Solar Update: Grid-Tie HO!!!
May 26th, 2002
Well, at last I have a problem: The batteries are all full, the shed is in use, and I just can't seem to use all that power coming off the roof...
What to do?
Go GRID TIE!!!
As some of you might know, Grid tie is the selling of excess solar power output directly to the grid. In order to do this right, one needs a Grid Tie inverter with a special connection to the power company. Big bucks for the inverter, and bigger bucks to hire an electrician. Also most grid tie inverters cost in the range of $2,000-$3,000, require 48 volts, require arrays in the 1,000-2,500 watt range, and basically have to be alone with the solar panels. Typically they don't work with battery systems.
The only other option is to get a MicroSine type inverter. They cost less (about $240.00) and they can be installed by basically plugging them into the panel and power. The problem is they are rated at 100 watts output at most, so I would need 6 of them for my 600 watt system.
Or would I...
The breakthrough in my mind is the adage that I have been writing about on this site: The main strength of solar power is not power output, but power output over *TIME*. My 600 watt panels can produce 2,400 watts of power on a typical day. If I used the micro-sine attached to the panels alone, I would get one hundred watts of power per hour, or 400 total. Leaving 2,000 watts to go to waste.
HOWEVER: What if instead of hooking the micro-sine to the panels, I hook it to the batteries?
Apparently this will work. 24 volts of battery would be like 24 volts of solar. The current would allow the MPPT of the inverter to suck 100 watts out of the batteries while the sun was down, and suck power from the panels while the sun was up. With 24 hours in the day, I could pull down 2,400 watts of power over a period of 24 hours. However there are a few problems
Problem 1: Battery core size.
In order for this to work, I can't be burning down the batteries every night. In fact I wouldn't want to go below 50% DOD on the battery core. Thus my battery pile would have to be at least 5,000 watt/hours in size.
Currently I have 300amp/hr at 24 volts which equals 7,200 watts of battery power. Plenty to handle a 2,400 watt per day downdraw/updraw.
Problem 2: Shady days
While the system would be able to regenerate itself on fully sunny days, 2,400 watts is the practical limit of my panels, and output will be less if the batteries need a charge, if I use the shed, or if the tractor needs charging. All of these happen, so what I really need is a regulation system to not draw down the batteries unless they are basically full.
My thought is this: My Pro-Solar charge controller on the 24 volt side has three indicator lights. The red light comes on when the batteries are dead (20% SOC). The yellow comes on at 50% SOC, and the green light comes on above 80% (charged). Now, if I tie two relays into the green and yellow lights, I can do the following logic:
When the green LED is on, relay two (which is connected to the yellow LED) is open. This allows a current to latch from the 12 volt source of the ProSolar (what powers the LED) through the NO connection of relay two and into the coil of relay1. When relay 1 is closed, the output is connected to the 100 watt inverter, and we sell power to the utility.
When the batteries fall to a 50% SOC, the yellow light comes on. This closed relay two, which breaks the connection to relay one, and the inverter is shut off. The batteries can then charge. Normally we would not get to 50% SOC due to the size of the battery pile, however if we do have clouds or whatever, the inverter will be shut down till the batteries can get back above 80%.
This allows us to sell all surplus power to the grid. In theory, as long as I am getting <2400 watts per day I can keep adding panels secure in the knowledge that all the spare power is going to the utility, and not going to waste.
I ordered the inverter today, and will install it over the weekend. I'll post a June update letting the world know how it works out.
Chris