19-Solar Thermal Basics ECEGR 452 Renewable Energy Systems
Overview • • • • • •
Introduction Concentrating Solar Power (CSP) Plant Principles CSP Technologies Concentration Ratio CSP Efficiency Stagnation
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Introduction • Solar radiation is converted to thermal energy when it is absorbed by an object • Typical irradiance value is 1000 W/m2
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Introduction • Three types of solar-thermal systems Active solar heating: a separate collector is used Passive solar heating: collection is integrated into the design of a building Solar thermal engines: similar to active, but the thermal energy is used to drive an generator
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Concentrating Solar Power (CSP) direct solar radiation optical concentrator concentrated solar radiation
receiver
heat Heat engine
(Brayton/Rankine)
electricity Dr. Louie
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Solar Thermal Generation • Concentrator: none • Receiver: a small pool (1 m2) laying on the ground containing 100 liters of water • Assume: no reflection no energy lost to surroundings
GHI = 1000 W/m2
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Solar Thermal Generation • Ambient water temperature: 15o C • After one hour: 3.6 MJ of solar radiation have been absorbed • Assume: no reflection no energy lost to surroundings
• What is the temperature rise?
GHI = 1000 W/m2
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Solar Thermal Generation • How much has the temperature risen? T
Q 3.6MJ 8.637 C mch (100)(4186) (specific heat of water is 4186 J/K)
• We now have water at 23.6 oC • Can we use this heated water to create electricity?
GHI = 1000 W/m2
horizontal surface
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Solar Thermal Generation • We cannot use it to generate electricity efficiently • Temperature is too low
GHI = 1000 W/m2
horizontal surface
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Carnot Efficiency • Upper limit on the efficiency of a process operating between two temperatures is dictated by the Carnot Efficiency TL TH TL hc 1 TH TH
• Where hC: is the Carnot efficiency TL: is the cold reservoir temperature (K) TH: is the hot reservoir temperature (K)
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Solar Thermal Generation • Assuming the cold reservoir is at ambient temperature, then the maximum efficiency is TH TC 296 288 hC 3% TH 296
• Can you think of a more efficient design?
GHI = 1000 W/m2
horizontal surface
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Concentrating Solar Power (CSP) • Concentrator: lens Assume the area of the lens shadow is 2m2 (note this is NOT the surface area) Diffuse irradiance (assumed to be 100 W/m2 is not concentrated)
• Only direct radiation is focused • Irradiance on the pool: 1800W/m2
Gb = 900 W/m2
2m2
1m2 Dr. Louie
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Concentrating Solar Power (CSP) • What is the temperature of the pool after one hour under these conditions? same assumptions as previous case Tamb = 15 oC Gb = 900 W/m2
2m2
1m2
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Concentrating Solar Power (CSP) • What is the temperature of the pool after one hour under these conditions? T
Q 6.48MJ 15.5 C mch (100)(4186)
TH 30.5 C
hC
TH TC 5% TH
increased efficiency
2m2
1m2
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Concentrating Solar Power (CSP) • Concentrator: mirrors • Modeled from Snell’s Law • We will use GDNI, assuming the mirrors track the sun absorber direct irradiance
reflected
1
2
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Direct Normal Irradiance Direct Normal Irradiance: beam irradiance on a surface that is normal to the beam
direct normal irradiance
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Concentrating Solar Power (CSP) Angle of reflected beam determined from Snell’s Law
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Concentrating Solar Power (CSP) • Fundamental operating premise: Concentrated Solar Power (CSP) allows for “higher quality” energy to be converted due to the higher operating temperature of the heat engine • What CSP configurations can you think of?
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Types of CSP • Four commercial or pilot project types:
Parabolic trough collector (PTC) Centralized receiver systems (power towers) Dish/engine systems Linear Fresnel reflector
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PTC
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Power Tower
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Dish
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Concentration Ratio • Geometric concentration ratio AC C Arec
AC: area of collector Arec: area of receiver
• “area” is interpreted as aperture area • To heat a liquid to a high temperature, you need large collector area and/or a small receiver area
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Example • 25 mirrors with dimension 10m x 10m are used to concentrate beam irradiance (DNI) of 1000 W/m2 on a receiver that is 5m x 5m. Find the: geometric concentration ratio irradiation received by the receiver
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Example • 25 mirrors with dimension 10m x 10m are used to concentrate beam irradiance (DNI) of 1000 W/m2 on a receiver that is 5m x 5m. Find the: geometric concentration ratio irradiation received by the receiver 25 (10 10) 100 (5 5) irradiance 100 1000 100kW C
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Concentration Ratio • What prevents us from using an arbitrarily small receiver to increase the concentration ratio? C
AC Arec
• Consider a surface with aperture of 100m2 that focuses solar radiation on a receiver with area 0.001m2 • What is the concentration ratio?
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Concentration Ratio • What prevents us from using an arbitrarily small receiver to increase the concentration ratio? C
AC Arec
• Consider a surface with aperture of 100m2 that focuses solar radiation on a receiver with area 0.001m2 • What is the concentration ratio? 100 C 100, 000 0.001
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Concentration Ratio • What is the irradiance on the receiver (if GDNI =1,000 W/m2)?
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Concentration Ratio • What is the irradiance on the receiver? 100, 000 1000 W
m
2
100 MW
m2
• This greater than the irradiance at the surface of the Sun ( about 63 MW/m2)! • Second law of thermodynamics prevents this
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Concentration Ratio • The actual limit is 2
R C 45, 000 r
R: mean distance from the Earth to the Sun r: radius of the Sun
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Concentration Ratio • Actual concentration ratios are much smaller (< 5000), depending on the collector arrangement • It is useful to consider the size of the reflected image of the Sun • Sun does not appear as a point, it appears as a disc
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Concentration Ratio subtended solid angle of 0.53o
s 0.53
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Side Note • Angle subtended by the moon is also about 0.53o • This is why a “Ring of Fire” eclipse can occur
Source: National Geographic
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Reflected Image reflected image of the Sun will always have a target-to-mirror solid angle equal to 0.530 Sun’s rays cannot be focused to a single point! receiver
Sun’s image
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CSP Process • CSPs concentrate irradiation, GDNI, from the collector to receiver GC CGDNI
Gc = irradiance from the collector (W/m2) is the absorptivity (unitless, ≤ 1)
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CSP Efficiency • Temperature of the receiver, Trec, increases thermal energy, Q, is transferred to a working fluid in the receiver as Trec increases above the ambient temperature, Tamb, it begins to transfer heat via radiation (convection and conduction heat transfer are ignored) Thermal energy is lost How is the radiated power determined?
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CSP Efficiency • Recall the Stefan-Boltzmann Law: G T 4 W/m2
5.67 108 J/(sm2K 4 )
• If the object is not a black body, then: G eT 4 W/m2
e is the emissivity of the object (unitless)
• Surrounding environment is also radiating energy toward the receiver • net irradiance: 4 4 Grec e (Trec Tamb )
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CSP Efficiency • Net flow of heat: Q AC (GC Grec ) W 4 4 Q AC ( CGDNI e (Trec Tamb )) W
• Receiver efficiency: hrec Q
AC CGDNI
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CSP Efficiency • Efficiency: hrec Q
ACC GDNI
4 4 (Trec Tamb ) e CG DNI
• Assume Trec > Tamb • What happens to the efficiency of the receiver if: Concentration ratio (C) increases? Trec increases?
• What is the maximum efficiency?
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CSP Efficiency • Efficiency: hrec
Q
ACGDNI
4 4 (Trec Tamb ) e CG DNI
• assume Trec > Tamb • What happens to the efficiency of the receiver if: Concentration ratio increases? • efficiency increases
Trec increases? • efficiency decreases
• What is the maximum efficiency? Dr. Louie
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CSP Efficiency
Receiver Efficiency
Efficiency (%)
100 80
C = 1000
60
C = 100
40
C = 10
20 0
0
Stagnation temp
500
1000
1500
2000
Receiver Temp (C)
With: Tamb = 20 C; e = = .95; GDNI =770 W/m2 Dr. Louie
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CSP Efficiency • CSPs use heat engines • System efficiency: hCSP hChrec
• Carnot efficiency increases with temperature hC
Trec Tamb Trec
• Which efficiency dominates?
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CSP Efficiency
Total CSP Efficiency
Efficiency (%)
100 80 60
C = 1000
40
C = 100
20 0
C = 10 0
500
1000
1500
2000
Receiver Temp (C)
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Receiver efficiency: hrec
4 4 (Trec Tamb ) e CG DNI
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Receiver efficiency: hrec
4 4 (Trec Tamb ) (6734 2934 ) e 1 76% CGDNI 70 800
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Carnot efficiency: hC
Trec Tamb Trec
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Carnot efficiency: hC
Trec Tamb 673 293 56% Trec 673
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Total efficiency: hCSP hChrec
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CSP Efficiency Example • A typical concentration ratio for a PTC CSP is 70. Find the receiver efficiency, Carnot efficiency and total efficiency if: Trec = 400 oC, Tamb = 20 oC; e = = 1; GDNI =800 W/m2
• Total efficiency: hCSP hChrec 42%
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CSP Efficiency
Total CSP Efficiency
Efficiency (%)
100 80 60
C = 70
40 20 0
0
500
1000
1500
2000
Receiver Temp (C) Dr. Louie
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