GREENHOUSE EFFECT AND ITS IMPLICATIONS

By: Tobi Nagy

What are the results of the Greenhouse Effect?

By using very sophisticated computer modeling, scientists have been able to predict what the world’s climate will be like when carbon dioxide levels have doubled.

1. Increase in world’s temperature It is thought that there will be an increase in the average global temperature by between 1.5 and 4.5 deg C . • By year 2030 an increase of 2 deg C, by 2100 an increase of 6 deg C. The warming will be greater at higher latitudes and in winter. This will lead to the melting of polar ice caps and glaciers which is already evident, in places like Greenland, the Arctic and Antarctic.


2. Changes in World’s climate The Greenhouse Effect will lead to world-wide changes in weather and climate. Some places may get more rain and storms while other places may get less. Not all changes will be bad. However, almost everywhere in the world will have changes in weather, which will have a big impact on our lives


3. Rising Sea Level It is estimated that by the year 2030, the average sea level will increase by approximately 20 centimetres. This will be due mainly to the melting of the polar ice cap, but also warming of the atmosphere will heat the upper layers of the oceans, which will expand when heated. For low lying countries in the Pacific like Tuvalu and Kiribati, and in the Indian Ocean like Maldives and other countries like Holland may altogether disappear.


4. Other impacts Other impacts could be the dieing out of some species of animals and plants, such as coastal marine environments and coral reefs. Some plants would not be able to survive temperature increases. It takes thousands of years for forests to move north or south to cooler climates. According to Joel B. Smith, co-author of an EPA report states that “such a warming over a century would require forests to move five times faster than the fastest rate recorded by paleontologists since the end of the last ice age”.

What are the primary Greenhouse gases?

They are a number of organic compounds which have more than two bonds (i.e 3 atoms). The seven major Greenhouse gases are:

1. Carbon Dioxide (CO2)


2. Ozone (O3)


3. Methane (CH4)


4. CFC’s (Freons F11 & F12)


5. Water vapour (H2O)


6. Nitrous Oxides (NOx)


7. Ethane (CH3CH3)

Freon

"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced, such as sulfur dioxide. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer refrigerants that have reduced ozone depletion effect include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine. However, CFCs, HCFCs, and HFCs all have large global warming potential.

Newer refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744.[4] These have similar efficiencies compared to existing CFC and HFC based compounds, and have many orders of magnitude lower global warming potential.

The thermodynamics of the vapor compression cycle can be analyzed on a temperature versus entropy diagram as depicted in Figure 2. At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (i.e., compressed at constant entropy) and exits the compressor as a superheated vapor.

From point 2 to point 3, the superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturated liquid. The condensation process occurs at essentially constant pressure.

Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (i.e., occurs at constant enthalpy).

Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporator operates at essentially constant pressure. The resulting saturated refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

It should be noted that the above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).