Enhancement options for separate sensible and latent cooling air-conditioning systemsOptions pour l'amélioration des systèmes de conditionnement d'air avec séparation de la chaleur latente et de la chaleur sensible
Highlights
► Divided heat exchangers improve SSLC system COP by reducing the discharge pressure. ► The evaporative cooling process benefits more to the CO2 SSLC system than R410A. ► The CO2 enthalpy wheel-desiccant assisted SSLC system achieves a high COP of 5.9. ► Sensible wheel is not suitable for the SSLC-DOS system.
Introduction
Researchers have developed various technologies to improve the energy efficiency of heat pump systems. Among those technologies, the low temperature lift heat pump systems reduce the temperature lift of vapor compression cycles (VCC) and hence decrease the compressor power inputs. In the air-conditioning application, a conventional VCC operates roughly between 7 °C evaporating temperature and 45 °C condensing temperature when assuming the ambient temperature to be 35 °C. The temperature lift of the cycle is 38 K. When applying different techniques such as separate sensible and latent cooling (SSLC) and evaporative cooling (EC) for the incoming air toward the condenser, the evaporative temperature of the VCC can be elevated and the condensing temperature of the VCC can be reduced. This leads to a reduced temperature lift of the cycle and consequently the compressor in the VCC works under a decreased pressure ratio. The compressor power inputs are therefore reduced.
As stated above, the SSLC is capable of increasing the evaporating temperature in a sensible cycle and therefore, improves a system's coefficient of performance (COP). An SSLC system generally consists of a VCC and a dehumidification device, which can be a secondary VCC or a desiccant wheel (DW). A DW utilizes an adsorption material, such as silica gel, zeolite, etc., to remove water vapor in air stream (process air) and requires another much hotter air stream (∼50 °C or higher, called regeneration air stream) to regenerate the adsorption material. Unlike conventional VCCs, the VCC in the SSLC system removes only sensible heat while the dehumidification device removes latent heat. Thus, the evaporating temperature of the VCC can be set higher than the dew point temperature of return air (example: dew temperature is 15.7 °C at 27 °C dry-bulb temperature and 50% relative humidity (RH)). A higher evaporating temperature reduces the pressure ratio of a compressor, and consequently reduces the compressor power input. There exist different methods to achieve the separation of the sensible cooling and latent cooling. Ling et al. (2010) proposed the most straightforward method. Their idea is to separate the two forms of cooling by two vapor compression systems. The first system removes sensible load only, while the second system removes both latent load and a small amount of sensible load. Under the ASHRAE standard ambient condition (35 °C, 44% RH), the energy consumption of such an SSLC system was reduced by 30% compared with that of a conventional VCC system, and the savings was reported to be up to 50% under the hot and dry condition (37 °C, 15% RH). Although this separation method is straightforward, there are two problems need to be addressed. First the sensible cycle cannot remove the entire sensible load in the system. There is always a small amount of sensible load associated with the process of latent load removal in the latent cycle. Because the energy savings of the SSLC system comes from the high COP of the sensible cycle, an incomplete separation means such configuration is not the best option (Ling et al., 2010). Second, an internal heat exchanger is required in the SSLC configuration to recover the cooling from the latent cycle. In this paper, the investigated SSLC systems use DWs to remove latent heat. The continuous operation of DW requires heat for regeneration. Various heat sources to provide hot regeneration air at the temperature ranging from 70 °C to 120 °C are described in the literature (Jia et al., 2006; Casas and Schmitz, 2004; Neti and Wolfe, 2000). With the advance in desiccant material research, recent DW products can be regenerated at or even under 50 °C. Such low temperature heat can be achieved from waste heat rejected from a condenser or gas cooler in the VCC. Ling et al. (2011) provides a theoretical and experimental study on the integration of VCC and low temperature-regenerated DW. A major challenge for this kind of system is to balance the performance of the DW and the COP of the VCC. As a solution to such challenge, Ling et al. (2011) applied divided condensers (gas coolers) in the VCC. Instead of heating the entire amount of air to a required temperature, only a portion of air is heated by a high-temperature section of divided heat exchangers to the temperature required by the DW regeneration. The other sections of divided heat exchangers work as heat sinks and the temperature of discharge air can be much lower than the regeneration temperature.
The application of the divided heat exchangers makes other energy efficiency-enhancing options feasible to the SSLC system such as the EC. In the EC process, incoming air flows through certain water source and water vapor evaporates to the air stream. Air temperature therefore decreases toward its wet bulb temperature while its humidity ratio increases. The process can be utilized to reduce the incoming air temperature to the condenser/gas cooler. Zadpoor and Golshan (2006) simulated the effect of applying desiccant-based EC system to a gas turbine cycle. The outdoor air, in this study, is fed to a desiccant wheel first and then to an EC device. This results in a lower outlet air temperature than a stand-alone EC device can provide. The system is useful especially in a hot and humid ambient condition in which the wet bulb temperature is not low enough compared to the air temperature. Lazzarin (2007) numerically investigated both the direct EC and indirect EC under various ambient conditions. The paper proposes a new diagram-based analysis method to demonstrate whether using EC is profitable or not.
Finally, the integration of heat recovery wheels, i.e., sensible wheels (SW) and enthalpy wheels (EW) to the SSLC systems are investigated. Jeong and Mumma (2005) developed practical EW effectiveness correlations based on some published complex formulations and models using statistical methods. The correlations relate the sensible and latent effectiveness of the EW to six variables such as entering air temperature and relative humidity. Two types of EW material used in the paper are silica gel and molecular sieve. Nobrega and Brum (2009) developed a mathematical model for the adiabatic adsorption within silica-gel and used it to simulate the performance of an enthalpy wheel and found an optimal non-dimensional revolution at which the enthalpy recovery is maximized. Enteria et al. (2010) experimentally investigated the performance of a SW (heat wheel). Under various rotation speeds from 2.5 rph to 20.0 rph, the sensible heat effectively keeps constant around 82% and 70% for 100 m3 h−1 and 200 m3 h−1. The paper also studies the configuration of combine a desiccant wheel and the enthalpy wheel and concludes that the SW helps improving the COP of the desiccant wheel.
Section snippets
System configurations
Several system configurations were selected as candidates for a detailed study. Fig. 1 shows the schematic of air-side processes of conventional four components VCC. In addition, seven SSLC system configurations are considered. Fig. 2 shows the DW-assisted SSLC system with single condenser or gas cooler without dividing the heat exchangers (HXs). This system configuration is called “SSLC option 1” hereafter. As introduced in Ling et al. (2011), the DW, which was a cylindrical shape with
Modeling approach
The modeling approaches used for the conventional system, SSLC system and DW have been discussed in the previous publication (Ling et al., 2011). Hereby, only a brief description of the model is presented. All the VCC were modeled and simulated using Engineering Equation Solver (EES, F-Chart Software, 2012) and an in-house heat exchanger simulation tool, CoilDesigner (Jiang et al., 2006). The compressors were modeled based on a three-efficiency model which includes the isentropic efficiency,
Modeling results and discussion
Before simulating those options, the model was validated by experimental data. Tables 5 and 6 compare the heat exchanger simulation results and experimental data, and Tables 7 and 8 compare the COP of R410A and CO2 SSLC option 1, respectively. It should be noted that, the comparison of COP in Tables 5 and 6 does not account for fan power consumption. The experimental facility had several nozzles for air flow rate measurement and therefore requires extra fan power to overcome the pressure drop
Conclusions
In this paper, different options of DW-assisted SSLC systems were discussed in detail. The application of divided heat exchangers, evaporative cooling, and EW improves the system performance. The CO2 system has more benefits from using the evaporative cooling than the R410A system, which is supported by 7% COP improvement for the R410A system as compared to a 14% COP improvement for the CO2 system when the SSLC option 3 is compared to the SSLC option 2. The reason for the outperformance of the
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 Air-Conditioning and Cold Chain Development Center in Panasonic Corp. is gratefully acknowledged.
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