Part B4: Storage

B4.1Storage

B4.1Storage Types Sensible heat –Water –Pebble bed –Ground Latent heat of phase change Chemical reaction

B4.1Storage Heat balance QcQc Heat from collector (W) QuQu Heat to load (W) QwQw Heat lost to ambient (W) Store TsTs M CpCp TATA Ambient temperature (K) Mass (Kg) Store temperature (K) Specific heat (J/kg/K)

B4.1Storage Heat balance U = heat transfer coefficient (W m -2 K -1 ) = surface area (m 2 )

B4.2Storage Water Convenient, cheap, only tank and insulation Good storage density –C p = 4.18 kJ kg -1 K -1 ) –1m 3 at 30°C =126 MJ (35 kW hr)

B4.2Storage Water: stratification

B4.2Storage Water: stratified tank

Hot water is produced Hot water is used

B4.2Storage Water: stratified tank: Modeling Divide into 3 layers T s1 T s2 T s3 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

B4.3Storage Pebble bed: A stratified air store

B4.3Storage Pebble bed: Performance

B4.3Storage Air – 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

B4.3Storage Phase 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 (Na 2 SO 4 ) at 32°C High cost Na 2 SO H 2 O = Na 2 SO H kJ/kg

B4.4Storage Inter 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