Revisiting Control, Storage & Backup for a solar cooling system with 1e absorption chillerAli Shirazzi, Stephen White & Rob Taylor

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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 stephen.d.white@csiro.auw www.csiro.au

ENERGY TECHNOLOGY

Thank you