Integration of variable refrigerant flow and heat pump desiccant systems for the cooling season

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Abstract

Energy saving and indoor air condition enhancing potentials by integrating the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems were investigated in a field performance test during a cooling season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–dehumidification assisted VRF systems were investigated. The HPD systems operated in the ventilation–dehumidification mode dehumidify the outdoor air and supply it to the indoor air during the ventilation. It was found that the VRF systems provided an average of 97.6% of the total cooling energy for the HPD ventilation assisted mode. The remainder was the recovered cool by the HPD systems during ventilation. The VRF systems provided an average of 78.9% of the total cooling energy for the HPD ventilation–dehumidification assisted mode. The remainder was covered by the HPD systems which provided additional sensible and latent cooling. Overall, among the three operating modes, it is concluded that the HPD ventilation–dehumidification assisted VRF outdoor units consume less energy than the HPD ventilation assisted ones, but more than the non-ventilated ones, while providing the best indoor thermal comfort and indoor air quality conditions. For the total system, the HPD ventilation–dehumidification assisted VRF systems consume less energy than the HPD ventilation assisted ones.

Introduction

A great amount of world energy demand is associated with the built environment [1], and it is estimated that air conditioning systems, which are required by the residential and commercial buildings due to modern societies’ demand for thermal comfort and healthy indoor environments [2], consume about 50% of the total electricity use in the office buildings [3], [4]. That is why; reducing the energy use for the space cooling and ventilation in buildings is a key measure for energy savings [5].

The variable refrigerant flow (VRF) systems, first introduced to the market more than 25 years ago [6], are now widely used in both residential and commercial buildings, because these systems have precise capacity control and individualized thermal comfort capabilities [2]. A VRF system is a refrigerant system that varies the refrigerant flow rate with a variable speed compressor and electronic expansion valves to match the capacity of the system to the space cooling loads in order to maintain the zone air temperature at the set temperature [5]. Schematic drawing of a VRF system and the refrigerant flow direction of the cooling mode can be found in Refs. [7], [8]. Due to the long history, the VRF systems have been widely studied experimentally and numerically [2], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. The literature survey indicates that the ventilation is one of the main drawbacks of the VRF systems [6], [7], [8], [23], [24], [25], [26]. Since the VRF systems cannot provide any ventilation, additional ventilation systems are necessary to be installed with the VRF systems. Heat recovery ventilation systems are widely used in conjunction with the VRF systems [2], [5], [7], [8], [23], [24]. A schematic drawing of a heat recovery ventilation system can be found in Refs. [7], [8], [24]. These systems provide fresh air while recovering cool from the exhaust air stream in order to reduce the ventilation loads [7], [8], [27]. However, they cannot provide any effective dehumidification during the heat recovery [8], [24], [25]. On the other hand, a new ventilation system, a novel self-regenerating electric vapor compression heat pump desiccant (HPD) system was recently introduced [25], [26], [28]. During the ventilation in the cooling season, the HPD system dehumidifies the outdoor air, and supplies the outdoor air with low humidity to the indoor air as the supply air [25], [28]. The detailed system description and operational characteristics of the HPD system can be found in Refs. [25], [28]. Preliminary experimental data related to the integration of the VRF and HPD systems was provided in the study of Ref. [25]. The integration was found to be promising in terms of energy savings of the VRF systems and better indoor thermal comfort [25].

The current study is an extension of Ref. [25], and addresses the field performance evaluation of the VRF systems integrated with the HPD systems in an existing office suite under varying outdoor conditions for the cooling season.

Section snippets

Existing building

Experiments were carried out in an office suite located in College Park, campus of University of Maryland, 16 km northeast from Washington DC, the Capital City of the US. The office suite has five office rooms and three open spaces. The floor layout and the area of the zones can be found in Fig. 1a, and Table 1, respectively. Building characteristics can be found in Ref. [2].

VRF system

Two VRF systems were used for the air conditioning of the existing office suite. Each VRF system had one outdoor and four

Evaluation methodology

A compressor performance map, extensively used in previous studies [8], [18], [20], [23], [24], [26], was used for the calculation of the total refrigerant mass flow rate supplied from the VRF outdoor unit. The refrigerant mass flow rate is defined with Eq. (1).m˙T=m˙T(Tc,Te,Tsuc,f,STNsta)

In order to obtain the individual cooling capacities of each indoor unit and the total cooling capacity of the VRF system, the individual refrigerant mass flow rates of each indoor unit are needed. An

Results and discussion

The current study is based on a field test and the comparisons are performed under similar outdoor conditions. Fig. 2a and b shows the variation of the percentage of data points with respect to the outdoor temperature and outdoor humidity ratio, respectively. Data are provided with 10 min averages. As can be seen from Fig. 2a and b, the outdoor conditions of all three operating modes have similar profiles.

Fig. 3a shows the seasonal variation of the daily energy consumption of the VRF outdoor

Conclusions

Energy saving and indoor air condition enhancing potentials by integrating the variable refrigerant flow (VRF) and heat pump desiccant (HPD) systems were investigated in a field performance test during a cooling season. Three different operating modes: non-ventilated, HPD ventilation assisted and HPD ventilation–dehumidification assisted VRF systems were investigated. The following conclusions were deduced from the evaluations:

  • The outdoor unit energy consumptions of the three operating modes

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