Energy harvesting 101, looking around for small solar pannels or other ideas. - Page 1
Forum section: electronics / projects

author: senso
author_rank: Frequent Contributor
timestamp: December 02, 2015, 03:02:49 pm
content: Good evening, so I've been tasked to make a smallish sensor module with an embedded radio to form a sensor network, and also to be able to last as much time as possible.For now it will only have a Bosch BME280(pressure, temperature and humidity), choosen in part because its an all in one, combined with a low power consumption.It will use an atmega128rfr2 for the comms part and either an EEPROM(1Mbit) or a Nand Flash(64Mbit), the EEPROM uses much less power, and even 1Mbit allows me to transmit 1 big packet every hour or even more time between transmissions.I sort of discovered the Ixys SMD solar panels, they are cute(lol), but provide too little power, max theoretic power output is 400uW, the Atmega+ PCF85063TP + BME280 + CAT24M01 have at least a power consumption of 2.3mW all in sleep/deep sleep with only the RTC ticking away to wake up the MCU.I was going to use CR2032 batteries, but they are rechargeble(the LIR2032 are but not available via the normal distributors) and they power density is way too low, maybe a 500-600mAh LiPo will be a better bet, but I can't find some small solar panels that would be acceptable in this project(max size around 20x30mm) that can be acquired via some legit distributor in Europe(eBay, Aliexpress can be used, but I prefer to avoid them), the energy harvesting solution that I'm thinking of using is the bq25504, it does everything(MPPT, LiPo charging all in one go).Any ideas?Best regards.

author: Inaho
author_rank: Contributor
timestamp: December 02, 2015, 04:31:20 pm
content: Have you considered a larger battery such as a CR2. Then use a solar panel to supplement power during the day like a calculator with the built in solar and dry cell? That should extend the life of the battery by quite a bit. I would also look at the camping solar panels or the ones to run game cameras or deer feeders. Those are cheep plentiful.Bobby

author: senso
author_rank: Frequent Contributor
timestamp: December 02, 2015, 04:56:35 pm
content: A calculator cell would be better, but I cant use calculator solar cell as a search term anywhere, the results are just the expected, millions of casios with solar cells, and not only the solar cell.There are lots of solar cells in ebay/Ali and dozens other sellers, but they are all way too big.I found this ones yesterday, might be usefull:http://www.flexsolarcells.com/index_files/OEM_Components/Flex_Cells/pages/00-PowerFilm-Solar-Cell-Module-SP3-12.phpThe problem is the voltage, bq25504 as a maximum voltage of 5v and even then the recommend input is just 3v, a bit afraid of frying it.

author: Inaho
author_rank: Contributor
timestamp: December 02, 2015, 05:31:34 pm
content: ok. So what about using a smaller cell and a super cap to build up to the operating point? How often are you trying to take a reading? You could also use the micro controller to directly control when power is applied to the other devices so they are really off when the micro is sleeping to cut off reading usage.Bobby

author: station240
author_rank: Unknown
timestamp: December 02, 2015, 05:43:13 pm
content: Closest I've found is a 30x30mm solar cell, used in a cheap solar pathway light. Does have decent glass protecting the cell, instead of the usual epoxy.I've got one here I can test for you, will be tomorrow before I can do that.The light looks something like this onePerhaps you can find one of the companies that make this product, and ask where you can get the cells from.

author: splin
author_rank: Frequent Contributor
timestamp: December 02, 2015, 05:56:29 pm
content: Where is this sensor to be located? If outdoors then what life expectancy do you require? Most small panels are epoxy encapsulated which degrades with UV and weather so they may not last more than 3 to 5 years. Others are amorphous thin film on glass. They are less efficient but have the potential to last much longer - provided they are properly sealed against moisture ingress around the edges. You might be able to get away with running a bead of good quality silicone around the edge but it may well not be adequate if this has to be very reliable.You could take a look at solar garden lights, which use both types of panels, around your area to see how they've fared over time.You can also get small panels encapsulated with PET with a life expectancy of 5 to 7 years, but they're a bit harder to find. Eg. this 110mW 36mm x 24.5mm panel:http://www.alibaba.com/product-detail/Super-Quality-Custom-ET-0-1W_60365589671.html?spm=a2700.7724838.30.38.kpNM69If indoors then remember that the panel's output ratings are in strong sunlight - indoors with indirect light from a window, or worse artificial lighting, you might only get .1%, .01% or even less of the rated output. On Alibaba you will find some small amorphous glass panels targeted for indoor use but I don't know if they really are more efficient in low light conditions. They are typically rated to provide 2 to 6V at uA levels.This supplier sells various small panels but they're no different to the Chinese panels on Ebay and Aliexpress.

author: mark03
author_rank: Frequent Contributor
timestamp: December 02, 2015, 07:04:12 pm
content: A while back I was looking at solar panels in the 1W to 50W range and was amazed at the high cost. The rooftop variety are now around $1 US per watt in single quantities purchased domestically. One expects a discount curve, of course, and the panel at that Alibaba link is really small, but still, $100 per watt!? Can a factory only manufacture panels of a certain size? It almost seems economic for a Chinese entrepreneur to buy the rooftop panels, slice them into smaller pieces, and make a killing. I'm sure I'm missing something.

author: splin
author_rank: Frequent Contributor
timestamp: December 02, 2015, 07:46:37 pm
content: Quote from: mark03 on December 02, 2015, 07:04:12 pmIgnore the prices on Alibaba - they are often unreliable or just a starter for negotiation. In any case, in that link it states:'Num. of cell :10=2*5pcs' so probably $10 to $15/W.Here's a 1W PET encapsulated panel for $2.6$/W:http://www.aliexpress.com/item/HOT-Sale-Mini-1W-5V-Solar-Cell-PET-Monocrystalline-Solar-Panel-Solar-Module-DIY-Solar-Charger/32515863142.html?ws_ab_test=searchweb201556_7_79_78_77_80,searchweb201644_5,searchweb201560_7

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 02, 2015, 09:19:48 pm
content: I've seen small solar panels like that in toys/kits usually for powering small motors. They might sell them for school lab supplies or electronics kit accessories.

author: splin
author_rank: Frequent Contributor
timestamp: December 02, 2015, 09:22:11 pm
content: Quote from: splin on December 02, 2015, 05:56:29 pmSorry forgot the link:http://www.cpssolar.co.uk/

author: splin
author_rank: Frequent Contributor
timestamp: December 02, 2015, 10:29:19 pm
content: And here's a link to an 'indoor' amorphous panel:http://www.alibaba.com/product-detail/4V-8uA-108x14mm-Thin-Film-Amorphous_60024458111.html?spm=a2700.7724838.30.135.kl5jKd108mmx14mm, but look at the output under 220 lux illumination - 4V @ 8uA!

author: station240
author_rank: Unknown
timestamp: December 03, 2015, 04:43:04 am
content: OK specs for the 30x30cm solar cell off the path light are ready.Under load (LED) 1.7V 20mAMax current (short circuit) 25mA

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 03, 2015, 05:33:12 am
content: Some simple reality checking says that it is going to require a lot of attention to detail to make this work. The good news is that a device doing roughly what you want is sitting in my garden, from a commercial vendor, so there is proof that it is possible.Noontime solar flux is roughly one kW/sq meter. Your spec says a max .0006 sq meter solar cell, so there is a noon potential of about .6 Watt. Crystalline solar cells are roughly 10% efficient, amorphous roughly 1% so that potential is reduced to something like 6 to 60 milliwatt. Still sounds pretty good doesn't it? But there are more losses. Depending on where your sensor is used, only a fraction of the noon potential is available. This fraction ranges from about a third in the most favorable locations, down to less than a tenth in many highly populated, but unfavorable solar locations. Leaving somewhere between .6 and 2 milliwatt for amorphous cells, and 6 to 20 milliwatts for crystalline cells. Amorphous cells are dropping out at this point, but solar power still seems feasible for crystalline cells. But the above numbers also assume favorable orientation and no shading. If your application allows setting the cell angle by the latitude and pointing it south, in an area that has little or no shade you are in good shape. Of course you are measuring temperature, so you need to properly shade your sensor to get meaningful temperature while exposing your solar cell. This may or may not suit your mechanical limits. You also need to factor in losses in the battery and battery management electronics, the power consumption of the battery management circuit and the average power of the rf link and measurement. The rf link, while used only briefly, may well have significant average power due to the high power when on. Storing data to allow longer intervals between transmissions may not save huge amounts of power. All you really save is the handshakes to establish a link. Depending on the link design that may not be a large fraction of the transmission time for a single measurement packet.Consistent with this rough analysis, the device in my garden uses a crystalline solar cell of roughly the dimensions you specify. It also has a battery, which might even be one of the Tadiran long life lithium types so I can't actually say that the solar cell provides the full power budget. The whole thing may die in a year or two. This device also avoids the temperature measurement problem by partitioning that function into a completely separate module which is unabashedly battery operated and mounted out of the sunlight. The solar powered module measures wind speed and direction. History is that the two C cells powering the temperature measurement element last about 18 months.

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 10:33:25 am
content: Quote from: senso on December 02, 2015, 03:02:49 pmIf you can power-down the sensors when the ATmega is asleep then you can get that power consumption down to single-digit microamps.Depending on how often you wake up, a set of AA batteries could last for several years.I've made ATmega based devices that run on internal clock and wake up every couple of minutes. They use about 10mA for a few seconds then go back to sleep again. The first one I made has been running for 2.5 years on a single CR2032 (and the battery voltage is still over 3.0V so who knows how long it will last)Quote from: senso on December 02, 2015, 03:02:49 pmAn if you have an external wake-up signal they can sleep in the nanoamp range .

author: senso
author_rank: Frequent Contributor
timestamp: December 03, 2015, 03:41:55 pm
content: So, a bit more information and some more data points.The boss says that 15 minutes between samples is OK, with data transmission being done every 6 hours(4 times a day).For now the devices are supposed to go inside a big greenhouse(around 10m x 550m), and placed where they will get sunshine.Now for some data:In deep sleep states the RFR2 + PCF85063 + APDS-9301 + BME280 + EEPROM use about 0.705mA, almost all that is from the Atmega, the RTC is always running, its the wake-up call to the micro.When running the atmega uses about 4.1mA@16Mhz, and all the sensors use just a little bit more than the baseline and total at 4.14mA for atmega and everybody woke up.Reading all of them/starting the conversions/acquisitions result in a total of 8.65mA and then turning on the radio to blast some data out shoots up to a total of 21mA.All in all, after some talking we/I have decided to go with a LiPo from Renata:ICP582930PR-01430mAh in a small form factor, way better than LIR2032's.I can also use a 20x20mm solar panel, just need to order some from China to test them.Now, new problems, using the LiPo means that the voltage can go up to 4.1-4.2v if she ever gets fully charged, almost all the chips used are 3.3v, should I throw in an LDO and feed them all at 3.3v or maybe at 3v,this LiPo from Renata already as built-in protection, so one less thing to worry about.I have also though of using some high side/p-channel FET's to cut power from maybe the sensors, but they have slow-ish start-up times, and at 3.3v the RDSon won't the greatest thing in the universe so I don't know if its a worthy trade with such low(in the uA range) consumptions.Will also design a preliminary prototype to test the power consumption and tweak any fail that I encounter, and also to field test the ability to recharge.Thanks for all the help

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 04:11:30 pm
content: Quote from: senso on December 03, 2015, 03:41:55 pmYou're doing something wrong. Thedatasheetsays:"Deep Sleep Mode: <700nA".

author: senso
author_rank: Frequent Contributor
timestamp: December 03, 2015, 04:29:04 pm
content: mili -> micro -> nano fail -.-That makes the deep sleep much much better, more headroom for charging

author: CatalinaWOW
author_rank: Super Contributor
timestamp: December 03, 2015, 04:48:57 pm
content: It is good that your power consumption is going down. The glass or plastic from the greenhouse will make a significant reduction in available solar. Losses an be as low as a few percent if sun is directly overhead and the glass is clean. Late in the day when reflection is an issue losses can be 90% or more. I would assume only 25% available over the day and year. Probably would be OK to assume 1 to 2 milliwatts average power from a crystalline cell of that size in your application. Remember, there is little penalty from having more power than you think is available, but a huge penalty for having less.

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 04:50:34 pm
content: Quote from: senso on December 03, 2015, 04:29:04 pmIf you can get it down to 700nA consumption during sleep with a 21mA burst every 15 minutes then there's no need to charge. A CR2450 battery will last for many years (decades...?)If you need 5V you could use a DC booster. It increases your baseline current a lot but I've measured thesecheapo eBay 5V booster boardsat 60uA consumption in standby mode (they automatically shut down at less than 1mA load). One of those with a CR123A battery should go for over two years (1500mAh @ 60uA is 25000 hours. 25000 hours is over 1000 days). Put plenty of capacitance on the output and it should be fine.Edit: Be sure to test that before building 1000 devices - I haven't actually used one on a sleeping ATmega!

author: senso
author_rank: Frequent Contributor
timestamp: December 03, 2015, 05:24:14 pm
content: Maybe I'm doing my math wrong and being led into much higher numbers then, so sorry if I'm nagging you guys, but the help as been great.So, lets assume 20mA and a 20ms burst to send the data.At 3v lets say that is 3*20mA = 60mWThen60mW * 20ms = 1200mW/s that translates into 0.3(3)mW/hFor example a CR2450 from Energizer(first google hit) as a rated capacity of 620mA/h at an average 2.8 thats about 1736mW/h.If I divide total "power" with the used in the burst 1736/0.3(3) that gives 5208h, divide by 24 and I get 217 days.My other thing with those small batteries is their high resistance/low peak current, maybe some 10-22uF ceramics will ride it through and not brown-out.I think I'm mixing something up.Best regards.

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 05:28:20 pm
content: Quote from: senso on December 03, 2015, 05:24:14 pmIt's 1200mW/ms, that's 0.0003(3)mW/hEdit: That's 217000 days, over500 yearsfrom a single battery!

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 05:31:30 pm
content: Quote from: senso on December 03, 2015, 05:24:14 pmYep, hence the idea to use a CR123A instead. They have loads of current capability and three times more capacity than a CR2450 (about 1500mAh).

author: senso
author_rank: Frequent Contributor
timestamp: December 03, 2015, 05:53:30 pm
content: Thanks you for your patience FungusWill suggest a change in perspective then, because that is a much, much better battery life, the only problem with the CR123 is that its "big", and my boss will say that its huge, as always, always asking for something tiny, then the boards arrive and he gets made because its too small

author: Fungus
author_rank: Super Contributor
timestamp: December 03, 2015, 05:55:44 pm
content: Quote from: senso on December 03, 2015, 05:53:30 pmIt's a lot smaller than a solar panel...

author: senso
author_rank: Frequent Contributor
timestamp: December 04, 2015, 10:06:57 am
content: I know, lets see what he says.Thanks, once again.

author: mark03
author_rank: Frequent Contributor
timestamp: December 04, 2015, 04:35:20 pm
content: Quote from: Fungus on December 03, 2015, 05:28:20 pmWhat are all these slashes? It looks as if you are putting time in the denominator, i.e. a measure of acceleration of energy (joules per second squared)! Surely you mean * instead of /.

author: senso
author_rank: Frequent Contributor
timestamp: December 10, 2015, 11:10:06 am
content: So, if I did everything correct this time around, with a single CR2 I get an estimated battery life of 19 years, account for a lot more data transmissions, and a lot more time to read all sensors, I have attached the .xls if anyone wants to take a look of how I have calculated it.Thanks for all the info.

author: Delta
author_rank: Super Contributor
timestamp: December 10, 2015, 11:35:11 am
content: As Mark has said, energy is measured in WattHours - Wh or derivatives thereof, kWh, mWh, Ws, mWs etc. NOT WATTS PER HOUR or mW per second or anything PER unit of time!Some people might think this is just semantics, but not grasping the correct units and what they actually mean, WILL make you screw up a calculation at some point!PS. Look at the self discharge rates of the cells you have shortlisted, they may well drain themselves quicker than your product does!

author: Fungus
author_rank: Super Contributor
timestamp: December 10, 2015, 12:00:47 pm
content: 19 years is perfectly plausible for something that powers on for less than one second per hour.But...Quote from: Delta on December 10, 2015, 11:35:11 amThis.(PS: I've got a gadget with an 18-year-old CR2032 that's still going. It has a physical on/off switch on it so there's normally zero power drain, but still: 18 years and counting...go figure!)

author: coppice
author_rank: Super Contributor
timestamp: December 10, 2015, 12:25:09 pm
content: Quote from: Delta on December 10, 2015, 11:35:11 amFor anything expected to run more than about 10 years self-leakage is the killer. The self discharge rate varies with temperature, and you might get a lot more life than you expect. However, you can't generally guarantee more than about 10 years operation if you expect to use a fair amount of the cell's capacity, or maybe 15 years if you are prepared to lose >90% of capacity to self-leakage. Beyond that and you can't even expect the cell not to leak.There are some specialist long life cells these days, that give a guarantee of a reasonable amount of energy being left after 20 years, but they are pricey.

author: senso
author_rank: Frequent Contributor
timestamp: December 10, 2015, 02:11:26 pm
content: I'm aware of self discharge/batteries just oozing out, but its over 2 years, so its OK, and I learned something with the help of all of you guys.So, thanks for you patience.The layout is a bit more challenging, 2 layers, 25x30mm, a bit cramped.

author: Fungus
author_rank: Super Contributor
timestamp: December 10, 2015, 02:17:03 pm
content: Quote from: senso on December 10, 2015, 02:11:26 pmThat's alkaline.All the batteries you looked at are lithium so they last a LOT longer than 2 years.