Elsevier

Energy

Volume 36, Issue 8, August 2011, Pages 5261-5273
Energy

On the performances of a hybrid air-conditioning system in different climatic conditions

https://doi.org/10.1016/j.energy.2011.06.030Get rights and content

Abstract

In previous papers the authors demonstrated that significant energy savings can be achieved in air-conditioning through the use of a hybrid plant in which a vapor-compression inverse cycle is integrated with an air dehumidification system working with hygroscopic solution and hydrophobic membrane. The advantage of this system lies in the fact that the refrigeration device operates at a higher evaporation temperature than that of a traditional system, in which dehumidification is achieved through condensation.

In the proposed hybrid system the supplied air is simultaneously cooled and dehumidified in an air–solution membrane contactor. The LiCl solution is cooled by means of a vapor-compression inverse cycle. The solution is regenerated in another membrane contactor by exploiting the exhaust air and the heat rejected by the condenser.

The paper reports a study of the steady-state behavior of the system in summer climatic conditions on varying some significant climatic parameters, such as the latent load of the conditioned space and the outdoor and indoor relative humidity. The performances of the hybrid system are compared with those of a traditional direct-expansion air-conditioning plant. Results of the simulations reveal that energy saving may exceed 60% when the latent load in the conditioned environment is high.

Highlights

► A hybrid air-conditioning plant is simulated in summer climatic conditions. ► A vapor-compression inverse cycle is integrated with an air dehumidification system working with liquid desiccant and hydrophobic membrane. ► The performances of the hybrid system are compared with those of a direct-expansion air-conditioning plant. ► The energy saving may exceed 60% when the vapor load in the conditioned environment is high.

Introduction

The planning of air-conditioning systems for civil indoor environments has aroused considerable technical interest. Particular attention has been focused on innovative solutions that enable both the thermohygrometric parameters and the quality of the indoor air to be controlled, while limiting energy consumption as far as possible. Indeed, recent recommendations concerning the quality of indoor air have imposed higher renewal air flow rates and, consequently, greater energy consumption [1], [2].

In summer conditions, the input air flowing into conditioned environments must be cooled and dehumidified. Dehumidification is traditionally achieved through condensation as the air is cooled below the dew-point temperature. Inverse cycle vapor-compression devices are commonly used for this purpose. This simple solution, however, requires high consumption of electrical energy. In addition, the air often has to be reheated to a suitable temperature before entering the conditioned environment. Alternatively, the air can be chemically dehumidified by means of solid or liquid desiccants. This approach enables the specific humidity of the air to be controlled independently of its temperature, thereby reducing the sensible and latent loads separately.

It has been demonstrated in the literature that significant energy savings can be achieved through the use of so-called hybrid air-conditioning systems, in which a chemical dehumidification system is combined with an inverse cycle vapor-compression device [3], [4]. The advantage of such systems lies in the fact that the refrigeration device can operate at a higher evaporation temperature than that of a traditional system, thereby achieving higher coefficients of performance.

In hybrid systems that use a liquid desiccant, the air is usually dehumidified by a hygroscopic solution in direct-contact air–solution exchange components (spray towers, packed-bed towers, falling-film columns) [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. In [10], a hybrid liquid desiccant system is described in which the sensible and latent exchanges take place simultaneously inside the air–solution exchange component, and the solution is cooled in the evaporator of the refrigeration device. In others studies [7], [8], [11], [14], only the latent exchange takes place in the air–solution exchanger, and the air is preliminarily or subsequently passed through the cooling coil. The solution can be regenerated either in another direct-contact exchange component by means of the exhaust air or the outdoor air [10] – in some cases, exploiting the heat rejected by the condenser [11] – or in open or closed solar collectors [7], [8], [12]. In [15] a multilayer artificial neural network based model is used in order to investigate the performance of a packed-bed tower dehumidifier.

The use of liquid desiccants offers an important advantage; in addition to reducing humidity, the quality of the air can be controlled through the co-absorption of pollutants into the solution [16]. However, direct air–solution contact gives rise to other problems, in that droplets of solution or spores and bacteria present in the liquid phase may be transferred to the air, resulting in the corrosion of ducts and worsening the quality of the air. Consequently, direct-contact devices are chiefly used in industrial applications, their utilization in civil/domestic settings being still limited. Such drawbacks can, however, be overcome by using mass exchangers in which the air and the solution are separated by a hydrophobic membrane [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Known as membrane contactors, these exchange devices have large exchange surfaces per unit of volume and display modest friction losses on both the air side and the solution side.

Research into the application of hybrid air-conditioning systems in which the air–solution exchange takes place in membrane contactors, instead of in direct-contact components, may prove very fruitful.

At present they are not available in literature studies concerning performance analysis of hybrid air-conditioning plants working with membrane contactors, with the exception of two papers [27], [28] carried out previously by the authors. In these papers a hybrid air-conditioning plant in which a system of air dehumidification operating by means of a liquid desiccant and a hydrophobic membrane is combined with a vapor-compression inverse cycle was proposed. The performances of the system were analyzed on varying some significant operating parameters of the plant itself, such as the air recirculation ratio, the flow rates of air and solution through both the dehumidifier and the regenerator and the exchange areas of the contactors. The performances of the hybrid system were compared with those of a traditional direct-expansion air-conditioning plant in summer climatic conditions. For this purpose a SIMULINK computer code was developed by the authors.

The aim of the present study was to analyze the steady-state behavior of the system in summer climatic conditions on varying some significant climatic parameters, such as the latent load of the conditioned environment and the outdoor and indoor relative humidity. Results are expressed in terms of the compressor power and coefficient of performance COP of the refrigeration device, and the percentage power saving in comparison with a traditional direct-expansion air-conditioning system functioning in the same climatic conditions.

Section snippets

Description of the hybrid air-conditioning system

Fig. 1 shows the hybrid air-conditioning system under examination. The air in thermohygrometric conditions (m) is simultaneously cooled and dehumidified in the air–solution membrane contactor (dehumidifier) until the input conditions (i) are reached. A thermal exchange and a mass exchange occur simultaneously inside this component on account of the temperature and the vapor pressure gradients present between the air and the hygroscopic solution, which is cold and concentrated (2) when it enters

Mathematical model of membrane contactors

In the hybrid air-conditioning plant the dehumidifier and the regenerator are considered to be two cross-flow membrane contactors in which the air and solution flows are separated by means of plane membranes.

Fig. 2 shows a sketch of the membrane contactor. The dimensions reported in the figure are those of the prototype produced by the company GVS [29] and subjected to experimental tests [21], [22] in the laboratory of DIPTEM, Division of Thermal Engineering and Environmental Conditioning of

Summer air-conditioning example

The performances of the hybrid system were compared with those of the traditional direct-expansion plant represented in Fig. 6 in a typical example of summertime air-conditioning. The air in thermohygrometric conditions (m), which is the result of mixing the external renewal air (e′) with the recirculation air (a′), is first cooled and dehumidified until state (i*) is reached; it is then reheated in order to obtain the conditions required for input into the conditioned space (i). The process of

Results and discussion

We analyzed the performances of the hybrid air-conditioning system on varying some significant climatic parameters, i.e. the indoor latent load gv, the relative humidity of the outdoor air ϕe and the relative humidity of the indoor air ϕa, at the same values of outdoor temperature te = 32 °C, indoor temperature ta = 25 °C, input temperature ti = 15 °C and sensible heat flux φsen = 5.7 kW.

In all the simulations, the flow rate of dry air through the dehumidifier and the regenerator was considered to be the

Conclusions

The present study analyzed a hybrid air-conditioning plant obtained by combining an air dehumidification system that uses a hygroscopic solution and hydrophobic membranes with an inverse cycle vapor-compression device. The performances of the plant on varying some climatic parameters in a typical example of summertime air-conditioning were analyzed and compared with the performances of a traditional direct-expansion plant The following conclusions can be drawn from the simulations carried out.

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