Batteryless and off-grid system for air conditioning - Page 1
Forum section: electronics / general

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 31, 2017, 07:32:38 am
content: Hi,As I want to stay away from the weight and chemical content and maintenance requirements of batteries, I am considering a battery-less system that will only operate the air conditioning unit. The logic is that it will only have to operate when there is a lot of sun, so no energy storage would be needed. Just the Solar panels, going to a DC_DC converter which would convert their output to a reasonable stable voltage for an inverter. And then the AC unit would be connected to the inverter.The AC unit I have now draws some 1.5kW. Considering some margin and considering I might want to replace it with a larger unit in the future, let's talk about a 2kW limit.So the questions are:1. Has anyone else tried such a setup? Is it viable in your experience?2. How many solar panels do I need for that? Is it 3kW instant power? Is it 10kW? (each conversion on the PV--to-220V AC string would have its losses, and also the sun will not always be DIRECTLY on top of the panels)3. Can you suggest some scenarios considering 250W panels? How should they be connected and in combination with what type of DC-DC converter?4. Ar there any known and affordable inverter manufacturers which provide pure sine-wave? (this is a must, in order not to damage the AC unit)Of course, I realize detailed calculations will be needed, but I just wanted to lay out the framework for the initial discussion.Regards,Cristian

author: Codebird
author_rank: Regular Contributor
timestamp: July 31, 2017, 07:42:45 am
content: You are going to need some energy storage, because the AC isn't going to be happy about shutting down every time a cloud passes the sun. You shouldn't need much though, as there is no reason to run AC at night - conveniently, the available energy each day should correlate very well with how much you need. It would also reduce the cost, because the AC will not need to run continuously - you will be able to get away with a smaller panel area, as on a moderately warm day you can charge for ten minutes in order to run for five. You might even be able to do it with supercaps. It's an interesting project, and you are going to need some electronics to manage it (ie, a charge controller of some sort), but engineering-wise I see nothing insurmountable.I'm assuming that grid power is unavailable, as it would probably work out cheaper.

author: brumbarchris
author_rank: Regular Contributor
timestamp: July 31, 2017, 08:36:20 am
content: QuoteWell, the AC unit already exists, and it is not an inverter based one. And even if it were, my goal would have been not to modify it, I want to maintain "backwards compatibility" with the grid.QuoteTurning OFF the AC each time a cloud passes is perfectly acceptable. When it's hot here then it is really HOT and we would love some clouds but we do not get them. This does, however translate into a requirement for the DC-to-AC inverter, which would have to have among its features a relatively smart undervoltage protection (i.e not just decrease the amplitude of the output sinewave, or scramble the frequency or something or anything like that; just pure shut-down.Anyway, that's one of the reasons for which I consider over-sizing of the PV panel area, to get sufficient energy maybe during the occasional cloud during a bright day. And if I really oversize it, meaning the whole roof covered with panels, then I might even consider using the unit for heating during winter. But of course, that is just a secondary objective.The grid is available, but I do not want to mingle with it a all. Minly because of the potential regulations regarding pushingcharge on the grid.Regards,Cristian

author: fourtytwo42
author_rank: Super Contributor
timestamp: July 31, 2017, 08:42:13 am
content: I think there are two things you need to investigate more fully here1/ what will be the shape of the power delivery curve each day considering the location of your panels, this will also show you what peak installed power you will need to maintain 1.5Kw for the number of hours you want and the months you want. There are many websites for this.2/ Determine the characteristics of your AC, for example is it an old capacitor start motor, then you will have trouble due to the very heavy inrush and no protection against frequent starts. Or is it a more modern inverter type, these have a lower inrush and can sustain frequent starts as they have inbuilt timers etc.Also not all inverters you might use for supply are the same when it comes to motor start so depending on your AC characteristics be careful what you buy. I am with you on the battery front BUT for this application it depends if the PV costs for the extra peak power required without a battery is cheaper than the alternatives (battery and charge controller).I just saw your latest reply, it's a capacitor start motor.......well I think the only practical way to avoid batteries is to use the grid to start it (as you have a grid), in other words invest in a GTI. Yes I understand that means investigating the regulatory requirements but that may be time well spent. There are a lot of "plug in" GTI's that are normally acceptable up to 1kW and there's nothing stopping you using multiplesThe only problem is sometimes there input range is a bit restricted meaning you have to use expensive 12V leisure panels but again a bit of research can pay dividends.Another thing I forgot to mention is check your electric meter specification on the web and make sure its not going to do anything nasty with export power, some of them treat it as fraud or charge you for it but a quick word with your local company normally gets this sorted out (meter change etc).

author: Codebird
author_rank: Regular Contributor
timestamp: August 01, 2017, 10:33:50 am
content: QuoteNot that smart. Any hobbyist could put together a circuit for that - it's an LM393, some resistors and transistors, and a bloody big MOSFET or relay. But there is an easier way.Give up on 'no batteries.' It really complicates things, because it means you can't use a standard charge controller, can't do MPPT, and you can't operate the AC at all under low-sun conditions (as opposed to operating it at reduced capacity), and the start-up surge is going to be a serious problem. Instead you should design a 'minimal battery' system. Use a battery, but make it a really tiny one - just a couple of cheap SLAs is all you need. Yes, they'll eventually wear out after a few years of use, but they are cheap enough that replacing them won't be a problem. You only need enough battery capacity to meet that start-up surge load and give you about five minutes runtime. Go that route and all you need are panels, battery, off-the-shelf charge controller and off-the-shelf inverter. No custom electronics required.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 01, 2017, 10:37:09 am
content: How about using the Einstein–Szilard cycle for refrigeration. Then all you need for electricity is enough to run a fan.

author: coppice
author_rank: Super Contributor
timestamp: August 01, 2017, 10:46:13 am
content: Quote from: Codebird on July 31, 2017, 07:42:45 amYou are right that air cons do not like short down times, but many have internal electronics which will inhibit the heat pump from restarting for a few minutes after the power cuts. If the environment is not subject to endless cloudy, clear, cloudy, clear cycles that internal protection might be sufficient to protect the heat pump and provide adequate cooling through the day. You do need to ensure that the inverter switched cleanly, and either provides proper power or cuts it completely. Trying to run an air con on sagging power is not good for it.Quote from: Codebird on July 31, 2017, 07:42:45 amThat's a major assumption. In many places you ONLY need the air con running at night. In this case substantial storage is not really avoidable.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2017, 05:27:47 am
content: Quote from: Codebird on August 01, 2017, 10:33:50 amYou can still do MPPT to some degree with V/Hz scaling - something like down to 30Hz for a standard compressor. Just expect a rather high threshold below which the A/C will not operate - it would probably make sense to start using some power from the grid to meet the minimum. You'll still be making use of the solar power and getting savings from V/Hz scaling.That said, do also take a look at Nissan Leaf batteries since they're fairly cheap and are rated for rather high current. Cheaper than sealed lead acid, in fact. They'll handle 1/2C charge rate no problem whatsoever and can take more than 1C under the right conditions, something you can't do with lead acid.I think a good direction to go in is a small, high dynamic range BLDC compressor (something like 250W max) along with a TXV and water cooled condenser. The condenser is cooled by an evaporative cooler located outside and connected via cheap plastic tubing. Then use a Raspberry Pi or similar along with sensors and stepper/servo motors to track the people in the room and deliver the cooling where needed. Keeping in mind that solar panels tend to be around $1/W (half that if you're really lucky), going from 2kW to maybe 300W or so would be a huge savings that would easily pay for the the bits to make it "smart". Such a low power usage also means that getting a few hours of runtime from a reasonable amount of battery capacity would also be pretty easy.

author: noidea
author_rank: Unknown
timestamp: August 03, 2017, 05:36:04 am
content: Quote from: brumbarchris on July 31, 2017, 08:36:20 amIf the AC is not an inverter one then you will need to think about inrush/starting current every time the compressor starts. As a rough guess make sure your supply can handle at least 6 times the running current of the unit.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2017, 06:04:09 am
content: Quote from: noidea on August 03, 2017, 05:36:04 amThere's an inverter being added. While it won't get the dynamic range a good BLDC compressor can, about 2:1 dynamic range and V/Hz soft start are pretty significant.

author: noidea
author_rank: Unknown
timestamp: August 03, 2017, 08:04:09 am
content: Quote from: NiHaoMike on August 03, 2017, 06:04:09 amThere's no mention of that by the original poster?

author: brumbarchris
author_rank: Regular Contributor
timestamp: August 03, 2017, 10:33:49 am
content: The inverter I was talking about was referring to making a 220V/50Hz output out of the regulated solar panels DC voltage.Anyway, I think I came across something off the shelf I could use, admittedly, battery based. But I am considering maybe replacing the battery with a super-super-super-capacitor.Its the TN 1500 series from Meanwell:http://www.meanwell.com/productPdf.aspx?i=44I t includes charger, DC, DC converter and also AC-DC inverter. And the brochure I have says it can draw 30A max from the solar panels; not enough, considering I have to run a 1500W load, given losses; but if I can replace the battery with supercaps, then I think it is the way to go, as it can charge the supercaps from the AC mains in case a cloud passes above the PV panels.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2017, 02:40:08 pm
content: Look up "VFD" for an off the shelf inverter that has user adjustable V/Hz settings.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2017, 03:17:04 pm
content: The inverter needs to be sized to handle the surge current rating of the compressor *and* any fans.Surge current requirements will either require massive over provisioning of the solar panels or the addition of a capacitor or battery bank. Over provisioning the solar panels would be expensive but help with clouds.I would be leery of compressor restart timers. Do they work correctly if power is completely lost?How do RV air conditioners handle unreliable power?At some point every morning and evening, the inverter is going to be operating without enough power to supply the air conditioner which will try to start anyway. How is this going to be handled?

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2017, 03:20:11 pm
content: V/Hz ramp up negates startup inrush.

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 03, 2017, 04:32:20 pm
content: Quote from: brumbarchris on August 03, 2017, 10:33:49 amGiven it can only produce 3Kw for 30 cycles I think the chances of it being able to start your 1.5Kw (2hp) capacitor start motor are about zero. I am not sure how you think this product will help you, there are no solar panels I am aware of capable of delivering more than 10A so you would require a parallel arrangement. I think this unit intends the solar input to be a mere float maintainer, not the main charging source, that is intended to be external as in a vehicle alternator.As I and a few others mentioned before if you are keeping your existing AC unit you need an inverter capable of starting compressors, as for the scientific stuff mentioned elsewhere on this thread there simply quoting lectures they have heard, nothing to do with reality

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 04, 2017, 01:10:25 am
content: Just start with a surplus VFD running from mains. The savings from having it is pretty significant. (And the very good temperature regulation pure luxury!) Devices sold as off grid inverters, at best, have a very narrow adjustment range and very basic V/Hz ramp up if you're lucky. A VFD lets you program loads of settings like the startup ramp rate and exactly what frequency range to run at, usually accepting something simple like a 0-10V signal to allow an external circuit to set the output.Then to add solar, you'll need a DC/DC converter to perform MPPT and supply voltage to the DC bus. If your mains voltage is already near the top end of the VFD rating, you might need to switch in an autotransformer to avoid going over the limit.

author: brumbarchris
author_rank: Regular Contributor
timestamp: August 07, 2017, 07:28:24 am
content: QuoteWell, I did a bit of reasearch over the weekend regarding my existing AC unit. It seems it draws some 24A during compressor start, that's almost 6kW. It therefore seems I need the higher end version of this Meanwell unit, namely one of the variations from the TN-3000 series:http://www.meanwell.com/productPdf.aspx?i=46This is capable of surge current of 6kW for 30 cycles so it should do it. Regarding to solar panel arrangement, I am starting with a blank sheet of paper, so I can arrange them anyway I like and need.I don't think the unit is intended to use the solar input as float. It actually has several modes of operation: one of them is UPS mode, where keeping the load operational has the highest priority and the battery is charged mainly from the mains; but the other mode is "Energy Saving mode" , where the solar energy has the highest priority to charge the battery and run the load. I just need to research the tiniest battery I can use with it, or if I can do some ultracapacitor replacement.Regards,Cristian

author: Rerouter
author_rank: Super Contributor
timestamp: August 07, 2017, 08:52:18 am
content: Pump water up a hill, hook up a turbine generator, pump up when the sun is shining. Use at your own discretion.Lots of energy in 1 tonne of water raised 5m

author: Jeroen3
author_rank: Super Contributor
timestamp: August 07, 2017, 09:07:37 am
content: That is draws 24 Amps does not mean it is 6 kW. When motors start, they use no power.If you remove off-grid from the equation it is much easier. Get an PV inverter and some current transducers.When the sum of the currents is positive, power is solar. When the sum of the current is negative, power is grid.When the sum is 0, load is shared, or AC if off.You will both get the benefit or net-metering, and no unnecessary shutdowns on clouds. You can add a timer for the amount of time the AC is allowed to run on grid.I think you can do this with a Easy PLC and isolated 0-10V current transducers.The price of this project however, it might be enough to run AC for years continuously.

author: brumbarchris
author_rank: Regular Contributor
timestamp: August 07, 2017, 12:12:18 pm
content: Quote220V*24A=5280W; if one considers margin, it would be some 6kW.Not sure I understand the part about "they use no power"Cristian

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 07, 2017, 01:22:48 pm
content: Quote from: brumbarchris link=topic=92725.msg1275338Yes I am starting to realize you have to sort the wheat from the chaff on this site too

author: Jeroen3
author_rank: Super Contributor
timestamp: August 07, 2017, 06:29:46 pm
content: When motors start, the power factor is awful. Thus reducing real power. The meters look really weird when starting big motors.Does not apply to variable frequency driven motors.

author: brumbarchris
author_rank: Regular Contributor
timestamp: August 07, 2017, 07:26:54 pm
content: So what I can understand from this graph is that the motor of the AC, while drawing some 24A from the mains, which would mean 5.2kW actually sucks 4 times more power from the mains (so some 20kW)? (the meaning of the 25% power rating during standstill in your graph below).Of course, I do understand this graph is just an example... But to be honest, I doubt my entire current home network is capable to provide 20kWat this very moment!Regards,Cristian

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 08, 2017, 04:11:26 am
content: Quote from: Jeroen3 on August 07, 2017, 06:29:46 pmAnd that's the key - there's going to be an inverter since solar panels output DC, so it makes sense to do V/Hz scaling!

author: noidea
author_rank: Unknown
timestamp: August 08, 2017, 05:33:03 am
content: Quote from: NiHaoMike on August 08, 2017, 04:11:26 amAnd whilst you keep posting this what you seem to be missing is that running a single phase motor with start and run windings from a VSD is a kludge at best.....

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 08, 2017, 05:39:24 am
content: It worked well enough for the DIYer in the video I linked earlier. Some of the better commercial inverters do V/Hz ramp up.To do even better, drive the motor as the two phase motor it really is. It's probably possible using a common 3 phase VFD and a transformer or two to synthesize the quadrature phase.