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Part B4: Storage
B4.1 Storage
B4.1 StorageTypes
• Sensible heat– Water– Pebble bed– Ground
• Latent heat of phase change
• Chemical reaction
B4.1 StorageHeat balance
Qc
Heat from collector (W)
Qu
Heat to load (W)
Qw Heat lost to ambient (W)
Store
Ts
M
Cp
TA
sp c u w
dTMc Q Q Q
dt
Ambient temperature (K)
Mass (Kg)
Store temperature (K)
Specific heat (J/kg/K)
B4.1 Storage Heat balance
U = heat transfer coefficient (W m-2 K-1)
= surface area (m2)
w s AstoreQ UA T T
B4.2 StorageWater
• Convenient, cheap, only tank and insulation
• Good storage density– Cp = 4.18 kJ kg-1 K-1)
– 1m3 at 30°C =126 MJ (35 kW hr)
B4.2 StorageWater: stratification
B4.2 StorageWater: stratified tank
B4.2 StorageWater: stratified tank
Hot water is produced
Hot water is used
B4.2 StorageWater: stratified tank: Modeling
• Divide into 3 layers
Ts1
Ts2
Ts3
• Incoming fluid from collector or load goes to sub-volume nearest its temperature
• Hot water leaves from the top, fluid to the collector goes from the bottom
B4.3 StoragePebble bed: A stratified air store
B4.3 Storage Pebble bed: Performance
B4.3 StorageAir – Pebble bed
• Heights: 1.2 to 6m
• Typically 20-40mm pebbles
• Taller beds use larger rocks to reduce pressure gradient
• Storage density is about 1/3 water
B4.3 StoragePhase change/chemical storage
• Good storage density up to 1.5x water
• Can use chemical reactions such as paraffin wax or Glauber’s Salt – E.g.Glauber’s Salt (Na2SO4) at 32°C
• High cost
Na2SO4 + 10 H2O = Na2SO4 + 10 H20
+ 251kJ/kg
B4.4 StorageInter seasonal storage
• Generally large and crude exploiting V/A ratio
• Netherlands store in 50 x 20m of wet soil for 100 – 200 houses
• Sweden covered lakes have been used as thermal stores