Optimizing the Photovoltaic Solar Energy C S d Cl d DCapture on Sunny and Cloudy Days

Using a Solar Tracking System

Nelson A. Kelly and Thomas L. GibsonCh i l S i d M t i l S t L b tChemical Sciences and Material Systems Laboratory

General Motors Research & Development CenterWarren, MI 48090

Annual Technical Meeting of the Council for Optical Radiation Measurements

(CORM)Las Vegas Nevada

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Las Vegas, NevadaMay 9-11, 2010

Statement of the Problem• Future transportation systems need to: 1) become sustainable, 2) be

removed from the air pollution and global climate change debates, and 3) reduce their dependence on petroleum. GM envisions future transportation with electric drive systems, such as fuel cell electric vehicles (FCEV) and y , ( )battery powered extended-range electric vehicles (EREV). If solar energy is used to make the hydrogen, i.e. solar-powered water electrolysis, or to charge the batteries, then the vehicle uses no fossil fuels and emits no pollutants. Solar energy is also one answer to the “terawatt challenge” for future energy needsfuture energy needs.

• We have addressed the optimization of the solar hydrogen and solar battery charging initiatives.

• In the present work we address the portion of that process involving optimizing the solar energy capture by a photovoltaic (PV) system (see Solar Energy, 83, 2009, pp. 2092-2102 for background information)

• Generally, solar tracking systems are optimized for sunny conditions when the solar disk is visible. A question of interest to those using solar energy on a day-to-day basis, is: can we improve the solar PV energy capture on

l d d ?

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cloudy days?

Solar Tracking Systems

• 2-axis tracking -- maximize the solar energy capture on sunny days by keeping the solar module pointed at the solar disk so the solar module is perpendicular to the beam normal solar radiation, Ibnp p , bn– Iθ = Ibn x cosine (Θ) = Ibn if θ = 0° (Θ is the angle between the

direct solar ray and a normal to the module surface)

hi h t l t DC l t i l– highest solar energy to DC electrical energy

• What happens on cloudy days?“open loop” tracker still points at the obscured sun using very– open-loop tracker still points at the obscured sun using very exact algorithms to “know” position of the sun in the sky

– Astronomically-determined location of the solar disk, i.e.,http://www.usno.navy.mil/astronomy p y y

– Random nature of clouds – no general model for solar positioning on cloudy days - empirical models are available

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Four Solar Arrays and Electrolyzer “GM Solar Hydrogen Fueling Station”

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Solar Measurements• Four solar arrays containing solar modules wired in parallel and

including a solar sensor mounted the plane of the array– PV system (Sanyo 190-BA3, 190 watt solar modules)– Solar sensors (LI-COR pyranometers, LI-200SL)

• Array tilt angle can be manually adjustedAll four with same tilt to see variability in measurements– All four with same tilt to see variability in measurements

– Four different tilt angles (57°, 42°, 27°, 0°) can be used to simulate 2-axis tracking

• Measure solar irradiance over a one-year period

• Analyze measurements to improve the solar energy capture and ffsmooth the day-to-day differences in solar energy capture

– Minimize system size and cost– Minimize storage (batteries, hydrogen)

Reduce day to day variability in solar energy due to clouds

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– Reduce day-to-day variability in solar energy due to clouds

Data Analysis

• Measure solar irradiance over a range of ambient conditions with four solar sensors (pyranometers) and four solar arrays (PV modules) with different four different tilt anglesmodules) with different four different tilt angles

• One array and sensor was always horizontal, H

• Near solar noon (sun directly south, β=180°), one of the arrays was nearly perpendicular to any direct solar irradiation (it has a tilt angle, Θ, such that the cosine error was negligible); this array was thus pointed directly toward the sun DTSwas thus pointed directly toward the sun, DTS

• We will analyze the data to determine the optimum solar collector orientation for sunny and cloudy conditions over the study periody y y p

• The variables we will utilize are the solar irradiances (W/m2) for the LI-COR sensors and short-circuit current , Isc, for the Sanyo modules for the H and DTS configurations and including the

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modules for the H and DTS configurations, and including the H/DTS ratio

Details on the LI-COR Pyranometer Sensor Pl t (I Pl S l I diPlacement (In-Plane Solar Irradiance

Measurement)

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Components of the Solar radiation on at a H i t l C ll t t th E th’ S fHorizontal Collector at the Earth’s Surface

Gh = Ibh + Idh

I I x cosine (Θ)Ibh = Ibn x cosine (Θ)

where Ibn is the beam normal radiation where Θ i th l ith l

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is the solar zenith angle

Solar Energy Models for Beam and Diff R di tiDiffuse Radiation

• Beam (direct) radiationO d 90% f h l l i b– On a sunny day, up to 90% of the total solar energy is beam radiation, so

– 2-axis tracking maximizes the collection of beam radiation by keeping the cosine term near 1, i.e.,p g , ,

– Iθ = Ibn x cosine (Θ) = Ibn if θ = 0°

• Diffuse (sky) radiation– On a cloudy day, nearly 100% of the solar energy is diffuse (sky)

radiation (Ibh = 0); – Isotropic Diffuse Model (Liu-Jordan Model)

I = I x (1 + cosine(Θ))/2Iθ = Idh x (1 + cosine(Θ))/2where θ is the tilt from the horizontal

– Iθ should be maximized for Θ=0 (0° tilt, horizontal surface)

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Specifying the Position of the Solar Disk Requires Two Angles, α and β

Some people use the solar zenith angle, Θ

Θ = 90° α

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Θ = 90 - α

Four Solar Arrays Used for TestingArrays with four different tilt angles

(57°, 42°, 27°, 0°)MAJOR POINT: AT SOLARMAJOR POINT: AT SOLAR NOON, ONE ARRAY WAS

ALWAYS VERY CLOSE TO A DTS CONDITION AND ONE ARRAY

WAS ALWAYS HWAS ALWAYS H

Same tilt on all four arraysSame tilt on all four arrays(and LI-COR sensors)

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Solar Irradiance with all Four Arrays Having Id ti l Tilt A lIdentical Tilt Angles

1000

1200

m2

600

800

1000

radi

ance

, W/m

Cloudy day – good agreement

0

200

400

6 8 10 12 14 16 18 20

Sol

ar Ir

r

350400450

, W/m

2

Cloudy day good agreement

6 8 10 12 14 16 18 20

Time of day

100150200250300

ar ir

radi

ance

,

Sunny day --- good agreement

050

6 8 10 12 14 16 18 20S

ola

Time of day

y y g g

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Solar Irradiance with all Four Arrays Having Diff t Tilt A l S DDifferent Tilt Angles on a Sunny Day

1200

800

1000

1200

ce, W

/m2

200

400

600

Sola

r irr

adia

nc

Licor #1

Licor #2

Licor #3

Licor #4

0

200

6 8 10 12 14 16 18 20

S

Time of day

LI-COR Tilt angle, ° Insolation(kWh/m2)

1 57 6.84

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2 42 6.423 27 5.534 0 3.41

Solar Irradiance with all Four Arrays Having Diff t Tilt A l Cl d DDifferent Tilt Angles on a Cloudy Day

300

350

200

250

300

ance

, W/m

2

Licor #1

0

100

150

Sol

ar ir

radi

a Licor #1Licor #2

Licor #3

Licor #4

0

50

6 8 10 12 14 16 18 20

Time of dayTime of day

LI-COR Tilt angle, ° Insolation(kWh/m2)

1 57 1.00

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2 42 1.083 27 1.204 0 1.30

Analysis of the H/DTS variable

• 181 days in the data base; 164 were used for the LI-COR and 161 for the solar arrays (Sanyo modules)

• One of the four LI-CORs and arrays was always H

• For a time period of ½ hour around solar noon (β = 180°), for the p (β ),south-facing arrays, one of the tilted arrays (57°, 42°, 27°) was within 10° of DTS

• The cosine error was only 1 5% for the DTS measurement• The cosine error was only 1.5% for the DTS measurement

• The solar irradiance was integrated for the ½ hour around solar noon for the H and DTS measurements to compute the solar pinsolation (kWh/m2) during this period

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H/DTS Ratio as a Function of the Solar Insolation

1.4

1.2

Aprily = 0.771 * x-0.146

1.0

DTS

ratio

pMayJuneJuly

AugustS t b

0.6

0.8H/D September

OctoberNovemberPower fit

0.40 0.1 0.2 0.3 0.4 0.5 0.6

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Insolation, kWh/m2, latitude tilt array

H/DTS Ratio vs. Tilt Angle on Sunny Days

1.0

0.8

0.4

0.6

/DTS

ratio

LI-COR

Sanyo

0.2

0.4

H/

cosine(θ)

0.00 10 20 30 40 50 60 70 80 90

Angle θ, degrees

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Isotropic Diffuse Model (IDM) Describing the Dependence of H/DTS on Array Tilt On Cloudy DaysDependence of H/DTS on Array Tilt On Cloudy Days

2 0

1.8

2.0

ys

LI-CORSanyo

1.6

clou

dy d

ay LI-COR, avg.Sanyo, avg.IDM

1.2

1.4

H/D

TS c

1.00 10 20 30 40 50 60 70 80 90

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DTS array tilt angle, degrees

Summary and Conclusions

• Four identical solar arrays and sensors were oriented at four different angles for a period of approximately 8 months at the GM Milford, MI P i G d i 2008 t t d th ff t f th tilt l thProving Ground in 2008 to study the effect of the tilt angle on the amount of solar energy captured.

• On sunny days, with predominantly direct solar radiation, the H/DTS ratio approximately obeyed the well-known cosine response law. On sunny days, H/DTS approached 0.5 at low solar altitude angles, i.e., t ki th ld i ld t i h l fi dtracking the sun would yield twice as much solar energy as a fixed horizontal tilt.

• On cloudy days or during cloudy periods, our analysis shows that 2-axis tracking will reduce the solar energy capture versus a horizontally tilted sensor (or array). We observed that the H/DTS

ti h d l f t 1 37 f th l di t d

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ratio reached values of up to 1.37 for the cloudiest days.

Summary and Conclusions• A simple model for diffuse radiation, referred to as the Isotropic

Diffuse Model, agreed well with the average angular dependence that we measured for the H/DTS ratio on cloudy days.that we measured for the H/DTS ratio on cloudy days.

• An optimized solar energy system would utilize 2-axis solar tracking during sunny conditions to capture the direct irradiance, but would orient the modules toward the zenith for cloudy conditions.orient the modules toward the zenith for cloudy conditions.

• In the past, the emphasis has been on optimizing the capture of direct solar radiation on sunny days because it is by far the largest overall component of the solar irradiance.overall component of the solar irradiance.

• Maximizing the capture of diffuse solar radiation on cloudy days is important in order to minimize the system size and level out the day-to-day fluctuations in system output. to day uctuat o s syste output

• The dramatic improvement on cloudy days is especially useful for a home solar-powered FCEV hydrogen fueling system or EREV battery charging system that needs to the energy for daily

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y g g y gy ycommuting on a continuous basis throughout the year.