccant air-conditioning system. Kozubal et al. [2] developed a DEVAP system with the objective of combining the benefits of liquid desiccant and evaporative cooling technologies into an innovative “cooling core.” They tested and modeled the device at NREL and resulted that, depending on whether, it can be applied in the humid or dry climates, the DEVAP technology can reduce the annual combined source energy for the thermal and electrical energies so that it consumes 30–90% less energy than the state-of-the-art direct expansion cooling systems. Woods and Kozubal [7] experimentally and numerically analyzed the DEVAP system over a range of inlet temperatures and humidities, process and flow rates of the exhaust air, and desiccant concentration and flow rates. They showed that the numerical model predicts the prototype performance with ±10% deviation from the test results. Gao et al. [8] experimentally investigated a liquid desiccant indirect evaporative cooling which its second part was the perforated counter flow M-Cycle, PCoFMC, indirect evaporative air cooler. They showed that the cooling capacity in the second stage is directly influenced by the variation of dehumidification capacity of the first stage, which is dependent upon the inlet parameters of the air or desiccant. Furthermore, they found that for attaining the best performance in the second stage, the supplied water flow rate to the wick surface should be approximately five times that of the evaporated water. In their experimental study, the actual performance of both the dehumidifier and M-Cycle stages deviated from the energy balance condition in the range of ±20% for all experimental runs (the energy balance condition is the condition in which enthalpy change of the product air is equivalent to energy transfer of the water and desiccant in the first stage. For the second stage, the enthalpy change of the product air is equivalent to evaporation enthalpy of the water in the wet channel at energy balance condition). Elmer et al. [9] carried out an experimental study to develop a novel integrated desiccant air-conditioning system with an environmentally friendly CHKO2 desiccant for small scale built environment applications. The values of 1.26 and 3.67 were achieved for the adjusted thermal COP and electrical COP, respectively. Moreover, some studies were carried out either to utilize the DEVAP system in building applications or to compare it with other types cooling systems. Hasani Balyani et al. [1] provided the analysis of the foremost cooling strategy based on 3E (energy, economic and environmental) analyses and thermal comfort. It was demonstrated that the DEVAP system was the best solution in temperate and humid, very hot and semi-humid and temperate and wet cities since it reduced the annual primary energy consumption and carbon dioxide emission up to 13,970 kW h and 3.3 tons of CO2, respectively. Kim et al. [10] investigated a retrofit of a liquid desiccant and evaporative cooling-assisted 100% outdoor air system for enhancing energy-saving potential. They found that by adding a membrane enthalpy exchanger before the liquid desiccant unit of the DEVAP system, the cooling capacity, thermal COP, and primary COP could be improved without any significant increase in the heating energy consumption for regenerating the desiccant solution. Cui et al. [11] carried out a theoretical analysis of a liquid desiccant based indirect evaporative cooling system. The simulation results illustrated that the outlet product air temperature was influenced by the working-to-intake air flow rate ratio. Moreover, it was found that the system dimensionless channel length has impact on both product air temperature and humidity ratio.

The review of previous studies shows that there is a strong need to conduct further investigations and also to present simple and accurate approaches for developing the integrated desiccant indirect evaporative cooler. In few works that were previously cited, some theoretical and numerical models were proposed, which are not only complex, cumbersome and requiring substantial computational time but also those require to solve conventional mathematical models consisting of complex differential and analytical equations. Thus, it is vital to develop a practical, simple and accurate model for evaluating the various performance parameters of the DEVAP system applicable for engineers.

Section snippets

Desiccant-enhanced evaporative, DEVAP, cooling system

Fig. 1 is a schematic of a desiccant-enhanced evaporative, DEVAP, cooling system, which was patented by Slayzak and Kozubal [12]. As shown in Fig. 1, the system consists of two parts: the dehumidifier, DEH, and the indirect evaporative cooler, IEC. Both parts comprise of numerous pairs of channel stacked together. For inlet air, there are options such as implementing the total fresh air or a mixture of ventilation air and the indoor return air. In Fig. 1, the mixing of the outdoor air and

Soft computing and statistical tools, SCST

In modeling the complete set of a DEVAP system, it is required to determine the outlet air conditions in each stage (DEH and IEC), separately. Since the heat and mass transfer processes simultaneously occur in each part; therefore, there is complicated phenomena and this lead to the lack of accurate correlations to predict the outlet conditions of each stage. In this regards, numerical model or experimental data are used. The experimental data are limited to the cases that experiment is

Model development

It is needed to examine extensive data for evaluating the performance of a system by statistical and soft computing approaches. In this regard, a comprehensive investigation on published literature was conducted to gather a complete collection of data required for modeling the DEVAP. For accurate and thorough evaluation of the DEVAP system, each stage should be evaluated separately; however, some studies reported the performance of the DEVAP system by considering it as an integrated system [2],

Validation

For validating models, 50 experimental data set that had not been used for developing the SCST models were considered. The outputs of models were compared with those experimental data and errors of models were computed. This validation was given in Table 4a, Table 4b for counter flow regenerative M-cycle, CoFRMC, and perforated counter flow M-cycle, PCoFMC, respectively. As seen, the most accurate model for the CoFRMC is the GMDH model with 2.38% error. The corresponding minimum error for

Conclusion

A comprehensive investigation on parameters having the potential impact on various performance parameters of the desiccant enhanced evaporative air-conditioning system, DEVAP, was conducted. Five soft computing and statistical tools, SCST, including the artificial neural network, ANN, group method of data handling, GMDH, genetic programming, GP, multiple linear regression, MLR, and stepwise regression method, SRM, were employed to predict the overall performances of the DEVAP system in a simple

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