Air conditioning may seem like an aspect of the build environment that is inextricably bound to the increased carbon intensity of our building stock due to energy input requirements and the utilization of refrigerants that are, in aggregate, an enormous contributor to increasing global carbon emissions.
Yet, there is one type of refrigeration cycle that has the potential to provide clean, low-carbon air conditioning: absorption refrigeration.
Three principles of absorption refrigeration
The absorption refrigeration cycle depends on the same two phenomena as the vapor-compression refrigeration cycle:
1. A large quantity of thermal energy (via heat of vaporization) must be added to change a liquid into a gas; however, the same amount of thermal energy (via heat of condensation) is released when the gas condenses back into a liquid.
2. The boiling/condensation point temperature of any material varies with pressure. When pressure is reduced, the boiling/condensation point temperature is also reduced.
Absorption refrigeration also depends on a third principle:
3. Some liquids (for example, lithium bromide or ammonia) have a strong tendency to absorb water vapor. Once saturated with water, these absorbers can be regenerated by heating the evaporate the water.
Absorption refrigeration uses a low GWP refrigerant
In the broader spectrum of common refrigerants, the absorbent solutions utilized by absorption refrigeration units have an extremely small global warming potential (GWP). For instance, absorption refrigeration units may utilize ammonia (i.e., R-717), which has a GWP of 1. Compare this to the HFC refrigerant R-134a, which is often utilized in vapor-compression units and has a GWP of 1,430.
Absorption refrigeration requires a lot of energy input - so make it clean energy
Unlike the vapor-compression cycle, the absorption refrigeration cycle does not require a compressor pump. However, the cycle does require a major source of heat to vaporize the absorber solution in the Generator (see figure). This source of heat is critical from the standpoint of carbon intensity and energy costs. This heat input may be provided by a gas flame, waste industrial heat, solar energy, or otherwise. In the interest of environmental responsibility and minimum carbon intensity, a clean energy source should be utilized.
How does the absorption refrigeration cycle work?
In the interest of clarity and at the risk of oversimplification, a very basic diagram of the absorption refrigeration cycle is offered in the accompanying figure. The cycle functions as follows:
Absorber: Consider the Absorber and Evaporator in concert with one another. These are two connected, closed tanks with an absorbent solution (e.g., ammonia or lithium bromide) in the Absorber and water in the Evaporator. Just as common table salt absorbs water, this solution in the Absorber draws cold, low-pressure water vapor from the Evaporator.
Evaporator: The refrigeration effect is utilized by putting a coil in the evaporator chamber. Water from the Condenser is pumped into the Evaporator to spray headers which wets the coil. As water evaporates it draws heat from the incoming coil, which cools the liquid in the coil, creating a chilled fluid output.
Generator: The solution in the Absorber is continuously becoming diluted by taking on water vapor. To keep the solution at the proper concentration, it is pumped to the Generator where excess water vapor is boiled off. This boiling off requires considerable heat input (which, again, would ideally come from a clean energy source such as solar). The concentrated solution is pumped back to the Absorber tank, where it mixes with the solution.
Condenser: Hot, high-pressure water vapor boiled off from the absorbent solution is condensed back into liquid in the Condenser. A coil in the Condenser induces heat exchanger by which the fluid in the coil takes on thermal energy and the vapor in the tank condenses back into a liquid, which can then be released into the Evaporator to facilitate heat exchange and vaporization - completing the cycle.
Absorption refrigeration has immense potential
By utilizing a refrigerant with almost no global warming potential and powered by a clean energy source, absorption refrigeration has the potential to offer buildings a clean, low-carbon refrigeration solution.
Moore, Fuller, 1993, Environmental Control Systems: Heating Cooling Lighting. McGraw-Hill, Inc., p. 225.
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