Solar Panels on residential Homes

[Open Neutral]

In theory, you could have enough panels on your house to gurantee that the meter went net backwards over a 12 month period. However, utilities have negotiated sweet deals with State Utility commissions that say they get to keep the excess over a 12-month period without having to send you a check.

I believe that was changed by federal legislation. Now, the PoCo must send you a check. But the buy prices is not the same as the sell price...

 

[mull982]

View the inverter output as a constant current source. The grid and its distribution transformer is a relatively low impedance source relative to the impedance of the inverter.

Why do we view the inverter as a constant current source?

If the inverter does not have enough power at any instant to supply all that is required of the house, then all of the inverter power goes to the house, and the remaining needed is supplied from the grid. This is all automatic based on the electrical circuit sources and impedances.

Are both the inverter and grid voltages equal for this case? If not how are they adjusted?

If by chance the exact amount of power required by the house is supplied by the inverter, then of necessity the house voltage exactly equals the grid voltage, and no current flows from the grid because the voltage difference is zero.

I'm not sure why you are stating that out of necessity the house voltage equals the grid voltage. If the voltages were equal and the grid had a lower souce impedance as you mentioned wouldn't more current want to flow from the grid than it would the inverter?

If the house requires less power than the amount being produced by the solar system, then the house voltage will be somewhat greater than the grid and power will flow to the grid.

When you mention "House Voltage" is this the same voltage as the "Inverter Voltage" . In other words are you saying that the house voltage is always the same as the inverter voltage?

In this case how does the house voltage become higher than the grid voltage.

I think it will make sense to me after I clarify these 3 cases.

 

[sgunsel]

Our utility will generously pay $.015 per KWH, they charge $0.135 for the same. There is no way this can be considered economically viable for a homeowner, unless somwone gives you the solar system essentially free. Their generating costs are probably between $0.035 and $0.04 per KWH. Also, solar panel life expectancy is seldom discussed. I doubt they will perform 20 years to 80% capacity. Cost has to come down a lot before they can replace the grid. Of course "incentives" can change the math, but only for the select few that can take advantage of them.

 

[gar]

110323-1336 EDT

sgunsel:

I agree with you.

In the State of Michigan the government has required two major utilities to provide some very good incentives. But these are only going to last a limited time.

As good as the Detroit Edison incentive is I can not economically justify it in my case. You would have to be in the open country with open sky and optimum orientation, or be lucky in the city, and have excess capital and a high income tax bill to get a reasonable payback. This won't include any battery storage capability.


As an aside. The Ford Motor Company at the Wayne Michigan assembly plant for the Focus is installing a large solar array. Someone told me this is in excess of 1,000,000 watts capability. I believe it is associated with some incentive. Also the adjacent plant that was formerly Michigan Truck where the Expedition and Navigator were previously made has been converted to produce the Hybrid Focus. It probably also can build the standard focus and possibly other models. It is now producing cars based of the employee lot and the assembled car lot. I suspect that some of these are the hybrid.


mull982:

First, a part of the inverter is designed to extract maximum power from the photovoltaic array. This is roughly the knee of the photovoltaic V-I curve. This is constantly changing and the inverter adjusts for the array change. Then within a useful, but limited range, the inverter output works like a constant current source.

Consider a 10 V battery with an internal resistance of 1 ohm and a load resistance of 9 ohms. The load current is 1 A and its voltage is 9 V.

From a 100 V battery I connect an 81 ohm resistor to the 9 ohm resistor. If the 10 V battery is disconnected the voltage across the 9 ohm resistor from the current from the 100 V battery is 10 V. Reconnect the 10 V battery and no current flows thru the 1 ohm internal impedance of the battery.

Change the 9 ohm resistor to other values and you can see how the current distributes between the two batteries and how the direction changes to the 10 V battery.

Why do we view the inverter as a constant current source?

Because it is designed that way.

Are both the inverter and grid voltages equal for this case? If not how are they adjusted?

You need to view the primary voltage to the pole transformer as a constant. Combine with this constant voltage source the impedance from the transformer primary point to the house main panel point. The main panel is where the inverter interfaces. All house loads emanate from the house main panel. Thus, three things interface at the main panel --- the grid supply, the solar supply, and the house loads. All these voltages are equal at the main panel at any time, because it is one and the same place. Under load conditions the voltage at the main panel is different than at the sources or the loads.

I'm not sure why you are stating that out of necessity the house voltage equals the grid voltage. If the voltages were equal and the grid had a lower source impedance as you mentioned wouldn't more current want to flow from the grid than it would the inverter?

If there is no energy flow from the grid to the house, then there is no current and the voltage drop thru the pole transformer and distribution wires has to be zero. Therefore, under these conditions the transformer secondary voltage has to equal the voltage at the main panel. Note: no load current on the transformer and there is no voltage drop across the internal impedance of the transformer.

When you mention "House Voltage" is this the same voltage as the "Inverter Voltage" . In other words are you saying that the house voltage is always the same as the inverter voltage?

In this case how does the house voltage become higher than the grid voltage.

I think it will make sense to me after I clarify these 3 cases.

When I refer to house voltage it is the voltage at the house main panel.

 

[mull982]

.

Consider a 10 V battery with an internal resistance of 1 ohm and a load resistance of 9 ohms. The load current is 1 A and its voltage is 9 V.

From a 100 V battery I connect an 81 ohm resistor to the 9 ohm resistor. If the 10 V battery is disconnected the voltage across the 9 ohm resistor from the current from the 100 V battery is 10 V. Reconnect the 10 V battery and no current flows thru the 1 ohm internal impedance of the battery.

Change the 9 ohm resistor to other values and you can see how the current distributes between the two batteries and how the direction changes to the 10 V battery.

O.K. this makes sense. There is no current flow throgh the 1ohm impedance of the 10V battery because the voltage difference betwee the 10V battery and the 10V across the 9ohm load resistor (sitting at 10V) is 0V therefore no current will flow.

I guess I was thinking of two sources more or less as being a current divider and as long as there was sources and impedances there would be some current flow even on the lesser voltage source. Your example makes sense now.

So if we take the 100V battery to be the grid, and the 10V battery to be the inverter we can see that the nature of this circuit dictates that all the current and therfore power will come from the grid with the 81 ohms representing the transformer and wiring to the main panel. So if this was the case, how would the inverter react to start supplying power from the PV array? Would it raise its voltage of 10V or would it somehow lower its 1ohm impedance?

Lets say for instance we raised the 10V battery to 50V?

Are these source impedances taken into consideration during the design stages of such an application?

Would the same principal apply if were were talking about a generator instead of an inverter?

 

[gar]

110323-1609 EST

mull982:

No.

The 10 V battery is the grid, and the 100 V battery and its 81 ohm resistor approximates a current source for reasonable values of the load resistance.

Make the load resistance 0, then the grid supplies 10 A and the 100 V source about 1.23 A. When the load is infinite the 100 V source is feeding 90/(81+1) = 1.097560976 A into the 10 V battery (grid). The % change in the current source current was about 10% for a load resistance change of 0 to infinity.

.

 

[sameguy]

Here in Syracuse NY we get about 2.3 solar days a week and solar will not pay for itself.
My understanding is with grid tie the power from both POCO and inverter go to the panel, if any power from the POCO is not needed due to lack of load the inverter will power any loads or push back to the POCO if it is producing more voltage than needed by the loads.

 

[dmagyar]

No added appraisal for solar in California

No added appraisal for solar in California

In Wisconsin renewable energy systems are exempt from property taxes by state statue.
Other states should do this as well. But the greeds are always looking out for the big guys first.

California is likewise not increasing appraisals for solar power add on's.