Realistic minimum desorption temperatures and compressor sizing for activated carbon + HFC 134a adsorption coolers

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Abstract

A low thermal diffusivity of adsorption beds induces a large thermal gradient across cylindrical adsorbers used in adsorption cooling cycles. This reduces the concentration difference across which a thermal compressor operates. Slow adsorption kinetics in conjunction with the void volume effect further diminishes throughputs from those adsorption thermal compressors. The problem can be partially alleviated by increasing the desorption temperatures. The theme of this paper is the determination the minimum desorption temperature required for a given set of evaporating/condensing temperatures for an activated carbon + HFC 134a adsorption cooler. The calculation scheme is validated from experimental data. Results from a parametric analysis covering a range of evaporating/condensing/desorption temperatures are presented. It is found that the overall uptake efficiency and Carnot COP characterize these bounds. A design methodology for adsorber sizing is evolved.

Highlights

► Evaluation of realistically minimum desorption temperatures. ► Design of adsorption compressors. ► Activated carbon + HFC 134a system. ► Validation of the model through experimental data. ► Identification of critical processes.

Introduction

Sorption refrigeration cycles are construed to be one of the means of waste heat recovery. Among them solid sorption cycles have the benefits of dispensing with solution heat exchangers and solution pumps. Solid sorption cycles based on silica gel, zeolite and activated carbon as adsorbents and water, alcohols, ammonia, carbon dioxide and HFC refrigerants as adsorbates have been investigated extensively in the literature [1], [2], [3], [4]. Industry generally prefers operation of refrigeration cycles under pressures above but close to atmospheric pressures. Although, much has been said about the positive aspects of adsorption cooling, seldom a realistic appreciation of thermal exigencies has been provided. Fig. 1 shows a schematic diagram of a typical adsorption cooler. Saha et al. [5] derive conditions of minimum desorption temperature for a few adsorbent + refrigerant combinations based on the assumptions of equilibrium conditions prevailing in the adsorption beds, no thermal gradients between the heating medium and the core of the adsorption bed and the absence of void volume effect. Saha et al. [6] expand that approach to multistage thermal compression which further reduces the temperature at which the heat source should be. Banker et al. [7] have shown that the core of a cylindrical adsorber never reaches the heating medium temperature within finite cycle times that are practical. As a result, adsorption occurs at a temperature higher than the purported adsorption temperature and desorption occurs at a lower temperature. Fig. 2 illustrates the differences between ideal and real cycles on pressure–concentration–temperature plane. Here abcd is an ideal adsorption cycle which shrinks to a′–b′–c′–d′ because of the differences between the cooling/heating media and the mean temperature of the adsorber. It gets further modified as to a″–b′–c″–d′ due to void volume effect [8]. The solution of Saha et al. [6] is based on a temperature at which Cb = Cd. Srinivasan et al. [9] introduce the concept of uptake efficiency (similar to volumetric efficiency of a positive displacement compressor) which is the ratio of actual to ideal uptake difference across which the adsorber operates. Thus, with reference to Fig. 2, the overall uptake efficiency can be defined asηuoverall=CbCaCbCa

Banker [10] has shown that the measured overall uptake efficiency is only of the order of 20–40% for the case of activated carbon + HFC 134a experimental heat recovery cooler. A designer has the input data of required evaporating temperature for a given ambient condition (which dictates the adsorption and condensing temperatures). To weigh the potential of a thermally driven solid sorption cooler as an effective waste heat recovery device, it is imminent to realistically assess what the minimum temperature of the heat source should be that will drive the adsorption cooler. This paper attempts to provide a criterion that links the Carnot COP and the overall uptake efficiency. A practical design approach is also suggested.

Section snippets

Formulation of the problem

The requirement of the temperature at which refrigeration is required (tev), the cooling load (Q) and the temperature at which heat rejection occurs (tad and tcon) are the primary inputs. The last parameter is governed, broadly, by the local ambient conditions. In the case of adsorption refrigeration cycles, though heat rejection occurs in the adsorber and the condenser, invariably the ambient forms the heat sink and hence in further analysis the adsorption and condensing temperatures are taken

Solution procedure

A range of conditions under which the cooler is likely to operate is chosen a priori. In the present analysis it is 5 ≤ tev ≤ 20, 30 ≤ tad ≤ 40, 75 ≤ tdes ≤ 100. The lower limit of tdes arises from uptake efficiency consideration and the maximum from a typical water heating flat plate solar collector. Generally, the cooling phase is more critical than the heating [7]. The Fourier number for cooling phase (Foc) needs to be optimized to secure a radial temperature non-uniformity to be less than a

Results and discussion

The case of Maxsorb II specimen of activated carbon as the adsorbent and HFC 134a as the adsorbate is taken up for further discussion. Properties of HFC 134a are evaluated using REFPROP [17]. The adsorption isotherm data of Akkimaradi et al. [18] are taken to determine the adsorption parameters using least squares regression process of Dubinin–Astakhov equation which has the following form:C=(W0/va){RTEln(psp)}n

The adsorbed phase volume (va) is treated as an extrapolation of the saturated

Conclusions

A method of determination of realistic minimum desorption temperature for adsorption coolers for a given set of evaporating, condensing/adsorption temperatures is evolved for adsorbers heated/cooled from outside. The poor thermal diffusivity of the bed results in appreciable thermal gradients across the thermal compressors which need to be accounted for. The link between the Carnot COP and uptake efficiency shows that the former needs to be at least 1.8 to build a viable thermal compressor

References (26)

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Also with Department of Mechanical Engineering, University of Melbourne, Vic 3010, Australia.

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