Designing
for Uninterruptible Power: Opportunities for Battery-Based Photovoltaic
Systems
By Jim Dunlop
We asked Jim Dunlop of FSEC’s Photovoltaics
and Distributed Generation Division to write an article for
the Energy Chronicle on using battery-based photovoltaic systems to
provide power during outages. The following story describes the typical
alternatives for emergency power used during power disruptions and looks
at the ways PV systems can meet energy needs.
Over the past few years, utility power outages have made headline news,
and not only caused inconveniences but loss of business, revenue and hardships
for many. Whether these outages are caused by hurricanes, ice storms,
power shortages or other utility disturbances, our way of life is significantly
disrupted without electrical power.
|
Photovoltaic arrays may serve as temporary,
stand-alone power sources in a disaster area.
(Photo: Robin Flynn ) |
In the aftermath of major storms and the expected damage to local electrical
distribution systems, it is not uncommon for utility power to
be out for several days, if not for weeks or even over a month
in some cases, and the longer the power remains out, the greater the problems
that result. As a solution, many businesses and homeowners are
looking into alternative sources of electrical power to mitigate potential
— and probable — utility outages.
The high availability and low cost of energy is something Americans have
taken for granted for some time, and our entire economy and way of life
are predicated on this seemingly inexhaustible supply of energy. While
we are accustomed to this panacea, most do not consider the value of energy
until supplies become limited or unavailable, like during extended utility
outages. In these cases, people are often willing to pay several times
what it normally costs to have energy available when we need it.
For example, consider the cost of energy from common alkaline batteries,
which we seem to use more and more of over time. A typical “D” cell
stores about 4 amp-hours at 1.5 volts, equating to about 0.006 kWh of
electrical energy. At a cost of about one dollar apiece, the cost of energy
from a “D” cell amounts to about $167 per kWh – up to
2,000 times what most pay for utility-supplied electricity ($0.08-0.10/kWh)!
So the cost of energy is relative to availability and convenience, and
most of us are willing to pay much more for energy when it’s not
readily available.
Several backup power options are available to businesses and homeowners,
with each having limitations on their application as well as operational
issues. This article describes three typical back-up power alternatives:
uninterruptible power supplies, engine generators and battery-based solar
photovoltaic (PV) systems.
Uninterruptible Power Supplies
Uninterruptible power supplies (UPS) have become increasingly used by
homeowners and businesses to back up computers, security systems,
office equipment and other critical electrical loads in the event
of utility outages or disturbances. UPS systems are available in a variety
of sizes ranging from very small units designed to power small
loads for short periods of time, to much larger systems designed to power
much larger equipment and even entire facilities for extended periods
of time.
A typical UPS system includes a battery, charger, inverter and automatic
transfer switch, where the battery is used to store electrical energy.
While some UPS systems are designed to power critical loads directly from
the battery to isolate these loads from the utility service and potential
surges at all times, most UPS systems operate in standby mode, while the
batteries are kept at full state of charge from utility power. When a
utility disturbance occurs, the automatic transfer circuit isolates critical
loads from the utility, and connects them directly to the inverter which
converts DC power from the battery to AC power at utility service voltages.
At this point, there is a limited supply of energy in the battery, which
dictates the magnitude and duration of the critical loads that can be
operated from these systems.
UPS systems with small batteries may only be able to sustain loads of
a few hundred watts like computers for only a few minutes, to allow time
to backup work and properly shut down equipment. Larger UPS systems include
larger battery banks and inverters capable of sustaining much larger loads
for longer periods of time. Ultimately, most UPS systems rely on utility
power to recharge the batteries, and without an alternative source to
recharge the batteries, these systems are unable to sustain backup of
electrical loads for long-term operation.
Engine Generators
Many homeowners and small businesses are purchasing engine generators
as a solution to power outages. Generators are available in a variety
of sizes from a few hundred watts to hundreds of kilowatts, and can be
portable or permanently installed in service, making them very versatile
in meeting a variety of electrical loads in many applications. Prime duty
machines are generators designed to operate in continual service, while
stand-by machines are designed for backup power applications of limited
duration. While generators may be the least initial cost option for back-up
power, they can present a number of operational and maintenance issues
for users, as well as potential electrical and fire safety hazards if
they are not installed and operated properly.
"While generators
may be the least initial cost option for back-up power,
they can present a number of operational and maintenance
issues for users ." |
First of all, engine-generators need a fuel source. Smaller portable
units are typically fueled by gasoline, while larger and stationary units
are typically fueled by diesel, propane (LP gas) or natural gas. During
natural disasters, gasoline supplies are often disrupted, there is high
demand for product, and many gas stations are closed or operating at reduced
capacity due to loss of power or delays in fuel deliveries. Long lines
at the few open stations are the norm under these circumstances. To mitigate
these supply problems, some opt to store large quantities of gasoline
on site in portable containers for stand-by generators, which can be unsafe
to transport in vehicles, and hazardous to store in quantities anywhere,
particularly interior to buildings. Due to the limited fuel tank size
on many small generators and their rate of fuel consumption, users are
often required to refuel generators many times a day, which increases
the chance for fuel spills, and the risk of fire, personal injury and
property damage. All it takes is some gasoline spilled on a hot exhaust
manifold and a stray spark or ignition source to ruin someone’s
life. Permanent generator installations done by professionals often include
means to store fuel as well as a means to deliver fuel automatically to
the engine, with diesel or propane being the fuel of choice for these
systems. As opposed to gasoline, large quantities of diesel and propane
can be stored safely on site in approved containers, and degrade much
less over time than gasoline. Diesel fuel is also less flammable than
gasoline, and less hazardous when spilled. Incidentally, propane and natural
gas can also be used to directly power a number of non-electric appliances,
including water and space heating, cooking equipment such as gas ranges,
ovens, and outdoor grills, and even refrigerators and freezers. However,
some of these appliances may incorporate ignition systems or valves that
may require electric power to operate. Clothes dryers and certain air
conditioning equipment can also be powered by propane or natural gas,
although this equipment also requires electrical power to operate motors
and fans.
|
Due to the limited fuel tank size on
many small generators and their rate of fuel consumption,
users are often required to refuel generators several times
a day.
(Photo:
Sherri Shields ) |
In addition to a requiring a readily available fuel source, generators
present a number of other operational and safety issues. For
one, generators operated in the open can be quite loud, and many
can agree this incessant noise is quite bothersome and even unnerving
to some in close proximity to the generating equipment. While improved
muffler systems and certain generator designs produce less noise, complaints
from neighbors are common where large numbers of generators are used after
a storm in high density residential areas. Like other internal combustion
engines, generators also produce large quantities of deadly carbon monoxide
gas in their exhaust. In fact, carbon monoxide poisoning and deaths are
quite common in the aftermath of disasters, as folks are operating generators
within buildings to prevent theft of the generator or to minimize
noise for neighbors, and otherwise not making proper provisions for ventilation
of exhaust gases. Many install generators on porches, adjacent
to bedroom windows or entrances that may be opened, increasing the potential
for carbon monoxide poisoning.
Generators also require regular maintenance and exercise to ensure that
they will operate properly and safely when needed. This required
maintenance typically includes oil and filter changes after a
number of hours of service or storage period, replacement of
spark plugs, and regular inspections of fuel systems for leaks,
servicing of starting battery, etc. While smaller units are typically
air cooled, larger units are typically water cooled, requiring
proper maintenance of antifreeze/coolant levels, and periodic flushing
of the entire cooling system. Like any automobile engine, generators must
also be routinely exercised to ensure reliable and safe operation. Fouled
fuel and carburetors are a common problem for any internal combustion
engine that is not routinely and properly operated and maintained. Many
automated generator systems incorporate a weekly exercise cycle for 10-15
minutes to ensure they will start and operate when needed. Automatic start
generators depend on a starting battery, which when not charged for extended
periods will slowly lose capacity to the point where it will no longer
start the engine.
Another great concern with portable stand-by generators is they are often
not interconnected with building electrical systems in a safe and code
compliant manner in accordance with the National Electrical Code. Many
utilize extension cords and other non-approved methods for connecting
emergency loads, including wiring the generator’s output directly
to an electrical panel without the appropriate switchgear or overcurrent
protection. Many times these cords and connections are undersized for
the load intended to be carried, creating a potential circuit overload
and fire safety hazard. When generators are connected to an electrical
system without the appropriate transfer switchgear, the potential exists
for back feeding the utility system and neighbors who may not be expecting
their services to be energized. Professional generator installations always
include either a manual or automatic transfer switch to ensure that the
generator is only connected to loads that have been fully isolated from
the utility and other electrical systems.
Like other standby power systems, generators must also be adequately
sized for the electrical load they are intended to power. This requires
analysis of the building equipment and appliances to estimate to total
connected electrical loads, and to ensure that the generator is properly
sized and not overloaded. One of the fundamental problems with utilizing
generators for backup power is that this sizing often requires the generator
to be significantly oversized for the average load, which results in the
engine operating at reduced capacity, and at lower than optimal fuel efficiency.
These are just some of the operational and safety issues associated with
utilizing engine-generator systems. Those considering the use of generators
for standby power are encouraged to consult with the professionals in
this business, and to have any permanent generator installations done
by licensed electricians.
Solar Photovoltaic Systems
|
Homes equipped with photovoltaic systems
can produce their own power. (Photo:
Steven C. Spencer ) |
Solar photovoltaic (PV) systems, or solar electric systems as they are
sometimes called, are highly modular electrical power systems that can
be designed and configured for a variety of electrical loads and services.
PV systems are becoming increasingly used as a supplemental energy supply
for residential and commercial facilities throughout the U.S. and abroad.
And while their cost is still somewhat higher relative to conventional
utility sources in most applications, the performance and reliability
of PV systems and equipment have been ever improving. Many states and
utilities are providing considerable subsidies to homeowners and businesses
that install PV systems, who may also be eligible for other financial
incentives including sales and property tax exemptions. In 2006, federal
tax credits are available for those who install PV systems on residential
and commercial buildings.
PV systems have several merits over conventional energy sources. They
are fueled by a free source of energy — the sun, and they produce
no noise or pollution and require very low maintenance. Certain
types of PV systems are designed to provide backup power in the
event of utility outages, and have proven quite successful in these applications.
However, not all PV systems are designed to provide standby power
- only those systems that include battery storage and an inverter
and control system designed for that purpose.
To better understand the differences, PV systems are often categorized
in terms of the loads they are designed to power, or their connections
with other electrical systems. Stand-alone PV systems operate
independently of other electrical services and are commonly used for remote
power applications, including lighting, water pumping, transportation
safety devices, communications, off-grid homes/facilities and many other
electrical loads. Interactive PV systems operate in parallel, or interconnected
with the utility grid, and supplement utility-supplied energy to a building
or facility. Most simple interactive PV systems are not designed to provide
backup power, as they are required by electrical codes to disconnect from
the grid during outages or disturbances for safety reasons. A third-type
of PV system, referred to as a bi-modal or battery-based interactive system,
operates normally in interactive mode, but switches emergency loads from
the utility to the inverter-battery system during a utility outage, in
a manner similar to how most UPS systems operate. The main advantage of
a PV system over a UPS system is now we have a means to recharge the battery
and power loads for longer periods.
Figure 1 shows a basic diagram of a PV system and the relationship of
individual components. The primary component is the PV array, consisting
of individual PV modules that are electrically connected and mounted on
the ground or rooftop support structure. Array designs can be produced
to meet high wind load requirements, with most commercial PV modules constructed
with high-strength tempered glass and certified to withstand nominal impacts
from hail, etc. A number of other components are required in any PV system
to conduct, control, convert, distribute, and store the energy produced
by the PV array. The specific components required depends on the type
of system and functional requirements, but typically includes major components
such as an inverter to convert DC to AC power, a battery bank, a charge
controller, as well as balance of system hardware, including wiring, switchgear
and overcurrent protection.
Figure 1. Solar photovoltaic (PV) system and typical components
Stand-alone PV systems are designed to operate
independent of the electric utility grid, and are sized to supply
specific DC and/or AC electrical loads. These types of systems
may be powered by a PV array only, or may use a wind turbine,
an engine-generator or utility power as an auxiliary power source in what
is called a PV-hybrid system. Stand-alone PV systems can only use the
utility as a charging source – they
can not send excess energy back to the utility grid. As the energy
produced by a PV array varies with the sunlight intensity and
is not available at night, batteries are used in most stand-alone
PV systems to store energy produced by the array for later use
by the electrical loads as needed. Figure 2 shows a diagram of a typical
stand-alone PV system powering DC and AC loads. Figure 3 shows how a typical
stand-alone PV hybrid system may be configured.
Figure 2. Stand-alone PV system with AC/DC loads.
Figure 3. Stand-alone PV-hybrid system.
Grid-connected
or utility-interactive PV systems are designed to operate interconnected
and in parallel with the electric utility grid. The primary component
in grid-connected PV systems is the inverter, which must be listed
to UL1741 as an interactive photovoltaic inverter, and clearly labeled
as such. In these systems, the DC output of the PV array is directly connected
to the inverter, which produces AC power output consistent with
the requirements of the utility grid and typical appliances. A bi-directional
interface is made between the PV system output and electric utility network,
typically at the site distribution panel or electrical service entrance.
This allows the AC power produced by the PV system to either supply on-site
electrical loads or to back feed the utility grid when the PV
system output is greater than the site load demand. At night and during
other periods when the electrical loads are greater than the PV system
output, the balance of power required by the loads is obtained from the
utility.
One important feature of interactive inverters is that they must de-energize
if the interactive source of power is lost, and must stop supplying power
to the grid when the utility grid is not energized. In other words, these
systems are not designed to operate in the event of a utility failure
and can not provide power to backup critical loads during utility outages.
This safety feature is required for all grid-connected PV systems, and
ensures that the PV system will not continue to feed power onto the utility
grid when the grid is down, to prevent potential injury to linemen working
to restore service. Figure 4 shows a simple diagram of a simple interactive
PV system.
Figure 4. Simple interactive PV system.
Battery-Based PV Systems for Uninterruptible Power
Bi-modal, or battery-based interactive PV systems, include the benefits
of stand-alone as well as interactive PV systems, and are becoming popular
for homeowners and small businesses as a backup power supply option for
critical loads such as refrigeration, water pumps, lighting and other
necessities. Under normal circumstances, these types of PV systems operate
in interactive mode and interconnected with the utility, serving the on
site loads or sending excess power back onto the grid while keeping the
battery fully charged. In the event the grid becomes de-energized, control
circuitry in the inverter opens the connection with the utility through
an automated bus transfer mechanism, and operates the inverter from the
battery to supply power to the critical loads only. In this mode of operation,
the critical loads must be supplied from a dedicated subpanel separate
from the utility-supplied electrical system, and the total load connected
to the subpanel must not exceed the inverter power rating. A bypass circuit
is always included to power the critical load subpanel directly from the
utility service, and to fully isolate the PV-battery-inverter system for
maintenance or service. Figure 5 shows a typical configuration for a battery-based
interactive PV system.
Figure 5. Battery-based interactive PV system
The primary advantage of these systems is the PV array provides a source
to recharge the batteries, allowing for extended operation of these systems
as compared to simple UPS systems that only provide power for as long
as the battery lasts. They also provide supplemental site power during
normal utility operations, but their ability to provide backup power is
the real advantage over simple interactive PV systems, despite their higher
costs (primarily for the battery). The only caveat it that during backup
mode, there is only a limited supply of energy available on a daily basis
from the PV array to recharge the battery, and the battery has a limited
amount of capacity – based on the overall size of the PV and battery
systems. Regardless of sizing, careful load management and conservation
are essential to ensure the system adequately meets the intended critical
loads. Due to the high electrical demand for appliances such as air-conditioners,
electric water heaters and electric ranges, these loads are typically
not powered from these systems in backup operation, while lower power
alternatives such as ventilation fans, solar water heating, and microwaves
or gas grills are frequently used instead.
Commercial bi-modal PV inverters are available in sizes from 2-6 kW,
and some can be combined in parallel to provide greater overall peak power
output. The inverter DC voltage and corresponding battery bank voltage
for these systems is typically 24 or 48 VDC. The sizing of these systems
depends on a number of factors, but mainly the critical loads that are
to be powered in backup operation. Once the peak load and total daily
energy have been determined, the PV array must be sized to produce enough
energy on an average daily basis to meet that load. The sizing of the
battery is somewhat arbitrary, but the amount of storage should be at
least equal to the daily load energy requirement.
For example, consider a residential application to backup a refrigerator,
lighting, microwave, television, small fans, and a water pump. Assume
the electrical loads and average daily energy consumption is as follows:
Load |
Power (watts) |
x Time (hours/day) |
= Daily Energy (watt-hours) |
Refrigerator |
300 |
12 (50% duty cycle) |
3600 |
Lighting |
200 |
4 (evening only) |
800 |
Microwave |
1000 |
0.5 |
500 |
Television |
200 |
4 |
800 |
Fans |
200 |
8 |
1600 |
Water Pump |
1000 |
0.5 |
500 |
Total |
2900 (2.9 kW) |
|
7800 (approx. 8 kWh/day) |
Based on this 2.9 kW peak load, a 4 kW bi-modal PV inverter would be
adequate for this application, and would support some future
addition of load as required. The PV must be able to produce 8 kWh per
day, and the sizing will be based on average sunlight conditions in the
given area. In Florida for example, a nameplate 1 kW PV system is capable
of producing at least 3.5 kWh, so for this application a PV array size
of approximately 8/3.5 = 2.3 kW would be required, which would take some
200 square feet of surface area. For one day of battery storage equivalent
to the daily load, 8 kWh of battery would be required, which can be supplied
by approximately 12 typical flooded lead-acid golf cart batteries – which
incidentally are a good deep-cycle battery for this application. Another
popular option is the more expensive valve-regulated or sealed lead-acid
battery, which do not require the maintenance and water additions like
flooded types, and are suited for indoor applications.
Summary
Battery-based interactive PV systems present an alternative to generators
and simple UPS systems for emergency backup power. Like any other electrical
equipment, PV systems should be designed and installed in accordance with
the National Electrical Code, as well as meeting other applicable building
code requirements. In general, it is highly advisable to have qualified,
licensed contractors install PV systems. In addition to the local electrical
permitting and inspection process, interconnection agreements must also
be completed with the local utility for any interactive system. Most utility
companies offer a simple interconnection agreement for small PV systems,
which may include special requirements for inspections, disconnects, metering
and insurance.
Following are major U.S. manufacturers of bi-modal interactive battery-based
PV inverters:
Xantrex: http://www.xantrex.com
Outback Power: http://www.outbackpower.com
Beacon Power: http://www.beaconpower.com
Alpha Technologies: http://www.alphatechnologies.com
These companies have authorized distributors for their products, and
those distributors often recognize dealers and contractors who
sell and install their products and systems locally. Those interested
in pursuing PV systems for either supplemental energy production or back-up
power are encouraged to contact their local supplier or contractor. In
Florida, the Florida Solar Energy Industries Association (FLASEIA) maintains
a list of certified Florida solar contractors: http://www.flaseia.org.
Nationally, the North American Board of Certified Energy Practitioners
(NABCEP) maintains a list of PV installers having passed a national exam
and meeting certain other eligibility requirements: http://www.nabcep.org.
For additional information on PV systems technology, visit the
following Web sites:
Florida Solar Energy Center: http://www.fsec.ucf.edu/en/consumer/solar_electricity/basics/index.htm
Solar Energy Industries Association - http://www.seia.org/
U.S. Department of Energy Solar Energy Technologies Program: http://www.eere.doe.gov/solar/
Sandia National Laboratories PV Program: http://www.sandia.gov/pv
National Center for Photovoltaics/National Renewable Energy Laboratory:
http://www.nrel.gov/ncpv/
The Source for Renewable Energy - http://www.sourceguides.com/energy/
World Directory of Renewable Energy Suppliers and Services - http://www.jxj.com/suppands/renenerg/index.html
Home Power Magazine: http://www.homepower.com