Solar Microgrid
Today I’m sharing with you a
project I’m particularly proud of. As final stage of my Renewable Energy course
I was asked to complete a project of a stand-alone, hybrid system. Stand-alone
means that the plant is not connected to the grid, like in a remote area. Hybrid
means that more than one source of energy is used. In this case I designed a
27kW solar system with a backup diesel generator.
A microgrid is an independent
grid where more than one unit can produce and use the power of the grid.
The purpose of the system is to
power the irrigation system of a cherry farm, the owner’s house, a shed, and
the farmstay for workers. The farm really exists and I worked in it, in
Griffith NSW. It’s grid connected but I designed this system pretending that it
wasn’t.
This is the final diagram of the
complete system:
LOAD ASSESSMENT
The first step of a power system design is a load assessment. This stage allows us to understand the needs and the size of the system, but also to improve the energy consumption. In this case I was able to reduce it up to 30% in summer, improving the efficiency of the air conditioning.
Decreasing the energy consumption
decrease the size of the system and of the components.
The next chart shows the load
assessment of the 4 units (pump, house, shed and farmstay) combined, in
different seasons.
DESIGN
To design this system I followed
the Australian standards and the Clean Energy Council guidelines but I exceeded
their requirements. I created a complex spreadsheet to help me in future design
of this size.
The microgrid is a 3 phase
system, the loads are spread across the phases equally, and only the irrigation
pump uses the 3 phase power.
The following charts compare the
power used in each phase at different time.
The system is designed to be
expanded easily in the future.
In good weather conditions the
sun (daytime) and the batteries (nightime) are enough to power all the loads. The
generator starts automatically to power the loads and recharge the batteries in
rainy days.
To calculate a perfect system I analysed
the solar data (from NASA) and the load across the year and I considered the
worst scenario. In this spreadsheet you can see the comparison between the load
and the energy from the sun:
I
also compared the performance of my arrays at different angles. I calculated
that changing the tilt of the array just twice a year the annual output of the
system increase considerably. To do that, we need an adjustable tilt array. The
following chart compares the energy output of the system with a different tilt:
I made the following spreadsheet
to help me in matching the inverter with the array, using the calculation
suggested by the Australian standards and CEC. It also calculates the size of
cables and circuit breakers. I need 6 inverters to collect the power from the 3
arrays and put it on the 3 phase grid.
ARRAY SIZE
To calculate the distances between
the arrays (needed to calculate the length of cables and the voltage drop),
avoiding shading between 10am and 2 pm, I made a spreadsheet that can be used in
any area in Australia. You just insert the size and number of panels in the
array, the tilt of the array, and the latitude of the location. The result will
be the minimum distance between arrays to avoid shading when the system is more
productive.
I copied the formula from the Clean Energy Council tech info
released in March 2010. For some reason it’s not on their website anymore so you
can download it here.
CUSTOMER MANUAL
As a final task I was required to
write a customer manual, according to the Australian standards. It’s written in
simple words to explain the basics of how the system works. It has to include
important instructions and procedures.
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