Elsevier

Energy

Volume 123, 15 March 2017, Pages 432-444
Energy

Impact of district heat source on primary energy savings of a desiccant-enhanced evaporative cooling system

https://doi.org/10.1016/j.energy.2017.02.005Get rights and content

Highlights

  • District heat from CHP plant was used as a heat source of the DEVap cooling system.

  • The primary energy consumption and CO2 emission of the DEVap were evaluated.

  • The results are compared to those of the system served by a conventional gas boiler.

  • The DEVap with district heat source saved 46.2% primary energy.

  • The DEVap with district heat source reduced 40.5% CO2 emission.

Abstract

The purpose of this research is to evaluate the primary energy savings of a desiccant-enhanced evaporative (DEVap) cooling system with a district heat source. The DEVap system consists of an internally cooled liquid desiccant dehumidifier and dew point evaporative cooler connected in series. The liquid desiccant unit requires a heat source for regenerating the weak desiccant solution, which means that the DEVap cooler is a thermally driven cooling system. It can provide energy benefits when the supplied heat comes from waste heat or renewable heat sources. In this research, district heat obtained from a combined heat and power (CHP) system was used as the heat source for the DEVap system. The primary energy consumption and CO2 emission rate of the proposed system with a district heat source were estimated using a detailed energy simulation and compared with those powered by a conventional gas boiler. The results showed that the DEVap system with district heat source consumed 46.2% less primary energy and produced 40.5% less CO2 compared with the system using the conventional gas boiler.

Introduction

For the last decade, alternative air-conditioning technologies to conventional vapor-compression-based systems have been studied in the relevant research community. Non-vapor-compression technologies could replace conventional air-conditioning systems that cause negative environmental impacts if these new technologies provide reliable energy savings and low carbon emission potentials. Among several non-vapor-compression technologies, the liquid-desiccant and evaporative cooling technologies have been attracting interest for commercialization and practical use [1] because of their technical maturity.

Liquid desiccant systems dehumidify the process air through the vapor pressure difference between the desiccant solution and process air. They provide some advantages, such as lower airside pressure drop and lower regeneration temperature, compared with solid-desiccant systems. Evaporative coolers cool the process air using the latent heat of vaporization of water. When combining the liquid desiccant unit with the evaporative cooler, the dehumidification in the liquid desiccant unit enhances the cooling effect in the evaporative cooler. Previous studies [2], [7], [8], [9], [10] have proposed liquid desiccant assisted evaporative cooling systems for non-vapor compression HVAC systems. Kozubal et al. [2] proposed a desiccant-enhanced evaporative (DEVap) cooling system. This system combines a liquid desiccant (LD) dehumidifier and a dew point indirect evaporative cooler (DP-IEC). In the DEVap system, an internally cooled type LD was considered for reducing the temperature increase of the process air during dehumidification in the LD, and the DP-IEC was used to cool the process air down to its dew point. A portion of the process air leaving the primary channels is redirected to the wet channels for enhancing the evaporative cooling effect in the DP-IEC [3], [4], [5], [6]. They showed that the DEVap system could reduce the cooling energy consumption by 61% compared with a high-efficiency vapor-compression system. On the other hand, Kim et al. [7] proposed a liquid desiccant evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) consisting of a liquid desiccant unit, a direct evaporative cooler, and an indirect evaporative cooler connected in series. They showed that this system could provide 51% operating energy savings compared with a conventional variable air volume (VAV) system. Ham et al. [8] evaluated the energy savings potential of a liquid desiccant and dew point evaporative cooling-assisted 100% outdoor air system called LDEOS. Their system was a VAV system consisting of a membrane enthalpy exchanger, a liquid desiccant, a dew point indirect evaporative cooler, and a sensible heat exchanger. The LDEOS was developed to overcome undesirable humidification of the process air passing the direct evaporative cooler of the LD-IDECOAS. The membrane enthalpy exchanger was installed upstream of the LD to enhance the dehumidification performance of the LD. Their system showed a 12% primary energy savings in comparison with the conventional VAV system.

Gao et al. [9] conducted experimental study on a LD-integrated Maisotesenko-cycle, called M-cycle, an indirect evaporative cooler. They observed the inlet and outlet temperature and humidity ratio variations in both air and desiccant solution to evaluate the cooling performance of their system. The experimental results showed that the LD improved the dehumidification performance, but the cooling effect of the M-cycle indirect evaporative cooler was weakened as airflow rate and/or humidity ratio of the process air increased. Increasing the inlet process air temperature affected both the liquid desiccant dehumidifier and the M-cycle indirect evaporative cooler negatively.

Cui et al. [10] proposed a compact desiccant-evaporative heat and mass exchanger that is a LD-integrated regenerative indirect evaporative cooler. They investigated key parameters affecting the cooling performance of their system, and found that the process air leaving the proposed system was mainly influenced by the working-to-intake airflow ratio and the dimensionless channel length, and the dehumidification performance was enhanced by increasing the dimensionless channel length.

When the liquid desiccant system is commercially used, utilizing a relatively low-grade heat source can be advantageous, because it allows the effective use of renewable heat sources, such as waste heat from a combined heat and power (CHP) system, solar heat, or district heat sources. Dong et al. [11] evaluated the applicability of a proton exchange membrane fuel cell applied to the DEVap cooling system compared to the conventional VAV system. They indicated that the DEVap cooling system with the fuel cell could reduce primary energy consumption and carbon dioxide generation by 33.5% and 34.7%, respectively. Kim et al. [12] also integrated a proton exchange membrane fuel cell with the LD-IDECOAS and investigated primary energy saving potentials. They showed that their system combined with the fuel cell reduce primary energy consumption by 21% compared to the system powered by grid electricity and a conventional gas-fired water heater.

District heating systems have been attracting much interest in building applications, because they have a rational delivery mechanism to distribute the heat from a central plant to the residential, commercial, and industrial areas in the form of steam or hot water through long and insulated pipelines. The distributed heat is finally transferred to building systems via onsite heat exchangers [13]. The district heating system safely distributes heat at constant temperature (i.e., approximately 80–120 °C) [13]. Large-scale CHP plants, used as central plants for providing district heat, commonly have good energy efficiency caused by the recovery of waste heat. According to the open literature [14], the energy efficiency of a typical power plant is 50%, while that of the CHP plant is approximately 81% (i.e., 42% for power and 39% for heat).

Consequently, during the cooling season, adopting thermally driven air-conditioning systems integrated with the district heating system can lower the peak electric load for building air conditioning. In the building sector, thermally driven cooling systems, such as absorption chillers, can be supported by district heat from the CHP plant [15], [16], [17], [18]. Similarly, a liquid desiccant-assisted evaporative cooling system, such as the DEVap cooler, can also be an effective application for the use of district heat because low-grade heat sources (i.e., approximately 40–70 °C) [19] are sufficient for regeneration of the desiccant solution [20].

Among the systems that use an LD integrated with an evaporative cooler, the DEVap system showed the highest energy saving potential compared with the conventional system. However, there has been little open literature addressing the impact of a district heat source or a combined heat and power (CHP) system on the primary energy savings in DEVap system operation. Consequently, this research estimated the primary energy consumption and carbon dioxide emission rate of a DEVap system powered by a district heat source based on a detailed energy simulation. District heat from a large-scale CHP plant is used as the heat source for regeneration of the desiccant solution and as the terminal reheating coil of the DEVap cooling system. The acquired results are compared with those for the same system served by a conventional gas boiler. Detailed energy simulations are carried out with the assumption that both systems operate during the cooling season under identical operating conditions.

Section snippets

DEVap cooler

Fig. 1 shows the schematic diagram of the DEVap cooler conditioning the introduced process air via two thermodynamic processes. The first stage is the dehumidification process in the LD dehumidifier, and the second stage is the sensible cooling process in the DP-IEC. Fig. 2 illustrates the top view of a single-channel pair of the DEVap cooler. It consists of a stack of process air channels and secondary air channels. Pairs of single channels make up the DEVap cooler to provide the cooling

Primary energy factor

District heating systems only supplies heat energy to users; therefore, the electricity for building operation should be provided by the existing power grid. The overall energy performance of the DEVap cooling system with district heat source was compared to the system served by a conventional gas boiler by converting the consumed energy into primary energy. Different types of energy such as electricity, district heat, and liquefied natural gas (LNG) can be converted into primary energy using

Model building

The model building used for the simulation is an office building located in Seoul, South Korea, which is served by the DEVap cooling system. The primary energy consumptions of the DEVap cooling system with two different regeneration heat sources, namely district heat and conventional gas boiler, were estimated for the cooling season operation via detailed energy simulations.

In Table 2, the physical information of the model building is summarized. The hourly cooling load profile of the model

Energy demands

The energy performance evaluation of the DEVap cooling system was conducted during the cooling season (i.e., June to August). Fig. 8 presents the electricity and heat demands of the DEVap cooling system using the energy calculation procedure shown in Fig. 7 before they are converted into primary energy consumption.

In terms of the electricity demand, it can be observed that the chiller energy consumption was only 20.1% of the total electricity demand in the DEVap cooling system operation, which

Conclusions

This research was conducted to evaluate the primary energy savings of a DEVap cooling system with a district heat source during the cooling season. In terms of energy demand, the DEVap system needed much more heat energy than electricity, which can be a great advantage during the cooling season. The DEVap cooling system with a district heat source showed 46.2% primary energy savings compared to the same system using a conventional boiler. Considering the environmental aspect, the DEVap cooling

Acknowledgments

This work was supported by a National Research Foundation (NRF) grant (No. 2015R1A2A1A05001726), the Korea Agency for Infrastructure Technology Advancement (KAIA) grant (16CTAP-C116268-01), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) of the Republic of Korea (No. 20164010200860).

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