EAI presents Cleantech Snapshots: a quick summary of some of the most interesting and innovative areas in clean technology that will drive the sustainability movement in future.

This snapshot focuses on Vapour Absorption Cooling.. Within this page you will find

• An overview of vapour absorption cooling
• It’s Working
• Characteristics
• Advantages
• Cost
• Applications
• Disadvantages

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Overview

• VAM, referring to vapor absorption machine, is a cooling/refrigerating technology that uses typically waste heat for air-conditioning or refrigeration
• The heat evaporates a low boiling point refrigerant, which produces the cooling effect. The evaporated refrigerant is absorbed in a liquid to be converted back into liquid form
• With cogeneration and CHP becoming important aspects of the global industrial energy strategies, VAM is fast becoming a feature in most industrial setups
• Unlike vapour compression systems, which use electrically-driven compressors, absorption cooling technologies use a source of heat to produce cold
• This characteristic makes sorption cooling machines a very useful sink of waste heat or solar energy. The term sorption cooling technologies encompasses both absorption and adsorption cooling

Vapour absorption cooling cycle (Image source)

 

How it works

• Absorption system employs heat and a concentrated salt solution (lithium bromide) to produce chilled water. In its simplest design the absorption machine consists of 4 basic components:
  1. Generator
  2. Condenser
  3. Evaporator
  4. Absorber
• Just like the vapor-compression refrigeration cycle, the absorption machine operates under two pressures – one corresponding to the condenser-generator and the other corresponding to evaporator-absorber
• The pressure on the high-pressure side of the system (condenser) is approximately ten times greater than that on the low-pressure side to allow the refrigerant to reject heat to water at normally available temperatures
• Typically the condensation of water in the condenser-generator takes place at a pressure of 75 mmHg and temperature of about 45°C

Characteristics

• The absorption systems use heat energy in form of steam, direct fuel firing or waste heat to achieve the refrigerant effect
• It uses a liquid pump to create the pressure rise between evaporator and condenser
• Pumping a liquid is much easier and cheaper than compressing a gas, so the system takes less work input
• The absorption cycle uses different refrigerants that have no associated environment hazard, ozone depletion or global warming potential (for example lithium bromide absorption system use distilled water as the refrigerant). The vapor compression refrigeration cycle generally uses a halocarbon (such as HCFC-123, HCFC-22, HFC-134a, etc) as the refrigerant
• Absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat
• An absorption chiller is the equivalent of a heat engine – absorbing heat at a high temperature, rejecting heat at a lower temperature, producing work – driving a heat pump

Advantages

• Moving part is pump
• Reduced vaporization pressure & temperature
• Load variation does not affect performance

Cost

• Depending on the unit size, the capital cost of single-effect absorption chillers is roughly 20-50% higher compared to an equivalent electric or engine-driven chiller
• The cost will be still higher for direct fired units and for double- and triple-effect units

Applications

• For facilities that use lot of thermal energy for their processes, a large chunk of heat is usually discarded to the surrounding as waste. This waste heat can be converted to useful refrigeration by using a VAM
• For facilities that have a simultaneous need for heat and power, absorption chillers can utilize the thermal energy to produce chilled water
• Absorption chillers minimize or flatten the sharp spikes in a building’s electric load profile, and can be used as part of a peak shaving strategy
• They uses very little electricity compared to an electric motor driven compression cycle chiller

Disadvantages

• Efficiency is low
• Takes long time to cool the application