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Part B4: Storage

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B4.1Storage

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

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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)

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

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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)

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B4.2Storage Water: stratification

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B4.2Storage Water: stratified tank

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Hot water is produced Hot water is used

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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

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B4.3Storage Pebble bed: A stratified air store

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B4.3Storage Pebble bed: Performance

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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

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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 4 + 10 H 2 O = Na 2 SO 4 + 10 H 2 0 + 251kJ/kg

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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

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