4_storage

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