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By Sep 12, 2010

Welcome back to GreenergyStar Report. This week we will look into the difference between a battery and “batteryless” solar system. We will also introduce Grid Tie inverters, the benefits they provide and the upfront cost that needs to be dealt with by homeowners for utilizing such technology.

PV Systems – with battery

Traditionally we use batteries to store the charge from our photovoltaic (PV) systems. They provide energy when the sun is down and can be use as a backup during blackouts. This is the impression that imprints in everyone’s mind about a PV system. And often, our mail box gets flooded with questions on something like: “I would like to go solar, and I need a system that can support my TV, lights, fridge and etc… ” People asked this question because they were taught that battery is their only option when going solar!

However, as we soon learn, when the system gets bigger (…to power a house), using batteries as storage devices is just more complicated than an average household owner would want.

First of all, batteries are expensive. Depending on the type of batteries you get, they range from US$100.00 for a lead acid battery to the more expensive $200.00+ deep cycle batteries. Due to the fact that these cells will be discharged pretty often, it is recommended to get the deep cycle batteries.  Now, we will probably need at least 10 of theses cell to form a battery bank so our spending will be in thousands.

Secondly, batteries need maintenance. They have a set number of cycles before they die out and replacements are costly. A high-quality charge / discharge controller, combined with good knowledge on sizing will probably help squeeze more cycles but for folks that are new to this field, their first set of batteries are expected to have a life span shorter than average.

Another problem is, when a battery has a state of charge (SOC) over 70% (in other words, 70% full), the charging efficiency is 20% and will keep decreasing as the battery gets charged up. This means holding solar energy with batteries will only have 60 - 70% efficiency and the rest are lost as “heat”. After a complete charge, the on-going chemical reaction in the cell will cause the cell to slowly discharged overtime. Not to mention there is further energy lost in the charge controller and inverter during charging / discharging. On top of this, a battery bank will take up quite a bit of space for storage and when disposing them at the end of their lives, if they are not handled properly, heavy metals such as lead can cause dire environmental problems as they are extremely toxic. This list can goes on and on…

And finally, the biggest problem is to converting energy stored in the 12VDC batteries to 120VAC for household electronics with a traditional inverter. Since the electric generated by the inverter is not in sync with the electric provided by the grid, they cannot be used together. What this means is we can only either draw power from the grid, or the battery bank, but not both at the same time. That is very inconvenient. Isn’t there a better way to do this? Why can’t there be a large battery bank that is affordable, maintenance free and works well with household electronic?

…Hey, wait a minute… isn’t that something we called “the grid”? What if we can turn this grid which is readily available into our new “battery bank”?

Ah-Ha! So here is where Grid Tie inverters kick in…

Stay tuned for part 2 – Batteryless PV systems.

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