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SOLAR THERMAL

PART 1

Prof. Vanni Lughi

Department of Engineering and Architecture

University of Trieste, Italy

Solar Thermal Heating

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Solar Thermal Heating

Process Heat

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Heat Quality

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Solar Thermal: Main Components

• Collectors

• Storage

• Balance of Plant

• Boiler

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Solar Collectors

• Flat plate collectors

• Vacuum tubes

− Heat pipes

− Vacuum tube forced circulation

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Flat Plate Collectors

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Flat Plate Collectors – Energy Balance

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Flat Plate Collectors

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Heat Pipes

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Heat Pipes

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Vacuum Tubes Forced Circulation

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Vacuum Tubes Forced Circulation

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CPC – Compound Parabolic Concentrating

Heat Pipes

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Vacuum Tubes Forced Circulation

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Choice of Collectors

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Flat panels

• Low/medium grade heat required

• Lower cost

• Larger collector surface required

• Easier integration in roof/structures

• Good efficiency provided mainly

with high ambient temperature

Vacuum tubes

• High/medium grade heat required

• Higher cost

• Smaller collector surface required

• Integration in roof/structures more

difficult

• Good efficiency provided with low

ambient temperature as well

Collector Efficiency

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Flat collector

Heat pipe

Vacuum tube

forced circulation

Design principles

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Q

η·HA =

Required energy

System’s efficiency

Limiting Irradiation [Wh/m2 day]

Collector area

Design principles

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Q

η·HA =

Required energy

System’s efficiency

Limiting Irradiation [Wh/m2 day]

Collector area

Example: Solar collector system in Rome

Required energy: 40 kWh/dat(from building’s plans and/or energy certifications; UNI 10344)

Irradition H in Rome, february (limiting): 5.87 kWh/(m2 day)

� Average irradiance Gk over daily time (8 hrs): 5.87/8 kWh/m2

Desired temperature differencebetween fluid and environment

� (Tm – Ta)/ Gk = 0,0517 K m2/W

� ηcoll = 0.53

� η = 0.53 * 0.9 = 0.48

η = ηcoll

ηs

from datasheetsOther factors

(approx. 0,9)

: 38 K

40000

0.48·5870A = = 14.2 m2

Design principles: Tilt

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Solar Cooling

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Cooling energy demand

Solar Cooling

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CHP – Combined Heat and Power

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Concentration on cooled solar cells:

Electric power + Heat

CSP – Concentrating Solar Power

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• CSP needs large amounts of direct irradiance (latitudes 15 – 40 °)• CSP includes storage� is suitable for large power generation plants

Foresights:

- up to 10 – 25% of global electricity demand (40000 TWh)

CSP – Concentrating Solar Power

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CSP – Parabolic trough

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• 95% of existing plants• 75% of plants under construction• Size: 50 – 250 MW• Capex: 4000 – 7300 $/kW (OECD); 3100 – 4050 (non-OECD)• LCOE: 0.19 – 0.38 $/kWh (no storage); 0.17 – 0.37 $/kWh (6 hrs storage)• Capacity factors: 20 – 40% (no storage); 35 – 75% (with storage);

Fluids for heat storage and transfer:• Synthetic oils (350 – 400 C)• Molten salts (up to 540 C)

CSP – Solar towers

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• 18% of plants under construction• Size: 20 – 250 MW• Capex: 6300 – 10500 $/kW (with 6-15 hrs of storage)• LCOE: 0.20 – 0.29 $/kWh (6-7 hrs of storage); 0.12 – 0.15 $/kWh (12-15 hrs of storage); • Capacity factors: 20 – 40% (no storage); 35 – 75% (with storage)• Two-axes tracking

Temperatures over 600 C� High conversion efficiency

� Cheaper storage� high capacity factors

� Expensive supercritical steam turbines

Alternative: direct steam generation

CSP – Fresnel reflectors

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• 6% of plants under construction• Size: 10 – 100 MW• Capex: ? (lower than the others)• LCOE: 0.19 – 0.38 $/kWh (no storage); 0.17 – 0.37 $/kWh (6 hrs storage)• Capacity factors: 20 – 40% (no storage); 35 – 75% (with storage);

CSP – Solar Dish

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• Two axes tracking • Currently limited by scaling of conversion engine technology: Stirling and microturbines

LCOE of CSP

LCOE of CSP

CSP - Storage

CSP - State of Technology