**4. Advanced adsorption cycles**

In view of the fact that the basic adsorption cycle produces intermediated cooling output and its COP is low, many advanced adsorption refrigeration cycles have been proposed and developed to help in overcoming these main drawbacks such as the heat recovery cycle, mass recovery cycle, thermal wave cycle, cascade cycle and multi-stage cycle. However, the basic cycle is mostly used in the solar powered adsorption system for its simplicity.

## **4.1. Two-bed adsorption cycles**

The flow diagrams of a conventional two-bed cycle representing one complete cycle are illustrated in **Figure 9**. Each adsorber undergoes four operating modes: pre-heating, heating (desorption), pre-cooling and cooling (adsorption) processes in repeating cycles and according to the sequence shown in **Table 1**. The system consists of two adsorbent beds, a condenser, an evaporator and an expansion valve, in addition to the four connecting valves and connecting pipes. The working principle of the basic cycle is discussed in detail in Section 1.1 for one-bed ARS. In a two-bed ARS, while hot water is used to heat up Bed-A during the first two processes: cooling water is used to cool down Bed-B. The hot water is switched to Bed-B in the last two processes, as Bed-A is subjected to cooling water. The four valves are completely closed during the two switching modes. Circulating the cooling water and chilled water in the condenser and evaporator, respectively, are supposed to be continuous during the whole cycle.

typical two-bed cycle equipped with mass recovery system, while one bed at the end of desorption mode at higher pressure and temperature, the other bed at the end of adsorption mode at lower pressure and temperature. Internal mass recovery process is started via connecting the high-pressure adsorber to the low-pressure one typically by means of a valve. The process ends when the two pressures become equal. The combined heat and mass recovery procedures may increase COP more than 10% [62], compared to heat recovery cycle. Thermal wave cycle is another way to recovery the heat inside the cycle. A typical thermal wave cycle is composed of two adsorbers, an evaporator, a condenser, a cooler and a heater as shown in **Figure 10** [63]. Experimental results showed that the COP of a twobed adsorption air conditioner (zeolite-water) with thermal wave cycle was approximately

**Figure 10.** Schematic representation of the thermal wave adsorption heat pump during the first half of the cycle [63].

Mode-A switching Pre-heating Pre-cooling X X X X Mode-B Des/Ads Heating/Des Cooling/Ads O X O X Mode-C switching Pre-cooling Pre-heating X X X X Mode-D Ads/Des Cooling/Ads Heating/Des X O X O

**Bed-A Bed-B V1 V2 V3 V4**

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167 87

1.0 in cooling season [12].

**Mode Component**

**Table 1.** Cycle modes and valve positioning.

In a system of two or more adsorbers work between an evaporator and a condenser, there is a hot adsorber under cooling process and a cold one under heating process which offers the availability to recovery the heat inside the system. The experimental results show that the COP of the system will increase by up to 25% with the heat recovery cycle [62]. In a

**Figure 9.** Schematic diagram of a conventional two-bed adsorption chiller, as Bed-A in desorption mode, Bed-B in adsorption mode.


**Table 1.** Cycle modes and valve positioning.

**4. Advanced adsorption cycles**

86 Sustainable Air Conditioning Systems

**4.1. Two-bed adsorption cycles**

the whole cycle.

adsorption mode.

In view of the fact that the basic adsorption cycle produces intermediated cooling output and its COP is low, many advanced adsorption refrigeration cycles have been proposed and developed to help in overcoming these main drawbacks such as the heat recovery cycle, mass recovery cycle, thermal wave cycle, cascade cycle and multi-stage cycle. However, the basic

The flow diagrams of a conventional two-bed cycle representing one complete cycle are illustrated in **Figure 9**. Each adsorber undergoes four operating modes: pre-heating, heating (desorption), pre-cooling and cooling (adsorption) processes in repeating cycles and according to the sequence shown in **Table 1**. The system consists of two adsorbent beds, a condenser, an evaporator and an expansion valve, in addition to the four connecting valves and connecting pipes. The working principle of the basic cycle is discussed in detail in Section 1.1 for one-bed ARS. In a two-bed ARS, while hot water is used to heat up Bed-A during the first two processes: cooling water is used to cool down Bed-B. The hot water is switched to Bed-B in the last two processes, as Bed-A is subjected to cooling water. The four valves are completely closed during the two switching modes. Circulating the cooling water and chilled water in the condenser and evaporator, respectively, are supposed to be continuous during

In a system of two or more adsorbers work between an evaporator and a condenser, there is a hot adsorber under cooling process and a cold one under heating process which offers the availability to recovery the heat inside the system. The experimental results show that the COP of the system will increase by up to 25% with the heat recovery cycle [62]. In a

**Figure 9.** Schematic diagram of a conventional two-bed adsorption chiller, as Bed-A in desorption mode, Bed-B in

cycle is mostly used in the solar powered adsorption system for its simplicity.

typical two-bed cycle equipped with mass recovery system, while one bed at the end of desorption mode at higher pressure and temperature, the other bed at the end of adsorption mode at lower pressure and temperature. Internal mass recovery process is started via connecting the high-pressure adsorber to the low-pressure one typically by means of a valve. The process ends when the two pressures become equal. The combined heat and mass recovery procedures may increase COP more than 10% [62], compared to heat recovery cycle. Thermal wave cycle is another way to recovery the heat inside the cycle. A typical thermal wave cycle is composed of two adsorbers, an evaporator, a condenser, a cooler and a heater as shown in **Figure 10** [63]. Experimental results showed that the COP of a twobed adsorption air conditioner (zeolite-water) with thermal wave cycle was approximately 1.0 in cooling season [12].

**Figure 10.** Schematic representation of the thermal wave adsorption heat pump during the first half of the cycle [63].

#### An adsorption chiller installed in the University hospital in Freiburg [64]

Working pair: silica gel-water

CC(kW): 70 and COP –

Backup: thermally driven by steam network of the hospital

Daily solar radiation: temperate, central European climate

Solar collector type: evacuated tube

Area: 170 (m2 )

Main features: solar system is working on assisting the main driving heat system and can deliver about 90% of the heating required in mid-day hours (4 hours)

**4.2. Other advanced cycles**

adsorption chillers.

**5. Conclusions**

**Acknowledgements**

**Nomenclatures**

There are many of advanced and novel cycles proposed in literature for ARSs. The advanced cycles such as multi-bed cycle, multi-stage cycle and dual-mode cycle are originally developed to make utilize of lower temperature heat sources applicable and more efficient. Another trend in advanced cycles is eliminating the vacuum valves by putting the adsorber, condenser and evaporator in a single chamber to increase the reliability of the system, particularly under the vacuum operating conditions. **Table 2** summarizes data about some applied or prototype

Adsorption Refrigeration Technologies http://dx.doi.org/10.5772/intechopen.73167 89

Adsorption refrigeration systems have a lot of advantages making them more and more competitive when compared to conventional vapor compression refrigeration systems. Apparently, the environmental regulations and local safety considerations, the expensive and limited petrol energy resources, solar driven possibility and the increasing of industrial waste heat availability are all in favor of thermally driven refrigeration systems, particularly adsorption systems. The adsorption refrigeration technology has not been carrying out in mass production level yet. That justifies its higher initial cost compared to the conventional technology. On the other side, there is a serious need to consider together all aspects of energy, exergy, environment and economy in the future comparative studies. Also, it should be noticed that the thermal COPs of ARSs are around 0.6 which is low. However, the electrical COP values of ARSs can reach up to 10 which is high compared to that values of conventional systems typically between 3 and 5. In order to find out new direction of adsorption refrigeration systems developments, previous

This work is funded by the Science and Technology Development Fund (STDF) program in Egypt under the UK-Newton Institutional Links Grants, project ID 26148, in collaboration

related researches are reviewed and classified in this chapter.

with University of Birmingham, Birmingham in UK.

ARSs adsorption refrigeration systems

COP coefficient of performance

HEX heat exchangers

Adsorption chiller installed in the University hospital in Freiburg

Two-bed adsorption chiller developed by SorTech [65]

Working pair: silica gel-water

CC (kW): 7.5 and COP: 0.55 (system)

Electricity consumption: 9 W.

Heat supply circuit: 72/67°C at 1.6 m<sup>3</sup> /h. Heat rejection circuit 27/32°C at 3.7 m<sup>3</sup> /h. Chilled water circuit 18/15°C at 2.0 m<sup>3</sup> /h.

Daily solar radiation: temperate, central European climate

Solar collector type: flat plate

Area: 3.5–4.5 (m2 /kW)

Main features: developed by SorTech AG, and it enables to paste the heat exchanger surface with silica gel pellets with the aid of epoxy resin without blocking the entrance pores of the pellets

The Fraunhofer ISE institute building and SorTech adsorption chiller in the technical room (sources: Fraunhofer ISE) [34]

Silica gel-water adsorption chiller developed in SJTU [66]

Working pair: silica gel-water

CC(kW): 15

COP: 0.35 (system) 0.15 (solar)

Regeneration temp. (o C): 85

Solar collector type and area: 90 m2 of U-type evacuated tube and 60 m2 of heat pipe evacuated tubular

Main features: the adsorber is a compact finned-tube heat exchanger, the condenser is a shell and tube heat exchanger, and the evaporator cooling is output through a methanol chamber, which acts as a gravity heat pipe. Power consumption for pumps was 1.87 kW

Photo of the adsorption chiller

**Table 2.** Summary of some adsorption prototypes.
