Daad, Presentation, W_g Le Roux

1

UNIVERSITY OF PRETORIA

Solar thermal power generation

using the Brayton cycleW.G. le Roux (MEng)

Prof. J.P. Meyer

Prof. T. Bello-Ochende

Thermofluids Research Group

Department of Mechanical and Aeronautical Engineering

University of Pretoria

German South African Research Lecture Series: ‘Energy Sciences’

Tshwane University of Technology, 10 April 2013

Scope of research – Thermodynamic

optimisation• Open and direct solar thermal

Brayton cycle

• Second Law of Thermodynamics

• Entropy Generation Minimisation

• Maximise net power output

• Optimise geometry of

recuperator and receiver

• Heat Transfer & Fluid Flow

Irreversibilities

• Prof. J.P. Meyer (Head of

Department) and prof. Bello-

Ochende as study leaders

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnetWWW &&&

−=

Receiver

11

5

*Q&

2

Turbine

Solar resource – South Africa

Why Solar?

Solar resource - World• According to DLR

Solar resource – South Africa

Why Solar?

The Department of Minerals and Energy places South Africa’s annual direct

normal irradiation (DNI) between 2 500kWh/m2 and 2 900 kWh/m2 with an

average of almost 300 days of sunshine per year.

Solar resource - Pyranometer

• According to Stine and Geyer

(2001)

• Global horisontal irradiance

• Pyrheliometer measures DNI

0

200

400

600

800

1000

1200

1400

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Irra

dia

nce

(W

/m^

2)

Time (h)

Irradiance of beam

Mean irradiance of global

radiation, tracked

Mean irradiance of global

radiation horizontal

Solar resource – South Africa, Pretoria

Meteonorm

Solar power

• Photovoltaics

• CSP

• Solar water heaters

CSP - Concentrating methods

Solar Tower

CSP (Concentrated solar power)

Power conversion cycles:

• Stirling Dish

• Steam Rankine Cycle

• Brayton cycle

Stirling Dish

Stirling Engine

Stirling Dish

• Ripasso Energy, a solar

technology provider based in

Sweden, has commissioned the

first units of their Concentrated

Solar Power (CSP) installation in

Upington, South Africa.

• The technology is based on

parabolic mirrors and Stirling

Power Converters (SPC). During

the initial operation a new solar-

to-grid-quality-electricity

efficiency world record of 32 %

has been achieved.

Rankine cycle

• Sandia National laboratories

• Fluid – water

• Lower temperatures

• Trough

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW &&&−=

Receiver

11

5

*Q&

2

Turbine

• Air/Gas

• High temperatures

• Dish/Tower

Brayton Cycle

Small-scale open and direct solar thermal Brayton

cycle with recuperator

• Advantages– High recommendation [1, 2] (cost, efficiency)– Air as working fluid– Hot air exhaust

• Water heating• Space heating• Absorpsion refrigeration

– Recuperator• high efficiency and• low compressor pressure ratios

• Disadvantages– recuperator and receiver pressure losses– turbo-machine efficiencies– recuperator effectiveness [3]– Heat losses

[1] Chen L., Zhang, W. and Sun, F., 2007; [2] Mills, D., 2004; [3] Stine and Harrigan, 1985

irreversibilities

Why small-scale?

Advantages of micro-grid

• Smaller financial commitment

• No municipality or larger corporation

Disadvantage of micro-grid

• Grid management

Small-scale solar thermal Brayton cycle

– design considerations

• Dish

• Tracking

• Sensors

• Receiver

• Recuperator

• Heat loss, pressure loss

• Micro-turbine

Solar Brayton - Dish

• Soltrace

• Reflectance

• Shape

Reflectance of aluminium - 79% to 86%.

Polished aluminium - 91% specular reflectance.

Solar Brayton – Dish

Solar Brayton – Dish Reflectivity vs

time

Tracking - Elevation

• SunEarthtools

Tracking - Azimuth

Mousazadeh et al. (2004), Poulek and Libra (2000)

Two-axis solar tracking requiredSolar tracking

Active

PassiveMicro-

processor

and electro-

optical

sensor

basedAuxiliary

bifacial solar

cell based

Date and time

based or a

combination of

sensor and

date/time based FluidBi-

metallic strips

Solar thermal Brayton - Receiver

• Receiver types

– Pressurised tube receiver (cavity) –

air/water/oil/salt

– Open volumetric pressurised receivers (sucks air in

through a fan)

– Closed volumetric receiver (sucks pressurised air

in, using quartz glass cover)

– Solar particle receivers (air particle mixture

absorbs solar radiation)

Receiver types

• Pressurised tube

receiver (cavity) –

air/water/oil/salt


• Open/closed

volumetric

receiver

http://www.volker-quaschning.de/articles/fundamentals2/index.php


• Particle receiver (air-particle

mixture absorbs sunlight)

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnet WWW &&&−=

Receiver

11

5

*Q&

2

Turbine

Solar thermal Brayton - Recuperator

Solar thermal Brayton - Recuperator

Solar thermal Brayton – plate type

recuperator

Lreg

a

b

t H

Solar thermal Brayton – plate type

recuperator

Micro-turbine, coupling

Micro-turbine – compressor map

Micro-turbine – turbine map

Research

• Determination of cavity receiver wall temperatures – effects

of tracking errors, slope errors, specularity errors

• Modelling – Matlab (steady), Flownex (time)

• Determining maximum net power output – total entropy

generation minimisation (optimising components)

• Experimental setup

Compressor

Recuperator

Load

Air in 1

Air out

3

6

4

7

8

9 10

ctnetWWW &&&

−=

Receiver

11

5

*Q&

2

Turbine

Research results – cavity receiver

temperatures as function of errors

500

600

700

800

900

1000

1100

1200

1300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Tem

pe

ratu

re (

°C)

Position

0.0067

0.021

0.0335

0.0067 (Average flux

assumption)

Tracking error: 0°Optical error (rad)

500

600

700

800

900

1000

1100

1200

1300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Te

mp

era

ture

(°C

)

Position

0.0067 0.021 0.0335

Tracking error: 3°

Optical error (rad)

500

600

700

800

900

1000

1100

1200

1300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Tem

pe

ratu

re (

°C)

Position

0.0067 0.021 0.0335

Tracking error: 2°

Optical error (rad)

500

600

700

800

900

1000

1100

1200

1300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

Te

mp

era

ture

(C

°)

Position

0.0067

0.021

0.0335

Tracking error: 1°

Optical error (rad)

Research results – Maximum net power output as

function of solar power and maximum receiver surface

temperature

0

1

2

3

4

5

6

7

8

0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

(kW

)

(kg/s)

t3

t4

q1 = 50 W/K

q2 = 45 W/K

q3 = 40 W/K

q4 = 35 W/K

q5 = 32 W/K

q6 = 27 W/K

q7 = 23 W/K

t1 = 3.80

t2 = 3.60

t3 = 3.15

t4 = 3.00

t5 = 2.76

q1

q2

q3q4

q7 q5

q6

t1

t2

t5

Research results – Maximum net power output as

function of weather condition and maximum receiver

surface temperature

0

1

2

3

4

5

6

0.04 0.05 0.06 0.07 0.08 0.09 0.1

(kW

)

(kg/s)

C, Ts,max = 1200 K

C, Ts,max = 1100 K

C, Ts,max = 1000 K

B, Ts,max = 1200 K

B, Ts,max = 1100 K

B, Ts,max = 1000 K

Current setup

• Experimental work

Experimental setup –

University of Pretoria

Experimental setup – University of Pretoria

For interest - Storage• Batteries

• Rock storage

• Molten Salt

Compressor Turbine

Recuperator

Load

1

2 3

4 e

netQ&

c

Receiver

Solar Heat

Exchanger

Air in

Air out

Ismail and Stuginsky (1999), Applied Thermal

Engineering

For interest –

Large scale solar in South AfricaAccording to SASTELA

• Abengoa:o KaXu Solar One, a 100 MW parabolic trough plant with three hours of storage,

close to Pofadder;o Khi Solar One, a 50 MW solar power tower with storage capacity of two hours, in

Upington.

• Bokpoort CSP, a 50 MW parabolic trough power station with nine hours of storage, located near Groblershoop, by the Saudi group, ACWA, in partnership with South African solar and investment groups

• Eskom is developing a 100MW power tower plant with nine hours of storage in Upington; and

• The Department of Energy is investigating the feasibility of establishing solar power plants with capacity of up to 5 000MW in the Northern Cape.

Brayton – oversees – Solar CAT, Brayton Energy, Google

Questions?

• [email protected]

Daad, Presentation, W_g Le Roux

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