ASHRAE Journal - September 2011 - 20

Ln(P) Pc

Heating and Pressurization TH

High Pressure Desorber Open

Low Pressure Adsorber TM

Refrigerant Vapor

PE

Figure 3: Diagram of mass recovery cycle.
Cold Adsorber –1/T Temperature B Energy Supplied by The Heat Source Heat Source Adsorber 1 A Condenser C Adsorber 2 Heat Sink D Energy Released To the Adsorber

Ln(P) Pc Cooling and Depressurization PE

Hot Adsorber

–1/T

Figure 2: Pressure-temperature diagram of heat recovery cycle.
Evaporator

is connected to the low-pressure adsorber in a closed loop. The refrigerant in the high-pressure adsorber will be re-adsorbed by the adsorbent in the low-pressure adsorber due to the pressure difference between the two adsorbers. In a mass recovery process, the adsorption quantity of adsorbent is increased, which causes the cooling capacity and COP to increase. The experimental results showed the mass recovery cycle may help obtain a COP increase of more than 10%.21 The concept of thermal wave cycle, proposed by Shelton, et al.,23,24 is shown in Figure 4.25 The heat transfer fluid circulates through four components: (1) Adsorber 1 in adsorption phase, (2) the heat source; (3) Adsorber 2 in desorption phase, and (4) heat sink. The adsorption heat released from Adsorber 1 is recovered by the heat transfer fluid and transferred to Adsorber 2, and only limited thermal energy is required from the heat source since about 65% of the total energy received by each adsorber can be internally recovered.26 Experimental results showed the COP of a two-bed adsorption air conditioner (zeolite−water) with thermal wave cycle was approximately 1.0 in cooling season.27 Critoph28 invented and theoretically investigated the convection thermal wave cycle, which uses refrigerant as a heat transfer medium for internal heat recovery. The simulation results predicted a COP of 0.95 for this system when the evaporating temperature and condensing temperature are 0°C and 42°C (32°F and 107.6°F), respectively. Table 125 summarizes the performance of some typical adsorption refrigeration systems that were manufactured and tested in the last 20 years for use of waste heat and solar energy. These results were obtained under various operating conditions; hence they should not be compared to one another.
20 ASHRAE Journal

Reversible Pump

Figure 4: Thermal wave adsorption cycle. However, they could be used as a reference to what can be expected from adsorption refrigeration systems.

Summary
Compared to the vapor compression refrigeration systems, adsorption systems have the following advantages: 1) they can be driven by waste heat and low-grade heat such as solar energy; 2) they use environmentally friendly fluids such as water or ammonia as refrigerants. The major drawbacks of adsorption systems are their low energy efficiency (low COP and SCP). Silica gel−water and activated carbon−methanol are suitable working pairs for low temperature waste heat and solar energy due to their relatively low desorption temperatures. Zeolite− water, activated carbon−ammonia, and metal chlorides−ammonia, as well as composite adsorbents−ammonia can be used in adsorption systems driven by high temperature waste heat. Since the typical pressures in silica gel−water, zeolite−water, and activated carbon−methanol systems are subatmospheric, a hermetically sealed outer vessel is essential to maintain good machine performance. 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 the desorption phase). Several advanced adsorption cycles (such as heat recovery cycle, mass recovashrae.org September 2011

Performance of Adsorption Systems
ASHRAE Journal - September 2011

Table of Contents for the Digital Edition of ASHRAE Journal - September 2011
