i got an idea for using excess heat - Page 1
Forum section: electronics / beginners

author: willbanks
author_rank: Contributor
timestamp: June 20, 2017, 12:54:49 am
content: this seemed like the best place to post this to.(im pretty new to electronics so there are probably some factors im not thinking of)so resistors convert wasted energy into heat. so what if you had tiny thermoelectric generator attached to the resistor somehow, and then that generator produces a (very) small amount of electricity. But if you had multiple of these components could you add them together to produce a larger amount of electricity?im sure there is something wrong with this idea, which is why im asking. you learn from mistakes yeah?

author: jwm_
author_rank: Frequent Contributor
timestamp: June 20, 2017, 01:14:49 am
content: The main issue is you are now insulating your resistors. Thermoelectric generators don't conduct heat well, so you need a very large temperature differential to push the heat through them meaning big heat sinks on the other side of the thermoelectric generator.Remember, you cannot extract work from heat, only from equalizing a temperature difference. As soon as your heat sink warms up to the resistor temp no useful energy can be extracted.

author: willbanks
author_rank: Contributor
timestamp: June 20, 2017, 01:23:07 am
content: ok i get it, thanks for explaining!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 20, 2017, 01:30:04 am
content: There is nothing in your idea that violates physics. You can recover some of the heat as electricity.One of the better ways to think about what is wrong with the idea is to put a price on implementing it. The thermo-electric generator will have a cost, not less than the cost of the resistor, and probably several times higher. It will need a heat sink, also not free. Wires to connect all of the generators. You will easily double to cost of your bill of materials, and more likely tentuple it.How much electricity will you recover? Thermodynamics says you will recover less than you put in. Reality says that value will be far less, only a few percent at best. Two percent is a nice round ballpark that won't be that far off what you can achieve.Now, is the value of what you have recovered. Assume you have a nice little 100 watt device. You can get two watts back out. Electricity costs in the neighborhood of 20 cents a kilowatt hour. So you recover 0.0004 cents for every hour of operation. You have to run your device for a very, very long time to recover the extra costs of manufacture.That doesn't take into account the extra weight and volume which make the product less desirable, the reduced reliability from all of those added components, and how much harder it is to service because of all of those things bonded to the sides of the parts.

author: willbanks
author_rank: Contributor
timestamp: June 20, 2017, 01:46:19 am
content: didnt even think of the cost portion! so is there any possible way to use the excess heat for less wasted energy?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2017, 02:24:23 am
content: Waste heat of low grade (i.e., the hot temperature is not much above the sink temperature) can only possibly yield a few percent, and that's with an ideal (Carnot cycle) generator.TEGs are massively below ideal, maybe 10% of ideal at best, so the cost outweighs the benefit by orders of magnitude. But even using an optimized Stirling engine, it's way off.Much better to simply avoid dissipating that heat in the first place. If it's dissipating DC, replace it with a DC-DC converter. If it's AC, rectify it and convert it back into useful energy (DC or AC).Many common applications of power resistors fall into this category: most of the power can be conserved for not much added cost and complexity. More and more often, these days, SMPSs have quasi-resonant snubbers in them, in contrast to passive snubbers (that waste the reactive AC switching power as heat).Tim

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 20, 2017, 04:08:19 am
content: As T3sl4co1l says the best approach is to avoid the heat generation in the first place. Use larger value resistors in voltage dividers. Use active architectures instead of passive power dissipation. Use lower voltages overall, in combination with switch mode power supplies to get to the lower voltage. Design in sleep modes that turn your circuit off when it isn't being used. All easier and more effective than recovering waste heat.If you can't do that you are left with things like keeping your coffee or tea from cooling quite so fast, or gently warming an epoxy to make it cure faster. Not horribly effective or useful, but something.

author: lapm
author_rank: Frequent Contributor
timestamp: June 20, 2017, 07:00:32 am
content: Nasa used to send probes to space with nuclear decay thermoelectric generators. Basicly nuclear decay produced heat that was converted to electricity...https://solarsystem.nasa.gov/rps/rtg.cfm

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2017, 07:44:42 am
content: Quote from: lapm on June 20, 2017, 07:00:32 amTrue but note that the heat was generated deliberately to generate electricity. It wasn't waste heat.

author: CCitizenTO
author_rank: Contributor
timestamp: June 20, 2017, 03:58:46 pm
content: Quote from: willbanks on June 20, 2017, 01:46:19 amSometimes it's easy to answer your own questions by asking the right question... Why hasn't anyone done it this way before? Usually the answer is that it's very inefficient and not cost effective. When you have a community working on things all the time certain things get done a certain way because it's the best way after they've tried hundreds of other ways to do the same thing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2017, 05:46:10 pm
content: Quote from: CCitizenTO on June 20, 2017, 03:58:46 pmway that the community norm is maintained.FTFY.If you think this sounds cynical, or pedantic, or silly, consider some example communities: Tesla coil builders tend to reproduce the same designs, without putting real development into them. Ham radio operators continue to buy and build antenna and radio designs that, simply put: stink. Audiophiles, that build their own amps, seek novelty in circuitry as well as in sound; typically at the expense of cost, efficiency, distortion, and even basic considerations like stability and safety.These are all communities that, you would hope, would tend to improve their collective designs over time, but alas, that isn't the case. The designs here are actually memes, and tend to persist on their own inertia, rather than for any objective reasons.On a related subject: this is the second most important reason I hate on the 2N2222 and 2N3055 whenever I see them. They are as perfect a warning sign as any, that a user is repeating a meme, rather than understanding a design.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 20, 2017, 06:39:55 pm
content: You forgot to mention the 741Back to the original question;Generating energy only becomes cheap when is generated on a massive scale.

author: Brumby
author_rank: Unknown
timestamp: June 21, 2017, 02:22:17 am
content: To the OP:Coming up with the idea of utilising excess heat is a good indication - it means you are thinking about the situation. This can never be a bad thing.The next step is to address the process of doing so and working out things like efficiency, cost, benefit and so on. As indicated above, these numbers will not be encouraging, especially at the scale we are talking about.Somewhere along the line, there is an important question that is so obvious, you might not even think about asking it - but it iscrucialthat you not only ask it, but answer it as well. That answer will have a world of further thinking come out of it....The question is "WHY am I trying to do (this)?"An answer to the specific scenario given, I would suggest, is that you want to make the most out of the power available. An admirable quest.The next step - is taking a step backwards and having a look at the broader picture and asking yourself whether there are other ways of looking at the issue ... and what other approaches might be considered. This is where you will find phrases like "lateral thinking" and "thinking outside the box" getting used. It is also where "brainstorming" comes in. (If you've never been involved in or witnessed a brainstorming session, I strongly recommend you take up the opportunity if it comes your way. They can be crazy, fun and surprising.)One such alternative line of thought is - Can I reduce the power used in the first place? That will reduce the excess heat generated and allow the power that would be otherwise expelled in creating some of that heat to be used directly, without any recovery process required. To get a feel for how good or bad this idea might be, throw some numbers in. Back of a napkin numbers will do for starters.Let's say we have a resistor dissipating 10W. Compare the effort of trying to recover 50% of that from the waste heat against reducing the power dissipation of the resistor by 50%. The first option hits a brick wall very quickly. The efficiencies are just not there. The second option might require a redesign of the circuit around the resistor. This might be a challenge - but it's not a brick wall.A practical result from efforts on both of these fronts soon shows which is the best value approach. A recovery system would be lucky to provide a 5% benefit, with the complexity and cost of implementing it and delivering the power recovered in a useful way. A 5% benefit through a redesign may well be fairly easy (in comparison) with 50% and maybe even 90% being a possibility. No recovery system is required and consideration for delivery of the power is unnecessary - it's already available at the existing terminals!We have now reached a point where the preferred development path - on the basis of the original objective - has been clearly identified. What we then do is assess all the implications of travelling down this path and work out whether this path is a good idea - or if it will lead us into a world of pain because of some incidental consequence.The addition of an energy recovery system will add bulk, complexity, cost and an increased risk of failure (because there are more things to go wrong). The reduction of power consumption approach will mean the existing power supply can do more - and/or you can reduce the requirements of the power supply. If we opt for the latter, we can have a smaller, cheaper power supply, which could likely be more reliable. The overall heat generation will be less and the measures required to remove unwanted heat will be far more modest. Since heat is the number one enemy of electronics, this is a very good thing. A reduction in size and weight is also very useful - especially in aerospace applications.However, there is still one hurdle to travelling down our obviously superior path. The long-standing conflict of the engineering ideal against the commercial reality. But this conflict is not just an engineering one - it exists in every industry and there are a lot of factors that can come into play. I won't attempt to go into any details here ... it's a subject all its own ... but you would be hard pressed to find anyone here who's been in the workforce for more than a year or two that doesn't have a tale to tell.

author: MrAl
author_rank: Super Contributor
timestamp: June 21, 2017, 11:35:05 am
content: Quote from: willbanks on June 20, 2017, 12:54:49 amHi,It's not only a good idea, it's being used now and has been for many years now. In fact, there was research a few years back being done on making LED's more efficient by incorporating thermoelectric generators right onto the LED chip. There are also thermoelectric alternators.Your main problem as a non mainstream inventor though is that the thermoelectric devices available all have very limited efficiency, and also very importantly they have an associated finite surface area, and that means there is a certain surface area to efficiency (SAE) ratio. This SAE rating limits the electrical output you can get from a given resistor or other heat generating device because you only have a certain area to collect the heat from and that is the surface area of the resistor or other device. Small resistors have a fairly small surface area, so it will be hard to get much energy out of the heat they produce. You should be able to get a little but would it be worth it. Most of the thermoelectric devices i've seen in the past are also flat, while most resistors have a round cross section, so there would be a poor surface area contact without adding some sort of extra heat conductive medium.Yes, it can and has been done, but the question is, is it worth the effort. Your application will define that.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2017, 02:36:44 pm
content: I remember an article from my youth (ca. 1960) in Popular Science where a missionary to a region where hookah-smoking was popular developed a transistor radio for the locals that was powered from a thermopile (multiple thermocouples wired in series) heated by the hookah combustion. Efficiency was not too important, since the very low-power transistor circuit only drove a small headphone.