A very silly question about heating water - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: May 17, 2015, 04:44:18 am
content: Hey guys,A good friend has reached out to me about this. Please bear with me, I swear it's not a troll question, I just want to report back with community expertise so I can save his boss from himself.His boss wants to get into the carwash business, and is intent on devising a more efficient method of heating water. He would like to use an electric motor (I do not know which type) to act as the heating element, running it with it's cooling fins mechanically removed...I am well aware of how this sounds, and Dave - I'm sorry :C but this is probably the only way I can make him see sense as I've already tried to explain why this is a terrible idea. If 5 to 10 very knowledgeable members can shoot this idea down and kill it then I'm happy for the thread to be closed. Just give me something I can link my friend to to show his boss why this is an awful idea so he'll do what I tell him :CIf it helps, consider it "peer review"... go nuts. Shoot it down.EDIT:To clarify, said friend has only just learned the difference between volts and amps, and is currently studying what is roughly equivalent to a BTEC level 2, and is being forced to research stuff by a boss who from what I can understand, doesn't know his arse from his anode.He's based in Portugal, so the language barrier in explaining WHY this is a terrible idea is possibly being a problem.

author: tautech
author_rank: Super Contributor
timestamp: May 17, 2015, 04:54:12 am
content: Now IF one of the motors in the carwash runs quite warm, you could conceivably water cool it and REDUCE total water heating costs.But will the outlay be recovered with lower heating costs?

author: Unknown
author_rank: Unknown
timestamp: May 17, 2015, 04:56:54 am
content: Tautech, this is exactly where I was going! My suggestion was to use an off-the-shelf system, but my friend is dealing with a boss who wants to design a bespoke system and won't be told. He's a good person but lacking in technical expertise.

author: tautech
author_rank: Super Contributor
timestamp: May 17, 2015, 05:00:33 am
content: A much better option would be solar.

author: Rick Law
author_rank: Super Contributor
timestamp: May 17, 2015, 05:03:00 am
content: Quote from: BulletMagnet83 on May 17, 2015, 04:44:18 amThis is nutty.If he is using a motor coil as heat element - ie, using it as a heating coil, why not just get a heat element directly.Law of Thermodynamic - all your lost in efficiency is in the form of entropy dissipated as heat. In this case, you have darn near 100% efficiency because all you want is heat. (Assuming the whole contraption is submerged which means the water get to capture all its heat.)Then the question is,where do you want the heat? A motor coil will likely keep more heat inside the coil than a water heater element which is designed to spread the heat in the water. So, a motor coil can perhapskeep enough heat withinit to melt something it should not (which would not be very nice), whereasa heat element is designed to spread the heat.Now, if he really wants efficiency, why notsolar pre-heat the water(like running it through a "shallow pool" with black paint on bottom and plexi-glass on top), and/oruse a heat-exchange mechanism to recapture the heat of the used water?

author: Unknown
author_rank: Unknown
timestamp: May 17, 2015, 05:06:09 am
content: Rick Law,Thank youIt's mad, I know... please keep it coming :3 I just want some backup so my friend can talk to his boss and put this idea to bed before he kills innocent motors trying to make it work.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 17, 2015, 05:12:53 am
content: Heating water is one of the few viable uses for low content "waste" heat. Electric motors are at best around 90% efficient, so the remaining 10% could in principal be used to heat water (as opposed to the air which usually gets rid of this waste heat).The idea falls apart in several places. Most importantly, it is unlikely that his car wash has enough motors to supply the necessary heat. As stated already it makes no sense to use a motor only as a heater, it just isn't any more efficient for that than a resistance wire. Roughly a dozen 1 horsepower motors to get a kilowatt of waste heat. Even if there are enough motors the plumbing and extra difficulty in maintenance will far outweigh the energy costs saved. You might be able to get some green publicity credit, but no way there will be actual financial benefit.

author: Rick Law
author_rank: Super Contributor
timestamp: May 17, 2015, 05:20:50 am
content: By the way, he should think about usingnatural gasheat - unless you want to pay big bucks whenever the sun is not out.Cost-effectiveness (BTU/dollar) of natural gas heating is many fold that of electrical heat. It will however depend on state and federal regulation of today and for your locale. So he needs to look into how his state is in term of fossil fuel friendliness.

author: IconicPCB
author_rank: Super Contributor
timestamp: May 17, 2015, 05:34:52 am
content: The boss has a reasonable idea. He needs to modify it ever so slightly and he will be on a winner.He should use an electric motor interposed between clean supply and warmish waste water. He should attach a heat pump as is done in many a domestic heating system and shuffle energy between the two water flows.In the process he can harvest motor waste heat too.For every unit of electrical energy he will extract at least two units of heat energy.Clever boss.

author: Rick Law
author_rank: Super Contributor
timestamp: May 17, 2015, 05:37:39 am
content: Quote from: IconicPCB on May 17, 2015, 05:34:52 amHe may not be thinking about capturing waste heat!Read again and you will see it can mean he is actually thinking about usingmotor as heat element- not as motors to run something and capture waste heat.Quote from: BulletMagnet83 on May 17, 2015, 04:44:18 amAs CatalinaWOW pointed out a couple of replies back, the pumping and the maintenance (for waste heat) will kill any saving you got.A heat-exchange mechanism to pre-heat with re-capture heat from used water would probably be the biggest saving "low hanging fruit".

author: IconicPCB
author_rank: Super Contributor
timestamp: May 17, 2015, 05:48:22 am
content: Yes Rick...but pointing to the heat pump is a friendly way to not tell the boss he is a silly person

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2015, 05:53:39 am
content: Quote from: CatalinaWOW on May 17, 2015, 05:12:53 amyou havent used a motor for an air compressor, it got fin installed and if it runs for 10 minutes you are lucky if you can survive touching it for 5 seconds. the fin will undoubtedly further increase torque resistance and hence power usage, an inevitable waste for the cause. as of OP's friend's boss, if he want to recycle energy of such motor, i 110% support it, but piping and all will be something. but if he intend to use the motor to ONLY heat the water, then he have lack of knowledge, or in rough word people might easily say... stupid.

author: Rick Law
author_rank: Super Contributor
timestamp: May 17, 2015, 06:15:18 am
content: Capturing heat from a compressor instead of motor makes sense. There is a lot of heat with compressors, but I am not sure if a lot of air compressors are in used for a car-wash.A thought occur to me that you need to check into even with compressors.If you modify it, will warranty and insurance be impacted?If your compressor fails, you would be on your own. If your compressor fails to a point where the building burns down, would insurance be a problem since you tempered with the equipment in use.Your boss needs to look into that!

author: cs.dk
author_rank: Unknown
timestamp: May 17, 2015, 06:31:39 am
content: Quote from: Mechatrommer on May 17, 2015, 05:53:39 amA regular induction motor? Did some logging on my old compressor, it never came above 40C at full load. Haven't tested on my "new" (used) one, but i think it will be the same.

author: C
author_rank: Super Contributor
timestamp: May 17, 2015, 06:55:28 am
content: Quote from: BulletMagnet83 on May 17, 2015, 04:44:18 amFor most motors with air cooling, the cooling fins cool the windings and rotor. Removing the fins will burn out motor or shorten it's life.There are motors that use water for cooling.https://www.google.com/#q=water+cooled+motorSo a water pump could use a water cooled motor in place of an air cooled motor. Need flow of water and one quick look says 5 c rise in water temp at proper flow rates.Non standard setup could be more trouble then the gains. It could be cheaper and easer to to use a heat pump recover waste water heat and the exhausted air from the room that contains all the heat producing equipment.C

author: max_torque
author_rank: Super Contributor
timestamp: May 17, 2015, 10:30:02 am
content: Ask your Boss how he's going to pay the insurance payouts to the customers he's electrocuted by trying to "water cool" a motor clearly not designed for such a purpose?

author: timofonic
author_rank: Frequent Contributor
timestamp: May 17, 2015, 11:17:36 am
content: He's your boss, not your friend. He gets more money than you and he's acting that dumb, right? Is he dumb in any other areas too?Just explain him what can happen, if not agrees obey and let the disaster happen.That's what Dilbert would do!

author: Unknown
author_rank: Unknown
timestamp: May 17, 2015, 12:32:01 pm
content: Thanks very much for the replies, allI will pass them on. So I heard from my buddy, his boss is on the same beginner's electronics course as he is.... and after bringing the idea up in class, their tutor made a face like this "O___________o"I apologise to anyone who's left a head-print on their desk reading this thread. But you know how sometimes you encounter people who treat "that's stupid, you can't/shouldn't do that" as a challenge? Well...

author: xrunner
author_rank: Super Contributor
timestamp: May 17, 2015, 03:32:50 pm
content: Quote from: BulletMagnet83 on May 17, 2015, 04:44:18 amEven more heat can be generated if the first electric motor is used to run a generator (with any cooling fins removed), and that generator's output to run a second motor. The more you connect like this the more heat you will produce.

author: Rick Law
author_rank: Super Contributor
timestamp: May 17, 2015, 10:33:54 pm
content: Quote from: xrunner on May 17, 2015, 03:32:50 pmEntropy is just like tax - VAT tax.Should try that on motors with bad bearings, the axle will be a "hot rod" as well.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: May 17, 2015, 11:54:19 pm
content: I got the impression that you're in Spain.If so and there is enough land area available then bury enough pipework to extract ground heat to a central insulated tank. Supplement this with roof mounted solar panels to power the required pumps.Car wash for one maybe. Company jacuzzi more likely.

author: miguelvp
author_rank: Super Contributor
timestamp: May 18, 2015, 01:04:09 am
content: OP is from Wales, maybe he is on vacation or planing to go on vacation to Spain to catch some sunI know I am!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 18, 2015, 02:06:00 am
content: Hi group,Time for a little physics.The specific heat capacity of water is 4200 (J/(kg * K))1 kW = 1 kJ s-11 Imperial gallon = 4.546 litresdensity of water = 1.001 minute = 60 seconds.so with a flow rate of 1 gal / minute and 1 kW dissipation the temperature rise will be:Trise = 60 x 1kJ / 4200 x 4.546 = 3.1oCWater is pretty good for cooling, but it requires a lot of energy to raise the temperature.Regards,Jay_Diddy_B

author: max666
author_rank: Frequent Contributor
timestamp: May 18, 2015, 02:46:05 am
content: Quote from: Mechatrommer on May 17, 2015, 05:53:39 amAt the air compressor it isn't the motor that's getting that hot, it's the cylinder where the air is compressed. Partly because reciprocating compressors have friction, but mostly because gases heat up when they are compressed --> Thermodynamics.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 18, 2015, 03:29:40 am
content: Nothing inconsistent here. Compressing air is less efficient than running an electric motor. Don't know the number but suspect it is not that far above 50%. Using 50% and 90% as bounds on the number says that a 1 HP compressor (~750 W) will be dumping between 75W and 325 W as heat. Don't know anyone here that would want to grab either an incandescent light bulb or a soldering iron of those wattages. But as the prior poster says, it won't heat much water.

author: IconicPCB
author_rank: Super Contributor
timestamp: May 18, 2015, 03:33:00 am
content: it takes as much energy to heat as it does to cool ( on the understanding of direction of flow of energy ) a bunch of water.It is the phase changes which tend to be energy hogs.

author: Rick Law
author_rank: Super Contributor
timestamp: May 18, 2015, 04:32:08 am
content: Quote from: IconicPCB on May 18, 2015, 03:33:00 amDoubt he wanted to turn it from ice to water or from water to steam.As to the amount of energy, it really doesn't matter in a sense. If he is considering waste heat recovery, x BTU from waste heat is x BTU he doesn't have to buy regardless of how small X is. That is money he doesn't have to spend - as long as ROI (Return On Investment) is positive.I think pre-heating the water with a heat-exchange with the used-water is probably easiest and biggest bang for the buck. Even with heat-exchange, getting a positive ROI is probably going to be difficult.

author: mtdoc
author_rank: Super Contributor
timestamp: May 18, 2015, 04:53:07 am
content: Cogenerationis a mature technology that captures waste heat from an internal combustion engine or boiler that is generating electricity. I believe Honda even makes a home sized unit. But I've never heard of using waste heat from an electric motor. Seems doubtful that it would be worth the effort - they're too efficient.

author: IconicPCB
author_rank: Super Contributor
timestamp: May 18, 2015, 05:58:59 am
content: The larger the motor.. the more efficient it is.Many a small motor would be better for heating the water.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 18, 2015, 06:06:47 am
content: Quote from: cs.dk on May 17, 2015, 06:31:39 ami dont know induction or what. whats certain its AC mains motor connected right to the compressor in a single body, not with that rubber band type. to be precise, i'm not sure if the motor get hot or the compressor, maybe both, whats certain both single piston compressor and the motor wil get very very hot in 10-20 minutes of operation, havent timed that precisely.btw, if the heated water is for drinking purpose, piping, coupling and sealing will be something. but if not for drinking, it will still be something, but much easier than drinking purpose i guess.

author: eas
author_rank: Frequent Contributor
timestamp: May 22, 2015, 07:08:04 pm
content: In the US, or my part of it, at least, automatic car washes have become continuous engineering projects, with ongoing optimization of capital expenses and operating expenses. It seems like they are constantly tweaking them. I'm sure they reuse water multiple times, and in so doing, reuse the heat. If significant waste water goes to the sewer, they must use counter-current heat-exchangers to recover the heat for new supply.Recovering waste heat from the electric motors may make sense at some point, but I'm sure there are many optimizations with better ROI.

author: eas
author_rank: Frequent Contributor
timestamp: May 22, 2015, 08:16:10 pm
content: Quote from: CatalinaWOW on May 18, 2015, 03:29:40 amI'm pretty sure that most of the heat produced by an air compressor is due to the thermodynamics of compressing a gas, rather than mechanical inefficiency due to friction. My "hybrid" hot water heater, which uses and air-source heat pump meets our hot water needs by 1/4-13rd the electricity of our old resistive electric HWH. Probably still not enough for a car wash, but nothing to sneeze at, and even more useful if you can put the cold air to good use too.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 22, 2015, 09:04:32 pm
content: Quote from: eas on May 22, 2015, 08:16:10 pmI agree, it is gas thermodynamics. But that is where the heat comes from when you burn yourself on a compressor, whether it is your bicycle pump or the big one that drives the jackhammers. And it is part of the overall efficiency of a compressor. It is also one of the fundamental flaws of the compressed air cars that you see from time to time touted as a solution to the transportation problem du jour. That energy that went out as heat isn't recoverable for propulsion.