Adsorption heat StorageCurrent status and future developments

Ferdinand Schmidt1, Stefan Henninger2

1) Fraunhofer-Institute for Solar Energy Systems ISE2) University of Freiburg, Freiburg Materials Research

Centre FMF

PREHEAT Symposium at Intersolar 2007Freiburg, 22 June 2007

Scope of talk

Operating principle of adsorption heat storage

Thermodynamic limits of storage density

Novel adsorption materials

Some remarks on economics of heat storage

New system approaches

Principle of adsorption heat storage

Storage densities: Dependance on temperature lift

Sorption heat storage: During heat extraction, low temperature heat needs to be added Application determines lowest usable ΔT

Hot water storage sorption heat storage

Energy density achievable with some “classical” adsorbents

From: Núnez, ISHPC 1999

Thermodynamic Limits to storage density

Evaporation enthalpy of water is highest of all known fluids (0,68 kWh/kg)

Adsorption enthalpy can be approx. 30% higher than evaporation enthalpy (due to intermolecular forces in micropores)

Volume fraction not available for water adsorption:- Material skeleton (at least monomolecular walls)- Heat Exchanger- Space for vapor transport

=> 60 % of pore volume are very optimistic estimate

Max. energy density 680 x 1,3 x 0,6 = 530 kWh/m3

ESTTP-Vision 2030: “Factor 8” => 60 x 8 = 480 kWh/m3

Comparison of novel adsorption materials (I)Results of research network funded by german fed. min. of research (BMBF)

Loading spread (g/g) at two cycle conditions

condensation / evap. always at 35°C / 10°C

Front row of bars: Desorption: 95°CAdsorption: 40°C

Back row:Desorption 140°CAdsorption 30°C

1

3

Na-Y Li-LSX Ni-Y Li-Y SAPO-34 AlPO-180

0,05

0,1

0,15

0,2

0,25

0,3

0,35

Comparison of novel adsorption materials (II)

SAPO-34 and AlPO-18 highly interesting for heat pumps, cooling machines

For heat storage, all known syntheses are far too expensive (organic template, calciningprocess step)

1

3

Na-Y Li-LSX Ni-Y Li-Y SAPO-34 AlPO-180

0,05

0,1

0,15

0,2

0,25

0,3

0,35

Novel materials: 1. Hydrophilic carbon fibres

Work at Uni Leipzig (within BMBF network): Post-synthetic treatment of activated carbon fibres (ACF)

Reference material: ACF of Kynol, Inc.

Best results of hydrophilic treatment with nitric acid (HNO3)

Variation of duration and temperature of treatment

Novel materials: 1. Hydrophilic carbon fibres (cont’d)PhD thesis Stefan Henninger: molekular simulation (Monte Carlo) of water adsorption in micropores

Simulation shows: In hydrophic slit pores, 4-7 layers of water molecules are ideal for heat storage application

In activated carbon pores, treatment with nitric acid leads to hydrophilic pore surface

Pore size distribution depends in complicated way on carbonisation process of original material (difficult to influence)

Novel materials: 1. Hydrophilic carbon fibres (cont’d)

SG Grace 127B Kohle 1h HNO3 Kohle 24h HNO3 SAPO-34

0

0,05

0,1

0,15

0,2

0,25

Beladungs-hub [g/g]

SG Grace 127B Kohle 1h HNO3

Kohle 24h HNO3 SAPO-34

Loading spread (g/g) at two cycle conditions

condensation / evap. always at 35°C / 10°C

Front row of bars: Desorption: 95°C (SG, SAPO); 90°C (carbons) Adsorption: 40°C

Back row:Desorption 140°CAdsorption 30°C

Research needs:

How can pore size distribution be influenced during carbonisation

Less expensive base materials and processes

Optimisation of hydrophilic treatment process

Outlook:

Tailored granular carbons from biomass / waste could be cheap storage material

Novel materials: 1. Hydrophilic carbon fibres (cont’d)

Novel materials: 2. MOF’s

Metal Organic framework (MOF): New class of materials, so far mainly researched for hydrogen storage

At Univ. Mainz (within BMBF network) Cu-BTC was identified as suitable for water adsorption

Excellent adsorption properties, but stability unclear (monomolecular walls!)

S. Kaskel, Nachr. Aus d. Chemie, 53, April 2005

Loading spread (g/g) at two cycle conditions

condensation / evap. always at 35°C / 10°C

Front row of bars: Desorption: 95 / 40°C (Silica gel)90 / 40°C (Cu3(BTC))90 / 35°C (nanoscaled)

Back row:Desorption 140°CAdsorption 35°C(except silica gel: 30°C)

1

3

SG Grace127B

AlPo18-nano Cu3(BTC) CuBTC-nano0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

Novel materials: 2. MOF’s (cont’d)

Research needs

Cycle stability?

Finding other MOFs with similar properties that can be cheaply synthesised

Optimising synthesis routes, Upscaling

Forming of pellets or other suitable coupling to heat exchanger

Novel materials: 2. MOF’s (cont’d)

Work at ILK Dresden, Bauhaus-Univ. Weimar, HITK (Hermsdorf) within BMBF-network

Starting from “SWS”-type materials (silica gel impregnated with calcium chloride); avoid corrosion problems!

Goal: New, inexpensive support structures for heat storage (PCM or sorption)

Goal for adsorption: Find support structure preventing corrosion and leakage of salt solution from pores

Novel materials: 3. Salt hydrates in porous supports

Porous support struct.

Heat storage component

Hull material / hydrophobic coating

Salt hydrates: Porous support structures

Large pore ceramics granulate Fine pore ceramics granulate Composite granulate(hierarchical pore structures)

Novel materials: 5: hydrophilic/hydrophobic-transitionWork at Univ. Dortmund (Dr. Brovchenko, Dr. Oleinikova, Prof. Geiger)

Idea: Mesoporous material coated with chain molecules on pore surface

Chains are hydrophilic at low temp., become hydrophobic at higher temp. and collapse

Change of contact angle leads to evaporation from pore at defined temperature

Theory: High loading spread within small temp. change possible

Novel materials: 5: hydrophilic/hydrophobic-transition

State of art: Idea came from fundamental research on phase transitions and thermodynamic limits to adsorption heat storage

Patent filed, no porous materials with such coatings synthesized yet

Research needs: Basic research: Overlap with simulation and synthesis groups in molecular biology, biophysics

Proof of principle for heat storage needs to be delivered0.5 1.0 1.5

0.00.10.20.30.40.50.60.70.80.9

ρ / g

cm

-3

p/p0

Surface interactions:Red: U = -1.9 kcal/mol (carbon)Blue: U = -0.4 kcal/mol (hydrocarbon)

Remarks on economics of heat storageAmortisation of storage has to be achieved over storage cycles during system lifetime => long-term storage requires inexpensive materials

(below 1€/kg)

Most of the novel materials are by far too expensive for long-term storage (AlPO’s, SAPO’s: 20-50 €/kg with upscaled syntheses)

Coupling of slow storage cycle with fast heat pump cycle appears economically attractive

Heat from storage at high T can be fed to sorption heat pump for leverage effect (use some free ambient heat for heating)

Storage at high temp. (150-250°C) could be achieved with cheap zeolite (4A)

Preliminary conclusion

Getting storage density up to 250 kWh/m3 appears achievable through further research on Materials identified by now.

At that stage, the goal from the ESTTP vision for 2030 is still missed by factor of 2

New system concepts are needed!

Integral systems for heating and cooling of buildings: how can sorption storage, solar collectors, sorption heat pump, and ground heat exchangers be combined in optimal way?

Goal should be > 50% solar heat (from collectors and ambient heat!)

Long term adsorptive heat storageEU FP5 project HYDES:

Storage tested at “Solar

house Freiburg”

Energy density achieved:

135 kWh/m3 (to be

compared to water

storage with approx.

47 kWh/m3 at usable ΔT

of 40 K)

Example: Ground-source sorption heat pump

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

ener

gy, k

Wh

cooling/not coveredcooling/solarheating/AHPheating/solarDHW/gasDHW/solar

Heiz-/Kühl-system

Gaskessel

Adsorptions-wärmepumpe

Erd-sonde

Brauchwasser-speicher

Puffer-speicher

Development within EU FP6 project MODESTORE

Results of system simulation of single family house in Madrid

Example for new system concept using stratified storage

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80

20,0 40,0 60,0 80,0 100,0 120,0 140,0 160,0 180,0 200,0Temperatur [°C]

diff.

Ads

orpt

ions

wär

me δQ

/ δT

[kJ/

K]

Qdes_gesamtQads_gesQWRG

Coupling of adsorption heat pump with stratified storage

Optimised heat recovery between adsorption and desorption

Heat curves zeolite 13X, 200 / 35 / 35 / 10°C

At ideal heat recovery,COP > 3,5 possible!

COP=2 would already be a big step forward!

StratiSorp: New system concept using stratified storage

Heat recovery: flow and return from Adsorberdrawn from / fed into stratified storage

External heat source (e.g. blower) active only at end of desorption cycle when T in stratified storage is too low

Preliminary simulation results show that good heat recovery can be achieved

Complete revolving of strat. storage within a few minutes: Optimising feed and extraction pipes to/from storage is rewarding task!

Patent filed by ISE, industry partners wanted

Summary

Significant progress has been made on sorption materials in recent years, large potential for further improvements exists (research funding needed!)

Through new sorption materials alone, goal of factor 8 in storage density (ESTTP vision) will probably not be achieved

New system concepts are needed und should focus on intelligent exergy utilization (e.g. with sorption heat pumps)

Stratified storages can play a key role for enhancing the COP of sorption heat pumps (and cooling machines)

Thank you for your attention!

Funding by german research ministry BMBF is gratefully acknowledged (FKZ 01SF0303)

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5

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35

FAM-Z01

FAM-Z02

SWS

SG 127

BSG M

ayek

awa

UOPD DDZ 70

Zeolit

h 13 X

UOP SC-Y

1/16

Bel

adu

ng

shu

b in

%

90-40-590-40-15130-40-5130-40-15

Comparison of loading spread of materials

Materialien

FAM-Z01: Functional Ad-sorbent Material (Mitsubishi) ==> AlPO with Fe in framework (7.3 Å)

FAM-Z02: SAPO (3.8 Å)

SWS: Selective Water Sorbent (mesoporous silica gel impregnated with salt)

SG: various silica gels

UOP: zeolite Y

Zeolith 13 X: Faujasite

Temperaturen [°C]

Desorption - Adsorption/Kondensation - Verdampfung

275 300 325 350 375 400

10

100XminXmax

4

3

2

1

QEvap.

QAds.

Q3

QDes.

Q1

QCond.

pEvap.

pCond.

TAds,maxT,Ads,minTDes,maxTDes,minTCond.TEvap.

Isosteres [g/g]:water vapour pressure0.100.200.400.60

Dru

ck [h

Pa]

Temperatur [K]

Isosteric heating

Desorption and condensation

isosteric cooling

Adsorption and evaporation

Thermodynamic process