×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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

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

(OP)
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)

RE: Idea for offsetting solar/wind with grid power...

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

RE: Idea for offsetting solar/wind with grid power...

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: Eng-Tips.com Forum Policies


RE: Idea for offsetting solar/wind with grid power...

(OP)
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.)

RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

(OP)
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).

RE: Idea for offsetting solar/wind with grid power...

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

RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

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?????

RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

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: Eng-Tips.com Forum Policies


RE: Idea for offsetting solar/wind with grid power...

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????

RE: Idea for offsetting solar/wind with grid power...

Use an alternate energy system to charge the batteries of a Uninteruptible Power Supply. Set the charging voltage of the alternate energy source higher than the charging voltage from the grid powered charger.
If the UPS and its load are large in relation to the alternate energy source most of the energy from the alternate source will be utilized.
Automotive type lead-acid batteries have some flexibility in the charging voltage. The two charging voltage levels will only allow grid charging when the alternate energy source is insufficient to carry the load.
This scheme should allow almost full utilization of the available alternate energy and avoid the safety issues inherent with feeding alternate energy directly into the grid mains.
You can probably go with an off the shelf UPS. Change out the batteries for larger lead acid batteries and adjust the maximum charging voltage. Disable the bypass mode or use logic to inhibit bypass when the alternate energy supply is above a set minimum level.
respectfully

RE: Idea for offsetting solar/wind with grid power...

The whole purpose of net metering is to account for the fact that most houses don't use much electricity during the periods when solar generates the bulk of its electricity.  That's why you sell it back to the utility and buy it back later.

If you tried to store all the generated power, you'd need a truckload of batteries, which is hardly economical nor exactly low maintenance.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies


RE: Idea for offsetting solar/wind with grid power...

That's the problem I have with this IR. You want to use all of the available green energy you can. So, you either grid tie and net meter or you store it and balance the energy production and storage with the usage. Storage requires batteries and the negatives that go with them.

RE: Idea for offsetting solar/wind with grid power...

Thats why I want to approach it from the other end. Simple installation, no discernable difference to user, very minimal loss of power from the grid and when available the alt source is used if needed.
It does make very poor use of the potential of the alt source but in our current situation it seems like pain free (other than cost) systems don't exist.
If the loads were selected based on daytime usage maybe it could someday pay off. ( televisions, computers, refridgeration...)

Just think if one day some wise guy really makes solar panels cheaply then this would become much more attractive.



One possible usage is pre-heating the incoming water in a seperate tank before it reaches the main water heater. This would allow some energy storage and usefullness as hot water is a major draw of electrical energy.

RE: Idea for offsetting solar/wind with grid power...

Unless you're running a laundry from your home, that basically means that you're wasting all the solar.  And, you'd only need about 2 m^2 to heat 50 gal of water, so the economy of scale isn't even there.

What's wrong with net metering?  You get to sell your electricity at premium rates and buy it back at a lower rate.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies


RE: Idea for offsetting solar/wind with grid power...

Part of the net metering problem is the whole torrent of bureaucracy one subjects themselves to.  The POCO often charges some exorbitant "meter charge".  Permits...  Inspectors crawling around, etc, etc.  Basically violating the premise of allowing the average Joe to make some little contribution to The Problem and to his wallet.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Idea for offsetting solar/wind with grid power...

Net metering meets with much resistance from the utilities. This example from kansas as much as states openly that the utility cannot afford the competition for power production.

http://www.kepco.org/news/netmetering.html

This should not be an allowable argument. How would they adress a person who gets most of his power from alt energy and then occcasionally buys some from the grid. They clearly cannot oppose this on some moral/legal ground. They can make a claim that they have fixed costs from providing the availability of power and they would be right to charge enough to keep suppply available.

If this argument is allowed to prevail it means there will be no independent power production because everyone has to buy their share in order to keep the infrastructure operating. In other words their monopoly is a neccessity.
Doesen't sound like the free market to me.
Oh I forgot that only applies when you loose your job or pay high prices at the pump.

RE: Idea for offsetting solar/wind with grid power...

(OP)
2dye4 - An OS project would be ideal. My vision would be to have the plans available online, then work out a deal with a supplier to provide DIY packages of all the parts needed to build it yourself. This would allow people to just print the plans and source the parts themselves, or buy a ready-to-build kit, or even sell assembled kits.


To address another thing...

I use solar merely as an example. Obviously, it doen't work at night when most people are actually at home. But, during the heat of the day, it can offset the power needs of a refrigerator, window A/C, fans, pumps, etc. etc. Basically, all of the stuff that needs to run even when no one is home.

However, my thinking doesn't stop there. Since any dc supply can be easly plugged in, it will allow people to develop new, novel ways of generating electricity. For instance, I saw on Instructables.com that, as a school project, someone made a wind generator out of a Pringles can. It doesn't produce a lot of power, but they're really cheap to make, and a small farm of them on an outdoor balcony could be a decent contribution.

But there's also a lot of other things you could build... What about power generating exercise equipment? Or scavenging power from heat sinks? Or even mini water turbines on shower heads and faucets...? The ideas are endless, but there's no suitable way to implement all of these tiny ideas so that they can actually be of benefit past a science experiment.

That type of ingenuity, and the fact that to offset a little bit of power wouldn't take a lot of money in the DIY world, would really get this idea off the ground on a larger scale. Right now, to even get in the game takes a huge investment. With this 'box' you have a choice between the commercial panel/generator route, or the diy route.

One thing to remember is that this isn't supposed to be a whole-house system; each 'box' is only supposed to offset a few devices, or basically replace only a single wall outlet.

RE: Idea for offsetting solar/wind with grid power...

You want to see realll poco fighting and back peddling and from the company that runs the most gratuitous, pro solar ads on TV, check this very interesting site out.

Note the page on the fight with the power company.
Where the poco makes its real intentions abundantly clear.

http://www.solarwarrior.com/

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Idea for offsetting solar/wind with grid power...

spdracer22

I agree that a "box" with this functionality would open up many opportunities for alt energy. However to make this thing power real loads with the necessary source transfer capability   means it will not be cheap. Even the parts cost will be in the range of $200.00. I still think it is a worthwhile project.
Is there a web location for people to gather and pursue projects like this??
I am already working on a block diagram for how I imagine the device to function and will welcome all the eng-tips feedback I can get.
There could be a real market for this device given the fact that it allows alt energy to be utilized without all the usual problems. ( No financial payback, just satisfaction )
Lets get started

RE: Idea for offsetting solar/wind with grid power...

(OP)
Kieth hits a very, very good point, and one that this proposed system circumvents. Since you're not feeding back to the grid, you don't have to worry about interconnection issues. The poco will only see a drop in consumption, much like they'd see if you simply turned off a few big devices, or swapped them for more efficient models. You're not trying to feed the grid, so they can't do anything about it...

RE: Idea for offsetting solar/wind with grid power...

I think that any battery will add a large undesirable to the scheme.

For starters a 40% energy loss!  They have lousy charging efficiencies. Someone lays out $300 for a 40W panel and then from then on it acts like a 25W panel?  Yuck.  The need to replace the battery every few years.. Yuck.  The system costs more. Yuck.  The system is heavier. Yuck.  Bulkier. Yuck.

I like the aforementioned suggestion of converting the Line power to DC feeding instead a (small)capacitor bank.  Next an inverter stepping the solar power up to a slightly higher voltage also charging the capacitor bank. If the solar is available it trumps the line power. An inverter is fed from the capacitor bank.

The user would select a device that always runs or runs a lot during solar hours and plugs it into the inverter's output.  This would effectively remove that device or some part of it from the daily utility charge realm without back feeding.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Idea for offsetting solar/wind with grid power...

(OP)
So, something like (imagine the |'s are lined up):

AC--->Rectifier--->|
|--->Capacitor--->Inverter--->Load
DC---------------->|

Or:

AC--->Rectifier--->|
|--->Inverter--->Load
DC--->Capacitor--->|

RE: Idea for offsetting solar/wind with grid power...

The first. Not the second.

There's a lot of details that might also go into the Line to CapBank aspect.  Another words not just rectifiers.

This is certainly a much higher tech project than a feed the power to the Grid job...

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Idea for offsetting solar/wind with grid power...

As others have mentioned, these things all sure sound like UPS systems.  You've almost perfectly described an off-line UPS and a double-conversion UPS in the above.  The only one I haven't seen so far in the above is line-interactive.

If you ripped out the battery, and plugged in your DC generator (with sufficient voltage control), you'd be all set.  The only head-scratcher I see is, what happens if you have your DC generator connected and there is no load?  It's unclear to me what happens to a wind or solar generator if no load is attached....  overvoltages might be a problem....

Seems to me the best way to start tinkering with this would be to go buy a cheap APC UPS.  There's some available on ebay for $15-20.

RE: Idea for offsetting solar/wind with grid power...

I agree about the batteries Keith. But, you are going to need one very large capacitor bank to be able to get better utilization from that 40W panel without them.

I'd think you want a load that draws more power than the renewable source can provide. You use an AC-DC-AC inverter as the basis. Then, you add a DC-DC controller that pushes as much renewable energy as possible into the DC buss of this inverter. The DC buss of this inverter will simultaneously get the extra power it needs from the line. I think maybe this is what you were hinting at Keith in your description?

Switching between the two sources would be a waste of the potential capability of the RE in my opinion.

RE: Idea for offsetting solar/wind with grid power...

As for batteries.
How bout a small Ni-Cad pack like used in Hybrid Cars.
It even comes complete with a marketing hype built in.
"Hybrid Solar Power System"
Let the investors form a line to the left.

I have to disagree with Lionel. My main concern is that the line supply system not waste so much power when the alt source is idle as to render the whold think a wash.
Switching to direct line power without processing it during down time is a necessity to avoid the (80-90)% maximum effeciency when the power is processed.

My implementation.
Power Transformer with three isolated 120 VAC windings chosed for effeciency. The AC main is switched into one winding by a relay controlled by the system. The alt source feeds an inverter that connects to winding 2. The output to the load comes from winding 3.  This gives you the isolation for safety. When the alt source is available the relay opens the mains winding and supplies via the inverter winding.
The control system has to monitor the available alt energy and decide when it can switch as well as redundant safety measures to make sure the alt source doesn't feed the grid through the transformer. Also zero cross switching of the relay would be nice.

RE: Idea for offsetting solar/wind with grid power...

2dye4 -- your solution starts to sound like the line-interactive UPS system that was missing in the above.....

RE: Idea for offsetting solar/wind with grid power...

I hear what you're saying but I still think that you will need to match a load to a much bigger RE system to get significant RE only run time. Then, you're wasting a lot of the potential energy production you paid for. And, last time I checked solar panels were far from cheap and wind doesn't give a nice constant output.

I'd be curious to see some preliminary data comparing the required RE source vs the possible savings achievable when suppling some typical house loads.

RE: Idea for offsetting solar/wind with grid power...

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: Eng-Tips.com Forum Policies


RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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: Eng-Tips.com Forum Policies

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

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.




RE: Idea for offsetting solar/wind with grid power...

"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.

RE: Idea for offsetting solar/wind with grid power...

(OP)
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.

RE: Idea for offsetting solar/wind with grid power...

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.  

RE: Idea for offsetting solar/wind with grid power...

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: Eng-Tips.com Forum Policies

RE: Idea for offsetting solar/wind with grid power...

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.

RE: Idea for offsetting solar/wind with grid power...

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources