K. Sumathy

Department of Mechanical Engineering, University of Hong Kong, Hong Kong

Introduction

Refrigeration is an attractive application of solar energy, because, the supply of sunshine and the need for refrigeration reach maximum levels in the same season. One of the very effective form of solar refrigeration is the production of ice, because ice accumulates much latent heat in it, so that the volume of ice maker can be small. In 1981, Pons and Grenier [1] worked on a solid adsorption pair of zeolite and water, to produce the refrigerating effect and the coefficient of performance was about 0.1. Lately, they had successfully experimented with the adsorption pair of activated carbon and methanol. Sakoda[2], had presented the advantages and limitations of the simultaneous transport of heat and adsorbate in closed type adsorption cooling system, utilizing solar heat. This paper focus on a solar-powered ice-maker with solid adsorption pair of activated carbon + methanol. A simple flat-plate collector having an exposed area of 0.92 m2 is employed to produce ice of about 4-5 kg/day.

System Description

The system consists of a flat-plate collector, a fin-type condenser/heat exchanger and an evaporator which acts as a refrigerator, as shown in Fig. 1. The collector is supplied with activated carbon which is adsorbed with methanol. During the daytime, when solar energy is available on the collector, methanol gets heated up and evaporates from the activated carbon. The vapour is then condensed through the heat exchanger(condenser), and the liquid methanol is stored in the evaporator, which is said to be the regenerating course of the activated carbon. During the nighttime, the collector temperature begins to decrease
due to the heavy heat loss to the atmosphere. In this period, methanol begins to evaporate by absorbing heat from the liquid (water) and gets adsorbed by the activated carbon inside the collector. Since adsorption is a process of releasing heat, the collector must be cooled efficiently at night. As mentioned above, the ice maker operates in an intermittent way to produce the refrigerating effect.

The principle of the solid-adsorption ice-maker is explained using a P-T-X diagram as shown in Fig.2. To begin with, the adsorption bed along with the refrigerant gets heated up, and when it reaches the required desorption temperature(Td1), the methanol gets desorbed. In the evening, the flat-plate collector (adsorption bed) looses its heat to the surroundings and hence the temperature of the adsorbent bed is reduced rapidly (Td2 ? Tai), and the pressure in the adsorber drops to a value below evaporation pressure (Pe). During this period, evaporation could happen, and ice will be made in the refrigeration box.

As explained above, the system works in an intermittent way. This system can be made to operate continuously to produce ice, by incorporating two absorbers such that while one works in desorption mode, the other would be set in for absorption.

Line A-B : Represents the heating of AC along with methanol.

Qa-B = (C pa + CpmWA )(TB — TA) (3)

Line B-D : Shows that, the collector is connected with the condenser and progressive heating of the adsorbent from B to D causes some adsorbate to be desorbed and its vapour to be condensed.

Line D-F : The collector is closed and cooled. The decrease in temperature from D to F induces the decrease in pressure from PC to PE.

Line F-A : The collector is connected with the evaporator during when the adsorption as well as evaporation occur while the adsorbent is cooled from F to A.

The total energy input (QT) to the system is given by,

TOC o "1-5" h z Qt = Qa-b + Qb-d (3)

The cooling effect (for ice production) is given by,

Qc =(Wa — Wd )[l — Cpm (Ta — TE )J (4)

Therefore, the system efficiency is given by,

COPTH = (5)

TH Qt ( )