Absorption Process

ATequ ~ f (Tsol,^sol) Equ. 1

Figure 1. Two connected vessels containing liquid refrigerant and solution composed of the absorbent and the refrigerant.

Absorption occurs when one material, the refrigerant, is absorbed into another, the absorbent, to form a ‘solution’. The vapour pressure of the refrigerant for this solution (Ps0i) is lower than that for the liquid refrigerant (Pref). If two vessels are connected together as in Figure 1, one containing liquid refrigerant and the other the absorbent, refrigerant will be transported from the left hand vessel to the right due to this difference in vapour pressure. This results in evaporation and cooling in the refrigerant and absorption and heating in the solution, leading to a lower absorbent concentration. This transfer will continue until equilibrium is achieved when the vapour pressures in the two vessels are equal. However, the refrigerant temperature (Tref) will be lower than the solution temperature (Tsol). The difference between these two temperatures (ATequ) will be dependent on both the temperature and the concentration of the solution, as shown in Equ. 1, where ^sol is the mass fraction of the absorbent in the refrigerant.

If heat is applied to the refrigerant vessel at this lower temperature, the temperature will rise and with it the vapour pressure. This will result in transport of refrigerant vapour to the solution vessel where it is absorbed in the solution, releasing heat. If this heat is removed from the solution vessel, at a higher temperature than the refrigerant vessel, the process can continue. This is essentially a heat pumping process that can be used for cooling or heating. However, the solution gets weaker in terms of absorbent, and the temperature required to give a certain vapour pressure will decrease. Thus the temperature difference, the temperature lift, between the two vessels will also decrease, reducing the usefulness of the heat pump. In order to maintain the temperature lift, the solution needs to be regenerated by desorption, which in principle is the reverse process, with heat applied at a higher temperature to the solution vessel and removed at a lower temperature from the refrigerant vessel. Two vessels connected as in Figure 1 can be used for intermittent cooling, but in order to be able to simultaneously provide cooling while regenerating the solution, two pairs of vessels are required. For a single effect absorption chiller, these are connected to form a continuous cycle.

Different working pairs have been suggested in the literature (Macriss et al., 1988; Macriss and Zawacki, 1989), but only two are commonly available commercially: LiBr as the
absorbent and water as the refrigerant for comfort cooling, where the evaporation of water cannot go below 0°C; and refrigerators using water as the absorbent with NH3 as the refrigerant. Cycles using water/NH3 can also be used for comfort cooling, but they are not common. A number of different cycles have been developed and tested, and are treated in various studies (Herold et al., 1996; Srikhirin et al., 2001). Nearly all studies have worked on the cycles themselves, and very few have looked at the possibility of energy storage, although it is possible by storing the relatively concentrated solution between the generator and the absorber. Although this requires several extra vessels, it could be used instead of external storage devices (Berlitz et al., 1998). However, the potential for this type of storage is limited by the practical concentration variation achievable in a machine, where the heat exchanger in the absorber and generator are critical. In addition, crystallisation has to be avoided so that the solution can be pumped between vessels.

Adsorption Process

Adsorption, the binding of a sorbate onto the surface of a sorbent, can also be used in a similar way to absorption as in Figure 1. The major difference here being that adsorption is a surface phenomenon and can only be used with solid adsorbents, and thus a complete heat pump cycle cannot be built up in the same way. Instead the desorption/condensation phase, also called charging phase, and the evaporation/adsorption phase, also called discharging phase, must be separated in time. Again there are a number of different working pairs that have been studied (Dieng and Wang, 2001; Wongsuwan et al., 2001; Henning and Wiemken, 2003), and similar to absorption, those with water as the sorbate are limited to comfort cooling applications. Adsorption can be used in open or closed cycles. Common to all is the fact that the sorbate must be transported into the structure of the substance and also that the heat has to be transported to/from the solid. This creates practical problems for the design of heat exchangers and the matrix for the solid.

Thermal Storage

Adsorption has also been studied for thermal energy storage, especially for solar heating and cooling applications in recent years (Mittelbach et al., 2000). Due to their high energy density compared to sensible heat storage in water, the potential for long-term heat storage has been studied. This work has focussed mainly on water together with zeolite, silica gel or modified silica gels. An energy density of 134 kWh/m3 silica gel material has been achieved in a practical system (Nunez et al., 2003) whereas 160 kWh/m3 has been achieved for a small (1 kg) sample of zeolite and theoretically 233 kWh/m3 for impregnated aluminosilicates (Janchen et al., 2004). However, the storage density is dependent on the pressure in the system and thus the desorption temperature. Zeolites require, in general, higher desorption temperatures than silica gels. A comparison of storage capabilities for different materials for sorption systems (Mugnier and Goetz, 2001) showed that for refrigeration at -20°C, a solid-gas chemical reaction with ammonia gave the highest energy densities, whereas for comfort cooling the highest energy densities were achieved by water with NaOH for absorption, and for CaCl2, MgCl2 and Na2S for chemical reactions. These chemical reactions are the binding of water to hydrates of the salt.