Optimization of thermal design and geometrical parameters of a flat tube-fin adsorbent bed for automobile air-conditioning

Highlights

• A flat tube-fin adsorbent bed was studied to obtain the maximum system performance.

• The overall thermal conductance and the maximum SCC increased when reducing in flat tube thickness and fin pitch.

• The maximum SCC increased when increasing in fin thickness and water channel wall thickness.

• Adsorbent thermal conductivity is the most sensitive parameter to the specific thermal conductance.

• The system performance enhanced by reducing the mass of the metal bed and the heat transfer fluids.

Abstract

Adsorbent bed design and performance strongly affect the overall performance of adsorption systems. In the present study, an analytical model was developed to determine the optimum geometrical and thermal parameters of a flat tube-fin adsorbent bed to reach the maximum system performance. This types of heat exchangers offer substantial reduced in weight, cost, volume and thermal conductivity, which can make them a good choice for adsorbent beds in automobile applications. Results showed that the overall thermal conductance of the bed and the maximum practical specific cooling capacity increased when reducing in flat tube thickness and fin pitch as well as by increasing in fin thickness and water channel wall thickness. The specific thermal conductance increased by 2.5% when reducing the channel pitch from its design value to a minimum permissible (0.004 m). From thermal parameters that have been studied, the adsorbent thermal conductivity is the most sensitive parameter to the specific thermal conductance in beds. The system performance also significantly enhanced by reducing the mass of the metal bed and the heat transfer fluids as well as the desorption heat of the selecting working pair.

Introduction

Reduction of energy consumption by reducing the consumption of energy services and through efficient energy usage is turning out to be one of the important topics nowadays [1]. Research activity on thermally driven adsorption systems has received much attention in the recent years. Adsorption systems can be driven by waste heat and low-grade heat source (<100 °C) and they use environmentally friendly refrigerants such as ammonia or water [2], [3]. In addition to environmental benefits and energy saving, these systems have many other advantages such as lack of moving parts, low operating costs, vibration-free operation and simple control [4], [5]. These features make them suitable for applications in automotive air conditioning systems using waste heat from the engine operation without losing mechanical energy, which can also reduce the carbon footprint [6].

However, the major drawbacks of adsorption systems are their low energy efficiency (COP and SCP), large size and mass and high investment costs when compared to conventional systems. These drawbacks prevent adsorption systems from commercialization and widespread utilization, especially in automotive applications [7], [8]. Good heat transfer performance in adsorbent beds increases the total heat transfer coefficient and the rate of heat transferred between the adsorbent and the heat transfer fluid [9]. In a similar view, good mass transfer performance will reduce the diffusion time of the adsorbate in the adsorbent and shorten the cycle time, which will then cause an increase in overall system performance [10].

Therefore, the need for efficiency improvement of adsorption systems encourages scientists to search for possibilities of improving heat and mass transfer in the adsorbent beds during adsorption-desorption cycles. Most of searches focused on adsorption properties of adsorbent/adsorbate pairs, adsorption/desorption cycles, and design modification of the adsorbent bed [11]. Different types of heat exchangers are used in adsorbent beds such as plate-finned tubes, and a capillary tube bundle HE [12], a flat-plate type HE with fins [13], a serpentine flat-pipe HE, and spiral plate HE [14]. Many researchers have proposed various types of sorption bed designs in order to enhance the heat transfer rate in the adsorbent bed [15], [16], [17], [18], [19], [20], [21], [22].

Verde et al. [23], [24] studied theoretically and experimentally the performance of a proposed silica gel adsorption system for automotive air conditioning system. A flat tube-fin heat exchanger was used as an adsorbent bed. An improved non-equilibrium lumped parameter model was developed to predict the dynamic performance of the system under different real driving conditions. Different design configuration and operation strategies had been carried out in order to explore further improvements of the system performance. Results showed that, using two radiators in the system instead of one radiator increases the cooling capacity by 7.0% and decreases the cabin temperature by 9.1%.

There is relatively limited information regarding the effect of adsorption bed geometrical parameters with fins on the systems performance. Rezk [25] studied theoretically the effect of various adsorbent bed heat transfer enhancement techniques on the performance of a two-bed adsorption chiller. The results showed that cooling capacity and COP increased with increasing the fin spacing ratio to reach maximum of 25% and 10% respectively at fin spacing ratio of 2. Hamid and Iman [26] developed a transient two-dimensional model for adsorption bed with annular fins to study the effect of bed configuration on the system performance. They showed that the COP of the cycle is slightly influenced by the fins number and strongly dependence on the fins height. Hamid et al. [27] developed a three-dimensional model for adsorbent bed with square and annular plate fins. They showed that the cooling capacity can be increased by the increase of the number of fins and the decrease of the fins height. In addition, COP increases and SCP decreases with increasing fin height and fin pitch. Rezk and Al-Dadah [28] developed a lumped analytical simulation model for silica gel/water adsorption system. They showed that the cooling capacity increased by 3% and the COP decreased by 2.3% when varying the fin spacing from its design value to a minimum permissible. Khan et al. [29] studied the effect of overall thermal conductance and adsorbent mass on the performance of a two-stage adsorption system. They found that system performance can be strongly influenced by the bed overall thermal conductance. Furthermore, they showed that the COP can be increased by the reduction of the adsorbent mass.

The literature review indicates that, many of these studies have proposed various types of bed designs. Few studies carried out on the effect of bed geometrical parameters on the heat transfer rate and hence how it affect the system performance. However, there was no information about the optimization of the thermal and design parameters in order to enhance the heat transfer rate and hence the overall system performance.

Therefore the purpose of the present study is to optimize the different parameters that mostly affect the performance of a flat tube-fin heat exchanger for automobile adsorption systems. An analytical model has been developed in order to estimate the overall thermal conductance of the heat exchanger by using a thermal resistance network. A parametric study was performed using the model to determine the effect of different bed parameters on the specific thermal conductance and maximum practical specific cooling capacity of the adsorbent bed. Physically, the proposed flat tube-fin heat is constructed by flat channel tubes covered with uniformly spaced flat fins to increase the heat transfer area on the air side which will be employed to increase the heat transfer to the adsorbent. They are available in the markets where used normally as evaporator in the conventional automotive air conditioning systems, therefore they are available at a low cost. In addition these type of HE are made from aluminum, have the advantages of low weight and low thermal capacity (lower heat losses) compared with other heat exchangers. The literature survey is relatively limited to this type of adsorbent bed. Therefore, further studied on these types are needed.