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I live in Chicago and I walk past these trash cans every day that have a large solar cell on the top of them. These trash cans are supposed to collect energy which runs a trash compactor inside. This theoretically results in more room for more trash which means the city can spend less money picking up garbage.

My instinct tells me that this plan doesn't work. I just want to ask the community what your thoughts are about this.

The trash cans are in downtown Chicago, surrounded by skyscrapers. Do solar cells need direct sunlight in order to be able to generate a usable level of energy? I can't see the cells getting more than 3 hours a day of usable sunlight.

Also, the solar cells are covered with a plastic shell to protect the cells from dirt, grime, etc. However, the shells are thereby dirty which further inhibits solar energy from reaching the cells.

Is this idea technically feasible in the "hostile" environment that these trash compactors are in?

A little tid-bit on the side - I had a college professor once tell me that he worked for an oil company building a solar array to power radios that would go on oil rigs. A problem they had was bird feces. Apparently bird waste is high in uric acid which would stain the plastic covering of the cells, thereby reducing the amount of energy that could get to the cells. There's lots of pigeons in Chicago, although I've never seen one perched on a trash can :).

edit: spec sheet for the BigBelly compactor

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  • 2
    \$\begingroup\$ Your question is written to be very opinionated which is a reason to close it. However, you potentially have a good technical question. Please edit your question to be just about electronic design. \$\endgroup\$ – Kellenjb May 28 '12 at 1:21
  • \$\begingroup\$ Half my question got deleted. I hit the touchpad with my palm and erased half of it. I've updated it with more details to focus on the specifics. \$\endgroup\$ – user10005 May 28 '12 at 2:02
  • \$\begingroup\$ @kortuk or anyone else. Someone posted a link that was apparently to hard technical data on the Big Belly but I can't find the ref. May have been in meta but not found anywhere. I looked all over the Big Belly site (I think) and they seem to give very little technical data. Can someone provide the ref to a hard data source? \$\endgroup\$ – Russell McMahon Jun 9 '12 at 18:04
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The system will work.
How much rubbish you can compact how many times depends on

  • How much sun the panel gets.
  • How big the panels are (Watts peak power in full sun)
  • What technology they use (crystalline silicon, amorphous silicon, CdTe, ...) - And energy required for trash compaction (bin volume, rubbish type, rubbish volume)

Added long after:

Big Belly technical specification in my dropbox and on their site

In some places below I've added [Actual: xxx] figures next to my prior assumptions.


Based on calculations below it appears that

  • At a bad inner city location location (sun wise) in Chicago

  • in midwinter on an average December or January day

  • using a 20 W mono-crystalline silicon PV panel [Actual: 30 Watt]

  • and lead acid battery,

  • You'd probably get 10-15 compactions/day.

  • For all except 2.5 months of the year you'd get 2+ x that.

That sounds useful.

See below for the derivation of that result and the "assumptions" that it is based on.
As explained at the end, "assumptions" are initial conditions established based on best known information and past experience. They are stated clearly at the start so that the limitations of the system can be understood and so that they can be easily changed if other conditions apply.


The table below is from the wonderful www.gaisma.com site - provides solar and insolation & wind & more information from a vast range of sites worldwide.

The first line = insolation = equivalent hours of full sun daily on average, month by month.
Peak is 6.04 "sunshine hours" per day average in VII = July and lowest is 1.50 sunshine hours per day in December.

A "sunshine hour" will deliver 1000 Watts per square meter.
So an eg 50 Watt panel subject to 1.5 sunshine hours will deliver 50 x 1.5 = 75 Watt hours of energy if the sun is either full on or full off.
For reduced sunshine levels (cloud, shadow, rain, dawn/dusk, fog, ...) the light level will (of course) be lower.

As light levels reduce the best a cell can do is produce proportionately less power, but some cells are better at providing output at low levels than others.
The output of Silicon Monocrystalline cells reduces about in proportion to light level and they have the best output per sunshine hour of any technology commonly used.

Efficiencies vary with what you want to spend but the best cells have over 20% efficiencies and whole panel efficiencies of 17% but a target of 15% - 15% all up is reasonable. At 15% 1 m^2 = 150 Watts and 1 foot^2 = about 14 Watts. Lets assume a 20 W panel for now.

Gaisma Chicago

enter image description here

A 20 W panel in December gives an average of 30 W.h /day and in June - July it gives 120 Wh/day. By the time that gets stored and then used to power the compactor you'll get maybe 50%.

In winter 30 W.h x 50% = 15 W.h.
Assume the compactor uses a 1/4 HP or ~= 200 Watt motor [Actual: 1/6 HP, 130 Watt]
and that a compaction cycle takes 10 seconds (both of which I'd think should be very very adequate for a single garbage bin.).
200W x 10s x 1/3600 s/hr = 0.555 Wh/compaction - say 0.5 Wh.
So with 15 Wh available you get 15/.5 = 30 compactions/day.

BUT that's with a fully illuminated panel that gets what Chicago Winter sun it can when its available. At reduced light levels you get less. I'll put compactions/day in [[square brackets]] in the following assuming 30 on a good winters day with panel pointed at sun.

A bright clouded sky when you can't see the sun location but it almost hurts your eyes to look at can approach 0.5 suns (50,000 lux)[[15]]. A good bright clouded sky, sun not obvious and not dazzling bright can be 20% of a sun = 20,000 lux[[6]]. Dimly overcast and in deep shaded skyscraper valleys etc can go from 10% / 10,000 lux on down [[<= 3]].

I think the 200 Watts/10 seconds per compaction is probably rather higher than needed. 200 W = 20kg.metre/second. At say 50% electrical to mechanical that's say 10 kg force over 10 metres or 100kg over 1 metre with 10 seconds of operation. You'd have to have some rather stroppy rubbish and a big bin to need this - so you may be able to get say 3 to 5 x as many compactions/day as above.
ie 10 - 15 compactions on an average winter day in a rather unfavourable location.

The above was based on a 20 Watt panel. Resize as required.

I said 50% panel to output via storage.
Battery has current storage efficiency - say 85% for lead acid, and
and voltage conversion efficiency = Vbattery out / Vpanel_rated.

  • Using a lead acid battery (most usual) a panel rated at 18V (usually) delivers output from the battery at ~~ 12V so that's 2/3 efficient to start and current charge efficiency of LA is good but not 100% so say 2/3 x 85% =~ 57%. Add some wiring and connection losses and you have ~=50% panel to output.
    [Actual battery: 12 Volt. Type unspecified but wording used suggests lead acid.]

A lead acid battery is assumed only for getting a feel for charge/discharge efficiencies. There are many other factors in battery choice but the most significant one is liable to be operating temperature range. In subzero conditions none of the "traditional" batteries do really well.
Overall, if lowest lifetime cost is wanted plus operation in a wide range of temperatures, efficient use of the PV panel then the battery technology of choice is Lithium Ferro Phosphate (LiFePO4). About the only factor which may cause it not to be chosen is initial cost. The mass and volumetric energy densities are lower than for LiIon and for top NimH batteries, but this is unimportant in this role.

Gaisma Chicago

enter image description here


Related material:

"Assumptions"

A user question shows a misunderstanding of the engineering concept and implication of "assumptions".
A student of the art said ...

You make unnecessary assumptions about the system ...

This is extremely important.
An "assumption" is not a restriction per se but the assignment of an initial value to an equation set.

To "make an assumption" is NOT to set a value in stone but just the opposite - it is to say "this is the value we are using but you may wish to vary it depending on what parameters are considered important" etc. The initial values assigned to "assumptions" should not be random but would be expected to be the best available engineering guesstimates based on known data and conditions.

If you can assign a value to something AND if varying that value will affect the result then it is not "unnecessary". If you leave out something which can affect a result to "make things simpler" you risk making them, as Einstein warned "simpler than they can be". It may be that a variable has potential effect but that the solution is insensitive enough that it can be left as a constant or implied in other calculations. Here the volume of the bin may be decided to be unimportant NOT because it does not affect the end result but because all concerned have a general feel for the range of sizes that a rubbish bin is liable to take. My power & energy estimates carries an implicit "assumption" that eg we were not dealing with a 14 cubic metre dumpster.

By identifying factors that affect the result and assigning explicit values you make your answer usefully flexible and allow its limitations to be determined. By leaving possible factors unstated you deem them unimportant. If you find that you have included factors that the result is insensitive to they are simply assigned as constants.

Lets examine the "assumptions in my answer and see which ones are "unnecessary".

Depends on how much sun it gets,
how big the panels are and
what technology they use and
energy required for trash compaction.

4 points. All are key. Substantially vary any one and the result varies accordingly. Next ...

For a 20 W monocrystalline silicon PV panel and
lead acid battery
you'd probably get 10-15 compactions/day at a bad location (sun wise) in Chicago in midwinter on an average December or January day. For all except 2.5 months of the year you'd get 2+ x that.
That sounds useful.

All the above is a statement - its based on the following calculations. A 20 Watt panel is the sort of size seen on road signs and similar in typical city use. Lead acid is the battery technology of choice for industrial use. It's not the best by most measures, but it has low capital cost and some other advantages and it's liable to be what they use in the bins now.

The table below is from the wonderful www.gaisma.com site - provides solar and insolation & wind & more information from a vast range of sites worldwide.

The "assumption" here is that hard data on available solar energy will be "useful".

The meaning of the data table is explained. Degree of and language is targeted at typical site users.

...

Lets assume a 20 W panel for now
That's based on a paragraph of explanation .

A 20 W panel in December gives an average of 30 W.h /day and in June - July it gives 120 Wh/day. By the time that gets stored and then used to power the compactor you'll get maybe 50%.

Based on known real world performance.

In winter 30 W.h x 50% = 15 W.h. Assume the compactor uses a 1/4 HP or ~= 200 Watt motor and that a compaction cycle takes 10 seconds (both of which I'd think should be very very adequate for a single garbage bin.).

More assumptions. Stated so users can change them. Based on (my) real world experience, but clearly stated so that anyone change them.

200W x 10s x 1/3600 s/hr = 0.555 Wh/compaction - say 0.5 Wh.
So with 15 Wh available you get 15/.5 = 30 compactions/day.

Actual calculations so users can see how assumptions are used.

BUT that's with a fully illuminated panel that gets what Chicago Winter sun it can when its available. At reduced light levels you get less. I'll put compactions/day in [[square brackets]] in the following assuming 30 on a good winters day with panel pointed at sun.

That's all fact based.

A bright clouded sky when you can't see the sun location but it almost hurts your eyes to look at can approach 0.5 suns (50,000 lux)[[15]]. A good bright clouded sky, sun not obvious and not dazzling bright can be 20% of a sun = 20,000 lux[[6]]. Dimly overcast and in deep shaded skyscraper valleys etc can go from 10% / 10,000 lux on down [[<= 3]].

Facts from experience.

I think the 200 Watts/10 seconds per compaction is probably rather higher than needed. 200 W = 20kg.metre/second. At say 50% electrical to mechanical that's say 10 kg force over 10 metres or 100kg over 1 metre with 10 seconds of operation. You'd have to have some rather stroppy rubbish and a big bin to need this -

Assumption modification. Clearly stated. Clearly reasoned.

so you may be able to get say 3 to 5 x as many compactions/day as above.
ie 10 - 15 compactions on an average winter day in a rather unfavourable location.

Reassess based on the above.

The above was based on a 20 Watt panel. Resize as required.

...

I said 50% panel to output via storage.
Battery has current storage efficiency - say 85% for lead acid, and
and voltage conversion efficiency = Vbattery out / Vpanel_rated.
* Using a lead acid battery (most usual) a panel rated at 18V (usually) delivers output from the battery at ~~ 12V so that's 2/3 efficient to start and current charge efficiency of LA is good but not 00% so say 2/3 x 85% =~ 57%. Add some wiring and connection losses and you have ~=50% panel to output.

Figures based on experience.

Hmmm.

We seem to be at the end.
I don't see anything you'd want to risk leaving out.
I don't see any fudging, miracles, etc.
Uncertainties? Sure. But stated.
How many watts?
How long a compaction cycle ?
How many cycles a day? ...

If you can offer constructive way to improve on that I'd be pleased to see them.

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Why do you think it won't work? The only plausible reason I can think of is that there might be too little energy to drive the motor in the compressor. But if I were designing a solar powered trash compactor here's what I would do:

I would start with a solar panel that charged a battery. A common 7 amp-hour lead-acid battery would be fine. When there is enough charge in the battery to drive the motor for one "compression cycle", it does just that. By waiting until there is enough charge it can be assured that the compression cycle will complete, and not leave any of the mechanisms in the way of people throwing their trash away.

The number of "compressions per day" is limited by the size of the solar panel and amount of sunlight. On a cloudy day there would be less, and more on a sunny day. At night there might not be any (unless more sophisticated software saved some energy for the night). In a skyscraper-rich environment it would take longer to charge, but it would do it.

There does not need to be a sensor to detect how full the trash is, or how much stuff got thrown in there. Just have it compress when it has the energy to do so.

But here's the thing: on a cloudy day, or at night, it is possible that the trash can will fill up before there is enough energy to do a compression cycle. But I ask, so what? It will compress eventually, and that is still better than no compression on the old fashioned trash cans.

For this, even a tiny solar cell could eventually charge the battery. But a 15 watt panel (15 watts is approximately 1 square foot of panel) should be able to get a compression cycle every couple of hours. (I'm guessing.) For high-use areas you could use a larger panel, and a smaller one for low-use areas.

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  • \$\begingroup\$ +1 Though you're right, I'd also try to optimize it to compress only when it's reasonably full, to enhance also the usage of the battery. \$\endgroup\$ – clabacchio Jun 4 '12 at 8:23
  • \$\begingroup\$ For what values of tiny, should and guessing? \$\endgroup\$ – Russell McMahon Jun 5 '12 at 14:29
  • \$\begingroup\$ @clabacchio: The notion of compressing without worrying whether the bin is full is perhaps not unreasonable, if the amount of energy required to "compress" empty air is much less than the amount of energy required to compress "stuff"; that in turn would depend upon the mechanical design. Motors waste energy mechanically when driving a very small amount of torque, and electrically when driving a very large one. A variable-ratio transmission could greatly improve efficiency in this sort of application. \$\endgroup\$ – supercat Jun 5 '12 at 17:59
  • \$\begingroup\$ @supercat One thing to keep in mind is the overall reliability. Sensing if the can is full is difficult given the dirty environment. Ultrasonic and light based sensors could quickly foul. A mechanical sensor might not foul, but could get damaged with abuse. This thing would have to work reliably in all weather for years. It's hard to beat the reliability and robustness of absolutely nothing (no sensor). \$\endgroup\$ – user3624 Jun 5 '12 at 19:14
  • \$\begingroup\$ @DavidKessner: and a current sensor that stops the machine if current is too low -> nothing to compress? \$\endgroup\$ – clabacchio Jun 7 '12 at 13:49
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Okay, so got back back from visiting my college last week (IU (Bloomington, In))and saw the BigBelly in the downtown area as well so decided to do some homework; then came across this posting.

This system does take the energy from the sun but stores it in a battery (runs off of the battery only when needed). This product has been in various cities plus multiple climates/enviroments such as Vail CO, Philly,PA, Boston, MA, etc... It collects energy anytime light hits it (as solar energy panel do); it will collect energy on cloudy, rainy, snowy days, etc...so energy replenishment to the battery is not a problem. Check out this from the city of Philadelphia.

Also, I found out the BigBelly can report the collection data (tell you when you got to pick up garbage, give status reports). I would love to see the results, or lack of, in Chicago based on what other cities have reported and what I have found out about Streets and Santitation according to websites/articles.

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  • \$\begingroup\$ Welcome! I have just a couple of suggestions for your answer: 1) Try to be a bit more technical, as this is an engineering site, and this question is specifically about the energy autonomy of these cans; also the grammar is important, try to avoid abbreviations. 2) You can embed the link to be included in the text, it's better to do that. \$\endgroup\$ – clabacchio Jun 4 '12 at 8:28
  • \$\begingroup\$ @Paul As an introduction read: How to answer. This will help you to get better. \$\endgroup\$ – suha Jun 5 '12 at 0:31

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