Thermal storage in packed beds: pressure drop & heat transfer K.G. Allen, T.W. von Backström,...

Thermal storage in packed Thermal storage in packed beds: pressure drop & heat beds: pressure drop & heat transfertransfer

K.G. Allen, T.W. von Backström, D.G. KrögerK.G. Allen, T.W. von Backström, D.G. KrögerSolar Thermal Energy Research GroupSolar Thermal Energy Research GroupUniversity of StellenboschUniversity of Stellenbosch

Energy Postgraduate Conference 2013

Packed bed thermal storage for CSP

• Store thermal energy from gas turbine exhaust or central receiver

• Stored heat (500 - 600 ºC) used for steam power cycle

Example of packed bed use: SUNSPOT combined cycle

Packed bed characteristics

Prediction of packed bed characteristics:•Pressure drop (apparent friction factor)•Heat transfer coefficient & thermocline (temperature)Influence of:Particle shape?Roughness?Alignment?

Why? Estimate cost …

Experimental work

Sample materials and test section/wind tunnel

Friction factor – crushed rockRange of values obtained…

1.3 & 4.1 mm data from Allen et al., 2013. Powder Technology 246:590-600

How does a packed bed store heat?

Ideal bed Actual bed

Temperature profile?Heat transfer – h ?

ThermoclineRock size dependent

Temperature prediction? (I)

Glass spheres; Bi ≈ 0.18; Re ≈ 490

Temperature prediction? (II)

Crushed rock (ΣVp/ΣAp = 1.3 mm); Bi ≈ 0.12; Re ≈ 280

Thermal decomposition Thermal cycling – cracking and disintegration

Availability of suitable rock?

Other considerations

Conclusion• Pressure drop

Importance of shape, roughness, alignmentNo general correlation

• Temperature profile & heat transferExisting correlations work well up to ≈ 100 ºC

Thermal radiation at higher temperatures?