Technologies and materials for thermal energy storage

Peter Schossig

Fraunhofer-Institute for solar energy systems ISEand materials for the thermal energy storageFirst International Renewable Energy Storage Conference (IRES I)Gelsenkirchen, 31.10.2006

Energy consumption in europe

~ 50% of final energydemand in EU25+ isused for heating80 % of this thermal energy is used at temperatures below250°C

Source: ESTTP/ESTIF

heating49%

transport31%

electricity20%

Why thermal energy storage?• especially when using renewable energy sources, demand and

supply do not always correspond

• by shifting energy supply in time (called storage), the percentage of renewables used can be increased significantly

• but also conventional energies can be used with higher efficiency when using thermal energy storages

What defines the technology?• the wanted storage period:

Seasonal - week - 24 h storage

• the needed and the source temperature level:

high temperature > 250°Croom heating/ domestic hot water > 40°Ccold storages < 20°C

thermal

sensible latent sorptiv chemical

solidliquid organicinorganic

watertank

aquifer

Buildingmass

concrete

ground

Salthydrathes

paraffins

adsorption absorption

open closed open closed

Thermo-oil

gravel/waterstorage

technologies

thermal

sensible latent sorptiv chemical

solidliquid organicinorganic

watertank

aquifer

Buildingmass

concrete

ground

Salthydrathes

paraffins

adsorption absorption

open closed open closed

Thermo-oil

gravel/waterstorage

technologies

Conventional solar storage – combi system (DHW + heating)du

du

du

du

Cold water

boiler

controller

Hot water

Collectorloop

StorageSystem

heatingSystem

Source: itw/Drück

European 25+ market – solar thermal systems2005 in operation: about 16,000,000m²2005 newly installed: about 2,000,000m²

Assumption: 80% DHW, about 10% space heating, 10% others, simplified 85%DHW, 15% Combi

DHW: 3 to 6m² collector area per system, 50l storage volume per m²Combisystems: 8 to 15m² collector area per system, 70l storage volume per m²100% hot water storage vessels,

2,000,000m² / (0.85*6m² + 0.15*15m²) =~ 270.000 new systems in 2005

Source: ESTIF

European 25+ market – solar storages

DHW: About 230,000 hot water storage vessels with an average storage volume of about 300l (50 l / m²)-> ~ 70.000 m³ water storage per year installedCombisystems: About 40,000 hot water storage vessels with an average storage volume of about 1000 l (70 l / m²)-> ~ 40.000 m³ water storage per year installed

-> 110.000 m³ storage per year only for solar thermal

(existing capacity 2005 ~ 800.000 m³)Source: ESTIF

Saisonal storage

saisonal storage

heating network

solar network

heating station

solar collector

due to geometrical reasons (losses ~x², capacity ~x³)sensible storages are easier realised for bigger demands than single family housing

Source: solites/Mangold

Source: solites/Mangold

Tank thermal energy store (TTES)(60 to 80 kWh/m³) (60 to 80 kWh/m³)

(15 to 30 kWh/m³) (30 to 40 kWh/m³)

Pit thermal energy store (PTES)

Borehole thermal energy store (BTES) Aquifer thermal energy store (ATES)

Source: solites/Mangold

Example german Reichstag, Berlin

source: GTN/Kabus

connected to a heat pump, sensible saisonalstorage can be used for “dual use”, heating in winter and cooling in summer

thermal

sensible latent sorptiv chemical

solidliquid organicinorganic

watertank

aquifer

Buildingmass

concrete

ground

Salthydrathes

paraffins

adsorption absorption

open closed open closed

Thermo-oil

gravel/waterstorage

technologies

sensible versus latent storage

for most PCM,sensible heat < water

example: cold market in italy

• EU Study (EERAC) 1996: four times the floor area conditioned in 2020 ( for offices: 27% in EU, 80% USA, > 90% Japan)

• 2002 worldwide 15% of electricity for cold production (source: IIR)

Phase change Materials in walls

project with the partners BASF, caparol, maxit and Sto with Fraunhofer ISE 1/1999 - 9/2004

funded by BMWA

wall surface temperatures for 15 mm gypsum plaster with 20% PCM and 0% PCM

products on the market

some products based on microcapsules (plaster, plasterboard, dispersion based plaster)

two companies selling macro-encapsulated PCM:Dörken Rubitherm

BASF: micronal

www.micronal.de

active PCM-systems

passive systems suffer from two limitations:

wall to air heat exchange coefficient

only cold source is night air at dry bulb temperature

solution:

active driven systems

enhanced heat transfer coefficient

any source can be connected

development project 9/2004 - 8/2007

BASF, caparol, maxit, BTU Cottbus, Fraunhofer ISE

funded by BMWA

phase change slurries (PCS)

advantage : greatly enhanced storage/transport capacity in small temperature range, thus:

smaller storages

reduced losses due to isothermal storage

lower pumping energy due to lower mass flow

increasing the storage/transport capacity of a existing system just by exchanging the fluid

research-project 9/2004 - 2/2007 ; funded by BMWA

carrier fluid + PCM

e.g. water/water glycolas fluid und Paraffin-microcapsules as PCM or just paraffin emulsions

Heat capacity of PCS comp. to water

Small melting ranges

factor of 4 timesbetter than waterpossible today

Subcooling increasesthe temperaturerange of a PCS application

ready for demonstration plants/applications

30% 150 kJ/kg

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20melting range [K]

stor

able

hea

t - fa

ctor

to w

ater

[-]

50% 150 kJ/kg

40% 150 kJ/kg

35% 150 kJ/kg

thermal

sensible latent sorptiv chemical

solidliquid organicinorganic

watertank

aquifer

Buildingmass

concrete

ground

Salthydrathes

paraffins

adsorption absorption

open closed open closed

Thermo-oil

gravel/waterstorage

technologies

charge

storage

discharge

water vapour

water vapour

hightemperatureheat

dryadsorbent

lowtemperatureheat

lowtemperatureheat

liquidwater

desorption

evaporation

condensation

adsorption

hightemperatureheat

sorption storages

Sorption storage - examples

source: ISE/Nunez

source: AEE-Intec/Jähnig

2001

Energy densities achieved with commercial materials are about 2.5 to 3 times higher than a water storage

material as well as system research needed (heat exchangers, evaporators)

thermal

sensible latent sorptiv chemical

solidliquid organicinorganic

watertank

aquifer

Buildingmass

concrete

ground

Salthydrathes

paraffins

adsorption absorption

open closed open closed

Thermo-oil

gravel/waterstorage

technologies

thermochemical storageA + B <-> AB + heat

reversible

source: ECN/Visscher

Conclusion thermal storages are needed to increase the fractionof renewable energy as well as energy efficiency forconventional systems

different solutions for different tasks, depending on temperature level and time scale

still research needed, on material as well as systemlevel, with the goal to:

reduce the costs

increase the storage density

increase the efficiency

the rising cooling market requires new storagetechniques to increase the efficiency of cooling orair conditioning

Thank you for your Attention