design of solar absorption cooling system

1. Design of a Solar driven Absorption chiller for
School of Energy - CFD laboratory in PSG College of
Technology
Anirudh Bhaskaran
School of Energy
PSG College of Technology
Coimbatore, India
krrbanirudh@gmail.com
Abstract—Air conditioning is one of the primary systems which available solar collectors, solar assisted air conditioning can
are responsible for greenhouse gas emissions, ozone lead to remarkable primary energy savings, if the systems are
depletion and for energy guzzling. The use of solar properly designed [3,4].
energy in buildings is an important contribution to the Till 2007 there were 81 installed large scale SCS,
environment by the reduction of fossil fuel consumption
including systems which are currently not in operation. 73
and harmful emissions. This paper contributes to the
design of solar absorption chiller for School of energy – installations are located in Europe, 7 in Asia, China in
CFD laboratory in PSG college of Technology. Solar particular and 1 in America (Mexico). 60% of these
absorption cooling systems have the advantage of using installations are dedicated to office buildings, 10% to
absolutely harmless working fluids such as water or factories, 15% to laboratories and education centers, 6% to
solutions of certain salts. The primary goal is to utilize hotels and the left percentage to buildings with different final
zero emissions technologies to reduce energy use (hospitals, canteen, sport center, etc.). The overall cooling
consumption and CO2 emissions. The objective of this capacity of the solar thermally driven chillers amounts to 9
study is to design the Solar absorption chiller based on MW; 31% of it is installed in Spain, 18% in Germany and
the cooling load requirement and to evaluate the
12% in Greece [5].
techno-economics of the system to suggest the
institution to make use of the potential of solar energy Wide-ranging studies of different aspects of absorption
in air conditioning of buildings. system, such as performance simulations and experimental test
results, have been reported. Amongst the various types of
Keywords- fossil fuels; Solar absorption cooling system; CO2 continuous absorption SCS, LiBr–H2Oand H2O–NH3 are the
emissions; techno-economics; Air conditioning major working pairs employed in these systems. It is reported
that LiBr–H2O has a higher coefficient of performance (COP)
I. INTRODUCTION than that of the other working fluids [6].
Solar energy cooling systems for buildings have received However, for these applications to be economically
much attention from the engineers in the past few years due to interesting, in terms of payback period, it would be important
the world energy shortage. Especially, the solar driven to extend the system operation period as much as possible
absorption cooling system appears to be one of the promising throughout the year. Solar thermal collectors can also be used
alternative methods for conventional air conditioning system. for water or indoor space heating, thus making it possible to
The blackout situations faced by the Tamil Nadu electricity use an integrated system for building cooling and heating [7].
board due to power shortages can be partially overcome by the This study aims to evaluate the techno-economics of the
utilization of solar energy in air conditioning of buildings. cooling system for building applications and make it
The traditional refrigeration cycles are driven by economically feasible to incorporate in the educational
electricity or heat, which strongly increases the consumption institutions.
of electricity and fossil energy. The International Institute of
Refrigeration in Paris (IIF/IIR) has estimated that II. DESCRIPTION OF SOLAR ABSORPTION COOLING SYSTEM
approximately 15% of all the electricity produced in the whole Absorption is the process in which material transferred from
world is employed for refrigeration and air-conditioning one phase to another, (e.g. liquid) interpenetrates the second
processes of various kinds, and the energy consumption for phase to form a solution. The principle of the single effect
air-conditioning systems has recently been estimated to 45% system with water-LiBr as working pair is described below
of the whole households and commercial buildings[1,2]. [8].
Several thermally driven AC technologies are market
available by today, which enable the use of solar thermal  A pump brings the rich solution towards the high-
energy for this application. Based on current technologies, i.e., pressure zone.
market available thermally driven cooling devices and market

2.  The mixture is heated in the generator. A contribution iii) Auxiliary power
of heat (hot water from solar flat plate collector) iv) water tank volume
allows the separation of the refrigerant (H2O) from v) cooling tower type and power
the absorbent (LiBr solution).  Economical evaluation of optimized solutions
 The vapors of refrigerant are sent towards the
traditional cycle of condenser, expansion valve and
evaporator. Cold is produced by the evaporation of IV. ROOM DESCRIPTION AND ITS CHARACTERISTICS
refrigerant in the evaporator at low pressures and the The room studied in this paper is the CFD laboratory which
cool air is circulated in to the telecommunication is a part of School of Energy in PSG college of Technology.
shelter. The lab is currently cooled by a vapour compression air-
 The poor solution turns over in the absorber by conditioning system which is to be replaced by SAC. The lab
passing by a pressure-relief valve. has a surface area of 36m2 with heat generating equipment’s
 The vapors of refrigerant are absorbed by the poor such as LCD monitors, CPU, wall mounted racks for network
solution of absorber coming from the generator. The connection, LCD projector, lightings, ceiling fan. The detailed
cycle can start again. specification and heat generating capacity of the equipment’s
are given in table II. The knowledge of materials of the room is
necessary to conduct the cooling load calculation and is given
III. METHODOLOGY OF THE WORK in table I.
The methodology comprises of the following steps [9,10]: TABLE I. CONSTRUCTION MATERIALS OF THE ROOM
 Collection of the required meteorological data of the Type Building material Total U value
Thickness
examined area for the last 30 years. (W/m2K)
(mm)
These data include the monthly solar irradiation,
monthly dry bulb temperature, relative humidity. Ceiling RCC (400mm) and 404 2.75
tiles (4mm) on the
 Study of the maximum, minimum and average exterior surface
cooling energy demand of the building, for Floor RCC (400mm) and 404 2.75
determining the technical characteristics of the tiles (4mm) on the
interior surface
system.
In order to maintain stable humidity and temperature Wall1 Plaster 300 2.033
conditions within the building, the cooling loads (100mm),Brick
(100mm),
should be calculated. These depend on a great Plaster(100mm)
number of parameters, such as:
Wall2 Plaster 300 2.033
i) size and geometrical characteristics of the (100mm),Brick
building (100mm),
ii) orientation Plaster(100mm)
iii) construction materials Wall 3 Glass with 5 5.88
iv) activity aluminium frames
v) internal sources of heating Wall 4 Glass with 5 5.88
vi) ventilation aluminium frames
vii) infiltration
viii) lighting
ix) desired values of indoor temperature and The examined lab as shown in Fig.1&2 is in the first floor of a
humidity, during summer and winter block and it is enclosed by 2 seminar halls (air conditioned
x) meteorological conditions space) which is separated by the brick and plaster layers (wall 1
and 2) while the other 2 sides are enclosed by an internet lab
with 5 computers and office room separated by aluminium
 Selection of the solar cooling technology to be framed glass layers (wall 3 and 4). The space right below the
applied. The procedure adopted to select the optimum lab floor is occupied by another department (un-air conditioned
SAC technology depends on several building space) .The space above the lab ceiling is occupied by IT
parameters. The selected technology is also chosen department (un-air conditioned space). There is no direct solar
taken into account the type of the AC installation and heat gain in to the lab; therefore wall cooling load due to solar
the climatic conditions. heat gain is neglected.
 Sizing study of the solar assisted air-conditioning
The lab has an automatic door of 2.16m2 area which is
 Carryout studies on optimized solutions for the solar responsible for the infiltration of outside air in to the lab. The
fraction by varying the technical characteristics that lab will be occupied by a maximum of 15 people. The
mainly concern the: equipment’s operating time is from 8:30am to 5:00pm except
i) solar collector surface the LCD projector. It is assumed that 20% of the cooling load
ii) absorption chiller power

3. from lighting is directly absorbed in the return air stream Figure 2. 3D model of CFD lab (top cut section view)
without becoming room load.
V. COOLING LOAD CALCULATIONS
TABLE II. EQUIPMENT SPECIFICATIONS The cooling loads are calculated on component basis using
Equipment Manufacturer Avg. heat No. of Operating the RTS method. The following parts illustrate cooling load
with generating Equipments hours calculations for individual components of the CFD lab for a
specifications capacity particular day of a month.
Flouroscent 40W T5 lamp 40W 3 8:00am to A. Internal lighting cooling load using radiant time series
lamp with electronic 8:00pm
ballast The primary source of heat from lighting comes from
Ceiling fan 40W 5W 2 8:00am to light emitting elements, or lamps, although significant
5:00pm additional heat may be from associated appurtenances in the
LCD BENQ 250W 1 1 hr per day light fixtures that house such lamps [11]. Instantaneous rate of
projector corporation, M (avg) heat gain from electric lighting is given by [11],
series with
210W light
bulb
̇ (1)
LCD 17” TFT 33W 18 8:00am to Here the lighting use factor is taken as 1 and lighting special
monitors display HCL 5:00pm
monitors allowance factor as 1.1.
To determine the total sensible cooling load, the total heat gain
CPU Intel corp., 60W 18 8:00am to
Core 2 duo 5:00pm has to be split up in to convective and radiant cooling
processor components. The convective and radiant percentages are taken
E7200 to be 41% and 59% for fluorescent lamps recessed, vented to
@2.53Ghz
return air and supply air [11].Convective cooling load is given
Wall 24 port Giga 30W 1 8:00am to by [11],
mounted switch with a 8:00pm
rack for cooling fan
network ̇ ̇ (2)
connection
Radiant cooling load is given by [11],
̇
(3)
Total lighting load is given by [11],
̇ ̇ ̇ (4)
As assumed earlier, 20% of the lighting load is absorbed by the
return air stream, net lighting load is given by [11],
̇ ̇ (5)
Figure 1. 3D model of CFD lab (side cut section view) B. Wall,ceiling and floor cooling load
The conditioned space is adjacent to a space with
different temperature; therefore heat transfer through the
separating physical section must be considered and given by
[11],
̇ (6)
C. Equipment cooling load
The heat gain from the office and lab equipment can
create a significant amount of heat gains, sometimes greater
than all other gains combined. The individual equipment heat
generation can calculated from the average heat generation
capacity as specified by the manufacturer.

4. ̇ (10)
Figure 3. Schematic of Solar absorption chiller plant
D. Occupants load Latent heat gain corresponding to change in humidity ratio is
The sensible and latent heat gains comprise a large given by [11],
fraction of total load. Even for short term occupancy, the extra
(11)
heat gain brought in by people may be significant [11],
Total heat gain is given by,
Sensible heat gain is given by [11], ̇ ̇ ̇ (12)
̇ ̇ (7)
Latent heat gain is given by [11], VI. SOLAR ABSORPTION COOLING MODEL
The schematic model of the solar absorption cooling system is
̇ ̇ (8) shown in Fig.3. The thermal energy required by the absorption
chiller to handle the cooling load is given by [12],
̇
E. Infiltration load ̇ (13)
Automatic doors are a major source of air leakage in
buildings. They are normally installed where a large number Where, is the coefficient of performance of the
of people use the doors. They stay open longer with each use absorption chiller which varies with demand is given in a
fourth order polynomial for partial load efficiency of
than manual doors. Therefore, it is important to that designers
absorption chiller [12],
take in to account the airflow through automatic doors when
calculating the cooling loads in the space next to them [11]. (14)
To calculate the average airflow rate through an automatic
Where, is the ratio of the cooling load and the chiller
door, the designer must take into account the area of the door,
nominal capacity and given by [12],
pressure difference across it, the discharge coefficient of the
̇
door when it is open and the fraction of time it is open. (15)
The infiltration rate through the automatic door is given Energy balance applied at the chiller can be given by,
by [11],
̇ ̇ ̇ (16)
(9) The water leaving the chiller can be let through a flow control
valve to operate the chiller at partial load.
Here the airflow coefficient is taken as 25 L/(s.m2.Pa0.5) and the
pressure difference across the door is taken as 4 Pa0.5[11]. An Auxiliary heater is also provided at the inlet of the chiller in
order to attain the desired temperature of the heating medium
Sensible heat gain corresponding to change in dry bulb and also can be used in the absence of solar energy.
temperature is given by [11],

5. Auxiliary heater 4000 Rs./kW
Cooling tower 5000 Rs./kW
Storage tank 4500 Rs./m3
The characteristics of the required solar absorption air
conditioning system are given in table IV.
TABLE IV. CHARACTERISTICS OF THE SOLAR ABSORPTION AIR
CONDITIONING SYSTEM
Equipment Type Specification
chiller Absorption, LiBr-H2O 9.1kW
Solar collector Flat plate 16m2
Storage tank Hot water 0.4m3
heater Auxiliary cum pre heat 7.2kW
Cooling tower open 23kW
Figure 4. Monthly variations of dry bulb temperature TABLE V. INVESTMENT, OPERATING COST AND PAYBACK PERIOD
VII. THEORETICAL RESULTS OF THE COOLING SYSTEM Equipment Investment cost Rs.
Based on the 30 years dry bulb temperature data as shown in Absorption chiller 3,20,000
Fig 4, the monthly cooling load requirement is calculated and Solar flat plate collector 2,24,000
shown in Fig. 5
Storage tank 1,800
Auxiliary heater 28,800
Cooling tower 1,15,000
Total installation cost 5,000
Annual O&M cost 5,000
Annual electricity cost 10,000
Total cost* 7,09,600
Total annual savings* 1,08,917
Payback period 6.51 years
*cost of electricity =4.715 Rs./kWh
TABLE VI. ENVIRONMENTAL BENEFITS
Benefits Saving units
Figure 5. Monthly variation of total cooling load requirement Annual Electricity savings 23,100 kWh
The monthly cooling load helps us to find out the peak load CO2 savings * 12,173 kg
requirement and accordingly we can select the required
Working fluid in SAC LiBr-H2O
tonnage of solar absorption chiller. From Fig. 5 it is evident
that the peak load of 9.093kW occurs in the month of April and *CO2 emission factor = 0.527kg/kWh
the required tonnage of SAC is calculated for this cooling load The total annual savings is calculated by assuming the
and found to be 2.6 tons. conventional air-conditioning system runs for 300 days a year
VIII. ECONOMIC ANALYSIS and having a tonnage capacity of 5.5tons with 80% cooling
load. The electrical energy consumed by a solar absorption
The techno-economics of the cooling system is performed in chiller to pump the solution and water is also taken in to
this section. The financial data for all the equipment’s is given consideration for calculating the annual savings.
in table III.
The payback period is calculated by,
TABLE III. FINANCIAL DATA
Equipment cost (17)
Absorption chiller 32000 Rs./kW From the environmental aspect, the annual electricity savings
Compression system 25000 Rs./kW can lead to reduction in CO2 emission as shown in table VI.
Solar collector 14000 Rs./m2

6. IX. CONCLUSION
The design of solar absorption chiller for CFD laboratory has sa Special allowance
been carried out as per the ASHRAE standards and the cooling inf Infiltration
load was found to be 9.093kW for which a 3ton SAC can ch Chiller
handle the cooling load. The required cooling load needs a h Hot
thermal input from solar flat plate collector of area 16m2 which
Abbreviation
means a total of 6 flat plate collectors are needed to provide the
desired amount of hot water to the chiller. The economic CFD Computational Fluid Dynamics
analysis carried out in section VIII shows that in order to SCS Solar cooling system
implement this system, a total investment of 7 lakhs (approx.) SAC Solar absorption chiller
has to be invested. The replacement of vapour compression COP Coefficient of performance
system with the solar absorption system will provide a LCD Liquid crystal display
CLF Cooling load factor
significant annual electrical energy savings of 23,100kWh and RCC Reinforced cement concrete
total annual savings of Rs.1,08,917. The payback period for
this renewable cooling system is found to be 6.5 years
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W Lighting wattage, W
Greek symbols
θ Time, hours
Subscripts
c,θ Covective
r,θ Radiant
l Latent
s Sensible
ul Utilization