Thermodynamic and kinetic considerations

Transcription

1 Department of Chemistry Thermochemical Energy storage with storage salt hydrates: with salt hydrates: Thermodynamic and kinetic considerations Michael Steiger, Tanya Talrea, Kirsten Linnow Department of Chemistry, University of Hamburg Collaborators: Michael Fröba research group Department of Chemistry, University of Hamburg Konrad Posern & Christian Kaps Bauhaus University Weimar

2 The need for heat storage Domestic energy consumption (Germany, 2012) fuel 31% 9% 8% electricity water heating 52% space heating cumulative incident solar radiation can cover the heat demand solar radiation is intermittent (seasonal variation) gap between supply and demand requires storage

3 Storage of thermal energy Thermal storage Sensible heat Latent heat Thermochemical Solids metals, concrete Liquids water, oil, molten salts solid liquid water/pebbles Phase Change Materials Solid liquid - ice water - salt hydrates - paraffins - salts Adsorption Gas solid reactions - hydrates - ammoniates - hydroxides - carbonates Heat capacity (C p T) Heat of melting ( m H) Heat of reaction ( r H)

4 Gas solid reactions AB(s) dissociation back reaction (exothermic) A(s) + B(g) Hydrates: M m X x n A H 2 O M m X x n B H 2 O + (n A n B ) H 2 O < 260 C Ammoniates: M m X x n A NH 3 M m X x n B NH 3 + (n A n B ) NH 3 < 300 C Hydroxides: M(OH) 2 MO + H 2 O C Carbonates: MCO 3 MO + CO C Oxides: 2MO 2 M 2 O 3 + 1/2O 2

5 Working temperatures and storage densities Principle Sensible heat solid liquid Latent heat (PCM) Heat of sorption Heat of reaction Working temperature Storage density (GJ m 3 ) ( T = 50 K) !!!!! T / C

6 Heats of hydration MgSO 4 x A H 2 O + (x B x A ) H 2 O MgSO 4 x B H 2 O hyd H = f H B hydrate f H A hydrate (x B x A ) f H H2O(g) f H / kj mol hyd H 0 = 53.9 ± 0.5 kj mol 1 ( hyd H 0 = 44 kj mol 1 ) Calculations for mixed hydrates Hydration more exothermic than condensation! Number of water molecules ( hyd H 0 : average enthalpy of hydration per mole H 2 O) Astrobiology 2012, 12, 1042

7 Storage density of dehydration rehydration MgSO H 2 O MgSO 4 7H 2 O hyd H/V m = 2.9 GJ m 3 MgSO 4 H 2 O + 6 H 2 O MgSO 4 7H 2 O hyd H/V m = 2.2 GJ m 3 MgSO 4 H 2 O + 5 H 2 O MgSO 4 6H 2 O hyd H/V m = 2.0 GJ m 3 MgCl 2 2H 2 O + 4 H 2 O MgCl 2 6H 2 O hyd H/V m = 1.9 GJ m 3 CaCl 2 H 2 O + 5 H 2 O CaCl 2 6H 2 O hyd H/V m = 2.3 GJ m 3 CaCl 2 2H 2 O + 4 H 2 O CaCl 2 6H 2 O hyd H/V m = 1.9 GJ m 3 CaCl 2 H 2 O + H 2 O CaCl 2 2H 2 O hyd H/V m = 0.9 GJ m 3 ZnSO 4 H 2 O + 6 H 2 O ZnSO 4 7H 2 O hyd H/V m = 2.2 GJ m 3 ZnSO 4 H 2 O + 5 H 2 O ZnSO 4 6H 2 O hyd H/V m = 1.8 GJ m 3 CuSO 4 H 2 O + 4 H 2 O CuSO 4 5H 2 O hyd H/V m = 2.1 GJ m 3 for comparison: heat stored in liquid water ( T = 50 K) 0.18 GJ m 3

8 MgSO 4 hydrates MgSO 4 11H 2 O undecahydrate (stable <0 C) MgSO 4 7H 2 O epsomite MgSO 4 6H 2 O hexahydrite MgSO 4 5H 2 O pentahydrite MgSO 4 4H 2 O starkeyite MgSO 4 3H 2 O trihydrate MgSO 4 2.5H 2 O 2.5-trihydrate MgSO 4 2H 2 O sanderite MgSO H 2 O 5/4-hydrate MgSO 4 1H 2 O kieserite MgSO 4 magnesium sulfate (stable at high T)

9 Phase diagram MgSO 4 H 2 O solution p/p MgSO 4 H 2 O 1. Dehydration 2. Storage 3. Hydration T / C (Geochim. Cosmochim. Acta 2011, 75, 3600 )

10 Phase diagram MgSO 4 H 2 O 1.0 solution 0.8 MgSO 4 7H 2 O p/p ? MgSO 4 H 2 O 1. Dehydration 2. Storage 3. Hydration T / C Experimental conditions reported in the literature (Geochim. Cosmochim. Acta 2011, 75, 3600 )

11 Phase diagram MgCl 2 H 2 O MgCl 2 12H 2 O %! solution vapor p/p MgCl 2 2H 2 O MgCl 2 6H 2 O MgCl 2 4H 2 O T / C Hydration at low RH possible (can be realized in seasonal storage systems) (Geochim. Cosmochim. Acta 2011, 75, 3600 )

12 Kinetics of hydration reactions gravimetric water vapor sorption experiments In situ RH-XRD (controlled T and RH) 1. Na 2 SO H 2 O Na 2 SO 4 10H 2 O 2. MgSO 4 H 2 O + 5H 2 O MgSO 4 6H 2 O MgSO 4 6H 2 O + H 2 O MgSO 4 7H 2 O (51% RH)

13 Water vapor sorption measurements Hydration of Na 2 SO 4 (V) (particle size µm) solution n H 2 O 6 4 RH Na 2 SO 4 10H 2 O 2 Na 2 SO 4 (V) t / h T / C reaction only above DRH of Na 2 SO 4 (V) (through-solution) (Energ. Proc. 2014, 48, 394 )

14 Water vapor sorption measurements Hydration of MgSO 4 H 2 O (particle size µm) solution n H 2 O 6 4 RH H 2 O MgSO 4 H 2 O t / h T / C direct solid state reaction is slow and incomplete rapid hydration above DRH of MgSO 4 H 2 O (to MgSO 4 6H 2 O) formation of MgSO 4 7H 2 O only above DRH of MgSO 4 6H 2 O (Energ. Proc. 2014, 48, 394 )

15 Kinetic hindrance of hydration reactions Dehydration V < 0 Hydration V > 0 reaction interface dehydrated MgSO4 6H2O dense barrier product layer slow kinetics, clumping 408µµm m (Cryst. Growth Des. 2008, 8, )

16 Composite materials Problems with bulk hydrates: slow kinetics, incomplete reaction clumping during hydration Possible solution: increasing surface area (small crystals) dispersion in porous substrate (composite materials) Experimental study: glass frits (d m = 1.7 µm) porous glasses (d m = nm) (collaboration with Konrad Posern, Weimar)

17 Glass-MgSO 4 composites Heat of reaction Water uptake Q r / kj g condensation/sorption hydration n w /n s formation of liquid water significant increase in heat of hydrations in porous composites kinetic hindrance overcome in submicron pore size materials overall heat effect due to hydration, adsorption and condensation storage densities in the best materials only about 0.55 GJ m 3 (can be improved with higher pore fillings) (with Konrad Posern)

18 Loss of storage density in composites Storage densities in GJ m 3 1. Increase of total volume (dead volume of the substrate) Porosity Incomplete pore filling (after hydration) Pore filling Sorption with zeolites Sensible storage (water, T = 50 K)

19 New materials with high storage density 1. How can the porosity be increased? Use a densely packed arrangement of spherical pores porosity: max. 74% loss of density: 26% 2. Is it possible to make anything useful with the pore walls? Use a hierarchically structured material spherical macropores densely packed (embedding of the salt) micro- or mesoporous pore walls (as a sorption storage material) Stein, Microp. Mesop. Mater. 2001, 44-45, 227

20 Syntheses of meso-/macroporous systems (Collaboration with Michael Fröba research group) infiltration carbonization silica removal silica sphere array sphere / polymer composite sphere / carbon composite macro- /mesoporous carbon Stein, Microp. Mesop. Mater. 2001, 44-45, 227

21 Synthesis of porous carbon First material tested: porous carbon high mechanical strength (to support high pressures during growth of hydrated phases) high thermal conductivity Synthesis 1. Sedimentation 2. sintering PMMA particles 1. Impregnation 2. Polymerization 1. Calcination 2. Carbonization (Matthias Rogaczewski, Michael Fröba )

22 Synthesis of macroporous carbon (non-porous walls) Before and after sintering 3 µm Densely packed PMMA particles after sedimentation ( 500 nm) Porous carbon (M. Rogaczewski, M. Fröba))

23 Characterization and first experiments First experiments with macroporous carbon (no pores in the pore walls) 1. Characterization (Hg intrusion) porosity: 82% macro pore diameter: 737 nm pore throat diameter: 350 nm 2. Impregnation with MgSO 4 solution pore filling 30 40% 3. Powder XRD experiments 4. Water vapor sorption experiments (with M. Rogaczewski & M. Fröba)

24 First XRD results Sample stored at lab temperature and 43% RH (< RH eq of the monohydrate hexahydrate transition) MgSO 4. 6H 2 O composite Theta (Cu) formation of MgSO 4 6H 2 O at low RH

25 Water vapor sorption n w /n MgSO Pore filling: 30% 31% 40% 41% significantly increased water uptake at low RH full hydration to heptahydrate at 55% RH storage density at 40% pore filling: 0.74 GJ m 3 p /p 0 continuation of experiments with variable pore filling and pore sizes use of other materials (silica)

26 Summary Concluding remarks Use of salt hydrates for thermochemical storage very high theoretical storage densities with salt hydrates operating conditions have to be optimized for each salt and each application slow kinetics of hydration reaction, clumping of bulk hydrates (slow and incomplete hydration, decrease of storage density Composite materials (porous substrate with salt hydrate) improved kinetics if embedded in porous substrate decrease of storage density requires large porosities and very high degree of pore filling hierarchically structured materials offer the advantage of combined use of adsorption and hydration (under development) pore sizes (micro/meso and macro) need to be optimized (adapted to the salt used)