Atrex Energy Inc. - Spartan Controls

Transcription

Power
Atrex Energy Inc.
T EC H N I C A L W H I T E PA P ER: T H EO RY O F O P ER AT I O N S O L A R S TA N D -A L O N E S Y S T EM S
As a company, Atrex Energy does not sell photovoltaic (solar) stand-alone systems. Even though Atrex Energy made a
business decision not to do so, does not mean that Atrex Energy views solar negatively. To the contrary, Atrex Energy
believes solar to be a terrific source of energy and oftentimes uses PV (photovoltaic) modules in some of our different
system architectures.
As Atrex Energy often finds itself in situations where its customers compare solar standalone systems against Atrex Energy
SOFC-based systems, Atrex Energy wanted to be able to discuss and present what constitutes a properly designed solar
stand-alone system.
This Technical White Paper describes how a properly designed,
properly fabricated and properly installed solar system works.
The word ‘properly’ is key. Solar stand-alone systems that
have been properly designed for the environment they will be
deployed into are terrific.
Unfortunately, the solar industry has a very poor track record
in this regard as PV systems that were “inexpensive” were sold,
but turned out not to be properly designed and failed.
Photo at right shows a ‘properly designed’ solar stand-alone
system powering a telecom repeater in a location (US Mojave
Desert) that has a great solar resource. This is a perfect example
of a good application for a solar stand-alone system.
www.spartancontrols.com
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Power
Atrex Energy Inc.
Let’s discuss how properly designed solar stand-alone systems work
Solar energy is the only source of power generation. Solar stand-alone systems are ideal for remote power applications
where the loads are relatively small, typically under 400 watts continuous and there is a robust solar insolation resource
indigenous to the site where the system will be deployed.
Solar systems have been installed the world over. Depending upon location, solar resource, load requirements as well as
specific needs of the customer, solar systems come in a variety of different sizes and packages. They can be configured
for any system voltage. Most common system voltages are either 12VDC or 24VDC. 48VDC systems are also routinely
provided. For applications requiring 120 VAC, the solar systems are equipped with a DC-AC inverter.
How are solar systems sized?
To properly determine the size of a solar system, the best available solar insolation data, closest to the actual site location
should be utilized. Obviously, solar resources vary from site to site. A site to be located in Arizona, for example, would
naturally have a much better indigenous solar resource than a site to be located in northern Canada.
When sizing a solar stand-alone system, the PV array must be large enough to produce enough solar energy to support
the load and charge the batteries during the “worst-case” solar month of the year. The array must also be large enough to
provide enough “headroom” to handle periods of poor solar availability. This is necessary because array output projections
are based upon average historical data and the site could experience a “below average year”. In essence, headroom is
simply added “insurance” against such an occurrence.
As an example, the following graph plots of projected PV array output versus system load.
Power
Atrex Energy Inc.
This particular graph is based on a 200-watt continuous load. The solar resource utilized for this example was Portland,
Oregon. As can be readily seen from the graph, Portland has a terrific solar resource in the summer, but a very poor solar
resource during the winter months. Nonetheless, when sizing a solar stand-alone system for a place like Portland, the
system has to be designed to be able to function during those poor solar months.
The last major factor taken into account when sizing a solar array is how long it will take to recharge the system battery
bank from a very low State of Charge (SOC). In the event of a sustained period of bad weather where the system’s battery
bank discharges down to a low SOC, the solar array must be able to adequately recharge the battery bank, while continuing
to service the system’s load.
While there are no strict rules to apply here, good design practice dictates that the array should be sized such that it can
recharge the battery from 20% SOC up to 90% SOC within 30 days, while continuing to service the load full time. This
timeframe is often a customer judgment call, and there is no single right answer. The important point is that a properly
designed system must take this issue into account.
How do solar systems work?
The following three graphs demonstrate the methodology of how a solar stand-alone system actually works.
The first graph, shown above, shows the “theoretical” solar output. According to the U.S. Department of Energy’s National
Renewable Energy Lab (NREL), this site should have had an average solar insolation during May of 5.8 “sun-hours”.
The term ‘Sun-hour’ is a very specific unit of measurement. It is the internationally recognized standard used when sizing
solar arrays.
Power
Atrex Energy Inc.
In real world applications, however, the site’s solar resource will vary from day to day. Some days will be nice and sunny
while others may be overcast and rainy. The second graph shows what actually occurred on the site during a 4-day period
in mid-May.
The third graph shows how the system actually works. Looking at the x-axis (timeline) at midnight of the very first night,
the system is not producing any energy so the battery bank is supporting the loads. As can be seen from the graph, at
midnight, the battery bank line shows it is discharging (below the x-axis). Around 8 a.m. that morning the sun comes up
and the array begins to produce solar energy. The battery, although still providing most of the energy to the load, begins to
discharge less and less, until about 10 a.m. when the PV array is producing more energy than the load is consuming. At this
time, the battery actually begins to be charged. Following the battery line, it can be seen that the battery continues to
charge throughout most of the day until approximately 6 p.m. when the amount of PV energy is not sufficient to carry the
load and the battery, once again, begins to discharge.
Power
Atrex Energy Inc.
System Battery
The battery bank is a critical part of any solar stand-alone system. The battery stores any excess solar energy produced for
use during those hours of the day when there is no solar resource, or during times of cloudy or inclement weather.
When talking about remote power systems, the size of the battery bank is often referred to in terms of “days of autonomy”.
This is a measure of how long can the system continue to support the load when no solar energy is being produced,
assuming the battery bank is topped off (100% SOC). As with maximum recharge time there is no one right answer to that
question. For solar systems, a minimum of 7 days of system autonomy is typically specified, but Atrex Energy knows of
customers who have required up to 30 days of system autonomy.
Most properly designed solar systems these days utilize Valve Regulated Lead Acid (VRLA) batteries. These totally sealed
“maintenance free” batteries represent the state of the art technology for remote power applications. They are specially
designed for deep discharge applications such as are often experienced with solar-based systems.
Buyer beware: An easy and often used trick is to use ‘cheap batteries’. They will be ‘cheap’ until they fail and need to be
replaced after about a year.
System Controller
The last, and most important, component of a solar system is the system controller. As the name implies, the controller
is the “brains” of the system. With a well-designed controller, the customer will experience consistent reliable operation.
Without it, the customer essentially has a collection of parts that may or may not work all that well together.
Solar Sizing Considerations
1. “Worst Case” Solar Sizing
There are occasionally periods of unusually poor solar resource due to local weather conditions. In most cases they
are factored into the “system headroom” used in Atrex Energy’ system sizing practices. For practical reasons, the
system array sizing is not normally designed with enough capacity to cover a once per 10-20 year occurrence of
poor solar availability. In these rare instances, even the best-designed solar systems can experience interruption of
service. If it is absolutely necessary that the system remain in operation, even in these very unusual circumstances,
it may be necessary to increase the PV array size.
2. PV Array Shading - “Unfettered” Access to the Sun
Normally solar systems require full access to the solar resource to operate efficiently and reliably. To make this
possible, sites must be cleared (and kept clear) of trees, and vegetation that might create shading. Additionally,
arrays are positioned such that towers or other structures at the site do not shade them. As a result, shading is not
generally taken into consideration in a standard system configuration.
Power
Atrex Energy Inc.
The photo at right is of a system along an oil pipeline
in Ecuador. When the system was installed the area
was cleared of vegetation such that there was no
shading. In just a very few years’ time however,
the performance of the system had significantly
degraded. As can be readily seen from the photo,
the fast growth of the rainforest was choking out the
solar
resource and was responsible for the associated
degradation in system performance. Appropriate site
maintenance can be just as important as regularly
scheduled system maintenance.
As little as 5% shade could result in as much as a 50%
loss in power generation capability.
3. PV Array Shading –Example #2
The table shown on the following page is another example of the impact of shading on a solar system’s performance.
The data shown represents the PV array output for one day. This particular solar system had an array of 1350 watts
and was located at a site that had an average daily solar resource of 5.8 Sun-Hours. Based on this solar resource, the
daily array output should have been approximately 230 amp-hours.
This particular system, however, was not properly installed and did not have “unfettered access to the sun”. Rather, it
was placed behind a telecom tower. During the hours of between 10 a.m. and 2 p.m., the tower partially obstructed
sunlight from reaching the PV array. Approximately 10% shading was occurring. The impact of this shading affected
PV array output, as well as the functioning of the system’s battery bank (the amount it is being “charged” versus the
amount it is being “discharged”) as shown in the following chart.
Power
Atrex Energy Inc.
4. Bird Excrement
If a picture is worth a thousand words, the photo at right speaks for
itself.
Bird Excrement is a huge problem for solar arrays, particularly solar
arrays on offshore platforms like this one. The only real solution is
continual cleaning.
In case anyone is interested; the answer is NO, that solar panel is not
putting out a whole lot of electricity.
Theft and Vandalism
No discussion of solar systems can be had without bringing up solar’s ‘Achilles Heel’. Solar systems are prime targets for
theft (stealing of the PV modules) or vandalism (malicious damaging the PV array).
While extensive (and costly) efforts are routinely made to ‘protect’ the solar arrays, theft and vandalism, have always, and
will likely always remain huge concerns.
Power
Atrex Energy Inc.
System Environmentals
It is not the intent of this Technical White Paper to describe all of the different types of methods to environmentally
protect a solar stand-alone system. That said Atrex Energy wants to empathically make one very important point:
The heart of a solar stand-alone system is its battery bank. A battery bank is only as good as the environment in which it is
placed. Properly protecting the system’s battery bank is critical if the system is to function.
Heat kills batteries and cold robs a battery of its ampacity.
With solar stand-alone systems, it is imperative to design in environmental techniques that use little of no electrical
energy. Atrex Energy Technical White Papers on different techniques it uses for extreme hot and extreme cold weather
environments is available upon request.
A “1-line” electrical block diagram for what a typical solar system might look like is shown below.
Power
Atrex Energy Inc.
Summary
Although Atrex Energy does not supply solar stand-alone systems, it is very familiar with them and believes properly
designed solar systems are a proven method for reliable power generation in situations that require remote power. Solar
stand-alone systems are ideal for applications where the load is relatively small and the solar resource is good. Once
again, Buyer Beware. The world is littered with solar systems that were not properly designed, were not properly
fabricated andwere not properly installed in an appropriate site location.

Atrex Energy Inc. - Spartan Controls

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