Download ECE 7800: Renewable Energy Systems

ECE 7800: Renewable Energy
Systems
Topic 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 gridtied PV system is shown below:
Residential Grid-Connected PV Systems
City of Tallahassee
City of Lakeland
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, standalone PV system is shown below:
Examples of Stand-Alone PV Systems
PV Lighting
Courtesy: Kevin Lynn, FSEC
Eco-Resort
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 overdischarged. 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 gridtied 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
Remote Village Water Supply
In West Africa [Senegal]
Livestock watering
(http://www.agr.gc.ca/pfra/water/facts/solar.pdf)
(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 currentvoltage 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 gridtied 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 gridconnected 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
Grid-Tied PV System Sizing (cont’d)
Let us extend the example 9.6
calculation taking real module and
inverter specs into account (pp. 538539).
Grid-Tied PV System Sizing (cont’d)
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 Basics (cont’d)
Lead acid battery capacity depends
on discharge rate and temperature
as shown in the below figure:
Lead Acid Battery Basics (cont’d)
Example 9.16
Lead Acid Battery Basics (cont’d)
Batteries are wired in series and
parallel combinations to achieve the
voltage and Ah ratings required for
the PV system.
Lead Acid Battery Basics (cont’d)
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 derating 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.
Hydraulic System Curves (cont’d)
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:
Hydraulic System Curves (cont’d)
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
Hydraulic System Curves (cont’d)
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