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Experiences with solar cooling systems in Kuwait
WREC 1996
Experiences With Solar Cooling Systems in Kuwait A.A. Al-Homoud, R. K. Suri, Raed Al-Roumi and G. P. Maheshwari Kuwait Institute for Scientific Research POB 24485, 13109 Safat, Kuwait
ABSTRACT Solar space cooling is important in the countries of the Arabian Peninsula where nearly half of the total produced electricity is used for air conditioning of residential commercial and public buildings. In Kuwait, large proportion of funds were allocated for research in solar cooling applications. By the year 1985, several small and medium capacity demonstration projects were installed and tested and more were anticipated for the future. These systems used flat plate collectors and small vapor absorption refrigeration (VAR) system of 5 to 10 tons cooling capacity (TR). The first large installation in Kuwait was carried out in the early eighties for a school building. Immediately thereafter, an equally important installation comprising of 300 m* of flat plate collector area and three 10 TR VAR chillers, was completed in 1983 for an office building of the Ministry of Defense (MOD). These two well instrumented installations were tested for more than one summer season. The system at MOD is the most successful installation and it has been functioning excellently, to-date. Performance results of the system taken during the summer of 1995 have been presented in this paper. KEYWORDS
Solar cooling;
absorption chiller; flat plate collector; coefficient of performance; energy saving
INTRODUCTION Application of solar energy for power generation, desalination and air-conditioning, were the major areas of research during the mid-seventies at the Kuwait Institute for Scientific Research (KISR). A number of solar cooling systems were successfully installed and tested in addition to a 100 kW solar power system and a solar thermal energy powered multi-stage-flash desalination unit. The research and development work in the area of solar cooling was mainly confined to the use of vapor absorption chillers fired by hot-water at less than 1OO’Cusing flat plate collectors. For a country like Kuwait, it is an attractive proposition as the thermal energy collection subsystem has year-round utility for summer cooling, winter heating and hot water for services. As of today, the overall useful conversion of solar thermal energy to cooling is limited to 35 % , as the thermal energy collection and the cooling conversion subsystems have capabilities of 50 % and 70 %, respectively. These subsystems also need electrical energy for the auxiliary motors and consume water in their cooling tower, the later being an important factor in arid zone countries, where soft water is produced from seawater desalination. The overall realistic conversion of solar-to-cooling thus, gets reduced further. 664
WREC 1996 Despite the high capital investments, solar absorption cooling is attractive because of its ability to save electricity, as compared to an equivalent cooling capacity conventional vapor compression machine’. The operation experience with some of the solar cooling projects in Kuwait has shown that electrical energy savings of the order of 40% can be achieved. These savings can exceed 50% value if the absorption chillers operate at rated capacity and water for the cooling tower is available free of energy expense. Such operational conditions require energy collection and storage at a fried temperature, proper selection of plant capacity and thermal storage, use of appropriate controls and selection of pump and fan motors of high quality, thereby reducing parasitic power to a minimum level. The paper presents operation experience of over a decade with a medium capacity (40 TR) installation in Kuwait. SOLAR COOLING INSTALLATIONS The fist large size installation was erected in 1981 for a kindergarten. It comprised of 4 units, each 10 TR Water-Lithium Bromide chillers and 360 m* of roof mounted collectors. The system was in continuous operation till the Iraqi invasion of Kuwait in August 1990. It was shut down during the occupation period and a visual inspection in early 1992 showed serious damage to many of the collectors resulting in pin holes in their absorber plates, rusting of the piping in some of the places and pilferage of the pumps and data storage equipment during the Iraqi occupation of Kuwait. The most successful solar cooling system established in 1983 for a single-story MOD office building with an approximate area of 530 m* has been in continuous operation till date. The solar energy collection subsystem of this system consists of 172 flat plate collector panels, having a window area of 1.72 m* and a vertical hot water reservoir of 20 m3 (2.25 m diameter and 5.25 m height). The schematic diagram of the total system is shown in Fig. 1 The auxiliary thermal source of oil fired water heater of 130 kW thermal capacity automatically supplement the thermal energy input to the chillers in case the hot water temperature at the top of the reservoir or the heat supply temperature to the chillers falls below a set point, say 85°C. In such an event, the collector pump Pl continues to collect thermal energy as an independent circuit. For such an operation, the 3-way valve V9 connects the oil-fired water heater and the chillers for a short-circuit operation, using the same heat medium pump P2. The cooling generation and utilization subsystem consists of four water-lithium bromide VAR chillers of 10 RT capacity each. Three chillers are required to meet the cooling demand of the building and the fourth unit is a stand-by unit. The chillers and the two an-handlers (6115 m3/h capacity each) operate in a short-circuit with the chilled water pump. The heat rejection subsystem consists of a roof mounted cooling tower. Room thermostat controls the production of cooling by controlling the operation of the chilled water and the heat medium pumps. The system has an automatic water quality monitoring system that controls the input of the additive for different water circuits. Performance Analvsis: The flow (Ml to M4) and temperature (tl to t8) measuring instruments for the different water circuits are also shown in the Fig. 1. Simple orifice type flowmeters and the resistance temperature differential (RTD) have been used for the measurement of water flow rates and temperatures, respectively. These data were recorded every hour along with the ambient temperature and the solar radiation. Measurements for the daily consumption were also made for the electricity by the auxiliary equipment, diesel oil by the water boiler and water in the cooling tower. These data are then used to arrive on daily basis for the following performance parameters: ADDared
Cooline summation of the the period during produced using the
Production, Q, (kWh, d“): The daily apparent cooling production (Q,,) is the total cooling produced during a day. The amount of cooling produced with oil or which the auxiliary source supplies the heat input to the chillers and the cooling hot water reservoir are defined as (Q,) and (Q,,) respectively, such that,
665
WREC 1996
Q,. =
Qw + Qw
(1)
Thermal Enerev Availabilitv (kWh, d’k The daily thermal energy availability (Qa) is the summation of the total thermal energy used d&g a day. The amount of thermal energy produced with oil or the period during which the auxiliary source supplies the heat input to the chillers and the thermal energy produced by the solar system are defined as (Q,,) and (Q,) respectively, such that,
Q, = Coefficient of Performance (COP): and Q, as follows
Q, + Q, The daily average COP is defined as the ratio of the Q,.
COP = Q,, 1 Q, Parasitic Enerev, Er (kwh, d-l): Daily parasitic energy (Et,) is the total of the electrical energy consumed by the auxiliary equipment (E,) and the energy equivalent of the water consumed in the cooling tower (W). The later is evaluated using the estimate for a reverse osmosis desalination system producing water at the rate of 100 liter per unit of electricity (km). It is expressed as follows Ep = E, + W/l00
(4)
Svstem Performance: System performance is measured in terms of daily electrical energy saving capability of the overall system, E,, It is expressed as follows
Es = (Q,,/EER) - @J Where EER is the performance conditioning system.
rating of the
conventional, water cooled, vapor compression air-
Samnle Exnerimental Results’ ’ After erection in 1983, the system was commissioned during the summer of 1984. Since then, the system was in operation for the full summer and the experimental data collection was carried out till the summer of 1987. Table 1 gives performance data for three representative days of August 1984, alongwith the estimates for the energy saving. Figure 2 shows the average daily cooling production during the four years period from 1984-87. The summer of 1987 had a large number of high humidity days, resulting in degraded performance of the cooling tower and consequently, a less total cooling production as compared to the other years. To assess the year-to-year performance of the system, a 30days working period of the building was chosen from the period of June 1 to July 7, as a standard Figure 3 shows the average performance of the system on a weekly basis for period for comparison. the selected 30-days period for the 4-year period. The system has been in continuous operation to-date. The latest results for the summer of 1995 shown in Table 2 highlights the continued good performance of collector field and the chillers after more than a decade in operation. System was operated independently with solar energy without any back up from Incoming water temperature to the generator was well over 80 ‘C. Likewise, the chillers the boiler. were able to produce chilled water of less than 5°C with a reasonable COP. An important feature of this project has been a high quality maintenance by a well organized and professionally sound contractor.
666
WREC 1996
1 II”; I
___I
-
---I
‘6,
\
PZ
Hot water tank
pi
t3
-t5
Chiller
Auxiliary thermal sour-ce
Fig. 1.
600
Diagrammatic
sketch
of solar cooling
systems
r
500
2 400 z r 8 1
300
: F =o 8 200
100
0
-
1965
Fig. 2.
1966
Average daily cooling produced during different years.
667
by the system
M3
WREC 1996 Table 1: Electrical energy saving on daily basis for the representative days S.No.
Item
Day 1
Day 2
Day 3
Remarks
1.
Solar-to-oil ratio of thermal energy input
zero
0.5
1.0
2.
Heat input temperature,
81.6
82.1
76.5
3.
Cooling water inlet temperature ‘C
27.4
27.2
26.3
4.
Chilled water outlet temperature, “C
6.5
7.5
6.75
5.
Apparent cooling (Q,& kWh, d-’(RTh d-‘)
6.
Heat input, kWh,hdM’
330 (94) 710
460 (131) 850
330 (94) 610
7.
Auxiliary energy used; kwd-’
83
95
86
8.
Electrical equivalent of water, kmd-’
102
120
20
9.
Total parasitic energy (E,,); kmd-’
0.46
0.54
0.54
10.
Coefficient of performance (COP)
102
120
106
Item (7) + (8)
11.
Electricity for conventional system*
142
185
145
Item (5) / (6)
12.
Electrical energy saving (E,); kwd-’
40
65
39
13.
Percentage saving; %
28
35
27
“C
*Based on the following: 1. Compressor power input 2. Electricity requirement of heat rejection system 3. Electricity requirement of cooling distribution system
Item (12) I (11)
= 0.9 kW,/RT, = 40% of that used by the VAR system, = Equal to the solar system
Table 2. Sample Experimental Data for the year 1995 Date
8/21
Water Flow Rate l/s Incoming Water Temperature Temperature Difference ‘C COP _____________________-----___-_______________________~~~~~___~~~~ ______________________---____----__ Hot Chilled Condenser Generator Condenser Chiller Generator Condenser Chiller 5.9 4.8 8.9 83.6 27.5 6.7 2.5 3.5 2.0 0.64
8/27
5.9
4.8
8.6
87.6
26.5
6.7
3.0
3.9
2.6
0.70
8129
5.9
4.8
8.9
88.0
26.5
7.5
3.1
4.0
2.6
0.62
668
L8 6
IF
I
-
61
a 3 -
a -
i
9661 XXM
Experiences With Solar Cooling Systems in Kuwait A.A. Al-Homoud, R. K. Suri, Raed Al-Roumi and G. P. Maheshwari Kuwait Institute for Scientific Research POB 24485, 13109 Safat, Kuwait
ABSTRACT Solar space cooling is important in the countries of the Arabian Peninsula where nearly half of the total produced electricity is used for air conditioning of residential commercial and public buildings. In Kuwait, large proportion of funds were allocated for research in solar cooling applications. By the year 1985, several small and medium capacity demonstration projects were installed and tested and more were anticipated for the future. These systems used flat plate collectors and small vapor absorption refrigeration (VAR) system of 5 to 10 tons cooling capacity (TR). The first large installation in Kuwait was carried out in the early eighties for a school building. Immediately thereafter, an equally important installation comprising of 300 m* of flat plate collector area and three 10 TR VAR chillers, was completed in 1983 for an office building of the Ministry of Defense (MOD). These two well instrumented installations were tested for more than one summer season. The system at MOD is the most successful installation and it has been functioning excellently, to-date. Performance results of the system taken during the summer of 1995 have been presented in this paper. KEYWORDS
Solar cooling;
absorption chiller; flat plate collector; coefficient of performance; energy saving
INTRODUCTION Application of solar energy for power generation, desalination and air-conditioning, were the major areas of research during the mid-seventies at the Kuwait Institute for Scientific Research (KISR). A number of solar cooling systems were successfully installed and tested in addition to a 100 kW solar power system and a solar thermal energy powered multi-stage-flash desalination unit. The research and development work in the area of solar cooling was mainly confined to the use of vapor absorption chillers fired by hot-water at less than 1OO’Cusing flat plate collectors. For a country like Kuwait, it is an attractive proposition as the thermal energy collection subsystem has year-round utility for summer cooling, winter heating and hot water for services. As of today, the overall useful conversion of solar thermal energy to cooling is limited to 35 % , as the thermal energy collection and the cooling conversion subsystems have capabilities of 50 % and 70 %, respectively. These subsystems also need electrical energy for the auxiliary motors and consume water in their cooling tower, the later being an important factor in arid zone countries, where soft water is produced from seawater desalination. The overall realistic conversion of solar-to-cooling thus, gets reduced further. 664
WREC 1996 Despite the high capital investments, solar absorption cooling is attractive because of its ability to save electricity, as compared to an equivalent cooling capacity conventional vapor compression machine’. The operation experience with some of the solar cooling projects in Kuwait has shown that electrical energy savings of the order of 40% can be achieved. These savings can exceed 50% value if the absorption chillers operate at rated capacity and water for the cooling tower is available free of energy expense. Such operational conditions require energy collection and storage at a fried temperature, proper selection of plant capacity and thermal storage, use of appropriate controls and selection of pump and fan motors of high quality, thereby reducing parasitic power to a minimum level. The paper presents operation experience of over a decade with a medium capacity (40 TR) installation in Kuwait. SOLAR COOLING INSTALLATIONS The fist large size installation was erected in 1981 for a kindergarten. It comprised of 4 units, each 10 TR Water-Lithium Bromide chillers and 360 m* of roof mounted collectors. The system was in continuous operation till the Iraqi invasion of Kuwait in August 1990. It was shut down during the occupation period and a visual inspection in early 1992 showed serious damage to many of the collectors resulting in pin holes in their absorber plates, rusting of the piping in some of the places and pilferage of the pumps and data storage equipment during the Iraqi occupation of Kuwait. The most successful solar cooling system established in 1983 for a single-story MOD office building with an approximate area of 530 m* has been in continuous operation till date. The solar energy collection subsystem of this system consists of 172 flat plate collector panels, having a window area of 1.72 m* and a vertical hot water reservoir of 20 m3 (2.25 m diameter and 5.25 m height). The schematic diagram of the total system is shown in Fig. 1 The auxiliary thermal source of oil fired water heater of 130 kW thermal capacity automatically supplement the thermal energy input to the chillers in case the hot water temperature at the top of the reservoir or the heat supply temperature to the chillers falls below a set point, say 85°C. In such an event, the collector pump Pl continues to collect thermal energy as an independent circuit. For such an operation, the 3-way valve V9 connects the oil-fired water heater and the chillers for a short-circuit operation, using the same heat medium pump P2. The cooling generation and utilization subsystem consists of four water-lithium bromide VAR chillers of 10 RT capacity each. Three chillers are required to meet the cooling demand of the building and the fourth unit is a stand-by unit. The chillers and the two an-handlers (6115 m3/h capacity each) operate in a short-circuit with the chilled water pump. The heat rejection subsystem consists of a roof mounted cooling tower. Room thermostat controls the production of cooling by controlling the operation of the chilled water and the heat medium pumps. The system has an automatic water quality monitoring system that controls the input of the additive for different water circuits. Performance Analvsis: The flow (Ml to M4) and temperature (tl to t8) measuring instruments for the different water circuits are also shown in the Fig. 1. Simple orifice type flowmeters and the resistance temperature differential (RTD) have been used for the measurement of water flow rates and temperatures, respectively. These data were recorded every hour along with the ambient temperature and the solar radiation. Measurements for the daily consumption were also made for the electricity by the auxiliary equipment, diesel oil by the water boiler and water in the cooling tower. These data are then used to arrive on daily basis for the following performance parameters: ADDared
Cooline summation of the the period during produced using the
Production, Q, (kWh, d“): The daily apparent cooling production (Q,,) is the total cooling produced during a day. The amount of cooling produced with oil or which the auxiliary source supplies the heat input to the chillers and the cooling hot water reservoir are defined as (Q,) and (Q,,) respectively, such that,
665
WREC 1996
Q,. =
Qw + Qw
(1)
Thermal Enerev Availabilitv (kWh, d’k The daily thermal energy availability (Qa) is the summation of the total thermal energy used d&g a day. The amount of thermal energy produced with oil or the period during which the auxiliary source supplies the heat input to the chillers and the thermal energy produced by the solar system are defined as (Q,,) and (Q,) respectively, such that,
Q, = Coefficient of Performance (COP): and Q, as follows
Q, + Q, The daily average COP is defined as the ratio of the Q,.
COP = Q,, 1 Q, Parasitic Enerev, Er (kwh, d-l): Daily parasitic energy (Et,) is the total of the electrical energy consumed by the auxiliary equipment (E,) and the energy equivalent of the water consumed in the cooling tower (W). The later is evaluated using the estimate for a reverse osmosis desalination system producing water at the rate of 100 liter per unit of electricity (km). It is expressed as follows Ep = E, + W/l00
(4)
Svstem Performance: System performance is measured in terms of daily electrical energy saving capability of the overall system, E,, It is expressed as follows
Es = (Q,,/EER) - @J Where EER is the performance conditioning system.
rating of the
conventional, water cooled, vapor compression air-
Samnle Exnerimental Results’ ’ After erection in 1983, the system was commissioned during the summer of 1984. Since then, the system was in operation for the full summer and the experimental data collection was carried out till the summer of 1987. Table 1 gives performance data for three representative days of August 1984, alongwith the estimates for the energy saving. Figure 2 shows the average daily cooling production during the four years period from 1984-87. The summer of 1987 had a large number of high humidity days, resulting in degraded performance of the cooling tower and consequently, a less total cooling production as compared to the other years. To assess the year-to-year performance of the system, a 30days working period of the building was chosen from the period of June 1 to July 7, as a standard Figure 3 shows the average performance of the system on a weekly basis for period for comparison. the selected 30-days period for the 4-year period. The system has been in continuous operation to-date. The latest results for the summer of 1995 shown in Table 2 highlights the continued good performance of collector field and the chillers after more than a decade in operation. System was operated independently with solar energy without any back up from Incoming water temperature to the generator was well over 80 ‘C. Likewise, the chillers the boiler. were able to produce chilled water of less than 5°C with a reasonable COP. An important feature of this project has been a high quality maintenance by a well organized and professionally sound contractor.
666
WREC 1996
1 II”; I
___I
-
---I
‘6,
\
PZ
Hot water tank
pi
t3
-t5
Chiller
Auxiliary thermal sour-ce
Fig. 1.
600
Diagrammatic
sketch
of solar cooling
systems
r
500
2 400 z r 8 1
300
: F =o 8 200
100
0
-
1965
Fig. 2.
1966
Average daily cooling produced during different years.
667
by the system
M3
WREC 1996 Table 1: Electrical energy saving on daily basis for the representative days S.No.
Item
Day 1
Day 2
Day 3
Remarks
1.
Solar-to-oil ratio of thermal energy input
zero
0.5
1.0
2.
Heat input temperature,
81.6
82.1
76.5
3.
Cooling water inlet temperature ‘C
27.4
27.2
26.3
4.
Chilled water outlet temperature, “C
6.5
7.5
6.75
5.
Apparent cooling (Q,& kWh, d-’(RTh d-‘)
6.
Heat input, kWh,hdM’
330 (94) 710
460 (131) 850
330 (94) 610
7.
Auxiliary energy used; kwd-’
83
95
86
8.
Electrical equivalent of water, kmd-’
102
120
20
9.
Total parasitic energy (E,,); kmd-’
0.46
0.54
0.54
10.
Coefficient of performance (COP)
102
120
106
Item (7) + (8)
11.
Electricity for conventional system*
142
185
145
Item (5) / (6)
12.
Electrical energy saving (E,); kwd-’
40
65
39
13.
Percentage saving; %
28
35
27
“C
*Based on the following: 1. Compressor power input 2. Electricity requirement of heat rejection system 3. Electricity requirement of cooling distribution system
Item (12) I (11)
= 0.9 kW,/RT, = 40% of that used by the VAR system, = Equal to the solar system
Table 2. Sample Experimental Data for the year 1995 Date
8/21
Water Flow Rate l/s Incoming Water Temperature Temperature Difference ‘C COP _____________________-----___-_______________________~~~~~___~~~~ ______________________---____----__ Hot Chilled Condenser Generator Condenser Chiller Generator Condenser Chiller 5.9 4.8 8.9 83.6 27.5 6.7 2.5 3.5 2.0 0.64
8/27
5.9
4.8
8.6
87.6
26.5
6.7
3.0
3.9
2.6
0.70
8129
5.9
4.8
8.9
88.0
26.5
7.5
3.1
4.0
2.6
0.62
668
L8 6
IF
I
-
61
a 3 -
a -
i
9661 XXM