Isolated Micro grids and Hybrid micro
grids and tie with utility, assessment
Introduction
Rooftop system uses very less space and is also cost
effective. The major issue behind utility scale solar PV project is
availability of land. It takes around 5 acres of land for installing 1MW of
solar PV plant at times the land utilized for sola is productive kand , needed
for growing food grains to support the
growing population. Agricultural land conversion for solar project does not
seem a viable option. Another issue is the availability of grid in developing
countries significant number of people are still unconnected to the grid , electricity
generation is usually a costly affairs with fuels that are imported , which can
cause balance of payments issues. .Decreasing
solar costs have opened opportunities for solar panels to be installed on roof
tops to feed domestic demand. There are various options some of which are
briefly discussed in this presentation .What are the benefits of grid-connected
solar panels vs. living off the grid? Deciding whether or
not to grid-tie your solar panels is usually pretty straightforward – the
clear-cut benefits of being grid-tied are, however, some people that choose to live
off the grid.
Rooftop Solar PV system: How it works?
Solar PV rooftop system is basically a small power plant
at your rooftop. The Grid interactive Roof Top Solar Photo Voltaic (PV) mainly
consists of three major components. These are the solar PV modules, mounting
structure for the modules and the inverter or power conditioning units. Solar
PV modules form an array and it requires a mounting structure to hold PV
modules at the required angle for maximized generation. The solar panels
convert solar energy in the form of light into electricity in DC form (Direct
Current). The DC electrical energy is converted to AC (Alternate Current) power
by the inverter/power conditioning unit which is connected to the power grid
through AC distribution board. The AC power output can be measured through
metering panel connected to it. The 415 V AC output of the system can be
synchronized with the grid and the electricity can be exported to the grid
depending upon solar power generation and local consumption.
Grid-tied, on-grid, utility-interactive, grid
intertie and grid back feeding are all terms used to describe the same concept
– a solar system that is connected to the utility power grid.
Advantages of Grid-Tied Systems
1. Save more money with net metering
A grid-connection will allow you to
save more money with solar panels through better efficiency rates, net
metering, plus lower equipment and installation costs: Batteries,
and other stand-alone equipment, are required for a fully functional off-grid
solar system and add to costs as well as maintenance. Grid-tied solar systems
are therefore generally cheaper and simpler to install.
Power generated from the rooftop solar system during the
daytime can be utilized fully by powering the building loads and feeding excess
power to the grid as long as grid is available. Whenever, solar power is not
available due to shadow or a cloudy day, the building loads can be served by
drawing power from the grid or DG sets. Smart inverters automatically
understand the power situation and always give preference to power generated
from solar modules.
Your solar panels will often generate more
electricity than what you are capable of consuming. With net metering, homeowners
can put this excess electricity onto the utility grid instead of storing it
themselves with batteries. Net metering (or feed-in tariff schemes in some
countries) plays an important role in how solar power is incentivized. Without
it, residential solar systems would be much less feasible from a financial
point of view. Many utility companies are committed to buying electricity from
homeowners at the same rate as they sell it themselves.
2. The utility grid is a virtual battery
Electricity has to be spent in real time. However,
it can be temporarily stored as other forms of energy (e.g. chemical energy in
batteries). Energy storage typically comes with significant losses. The electric power grid is in many ways also a battery,
without the need for maintenance or replacements, and with much better
efficiency rates. In other words, more electricity (and more money)
goes to waste with conventional battery systems.
Lead-acid batteries, which are commonly used with solar panels, are only 80-90%
efficient at storing energy, and their performance degrades with time. Additional
perks of being grid-tied include access to backup power from the utility grid
(in case your solar system stop generating electricity for one reason or
another). At the same time you help to mitigate the utility company`s peak
load. As a result, the efficiency of our electrical system as a whole goes up.
Equipment for Grid-Tied Solar Systems
There are a few key differences between the
equipment needed for grid-tied, off-grid and hybrid solar systems. Standard
grid-tied solar systems rely on the following components:
- Grid-Tie
Inverter (GTI) or Micro-Inverters
- Power
Meter
Grid-Tie Inverter (GTI)
The
solar regulates the voltage and
current received from your solar panels. Direct current (DC)
from your solar panels is converted into alternating current (AC), which is the
type of current that is utilized by the majority of electrical appliances. In
addition to this, grid-tie inverters, also known as grid-interactive or
synchronous inverters, synchronize the phase and frequency of the current to
fit the utility grid. The output voltage is also adjusted slightly higher than
the grid voltage in order for excess electricity to flow outwards to the grid.
Electrical inverters convert direct current generated from
solar PV modules to alternating current. Therefore, solar modules need to be
connected to inverters. Nowadays we have smart Grid-interactive inverters.
These inverters can produce AC power that matches voltage and frequency of the grid
and the power line it connects to. There is another requirement of adding an
isolation transformer. For safety purposes, isolation transformer is used to
transfer electrical power from inverter to the connected load while isolating
the load from the power source. In addition, the injection of DC power into the
grid can be avoided by using an isolation transformer at the output of the
inverter.
Micro-Inverters
Micro-inverters go on the back of each solar panel,
as opposed to one central inverter that typically takes on the entire solar array.
There has recently been a lot of debate on whether
micro-inverters are better than central (string) inverters. Micro-inverters
are certainly more expensive, but in many cases yield higher efficiency rates. Homeowners who are suspect to shading issues should
definitely look into if micro-inverters are better in their situation.
Power Meter
Most homeowners will need to replace
their current power meter with one that is compatible with net metering. This
device, often called a net meter or a two-way meter, is capable of measuring
power going in both directions, from the grid to your house and vice versa. You
should consult with your local utility company and see what net metering
options you have. In some places, the utility company issues a power meter for
free and pay full price for the electricity you generate; however, this is not
always the case.
Off-Grid Solar Systems
An off-grid solar system (off-the-grid, stand alone)
is the obvious alternative to one that is grid-tied. For homeowners that have
access to the grid, off-grid solar systems are usually out of question. To
ensure access to electricity at all times, off-grid solar systems require
battery storage and a backup generator (if you live off-the-grid). On top of
this, a battery bank typically needs to be replaced after 10 years. Batteries
are complicated, expensive and decrease overall system efficiency.
Key facts about rooftop solar systems;
1.
The solar panels should be south
facing to maximize the production. The angle of panel should be 25-30 degrees
with respect to the land surface, both facing and angle will vary from country
to country.( Solar
panels should always face true south if you are in the northern hemisphere, or
true north if you are in the southern hemisphere. True north is not the same as
magnetic north. If you are using a compass to orient your panels, you need to
correct for the difference, which varies from place to place. Search the web
for “magnetic declination” to find the correction for your location. The angle
from horizontal should the panels be tilted should be equal to your latitude,
plus 15 degrees in winter, or minus 15 degrees in summer. It turns out that you
can do better than this - about 4% better. It is simplest to mount your solar
panels at a fixed tilt and just leave them there. But because the sun is higher
in the summer and lowers in the winter, you can capture more energy during the
whole year by adjusting the tilt of the panels according to the season,
adjusting the tilt twice a year gives you a meaningful boost in energy.
Adjusting four times a year produces only a little more)
2.
The weight exerted by the Solar
Panels and the structure is around 10 kg per square meter.
3.
The solar installation requires about
100-150 square feet of area per Kilowatt.
4.
No frequent maintenance is required
for the solar system except regular cleaning of surface of the panels.
5.
There are two types of rooftop solar
installations. First is for captive use and second is for feed in tariff based
grid connected system. In captive use, rooftop solar system installer himself
consumes all electricity generated by the system. However, in second type i.e. feed
in tariff based grid connected system, installer can also feed/sale excess
power to the grid.
6.
In captive rooftop solar systems,
there are two types. The first one is standalone systems and other is grid
connected system.
Net metering schematic
Benefits
of Rooftop Solar PV systems
1.
Cost of electricity is increasing.
Electricity utility companies are bound to increase the cost per unit of
electricity due to increase in cost of fuel.
2.
Many regions in the country are
facing severe power cuts extending to 12 -14 hours in a day.
3.
Grid tied solar PV systems are the
most common systems for captive power generation. It is technologically
feasible to have hybrid systems such as Solar PV +Grid or Solar PV +Diesel
Generator
4.
It cost about US $. 3000/kW with battery
storage and US $ 2200/kW without battery storage. It seems high as compared to
conventional diesel power generators. However, operational cost for diesel
generator is very high. Considering the replacement cost of power generation
from diesel generators, solar PV systems provide attractive returns.
Advantages of Off-Grid Solar Systems
1. No access to the utility grid
Off-grid solar systems can be cheaper than extending power lines
in certain remote areas. The economics will vary from place
to place.
2. Become energy self-sufficient
Living off the grid and being
self-sufficient feels good. For some people, this feeling is worth more than
saving money. Energy self-sufficiency is also a form of security. Power
failures on the utility grid do not affect off-grid solar systems. On the flip
side, batteries can only store a certain amount of energy, and during cloudy
times, being connected to the grid is actually where the security is. You
should install a backup generator to be prepared for these kinds of situations.
Equipment for Off-Grid Solar Systems
Typical off-grid solar systems require the following
extra components:
- Solar
Charge Controller
- Battery
Bank
- DC
Disconnect (additional)
- Off-Grid
Inverter
- Backup
Generator (optional)
Solar Charge Controller
Solar charge controllers are also known as charge
regulators or just battery regulators. The last term is probably the best to
describe what this device actually does: Solar battery chargers limit the rate
of current being delivered to the battery bank and protect the batteries from
overcharging. Good charge controllers are crucial
for keeping the batteries healthy, which ensures the lifetime of a battery bank
is maximized. If you have a battery-based inverter, chances are that the
charge controller is integrated.
Battery Bank
Without a battery bank (or a generator) it’ll be
lights out by sunset. A battery bank is essentially a group of batteries wired
together.
DC Disconnect Switch
AC and DC safety disconnects are required for all
solar systems. For off-grid solar systems, one additional DC disconnect is
installed between the battery bank and the off-grid inverter. It is used to
switch off the current flowing between these components. This is important for
maintenance, troubleshooting and protection against electrical fires.
Off-Grid Inverter
There`s no need for an inverter if you`re only setting
up solar panels for your boat, your RV, or something else that runs on DC
current. You will need an inverter to convert DC to AC for all other electrical
appliances. Off-grid inverters do not have to match
phase with the utility sine wave as opposed to grid-tie inverters. Electrical
current flows from the solar panels through the solar charge controller and the
bank battery bank before it is finally converted into AC by the
off-grid-inverter.
Backup
Generator
It takes a lot of money and big batteries to
prepare for several consecutive days without the sun shining (or access to the
grid). In most cases, installing a backup generator that runs on diesel is a
better choice than investing in an oversized battery bank that seldom gets to
operate at its full potential. Generators can run on propane, petroleum,
gasoline and many other fuel types. Backup generators typically output AC,
which can be sent through the inverter for direct use, or it can be converted
into DC for battery storage.
Hybrid Solar Systems
Hybrid solar systems combine the best from
grid-tied and off-grid solar systems. These systems can either be described as
off-grid solar with utility backup power, or grid-tied solar with extra battery
storage. If you own a grid-tied solar system and
drive a vehicle that runs on electricity, you already kind of have a hybrid
setup. The electrical vehicle is really just a battery with wheels.
Advantages of Hybrid Solar Systems
1. Less expensive than off-gird solar systems
Hybrid solar systems are less
expensive than off-grid solar systems. You don`t really
need a backup generator, and the capacity of your battery bank can be
downsized. Off-peak electricity from the utility company is cheaper than
diesel. The introduction of hybrid solar systems has
opened up for many interesting innovations.New inverters let homeowners
take advantage of changes in the utility electricity rates throughout the day. Solar
panels happen to output the most electrical power at noon – not long before the
price of electricity peaks. Your home and electrical vehicle can be programmed
to consume power during off-peak hours (or from your solar panels). Consequently,
you can temporarily store whatever excess electricity your solar panels in
batteries, and put it on the utility grid when you are paid the most for every
kWh. Smart solar holds a lot of promise. The
concept will become increasingly important as we transition towards the smart
grid in the coming years.
Equipment for Hybrid Solar Systems
Typical hybrid solar systems are based on the
following additional components:
- Charge
Controller
- Battery
Bank
- DC
Disconnect (additional)
- Battery-Based
Grid-Tie Inverter
- Power
Meter
Battery-Based Grid-Tie Inverter
Hybrid solar systems utilize batter-based grid-tie
inverters. These devices combine can draw electrical power to and from battery
banks, as well as synchronize with the utility grid. for the vast majority of
homeowners, tapping the utility grid for electricity and energy storage is
significantly cheaper and more practical than using battery banks and/or backup
generators.
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