Performance analysis of desiccant wheels assisted fresh air humidifiers in winter using natural gas boilers: Applied in cold and dry climate regionsAnalyse des performances des humidificateurs d’air frais assistés par roues déshydratantes à l’aide de chaudières à gaz naturel : utilisation en hiver et dans des régions à climat froid et sec

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Abstract

A fresh air handling unit is proposed for space humidification in winter for buildings located in cold and dry climate regions. The air handling unit has three-stage desiccant wheels to transfer moisture from dehumidification air (Adeh) to humidification air (Ahum), both of which are from outdoor, and heat from boilers is used to drive this process. Humidification performances of this device were discussed when ambient humidity ratio was from 1 to 4.5 g kg−1 and corresponding operating strategies under different ambient conditions were proposed to keep the supply humidity ratio in the range of 5.1–5.5 g kg−1. Then, effective heating systems, including air side heat recovery and step utilization of heat from low-grade waste heat, were investigated. Humidification efficiency (ηsys) of systems without heat recovery ranges from 0.24 to 0.9. With heat recovery at the air side, which is called improved system I, ηsys was enhanced to 0.4–1.2. Water circulation layout was redesigned to realize step utilization of heat. Return water temperature from humidification devices could be reduced, and ηsys can be obviously enhanced when low-grade waste heat sources are available. When air source heat pumps are used to preheat the return water, from Case A to Case H, primary energy efficiency can be improved by 5.8–26.1% as compared with the improved system I.

Introduction

Indoor air humidity plays a vital role in human health. During heating period, indoor relative humidity becomes very low for lack of humidification units, especially for dry climate regions. Adsorption desiccant wheel is an efficient air humidification method, which consumes no fresh water. With honeycomb air channels, it has large specific surface area and relatively small pressure drop (Cao et al., 2014). Vapor can be transferred from one stream of air, called dehumidification air to the other stream of air, called humidification air or regeneration air. The later needs to be heated before being supplied into the desiccant wheel. Renewable heat energy, such as solar collectors, can be effectively used (Goodarzia et al., 2017), making the method more attractive.

Desiccant wheels have been widely used for air dehumidification in air conditioning systems. There have been a lot of studies about efficient design of desiccant wheels and performance of desiccant wheel dehumidification systems. Giannetti et al. (2015) defined a new entropy generation number and obtained useful criteria for desiccant wheel optimization. O'Connor et al. (2016) redesigned the desiccant wheel to reduce flow resistance below 2 Pa. Narayanan (Narayanan, 2017) investigated the effect of geometry and shape of element channels on the transport process and the performance of desiccant wheel. Fong and Lee (2018) compared the performances of desiccant wheels using different adsorbents, such as RD silica gel (SG), AQSOA-Z02 and CECA-3A. Zendehboudi and Li (2018) proposed a two-step computational framework based on the combination of response surface methodology and multi-objective optimization to model the outlet-air state of desiccant wheels and subsequently optimize their operation. Kang et al. (2018) proposed an analytic solution to predict the outlet air states of a desiccant wheel with an arbitrary split ratio. Tu et al. (2016) investigated the performances of ventilation systems with desiccant wheel cooling from the perspective of exergy destructions. Liu and Tan (2020) proposed a novel desiccant wheel air conditioning system that uses high-temperature chilled water from natural cold source.

While desiccant wheels are primary used for dehumidification in wet season, they can provide a humidification function in dry season without using conventional evaporative humidification system (Hamed, 2003; Parekh et al., 2004). Wada et al. (2009) studied an outdoor air processor with the desiccant wheel, regeneration temperature of which was kept at 50 °C by the water source heat pump waste heat. La et al. (2011) and Zeng et al. (2014) constructed a solar heating and humidification system with a one-rotor two-stage desiccant unit. Antonellis et al. (2015) suggested that dehumidification side air flow rate should be higher than that of regeneration side. Similar results can be found in the study of Kawamoto et al. (2016), who reported that, to satisfy the minimum humidification needs, no additional water during the winter season was required if the dehumidification side air flow rate was 100–200% higher than that of humidification side. Tu and Hwang (2019A) and Tu and Liu (2019B) investigated an atmospheric water harvesting system that humidified the air through multi-stage desiccant wheels before being dehumidified by the evaporator of heat pump systems.

The heating of humidification air before entering desiccant wheels for regeneration has an important effect on energy consumption and efficiency of the system. Studies includes reducing heat souce temperature (Zhou and Reece., 2019; Tu et al., 2018A), reducing heating capacity (Tu et al., 2018B) or adopting efficient low temperature heat sources, such as heat pump systems (Sheng et al., 2015) or waste heat sources like solar energy (Goodarzia et al., 2017). Tu et al. (Tu et al., 2018B) investigated the efficiency of regeneration systems using three different heat sources, namely vapor compression cycle, electrical heater and natural gas burner. Zhou and Reece (2019) constructed and tested a non-adiabatic solid desiccant wheel, which can use low temperature heat sources for regeneration. Zhou et al. (2018) proposed an internally water-cooled desiccant wheel, performance of which can be improved by 48% as compared with a conventional desiccant wheel. Tu et al. (Tu et al., 2018A) investigated effects of adsorption isotherms and rotational speed on regeneration temperature of desiccant wheel systems. Goodarzia et al. (2017) presented an energy-efficient desiccant wheels system with solar energy and waste heat. Sheng et al. (2015) investigated the energy saving potential of a desiccant wheel system that was driven by a high temperature heat pump.

Literature review shows that there are many studies conducted on the air dehumidification processes using desiccant wheels, some studies conducted on the air humidification processes using desiccant wheels in winter season, and few studies conducted on the air humidification processes using desiccant wheels for dry climate regions. To fill the gap, this paper investigated an effective desiccant wheel-assisted humidification system especially for dry and cold climate regions. And system efficiencies were discussed under eight typical ambient conditions when natural gas water boilers are used as heating source. Air-air heat recovery and water-waste heat recovery were discussed. In this paper, the psychrometric process through the desiccant wheel is analyzed by using the mathematical model, which analyzes the transient heat and mass transfer processes between the air and adsorbent in the honeycomb structure of desiccant wheels. This model was developed from the previous research and validated by experiment results from two different dehumidification wheels operated under range of working conditions, details of test procedures and test results have been discussed in detail in previous studies (Tu and Hwang, 2018A, Cao et al., 2014). Tu et al. (2016, 2015A,B,C) and Tu and Hwang (2018A,B) provided its details including assumptions, four heat and mass transfer and conservation equations, which are used to describe transient heat and mass transfer processes between the air and the desiccant material, the equilibrium isotherm equation of adsorbents, and other important parameters.

Section snippets

Configurations and design parameters of proposed air humidification units

Schematic of the air handling unit for air humidification using desiccant wheels is shown in Fig. 1. The air handling unit has a three-stage configuration. There is one stream humidification air (Ahum) that enters into the three desiccant wheels in sequence, and three independent streams of dehumidification air (Adeh) that enter into each desiccant wheel separately in a reverse direction of Ahum. Both Ahum and Adeh are from outdoor. Adeh is dissipated after being dehumidified at one of the

descriptions of heat systems

In this part, energy efficiencies of the fresh air humidification system, which uses water as a heating fluid and natural gas boilers as a heating source, are discussed. In the discussion, treg is fixed at 50 °C. For different working conditions, ASN, Fr and X can be referred to Table 2.

H1, H2 and H3 in each air handling unit are used to heat Ahum to treg. A preheater is used to heat all the inlet air from ambient temperature to 10 °C to prevent water solidification at heat exchangers of air

Step utilization of heat and heat recovery from low-grade heat sources

For the basic system and the improved system I, inlet temperature of hot water in each heat exchanger is the same. However, inlet temperature of air is quite different. As shown in Fig. 9, for preheater 2, temperature level of air is pretty low, while heat exchange capacity is large. For H1, the change of air temperature is large. For H2 and H3, the change of air temperature is pretty small, while temperature level is high. Therefore, it is not energy efficient for those heat exchangers to use

Conclusions

This article discusses the energy efficiency of desiccant wheel assisted air humidifier when natural gas boilers is used as the heat source for buildings located in cold and dry climate region. The following conclusions are noteworthy:

  • 1)

    An air handling unit with three-stage desiccant wheels and independent Adeh for each desiccant wheel was studied. When ambient humidity ratio of which is ranged from 1 to 4.5 g kg−1 and heat source temperature is 50 °C, supply humidity ratio is in the range of

Declaration of Competing Interest

We received the financial support from the National Natural Science Foundation of China (No. 51706015), and from the Fundamental Research Funds for the Central Universities (FRF-IDRY-19-01)

Acknowledgments

The authors appreciate the support from the National Natural Science Foundation of China (No. 51706015), and from the Fundamental Research Funds for the Central Universities (FRF-IDRY-19-01).

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