Environmental Engineering Reference
In-Depth Information
to compress the working fluid, an absorption system utilizes heat. Assuming waste heat can be
recovered from a boiler or another source, in the net balance, the absorption system would
consume less energy than a conventional system. In addition, the working fluids for absorption
systems (ammonia/water and water/lithium bromide) have no GWP. The drawback of
absorption systems is their low efficiency. So if fossil fuels need to be burned exclusively to
run the cycle then there is no environmental advantage.
In refrigerated transport, refrigeration units are subjected to extreme operating conditions,
making it more difficult to improve them in terms of less energy consumption. When
transporting refrigerated or frozen products, the main goal of maintaining safety and quality
is achieved at the expense of higher energy use. One action that reduces energy consumption,
and at the same time, improves the quality of the food product is the use of modulated
refrigeration. Instead of having an on/off type of control, modulated refrigeration systems
adjust the level of refrigeration power to maintain the temperature at almost a constant level.
A subject that has no technical substitute at the moment is the use of hydrofluorocarbons
as working fluids for mobile refrigeration equipment. Hydrofluorocarbons are considered to
have a high GWP in relation to carbon dioxide, but there are no alternative gases for applications
in mobile refrigeration, and it is likely that their use will continue for a long time. Therefore,
the only option to reduce emissions of hydrofluorocarbons is to take measures to prevent the
intentional or unintentional venting of these gases to the atmosphere. Some actions that help
to prevent the release of hydrofluorocarbons are reducing the number of connections in the
circuit where the refrigerant flows, recover and recycle refrigerants, use tight leak tested
systems, and periodical leak monitoring (Alliance for Responsible Atmospheric Policy
[ARAP], n.d.).
Direct emissions from stationary refrigeration systems
Since the beginning of industrial refrigeration, ammonia (R-717) has been a favorite refrigerant
because of its low cost, high efficiency, smaller vapor lines requirement, water contamination
tolerance, and good heat-transfer coefficients (Stoecker, 1998). In contrast, ammonia is toxic,
flammable, and corrosive to some metals. Because of these negative qualities, ammonia was
substituted by chlorofluorocarbons first and then by hydrofluorocarbons in many small- and
medium-size applications. Large food industries, however, kept using ammonia systems.
After it was found that hydrofluorocarbons have high GWPs when compared to carbon
dioxide, ammonia became the star again. Of course, ammonia emissions are a concern for the
reasons mentioned at the beginning of this chapter; however, the fact that ammonia has zero
GWP and does not deplete stratospheric ozone makes it a good environmentally friendly alter-
native to chlorofluorocarbons and hydrofluorocarbons.
Unfortunately, ammonia cannot be used in all refrigeration systems because of its toxicity
(e.g., urban distribution centers, retailers, or confined spaces). Refrigeration experts started
looking at another old refrigerant that was used in early refrigeration systems and then
practically forgotten: carbon dioxide, or R-744. Carbon dioxide has the main disadvantage of
requiring high working pressures and not condensing at room temperature. On the other hand,
carbon dioxide is a natural compound with around a thousand times less GWPs than
hydrofluorocarbons.
Carbon dioxide as a refrigerant
Carbon dioxide can be used as a refrigerant in two kinds of cycles: subcritical and transcritical.
When working in the subcritical range of pressures, carbon dioxide cannot be liquefied after
compression by just using an air condensing unit like most systems using hydrofluorocarbons
do (Fig. 8.4). Instead the condensing unit of the carbon dioxide cycle needs to be coupled with
