Idea for offsetting solar/wind with grid power... 1

[spdracer22]

I'm an auto/mech engineer, and have only had a couple circuits classes, so please forgive me if I'm talking out of my you-know-what. But, easily-integrated 'green' power generation interests me, so here I am, trying to learn...

When a solar panel or wind turbine is added to a home, it's either:

A) Connected to the main supply, allowing all loads to feed off of it, and feeding any excess to the grid. It may have a battery bank between the generator and main.

B) Somewhat like a UPS. Charge a battery bank that feeds a few devices directly, then switches to main when bank dries up.


The idea:

Somewhat like B, only without the batteries. There would be a box that the load device(s) and generator would be plugged into, then the box would be plugged into a standard 110V wall outlet. 100% of the generated power would be used all the time, which would be supplemented by the wall outlet. So, for a 100W load, 10W may come from the solar panel and 90W from the wall, etc, etc.

So, my questions are:

1. Does anyone know of a company that builds such a device?

2. How would one go about building such a device? Is it as simple as just putting the supplies in parallel?

3. Or, is a better/simpler way just to invert the generated DC into AC and plug into a wall outlet? (I've heard this is a bad idea, but thought I'd throw it out there anyway)

 

[IRstuff]

daytime --> little or no household loads
--> lots of available sunlight

late afternoon --> tons of household load
--> little or no sunlight.

Anything that doesn't involve net metering or some daytime industrial load is economically and environmentally a poor investment. Why incur the environment cost of a 5 kW solar panel, only to drive a measley few hundred watts of average household load during the day?

TTFN

FAQ731-376

 

[spdracer22]

Come on guys...listen to what I've said. This is NOT SUPPOSED TO BE A FULL HOUSE SOLUTION. It's supposed to offset a few SMALL loads. Think about things that NEED to run during the heat of the day when electricity costs are highest. Sure, you're going to have a lot less load during the middle of the day when you're at work than when you're at home at night. BUT, the refrigerator still needs to run. A/C units still need to run. DVD players, cable boxes, etc. all have standby modes and draw power while they're 'off' and you're away, unable to use them. This is what this device is supposed to OFFSET. Plus, most people are home during the weekend, and there's a lot higher load during the day on sat/sun than during the week, so it will help there as well.

Plus, the system is not supposed to be able to use 5kW supplies, or even 1kW supplies. In MOST situations, you're not going to pull near 1kW from a single outlet. It's supposed to use a 50-200W max alt. supply to supplement, NOT replace grid usage.

Also, it's not meant purely to be used with solar. It's supposed to be used with any dc source, 'alternative' or not. You can charge a battery in your car on your way home from work and plug it in if you wish. As long as it's 12V, it'll work. Wind, water, solar, even human powered generators can be used. It's not limited to just solar.

-----
With that said, yet again...


peebee- Yes, it basically is an online-UPS. I just didn't know they existed.

http://www.pcguide.com/ref/power/ext/ups/typesOnLine-c.html

So, in this setup, as long as the charger and alt. source would play nicely together, it should work. The battery would act as the 'balancer' of the alt. source and AC source.

The only problem is the efficiency of doing it this way. You'd have to bank on the alt. source making up for the inefficiency of converting the AC, and then some. Though, check out the description of the 'delta converter' in that link as well.


For example:

Consider a 100W load. Inverter/Converter efficiency is 80%. Alt. source is supplying 50W to inverter.

100W/0.8 = 125W needed going into the inverter.

125W-50W = 75W needed from converter.

75W/0.8 = 93.75W needed from AC Source.

So, based on this scenario, we end up saving 6.25W on a 50W alt. source supply. Not exactly ideal. Efficiency is going to be key. At 90% C/I efficiency, we save closer to 32W, which it in the ballpark of being viable.

 

[2dye4]

My angle on it is much smaller as well. 300W of panels - 500 watt inverter powering refridgerator and lighting. It definitely is not a money making opportunity as is true still with most A/E projects.
I meant to add to my description a method for the grid to supplement the alt source that feeds the inverter. The control system would determine when to give up on alt power and use grid.
If people bought systems like this just as a hobby thing or a feel good greenie thing it could make a difference. The key is reliability and no wear out items (batteries).
Long after the glow wears off the system could be quietly doing its job of harvesting a few thousand watt-hours a day and causing its user no inconvienence.
Saving a bucket of coal or gallon of oil or milligram of nuke per day.

 

[spdracer22]

Exactly. 1kWh/day will only save me a little more than $3.65/yr. (In cali, it'd be more like $10/yr) Economically, probably not the best thing. But, it keeps 365kWh worth of energy from needing to be produced at the power plant every year (3% of my annual usage), and I feel just a little bit better. Plus, this would open up, as I've said before, the possibility of extracting power from other sources as well, both conventional and novel.

 

[IRstuff]

Sure, after 4300 days, you'll recoup the energy required to make the solar panels in the first place.

I don't have process recipes for solar cells, but if you assume 5 hrs of diffusion furnace time per lot of wafers, 100 wafers/lot and 1700 wafer total to get 7 m^2 of yielded solar panel, at 50 kW furnace power, you'd consume 4317 kWh in the manufacturing of the solar cells.

TTFN

FAQ731-376

 

[LionelHutz]

Hmm, so to make best use you're now requiring something even more complicated.

You would need an on-line and off-line mode. The UPS would run on-line only when there was enough RE available to make it save evergy vs just using line power. Sure, your numbers show that a 90% inverter section would work but only when you have 50W of RE available....what about when the RE available drops to 20W?

Put a 100W "RE Box" by your entertainment system and it would save the standby power for the TV, AV receiver, cable box and DVD player. So, you save say 20 watts = probably about 5kWh a month....this would be a waste of the 250W solar panel needed to make full use of the 100W "RE Box" when that equipment is on during the day.

The example of powering the fridge. Just what percentage of the time during the day does a fridge run? Say it's 1 hour during daylight hours. Then your 500W panel will only save you at most 0.5kWh a day....basically a waste of a 500W panel.

I'm not saying it's a bad idea but it would have to be properly applied. People just hap-hazardly slapping these boxes onto different loads without understanding the applciation will be a total failure and actually be the opposite of green power once manufacturing energy is accounted for. Might make for a warm-fuzzy good feeling but not much else.

Remember, you're calling this a plug-in UPS like box. So, it can't be powering different loads all over the house. It will only be powering the loads at one wall plug.

 

[2dye4]

Its true the effectiveness would be related to the variability of the load. The ideal load would be something only on when the alt source is available and capable of using nearly all of it.
How bout this scenario.
Box powers Entertainment center and TV in a home where there is daytime occupation. Both stay on a good portion of the day and the alt power is utilized at 50%.
It would fall to the user to understand and apply the thing effeciently.
IRstuff
You used a panel area of 7 meters ^2. This seems a bit oversize for a 300W collector. Tell me if I am wrong.

What if it were scaled up so that a house essentially had two groups of outlets. One group would be used for really high demand items. (vacuum cleaner,microwave,hair dryer...) the other group would be used for low demand and releatively constant loads. (TV,computer,lighting...) and run through a power box of the sort we are discussing.

I realize net metering and using the grid as a storage is the most sensible and usefull thing to do. However the utilities are never going to allow this on a large scale. Unless there is government intervention to require them to make it easy. However this would be a infringement of the "free market concept" and a "government intervention" and as we are told every day this would bring on the collapse of society. So a solution to incorporating alt energy into household usage has be a "go it alone" type thing as there will be no help establishing a cooperative arrangement whereby sytems trade power. That is my angle of attack. I would like there to be a way to use alt energy without batteries as they are a source of major enviromental concern and the only way is for the system to power the loads and pull what is necessary to make up for the shortfall from the grid.

 

[BJC]

"I'm an auto/mech engineer, and have only had a couple circuits classes..."

spdracer22
I wouldn't worry about circuit classes. If Engineering Economics wasn't required where you went to school I would find a class and take it.
The money for the system(s) your talking about would get a better return in several scheems. The money spent on a home solar array or windmill would have faster return on insulation upgrade, triple pane windows etc. A kilowatt that is never generated is just as good as one put back on the grid.

 

[spdracer22]

BJC- I've taken a few economics classes, as well as run my own business, just FYI. If you look at the discussion, the argument for this device is not to save money; it's to help save the environment (which, I will concede that, in practice, may have varying degrees of success, as with any 'green' generation program.)

If there is a 20 year investment recoup time and a 12 year production energy recoup time, why even pursue solar on any scale in the first place? If we could solve all of our problems by replacing windows and upgrading insulation (which, by the way, can have many disadvantages, as well), why are there any efforts at all for alternative energy?

I, for one, don't use my AC except for on the hottest days in the middle of the summer when I'm actually inside, instead opting to open windows at night and closing and covering them during the day. So, cooling costs me effectively nil, except for the small window fan I use to circulate air into the house at night. I use natural gas to heat my home and water, which is effectively green anyway.

 

[BJC]

Solar and Wind are both economically feasable. Are they feasable on the scale of a 20 amp receptacle? most likey not unless you have a lot of disposable income.

"the argument for this device is not to save money; it's to help save the environment..." that's a good ideal but in practice not many people want to spend their own disposable income to do it. They will pay higher utility bills ( because there always going up anyway), they will recycle pop cans etc.
I personally am not going to spend 2 or 3 grand on a system to save a KWH a day. That won't even pay the intrest on the loan.

 

[IRstuff]

I won't claim that my numbers aren't swags, but I think they're actually a bit conservative.

Solar energy, without some sort of buffer, simply follows what you can get from the sun. The peak total radiant power from the sun is 1120 W/m^2 at the equator; at the peak temperature point of the day, it's less than half that value. Only 1/2 of that power is usable in a solar cell; the rest is ouside of the responsivity of the cell.

Now, you need to account for the conversion efficiency of the system, which will be around 15% net. That means that a square meter of panel will net you 42 W, under those conditions. Therefore, 7 m^2 will only net you 294 W in the afternoon. At the peak, you'd get double that, so about 600 W, but your A/C probably wouldn't even kick in until 5 pm or so, to cool the house down for when you get home.

And that's with the max solar constant day. You'd need even more area to account for a typical day, or a partially overcast day. Southern Cal, whil it gets lots of sun, the sky isn't really all that clear. Much of the junk in the air will reduce the surface solar energy.

As for why doing it at all, it's the realization that fossil fuels will run out within the next century. It'll probably take us 20 to 40 yrs to develop a really efficient solar panel. Current technology barely gets over 15% to 20%, which is then further reduced by the converter efficiency. With a variable, and light, load, the converter may get less than 70% efficiency.

It's taken more than 20 yrs to get solar cells up to the current level of efficiency. Without a majot breakthrough, we're pretty much topped on with the standard photovoltaic solar cell. That would be the argument for starting now. By using the current technology in places that can really take advantage of the minimal efficiency, we can potentially postpone the depletion of our other fuel sources, allowing us more time to develop a better technology, assuming that it's even possible.

TTFN

FAQ731-376

 

[BJC]

So how much would it cost?
How much would it cost to put a corrigated aluminum roof over my existing roof? I would space the new roof about 4" over the existing roof. How much AC energy would it save?
My point is you use your limited resources to same the most energy the cheapest. For me and my limited budget a mini-solar/wind system ain't it.

 

[2dye4]

Ir
I checked a spec for a panel
Evergreen solar PVEC115 dims (62.5 X 25.7) inches
This works out to 1.03 M^2
Its rated at 115 watts.

I know they fluff their numbers with ideal situations but shouldn't one expect at least 70% of this or 80 W.

Do you see a discrepancy here or maybe they fluff their numbers even more than I expect.

 

[IRstuff]

That number is doing pretty good, since it doesn't appear to include the converter efficiency. It would seem to be a >20% efficiency solar cell, coupled with the responsivity, giving you 11.5% of peak solar power, which only lasts about an hour or so:

http://www.solardepot.com/pdf/EG_EC-100_120.pdf

TTFN

FAQ731-376