Conventional Chiller

Advantages of the conventional chillers include its high efficiency; Hammad and Alsaad (1999) found that conventional chillers using R12 refrigerant had a coefficient of performance (COP) of 3.6. Conventional chillers were found to be cheaper to procure and more efficient than thermoelectric and absorption chillers, albeit operated with much more noise (Bansal & Martin, 2000). However, reviews of the electro-mechanical conventional chillers as early as 1996 identified many problems and concerns, including the utilisation of environmentally harmful fluids as refrigerants, as well as high energy consumption (Radermacher & Kim, 1996). Suzuki (1998) noted that the mechanical operating nature of the conventional chiller produced noise, and this noise was difficult to reduce. Furthermore, conventional chillers have high operational costs related to the wear and tear of mechanical components and the cost of electricity (Sanz Gallego & Muller, 2010). For example, in Kuwait, 63% of electricity generation during peak demand period is consumed by conventional chillers in air-conditioners alone (Sebzali, Hussain & Ameer, 2010).

Adsorption Chiller

Adsorption cooling systems are developed based on the thermal compression of natural working refrigerants like water or alcohols and lies in its ability to operate with motive energy derived from fairly low temperature sources such as waste heat or sun light; the adsorption process is thus a method for avoiding the use of ozone depleting and hazardous refrigerants (Meunier, 1993). Wang & Vineyard (2011) offers a description of the adsorption cycle in an adsorption chiller. A basic adsorption cycle consists of four steps: heating and pressurisation, desorption and condensation, cooling and depressurisation, and adsorption and evaporation. Figure 2 offers a graphical depiction of these four steps. In the first step, the adsorber is heated by a heat source. The pressure of the adsorber increases from the evaporating pressure up to the condensing pressure while the adsorber temperature increases. This step is equivalent to the “compression” in the vapor-compression cycle. In the second step, the adsorber continues receiving heat and its temperature keeps increasing, which results in the desorption (or generation) of refrigerant vapor from adsorbent in the adsorber. This desorbed vapor is liquefied in the condenser and the condensing heat is released to the first heat sink at a temperature of TC. This step is equivalent to “condensation” in the vapor-compression cycle. At the beginning of the third step, the adsorber is disconnected from the condenser. Then, it is cooled by heat transfer fluid at the second heat sink temperature of TM. The pressure of the adsorber decreases from the condensing pressure down to the evaporating pressure due to the decrease in the adsorber temperature. This step is equivalent to the “expansion” in the vapor-compression cycle. In the last step, the adsorber keeps releasing heat while being connected to the evaporator. The adsorber temperature continues decreasing, which results in the adsorption of refrigerant vapor from the evaporator by adsorbent, producing the desired refrigeration effect. This step is equivalent to the “evaporation” in the vapor-compression cycle. The basic adsorption refrigeration cycle is an intermittent system and the cooling output is not continuous. A minimum of two adsorbers are required to obtain a continuous cooling effect (when the first adsorber is in the adsorption phase, the second adsorber is in desorption phase). These adsorbers will sequentially execute the adsorption-desorption process.

Fig. 2 Basic adsorption refrigeration system. A. Heating and pressurisation. B. Desorption and condensation. C. Cooling and depressurisation. D. Adsorption and evaporation(Wang & Vineyard, 2011) ("Current solution" analysis of adsorption chiller: complete)

Advantages of ADCs include that it can be developed to enable regenerative use of adsorption heat produced by the ADCs themselves, as well as its lack of mechanical moving parts, noise or vibration (Saha et al., 2009). In addition, adsorption chillers can be driven by waste heat or low grade heat sources (Wang & Vineyard, 2011). This makes it suitable for use in places that generates a source of waste heat, and require chilling at the same time, for example on board ships, cars, and in printing factories. In addition, the adsorption cycle can also be used for the purpose of desalination; it is cost-effective, environmentally friendly and require only low temperature waste heat to operate (Chakraborty et al., 2013). Current desalination systems include thermally activated, pressure-activated and chemically-activated, however, all current desalination methods have shortcomings such as energy intensiveness and high maintenance arising from the cost of membrane replacement and corrosion (Chakraborty et al. 2013). 

However, usage of adsorption chillers (ADCs) on a large scale is hindered by its relatively poor performances, as well as large size, due to limited properties of solid adsorbents (Saha et al., 2009). The COP of adsorption chillers outputting chilled water are generally around 0.4 (Wang & Vineyard, 2011). The maximum optimum COP theoretically possible is 0.8, using Zeolite 13X as adsorbent and ammonia zeolite as refrigerant (Saha et al., 2009). Even then, the COP of the adsorption chiller is significantly lower than the COP of conventional chillers (3.6 as cited above). The specific cooling capacity (indicated by the amount of cooling power achieved per unit mass of adsorbent) of the adsorption chiller is also low compared to conventional chillers (Wang & Vineyard, 2011; Saha et al., 2009). In addition, an adsorption chiller optimised for cooling is inefficient at desalination, and vice-versa; desalination chillers and cooling chillers requires the chilled water input to be at different temperatures (Chakraborty et al., 2013). This often leads to the two systems being separated from each other, with the cooling provided by the desalination chiller neglected instead of being utilised.