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)

 

[spdracer22]

Just found this link. It seems to suggest that the inverter can be plugged right into the wall outlet.

http://www.zonnepanelen.wouterlood.com/solar_pages_uk/inverters.htm

 

[davidbeach]

How does your proposed scheme differ - fundamentally - from what you describe in A)?

 

[IRstuff]

They're already selling these sorts of systems at Costco. $30K for a 5 kW system, including installation and all the hardware.

TTFN

FAQ731-376

 

[spdracer22]

It would be after the breaker box.

In my fan example, the box would be plugged-in between the fan and the wall. It will supply only the load device connected to it.

In A, the generator is connected before the breaker box and feeds the entire house.

The point would be to allow any normal joe to aquire this box, take it home, and plug it in himself, without the need for an electrician, just like a standard computer UPS. He'd only need to get whatever kind of generator he wants, be it a 500W turbine or a 10W solar panel, and plug it in.


In the link I posted, the guy is using a solar panel and an inverter and plugging the inverter directly into the wall socket... This would be ideal, but I'm curious about how the rest of the circuitry would handle it (breaker box, main supply, outlets, etc.)

 

[spdracer22]

Ok, doing some more searching, I've found that for a grid-tie system (#1 & #3), I just need a grid-tie inverter. It doesn't matter where in the circuit it's plugged in, but I would imagine that if there are multiple supply lines to the house (mine has 3) it would have to be connected to all three to supply the whole house.

The problem is, though, I can't find any inverters that are less than 1KW, or costing less than about $1900. From what I've read, this is due to UL requirements. There used to be one made by a Dutch company (the "OK4U") that was a 100W inverter and could be plugged directly into a wall socket, but due to the UL, it's not legal to sell in the US anymore. If anyone has a resource for a similar product, it'd be great.

This, though, is where the system I described is useful. It's not a grid-tied system, but uses the grid for supplemental power, if the generator can't provide enough. It can't supply a whole house, but could be used for big things like refrigerators and air conditioners, or multiple devices can be run off of power strips.

To put a number on it, say it will handle 500W of power from a combination of two sources, in any ratio of the two.

 

[jraef]

While the concept you (and that site) describe is the basic operating design of a grid connected PV system, the leap you are taking, that of an average Joe plugging in "a box" between an outlet and a load to have the load "share" power sources, is fraught with dangers. Electricity has this nasty tendency to go wherever it can. Once you connect the PV power to the utility line, it is available to everything else on that utility line. The "box" you are wondering about that would pump AC power in only one direction from 2 sources does not exist, at least in an economical sense (explained below).

If you had a DC powered fan, you could theoretically convert your utility power to DC with a rectifier, then feed that DC into a DC isolator along with your PV source. They will truly share the load without back feeding into each other (that is of course what a grid connected PV system does). This is done on marine systems all the time. The caveat is however, you can't "tell" the load how much to draw from one source or the other, it is going to just try to draw whatever it needs. So if you try to draw more from the PV system than it is capable of delivering, it will overload. That's why in a grid connected PV system, you need to have a PV system capable of powering everything you want on that system.

To take your single load idea one more expensive step further however, you could do the DC route mentioned above, forgo the PV system inverter (since DC is what PV panels actually produce anyway), have a smaller inverter at the load so then you are back to being able to use an AC fan. But now your cost for one fan power system has become extreme.

 

[LionelHutz]

So, take a computer UPS except build it to run on batteries and switch to line power if battery power is no good. Then charge the batteries with solar panels. Is this what you're asking? Except, get rid of the batteries?

It's a nice idea but I haven't seen anyone building solar or wind inverters that will work standalone without batteries.

 

[spdracer22]

The grid-tie inverters are made to work without batteries; they tie into the house mains.

I did a little more research this weekend and found that Tripplite (and probably others) makes a PDU/ATS with multiple AC inputs. In a sense, this is what I'm looking for. It has a primary and a secondary AC input, with the secondary as the backup. When the primary supply cannot supply enough power, it switches to the secondary. In my case, the primary would be the solar panel/generator. If the power from the panel drops, it'll switch to grid.

The only problem I see is this: If the load is 100W, but the panel is only outputting 20W, then it will switch to grid, and ignore the panel until it comes back up to 100w. Without some sort of battery to capture the power that isn't being used, it's just wasted.

So, at this point, it's not highly viable... My solutions end up being one the two existing mainstream solutions, or converting AC to DC, combining with DC, then converting back to AC, as jaref suggested (unless I'm running everying DC).

 

[itsmoked]

There was once exactly what you are talking about. You could by a solar panel and wire it to a dohicky module that you plugged directly into any wall outlet.

Had sun? Reduce your household consumption! Worked really, very well.

The power companies looked at this and I'm sure extrapolated to the extreme of every tom-dick-and-harry doing this and promptly pulled out their trump card "safety" and killed every maker of these devices, almost overnight.

It was really very unfortunate.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[spdracer22]

Yeah, Europe seems to be all over this kind of thing, and why shouldn't they? I keep finding links to inverters you plug directly into a standard (European) wall outlet, and the one I mentioned earlier, the 'OK4U', could either be wired to a plug, or directly into the main. Plus, it was cheap. According to what I've read so far, the UL pretty much put the kabosh on these cheap, easy ways for us to help save the planet.

I subscribe to my electric company's 'green generation' program, which costs me about an additional $10/month from the additional per-kWh cost, but at least they're required to give me my energy from a 'green' source.

 

[itsmoked]

It is indeed a shame as a lot of people would do this just for green reasons, whereas the regular Full-Monty hits $20k in a blink prohibiting any useful percent of The People from participating.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[2dye4]

This is really a significant idea. I've been thinking the same thing.
The key is that the power flow from the grid tie is one way. The power company cannot say a thing about this. The box just allows the alt source to supply all it can to supplement the grid source.
It certainly is possible to build just such a thing for 1kw loads in the $1000 dollar range.
You need to stay away from grid tie inverters because the power companies will allways have a legitimate and convieneient disagreement with them.
But making devices that supply primarily from alt sources and then supplement with grid in a one way arrangement cannot be argued with by the power companies.

 

[2dye4]

How I would do it.

120V from grid rectified to HV DC bus.
Alt energy source is boosted to supply HV DC bus.

Inverter from HV DC bus to output sockets.

Incorporate a switchover mechanism to feedthrough grid
power directly bypassing the inverter when no significant
alt power has been available for some time limit.

Wheres my soldering iron?????

 

[spdracer22]

I definitely think this is the way to go. The only thing is keeping the efficiency decent. I think the option to bypass the rectifier/inverter at low alt power levels (ie. night with solar), would be essential. You'd have to do some testing, measuring output vs. input with varying DC supply voltages to find the switching cutoff.

 

[IRstuff]

This site appears to be quite fly-by-night:

http://www.solarsystemsintl.com/pages/residential_sys.htm

but, they were selected by Costco to sell systems to Costco members:

http://www.solarsystemsintl.com/RoadShow/CostcoMain.htm

Anyway, about $30K, installed, along with direct hookup for net metering:

http://www.solarsystemsintl.com/pages/residential_sys.htm

TTFN

FAQ731-376

 

[itsmoked]

The key is just making sure you use every erg of the solar input.

A real cheapo method,(that would violate the above premise), would be to just run the solar to an inverter that runs the Plugged In and if it becomes insufficient just switch via a relay. It could probably happen fast enough to not be noticed by most devices.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[2dye4]

Anyone know of a dc -> sinusoidal ac inverter with a
high dc voltage input 100-200V

How about doing the switchover with zero crossing solid state relays. The switchover would occur as soon as the inverter and power line were in phase. Note there is no time limit on this operation so one could wait for the inverter to drift into phase then switch. I want to avoid the blinking lights problem.

 

[IRstuff]

Then, you could be waiting for a couple of days.

Don't you think it would simpler to simply design the inverter to sync its output to anything that it's connected to?

TTFN

FAQ731-376

 

[2dye4]

Ir
yes your probably right. However remember if the inverter actually output 59.9 hz it would sync up in about 10 seconds.
I doubt if production tolerances are very tight. But of course you could have the occasional perfect unit that actually had problems syncing.

To me this concept has enormous power to enable people to incorporate Alt energy on their own without red tape interference.

I think we engineers should start a sort of open source project with the design being public domain.

The goal is to create a box. Plug this into the household power then plug a subset of the household loads into the device and thereafter these household loads are incapable of
distinguishing the source of power and therefore the human users also cannot tell. Then attach whatever alt sources you can afford and this box will transfer the power when available and required.
The whole key to this is that it is imperceptable to the user and the utility cannot complain and it allows the substituting of alt energy for grid energy.
This is the missing link.

What do you guys think????