ECE 7800: Renewable Energy Systems Topic 9: Photovoltaic System Design Spring 2010 © Pritpal Singh,...

ECE 7800: Renewable Energy SystemsTopic 9: Photovoltaic System Design

Spring 2010

© Pritpal Singh, 2010

Types of PV Systems

There are three types of PV systems:

1. Grid-tied systems (generally without battery backup)

2. Stand-alone systems (may or may not include auxiliary power source)

3. Direct PV systems (e.g. water pumping systems)

Grid-Tied PV Systems The general block diagram of a grid-

tied PV system is shown below:

Residential Grid-Connected PV Systems

City of Lakeland

City of Tallahassee

Courtesy: Kevin Lynn, FSEC

Grid-Tied PV Systems (cont’d) The power conditioning unit (PCU)

serves several useful functions:

• Converts DC output of PV array to AC line power.

• Prevents flow back of power from utility to PV array at night.

• Provides peak power tracking capability.

• Minimizes harmonics to the utility grid.

• Provides automatic disconnect if utility voltage is not detected.

Grid-Tied PV Systems (cont’d)Advantages of grid-tied PV systems:

• High reliability

• High PV efficiency because of maximum power tracking unit

• Integrated with building => no

additional land costs for the PV system

• Deliver peak power in the middle of the day when electricity costs are the highest

• No additional costs for batteries

Off-Grid Stand-Alone PV Systems A block diagram of an off-grid, stand-

alone PV system is shown below:

Examples of Stand-Alone PV Systems

Eco-Resort

PV Lighting

Courtesy: Kevin Lynn, FSEC

Off-Grid Stand-Alone PV Systems (cont’d) Stand-alone PV systems usually use

battery back-up for energy storage. There may also be an auxiliary power source such as a diesel generator. The system may be a dc or ac system. For both systems a battery charge controller is required to ensure that the batteries are neither over-charged nor over-discharged. With an ac system, an inverter is also required.

Off-Grid Stand-Alone PV Systems (cont’d)

Stand-alone PV systems are cost effective in remote locations where the alternatives are expensive grid extension (~ thousands of dollars per mile) or large, noisy generators with expensive fuel costs. These systems are not as efficient as grid-tied systems because of losses in the batteries. They also require maintenance and periodic replacement of the batteries over the lifetime of the PV system.

Direct PV Systems In direct PV systems, the PV array is

directly connected to the load. A good example of this type of system is a PV water pumping system.

Examples of Solar Water Pumping Systems

(http://www.agr.gc.ca/pfra/water/facts/solar.pdf)

Remote Village Water SupplyIn West Africa [Senegal]

Livestock watering (http://www.solarwater.com/applications.htm)

Direct PV Systems (cont’d) The solar panel is directly connected

to the PV pump and the water pumped whenever sufficient solar intensity is available. The water may be pumped into a tank for use when solar energy is not available.

Current-Voltage Curves for Loads The load to a solar panel may be

resistive or may be a motor load or battery load. We will consider each of these loads and how they map to the I-V characteristics of the solar panel.

Simple Resistive Loads A simple variable resistive load

mapped to the I-V characteristic of a solar module is shown in the figure below:

Need for Max. Power Point Trackers

As shown below, the maximum power point changes with solar intensity. More importantly, the maximum power point voltage shifts with temperature. Thus, a maximum power point tracker is required to shift the effective operating voltage of the array.

DC Motor I-V Curve The steady state equivalent circuit of a dc

motor is an armature resistance in series with a voltage source representing the back emf. as shown below:

DC Motor I-V Curve (cont’d) The current-voltage curve for a permanent

magnet dc motor is shown below. Notice the high starting current requirement compared to the steady state current.

DC Motor I-V Curve (cont’d) When the motor I-V curve is mapped onto

the I-V characteristics of the module (see figure below) it is clear that for low insolation conditions, the starting current cannot be achieved. A linear current booster can be used to provide motor starting at low insolation levels.

Battery I-V Curves A simple equivalent circuit of a battery

may be taken to be a voltage source in series with a resistance. The current-voltage curves during charging and discharging are shown below:

Battery I-V Curves (cont’d) Since the battery voltage increases with

increasing state-of-charge (SOC) the voltage curve moves to the right. This will shift the operating point relative to the solar module I-V curve. A self-regulating module with fewer cells can automatically reduce current as the battery approaches full charge (see figure below).

Maximum Power Point Trackers A maximum power point tracker is a

dc-dc converter that can step up or step down the voltage from the solar array to the load. It is a switch-mode power converter in which a switching element is turned on and off in a controlled manner to adjust the effective “turns ratio” of the dc transformer.

Maximum Power Point Trackers (cont’d)

A buck-boost converter allows the voltage to either be stepped up or stepped down. The topology of such a converter is shown in the diagram below:

Maximum Power Point Trackers (cont’d) When the switch is closed, input voltage

drives current through the inductor, energizing the magnetic field in the inductor.

When the switch is open, the magnetic field diminishes and current flows out of the inductor into the load.

The switching duty cycle determines the effective “turns ratio” of the dc transformer. It can be shown (see text for derivation) that the ratio of the output to input voltage, V0/Vi = D/(1-D)

Hourly I-V Curves It is useful when designing a PV

system to have information about the hourly insolation on the array and how this translates into the array’s output I-V curves. The hourly insolation data can be estimated as described in topic 4. The hourly I-V curves can then be easily estimated since the short-circuit current is directly proportional to the light intensity.

Hourly I-V Curves (cont’d) The voltage may also be corrected for

insolation variation. The I-V characteristics for a standard 1 sun (1 kW/m2) and for a 677W/m2 insolation levels are shown below:

Hourly I-V Curves (cont’d) Hourly curves with different load type

I-V curves superimposed are shown for a 40º latitude for a module tilted at a 40º angle in April in the figure below.

Hourly I-V Curves (cont’d) The energy delivered in each of the

three cases is shown in the below table:

Hourly I-V Curves (cont’d) The dc motor is well matched to the 1

sun curve but does badly in the early morning and late afternoon.

The 12V battery is consistently below the max. power point.

The MPPT delivers the maximum energy to the load.

Note: These figures assume that the operating temperature of the module is 25ºC which is not a realistic assumption (in most cases).

Grid-Connected Systems The principal components in a grid-

tied PV system are shown below:

Grid-Connected Systems (cont’d)

Large grid-connected systems may have a single inverter per string or may have one large inverter (see below).

The Utility Interface The ac output of a grid-connected PV

system is usually tied directly into the main ac distribution panel of the house. It can therefore supply power to the grid or the house. The electric meter spins backwards when power is being supplied to the grid and forwards when power is being taken from the grid. This setup is referred to as net metering. Sometimes two separate meters are used.

The Utility Interface (cont’d) As mentioned earlier, all power

conditioning units for grid-tied PV systems must drop the PV system from the grid in the event of a utility power outage. In such an event, breakers automatically isolate a section of the utility lines in which the fault has occurred, creating an “island”. The PV system must not provide power to this “island” since it could be dangerous to work crew who are repairing the line.

DC and AC Rated Power The ac power output of a grid-tied

PV system is derated compared to the dc rated output of the PV array for several reasons:

• Efficiency of the inverter

• Mismatched PV modules

• Dirt collection on the modules

• Differences in ambient conditions

DC and AC Rated Power (cont’d)

The ac output power of a PV system relative to the dc power available under standard conditions can therefore be expressed as follows:

Pac = Pdc,STC x (conversion efficiency)

Example 9.3

Note: PTC conditions (PVUSA testing conditions) are 1-sun irradiance, 20ºC ambient temperature) and 1m/s wind speed.

Peak Hours Approach The “peak hours” approach to

estimating PV array output considers the average number of peak sun hours available at a location to give an average output energy of the PV array in kWh/m2.day.

Example 9.4

Temperature Adjustment

Example 9.5

Annual Energy Production in Different Cities

Monthly Energy Production in Different Cities (Fixed Tilt and 1-axis Tracking)

Capacity Factors for Grid-Connected PV Systems The annual performance of a grid-

connected PV system can be expressed in terms of its capacity factor CF given by:

CF = hours/day of “peak sun”

24 hrs./day

The capacity factors for a number of US cities is shown on the nex

Capacity Factors for Grid-Connected PV Systems (cont’d) Capacity Factors for grid-tied PV

systems for a number of cities is shown below:

Grid-Tied PV System Sizing With the grid serving as the backup power

and energy storage, it is not critical to exactly design the PV system to match the load. However, we do want to be able to accurately predict the energy output of the system. Various factors come into play:

- the size of standard solar modules;

- the rooftop area and orientations;

- how many peak watts of dc power

are needed to meet the annual kWh

load;

- available inverter specifications.

Grid-Tied PV System Sizing (cont’d)

Example 9.6

Grid-Tied PV System Sizing (cont’d) We need to now consider real module and inverter characteristics. The below tables give this information for some modules/inverters:

BP380 Solar Module

Fronius Inverter


Let us extend the example 9.6 calculation taking real module and inverter specs into account (pp. 538-539).


Examples 9.8 and 9.9

Stand-Alone PV Systems Stand-alone systems must be

carefully designed to meet the load requirements since there is no grid backup power available. The first step in the design process is to estimate the loads that the system is being designed to meet.

Load Estimation

Load Estimation (cont’d)

Example 9.14

Load Estimation (cont’d)

Inverter and System Voltage

See text pp. 554-557

Batteries for PV Systems Various energy storage technologies,

including batteries, flywheel storage, compressed air and hydrogen are possible. However, batteries tend to be the best choice today. Although various battery technologies could be used, lead acid is the chemistry most widely used in PV systems. A comparison of battery characteristics is shown in the table on the next slide.

Batteries for PV Systems (cont’d)

Lead Acid Battery Basics The most commonly used battery is

the start, light, ignition (SLI) automotive battery. Its purpose is to provide short, high current discharges which create a shallow discharge (<20% DOD) of the battery. They typically can do this for over 500 cycles. Nowadays they are usually maintenance-free and are about 125Ah in capacity at approx. 12V.

Lead Acid Battery Basics (cont’d)

Deep discharge lead acid batteries have thicker plates than SLI batteries and have larger cases which allow more debris to collect at the bottom (without shorting the plates) and more room for electrolyte in the top of the cells. These batteries therefore tend to be relatively big and heavy. The number of cycles that may be delivered by the battery depends on the DOD to which it is subjected (see figure on next slide).

Lead Acid Battery Basics (cont’d) Deep discharge batteries typically use

antimony alloy in the lead plates rather than calcium alloy (which does not tolerate deep discharges well).

Lead Acid Battery Basics (cont’d) The lead acid battery plate reactions

during discharge are as follows:

Positive electrode:

PbO2 + 4H+ +SO4 2- + 2e- -> PbSO4 + 2H2O

Negative electrode:

Pb + SO4 2- -> PbSO4 + 2e-


Lead acid battery capacity depends on discharge rate and temperature as shown in the below figure:


Example 9.16


Batteries are wired in series and parallel combinations to achieve the voltage and Ah ratings required for the PV system.


The Coulombic efficiency of a battery is the ratio of coulombs of charge out of the battery to the coulombs of charge into the battery. For a lead acid battery, its Coulombic efficiency is approx. 90-95%. The voltage efficiency is the charging voltage/discharging voltage ≈ 86% for a lead acid battery. Therefore the energy efficiency of a lead acid battery = 0.86 x 0.90 = 0.77 (77%).

Battery Sizing for Stand-Alone PV Systems

Sandia National Labs has developed an excellent handbook of recommended design practices for stand-alone PV systems. A chart in that book (shown below) gives the number of days of battery storage as a function of peak sun hours.

Battery Sizing for Stand-Alone PV Systems (cont’d)

The two curves are for 99% availability (over the 8760 h in a year) for more critical loads and for 95% availability for less critical loads. The curves can be replaced by the following polynomial equations:

storage days ≈ 24.0-47.3 (pk. sun hrs.)

(99%) + 0.3 (pk. sun hrs.)2

storage days ≈ 9.43-1.9 (pk. sun hrs.)

(95%) + 0.11 (pk. sun hrs.)2

Battery Sizing for Stand-Alone PV Systems (cont’d) The figure refers to days of “usable

storage”. This is related to nominal storage capacity as follows:

nominal battery capacity (C/20, 25ºC)

= usable battery capacity

(max. DOD)x(T,discharge rate factor)

Example 9.18

Blocking Diode A blocking diode is used in a PV

system between the battery and the PV array to prevent the battery from discharging through the array at night. This results in a loss of about 0.6V for a silicon diode.

PV Array Sizing for Stand-Alone System

The PV array in a stand-alone system must be sized to maintain the battery bank in an acceptable SOC range. A 1-sun PV I-V curve with a vertical line for the battery I-V curve is shown below.

PV Array Sizing for Stand-Alone System (cont’d) The operating point of the PV is

almost always above the knee of the I-V curve during battery charging. Thus a fairly conservative estimate is to simply set the rated current of the PV as the battery charging current at 1-sun insolation.

PV Array Sizing for Stand-Alone System (cont’d) A simple sizing procedure is to employ

the concept of “peak hours” of 1-sun radiation. In this way, the rated current x # of peak sun hours, gives the amp-hours of charge provided to the batteries. A de-rating factor of 10% is usually used to account for dirt accumulation and module aging. We must also multiply by the Coulombic efficiency of the battery. Thus,

Ah to load = IR x pk. sun hrs. x Coulombic

efficiency x de-rating factor

PV Array Sizing for Stand-Alone System (cont’d) Modules are added in series to meet

the desired system voltage.

Example 9.20

Hybrid PV Systems

See text pp. 579-580

PV-Powered Water Pumping A wide application of PV is for water

pumping. In such systems, the PV array is directly coupled to the pump. A diagram of such a system is shown below:

PV-Powered Water Pumping (cont’d) On the electrical side, a voltage, V,

from the PV array drives a current, I, through the pump motor that rotates at an angular velocity, ω.

On the mechanical (hydraulic) side the pump creates a head pressure, H, which circulates water at a flow rate of Q.

The water pumping system may be closed loop (as shown) or open loop.

Hydraulic System Curves The below figure shows an open

system in which the water is raised through a static head, H.

Hydraulic System Curves (cont’d)

The units of head are usually “feet of water” or may be measured as a pressure in psi. The conversion between these units is given below:

1 ft. of head = 0.433 psi or

1 psi = 2.31 ft. of water

City water pressure is usually about 60 psi or equivalent to 140ft. of head.


Static head measures the pressure need to raise the water to a certain level. In order to account for water flow, a dynamic head must be specified. This includes friction losses in the pipe which increases as ≈ Q2.

Hydraulic System Curves (cont’d) The pressure drop per 100 ft. of plastic pipe as a function of flow rate and tube diameter is given in the table below:


Valves, tees and elbows also create friction losses which may be expressed as equivalent lengths of tube as given in the table below:

Example: Each ¾” 90º elbow contributes the equivalent 2.0 ft. of straight pipe.

Hydraulic System Curves (cont’d) The sum of the friction head and the

static head is termed the “total dynamic head”.

Example 9.21


If the approach of example 9.21 is repeated for various flow rates, the plot of total dynamic head vs. flow rate is called a hydraulic system curve. An example is shown below:

Hydraulic Pump Curves The next piece of information we need

is how the pump head and flow rate vary with the input voltage/current supplied to the pump motor. These are pump curves and depend on the type of pump used. There are two common types of pumps used in PV systems –centrifugal and positive displacement pumps.

Centrifugal Pumps A centrifugal pump has a fast-spinning

impeller that literally throws the water out, creating suction on the input side of the pump and pressure on the delivery side. Float pumps are limited to about 20 ft. of vertical lift whereas submersible pumps can provide over 1,000 ft. of vertical lift. A disadvantage of a centrifugal pump is that the impeller can get clogged or damaged by grit in the water.

Positive Displacement Pumps Positive displacement pumps come in

several different types – helical pumps, jack pumps, and diaphragm pumps. Jack pumps are the traditional hand pumps and include flap valves which act like hydraulic diodes. The valve is opened to let water in and on the next stroke the valve is closed and the water is raised. Generally, positive displacement pumps are good for low flow-rate applications.

PV Water Pumps Pictures of a centrifugal pump and a

hand pump are shown below:

Centrifugal Pump Hand Pump

Comparison of Centrifugal and Positive Displacement Pumps

Hydraulic Pump Curves Hydraulic Q-H curves have similarities

to electrical I-V curves. The power required for the pump is given by:

P = ρHQ (analogous to IV)

where ρ is the fluid density.

In American units,

P (watts) = 0.1885 x H (ft.) x Q (gpm)

Hydraulic Pump Curves (cont’d) A pump curve for a centrifugal pump is

shown below:

For a PV system, the voltage will change

with the insolation level => pump curve will change with insolation level.

Combining System Curve and Pump Curve

Combining the hydraulic pump curve and the system curve on one plot is shown below.

PV Water Pumping System Design

A simple approach to design a PV water pumping system is as follows:

1. Determine water production goal (gallons/day) in the design month (highest water need and lowest insolation).

2. Use design-month insolation (hrs. at 1-sun) as the hours to find pumping rate:

Q(gpm) = Daily demand (gal./day)

Insolation (h/day@ 1-sun)x 60 min./hr

PV Water Pumping System Design (cont’d)3. Find total dynamic head H@ Q (gpm).

4. Find a pump capable of delivering desired head H and flow Q.

5. Number of series PV modules is given by:

Modules in series = Pump voltage (V)

15V/module

6. Number of parallel strings is given by:

# strings = Pump input power, Pin (W)

# in series x 15V/mod. x IR(A) x de-rating

PV Water Pumping System Design (cont’d)

Example 9.22

ECE 7800: Renewable Energy Systems Topic 9: Photovoltaic System Design Spring 2010 © Pritpal Singh,...

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