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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