REFRIGERATION AND AIR CONDITIONING: TECHNOLOGICAL TRENDS

François Billiard - Director of the International Institute of Refrigeration - http://www.iifiir.org

from IX European Conference on new technologies in Refrigeration and in Air Conditioning, Politecnico of Milano - Organized by Centro Studi Galileo

INTRODUCTION
Ninety-five percent of currently used refrigeration, air-conditioning and
heat-pump systems are based on the vapour compression cycle. Over the last
ten years refrigerant use, with the exception of ammonia use has radically
changed. The driving force providing impetus for these changes is primarily
regulatory measures on environmental issues: ozone depletion and global
warming. Technological trends are analyzed at the level of components,
systems architecture and refrigerants. Non-vapour compression cycle systems
are briefly analyzed.

1 VAPOUR-COMPRESSION CYCLE SYSTEMS

1.1 Components
New trends in component technology are based on:
-specific properties of new refrigerants (the high pressure of R407C, R410A
and R744 for instance);
-low fluid charge design;
-better component reliability;
-new lubricating oils with new refrigerants;
-low leakage level;
-high energy efficiency;
-electronic management of refrigerating systems using remote control.

1.1.1 Compressors:
-rapid development of screw and scroll technology;
-variable volume and speed;
-thicker compressor walls;
-aluminium coils for electrical engines.

1.1.2 Exchangers:
-plate exchangers (to reduce the thermodynamic difference);
-aluminium exchangers;
-microchannel exchangers;
-plastic exchangers;
-exchangers optimized for zeotropic mixtures with large glides;
-floating condensation for condensers.

1.1.3 Expansion valves
-electronic valves.

1.2 Architecture
The main trends in systems architecture are:

1.2.1 Secondary heat transfer fluid systems
Already common in fruit packing stations, abattoirs and liquid chillers,
these systems are now common in commercial refrigeration even if investment
costs are 10 to 20% higher and operating costs also generally higher because
of additional energy consumption. These systems enable the amount of
refrigerant to be reduced considerably and also make it possible to use
refrigerants such as ammonia and hydrocarbons, even if such use is not yet
common.
There are various types of heat transfer fluids:
-brines are of value in low-temperature applications (low viscosity);
-two-phase heat transfer fluids, these being either solid-liquid (ice
slurries) or liquid-vapour (carbon dioxide);
-widely used heat transfer fluids based on glycol.
Distributed systems are used in commercial refrigeration: several small
systems are located in several parts of the premises; accordingly, the
refrigerant charge is greatly reduced.
Cascade systems enable CO2 to be used as a refrigerant in the lower stage,
for instance.

1.3 Refrigerants
The best way to deal with refrigerants is to adopt a per application
approach. Here are some very rapid and wide-ranging comments on new trends
in refrigerant use.

1.3.1 Domestic refrigeration
After a long debate between defenders and opponents of hydrocarbons, a
certain balance between halocarbons and hydrocarbons has now been achieved.
HC600a is now well accepted for very low refrigerant charge in hermetic
systems. HC domestic refrigerators make up 35% of European output. Ten
percent domestic refrigerators worldwide are presumably hydrocarbon
refrigerators. We have not heard about accidents even that today there are
about 80 million hydrocarbon refrigerators in use worldwide.
However, HFC134a is still the most common refrigerant in this application.

1.3.2 Commercial refrigeration
HFC404A and HFC507 are the most common refrigerants. The favourable
properties of zero flammability displayed by these HFC mixtures make them
popular alternatives. The same refrigerant is generally used for medium- and
low-temperature applications for simplification purposes. The drawback of
these 2 refrigerants is certainly their high GWPs (3800).
Some supermarkets are using ammonia or hydrocarbons in secondary refrigerant
systems. There are over 50 supermarkets using ammonia and 10 using
hydrocarbons in Europe (UNEP, 1998).
Self-contained display cabinets generally use HFC134a as a working fluid for
medium-temperature applications; however hydrocarbon cabinets are developing
in certain countries (the UK).

1.3.3 Industrial refrigeration
Ten years ago, ammonia, R502 and HCFC22 were the 3 common working fluids in
this sub-sector, with hydrocarbons being used in the chemical industry. The
phase-out of CFCs and HCFCs means that ammonia has now become the most
popular refrigerant in this field. HFC404A is sometimes utilized in flooded
systems but it is not widely used because of its zeotropic properties, even
if its glide is small.

1.3.4 Unitary air conditioning
Broadly speaking, there are 3 main application fields:
Low capacity: HFC410A is developing rapidly, probably because of its
favourable energy efficiency properties. Hydrocarbons are used in some
countries for low-capacity air conditioners such as portable air
conditioners.
Medium capacity: HFC407C is widely used.
Large capacity up to 100 kW: HFC134a is commonly used.

1.3.5 Water chillers
HFC134a is used for flooded chillers because it is a pure refrigerant. It is
also highly suitable for centrifugal compressors.
Ammonia and HFC407C are also utilized for lower capacity chillers (not for
centrifugal applications).

1.3.6 Transport
In road transport the most commonly used refrigerant is HFC404A. As many
vehicles now belong to Class C of the ATP regulations for both chilled and
frozen foods, HFC134a is no longer well suited for low-temperature
applications because of the energy efficiency penalty at low temperatures.
HFC134a is suitable for transport of chilled foodstuffs only. HFC410A is
also used.

1.3.7 Mobile air conditioning
Since 1994, the most widely used refrigerant in this application is HFC134a.
Two others technologies are progressing well: CO2 as a refrigerant in
transcritical cycles and hydrocarbons with a secondary circuit. The system
with the best efficiency will have a leading edge in the future.
Even if this practice is not always permitted, hydrocarbons seem to be
largely used in retrofitting applications: 10 million cars per year in the
US and 500 000 in Australia according to WorkCover NSW (2000).

2 NON-VAPOUR-COMPRESSION CYLE TECHNOLOGY

Generally speaking, not-in-kind technologies are said to be less favourable
from an energy-efficiency viewpoint. However steady progress is being made
and the gap between vapour compression and alternative technology is
narrowing. There are also niches in which these alternative technologies are
more favourable. In addition, energy efficiency is not the only factor to
consider when selecting a technology.

2.1 Absorption
There are typical applications of absorption for chillers when heat in the
form of hot water or steam is available. This is also the case for
cogeneration systems where waste engine heat or steam is available. In
countries where natural gas price structures are particularly favourable
compared to electricity price structures, direct-fired, single-stage
absorption chillers are common (UNEP, 1998). Shortages of electricity,
deregulation and the high price of electricity during certain periods of the
day favour the choice of absorption. Absorption systems operate quietly and
in the lithium-bromide/water refrigeration cycle the refrigerant is water
which is of value for those who don’t want to have to worry about future
regulatory developments on greenhouse gases.
Absorption chillers appear to be much more popular in Asia thanks to policy
in favour of gas usage for large air-conditioning systems. For instance, in
Japan, 6667 absorption chiller units were sold in 1995, representing 24
times the number of centrifugal compressors. In Korea, the sales level of
1300 absorption chillers represents 3 times the number of centrifugals.
For lower capacity applications, current research is focused on developing
cost-effective double-effect and triple-effect systems. Some authors
consider that such systems could have a lower TEWI than vapour compression
cycles (Fisher, 1997).
In France gas-air-conditioned surface area are steadily increasing: 1% of
total air-conditioned surfaces in 1998, compared with 8% in 2000 with a
total surface area of 550 000 m2 (Grumel, 2001).

2.2 Desiccant cooling
New desiccant materials such as titanium silicate have much higher
affinities for water than previously used desiccants such as silica gel. The
higher efficiencies of these newer desiccants make it possible to design
practicable air conditioning systems (UNEP, 1998).
According to Wurm (1999), in the United States, annual production of
desiccant systems and components in 1997 was worth nearly 100 million USD
representing about 1% of all U.S. HVAC & R market shipments and is expected
to more than triple by the year 2003.

2.3 Air-cycle systems
Air cycle refrigeration is an attractive process since air is free, safe and
harmless to the environment. Today, applications are not numerous due to the
low energy efficiency of air-cycle systems.
Research is steadily developing in this field. An air-cycle group is
operated by the University of Bristol, UK (FRPERC) and workshops are held on
a regular basis.
Among existing applications:
? air-cycle systems are the basis of most aircraft cabin cooling systems;
? Normalair Garrett (Honeywell) has developed an air-cycle air-conditioning
pack for German ICE-2 high-speed trains;
? a demonstration unit for heating and cooling of buildings has been
designed and built by Bristol University and BRE (DTI, 2000).

2.4 Thermoelectric cooling
The refrigerating capacity of equipment using the Peltier effect is rather
low, but progress is being achieved. For instance, a Swedish firm is
manufacturing cooling trays for drinks and sandwiches, the capacity of which
is 300 W. Ten years ago, the maximum capacity achieved was only 150 W. The
temperature difference between the cold and the warm points is reported to
be 74°C.

2.5 Other systems
There are many other non-compression vapour systems on which significant
research is being performed; these include Stirling and Vuilleumier cycle
equipment and adsorption systems.

CONCLUSION

Profound changes in refrigeration, air-conditioning and heat-pump systems
are taking place in order to comply with new environmental regulations.
Changes involve components, architecture and refrigerants for
vapour-compresion cycles. Progress is also being achieved at the level of
non-vapour-compression cycle refrigeration, even if developments are still
taking place very slowly.

REFERENCES

1. DTI Sector Challenge Air cycle Technologies Newsletter, Issue 2, March
2000, 4 p.
2. Fisher S. K., Sand J.R., Baxter V. D., 1997,Energy and Global Warming
Impacts of HFC Refrigerants and Emerging Technologies, Sponsored by AFEAS
and DOE.
3. Grumel N., 2001, Gaz de France lance Climo’gaz, Clim Pratique, Avril
2001, p. 9.
4. IEA/HPP Workshop Proceedings, 1999, Ab-sorption Machines for Heating &
Cooling in Future Energy Systems, 154 p.
5. UNEP, 1998, Report of the Refrigeration, Air Conditioning and Heat Pumps
Technical Options Committee, 285 p.
6. WorkCover NSW Assessment Report of Clause 242 of the Dangerous Goods
(General) Regulation 1999, 2000, 93 p.
7. Wurm J., Kosar D., Czachorski, 1999, Quo Vaditus desiccants?, Proceedings
20th IIR Int. Cong. of Refrigeration, Sydney, Australia.