The document summarizes research on small-scale concentrating solar power (CSP) and thermal energy storage carried out by Professor Giorgio Cau and the Department of Mechanical, Chemical and Materials Engineering at the University of Cagliari. The research includes experimental and numerical studies of sensible heat storage using packed beds with air or CO2 as heat transfer fluids, as well as phase change material systems. Experimental facilities have been developed to test thermal energy storage concepts and optimize design of CSP plants. The research supports development of small-scale CSP installations in Sardinia combining solar thermal fields, thermal energy storage, and organic Rankine cycle or photovoltaic power generation.

1. SMALL SCALE CSP AND THERMAL ENERGY STORAGE:
RESEARCH ACTIVITY CARRIED OUT BY THE DIMCM
University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Giorgio Cau
Professor of Energy Systems
Department of Mechanical, Chemical and Materials Engineering, DIMCM
gcau@unica.it - http://people.unica.it/giorgiocau/
Solar Concentration Technologies and Hydrogen from RES Laboratory
Renewable Energy Platform – Sardegna Ricerche
1Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015

2. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Why energy storage?
Energy storage is an essential option to make independent production and use of
energy, or to match supply to demand regardless of the constraints that affect the
energy production
In particular it is necessary to make possible the use of energy during a period other
than that in which it is available to the production (renewable energies and non-
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 2
than that in which it is available to the production (renewable energies and non-
programmable energy sources)
In other words, it makes possible the deferred use of the energy produced in the
absence or deficiency of a contextual and compliant demand
Moreover, energy storage allows to make available the energy to the user at a power
level different than that to which it is provided during production

3. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Thermal Energy Storage (TES) systems
TES systems cover today a wide range of temperatures (from about -40 °C to more than
500 °C) for applications in energy production, industrial heat recovery and buildings
TES is based on the variation of the internal energy of a suitable material in form of:
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 3
Sensible heat
Latent heat (of phase transistion)
Thermochemical energy (breaking and rearrangement of chemical bonds)
Combined processes (sensible- latent-termochemical)

4. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Research on Thermal Energy Storage at DIMCM
Sensible heat (termocline systems with gas as HTF)
Latent heat (PCM systems)
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 4

5. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Research activity on Thermal Energy Storage (TES)
Sensible heat - thermocline systems with gas a HTF
TES systems based on termocline in packed bed of solid material using gas as HTF
Experimental and numerical studies on thermocline formation and TES performance
and their influencing factorsand their influencing factors
Evaluation of performance deterioration during repeated charge/discharge cycles
2-D and 3-D CFD modeling and analysis
Identification of the optimum management and control criteria of charge/discharge
processes
Experimental performance evaluation of thermocline TES prototipes
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 5

6. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Experimental facilities for testing sensible heat TES systems
based on the termocline principle using gaseous HTF
Open circuit packed-bed TES with air as HTF
Closed circuit packed-bed TES with CO2 as HTF (under construction)Closed circuit packed-bed TES with CO2 as HTF (under construction)
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 6

7. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
400 V
Heater
Control Panel Electric Heater
70 kW, 25 - 300°C NI cDAQ
TC
V3
Vent
Open circuit packed-bed TES with air as HTF
Bed height and diameter: 1.8 – 0.58 m
Solid material: alumina beads, 7-9 mm diameter
TES storage capacity: up to 72 kWh
Thermal power (charge): up to 70 kW
Maximum temperature of HTF: 300 °C
Atmospheric pressure
Inverter
PID
̴
400 V
Q
Screw
Compressor
255 – 940 m3/h
Compressor
Control Panel
70 kW, 25 - 300°C
Insulated
steel tank
TQ PQ
Thermocouples
TD
V1
V2
Vent
PB
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8. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
thermocouples rack
sintered alumina beads
7-9 mm diameter
reservoir inner diameter: 0.58 m
bed height: up to 1.8 m
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9. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Metallic
rack
19 T-type thermocouples placed along the vertical axis
5 more T-type thermocouples placed along a radius
15 K-type thermocouples along the external wall
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10. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Experimental studies have been carried out to investigate the influence of operating and construction
characteristics (mass flow rate, maximum HTF temperature, aspect ratio of the bed) on the
thermocline formation and of the TES behavior in case of repeated charging and discharging cycles.
0.6
0.8
1
θ
1/3 t
ch
2/3 t
ch
3/3 t
ch
3/3 t
disch
0.6
0.8
1
θ
1/3 t
θ
θ
Charge
Indicative of
the Useful
2/3 3/3
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
x/L
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
r/R
1/3 t
ch
2/3 t
ch
3/3 t
ch
3/3 t
disch
minmax
min
TT
TT
−
−
=θ
External
Wall
r/Rx/L
Discharge
BottomTop
Dimensionless temperature
the Useful
Energy
T=Tmin θ = 0
T=Tmax θ = 1
1/3 3/3
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11. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
The experimental investigation has highlighted the influence of the wall on the radial temperature
distribution, that dramatically affects the amount of energy stored by the accumulator.
This aspect has been analyzed by 2-D and 3-D CFD models developed to better predict the
temperature field inside the bed and the energy stored during charge and discharge phases.
0 0.2 0.4 0.6 0.8 1
τ
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12. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Closed circuit packed-bed TES with CO2 as HTF
Bed height and diameter: 0.9 – 0.30 m
Solid material: alumina beads 1.5-2.5 mm diameter
TES storage capacity: up to 5 kWh
Thermal power (charge): up to 6 kW
Maximum temperature of HTF: 150 °C
Maximum pressure of HTF: 5 bar
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13. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
sintered alumina beads
1.5-2.5 mm diameter
compared with beds of
7-9 mm diameter CO2 compressor
reservoir inner diameter: 0.30 m
bed height: up to 0.90 m
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14. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Research activity on Thermal Energy Storage
Latent heat - phase change materials (PCM) systems
Materials choice (temperature range of interest, melting and freezing characteristics,
thermal cycle, compatibility with other materials, useful life, etc.)
Development of storage systems, equipments and devices (numerical analysis,Development of storage systems, equipments and devices (numerical analysis,
experimental investigation and simulation models)
Evaluation of the performance deterioration during repeated cycles of
charge/discharge
Identification of optimum management and control criteria in function of type and use
Evaluation of performance improvements provided by heat transfer enhancement
techniques
Experimental performance evaluation of PCM-TES prototypes
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15. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
PCM thermal energy storage – shell and tube configuration
Thermal energy storage: ≈ 6 kWh
Charge temperature of HTF: 200 °C
Discharge temperature of HTF: 100 °C
PCM material: Hydroquinone
PCM mass: 104 kg
Heat transfer surface: 2.49 m2
Charge process Discharge process
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Lenght: 2.44 m
Diameter: 0.254 m
Number of tubes: 17
Charge process Discharge process

16. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
PCM thermal energy storage – comparison of three different configurations
Thermal energy storage: ≈ 3 kWh
Charge temperature of HTF: 180 °C
Discharge temperature of HTF: 100 °C
PCM material: Mannitol
PCM mass:
Case 1: 23.4 kg
Case 1 Case 2 Case 3
Case 1 – Double Tube
Case 2 – Triplex Heat Exchanger
Case 3 – Triplex with circular fins
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Case 2: 23.4 kg
Case 3: 22.8 kg
Heat transfer surface:
Case 1: 0.077 m2
Case 2: 0.568 m2
Case 3: 0.730 m2

17. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Case 1 – Double Tube
Case 2 – Triplex Heat Exchanger
Case 3 – Triplex with circular fins
Temperature profiles
Charge process Discharge process
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 17

18. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Phase Evolution - Charge process
Red area: solid PCM
Blue area: liquid PCM
Case 3Case 2Case 1
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19. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Phase Evolution - Discharge process
Red area: solid PCM
Blue area: liquid PCM
Case 1 Case 2 Case 3
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20. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Experimental facility for testing PCM
and other direct and indirect TES
Charge electrical power (EH): up to 40 kW
Discharge thermal power (CHE): 170 kW
PCM-TES storage capacity: up to 300 kWh
Maximum temperature of HTF: up to 350 °C
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 20

21. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Experimental facility for testing PCM
and other direct and indirect TES
Shell and tube thermal storage prototype filled with organic PCM
Melting temperature: 167 °C
Design storage capacity: 5.7 kWh
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22. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Research activity on CSP systems
Modeling, design, simulation, performance prediction and optimization, of:
Concentrating solar collectors and solar fields
Steam power plants for CSP applications
ORC power plans for CSP applications
Integration of CSP and fossil fuel power plants (also with CCS)Integration of CSP and fossil fuel power plants (also with CCS)
Thermal energy storage systems
Technical and scientific support to the design of the experimental CSP installations in
the industrial areas of Ottana, Villacidro and Tortolì (solar field, thermal engine and
thermal energy storage system)
Technical and scientific support to testing and management of the Ottana CSP plant
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23. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Ottana Solar Pilot Plant
Collecting area: 8400 m2,
Solar field thermal output: 4690 kWt;
TES capacity: 14.7 MWh;
ORC power output: 600 kWe
CPV power output: 400 kWe
Electrical storage: SoNick batteries, 430 kWhElectrical storage: SoNick batteries, 430 kWh
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24. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
THANK YOU FOR THE ATTENTION
gcau@unica.it
http://people.unica.it/giorgiocau/
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25. University of Cagliari
Department of Mechanical, Chemical and Materials Engineering
http://www.unica.it/
Conference on small scale Concentrating Solar Power in Sardinia – Cagliari – September 25, 2015 25