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
Best Answer
The system will work.
How much rubbish you can compact how many times depends on
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
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.
[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
Related material:
"Assumptions"
A user question shows a misunderstanding of the engineering concept and implication of "assumptions".
A student of the art said ...
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".
4 points. All are key. Substantially vary any one and the result varies accordingly. Next ...
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 "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.
...
Based on known real world performance.
More assumptions. Stated so users can change them. Based on (my) real world experience, but clearly stated so that anyone change them.
Actual calculations so users can see how assumptions are used.
That's all fact based.
Facts from experience.
Assumption modification. Clearly stated. Clearly reasoned.
Reassess based on the above.
...
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.