Paolo Silva Paolo Silva
Energia solare termodinamica Prof. Paolo Silva Politecnico di Milano – Dipartimento di Energia [email protected] Legnano, 12 Novembre 2013
Energia al Trasferimento tecnologICO
Paolo Silva
Presentation Outline 2
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
The Solar Resource
Paolo Silva
3 Solar energy
Advantages:
Renewable
Low environmental impact
Modular
Disadvantages:
Costly
Low density
intermittent energy source
Paolo Silva
Paolo Silva
4
on the earth surface 1000 W/m2
Solar radiation wavelenght: from 0.3 to 2.5 µm, peak at 0.5 µm The solar radiation through the atmosphere is weakened due to
scattering and absorption (CO2, H2O, O3).
The solar radiation is the electromagnetic energy emitted by the sun
The power emitted by the sun is 3.8*1014 TW
The maximum power density on a yearly average: outside the atmosphere: 1367 W/m2 (solar constant) it represent the
irradiation on a surface perpendicular to the line connecting the the earth and the sun
The Solar Resource
Paolo Silva
5 Solar spectrum
Visible=48% of the total radiation, UV=6%, IR=46% Annual average radiation 1,367 W/m2 out of atmosphere (±3% during year)
Clear sky, G=1000 W/m2
Paolo Silva
6
The effect of attenuation on the solar radiation is: • Albedo: is the amount of radiation reflected back by the atmosphere • Diffuse radiation (Gd): is the amount of radiation that arrives from all the
directions due to the scattering • Direct radiation (Gb): is the amount of radiation that is not deviated nor
absorbed and keep the direction of the sun rays
The Solar Resource
Paolo Silva
7 Solar radiation
Radiation reaching earth surface: Gglobal = Gbeam + Gdiffuse
Paolo Silva
8 The Solar Resource
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9
• TMY3 (typical metereological year) data, mainly for US sites (but not only): http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html • European Data-base of daylight and solar radiation: http://www.satel-light.com/indexgS.htm
Online DNI data
The Solar Resource
Paolo Silva
PHOTOVOLTAICS THERMAL COLLECTORS THERMODYNAMIC
SYSTEM
Concentrating Solar Power
(CSP)
Systems w/o concentration
ELECTRIC ENERGY THERMAL ENERGY
Systems w/o concentration
(PV)
Concentrating Photovoltaic
(CPV)
SOLAR ENERGY
Introduction on Concentrating Solar Power
Paolo Silva
Solar energy
PV versus CSP:
• EPC costs for large scale PV plants: 1600 €/kW • EPC costs for large scale CSP plants: 3500 €/kW • PV cumulative installed capacity: 70 GW (2011) • CSP cumulative installed capacity: <3 GW (2011)
However… • CSP costs should drastically reduce in the future due to economy-of-scale effects (the thermodynamic conversion uses well-known technologies!) • Efficiency and costs should benefit from an increase in the plant size • CSP EPBT is lower than PV (1 year vs. 4-5 years) • CSP has the key to obtain “dispatchability”: decoupling of electricity production from the availability of the source
Thermal storage tanks obtain this in a relatively cheap way
Paolo Silva
Presentation Outline 12
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
13
θz
• Zenith Angle (θz): angle between the zenith (an imaginary point directly "above" a particular location) and the sun direction
• Solar Altitude (α): complementar of the Zenith angle • Azimuth Angle (γ): angle between North (South) and the projection of the sun
direction on the horizon plane Zenith
The Solar Resource
Paolo Silva Andrea Giostri – Claudio Saccilotto
Effective solar radiation on a surface
Paolo Silva
α = elevation γ = azimuth
Paolo Silva Andrea Giostri – Claudio Saccilotto
Effective solar radiation on a surface
Paolo Silva
Gglobal,T = Gglobal * cos θ
θ
Paolo Silva Andrea Giostri – Claudio Saccilotto
Solar thermal collectors are non appropriate for thermodynamic conversion…
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Solar thermal collectors are non appropriate for thermodynamic conversion…
Paolo Silva
Tglobal
envrL
GTTU,
)( −−=ταη
Paolo Silva
Solar plants classification
Systems without concentration:
• Solar Pond use of salts in solution (NaHCO3) to create a vertical salinity gradient generating hot layers at depth (~ 80-90 ° C) and cold surface layers. In fact, the layers of salt solutions increase in concentration (and therefore density) with depth exploitation of the heat contained in the hot layers in a thermodynamic cycle (ORC "Organic Rankine Cycle") conversion efficiency solar-to-electricity is very low (~ 1 ÷ 2%)
• Plants based on “chimney effect“ – Updraft Tower consist of a greenhouse arranged around a tall tower with a wind turbine at the base the heat produced by solar radiation heats the air trapped in the greenhouse by means of natural convection air rises up the updraft tower. Large size of towers and huge extension of land is needed
Paolo Silva
Paolo Silva
Solar Pond at El Paso, Texas (USA)
Paolo Silva
Solar plants classification
Paolo Silva
Updraft Tower plant at Manzanares, Spain 50 kW 195 m height, 10 m diameter 46’000 m2
Paolo Silva
Solar plants classification
Paolo Silva Andrea Giostri – Claudio Saccilotto
Why concentration?
Flabeg Thick Glass mirror
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Absorber tube
Absorber tube with selective coating
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Concentrating solar collector
,optical peak clη ρ γ τ α= ⋅ ⋅ ⋅
Riflessività Specchio (ρ) Assorbanza
Ricevitore (α)
Trasmittanza Intercapedine (τ) Radiazione
Solare Diretta
Fattore di Intercettazione (γ)
Paolo Silva
Paolo Silva
Parabolic Trough: Incidence angle
Andrea Giostri – Claudio Saccilotto Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Type of losses/1 Optical losses indipendent of the operating conditions depend only on the technology peak optical efficiency is defined (mirror and tube clean, θ = 0)
,optical peak clη ρ γ τ α= ⋅ ⋅ ⋅ Where: ρcl is the reflectivity of the clean mirror γ is the intercept factor τ is the glass trasmissvity α is the selctive coating absorbivity
Paolo Silva
Concentrating solar collector
Paolo Silva Andrea Giostri – Claudio Saccilotto
Type of losses/2 Thermal losses related to the heat exchange between the absorber tube and the external environment related to the thermal properties of the absorber tube (emissivity) related to the thermodynamic properties of the fluid
Paolo Silva
R
CTbeam
envrL
AAG
TTU
,
)( −−= ργταη
Concentrating solar collector
Paolo Silva Andrea Giostri – Claudio Saccilotto
Concentrating solar collector
Paolo Silva
DNI = 800 W/m2
Paolo Silva
Punctual: conc. in a singol point, high concentration factors
Continuous Partitioned
Solar Concentration
Goal
Increase heat fluxes Higher temperatures Lower receiver’s size Lower heat losses
Concentration types: Linear : concentrates solar beams over a line Continuous Partitioned
Parabolic trough Fresnel reflector
Solar Dishes (small size) Central receiver
(Solar Tower)
Disadvantage: concentrates only direct solar radiation
Paolo Silva
Paolo Silva
CSP Technologies
Line focusing sys: Parabolic Trough Line focusing sys: Fresnel
Point focusing sys: Solar Tower
Point focusing sys: Parabolic Dish
Paolo Silva
Concentrating Solar Plants classification
Systems with concentration:
• Concentration types: • Puntcual (Solar Tower, Solar Dish) • Linear (Parabolic Trough, Fresnel)
• Degrees of Freedom (DOF) of solar collectors (SC)
•1 DOF tipically used with linear concentration collectors (“dense array”) tracking system based on a single axis (E-W if orientation of SC is N-S) simple and cheap, but less efficient than 2 DOF (due to incidence angle θ)
•2 DOF tipically used with punctual concentration collectors (“point focus”) double axis tracking system higher efficiency, but good precision is required, higher costs
Paolo Silva
Paolo Silva
Presentation Outline 31
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva Andrea Giostri – Claudio Saccilotto
Solar Field Heat Storage system
Power Block
Paolo Silva
Introduction on Concentrating Solar Power
Paolo Silva
Main components of a CSP plant using linear collectors • Power Block water Rankine cycle for electric power generation
• Heat Storage aims to increase the yearly operating hours of the plant. It decauples supply and demand
• Connecting pipes are the link between the solar field and the power block • Solar Field consists of several files ("loops") of linear collectors (PT o LFR) responsible for uptake of solar radiation and transfer heat to the fluid flowing inside the absorbers tubes (HTF)
Linear concentration systems: Components
33
Paolo Silva
The Collector
34
Goal Increase heat fluxes Higher temperatures Lower receiver’s size Lower heat losses
Disadvantage: concentrates only direct solar radiation
Solar collectors capture incident solar radiation energy and convert it to heat (thermal energy)
Paolo Silva Andrea Giostri – Claudio Saccilotto
Plant configurations • Indirect cycle the HTF heats the working fluid of the thermodynamic cycle
• synthetic oil (ongoing technology SEGS I – IX, Kramer Junction (US)) • molten salts (experimental technology Archimede Project, Priolo Gargallo, Siracusa)
• Direct Steam Cycle direct steam generation inside the receiver tubes of the collectors DSG technology (“Direct Steam Generation”) DISS Project, Almeria (Spain)
Paolo Silva
Plant configurations of a solar plant
Paolo Silva Andrea Giostri – Claudio Saccilotto
Power Block
Purpose: to convert the thermal energy brought by the fluid into mechanical energy with an appropriate thermodynamic cycle and, consequently, into electrical energy through the alternator.
Type of thermodynamic cycle The main thermodynamic cycle used in commercial plants is the Rankine steam cycle (given the limited steam temperatures of 380 ~ 370°C). The possible variations are: presence of reheat (RH) arrangement of RH number of high-temperature regenerators type of condensation In point focus systems the thermodynamic cycle can be also a Joule or Stirling cycle.
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Power Block
Paolo Silva
Solar field Solar field
Indirect cycle Direct cycle
Heat Transfer Fluid (HTF)
Paolo Silva Andrea Giostri – Claudio Saccilotto
Power Block (Rankine cycle)
ECO
EVA
SH RH HP LPSOLAR FIELD
HT REGENERATORS LT REGENERATORS
Paolo Silva
Paolo Silva
SOLA
R F
IELD
SO
LAR
FIE
LD
1
2
3
4
5
6
7
8
Power plant layout: Direct Steam Generation (DSG)
Paolo Silva
Power plant layout: Direct Steam Generation (DSG) – Saturated steam
40
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
Types of heat transfer fluid/1 • Synthetic oil: it is the fluid used in all commercial systems (es. Therminol VP-1 in SEGS VI, Dowtherm A in Andasol I)
• ADVANTAGES:
Low freezing temperatures (~ 12÷20°C) High thermal stability in the range of operating temperatures Low viscosity (enhances heat transfer, minimizes problems of start-ups and pumping) No corrosivity (does not require the use of stainless steel or special alloys)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
Types of heat transfer fluid/2
• DISADVANTAGES:
Limited maximum temperature (~ 400°C problems of thermal decomposition) Flash problems Problems of toxicity Working under pressure (~25÷35 bar) to prevent evaporation at the operating temperatures High costs (~4÷7 €/kg)
Searching for alternative heat transfer fluids
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
Types of heat transfer fluid/3 • Molten Salts: used in Archimede plant as HTF in the solar field or in Andasol I as storage fluid. There are various types of mixtures of molten salts: binary mixtures
• Solar Salts (%m/m: 60% NaNO3, 40% KNO3)
ternary mixtures • Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) • Hitec XL (%m/m: 7% NaNO3, 45% KNO3, 48% Ca(NO3)2)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
SOLAR SALT HITEC HITEC XL
T MAX [°C] 600 535 500
FREEZING POINT [°C] 238 142 120
DENSITY @ 300°C [kg/m3] 1899 1640 1992
VISCOSITY @ 300°C [cp] 3,26 3,16 6,37
HEAT CAPACITY @ 300°C [kJ/kg]
1495 1560 1447
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
Types of heat transfer fluid/4
• ADVANTAGES:
High maximum temperature (up to 600°C) No Flash problems No toxicity problems Low working pressure (~1÷10 bar) Low costs (~0,5÷2 €/kg)
• DISADVANTAGES:
High freezing temperature (difficulty in managing the night, eg. electric heating of tubes, or circulation of hot fluid)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
HTF
Types of heat transfer fluid/5 • Steam/Water: the case of the DSG technology in which heat transfer fluid and the working fluid in the power cycle coincide
• ADVANTAGES:
No freezing problems No corrosivity or flash problems Virtually no cost
• DISADVANTAGES:
High pressures inside the absorber tube (~100 bar) Difficult to control the temperature along the tube
Paolo Silva
Paolo Silva
Storage system
Andrea Giostri – Claudio Saccilotto Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
• Purpose: to ensure the operation of the system even during periods with no sunshine or during transients due to passing clouds mitigates the disadvantage of uncertainty / discontinuity of solar energy by increasing the operating hours of the plant
• State of the art: it is the component on which R&D efforts of the manufacturers are more focused. Still an open discussion about:
Optimal size of the storage Storage system technology Heat transfer fluid
Paolo Silva
Dispatchability is the real extra-value of CSP compared to other renewable energies
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
Paolo Silva
0
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we
r [k
W]
Hours
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we
r [k
W]
Hours
Mid-season dayTypical summer day
Storage 1,5 hours Storage 7,7 hours Net power output Solar energy
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
Optimal size of the storage related to the invetsment costs of the storage system depends on the plant operating strategy should minimize the “Levelized Electricity Cost” (LEC)
Storage system technology
• Type of heat transfer
Direct system (storage fluid = HTF in the solar field/receiver): does not require an intermediate heat exchanger Indirect system (storage fluid ≠ HTF in the solar field/receiver): requires an intermediate heat exchanger
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
• Type of tank/1 Thermocline tank: one tank containing a fluid with a vertical temperature gradient, where the hot fluid is at the top while the cold fluid lies on the base. It can feature a low-cost filler material that performs three basic functions: it is the "bulk" of the heat capacity of the storage system it prevents convective mixing between high and low temperature zones reduces the amount of fluid required
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
• Type of tank/2 Two tanks system: involves the use of two separated "tank", a hot tank and cold tank. The temperature levels depend on the maximum and minimum temperatures reached by the HTF in the solar field. Commercial plants adopting this technology are Archimede at Priolo Gargallo (direct system) and Andasol I near Sevilla (indirect system). In both cases the storage fluid is a molten salts mixture: Archimede (Thot,tank=550°C, Tcold,tank=290°C) Andasol I (Thot,tank=386°C, Tcold,tank=292°C)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Direct storage system (2 tanks)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Indirect storage system (2 tanks)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Indirect storage system (thermocline)
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Storage system
Paolo Silva
Paolo Silva
Storage system
Storage system in ANDASOL I plant 2 indirect tanks (molten salts)
Andrea Giostri – Claudio Saccilotto Paolo Silva
Paolo Silva
Presentation Outline 58
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
59
Parabolic Trough
The Parabolic Trough Collector
Paolo Silva Andrea Giostri – Claudio Saccilotto
Parabolic Trough
SUPER IMMAGINE COPERTINA PARABOLIC TROUGH
Paolo Silva
Paolo Silva
Working principle of parabolic trough collectors • Solar radiation is concentrated by means of a parabolic mirror on a receiver tube located on the focus of the parabola • Inside the receiver tube (“absorber”) a HTF flows, which is progressively heated up. The HTF coming from the solar collectors is used as a heat source for the thermodynamic cycle
• The mirror and the absorber tube rotate togheter to track the sun
Mirror Absorber Tube
Tracking system
Support beams
61 The Parabolic Trough Collector
Paolo Silva
Supporting structure: it is a sort of collector frame and must meet three basic requirements:
1. Support the reflective mirrors and the absorber tube ensuring
optical alignment 2. Tolerate external forces (e.g. wind action) 3. Allow the rotation of the collector to track the Sun in its daily
apparent orbit
62
There are several commercial technologies (Luz LS2, LS3 Luz, EuroTrough, Solargenix, SKyTrough, HelioTrough…) that differ in module length, aperture of the collector and the material used to support beams.
The Parabolic Trough Collector: supporting structure
Paolo Silva
EuroTrough ET-100 collector w=5.76m, L= 100/150m
Solargenix SGX-2 collector w=5.76m, L=65m
63
SenerTrough collector w=5.76m, L=144m
Consorzio XXI collector (ENEA) w=5.90m, L=100m
The Parabolic Trough Collector: Supporting Structure
Paolo Silva
64
The research aims to: 1. Increase aperture 2. Create simplified structure 3. Create stiffer steel structure 4. reduce invest costs 5. increase performance
The Parabolic Trough Collector: supporting structure
Paolo Silva
Reflecting mirror: should concentrate the greatest possible amount of solar radiation on the absorber tube
high reflectivity (90÷96%) lightweight (easier to handle) adequate stiffness (minimizes the deformation of mirror under the action of dynamic loads) durability low specularity errors
65
Microscopic surface roughness cause scattering of the reflected ray with respect to the specular direction
The Parabolic Trough Collector: mirror
Paolo Silva
Flabeg Thick Glass mirror
66
THICK GLASS MIRROR (3-4 mm thick) [Flabeg et al.] Reflectivity ≈ 93-96% Cost ≈ 43-65 $/m2
THE STANDARD...
The Parabolic Trough Collector: mirror
Paolo Silva
67
THIN GLASS MIRROR (1 mm thick) [Flabeg, ACG-Solar et al.] The Adhesive is a key point for durability Reflectivity ≈ 93-96% Cost ≈ 16-43 $/m2
ALUMINIZED MIRROR [Alanod et al.] Durability? (delamination) Reflectivity ≈ 90% Cost ≈ 22 $/m2
SILVERED POLYMERS (film) [SkyFuel, 3M et al.] Durability? Reflectivity ≈ 93% Cost ≈ 16 $/m2
... AND THE OTHER OPTIONS
• Only Aging tests have been performed
• The newst product have only 3 years of field experience
• The substrate cost is not negligible
The Parabolic Trough Collector: mirror
Paolo Silva
68
ReflecTech by SkyFuel: a sample of Film mirror
The Parabolic Trough Collector: mirror
Paolo Silva
Absorber tube: it is positioned in the focus of the parabola, and must transfer the captured energy to the HTF / working fluid that flows inside In a cross-section (transversal) can be recognized:
metal tube (Grade B, AISI 321H) of 70/80/90 mm selective coating (Ni, Cr, Al, Ti) air gap with a high vacuum (10-4 Torr) limits the convective heat losses protects the coating from oxidation coaxial tube of borosilicate glass
69 The Parabolic Trough Collector: the Absorber Tube
Paolo Silva
Schott PTR 70
70
Short wavelength: high absorptivity (and thus high emissivity) Long wavelength: low absorptivity (and thus low emissivity)
The Parabolic Trough Collector: the Absorber Tube
Paolo Silva
The Absorber tube is designed to optimize its optical properties, in order to maximize the optical efficiency of the collector: transmissivity of the glass tube ( τ = 93÷97% ) absorptivity of the selective coating ( α = 92÷97% ) In order to estimate the heat losses of the absorber tube a parameter of fundamental importance is: emissivity of the selective coating ( ε = f(T) ) for all the technologies is increasing with temperature The main loss mechanism are the radiative losses
71 The Parabolic Trough Collector: the Absorber Tube
Paolo Silva
0.05
0.1
0.15
0.2
0.25
0.3
300 350 400 450 500 550
Emis
sivi
tà ε
Temperatura [°C]
LUZ BLACK-CHROME
LUZ CERMET
UVAC 2008
UVAC MEDIO
SCHOTT
ENEA 0.89
0.9
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
LUZ BLACK
CHROME
LUZ CERMET
SOLEL UVAC
SOLEL UVAC 2008
SCHOTT PTR 70
ENEA
A
T
α
τ
Transmissivity and absorbivity of the absorber tube
72 The Parabolic Trough Collector: the Absorber Tube
Paolo Silva
Incidence Angle Angle formed by the normal to the opening of the mirror and the rays coming from the sun
Its value depends on several factors:
• Location of site (Latitude, Longitude) • day of the year • Time of the day • Direction of the tracking
strong influence on the energy performance of the system
73 The Parabolic Trough Collector: Optical Efficiency
Paolo Silva
74
Longitudinal Plane
Parabolic Mirror Aperture Area
Incidence Angle θ
Transversal Plane
θ
INCIDENCE ANGLE
The Parabolic Trough Collector: Optical Efficiency
Paolo Silva
75
Nominal Optical losses indipendent of the operating conditions depend only on the technology peak optical efficiency is defined (mirror and tube clean, θ = 0)
Mirror Reflectivity
(ρ) Coating
Absorbivity (α)
Glass Transmissivity (τ)
Direct Solar Radiation
Intercept Factor (γ)
The Parabolic Trough Collector: Optical Efficiency
Paolo Silva
There is an ‘image spread ’ due to:
• SUN SHAPE The sun view from the earth appear as a disk of angular radius of ∆s=4.7 mrad Usually its shape is approximated as a gaussian
0
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1.2
-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30
rela
tive
Inte
nsity
Angle (mrad)
CSR10
Normal, s=2.8 mrad
The Parabolic Trough collector: Optical Efficiency Intercept Factor
Paolo Silva
• SUN SHAPE • MIRROR SPECULARITY Microscopic surface roughness cause scattering of the reflected ray with respect to the specular direction
There is an ‘image spread ’ due to:
The Parabolic Trough collector: Optical Efficiency Intercept Factor
Paolo Silva
There is an ‘image spread ’ due to:
• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS
Deviation of the surface from the best fit parabola: The waviness, with typical wavelengths on the order of centimeters to decimeters; represents medium scale errors.
The Parabolic Trough collector: Optical Efficiency Intercept Factor
Paolo Silva
• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR
Is the difference between the design shape and the average shape: represents large scale optical errors, caused by gravity, wind, materials or manufacturing errors.
There is an ‘image spread ’ due to:
The Parabolic Trough collector: Optical Efficiency Intercept Factor
Paolo Silva
There is an ‘image spread ’ due to:
• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR • TRACKING ERRORS
The Parabolic Trough collector: Optical Efficiency Intercept Factor
Paolo Silva
• SUN SHAPE • MIRROR SPECULARITY • SLOPE (CONTOUR) ERRORS • SHAPE ERRORS • ALIGNMENT ERROR • TRACKING ERRORS
Each error type can be characterized by its rms angular width. The rms width σoptical for the total optical error is obtained by adding the squares of the individual widths:
OPTICAL ERRORS
Sun Shapeeam spread
Parabolic Trough: Optical Efficiency Intercept Factor
Paolo Silva
The Parabolic Trough Collector: Optical Efficiency (off-design)
Optical off-design losses (non zero incidence angle): effect of the incidence angle (K(θ)) decrease in energy density of radiation (cosθ) change in optical properties of the absorber (τ(θ),α(θ)) shadow projection of the support
mutual shading of the mirrors (ηshading) collector end losses (ηend_loss)
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Paolo Silva
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80
K(θ)
Incidenza (θ) [°]
LS2
LS3
ET-100
K(θ) trends for different collector technologies
83
( ) ( ) 4 5 2cos 5.251 10 2.8596 10ET
K θ θ θ θ− −= − ⋅ − ⋅
The Parabolic Trough Collector: Optical Efficiency (off-design)
Paolo Silva
Reciprocal shadowing of mirrors in different hours of the day
Importance of spacing between rows of collectors (usually distance = 2.5 times the aperture of the parabola)
Andrea Giostri – Claudio Saccilotto Paolo Silva
84
( )( )
cosmin max 0; ;1
coseff spacing Z
shadowing
W LW W
θη
θ
= = ⋅
The Parabolic Trough Collector: Optical Efficiency (off-design)
Paolo Silva
Collector end-losses
Influenced by: • Lenght of receiver tube • Incidence angle • Position of the receiver tube (focal distance)
Andrea Giostri – Claudio Saccilotto Paolo Silva
85
( )1 tan pmend loss i
abs
DL
η θ− = −
“Account for the spilling of radiation over the end of a finite trough”
From: P.Bendt et al, Optical analysis and optimization of line focus solar collector
Spillage of rays due to end losses
The Parabolic Trough Collector: Optical Efficiency (off-design)
Paolo Silva
Thermal analysis purpose Calculation of solar collector performance: evaluation of the thermal power transferred to the fluid (estimation of thermal losses) evaluation of the pressure drop along the absorber tube Study of the influence of some technological-environmental parameters: direct radiation wind speed presence of H2 in the cavity
86 The Parabolic Trough Collector: Thermal Efficiency
Paolo Silva
Thermal losses related to the heat exchange between the absorber tube and the external environment related to the thermal properties of the absorber tube (emissivity) related to the thermodynamic properties of the fluid Need to model heat transfer in a cross-section of the absorber tube (hypothesis of circumferentially uniform temperature) To this extent it is very useful to introduce an "electrical analogy“ considering an equivalent circuit diagram
87 The Parabolic Trough Collector: Thermal Efficiency
Paolo Silva
T_Fluid T_Cint T_Cext
T_GintT_Gext
T_Amb
T_Sky
q_conv_HTF q_cond_abs
q_rad_glass
q_conv_glass
q_cond_glass
q_conv_sky
q_rad_sky
q_cond_bracket
88 The Parabolic Trough Collector: Thermal Efficiency
Inputs of the 1D model: - Materials
characteristics - Solar radiation hitting
the receiver - HTF temperature - Ambient Temperature
Output: - Temperature in each knot of the circuit - Fluxes and losses
The Model can be solved in subsequent sections along the tube lenght to perform a 2D analysis
Paolo Silva
T_Fluid T_Cint T_Cext
T_GintT_Gext
T_Amb
T_Sky
q_conv_HTF q_cond_abs
q_rad_glass
q_conv_glass
q_cond_glass
q_conv_sky
q_rad_sky
q_cond_bracket
89 The Parabolic Trough Collector: Thermal Efficiency
Inputs of the 1D model: - Materials
characteristics - Solar radiation hitting
the receiver - HTF temperature - Ambient Temperature
Output: - Temperature in each knot of the circuit - Fluxes and losses
The Model can be solved in subsequent sections along the tube lenght to perform a 2D analysis
0 100 200 300 400 500 600 700 8000
50
100
150
200
250
300
350
400
450
Collector Length [m]
Tem
pera
ture
s [°C
]
T_HTFT_in_steelT_out_steelT_in_pirexT_out_pirexT_ambT_sky
Paolo Silva
90 The Parabolic Trough Collector: Thermal Efficiency
An example of sensitivity analysis as a function of the HTF temperature: model VS real measured data
Paolo Silva Andrea Giostri – Claudio Saccilotto
Parabolic Trough: on-design sizing
Example of sizing of an indirect cycle plant (synthetic oil Therminol VP1)
• Net electric power: 50 MWe
• Technology: Absorber tube Solel, Structure Eurotrough, Mirror Flabeg
• n° SF section: 4 (“H”)
• SF global thermal efficiency: 65,61%
• Electric efficiency of PB: 31,75%
• Net electric efficiency (solar-to-electricity): 20,83%
• Solar Field Area (mirrors): 299985 m2
• SF Size LxW: 1140 m X 649 m
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
Yearly simulation of the plant
N-S
E-O
• N-S higher annual electricity production
• E-O greater constancy of productivity
through the year
0
2000
4000
6000
8000
10000
12000
14000
ENER
GIA
[MW
h]
E-O
N-S
MEDIA E-O
MEDIA N-S
Effect of axis orientation:
Paolo Silva
Paolo Silva Andrea Giostri – Claudio Saccilotto
N-S axis orientation (site of Almeria (Spain) (36° 50′ 0″ N, 2° 27′ 0″ W))
TIPOLOGIA IMPIANTO
ENERGIA ELETTRICA [MWh]
ORE EQUIVALENTI [h]
CICLO INDIRETTO AD OLIO 94’045 1’881
CICLO INDIRETTO A SALI FUSI 81’442 1’629
CICLO IBRIDO DSG + OLIO 91’836 1’837
CICLO IBRIDO DSG + SALI 78’819 1’576
E-O axis orientation (site of Almeria) TIPOLOGIA IMPIANTO
ENERGIA ELETTRICA [MWh]
ORE EQUIVALENTI [h]
CICLO INDIRETTO AD OLIO 83’179 1’664
CICLO INDIRETTO A SALI FUSI 71’774 1’435
CICLO IBRIDO DSG + OLIO 82’224 1’644
CICLO IBRIDO DSG + SALI 70’988 1’420
for both cases electricity
production is higher with N-S
orientation
Paolo Silva
Yearly simulation of the plant
Paolo Silva Andrea Giostri – Claudio Saccilotto
Effect of geographic coordinate: Sites located at different latitude
0
2000
4000
6000
8000
10000
12000
14000
ENER
GIA
[MW
h] E-O
N-S
MEDIA E-O MEDIA N-S
0 2000 4000 6000 8000
10000 12000 14000
ENER
GIA
[MW
h] E-O
N-S
MEDIA E-O
MEDIA N-S
Dakar (Senegal) (14° 41′ 0″ N, 17° 27′ 0″ W) DNI = 1947 kWh/m2
Almeria (Spain) (36° 50′ 0″ N, 2° 27′ 0″ W) DNI = 1957 kWh/m2
• Influence of the angle of incidence • Meteorological differences
Paolo Silva
Yearly simulation of the plant
Paolo Silva
Cantiere
Collettori
Accumulo
Generatore vapore
Gruppo termoelettrico
Ripartizione dei costi per sottosistemi
Investment Cost parameters
€/m, €/m2
€/m2
€/kg, €/m2 (…)
Ricevitori
Pannelli riflettenti
Struttura
Inseguimento
Piping collettori
Controllo
Piping esterno
Fondazioni
Altre opere civili
Fluido termovettore
Solar Field cost=270 €/m2 Total Investment Cost
(EPC) = 3500 €/kWel
95 Analysis of system cost allocation for PT plants
Paolo Silva
Presentation Outline 96
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
The Linear Fresnel collector
97
Paolo Silva
98 98
• The concept: approximation of the parabola with a series of flat mirrors
• The origin: a technology with “Italian” Roots: First Patent on LFR by G.Francia in 1962
The Linear Fresnel collector
Paolo Silva
• The direct solar radiation is focused by a series of mirrors on a linear absorber tube at a certain height above the mirrors • Inside the absorber tube a heat transfer fluid circulates, which is gradually heated: it represents the heat source of a thermodynamic cycle similarly to what happens in a parabolic-trough system
Andrea Giostri – Claudio Saccilotto Paolo Silva
99 The Linear Fresnel collector
Paolo Silva
The REFLECTORS
• LFR use flat (or elastically bent) mirrors (w=0.5-1m) • The mirrors are cheaper , lighter and easier to clean • The number of mirrors can variate between 10 and 50 (higher CR)
100 The Linear Fresnel collector: the Mirrors
Paolo Silva
The SUPPORTING STRUCTURE The supporting structure is lighter beacuse the mirror are lighter Less foundation (lower wind load) The absorber tube is still above the primary mirrors (5-10 m) The absorber tower is kept still with a series of beams and steel wires
101 The Linear Fresnel collector: Supporting Structure
Paolo Silva
Fresnel collector: heat collection element (absorber)
Andrea Giostri – Claudio Saccilotto Paolo Silva
Glass tube
Absorber tube
Single absorber tube Secondary concentrator
Absorber tubeGlass plate
Secondary receiver
Cavity
Absorber tubes
Insulation Insulation
Paolo Silva
The ABSORBER The absorber tube can be either evacuated or non evacuated Can be a single tube or a boundle of smaller tubes Can mount a secondary reflector above it to enhance ray interception
103
SUPERNOVA 1 ENEA
Evacuated absorber are the same used for PT collectors, but usually have also the secondary reflector
The Linear Fresnel collector: the Absorber tube/tubes
Paolo Silva
The ABSORBER • The absorber tube can be either evacuated or non evacuated • Can be a single tube or a bundle of smaller tubes • Can mount a secondary reflector above it to enhance ray interception
(CPC, Circular, Two wings... )
104
Insulation Insulation Single absorber tube Bundle of absorber tubes
Secondary Mirror Secondary Mirror
NOVA 1 AUSRA- AREVA
The Linear Fresnel collector: the Absorber tube/tubes
Paolo Silva
The ABSORBER • The absorber tube can be either evacuated or non evacuated • Can be a single tube or a bundle of smaller tubes • Can mount a secondary reflector above it to enhance ray interception
(CPC, Circular, Two wings... )
105
Non evacuated absorber are sealed inside the secondary reflector cavity (filled with air) to reduce thermal losses (convection) They are usually used for low/medium temperature (300°C)
The Linear Fresnel collector: the Absorber tube/tubes
Paolo Silva
Nominal Optical efficiency • Takes into account:
• Reflectivity of the primary and secondary mirror • Transmissivity of the cover glass or envelope glass • Absorptivity of the absorber tube • Cosine effect (a) • Shading (b) • Blocking (c)
106 The Linear Fresnel collector: Optical Efficiency
Paolo Silva
0
0.2
0.4
0.6
0.8
1
0 15 30 45 60 75 90
IAM
Incidence Angle [°]
IAM(θi)IAM(θ⊥)
107
Off desing efficiency
The Linear Fresnel collector: Optical Efficiency (off-design)
Paolo Silva
108
0
0.1
0.2
0.3
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0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85
Effic
ienc
y
Transversal Incidence Angle (°)
Cosine losses Shading/blocking spillover from primary mirrors Spillover from secondary mirror Overall optical efficiency
Scomposition of the optical losses with the variation of the transversal inxidence angle
The Linear Fresnel collector: Optical Efficiency (off-design)
Paolo Silva
The Linear Fresnel Collector: Thermal efficiency
109
• The thermal efficiency is defined as the thermal efficiency for parabolic trough and the same parameter ifluence the thermal performance of a LFR • Linear Fresnel Reflector that use evacuated absorbers can be modeled in the
same way made for PT.
• Linear Fresnel with non evacuated absorber can be modeled with more complex electric circuits. Also convection within the cavity has to be considered
Paolo Silva
110
• The thermal efficiency is defined as the thermal efficiency for parabolic trough and the same parameter ifluence the thermal performance of a LFR • Linear Fresnel Reflector that use evacuated absorbers can be modeled in the
same way made for PT.
• Linear Fresnel with non evacuated absorber can be modeled with more complex electric circuits. Also convection within the cavity has to be considered
• Another option is the simulation of the secondary reflector and of the absorber tube using a CFD software
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
7.2
7.25
7.3
7.35
7.4
7.45
7.5
7.55
y [m
]
20000
40000
60000
30
210
60
240
90
270
120
300
150
330
180 0
Heat flux [W/m2]
The Linear Fresnel Collector: Thermal efficiency
Paolo Silva
Heat transfer fluids for Linear Collectors
111
Moving parts critical for solidification of salts The receiver and the connecting pipe are still
Types of Heat Transfer Fluid (HTF) • Synthetic oil (Therminol VP1, Dowtherm A): Tmax ~ 400°C
Is the ‘state of art’ HTF • Molten Salts: there can be solidification problems
Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C
Paolo Silva
112
Types of Heat Transfer Fluid (HTF) • Synthetic oil (Therminol VP1, Dowtherm A): Tmax ~ 400°C
Is the ‘state of art’ HTF • Molten Salts:
Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C • Steam/Water (DSG): High temperature mean high pressure. More promising for the Fresnel technology than PT.
Heat transfer fluids for Linear Collectors
Paolo Silva
Types of Heat Transfer Fluid (HTF) • Synthetic oil (Therminol VP1, Dowtherm A): Tmax ~ 400°C
Is the ‘state of art’ HTF
113
• Molten Salts: Solar Salts (%m/m: 60% NaNO3, 40% KNO3) Tmax ~ 600°C Hitec (%m/m: 7% NaNO3, 53% KNO3, 40% NaNO2) Tmax ~ 535°C • Steam/Water (DSG): High temperature mean high pressure. More promising for the Fresnel technology than PT.
• CO2 or other gas: just experimental facilities and some research on it There can be problems with the temperature distribution on the absorber tube
°C
0
20
40
60 Power per degree PT [W/m/deg]
LS-2, CR=22.74
Power per degree LFR [W/m/deg]
Heat transfer fluids for Linear Collectors
Paolo Silva
- Cheaper - Less land occupation - Easier to clean - Less drag-wind effect - More feasible for DSG and use of Molten Salts - No ball joints - Less tracking power consumption
Possible disadvantages of LFR VS PT - Lower on-design efficiency - Penalties at partial load
114
Possible advantages of LFR VS PT
114 Linear Collectors Comparison
Paolo Silva
115
The Andasol plants (Parabolic Trough): http://www.youtube.com/watch?v=bxCUYPzHsug The Puerto Errado 1 plant (Linear Fresnel Collector) http://novatecsolar.com/44-1-NOVATEC-TV.html
Linear Collectors Comparison: videos
Paolo Silva
Presentation Outline 116
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva Andrea Giostri – Claudio Saccilotto
Parabolic Trough: Solar Field
Paolo Silva
Paolo Silva
Purpose: to transport the fluid from the solar field to the power block with the minimum thermal dissipation (presence of insulation) and low pressure drops. It consists of three main elements:
Headers for distributing and collecting the fluid to the loops of the solar field “U” links inside the loops Connection from the headers to the steam generator of the power block
Used Materials Piping (carbon steel, P91, AISI 316L / 321H) Insulation (mineral wool low thermal conductivity)
118 Piping System and Solar Field Layout
Paolo Silva
Sizing criteria Input data
Solar field layout Mass flow rate of fluid maximum speed of the fluid Thermodynamic conditions at inlet/outlet (temperature, pressure) Specifications for pipes (ASME)
Output data
Pipe sizing (thickness, diameter) Thinckness of insulation pressure drops Heat losses to the environment
119 Piping System and Solar Field Layout
Paolo Silva
• Usally in solar fields using PT each loop has a ‘U’ shape so the outlet and inlet of the
collector are on the same side • Central feed configuration is usually used but also a inverse return configuration is
possible (even Dp in each loop, higher costs)
120
2 tubi 3 tubi
Layout of the solar field:
Third Tube
Central field Inverse return
Piping System and Solar Field Layout
Paolo Silva
“I” configuration the total flow rate is divided in 2 sections
SEZIONE DEL CAMPO SPECCHI
POWER BLOCK
COLLEGAMENTO CAMPO SPECCHI PBLATO CALDO
HEADER CALDO
FILA
HEADER FREDDO
LOOP
POMPA CIRCOLAZIONE
121
Layout of the solar field:
Piping System and Solar Field Layout
Paolo Silva
Layout of the solar field:
POWER BLOCK
L_specchi
COLLEGAMENTO CAMPO SPECCHI PBLATO CALDO
HEADER CALDO HEADER FREDDO
FILA
LOOP
SEZIONE DEL CAMPO SPECCHI
COLLEGAMENTO CAMPO SPECCHI PBLATO FREDDO
“H” configuration the total flow rate is divided in 4 sections
122 Piping System and Solar Field Layout
Paolo Silva
Andasol I (Spain) – 50 MW 123
Paolo Silva
Piping and layout of the solar field
• LFRs have greater focal lenght and thus greater end-losses. • Usually LFRs use long parallel rows (few plant built)
Layout of the solar field:
PB
Paolo Silva
• The diameters of the connecting pipe change after every loop (theoretically) • The optimum piping size is a compromise between investment costs (material)
and operating costs (pumping power) • Extra meters of piping are needed to compensate for thermal expansion and
to allow access to each loop
Piping characteristics:
Piping System and Solar Field Layout
Paolo Silva
Presentation Outline 126
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
Efficiency Parameters
The efficiency parameters typically used to define the energy performance of a concentrating solar thermal power plant are: Optical efficiency of the solar field: Thermal efficiency of the solar field: Piping Efficiency:
127
Paolo Silva
Net Power Block efficiency:
Solar-to-electric efficiency (“overall efficiency”)
128
, , , _ , __
el turb el pumps el aux cond el net PBnet PB
boiler boiler
E E E EE E
η− −
= =
, _ , __
, _
el net PB el aux SFaux SF
el net PB
E EE
η−
=
Efficiency of Solar Field auxiliaries
Efficiency Parameters
Paolo Silva
129
PNET
ESUN
Optical losses E@absorber
EHTF
Collector Thermal losses
E@BOILER
Piping Thermal losses
PGROSS Condenser
losses
∆H PUMP SF
PAUX SF
PAUX + other losses
129
∆H PUMP
Efficiency Parameters
Paolo Silva
130 Solar field control strategy (off-design)
Power increases with HTF max T
Power increases at constant HTF temp. with mass flow
Best control strategy: HTF outlet temperature is fixed at its nominal value (es. 390°C) and the HTF mass flow rate is varied at part load, until the minimum HTF mass flow is reached; for lower sun radiations HTF mass flow is kept constant reducing HTF temperature at the solar field outlet. (optimal value of minimum HTF mass flow is about 50% of nominal value)
Paolo Silva
Presentation Outline 131
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
132
FLABEG
SOLEL
ET - 100
OPTICAL EFFICIENCY: 75%
132 LFR vs PT: the chosen PT tehcnology
Paolo Silva
133
OPTICAL EFFICIENCY: 67%
NOVATEC SOLAR
133 LFR vs PT: the chosen LFR tehcnology
Paolo Silva
134
I
A
D
H
C
G
B
F
E
HTF: Therminol VP1
Tmax HTF =391°C p = 13 bar
Tin turbine=371°C p = 100/18.3 bar Tcond=41.5°C
p = 0.08 bar
Tcond= 235°C p = 100 bar
“H” configuration
134 LFR vs PT: overall plant layout – Indirect cycles
Paolo Silva
135
A
H
B
C
D
G
I
F
E
Tout SF=270°C p =55 bar
X = 0.8
Tcond=41.5°C p = 0.08 bar
T=196°C p = 66 bar
“I” configuration
135 LFR vs PT: overall plant layout - DSG
Paolo Silva
DSG 52.52 1.40 0.85 43 -
77.2 11.01 0.19 50
289,101 593,205
136
IND-PAR IND-FRE Gross power [MW] 54.87 53.45 Steam cycle aux cons. [MW] 1.72 1.68 Cooling tower aux cons. [MW] 0.65 0.63 Number of operating flow paths 72 75 HTF mass flow [kg/s] 616 601 Steam mass flow @ HP turbine [kg/s] 61.3 59.9 Pump head [bar] 23.34 10.86 Solar field aux cons. [MW] 2.51 1.14 NET POWER [MW] 50 50 Total SCA aperture area [m2] 235,899 268,596 Total required land area [m2] 683,902 594,465
ηoverall [%] 23.53 20.69 ηoverall_modified [%] 21.25 20.69
136
0 10 20 30 40 50 60 70 80 90 100
eta_overall modified (%)
eta_auxSF (%)
eta_netPB (%)
eta_piping (%)
eta_thermal (%)
eta_optical modified (%)
IND-PAR IND-FRE DSG
19.25 19.25
LFR vs PT: on design performance
Paolo Silva
137
0
0.2
0.4
0.6
0.8
1
0 15 30 45 60 75 90
IAM
Incidence Angle [°]
IAM(θi)IAM(θ⊥)
PARABOLIC TROUGH FRESNEL
137 LFR vs PT: solar field off design
Paolo Silva
138
IND-PAR IND-FRE DSG Available solar energy [MWh/y] 609464 696185 749336
Receiver solar energy [MWh/y] 321473 264173 289700 Net electric energy [MWh/y] 97818 74454 76136 ηoptical (%) 52.75 37.95 38.66 ηoptical_modified (%) 49.17 37.95 38.66 ηthermal (%) 92.73 85.82 92.08 ηpiping (%) 98.64 98.25 99.75 ηnet_PB (%) 34.45 34.00 28.77 ηaux_SF (%) 96.57 98.29 99.47 ηoverall (%) 16.05 10.69 10.16 ηoverall_modified (%) 14.96 10.69 10.16
138 LFR vs PT: yearly performance
Partial load penalization: PT: -32% LFR: -48% and -47%
Paolo Silva
Some of the of existing PT plants
IMPIANTO ANNO SITO W EL, NETTA [MW] FLUIDO S [m2]
SEGS I 1985 KRAMER JUNCTION (USA) 13,8 OLIO 82960
SEGS II 1986 KRAMER JUNCTION (USA) 30 OLIO 190338
SEGS III / IV 1987 KRAMER JUNCTION (USA) 30 OLIO 230300
SEGS V 1988 KRAMER JUNCTION (USA) 30 OLIO 250560
SEGS VI 1989 KRAMER JUNCTION (USA) 30 OLIO 188000
SEGS VII 1989 KRAMER JUNCTION (USA) 30 OLIO 194280
SEGS VIII 1990 KRAMER JUNCTION (USA) 80 OLIO 464340
SEGS IX 1991 KRAMER JUNCTION (USA) 80 OLIO 483960
DISS PROJECT 1996 ALMERIA (SPAGNA) 5 ACQUA -----
ARCHIMEDE 2007 PRIOLO GARGALLO (ITALIA) 5 + Accumulo SALI FUSI 30156
ANDASOL I 2008 ALDEIRE-LA CALAHORRA (SPAGNA) 50 + Accumulo OLIO 510120
NEVADA SOLAR ONE 2008 BOULDER CITY, NV (USA) 64 OLIO 357000
Plants under construction: 1562 MW (Spain and USA)
139
Paolo Silva
140
Find out about existing CSP plants on: http://www.nrel.gov/csp/solarpaces/by_technology.cfm
IMPIANTO ANNO SITE W EL, NETTA [MW] FLUIDO S [m2]
Puerto Errado 1 2009 Calasparra (Spain) 1.4 (gross) Saturated Steam (270°C) 25800
Puerto Errado 2 2012 Calasparra (Spain) 30 Saturated Steam (270°C) 302000
Kimberlina 2008 Bakersfield (CA, USA) 5 Steam 26000
Llo solar thermal porject 2012 Llo (France) 9 Steam 120000
Alba Nova 1 2012 Corsica (France) 12 Saturated Steam 140000
Fera 2010 Sicily (Italy 1 ? Saturated Steam ?
Some of the existing LFR plants
Paolo Silva
Plants under construction
25 75 110
1405
0
200
400
600
800
1.000
1.200
1.400
1.600
America del Sud USA Africa Europa
MW
Solar tower
Parabolic through
Europe and Spain in particular appear largely predominant…
141
Paolo Silva
The plants under planning
350 1050
5635
100
3116
1600
300
0
2.000
4.000
6.000
8.000
10.000
12.000
Austarlia Sud Africa Africa Europa USA
MW
Fresnel reflector
Parabolic dish
Solar tower
Parabolic through
... on the other hand considering the plants at the design stage (PPA: Power Purchase Agreement), the dominance of the U.S. is evident...
142
Paolo Silva
Presentation Outline 143
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
SOLAR TOWER Main Characteristics • 2 DOF point focus concentration • Suitable for high power output (> 10 MW) Typical components • Heliostat high reflectivity moving mirrors (ex. Sheliostat ~ 100 m2) that concentrate solar radiation over a heat exchanger positioned on the top of the tower, the “solar receiver” mirrors require supports and electromechanical devices, i.e. “trackers”, that are electronically activated
•Solar Receiver a heat exchanger positioned on the top of the tower in which the concentrated sunlight is converted in thermal energy that is transferred to the HTF finally the HTF gives the thermal input to the power cycle
Paolo Silva
Point focus systems: Solar Tower
Paolo Silva
• Tower its function is of supporting the solar receiver on the top. It can be made of steel (h < 120 m) or concrete (h > 120 m) Different technologies: • indirect system with molten salts Solar Two, Barstow (US) • indirect system with air TSA-Phoebus Project • direct system with air Refos Project • direct system with saturated steam (DSG) PS10 Project, Siviglia (ES)
Paolo Silva
Point focus systems: Solar Tower
Paolo Silva
• The concentration ratio is usually in the range of 500-1000 suns • The collector field is based on a large number of heliostats with a tracking control system to continuously focus direct solar radiation onto the receiver aperture area • Heliostats can be flat or also a parabolic shape with small curvature
• The optical efficiency of the solar field is equal to the ratio of the net power intercepted by the receiver to the product of the direct insolation and the total mirror area • Optical losses include the cosine effect, mirrors properties as reflectivity, shadowing, blocking, atmospheric attenuation and receiver absorbivity
Paolo Silva
Point focus systems: Solar Tower
mirrbeam
recopt AG
Q.
=η
Paolo Silva
CONFIGURATION: • North fields (the heliostats are just on the north side of the receiver): commonly used for high latitudes or high incidence angles •Surround fields (the heliostats surround the receiver): typical of location close to the equator
• Higher optical efficiency than linear collectors. Ex. Spain: nom. eff. = 77%, average eff. = 64%
Paolo Silva
Point focus systems: Solar Tower
Paolo Silva Andrea Giostri – Claudio Saccilotto
PS10 Concentrating Solar Tower, Sevilla (Spain) 10 MW electricity, DSG saturated steam 40 bar
Paolo Silva
Point focus systems: Solar Tower
Paolo Silva Andrea Giostri – Claudio Saccilotto
Point focus systems: Solar Tower (PS10 – Spain)
Paolo Silva
Paolo Silva
Indirect system with molten salts (%m/m: 60% NaNO3 - 40% KNO3 )
Solar Tower indirect cycle with direct storage (2 tanks)
Paolo Silva
Point focus systems: Solar Tower
Water steam Molten Salt Receiver
Outlet Temperature [°C] 250/525 566
Incident Flux [kW/m2] 350 550
Peak flux [kW/m2] 700 800
Maximum pressure [bar] 100-135 -
Thermal efficiency [%] 80-93 85-90
Paolo Silva
152 Solar tower (indirect cycle w/o storage)
SOLAR TWO (California)
Paolo Silva
Indirect cycle with air as HTF
Paolo Silva
Point focus systems: Solar Tower
Paolo Silva
Plant with gas turbine
Paolo Silva
Direct cycle with air (pre-heating of air coming from the compressor of a gas turbine)
Paolo Silva Paolo Silva
Point focus systems: Solar Tower
Paolo Silva
ISCC
Paolo Silva
ISCC (integrated solar combined cycle)
Paolo Silva
Beam-Down solar concentration
Paolo Silva Paolo Silva
Other concepts
BOILER
Heliostats
Central reflecting system
Heliostats
Solar Radiation Solar Radiation
Paolo Silva
Paolo Silva Paolo Silva
Solar tower data (source: NREL)
Paolo Silva
Paolo Silva Paolo Silva
Solar tower data
Paolo Silva
Paolo Silva Paolo Silva
Solar tower data
Paolo Silva
Paolo Silva Paolo Silva
Solar tower data
Paolo Silva
Presentation Outline 162
The Solar Resource Introduction on Concentrating solar power Concentrating solar power components
• Plant configuration, heat transfer fluids and storage • Linear Solar collectors
Parabolic Trough Collector Linear Fresnel Collector
• Piping system and power plant layouts Efficiency Parameters Linear Fresnel vs Parabolic Trough Solar Tower CSP plants Solar dish-Stirling systems
Paolo Silva
Point focus systems: Solar Dish
SOLAR DISH Main Characteristics • 2 DOF point focus concentration • Low power output for single collector (below 50 kW) Typical components • Concentrator paraboloid shape mirror with high reflectivity (ex. D=10 m, Pel ~ 25 kW) • Receiver delivers energy reflected from concentrator to the working fluid of the engine (designed for minimizing heat losses due to convection and irradiance)
Paolo Silva
Paolo Silva
Engine 2 different engines are adopted in commercial models:
•Stirling cycle pressurized circuit with a gas as working fluid (N2, He, H2) (Pmax ~ 20 Mpa, Tmax ~ 700°C, H2 and He yields high efficiency heat transfer)
• Joule-Brayton cycle low pressure gas as a working fluid (low efficiency heat transfer)
Paolo Silva
Point focus systems: Solar Dish
Paolo Silva
Dish Stirling
Paolo Silva
Paolo Silva SBP German/Saudi 50 kW
Cummins Power CPG 9 kW
STM e SAIC 25 kW
Dish Stirling
Paolo Silva
Dish Stirling – world efficiency record (31.3%)
Paolo Silva
Paolo Silva
168 Thermodynamics – ideal Stirling cycle
E
C
E
C
E
CE
E TT
QQQ
QL
−=−=−
== 11η ∫ ∫⋅1
2lnVVRT=
VdVRT=dVp=|L=|Q CCTT
1
4
3
2
1 2
3 4
Paolo Silva
Paolo Silva
169
CAUSES of NON-IDEALITY:
Heat transfer losses: (i) non-isothermal transformations (machines are adiabatic in nature, difficult to give / remove heat while the piston moves), (ii) heat exchanger ∆T (not infinite surfaces)
Fluid-dynamic losses: (i) pressure drops (regenerator), (ii) losses due to dead
volumes (volumetric efficiency not unitary)
Losses due to kinematics: adopted kinematics doesn’t allow to fully perform isochores
Electrical and mechanical losses
Paolo Silva
Thermodynamics – real Stirling cycle
Paolo Silva
170 Working fluid
Working fluid requirements:
Good heat transfer coefficients Low viscosity Thermal stability Low cost
Light-molecule fluids as H2 or He meet these requirements, but they do have
problems to escape through seals. H2 is also highly flammable
Heavy-molecule fluids as air or N2 are less suitable (bring lower η), however, is easier to make seals (corrosion at high T due to O2 presence in air has to be taken into account)
Paolo Silva
Paolo Silva
171 Configurations
α β
Configuration α: separate hot and cold piston Advantages: simplicity of the kinematics, good volumetric efficiency Disadvantages: difficulty in obtaining good seals
Configuration β: two coaxial pistons Advantages: seal are more simple to obtain, seals are less subject to wear (the
hot piston , said "displacer" , is subject to a ∆P equal to the pressure drops in the regenerator, the power piston is cold)
Disadvantages : kinematics is more complicated, volumetric efficiency is lower (dead volumes)
Paolo Silva
Paolo Silva
172 Configuration α
The regenerator is made of a metallic mass traversed by the fluid in both directions (other than a surface heat exchanger!): pressure drops, low volumetric efficiency, and is also subject to clogging (thermal cracking of oil that deposites in channels)
Paolo Silva
173 Configuration α
Paolo Silva
Paolo Silva
174 Configuration β
Paolo Silva
Paolo Silva
175 Configuration β − heater
Sometimes heat pipes with sodium can be used: it has excellent heat transfer properties (evaporates at 800°C at about 5 bar) Disadvantages: It is highly flammable (risk in case of failure)
Paolo Silva
Paolo Silva
176 Configuration β − free piston
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