linear solar receivers for csp - sfera2.sollab.eu summer... · –parabolic trough ... in an...
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Linear solar receivers
for CSP
François Veynandt
Centre RAPSODEE
Ecole des Mines d’Albi
avec la contribution de
Jean Jacques BEZIAN
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Summary
Overview
– Why linear concentration ?
– Various applications of linear systems
Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT
Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
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Summary
Overview
– Why linear concentration ?
– Various applications of linear systems
Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT
Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
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Why linear concentration ? One axis concentration is more simple, only one axis
movement to follow the sun
Maximum linear concentration on Earth is
46200 =210, 60 to 100 for commercial applications
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Why linear concentration ? Maximum temperature of black body is about 1150 K,
(835 to 950 K), good levels for industrial processes
Stagnation temperature as a function of concentration ratio C
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Why linear concentration ? Allows overheated steam at 500 °C (RANKINE cycle)
Optimal temperature as a function of concentration ratio C
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Solar power plants
Various applications
Andasol Puerto Errado
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Steam production for industrial processes
Solar assisted heating and cooling
Solar cogeneration (heat and power)
Linear CPVT
Various applications
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Various applications : small sizes Two axis concentrators
For small sizes, edge losses due to solar angle
a second tracking is interesting:
– improves optical efficiency,
– only one tracking needs to be precise
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Various applications : usually One axis tracking
The most common solution
For all applications: CSP, CPV, thermal
applications
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Summary
Overview
– Why linear concentration ?
– Various applications of linear systems
Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT
Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
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Two most common system: Parabolic Trough (PT) power plant
Typical design: thermal oil and molten salt
storage
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Two most common system: Linear Fresnel Reflector (LFR) power plant
Typical design: direct steam generation,
without storage
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Linear receivers design: considerations Very long distances involved: (1 km/MW in a
PT plant)
Depends on reflector geometry
Goal: Achieve High Performance, Low Cost,
Reliability and Durability
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Linear receivers designs: parameters High optical efficiency
– tracking accuracy
– reflective components
– absorptive element
High thermal efficiency
– glass cover
– vacuum
– coating
Low cost
– Fabrication
– Transport
– Installation
High durability
– Corrosion resistance
– Low weight / wind resistance
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Linear receiver for
parabolic trough Experience of SEGS
plants since the 80’s
Mature design
Optimization on
details
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Linear receiver for
parabolic trough: example
95 %: Schott PTR 70: 4 m long
Tube with selective coating
– 95 % solar absorption,
– 14 % IR emission 350 °C
In an evacuated glass tube
Mobile receiver
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Linear receiver for
parabolic trough: example
More than 3 Gigawatts capacity equipped with
SCHOTT PTR® 70 receivers (over 1 Million receivers)
More than half of the market (over 50 CSP projects
around the globe)
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Linear receivers for LFR collectors Many designs exist:
each company has developed its own concept
Advantage: fixed receiver
Geometry: tube, V shape, trapezoidal cavity
Number of tubes: one, two or more
Heat transfer fluid: air, water/steam, organic fluid,
thermal oil, molten salt …
Secondary reflector or not?
Glass window (or not?)
Evacuated or not?
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Linear receivers for LFR collectors Examples
reference Negi et al. (1990, 1989), Gordon and Ries (1993) and Abbas et al. (2012a,b).
Various geometries
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Compact Linear Fresnel Reflector (CLFR) concept
Mirror field optimization: etendue matched CLFR
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Linear receivers for LFR collectors Examples
reference Mills and Morrison (2000)
reference Horta et al. (2011)
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Linear receivers for LFR collectors Examples
reference Pye et al. (2003), Reynolds et al. (2004), Singh et al. (1999, 2010), Gordon and Ries (1993)
Trapezoidal receiver designs
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Linear receivers for LFR collectors Examples
reference Bernhard et al. (2008a,b), Selig and Mertins (2010)
Receiver with secondary reflector:
Fresdemo receiver equiped with
photogrammetric measurement
foil on secondary reflector
Novatec Solar receiver with
Composed Parabolic Concentrator
(CPC)
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Linear receivers for LFR collectors Examples
reference Grena and Tarquini (2011)
New receiver with flatter secondary reflector
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Linear receiver for CPVT Cogeneration (power and heat) with PV cells cooled
by a fluid
Low temperatures (60 to 80 °C)
Average efficiency: 15 % (or more) for power, 50 %
(or less) for heat
More conductive transfers
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Summary
Overview
– Why linear concentration ?
– Various applications of linear systems
Linear receiver for
– parabolic trough
– FRESNEL concentrators
– CPVT
Linear receivers’ design issues:
example for Linear Fresnel Reflector
– Energy efficiency: thermal transfers, losses
– Development trend
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Linear receivers’ design issues: Thermal transfer optimization Best solar energy collection
Least thermal losses
Depends on: – The level of temperature
– The fluid (air, water …)
– The solar angle aperture
– The flux map …
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Linear receivers’ design issues: Thermal transfers
Radiative transfers – Optical properties of selective coating
– Net incident solar flux
– Infra red emission (in the cavity)
– Infra red emission (external losses)
Convective transfers – In the tube (heat collection)
– In the cavity
– External losses
Conductive transfers, most often negligible, except for the tube
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Linear receivers designs Diagram of thermal transfers
An example of the various thermal
transfers
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- Radiative heat transfer Selective coating Absorber optical properties
Not suitable without glazing
Temperature range: - 70 °C, + 540 °C
Absorption: solar spectrum
Emission: black body at 400 °C
2 layers 3 layers 4 layers 5 layers 6 layers
Thickness 800 nm 900 nm
Absorption 0.87 0.90 0.91 0.91 0.92
Emission 0.22 0.23 0.23 0.24 0.24
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- Radiative heat transfer Incident flux map Depend on the concentrator optical efficiency:
tracking and quality of the optical components
Non homogeneity in the flux distribution
– Over heated lines (and problem on the durability of coating)
– Impact on the fluid temperature (heat exchange and local
vaporization)
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- Radiative heat transfer Incident flux map Results from
simulations using EDStar, Monte Carlo based radiative heat transfer simulation tool
sun Receiver
Mirrors
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- Radiative heat transfer Incident flux map Variability with
– date of the year
– hour of the day
– optical efficiency
of: – Total power
collected
– Homogeneity of flux distribution
=> Improve design for better efficiency and durability
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- Receiver energy balance Infra red exchanges
New repartition between internal
surfaces: best homogeneity
External losses
Depends on local conditions:
– Emissivity of surfaces,
– Temperature of surfaces (heat balance)
– Equivalent sky temperature
– Equivalent environment temperature
T4
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- Receiver energy balance Convection in the tube
Collection of solar heat by a fluid
Depends on the fluid (liquid, gas or 2 phases
flow), the temperature, the pressure …
Various local conditions
is given by various correlations,
depending on Reynolds number
For example : Colburn :
hST
h Nu /D
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- Receiver energy balance Fluid Mechanics in the tube: Pressure drop
With roughness (0.03 mm)
Colebrook correlation
Linear receiver are long, each loop may
exceed 1 km
=> Pumping power is important to consider
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- Receiver energy balance Convection in the cavity
If the cavity is not evacuated
Natural convection: h depending of Grashof number
Simplified hypothesis
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- Receiver energy balance Results
Temperature profiles along the
receiver pipe with air as HTF
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- Receiver energy balance Results
Temperature profiles along the
receiver pipe with water/steam
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Linear receivers’ design issues: Over heating of the secondary reflector
Good reflector (95 %), very bad emitter (1 %)
In the higher part of the cavity (bad convective
transfer)
Back insulation
=> Very high temperatures and deformations
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=>Thermal efficiency of the receiver Efficiency of the collector is the ratio between the heat
collected and the DNI x mirror area. It depends on: – the optical efficiency of the concentrator (50 %)
– the thermal efficiency of the receiver (80 %): heat collected divided by
solar flux absorbed by the receiver
Losses are mainly: – radiative losses: IR,
– convective losses: free or forced (wind) convection: from 5 to 50 W/m2K
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Development trends of Linear
Fresnel Reflector State of art:
– non-evacuated steel tubes (ex. Areva)
• suitable for 180-300°C (up to 480°C)
• significant losses over 400°C
– Direct Steam Generation • +: saves an expensive heat exchanger
• +: easier operation and maintenance
• -: only short time storage
Towards higher temperatures: – Evacuated pipes with secondary reflector
(demonstrated 520°C superheated steam ex. SuperNova, Novatec)
– Limits: • optical efficiency for higher concentration
• Materials’ reliability
Towards base load: – Molten salt as Heat Transfer Fluid and storage
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Conclusions Very long component of the plant (50 km for a
50 MW PT plant): /!\ cost, efficiency
Suitable for many industrial uses
Thermal efficiency very important
– Optical efficiency: Selective coating for high temperature
– Thermal efficiency: Evacuated tubes: expensive, efficient
Main receiver techniques:
– Mature evacuated pipe for PT • most commercial CSP power plants today
– More opened subject for LFR • towards base-load: evacuated tube, for high temperature
operation, with molten-salt as HTF and thermal storage
– Other solutions: cheaper, less efficient and not entirely mature, but with potential for improvement
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Zhu, G., Wendelin, T., Wagner, M. J., & Kutscher, C. (2014). History, current state, and future of linear Fresnel concentrating solar collectors. Solar Energy, 103, 639–652. doi:10.1016/j.solener.2013.05.021
Cau, G., & Cocco, D. (2014). Comparison of Medium-size Concentrating Solar Power Plants based on Parabolic Trough and Linear Fresnel Collectors. Energy Procedia, 45, 101–110. doi:10.1016/j.egypro.2014.01.012
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