Sand storage – Examination of a new storage conceptfor solar tower power stations

Storages forsolar thermal power plants

Sand storage concept

Air-sand heat exchanger

First results

Heat exchanger prototype

Dr. Joachim GöttscheAachen University of Applied Sciences

“Solar Two“at Barstow, California

10 MWel

Reference: Sandia National Laboratories

Storage capacity dependent electricity costs

75

80

85

90

95

100

105

0 2 4 6 8 10 12 14 16 18

Storage Capacity in Full Load Hours [h]

Rela

tive

Elec

tric

ity G

ener

atin

g Co

st L

EC [%

]

LEC-100% solar, 40 €/kWhLEC-100% solar, 20 €/kWhLEC-100% solar, 10 €/kWh

Calculation for a 50 MWel Parabolic trough plant – Mediterranean site

Reference: DLR 2002, H. W. Fricker

Breakdown of storage costs

Reference: Pacheco 2002

Cost estimation for2-tank salt storageFor parabolic trough plantCapacity: 688 MWhSimilar cost structure forsolar tower power plants

Estimated costs: 31 $/kWh

Storage materials for solar power towers

Text

Reference: DLR 2003Storage volume Cost of storage material Cost of storage tank

Rela

tive v

alu

e

Temperature rangeSand: 150 – 800 °CSaddle packing: 150 – 800 °CHitec XL: 150 – 500 °CSolar Salt: 220-600 °CStorage costs: 700 €/m³for all systems

Sand Saddle packing Salt Hitec XL Solar Salt

Tested storage concept: Packed bed of ceramic balls

Solar tower power with volumetric receiver

Storage with ceramic medium

Drawback: Pressure loss Storage capacity

Temperaturecharacteristic

Storagebodies

InsulationInsulation

Len

gth

in m

Storage full

Storage empty

Sand storage concept

hot storage

solar radiationfrom heliostat field

solar tower

fluid bedcooler

turbine

coldstorage

ambient air

air-sandheat exchanger

Fluid bed cooler

Reference: Richards Engineering

Multi-staged sand-waterheat exchanger

Good heat transfer

Multi-staged: required forlow temp. difference

Efficiency: - low pressure loss: < 5000 Pa

- good heat transfer: > 90 %

Dynamics: - controllability

Costs: - compact design

Air-sand heat exchanger:Desired properties

Air-sand heat exchanger

Cross flow arrangement

Porous walls for sandfiltration

Low-width designPorous wall

Throttle

Finite-elementsimulation

Darcy flow of air

Conductive and radiative heat transfer

Fixed sand velocityprofile

No leakages

No thermal losses

Simulated temperature field

Optimisation calculation for sand velocity

( )out,Sand

in,Sandout,Sandx

TTT

LP

LE −

⋅=&Exergy flow per hx length:

Simulation parameters: (v_Air_in = 1m/s, T_Sand_in = 200°C, T_Air_in = 800°C

0

100

200

300

400

500

600

700

800

0 1 2 3 4 5 6 7

Sand velocity [mm/s]

Tem

pera

ture

[°C

]

0

10000

20000

30000

40000

50000

60000

Exer

gy fl

ow p

er le

ngth

[W/m

]

T_Sand_outT_Air_outSand_Exergy

Output temperatures: TSand= 740 °C, TAir= 300 °C

Heat transfer: 91 %

Sand grain size ≥ 1mm of advantage

Power density P/V with 1 m/s air inflow speed:about 1 MWth/m3

Calculation results

Experimental air-sand hx unit

Experimental air-sand hx unit at SIJ

PropertiesHeat transfer area: 0,5m x 0,1 mAir flow:1-4 m/sAir temperature700 °C

ProblemsDynamics of sandSeparator materialLeakagesThermal losses

Summary

Storage capacity reduces LEC in solar thermal power plants

Higher operation temperatures allow higherefficiencies

Sand storage concept promises low storagecosts at high temperatures (> 800 °C)

Key component: Air-sand heat exchanger

New system under development at SIJ