1/48 STE-L4. Solar storage. water storage PCM stores volume design stratification heat losses

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Bernard Wright
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1 1/48 STE-L4 Solar storage water storage PCM stores volume design stratification heat losses

2 Heat storage for solar thermal systems 2/48 irregular heat supply irregular heat load during a day during a year HEAT STORE = HEART OF SOLAR SYSTEM highly efficient solar collector + inefficient store = inefficient system

3 Criteria 3/48 storage density (capacity) size of storage (space demand) efficiency (loss, usability of storage - exergy) price lifetime safety ekology

4 Possibilities 4/48 storage of sensible heat stored energy proportional to temperature change storage density: 100 to 300 MJ/m 3 on the market storage of latent heat use of phase change + sensible heat storage density: 200 to 500 MJ/m 3 sorption storage storage of water (humidity) in solid (adsorption) or liquid (absorption) component, sorption process = heat rejection, regeneration = heat supply storage density: 500 to 1000 MJ/m 3 storage with chemical reactions reversible chemical reaction with heat absorption and rejection storage density: 1000 to 3000 MJ/m 3 under development

5 Sensible heat storage 5/48 Q t 2 V t 1 c dt V c t 1 t 2 Working medium Temperature range [ C] Specific heat c [Wh/kg.K] Density [kg/m 3 ] Thermal capacity.c [Wh/m 3.K] water , air ,28 1,1 0,31 oil ,44-0, gravel, sand , granit , concrete , brick , iron , gravek-water (37% water) ,

6 Water as a storage medium 6/48 available cheap non-toxic non-flammable good heat transfer (conductivity) high thermal capacity limited temperature range (0 to 100 C) small surface tension (leaks) corrosivity

7 Water storage types 7/48 application hot water stores heating water stores, heat stores, combistores number of heat exchangers vessels (0), monovalent (1), bivalent (2), multivalent (...) pressure pressurized non-pressurized (free water level) storage period short-term (daily) long-term (seasonal)

8 Hot water stores heat exchangers 8/48 vessel monovalent bivalent

9 Combined tanks (HW + SH) 9/48 flow heat exchanger tank in tank flow-tank heat exchanger

10 Combined tanks (HW + SH) 10/48

11 What size? 11/48 domestic hot water 50 l/m 2 collector aperture area combined with heating 50 to 70 l/m 2 collector aperture area larger if backup heater supply heat into store biomass boiler (logs) 50 l/kw automatic biomass boiler (pellets) 25 l/kw heat pump 15 to 30 l/kw gas boilers 25 l/kw

12 solar fraction [-] solární pokrytí[-] Comparison of volumes 12/ trubkový kolektor 5 m2, odběr 160 l/den, tmax = 80 C, primární okruh 60 m different systems, different collectors plochý kolektor 5 m2, odběr 160 l/den, tmax = 80 C, primární okruh 60 m plochý kolektor 5 m2, odběr 160 l/den, tmax = 80 C, primární okruh 25 m l/m 2 plochý kolektor 5 m2, odběr 80 l/den, tmax = 65 C, primární okruh 25 m objem zásobníku / plocha kolektorů [l/m 2 ] specific store volume [l/m 2 ]

13 What size? 13/48 solar combitank one for solar system and back-up one for DHW and SH example solar system 10 m 2 biomass boiler 10 kw 2/3 volume = cca 500 l storage size 750 l 10 kw x 50 l = 500 l 10 m2 x 50 l = 500 l K

14 Which solar store? 14/48 tank in tank flow HX small heat transfer area small loads 1 2 persons more than 2x larger area larger loads 3-4 persons

15 Exergy = usable energy (temperature) 15/48 stratified mixed 150 l at 50 C 150 l at 30 C thermal stratification = high efficiency, high solar fraction

16 Factors influencing stratification 16/48 aspect ratio: height / diameter heated water inlet hot water load cold water inlet heat losses vertical conduction in storage wall vertical conduction in water content

17 Influence of supply and load 17/48 indirect charge direct discharge small solar systems for hot water heat exchanger in bottom part collector efficiency, use of volume solar collector solar store hot water cold water

18 Influence of supply and load 18/48 direct charge indirect discharge larger solar systems with external heat exchanger (combisystems) Fig: bad location of HW HX (obr), better to use full heigth solar collector solar store hot water cold water

19 Influence of supply and load 19/48 indirect charge indirect discharge simple combisystems large mixing effects solar collector solar store hot water cold water

20 Influence of supply and load 20/48 direct charge direct discharge advanced storages with stratification devices charging and discharging at layers (piston effect) solar collector solar store hot water cold water

Heat storage with stratification 21/48 stratification (thermal layering) of storage volume heat storage to layers with same or similar temperature upper part

21 Heat storage with stratification 21/48 stratification (thermal layering) of storage volume heat storage to layers with same or similar temperature upper part with significantly higher temperatures than bottom part (cold until fully charged) reduction of back-up heating increase of usable gains increase of solar fraction

22 Stratified storage 22/48 assumption: low flow to achieve higher temperature difference at solar collector (30 to 40 K) e.g. solar system control to constant collector output temperature, once through mode stratification devices passive active

23 Controlled thermal stratification 23/48 water enters the layer with similar density = similar temperature (passive) advanced control (active)

24 Stratification devices 24/48 DHW HX SH SOL HX

25 Stratification devices 25/48 HW insulation DHW heat exchanger SOL SH CW pipes return water stratified (SH) stratification device for solar collector loop integrating HX inlet of cooled water from DHW heat exchanger source: Solvis

26 Heat losses 26/48 influence storage effectivity thermal insulation pipe connections thermal bridges, degradation of stratification

27 Heat loss of storage 27/48 specific heat loss of cylindric storage tank U.A [W/K] UA D 2 s L 2 s D 2 s e 1 s a l i ins ins ins s w l w 1 a e ins 2 1 s a l i e ins ins s w l w ins 2 1 a e wall 2 x bottom D e storage diameter (no ins); D e + 2.s ins (with ins), L height s ins insulation thickness l ins insulation conductivity s w wall thickness l w wall conductivity a i heat transfer coefficient a e heat transfer coefficient (liquid) (ambient)

28 Heat loss of storage 28/48 heat loss of storage (power) Q l, st U A t st t a [W] heat loss of storage (energy) Q Q l, st l, st Q n i 1 l, st d U A t st, i t a, i [MJ, kwh] i heat demand to cover losses

29 Requirements (standard, legislation) 29/ specific heat loss q 24 [kwh/l.day] DIN : indirect EN 15450; SR EN (kategorie A) EN (category F) volume [l]

30 insulation thickness [mm] Minimum insulation thickness 30/ minimum insulation thickness at conductivity 0,045 W/(m.K) minimum insulation thickness at conductivity 0,035 W/(m.K) volume [l]

31 Influence of pipe connection 31/48

32 Influence of pipe connection 32/48 thermosiphon (convection) brakes natural check-valve, elimination of losses by in-pipe convection

33 Influence of pipe connection 33/48 convection brake

Trends 34/48 compact (integrated) solution minimizing of installation defects optimized hydraulics efficiency space savings placement into residential rooms Insulation Solar input Solar output

34 Trends 34/48 compact (integrated) solution minimizing of installation defects optimized hydraulics efficiency space savings placement into residential rooms Insulation Solar input Solar output Combustion chamber Burner Flue gases HX Controller Stratification device Expansion vessel solar DHW heat exchanger Solar heat exchanger Solar pump DHW output Cold water input Flow heating water Return heating water

35 Trends 35/48

36 Heat storage with phase change 36/48 phase change accompanied with heat absorption / heat rejection liquid gas: liquid solid: undesirable change of volume suitable (melting solidification) Q t t l c t t V s cs m 1 m m l l 2 m sensible heat solid phase latent heat sensible heat liquid phase where t m... phase change temperature t 1 < t m < t 2 l t... latent heat of melting - solidification

37 Heat storage with phase change 37/48 latent heat

38 Heat storage with phase change 38/48 42 MJ t = 10 K 124 MJ

39 Heat storage with phase change 39/ MJ t = 50 K 194 MJ

40 l m [kj/kg] Phase change materials (PCM) 40/ water hydrated salts salt hydrates and eutectic mixtures parafin salts t t [ C] anorganic PCM: salt hydrates (Glauber salt) + high latent heat + high thermal conductivity - corrosive - subcooling - phase segregation organic PCM: wax, parafin, fatty acids + chemically, thermally stable + noncorrosive - low thermal conductivity - low latent heat

41 Phase change materials (PCM) 41/48 required properties suitable temperature of phase change for solar systems: C, choice of temperature low thermal conductivity (parafins) irregular melting, reduction of storage capacity use of conductive matrix, composite materials (carbon fibres) change of volume at phase change not critical (up to 15 %) corrosion (salts) need for longterm stability (cyclic change of phase) subcooling: to use it or not?

42 Subcooling 42/48 limited nucleation of solid phase 42 MJ t = 10 K 16 MJ

43 Parafins 43/48

44 Applications - macrocapsules 44/48

45 Applications - macrocapsules 45/48

46 Applications - microcapsules 46/48 PCM slurries

47 Use of PCM in solar systems 47/48 solar collector store with PCM HW CW

48 Use of PCM in solar systems 48/48 solar gains water

1/58 Components of solar systems

1/58 Components of solar systems storage heat exchangers safety and protection devices air vents, check valve control & measurement Thermosiphon circulation system 2/58 circulation induced by buoyancy