|
 |
| |
SYSTEMS
OF REFRIGERATION BY ABSORPTION -
METROGAS |
THE
COMPANY
On 29 December 1992, MetroGas took over gas distribution for 35 years—renewable
for another 10 years—within an area of 2,150 square kilometers,
including the capital city of Buenos Aires and the nine
districts comprising the southern part of Greater Buenos
Aires. This area has a population of seven million, which
represents 21% of Argentina's total population.
MetroGas serves approximately 1,850,000 customers, with an average annual demand
of 6,000 million cubic meters, representing 25% of Argentina's
total consumption. The company’s network of low-and high-pressure
pipelines is 13,000 kilometers long and has more than
900,000 utility service lines.
THE
PROJECT
Abiding by the principles of ecoefficiency, which produce financial benefits
and link businesses together in their work for environmental
preservation, MetroGas has promoted the use of natural
gas as an energy source for air conditioning in buildings.
As
a part of this effort, the company decided to install
a 240-ton-capacity refrigeration system at its central
headquarters. MetroGas studied the options available on
the market and chose absorption air conditioning, which
uses lithium bromide (LiBr) as an absorbent, with water
as a refigerant. This technology decreases environmental
impact by complying with the guidelines for reducing the
use of refrigerant gases, which contribute to the greenhouse
effect.
THE
COOLING PROCESS
The Traditional Process
In the traditional cooling
process, a liquid circulates through a coil to absorb
the heat that is present. If the liquid is within a vacuum
packed system, its temperature drops due to the fact that
the liquid gives up its energy content, thereby lowering
the temperature of the medium. This process involves the
input of a certain amount of heat, which is consumed as
the liquid turns into vapor at a constant temperature
and pressure. When the pressure drops, evaporation temperature
decreases approximately 4°C at a pressure of 0.01 atm.
The process during which
a liquid expands and becomes vapor can occur in several
different ways. The most common involves the compression
of refrigerant compounds, which may be halogenated hydrocarbons
commonly known as Freons, at more than ten times atmospheric
pressure. When this occurs, the refrigerant heats up and
produces vapor, which then becomes liquid because the
ambient temperature cools it, thus eliminating the heat
caused by the compression effort from the medium. The
liquid refrigerant expands within a closed system, which
is in contact with the space to be cooled, and begins
the refrigeration cycle again.
The Ecoefficient Process
In the case of absorption
with LiBr, water is the substance that is compressed and
expanded. At ambient temperature, the concentrated LiBr
solution requires water to remain stable, and it absorbs
the vapor as though it were the compressor in the traditional
process. The following illustration shows the absorption
stage of the process:
The concentrated 65% LiBr
solution is mixed with water vapor and becomes diluted
to 60%. This dilution-absorption action causes a vacuum
in the vapor area and an increase in the volume of liquid.
The diluted solution is sent via pump to a generator,
where it is heated to concentrate the LiBr salt and begin
the process again. The energy required for the concentration
of salt and evaporation of water is equivalent to the
energy used to compress refrigerant vapor in the traditional
system.
To reach its liquid state
again, the water vapor must lose part of its non-useable
energy. This reaction occurs in a condenser, through which
water is circulated at 29.5°C in order to regenerate the
evaporation cycle. In this cycle, water is evaporated
by the vacuum effect generated by the absorber. Air cooled
to 15°C is extracted from a water loop at a temperature
of 7°C to service the building that is being air conditioned.
ECOEFFICIENT STRATEGY
When air conditioning its
headquarters, MetroGas ruled out the use of refrigerants
that damage the ozone layer and contribute to global warming.
The company adopted the LiBr absorption technology, due
to the fact that it works with natural refrigerants such
as water and a salt (LiBr), which, when vacuum-packed
within the equipment, do not present any hazards to the
environment.
If MetroGas had installed
a traditional air conditioning system, the company would
have had to use approximately 240 kilograms of environmentally
unfriendly refrigerants, such as chlorinated and/or fluoridated
(halogenated) products. They also would have run the risk
of refrigerant leakage through seals and/or total leakage,
if part of the system were damaged or if the equipment
were taken out of service.
RESULTS
As can be seen in the following
comparative table for both processes, for the same number
of hours per year of cooling equipment use at full load,
the alternative of a LiBr absorption air conditioning
system provides an operating savings of approximately
U.S.$14,700. Although the cost of these absorption systems
is higher than for compression systems with the same capacity,
this difference is offset within a short time, due to
lower operating and maintenance costs.
Comparison of Cooling
Processes
|
Variable
|
Traditional Process
|
LiBr Absorption Process
|
|
Electric Power (kW)
|
224
|
45
|
|
Hours of Operation
(h/year)
|
1,000
|
1,000
|
|
Electric Power
Consumption (kWh/year)
|
224,000
|
45,000
|
|
Natural Gas Consumption
(nm3/year)
|
0
|
76,800
|
|
Cost of Electric
Power (U.S.$/year)
|
30,400
|
6,100
|
|
Cost of Natural
Gas (U.S.$/year)
|
0
|
9,600
|
|
Total
Cost (U.S.$/year)
|
30,400
|
15,700
|
|
Operating Savings
(U.S.$/year)
|
-
|
14,700
|
For an air conditioning system
providing 240 tons of refrigeration, traditional equipment
uses approximately 224 kW of power, whereas absorption
units use 45 kW. As a result, not only are electrical
system requirements minimized, but so are the hazards
involved in handling high current loads, the heat losses
due to the Joule effect, and the reactive power that must
be compensated for. Absorption equipment requires less
power from an alternative source in order to operate without
a utility network.
The lower energy use shown
by absorption equipment can also be observed in the comparative
table. Its performance coefficient (PC), which
relates energy consumed to energy actually taken advantage
of, is close to 1. On the other hand, compression equipment
units driven by electricity and using halogenated refrigerants
have a PC of around 4.5. It must be realized that in a
traditional air conditioning system, 75% of fuel energy
is wasted when it is transformed into electricity, whereas
in the absorption system, only 10% of the energy from
the natural gas being used is wasted.
BENEFITS
OF THE PROJECT
Following are some of the
project’s benefits:
-
Use of refrigerants unfriendly
to the environment is avoided.
-
LiBr
can be marketed after it has been in the process, because
it is an easily recovered stable salt with good resale
value.
-
Savings
are achieved on a long-term basis, according to cost
recovery time, due to the fact that the absorption method
is cheaper than the traditional method.
-
Lower
operating costs are assured.
-
Dependence
on the absorption units of electric power networks is
circumvented, given their low power consumption.
-
Proper
system functioning is assured during the summer, because
this equipment compensates for energy demand by using
it when the maximum flow of gas is available.
-
Increased
durability and less maintenance are assured with this
equipment because of its proper design.
-
Noise
is eliminated, and therefore even more money is saved
because sound insulation is unnecessary.
-
Complete,
optimum use is made of this equipment because it serves
both heating and cooling purposes.
CONCLUSIONS
The
absorption system has been in use for more than forty
years in Argentina. From the experience that has been
gained, it has been shown that the useful life of equipment
of this kind is longer, which more than compensates for
whatever cost difference there may be when compared to
that of traditional air conditioning equipment. When its
lower energy requirements and the proven advantages of
greater friendliness to the environment are added to the
abovementioned factors, it can be predicted that systems
of this kind will eventually replace others currently
in use.
CONTACT
Fernando
Iuliano
MetroGas
Magallanes
1491, PB
1267
Buenos Aires, Argentina
Tel.
(54-11) 4309 1685
Fax
(54-11) 4309 1680
|
|
