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There are basically three types of PV systems in common use:
Off-Grid systems are independent of the utility power grid. They generate electricity, store it, and make it available for use without any connection to the power grid.
Off-Grid systems use PV panels connected to a Charge Controller to charge a set of batteries. The stored energy from the batteries is usually converted to regular 120 volt, AC power by an inverter. But, some small systems just use the DC power directly from the batteries.
Grid-Tied systems use PV panels to generate DC power. The DC power goes to a grid interactive (grid-tied) inverter which converts the PV panel DC power to 240 volt AC power that is compatible with the power grid. The power from the PV panels goes (via the grid tie inverter) to supply the household power needs. If the PV system is generating more power than the house can use, the excess is sent out over the grid to supply others. If the house needs more power than the PV system can supply, then the extra is drawn from the grid as usual.
Grid tied systems only work when the grid is up. If the grid power goes out, the grid tie inverter is required to shut down immediately.
Grid-Tied with Batteries:
A grid tied with batteries system is kind of a mix of the two system. It basically operates like a grid tied system when the grid is up, but it also charges a set of batteries. If the grid goes down, the inverter disconnects from the grid (to protect line workers), but it continues to supply power to the house from the batteries and inverter -- essentially behaving like an off-grid system when the grid is down.
Here is a run down on the pros and cons of each system:
- Lowest initial cost (because there is no need for batteries and charge controller)
- Lowest ongoing maintenance cost (no batteries to maintain and replace)
- Simplest to install
- Most efficient (because there are losses associated with charging batteries)
- You can start small and add (with some limitations)
- No power when the grid is down.
- Access to the utility power grid is required.
- Provides power independently of the utility grid -- you still have power when the grid is down.
- Don't have to deal with the utility company
- Can save significantly on initial cost if a long grid extension is needed to get to your house
- Really encourages conservation and efficiency in the use of electricity -- off-grid people typically get along on far less power than on grid and do it without any significant life style changes.
- Higher initial cost (needs a set of batteries and charge controller)
- Higher ongoing time and cost (the cost of batteries over time is significant -- some say its about as much as buying grid power)
- A good generator will likely be necessary from time to time
- The system must be large enough to supply your full power needs during the lowest sun part of the year (although a generator can be used to supplement during the worst times).
- You are the power company, and responsible for safe and reliable operation, and maintenance.
Grid-Tie with Battery Backup:
- Provides backup power when the utility grid goes down.
- Provides some of the advantages of both systems.
- Still has a battery system (usually smaller) that is expensive to buy and maintain.
While I would like to have tried an grid-tie system with batteries, our conclusion was that it was a lot of extra bother and inefficiency just to have power during the very few times our grid power is down. Even though we live in a somewhat rural area, our power has probably been off only at total of about 10 hours over the last 10 years. We have a small generator that supplies enough power to run the fridge and a few small things. It seems like most of the time when we have a power failure, by the time I get the generator out and gassed up, the power has come back on.
While I would not mind the maintenance involved in keeping a set of batteries healthy, the fact that the battery set pretty well wipes out any savings the system would produce and caused it to operate less efficiently made tipped the scales toward the grid-tie.
So, this write-up concentrates on our grid-tie system, but here are some examples of the other systems from the PV section:
Some off grid systems...
Some grid-tie with battery systems...
Some other grid-tie systems...
It is a fact that people who live off-grid generally use a lot less power than people who live on grid. I guess when each KWH you use has a large cost initial system cost, and when the consequences of using too much power is that you run out of power it makes you very conscious of power consumption. Power becomes a precious resource that you learn to use with care. The interesting thing is that this typically does not mean any significant changes in lifestyle -- it just means paying attention to things that use electricity, improving efficiency, and eliminating waste. If we could somehow get everyone to live as though they were off-grid, I am convinced it would cut residential power consumption to less than a third of what it is now. There must be some good way to do this? Any ideas?...
|Next -- How big to make the system?|
Gary November 20, 2009