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Revisiting Control, Storage & Backup for a solar cooling system with 1e absorption chillerAli Shirazzi, Stephen White & Rob Taylor
ENERGY FLAGSHIP
Generic flow-sheet for matching an intermittent heat source and a variable demand for cooling
Solar Collector
Evaporator(+possible backup AC)
Cooling Tower
Aim• How much storage is the right amount ?• What is the best control strategy ?• What is the best gas booster flowsheet ?
Sparber et al, IEA Task 38 Report B1
• Sydney hotel: (maximum cooling demand = 1000 kW, no potable hot water )
24 hr a day load or 7am to 6pm load
1023 kW single effect chiller
Evacuated tube or flat plate collectors
Gas burner backup in series or parallel
Tank insulation (0 to 30 kJ/hr-m2-K)
Parametric sensitivity 1 < m2/kW < 5 10 < l/m2 < 180
TRNSYS Model Scenarios
Optimising solar fraction(shifting to chiller capacity utilization)
C1: Fixed collector outlet temperature with a VSD pump:• collector set point temperature:100 °C (summer), 70 °C (winter)• Pump is off if G < 150 kW/m2 or ΔT < 3K
C2: Variable collector outlet temperature with a VSD pump:• Pump is off if radiation < 150 kW/m2 or ΔT < 3K
• Set point temperature = Tgen + 5°C
C3: Single speed pump with an on/off controller• Pump is on if ΔT < 5K and off if ΔT < 2K
Collector Control Loop Options
Parallel burner/ Evacuated tube/ 24 hr a day load
0,18
0,28
0,38
0,48
0,58
0,68
0,78
0,88
0 0,03 0,06 0,09 0,12 0,15 0,18 0,21 0,24
Tota
l Sol
ar F
ract
ion
(-)
Specific Storage Tank Voulme (m3/m2)
P_C1, Asc=1P_C2, Asc=1P_C3, Asc=1P_C1, Asc=3P_C2, Asc=3P_C3, Asc=3P_C1, Asc=5P_C2, Asc=5P_C3, Asc=5
70 l/m2
Medium solar fraction
Low solar fraction
High solar fraction
Series burner/ Evacuated tube/ 24 hr a day load
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0 0,03 0,06 0,09 0,12 0,15 0,18 0,21 0,24
Tota
l Sol
ar F
ract
ion
(-)
Specific Storage Tank Voulme (m3/m2)
S_C1, Asc=1S_C2, Asc=1S_C3, Asc=1S_C1, Asc=3S_C2, Asc=3S_C3, Asc=3S_C1, Asc=5S_C2, Asc=5S_C3, Asc=5
70 l/m2
Series vs. parallel gas burner
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
0 1 2 3 4 5 6 7
Tota
l Sol
ar F
ract
ion
(-)
Collector Specific Area (m2/kWc)
P_C2, υst=0.07
S_C2, υst=0.07
Series vs. Parallel collector efficiency
0,39
0,41
0,43
0,45
0,47
0,49
0,51
0 1 2 3 4 5 6 7 8
Col
lect
or E
ffic
ienc
y
Collector Specific Area (m2/kWc)
P_C2, υst=0.07
S_C2, υst=0.07
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 0,03 0,06 0,09 0,12 0,15 0,18 0,21
Tota
l Sol
ar F
ract
ion
(-)
Storage Tank Specific Volume (m3/m2)
ETC, Asc=1
FPC, Asc=1
ETC, Asc=5
FPC, Asc=5
Parallel burner/ C1/ 24 hr a day loadComparing evacuated tube and flat plate collectors
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 0,03 0,06 0,09 0,12 0,15 0,18 0,21
Tota
l Sol
ar F
ract
ion
(-)
Storage Tank Specific Volume (m3/m2)
Asc=0.5
Asc=1
Asc=1.5
As=2
Asc=3
Asc=430 l/m2 Low collector area
High collector area
Parallel burner/Evacuated tube/C1/ 7am-6pm load
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1
0 0,03 0,06 0,09 0,12 0,15 0,18 0,21
Tota
l Sol
ar F
ract
ion
(-)
Storage Tank Specific Volume (m3/m2)
U=0, Asc=2U=3, Asc=2U=10, Asc=2U=30, Asc=2
25 l/m2
50 l/m2
Parallel burner/ Evacuated tubes/C1 / 7am-6pm loadComparing storage tank insulation
Triple effect chiller/ C1/ PT Collectors/ 7am-6pm load
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250 300 350
Sola
r fra
ctio
n [-]
storage tank residence time (min)
0.615 A 1.23 SA 1.85 SA 2.46 SA
Backup in series
Backup in parallel
VST=24 l/m2
Storage Residence Time (mins)
Sola
r Fra
ctio
n (-)
Conclusion• Parallel gas burner is preferred
• A variable speed drive will ideally be used to control the outlet temperature of the solar collector
• A storage volume of around 70 l/m2 looks about right but may be too high if • The building is not occupied in the evening• Heat losses are expected (eg long pipe runs and/or multi-
effect absorption chillers)
• Next step: More simulations with vapour compression backup and absorption chiller part load (no gas back up)
CSIRO Energy FlagshipProf Stephen WhiteEnergy for Buildings Managert +61 2 4960 6070e [email protected] www.csiro.au
ENERGY TECHNOLOGY
Thank you