Transportation vehicles, the horse of the modern societies, are one of the biggest energy consumers. The world fleet of automobiles is about 1 billion,1 projected to double by 2035, and it consumes hundreds of billions of fuel gallons per year. In this context, transportation accounts for about 30% of all energy consumption worldwide,2 and more than 90% of this energy comes from fossil fuels.
The high dependence of vehicles on fossil fuels has produced serious negative impacts on the environment. Also the extended use of their air-conditioning (A/C) systems, due to climate changes, has led to a significant increase in the energy consumption. In fact, the United States alone consumes approximately 7.1 billion gallons of gasoline each year for vehicles (A/C) systems.3 This remarkable increase in consumption has placed vehicle (A/C) as the second largest consumer of the fossil energy after the vehicle propulsion. Furthermore, the automotive consumption of energy and their environmental impact will increase more in the near future as about 3 billion vehicles are predicted to be used. Based on the previous considerations, significant research efforts are devoted during the last 50 years to improve the automotive vapor-compression systems. In this article, the developments related to the vapor-compression (A/C) components and refrigerant (classification, evolution improvement, comparison) are reviewed and the future trends are presented. Also the different modeling approaches for these cooling systems are discussed. Furthermore, the development related to A/C test procedures is outlined.
Typical vapor-compression A/C system for automobiles
REFRIGERATION AND AIR CONDITIONING
Before 1830, few Am ericans used ice to refrigerate foods due to a lack of ice-storehouses and iceboxes. As these two things became more widely available, individuals used axes and saws to harvestt ice for their storehouses. This method proved to be difficult, dangerous, and certainly did not resemble anything that could be du plicated on a commercial scale.
Despite the difficulti es of harvesting ice, Frederic Tudor thought that he could capitalize on this new commo dity by harvesting ice in New England and shipping it to the Caribbean islands as well as the southern states. In the beginning, Tudor lost thousands of dollars, but eventually turned a profit as he constructed icehouses in Charle ston, Virginia and in the Cuban port town of Havana. These icehouses as well as better insulated ships helped reduce ice wastage from 66% to 8%. This efficiency gain influen ced Tudor to expand his ice market to othe r towns with icehouses such as New Orleans a nd Savannah.
This ice market further expan ded as harvesting ice became faster and cheap er after one of Tudor’s suppliers, Nathaniel Wyeth, invented a horse-drawn ice cutter in 1825. This invention as well as Tudor’s s uccess inspired others to get involved in the ice trade and the ice industry grew.
Ice became a mass-m arket commodity by the early 1830s with the price of ice dropping from six cents per pound to a half of a cent per pound. In New York City, ice consumption increased from 12,000 tons in 1843 to 100,000 tons in 1 856. Boston’s consumption leapt from 6,00 0 tons to 85,000 tons during that same period. Ice harvesting created a “cooling culture” as majority of people used ice and iceboxes to store their dairy products, fish, meat, and ev en fruits and vegetables. These early cold storage practices paved the way for many Ame ricans to accept the refrigeration technology that would soon take over the country.
CONCEPT OF REFRIGERATION
Refrigeration is a pro cess in which work is done to move heat from one location toanother. The work of heat trannsport is traditionally driven by mechanical work, but can also be driven by heat, magnetism , electricity, laser, or other means.
How does it work?
Thermal energy moves from left to right through five loops of heat trans fer:
1) Indoor air loop
2) Chilled water loop
3) Refrigerant loop
4) Condenser water loop
5) Cooling water loop
Refrigeration has had a large importance on industry, lifestyle, a griculture and settlement patterns. The idea of preserving food dates back to the ancien t Roman and Chinese empires. However, refrigeration technology has rapidly evolved in the last century, from ice harvesting to temperature-controlled rail cars. In order to avoid food spoilage, refrigeration plays an important role in day to day life, similarly, Air conditioning is also an important technological system to prevent the human from the hot atmosphere during summer seasons.
CLASSIFICATION OF REFRIGERATION SYSTEM
Types of Refrigeratio n
• Vapour Compression Refrigeration (VCR): uses mechanical energy
• Vapour Absorption Re frigeration (VAR): uses thermal energy
Vapour Compression Refrigeration
• Highly compressed flu ids tend to get colder when allowed to expand
• If pressure high enough
• Compressed air hotter than source of cooling
• Expanded gas cooler than desired cold temperature
• Lot of heat can be rem oved (lot of thermal energy to change liquid to vapour)
• Heat transfer rate re mains high (temperature of working fluid mu ch lower than
what is being cooled)
Vapour Compression Refrig eration Cycle
Low pressure liquid re frigerant in evaporator absorbs heat and changes to a gas
The superheated vapo ur enters the compressor where its pressure is raised
The high pressure sup erheated gas is cooled in several stages in the co ndenser
Liquid passes through expansion device, which reduces its pressure an d controls the flow into the evaporator
Type of refrigerant
• Refrigerant determine d by the required cooling temperature
• Chlorinated fluorocarb ons (CFCs) or freons: R-11, R-12, R-21, R-22 and R-502
Choice of compressor, design of condenser, evaporator determined by
• Required cooling
• Ease of maintenance
• Physical space requirements
• Availability of utilities (water, power)
Vapour Absorption Refrigeration
High pressure generator
• Air in contact with water to cool it close to ‘wet bulb temperature’
• Advantage: efficient cooling at low cost
• Disadvantage: air is ri ch in moisture
COMPARISON BETWEEN VAPOR COMPRESSION AND A BSORPTION SYSTEM
Assessment of Refrigeration
• Cooling effect: Tons of Refrigeration
1TR = 3024 kCal /hr heat rejected
• TR is assessed as:
TR = Q x×Cp x× ( Ti – To) / 3024
Q = mass flow rate of coolant in kg/hr
Cp = is coolant specific heat in kCal /kg °C
Ti = inlet, temperature of coolant to evaporator (chiller) in 0°C To = outlet temperature of coolant from evaporator (chiller) in 0°C
Specific Power Consumption (kW/TR)
• Indicator of refrigeration system’s performance
• kW/TR of centralized chilled water system is sum of
• Compressor kW/TR
• Chilled water pump kW/TR
• Condenser water pump kW/TR
• Cooling tower fan kW/TR
Coefficient of Performance (COP)
• The performance of refrigerators and heat pumps is expressed in terms of
coefficient of performance (COP), defined as
• Airflow Q (m3/s) at Fan Coil Units (FCU) or Air Handling Units (AHU): anemometer
• Air density r(kg/m3)
• Dry bulb and wet bulb temperature: psychrometer
• Enthalpy (kCal/kg) of inlet air (hin) and outlet air (Hout): psychrometric charts
APPLICATIONS OF REFRIGERATRION
Ø Petrochemical plants
Ø Transporting temperature-sensitive foodstuffs