solar thermal energy storage technologies

Folie 1

Hannover, 23.04.2008

Solar Thermal Energy Storage Technologies

Doerte Laing, German Aerospace Center (DLR)

ENERGY FORUM, 10,000 Solar GIGAWATTSHannover, 23. April 2008


Doerte Laing, Folie 2

Energy Storage for Concentrating Solar Power Plants

Higher solar annual contributionReduction of part-load operationPower management Buffer storage

Energy storage necessary forsuccessful market implementationof CSP technology

Solar-heat

ElectricityHeat

ENERGIE-SPEICHER

(optional)

KONZENTRIERENDER

SONNENKOLLEKTOR

WÄRMEKRAFT-MASCHINE

Solar

ENERGY -STORAGE

(optional)

CONCENTRATINGSOLAR COLLECTOR

POWER BLOCK

Fossilfuel



Highly specific design specifications regarding: primary HTF - pressure - temperature - power level - capacity

Storagesystem

ONE single storage technology will not meet the unique requirements of different solar power plants

Thermal Energy StorageChallenges



Direct activethermal energystorage

Molten salt sensible heat storage

Solid media sensible heat storage

liquid

solid

Heat transfer fluid as storage media

Two tank oil storage

Dual medium storage system

Passive thermal energy storage

phasechange Latent heat storage

Storage concepts for parabolic trough power plantsClassification

Steam Accumulator



Thermal Energy Storage for CSP Plants Status und Development

Commercially available storage systemsSteam Accumulator2-Tank sensible molten salt storage based on nitrate salts

Alternative materials and concepts tested in lab and pilot scaleSolid medium sensible heat storage - concrete storageLatent heat - PCM storageCombined storage system (concrete/PCM) for water/steam fluidImproved molten salt storage conceptsSolid media storage for Solar Tower with Air Receiver (e.g. natural rocks, checker bricks, sand)

Future focus for CSPHigher plant efficiency => Increase process temperatureNew fluids: steam, molten salt, gas/air



Steam Discharging

Isolated Pressure Vessel

Liquid Phase

Steam

Steam Charging

Liquid water Charging / Discharging

Charging process:raising temperature inliquid water volume bycondensing steam

Discharging process:generation of steamby lowering pressure insaturated liquid watervolume

→ Buffer storage for peak power

→ Inefficient and economically not attractive for high pressures and capacities

Steam AccumulatorsStorage of sensible heat in pressurized liquid water



Steam AccumulatorsPS10

Saturated steam at 250°C50 min storage operation at 50% load



Molten Salt Storage - Solar Two

Storage capacity (3h)1400 t of nitrate salts(60% NaNO3 + 40% KNO3)2 tanks: 12 m Ø, 8 m high



Molten Salt Storage – Andasol 1

Conventional steam turbineCollector field Molten salt storage

H2ONaNO3-KNO3Syn. Oil




Ø 38,5 m

14 m

292 °C 386 °C

Storage capacity 1010 MWh (7.7h)Nitrate salts(60% NaNO3 + 40% KNO3)

Salt inventory 28.500 tTank volume 14.000 m³6 HTF/salt heat exchangers




Source: ACS Cobra



Solid media concrete storage

Dual medium indirect storage system with regenerative heat transferPreferred for single phase HTF up to 400/500 °CModular and scalable design from 500 kWh to 1000 MWhEconomic and reliable TESCost target < 20 € / kWh TES capacityFlexible to large no. of sites and construction materials



2 year operation of 2 modules350 kWh castable ceramic350 kWh concrete

Second generation concrete400 kWh storage moduledeveloped with

Current investment cost~ 30 €/kWh(large scale, 6 h cycles)

Concrete storage is ready for scale-up and demonstration

System integration and operation strategy is an important issue

Solid media concrete storage – Current Status



18 m

2,6 m

4 m

134 mmBasic storage module

Solid media concrete storage – Storage Design



Storage Package

Solid media concrete storage – Storage DesignSet-up of storage units

Storage PackageInsulation

50 m

25 m



Solid media concrete storage – Storage DesignPiping



50.000 m³ Concrete Storage Material6 h – Storage for 50 MW-Power Plant

Solid media concrete storage Integration into power plant



Working fluid water/steam:=> Evaporation phase (T=const)

Phase change storage medium=> Melting phase (T=const)

Significant advantage of PCM technology in steam productiondue to constant temperature

Phase Change StorageWhy using Phase Change Material (PCM) ?

spec. enthalpyte

mpe

ratu

re

solid phaseliquid phase

two phasesolid / liquid

spec. enthalpy

tem

pera

ture

pressurized water

superheated steam

wet steam



Phase Change StorageSelection of Phase Change Materials

0

50

100

150

200

250

300

350

400

100 150 200 250 300 350Temperature [°C]

Enth

alpy

[J/g

]

KNO3

NaNO3NaNO2

KNO3-NaNO3

LiNO3-NaNO3

KNO3-LiNO3

KNO3-NaNO2-NaNO3

LiNO3

0

50

100

150

200

250

300

350

400

100 150 200 250 300 350Temperature [°C]

Enth

alpy

[J/g

]

KNO3

NaNO3NaNO2

KNO3-NaNO3

LiNO3-NaNO3

KNO3-LiNO3

KNO3-NaNO2-NaNO3

LiNO3For industrialprocess heat

For solar power generation



Approaches to realize PCM with superior thermal conductivity

Finned Tube Designeffective Lamda >10 W/(mK)

solid

liquid

Fluid

solid

liquid

Fluid

Heat transfer coefficient is dominated by the thermal conductivity of the solid PCM

→ Low thermal conductivity is bottleneck for PCM



On-sun demonstration of a 200 kWh PCM storage

Dimensions: 5 x 0,6 x 0,5 m3 PCM: 2 tons nitrate salt - 120 kg graphite platesMaximum pressure: 40 barCharging/discharging power 100 kWThermal capacity 200 kWhEstimated investment cost: 45 €/kWh th



from solar field

to power block

from power block

to solar field

concrete storage module PCM storage

module

concrete storage module

A B

C

D

A feed water inlet / outletB liquid water

C saturated steamD live steam inlet / outlet

preheating

evaporation/ condensation

Combined Concrete / PCM Storagefor direct steam generation in parabolic troughs

superheating



Conclusions

Energy storage is a key issue for CSP

Steam accumulators only economic as buffer storage

Molten salt technology is available, further improvements for costreduction needed

Concrete storage technology is attractive alternative – demonstration in pilot scale needed

PCM storage technology is the most promising technology for DSG plants

Continuous research and development effort is needed especially for higher process temperatures (> 400°C) and for further cost reduction

solar thermal energy storage technologies

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