Solar PV Stand Alone Design Project

1

Solar Energy: Photovoltaic Modules II

Renewable Energy Technology and

Systems

1

• For individual solar cells, we saw that they can be

connected in series adding the voltages, and in

parallel to add the currents

2

Arrays and Connections

• Same principles

apply for arrays of

modules as with

cells– treat each

module as a higher

voltage/current cell

3

• Blocking diode is simply a diode

in series between modules and

batteries

• Must be sized to handle array

current

• Introduces a voltage loss (forward

drop ≈ 0.5 – 0.6V) which is

significant for lower voltage

systems (≈5% for a 12V system)

• Higher voltage systems typically

incorporate blocking diode in

battery charge controller

Power Conditioning: Blocking Diode

Blocking Diode:

• Only allows current out

of the module array

• Prevents discharging of

batteries during non-

producing times

• Prevents current flow

into shaded module

from a parallel one

Module Connections: Blocking Diodes

5

Negative effects of increased temperature:

• Reduced power output (≈0.5% decrease in max power per oC for

silicon)

• Module stresses increase because of different material expansion

coefficients

• Degradation and failure mechanisms increase with temperature

Module temperature determined by balance between gain and loss:

• Heat gain sources: Absorbed radiation and Joule heating 

depends on insolation level and module operating conditions

• Heat loss mechanisms: conduction, convection and radiation 

depends on air temperature, wind speed, and details of module

construction

PV Module Temperature

6

Heat Loss Mechanisms:

• Conduction

• Convection

• Radiation loss

Heat Gain:

• IR absorption (sub-bandgap)

• Efficiency

• Absorption by module

PV Module Temperature

7

1. NOCT (Nominal Operating Cell Temperature) = temperature of open circuited

cells in a module with irradiance = 80 mW/cm2, air temperature = 20 °C, wind

velocity = 1 m/s, and mounting = open back side

2. NOCT varies with module construction. NOCT for a typical Si module = 48 °C

3. Empirical equation to determine cell temperature, given NOCT (°C), insolation (S)

in mW/cm2, and Tair (°C):

4. Example: Hot day in Phoenix, Tair = 45 °C, S = 100 mW/cm 2 , NOCT=48 °C

Tcell = 80 °C

Power relative to 25 °C rating = P/P25=76%

STT 80

20-NOCT aircell 

Sifor C/005. 1 o

 dT

dP

P

PV Module Temperature Estimation

8

Temperature increases, above ambient levels, with increasing solar irradiance for

different module types (J. Ross and M. Smokler, "Flat-Plate Solar Array Project Final

Report", Volume VI: Engineering Sciences and Reliability: JPL, pp. 86-31, 1986.)

NOCT=58 C

NOCT=33 C

NOCT=48 C

PV Module Temperature

9

SPR-205-BLK (sunpower)

Residential PV Module:

• 72 series-connected A-

300 solar cells with

conversion efficiency

up to 21.5%

• high-transmission

tempered glass; EVA

encapsulation; black

anodized aluminum

frame

Module Specifications (example)

10

Manufacturer's guarantee ≈ 20 years (for silicon module), but…

Category Component Cause

Cell degradation:

• Increases in series resistance

• Decrease in shunt resistance

• Metallization degradation

(corrosion, peeling, migration)

• Cell cracking

• Moisture ingress

• Thermal stress

• Physical damage (wind, hail

..)

• Latent cracks

Cell failure:

• Shorts

• Hot spot / thermal runaway

• Open circuit

• Cell to cell interconnect

• Mismatch / bypass diode

failure

• Metal corrosion / fatigue

• Thermal cycling

• Shading / inadequate design

• Moisture

• Physical damage

Module failure:

• Open circuit

• Shorts

• Delamination

• Hot spot / thermal runaway

• Physical damage

• Interconnect and wiring

• Encapsulation material

• Bypass diode failure

• Glass and frame

• Insulation aging

• UV damage to encapsulation

• Bypass diode overheating

• Cell mismatch / shading

• Hail, vandalism, wind, …

Failure Mechanisms

use inspired · transdisciplinary · intellectual fusion · social embeddedness

• PTL performs photovoltaic module qualification testing and related activities. Its staff is experienced in the analysis and full testing sequence of commercial Si solar cell technologies required by IEC 61215, IEC 61626, IEEE 1262, and UL 1703.

• PTL clients/partners span the globe, from the USA, Canada and Mexico to countries in Europe, South America, Asia and, of course, Australia. The modules they submit for qualification testing encompass all technologies both in marketplace and under development.

ASU Director: Govindasamy Tamizhmani (Mani)

Photovoltaic Testing Lab

PV Testing

Photovoltaics Test Laboratory

Solar Simulator Outdoor Exposure Environmental Chamber

24/7 Light Soaker 1000’s of Test Samples Wind/Snow Load Tester

Summary: Solar Modules

• Solar modules may be connected in series or parallel, and the

corresponding voltage and current modeled using the equivalent

circuit model

• The DC operation of solar cells with batteries and other cells

requires connections with blocking and bypass diodes

• The temperature of the module negatively affects performance, the

module temperature is a function of energy in (solar) and the

convective/conductive losses of the module

• The NOCT is a figure of merit defining how high a temperature a

module reaches under normal operating conditions

• Performance and lifetime testing of solar modules is critical in order

to warrantee performance over 20 years or more

13

1

Solar Energy: Photovoltaic Applications

Renewable Energy Technology and

Systems

1

Applications

• Several different ways to classify systems: based on size or based

on type of user.

• Common system classifications:

o Small consumer products

o Remote area power supplies

o Power for developing countries (village power)

o Residential and building integrated power

o Utility scale power

Consumer Products

• Cost of PV less of a factor and

more to do with perceived

value added to product.

• Added value usually related to

battery trickle charging,

transportable power supply or

addition of convenient small

power source

• Technology is often amorphous

silicon

• Very stable and competitive

market

• Solar cells can be high cost

(US $14 in 1994)

Consumer Products

Consumer Products

• Rechargeable lantern

systems are useful and cost

effective in developing

countries

Remote Area Power Supplies

• PV-based stand-alone power systems can be usefully divided into:

o industrial and private remote area power supplies (PV or hybrid

RAPS systems)

o PV in developing countries

• In 1993, remote power accounts for 80% of total PV market and 90%

of crystalline silicon market.

Industrial RAPS Systems

• Significant fraction of RAPS market is industrial applications.

• Applications include telecommunications, navigation, road signs, and

other general power requirements

• Advantages of PV are robust, reliable technology that can operate

unassisted.

• Key reason for installing PV is economics.

Industrial RAPS Systems

Home RAPS Systems

• Home-based PV systems are economic as utilities charge heavily for

grid extensions and maintenance.

• In general, PV economic for a single home if more than 1.5 km from

grid.

PV in Developing Countries

• PV addresses problems such as

health, education, lifestyle,

independence, movement to

cities.

• Key issue is that while PV is

economic for many applications,

the ability to afford it may be

relatively low.

• “Microeconomics” useful in

increasing accessibility of PV

• PV used for water pumping,

village electrification, solar light

systems, medical applications,

lighting and communications.

PV in Developing Countries

PV in Developing Countries

• Solar electric lighting systems

are lower cost than alternatives

(kerosene, candles)

• Financing systems (“micro-

credit”) offer way to afford large

initial costs.

Grid-Connected PV

• Grid connected PV today is a rapidly expanding market, with growth rates of 30%

• Reasons for installing PV include environmental and political attitudes of end users, and specific technical goals (distributed generation, peak power)

• Markets include:

o Building-integrated PV

o Residential PV

o Utility-scale PV

• Significant differences in installed equipment

o Inverters need to take into account islanding

Building-Integrated PV

• BIPV systems can have reduced costs since

cost of balance of system components (module

mounting, etc) are reduced.

• PV is also often lower cost than other building

cladding materials

• Specific modules for BIPV (roof tiles, colors)

• System attributes also depend on architectural

details – i.e., roof angle, etc.

Building-Integrated PV

• BIPV systems rapidly growing in Europe.

• In high latitude locations,still get good

power generation from nearly vertical

building facades, allowing them to be

installed in many locations on the building.

N

E

S

70%

60%

50%

40%

30%

º 30 10ºº50º70º90

80%

95%

90%

20%

Building-Integrated PV (BIPV)

Residential PV

• Eliminates need for batteries or storage

• All generated power is used

Residential PV

• Key parameter for success is level of subsidy required

• Typical value is on the order of 25-30%

• Solar Villages another option for residential PV

Utility Scale PV

• Fastest growth of PV worldwide

• Over 500 GW of utility scale

• Geographic distribution smooths

local intermittancies

Utility Scale PV

Space

• The ultimate “remote” application

• Key features are radiation resistance and specific power

• Technology is often high efficiency III-V multijunction cells

Summary: Photovoltaic Applications

Main applications of photovoltaics include

o Small consumer products

o Remote area power supplies

o Power for developing countries (village power)

o Residential and building integrated power

o Utility scale power

22

1

Solar Energy: Photovoltaic Systems

Renewable Energy Technology and Systems

1

Photovoltaic Systems Outline

o Systems Design • System Components • PV System Topologies • Key Design Issues and Trade-offs

– System Performance Indicators – Variability of Solar Resource – Key Trade-offs

• System Design Methodologies – “Rule of Thumb” – Worst Month Calculations – Computer-Based Calculations

PV System Components • PV System components:

– Loads – PV Modules – Batteries/Storage – Power Conditioning – Balance of Systems

• Key issues in components: – Power loss in each component – Implications of component on maintenance, lifetime of

PV system – Matching of components – How addition of component affects system cost

4

Comparison of discharge time and power rating of various EES technologies (EPRI, from Science 334, 928 (2011))

Electrical Energy Storage Technologies

Components: Batteries Batteries

o Main energy storage used in conjunction with PV

o Lithium ion batteries are the most common in energy applications, particularly electric vehicles (EVs)

o Battery characteristics for renewable energy systems: • Battery Capacity:

– The energy available from the battery, in Ahr. To find total energy of battery, multiple by nominal battery voltage

– Available battery capacity depends on temperature, history of battery, charging regime

• Battery voltage: Typically 12V, sometimes 6 or 24V. If the system voltage is different, need to put batteries in series

• Battery State of Charge: – Fraction of the total battery capacity that is in the battery – Battery state of charge depends on temperature

6

The most critical battery characteristics for photovoltaic systems include the battery voltage and capacity, how it responds to charging and discharging, lifetime of the battery, maintenance requirements, and efficiency.

Battery Voltage: – Typically approximated by the open circuit voltage – Varies with discharging – Temperature dependent – Sometimes used to estimate state of charge

Battery Capacity – Charge stored by the battery – Often units of amp-hr because voltage varies – Convert to watt-hr by integrating over V(t) – Depends on rate of discharge, temperature, age, and history

Battery Efficiency (coulombic vs. voltage efficiency) – coulombic = ratio of charge extracted to that input during charging – voltage = difference between charging and discharging voltages

Energy Density – Capacity divided by weight or volume (watt-hr/kg or watt-hr/cm3)

Components: Batteries

Batteries Battery characteristics (cont’d):

• Depth of discharge: Fraction of the total battery capacity that can be withdrawn from the battery without damaging the battery. – Manufacturers specify daily and total depth of discharge

o Charging regime, including: maximum charge/discharge current, gassing voltage, voltage difference between charging and use

o Lifetime of battery

Batteries • Key decision factor is depth of discharge (the level to which a battery

may be discharged without damaged

o Shallow cycle batteries, which cannot be discharged without damaging the battery by more than 10 -20%

o Deep cycle batteries, which can be discharged to 40-60% on a regular basis

• Other battery characteristics: o Vented/sealed: In vented batteries, gasses generated are designed

to escape o Gelled/flooded electrolyte

• Critical that batteries are used according to application for which they were designed

o Car batteries, which do not cycle well, are very poorly suited to deep cycle application, and last short time

Batteries • Impact of battery parameters on design

o Shallow cycle means that in extreme circumstances, the battery can fully discharged, hence providing power to critical loads at the expense of battery lifetime • Often systems with critical loads have shallow cycle

batteries. • Typical choice is deep cycle batteries.

o Battery choice affects choice of power conditioning equipment: complexity and accuracy of charging circuit, ability to be overcharged

o Type of battery affects standards that system must follow: ventilation, temperature standards, etc.

Li-Ion Batteries o Originally developed for portable

electronics o Characterized by high energy

density and long cycle life o They are deep discharge batteries

which is more adaptable for widely varying weather conditions in remote PV applications

o Work better at low temperature than lead acid batteries

o Basic technology for EV applications due to favorable weight to energy ratio

o Inventors of the Li-Ion (Yoshino, Goodenough and Whittingham) received Nobel prize in 2019

Li-Ion Battery Cost ($/kWh)

Summary: Photovoltaic Components: Batteries

• There are a number of storage technologies for power systems ranging from microwatts to GWs

• Batteries are the most common storage used in conjunction with photovoltaics

• Desirable properties include low cost, high energy density, deep discharge capability, and long cycle lifetime

• Li-ion batteries are the current technology most adopted in PV and EV applications

• Cost reduction with time of Li-ion similar to PV

12

1

Solar Energy: Photovoltaic Power Conditioning

Renewable Energy Technology and Systems

1

2

• Power requirements and matching to PV module. • Functions of power conditioning – Prevent battery from discharging through the

solar modules (blocking diode) – Keep the module operating at its maximum power

point (maximum power point trackers, MPPT) – Optimize charging of the battery to prevent

damage to the battery (battery charger) – Converting from DC power to AC power (inverter)

Power Conditioning

3

• Function of MPPT is to maximize the power generated by the solar panel by ensuring it operates at its maximum power point.

• The characteristic resistance is defined as Vmpp/Impp, and equals the resistance of an load for which the PV panel operates at the maximum power point.

• But what happens with different light intensities if Rload is fixed?

Rload = Vmpp

Impp Rload

Power Conditioning: Maximum Power Point Tracking

Power conditioning • Maximum Power Point Tracking

o Basic circuitry: DC-DC converter, combined with circuitry to look at power generated as load point is varied.

Dotted lines are constant power curves, solid lines are the IV curve from the solar array. A load which does not intersect with the maximum power point of the solar array (which is near the knee of the IV curve), is transformed into one on the maximum power curve by a DC-DC converter.

5

• MPPT operates the PV module at maximum power as light intensity changes or as effective load resistance changes.

• Consists of two basic components: – high frequency (usually 20 – 80 kHz) DC-DC converter – microprocessor control to find the maximum power point – 93-97% efficient

From Photovoltaic Systems Engineering.

TriStar MPPT • peak efficiency of 99% • systems < 2.4 kWp • ≤ 45 amps / 150 volts PV open circuit • $420

Power Conditioning: Maximum Power Point Tracking

6

Power Conditioning: Pulse Width Modulation

• Pulse width modulation used in modern DC-DC converters (MPPT) and DC-AC inverters

• DC level converted to AC digital pulse width

• Filtering with capacitor produces AC sinusoidal wave

7

• Function of inverter is to convert low voltage DC power from PV array to AC power as required by many loads.

• Characteristics of inverters – Power quality

• Waveform shape • Frequency • Voltage regulation • Total harmonic distortion

– Input and output voltage ratings / protection – Power / duty rating and surge power rating – Efficiency = ratio of output to input power (varies with load) – Idle current – Audio and RF noise

• Connection and interaction of inverter with other AC components, other inverters

• Grid power – Cost

Power Conditioning: Inverters

8

• Two types of inverters: Stand alone and grid-tied • Grid-tied inverters can include batteries, but many do not.

– If no batteries, inverter can have different functionality – can perform maximum power point tracking for array (otherwise MPPT goes between array and batteries), but the input voltage may vary by a greater amount than when batteries are present.

– If batteries, grid can be considered another AC power source. • Regulatory constraints of grid-tied inverters: Grid-tied inverters

must meet specifications set by the utility. – Typically includes specifications on power quality – Islanding: if power failure occurs when PV array is producing

power, a grid tied inverter may produce an “island” of live grid in the otherwise down grid lines. • Possible dangerous for workers of grid-line • Grid-tied inverters must have provision to isolate PV system

from grid in event of power failure.

Power Conditioning: Inverters

9

Xantrex™ GT Series Grid Tie Solar Inverter GT 2.8

AC output voltage / range 240 V / 211-264 Vac

Max ac power out 2800 W

AC frequency nominal / range

60 Hz / 59.3 - 60.5 Hz

Harmonic distortion <3%

MPPT operating range 193 - 550 Vdc

Maximum efficiency 95%

Idle power 1 W

Warranty 10 years

Cost $1930

Cost / watt = $0.69

Power Conditioning: Inverters

10

PV

HFT

Cf CPV

Grid

Li LgLboost

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CDCLink

DCLink Isolated Boost Type DC-DC

Converter Full-bridge

Inverter LCL Filter

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Courtesy of R. Ayyanar, ASU

Power Conditioning: Residential Power

Summary: Inverters and MPPT Inverter/MPPT

o Power electronics used for MPPT and DC-AC inverters o Based on pulse width modulation o Critical parameters are:

• Type of waveform: square, semi-sine, sinewave • Power quality of waveforms: harmonics, DC offset,

ripple voltage • Rated power • Efficiency at rated power • Efficiency as a function of power • Surge power: power and duration of surge power.

1

Solar Energy: PV System Configurations

Renewable Energy Technology and

Systems

1

PV System Configurations

• Types of Systems

o Direct-coupled PV system

o DC PV system with storage

o DC-AC PV systems

o Hybrid PV systems

o Grid-connected PV systems

• Increasing components in a system decreases reliability due to component

failure, increases the losses due to inefficiencies in components, but in

general significantly improves the match between various components

• Adding additional power resources increases the availability and increases

fraction of solar power used

• Increasing components increases cost of a system, but generally reduces

the cost to achieve a given availability

System Configurations

• Direct coupled DC system

o Simplest type of system

o Low cost, due fewer additional components required

o Reliability can be very high

o Ability to use direct coupled system depends on

• Match between load and solar resource or ability to tolerate low

availability

• Tolerance of load to range of input

voltage and currents

• Load must have DC as input

o Examples: some home power systems,

direct drive application including water

pumping and ventilation systems

• DC photovoltaic system with storage

o Traditionally most common type of system, still extensively used in smaller

systems or specific purpose systems.

o Requires all DC appliances, but efficiency of DC appliances may be higher than

that of conventional appliances (though usually more expensive)

o Requires different wiring, connector, fuses, and built to different set of standards

o In most cases, charge controller

as well as battery included. Use of a

power point tracker depends on

load and system

o High voltage DC connections

and wiring requires caution

o Examples: specific use

industrial systems, small

home power systems

System configurations

System configurations

• DC-AC system with storage

o Most flexible system: can use any appliance at any time

o Usually just AC used, but DC loads can be treated separately if desired

o Efficiency depends on other components used in system

• Maximum power point trackers increase efficiency

• Efficiency of batteries and inverters (including operating point of

inverters) affects system performance

System configurations

• Hybrid: AC system with alternate generating system (usually diesel generator).

o Diesel only-system performs poorly under part-load conditions, both from

efficiency and maintenance standpoint

o Addition of batteries reduces requirement to run generator under part load

conditions

o Addition of solar (or other small generating source) reduces need for diesel

o Ideally, a availability system, in which large fraction of solar resource is used

o Examples: stand alone home/village systems

System configurations

• Grid connected systems

o No storage included in systems: when PV system generates, power is

either used locally at generation source or fed into the grid

o Inverter design must be substantially different for grid-connected

systems in order to meet power quality requirements and safety

requirements of utility

• Key concern is that PV system continues to power grid, causing

safety hazard (called islanding).

o Several types of grid connected systems:

• Residential PV

• Building integrated PV

• Utility-scale PV

𝑊/𝑚2

Summary: PV System Configurations

• PV systems are generally of the types below

o Direct-coupled PV system

o DC PV system with storage

o DC-AC PV systems

o Hybrid PV systems

o Grid-connected PV systems

• Main considerations are reliability, system efficiency

and cost related to particular application

1

Solar Energy: PV System Design I

Renewable Energy Technology and Systems

1

Photovoltaic System Design System Design

o To make a successful system we need • A well-designed system • Reliable, appropriate and matched components • Suitable maintenance regimes • Conforming to legal, social, etc. expectations, including

relevant standards • Ensure that expectations and maintenance is realistic through

education o Well designed system means

• Appropriate choice of basic system configuration • Choice of array size, tilt angle, battery size and other

components to give “best” choice among trade-offs, as indicated by performance metrics

Photovoltaic System Metrics

Metrics for system performance

o Availability (different from capacity factor): fraction of time that energy is available compared to time energy is required by the load

• Critical availability requires availability > 99%.

• Availability of 95% for conventional applications • Loss of load probability = LOLP = 1 – A (may also be called loss

of power probability, LOPP)

o Utilization of incident solar energy • Solar fraction: amount of energy provided by solar divided by

the total energy required by the system

• Array-to-load ratio: Ratio of the energy collected by the solar panels to the energy used by the load per day

– Measure of how big to size system – Typical 1.2:1 is used under worst month conditions

Photovoltaic System Metrics o Losses in system

• Losses in incident radiation due to optical losses and angular losses.

o Losses due to operating point of solar array

Photovoltaic System Metrics o Losses due to temperature (usually calculated separate from design):

• Solar cells loose power as they increase in temperature

• The operating temperature depends on the ambient temperature, the incident solar radiation, and the temperature loss mechanisms of thermal conduction, convection and radiative cooling

1 0004 0005

P dP dT

to C forSi m

m » - °( . . )/

( )44 ambobj TTH -=es ( )ambsurf TThAQ --=

T L A

H D=kThermal conduction k is the thermal conductivity in W·m-¹Kelvin-¹, T is the temperature differential across some distance L, A is the surface area

Convection

Radiative

h is the heat transfer coefficient in W·m-²Kelvin-¹, the T’s are the temperatures at the surface and ambient, A is the surface area

e is the emissivity (number), σ is the Stefan- Boltzmann constant, and the T’s are the temperature of the object and ambient

Photovoltaic System Metrics o Losses due to electrical losses in components: efficiencies of power

conditioning, batteries, wiring, etc. o Losses in loads themselves: may not be included in design

calculations unless designer also has some input into or control over load

• Efficiency: not that commonly used due to the fact that the efficiency will vary dramatically over a day, but may be used as benchmark or comparison between systems if operating point or average is well defined.

• Reliability: Depends on the operating conditions, the component choice, matching between components and maintenance

• Cost: both initial cost and lifecycle cost • Specific applications may also have other performance indicators or may

use only a subset of these indicators o E.g.: Availability not a useful in residential grid connected PV system

Variability in Solar Resource Impact of variability in solar resource

o A key element in renewable energy systems is the design of one component that has inherent variation (the solar resource) to drive another component (the load) in which the variation should be minimized as much as possible

o The larger the variation in the resource compared to the load, the more difficult the trade-offs • Some loads have a match to solar resources, but often higher

loads are encountered in months with lower solar insolation o Largest variation if the night-day (diurnal) cycle o Large variation in solar radiation means that in order to get higher

availability, the system has to have • Substantial storage component • Higher cost

Variability in Solar Resource Variability due to latitude

o Every latitude has on average 12 hours of sunlight, ignoring local cloud cover, etc.

o High latitude locations have a larger variation in peak sunlight hours between summer and winter

N um

ber of hours of sunlight

Variability in Solar Resource o Variation in photovoltaic systems due to (cont.):

• Climate: The pattern on the solar resource, as well as the average depends on the local climate

o Impact of variability: • Design is statistical – 99% availability does not mean that

the system will give power 99% of time every year

Summary: PV System Design • Well design PV systems makes appropriate choices

in terms of array size/tilt, and components, with regard to performance metrics

• Performance metrics include the availability, the solar energy utilization (solar fraction)

• Losses in terms of tilt, operation point (without MPPT), thermal effects, losses in components

• Solar resource variability- high availability requires storage unless grid tied