Linear solar receivers for CSP - sfera2.sollab.eu SUMMER... · –parabolic trough ... In an...
Transcript of Linear solar receivers for CSP - sfera2.sollab.eu SUMMER... · –parabolic trough ... In an...
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
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
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
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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