Harvesting waste heat. - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: May 12, 2023, 01:00:35 pm
content: I can think of many sources of waste heat in the average home. Usually these just vent to the ambient indoor air and then have to be removed with air conditioning.Computers come to mind. Outdoors, solar panels produce a lot of unwelcome heat and could use cooling. Waste cooking heat. Empty ovens on cool down.The problem with all of these waste heats is they are low grade, low energy sources. The fact you are "cooling" something is going to require the coolant is also "cool". So running your computer cooling loop at 60*C and using it to heat your bath water is not an option!We have heat pumps however. Is there an "in reach" tech which can take low grade heat and concentrate or amplify it (to short useful bursts) or even store it accumulatively over time until it gets to a useful temperature.What scale does such a system need to be to be of benefit? Consider if you were to run the return line for your heatpump via the cooling system in a computer server rack. It might get a better GOP than it would going out into cold winter air or soil.If it would need to be large scale, it won't work domestically. However if you could capture accumulative heat with just a 20W compressor heat pump and a bucket of sand to store it, maybe you could use it like a storage heater for when it gets colder in the evenings. Maybe you could use that heat entirely on it's own to run a small air purifier/fan.I just hate pumping heat out into the air and spending money on air con moving it elsewhere when accumulatively the amount of energy being wasted to create the heat is also coming out of my pocket.Tech Ingredients did a video on "nearly free aircon" using a desiccant pump system to absorb moisture and then evaporating it. It used heat to dry the desiccant ... why can't we use waste heat from a server rack to drive that drier?

author: ejeffrey
author_rank: Super Contributor
timestamp: May 13, 2023, 06:35:03 pm
content: I think it's pretty hard to do this effectively for a bunch of small heat sources around the house. Still its an interesting idea. One idea I have thought about is drain water heat recovery. Use the heat in your waste water to pre-warm incoming water. This could work especially in climates where the cold water temperature is actually cold, even if you could warm incoming water from 45 to 60 you would save a reasonable fraction on your water heating bill.In the end I think there is so much more to be gained from big appliances. Variable speed AC units, AC economizer systems, heat pump water heaters and clothes dryers and induction cooktops will all save more than optimizing any of these small sources. After that, judicious venting of waste heat outside, but only if it's warmer than the makeup air.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 13, 2023, 07:15:55 pm
content: Sometimes the waste heat isn't waste heat. Your solar panel example for one. My solar panels on my roof are pretty typical. They convert somewhere between 10 and 20 percent of the incoming solar radiation to electricity. The rest is reflected or converted to heat. Since the panels are mounted with a sizeable air gap a lot of that heat is transferred to the air. If the solar panel were not there, roughly the same amount of incoming solar radiation that would have been converted to electricity is reflected, and the rest is directly deposited on the roof. So the solar panels deliver electricity and reduce the cooling bill. In a colder climate this would be a bad thing, but it generally works well at my location.You could use the sources you mention to preheat water going into the hot water heater, and the efficiency would be reasonable. But all of the plumbing would be expensive and increase the risk of leaks (not because this plumbing is particularly leak prone, but because every joint is another failure opportunity and you would be adding a lot of joints.) Hard to believe that economic payback would occur in the lifetime of the system, even if you don't incur the astronomical bill for home repairs after a leak has gone undetected for a weekend.

author: Someone
author_rank: Super Contributor
timestamp: May 13, 2023, 10:55:58 pm
content: Quote from: ejeffrey on May 13, 2023, 06:35:03 pmThere are quite a few manufacturers of such systems, and while the idea is sound it is only cost effective if installed at build time by a plumber who is already experienced with the device. Australia has some other plumbing rules that make it a little difficult but as an example is this NZ brand with their "human in the loop" thermal regulation:https://www.showerex.com/installationthat really only works properly when you have a trap/cistern sized to match the source volume and the user is happy to play with the mixer tap during the shower.

author: nctnico
author_rank: Super Contributor
timestamp: May 13, 2023, 11:27:02 pm
content: Quote from: Someone on May 13, 2023, 10:55:58 pmThermostatic mixers which regulate the temperature to be constant, are pretty much standard over here. If you don't have one of these, then get one because besides creating a lot of comfort they also save hot water from being wasted while waiting for the hot water temperature to stabilise.

author: Someone
author_rank: Super Contributor
timestamp: May 14, 2023, 12:14:19 am
content: Quote from: nctnico on May 13, 2023, 11:27:02 pmWe have tightly regulated water temperature out of the instantaneous heater, and set that rather than storing hot water for (speculated) future needs. One tap, no fuss.

author: paulca
author_rank: Super Contributor
timestamp: May 14, 2023, 09:11:34 am
content: I also have on demand hot water, which I set to 42*C. So when I have a shower I just use 100% hot water and the boiler throttles back.

author: Freesurfer
author_rank: Regular Contributor
timestamp: May 14, 2023, 09:13:12 am
content: This may depend on the country, but over here it is not uncommon to have a balanced ventilation system with a heat exchanger. It takes the heat from the air going out, and exchanges it into the fresh cold air coming in. It will indirectly recycle the heat from all your appliances that gets trapped inside your home.also, regarding low grade heat/energy, it is still possible to recover like 90% or more of the original heat from waste water or waste air, into new fresh air or water. It depends on the efficiency of the heat exchanger.Normally if you take a bucket of hot water, and a bucket of cold water and mix them, you will get an average temperature of the 2 and not be able to separate that energy back into their original cold and hot buckets. But what good heat exchangers do is they gradually exchange the heat along a length of the contacting surfaces between fresh and waste pipes/tubes, so that it is in theory possible to recover 100%.

author: nctnico
author_rank: Super Contributor
timestamp: May 14, 2023, 09:57:49 am
content: Quote from: Someone on May 14, 2023, 12:14:19 amThat setup is identical in function compared to using a thermostatic valve. If you loop the cold water supply towards your water heater through a heat exchanger that heats up the cold water, you have exactly the same result. You just make the wrong assumption with the 'human in the loop regulation'. It doesn't have to work that way; it is just bad wording by using the word 'mixer tap' where it should say 'water temperature regulator' (which can be any kind of device). The website you linked to just shows one example of what a heat recovery system can look like. Likely to match what is common in NZ. There are many possible ways you can employ such a heat exchanger as it is a passive device.And I may sound like a broken record but a serious downside I see is that you'll have warm water standing still for quite a long time leading to risk of legionella bacteria infection.

author: nctnico
author_rank: Super Contributor
timestamp: May 14, 2023, 10:06:43 am
content: Quote from: paulca on May 12, 2023, 01:00:35 pmThe best way is to produce as less heat as you can. Buy an induction stove for example and use a small oven (or use a pan with a cover on top).One of the things I'm looking at is to retrofit my home with a ventilation system (like Freesurfer describes) that heats up air coming in from the outside using the air that is extracted from my home. My home is well insulated but I have to keep windows open all year round in order to remove moisture.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 14, 2023, 11:19:37 am
content: If you wanted a project - I think a PC case that uses the waste heat from the CPU and GPU to heat coffee would work out quite well in terms of the temperatures involved and the overall thermal output.You'd start with a liquid cooling system, but instead of using an air-to-water heat exchanger and fans to simply dump the CPU heat outside the box, the heat would instead go into an insulated reservoir of water. Once that water reached coffee brewing temperature, a light comes on and it can be dripped through coffee grounds (or just deposited in a mug if you're OK with instant) in the usual way.

author: Circlotron
author_rank: Super Contributor
timestamp: May 14, 2023, 11:36:57 am
content: Quote from: Freesurfer on May 14, 2023, 09:13:12 amIndeed. The important part is to have the outgoing hot and incoming cold flowing opposite directions. Some birds uses this method to reduce body heat loss from their feet to a cold surface by having the blood descending to their feet cooled by the blood rising opposite direction back from their feet courtesy of the two veins having good thermal contact.https://web.stanford.edu/group/stanfordbirds/text/essays/Temperature_Regulation.html

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2023, 11:52:55 am
content: Quote from: paulca on May 12, 2023, 01:00:35 pmYes, that technology is called a heat pump, in whatever form that takes.

author: Someone
author_rank: Super Contributor
timestamp: May 14, 2023, 12:47:35 pm
content: Quote from: nctnico on May 14, 2023, 09:57:49 amAustralia has relatively simple plumbing systems yet strict rules on standing warm water (for legionella), so no, in general we cannot just feed warm water back into a heater (particularly an instantaneous one). But even if you do the engineering calculations then it still doesn't make sense for instantaneous heating with low flow shower heads (6l/min as installed at this residence) as the commercially available heat exchangers don't size to that volume and flow.

author: paulca
author_rank: Super Contributor
timestamp: May 14, 2023, 01:45:01 pm
content: Quote from: David Hess on May 14, 2023, 11:52:55 amYea. It's picking the right form etc.On "waste heat" in general. I see quite a few people from 50*N+ obviously they would understand that "waste" is a relative term. For example, I don't care a jot using the 110W PC through winter. The 110W is just heat. The office radiator hardly ever triggers for heating when I'm in the room and that PC is on.Nor would be concerned using the gas hob in winter as the heat will ultimately just help save gas heating!Other considerations of course is humidity. I tend not to have an issue with humidity in winter as the heating is running several hours a day. It's in summer when its' off. Simply opening the windows doesn't always work when it's 95% RH outdoors!The way to solve both problems. Is to convert the waste heat via a heatpump or phase transfer system to power a dehumidifer or air-con. However, that is not a small energy sink. Air-con and air-con/compressor based dehumifiers are multi kW loads!I have to revisit Tech Ingredients video on his desicant evapourating cooling. I can't see why I can't use exhaust heat from the PC to dry the desicant and refresh the cycle. The more heat the drier it is, the move it can absorb from the in-take air and the more evapourates in the exhaust air. As I say I may have his system backwards. Need to rewatch.

author: Marco
author_rank: Super Contributor
timestamp: May 14, 2023, 02:45:26 pm
content: For new housing blocks it might make sense to have a separate drain for showers, get all the warm water and use it to help out a communal ground source heatpump.

author: sokoloff
author_rank: Super Contributor
timestamp: May 14, 2023, 02:49:00 pm
content: Quote from: Someone on May 14, 2023, 12:47:35 pmIt's good to have rules that reduce the risk of legionella, but it seems to me that using drain water warmth to temper water in a pipe that's headed towards an instantaneous water heater is about as safe as it gets with respect to legionella risk. (There's no long-standing water; there's only significantly warm water in the drain stack if there is hot water being drawn at that moment, etc.)

author: nctnico
author_rank: Super Contributor
timestamp: May 14, 2023, 10:14:58 pm
content: Quote from: paulca on May 14, 2023, 01:45:01 pmAnd where is the water from the desicant going to end up? Same for the heat energy. I guess you will want to move the heat from inside to the outside of your home?

author: Someone
author_rank: Super Contributor
timestamp: May 14, 2023, 11:17:54 pm
content: Quote from: sokoloff on May 14, 2023, 02:49:00 pmThere is "long" standing water in the heat exchanger which will be right in the temperature window for maximum growth. The piping from a mixer tap to the outlet is limited in volume and the piping from the instantaneous heater to the point of use is limited in volume (likely along with its tank size). I've looked seriously at dumping low grade heat into water a few times here and after checking with plumbers they have all said it won't be acceptable for a domestic install.

author: sokoloff
author_rank: Super Contributor
timestamp: May 14, 2023, 11:42:58 pm
content: Not the way I’ve seen it plumbed. The copper cold feed to the instant water heater runs to a fitting which is a loop around the main DWV stack. From the water heater, there is no mixing valve (because none is needed), so anytime a gallon of hot water is used, a gallon of water flows around the DWV stack, into the heater, and to the consuming fixture. There’s no long-standing water anywhere.

author: Someone
author_rank: Super Contributor
timestamp: May 15, 2023, 12:04:50 am
content: Quote from: sokoloff on May 14, 2023, 11:42:58 pmThe volume of water from when it is first heated all the way to the point of use (since this is discussing domestic instantaneous systems where it never gets sterilised) is considered here, so you're adding everything from the heater inlet to the inlet of the heat exchanger (if it didn't get warm then what's the point?). Its that volume to point of use which is kept tiny in the installs here, narrow piping and short runs.

author: sokoloff
author_rank: Super Contributor
timestamp: May 15, 2023, 12:29:28 am
content: Similar here. Nonetheless, I havefarmore water in the pipes between the outlet of my water heater to the attic shower than I would from the start of this pre-heater to the outlet of my instant water heater.

author: Someone
author_rank: Super Contributor
timestamp: May 15, 2023, 12:39:12 am
content: Quote from: sokoloff on May 15, 2023, 12:29:28 amYes, other installs are different. Low flow shower heads and piping designed for those loads are likely a radically different situation to what you have. The local regulations here don't allow large volumes and/or high flow unless other precautions are used which gets outside domestic installs.Waste recovery is good, I'd like to use it despite the risks slightly elevated risks of legionella but its effectively illegal here as the plumbers won't certify it (depending how you squint it might be technically right or wrong). As I said, I've seriously looked into this stuff and couldn't find a way forward with the local regulations....WHICH ARE DIFFERENT TO YOURS

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2023, 12:42:41 am
content: Quote from: paulca on May 14, 2023, 01:45:01 pmThe problem is that there is nothing wrong with the efficiency of conventional heat pumps over limited temperature ranges. If there was a better technology, we would already be using it, so if your idea requires a better heat pump, then it might as well be a heat pump with greater than 100% efficiency.

author: sokoloff
author_rank: Super Contributor
timestamp: May 15, 2023, 12:59:51 am
content: Quote from: Someone on May 15, 2023, 12:39:12 amThese guys got approval for somewhere in Australia; might be worth a call/email:https://www.architectureanddesign.com.au/getattachment/69a82e64-718c-4bf5-be6f-b7d1fbed6df7/attachment.aspx

author: Someone
author_rank: Super Contributor
timestamp: May 15, 2023, 07:23:31 am
content: Quote from: sokoloff on May 15, 2023, 12:59:51 amAgain, you're talking about something entirely different from what I was describing:instantaneous heaterlow flow rateheated only to end use temperatureThe system you link to is being promoted in Australia but with a flyer that is non-representive of the hot water systems installed here, 65 degree distribution is not done, shower water is considered potable, and tempering valves are mandatory. It looks like a thin localisation veneer over a UK product/system. Yes, a system with central high temperature storage is a much better match for heat recovery as the long dwell time at >60 degrees sanitizes the bacterial concerns. Additional losses from the storage and higher temperatures may, depending on each specific installations parameters, or may not be offset by the waste heat recovery.

author: nctnico
author_rank: Super Contributor
timestamp: May 15, 2023, 07:41:25 am
content: Then again, if you make hot water using a heat-pump it will need to go to 65 degrees due to regulations. If you start out with warmer water it will help. In the end the shower heaters have a very low efficiency (I assume these run on electricity?). Using a heat pump (which can be part of an AC system) + heat exchanger you can likely reduce the electricity consumption for showering by a factor 5. The heat exchanger is not a product that is going to be mass installed today but that doesn't mean it won't become popular in the future.

author: Someone
author_rank: Super Contributor
timestamp: May 15, 2023, 08:02:23 am
content: Quote from: nctnico on May 15, 2023, 07:41:25 amSCOP of an air source hot water unit might be 5 in the future, but today 3-4 is more realistic. Higher upfront costs for lower energy use, so waste energy recovery becomes less economically viable!

author: nctnico
author_rank: Super Contributor
timestamp: May 15, 2023, 03:43:41 pm
content: Not just the SCOP... a heatexchanger recovers more than 50% bringing the total efficiency to well over double of what the heatpump alone can achieve

author: paulca
author_rank: Super Contributor
timestamp: May 15, 2023, 03:51:02 pm
content: I got lost there, are you guys referring to using a waste heat tank and passing the heatexchanger pipe of the heat pump through it?So, for example, the shower drain, or possibly the dishwasher and the washing machine too... drops into a 20 gallon drum in the basement. The house ground source heat pump's main refrigeration loop goes around through that before heading off through the rest of the ground loop.Considering those pumps are sourcing heat from 10*C ground 1 meter down in many cases, giving them a little boost with 30*C ex-shower water or 60*C ex-dishwasher water would surely give them a massive boost of efficiency?

author: Marco
author_rank: Super Contributor
timestamp: May 15, 2023, 06:07:47 pm
content: You'll need a sleuce to release shower water from any temporary storage and even then the standing water might cause problems with buildup because it's contaminated, the shower recovery heat exchangers are just a copper pipe with the waste water clinging to the inside wall, relatively fool proof.I think more complex mechanisms can only really be justified for multiple homes.

author: Someone
author_rank: Super Contributor
timestamp: May 15, 2023, 10:49:45 pm
content: Quote from: nctnico on May 15, 2023, 03:43:41 pmSo more exaggeration on top of exaggeration?https://www.showerex.com/save-money"up to 48%*"* under ideal circumstancesIts all install specific, even halving the energy input cost to a heat pump it still wouldn't pay off for my use (or others) but could pay off for some others. When the marginal cost is already so low (from the heat pump) further improvements to efficiency can be poor or negative investments.

author: Circlotron
author_rank: Super Contributor
timestamp: May 15, 2023, 11:55:36 pm
content: Waste water to incoming water heat exchangers might work as claimed when new, but after a while the surfaces of the waste water side are going to become coated with all sorts of yucky stuff. I expect that would degrade their effectiveness significantly.

author: sokoloff
author_rank: Super Contributor
timestamp: May 16, 2023, 12:42:37 am
content: My cast iron stack is probably original to the house (~100 years), but is surely unchanged for the 17 that I’ve owned it. When someone is showering, the outside of the stack gets noticeably warm, which is presumably all you’d need to make this work. (Surface tension tends to pull the water to the side of the stack rather than free-falling down the middle.)Edit to add: A family member is showering. The basement ambient temp is 65°F. The cast iron stack is 88°F (using the same IR non-contact thermometer for both). The incoming cold water service is 63°F right now. If I could temper that to 75°F (splitting the difference), that would mean about 21% less fuel used to heat the water to 120°F. Maybe that only applies to 2/3 of the hot water usage. I’ll still take 14% (or more if the tempering is more efficient than just half the delta-T).

author: Freesurfer
author_rank: Regular Contributor
timestamp: May 16, 2023, 10:29:36 am
content: I am no expert on heatpumps, but my basic understanding tells me that using waste water to pre-heat the cold side of a heatpump isn't the best way to regain the energy. If doing so had any real benefits, chances are they would already be cycling heat from the hot side back into pre-heating the cold side of any heatpump, and that you could almost make a perpetual motion device this way.It surely is better than letting the water go to waste, but the heatpump still needs high grade energy to run the compressor, to regain low grade energy from the waste water, so recycling the heat that way isn't "free" energy. As mentioned I don't know a lot of heatpump theory, but I am sure if you run the numbers you will see that you are essentially ending up swapping high grade energy for low grade energy 1:1, or otherwise not gaining more energy than the heatpump normally would by operating on ambient outdoor temperature from the cold side.In any circumstance I think that using a heat exchanger is the only good way to do it. That way you can in theory do a near 1:1 exchange of low grade to low grade energy.When you shower, a lot of the heat from the water is dumped into the air in the bathroom, and some of it is further transferred into the floor and drainage pipes that are partially inside the building, so I am not sure if the gain overall is that great.It would be cool if someone with good knowledge on heatpumps could throw together some numbers on it.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 16, 2023, 10:52:05 am
content: Heat recovery from shower water is mandatory in new buildings here and there exist a few ways to do so. In the tray from your shower cabin or in a pipe within the floor.To answer the initial questions asked, can we recover heat from solar panels or indoor appliances?Yes, PVT panels exist and are gaining popularity. And you can install heat pump water heater.Such water heater is basically the same as any other water heater tank, but with a heat pump. It takes ambient air from the room it's in, and makes hot water at a better efficiency then normal water heaters.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 16, 2023, 12:20:16 pm
content: Quote from: Freesurfer on May 16, 2023, 10:29:36 amOf course it isn't free, because you are getting more heat out than in; you have to put the difference in. E.g. drain water is cooled from 25degC->5degC, supply water is heated 10degC->60degC, flow rates are similar; there needs to be an extra energy source.The simple fact you miss is that the energy you put in the compressor comes back to heat your water, too. So you are getting the free part,plusthe non-free part, and losing barely anything. Just like if you use a passive heat exchanger, you get the free part, plus then you would use resistive heating to get to the final temperature, hence the non-free part.It's just that with a heatpump, you can cool the outgoing waste water close to freezing and extract more energy from it. Passive heat exchangers suffer from the fact that the output temperature is somewhere in the middle of hot and cold sides, so you leave some unused energy and have to use more energy for post-heating. This is why a heatpump will win this particular case.For the same reason,exhaust air heat pumps, really popular here for last 10 years or so, beat classic passive heat recovery ventilators in total system efficiency. They can cool the exhaust air down to outdoor temperature, or even below. Of course, if going significantly below, then it's better to install an additional air source heatpump that circulates the outdoor air in its evaporator, and cool down the exhaust air just to match outdoor temperature but no further.

author: paulca
author_rank: Super Contributor
timestamp: May 16, 2023, 02:08:37 pm
content: Yeah, you see is where I am thinking too.Using the shower water that left the boiler at 42*C, dropped over me at 40*C and by the time it makes it down the plumbing it's maybe 25*C. The tap water is around 15*C. Passive thermal transfers are relative to the delta. Jm2C or JKgCT units of heat transfer, just like datasheets. Just like PC radiators they are pathetically poor and I expect running the 25*C shower water around the outside of a pipe will do almost nothing to the 15*C water coming in.You need a heat pump to harvest as much heat as possible and take a lot of low quality heat and convert it into a small amount of concentrated "useful" heat. Exactly as air and ground sourced heatpumps do.I can't see why applying hat technique to waste water and other waste heat can't be an advantage. Instead of dumping the 24*C water after the heat exchanger, freeze the stuff in a grid completely and then dump it mechanically or with a heat pulse.Taking that loop further, which not put the ice on a belt and send it through a refridgeration unit for your food before finally going down the drain.The problem is when you start exploring options, not all of them add up, but you still get people thinking you are trying to over-unity the energy equations. I am not expecting "free" energy". I am expecting to pay some energy to move waste heat (or cold) from where it's not wanted and considered waste to somewhere it is wanted and isn't waste.The main issue with application is that most waste heat is "low grade", it's high entropy. There is a lot of it around, but it's spread far and thin. You need a way to collect, amplify and maybe aggregatively store it.Maybe a heatpump ala charge pump to generate smaller amount of mass at higher temperatures compared to the input heat.A 25*C waste heat stream 24/7 .... via heat pump ... create 1 minute pulses of 600*C into a barrel of sand. That kind of thing. You pay for the energy required to collect, focus and amplify that heat, you get back whatever heat transfer you can get back from the barrel of sand.I can't make any claims in these cases for the numbers and if they can be made to add up, but that's why I asked.

author: tszaboo
author_rank: Super Contributor
timestamp: May 16, 2023, 02:09:30 pm
content: Quote from: Someone on May 13, 2023, 10:55:58 pmThat's actually quite clever, I always imagine these systems that you need to wreck your house trying to fit in all the new appliances and pipes, but this could work without that. I wonder what's the ROI of such a system is, I think my showering is like 5% of my total heating bill. It definitely looks too expensive for just a bunch of pipes.

author: paulca
author_rank: Super Contributor
timestamp: May 16, 2023, 02:25:22 pm
content: The higher output temperature could be achieved by a higher compression ratio on the refrigerant, but I expect the numbers do start to fall apart and it's why heat pump heating systems have typically been "mid grade" heat until recently. The higher the compression ratio the higher the phase change temperature and the less efficient it will be collecting lower grade heat.Can this be stacked just like charge pumps? Using different classes of refridgerant to progressively condense the heat into smaller masses of higher temperature?I scoffed originally at sand batteries. "Sand thermal storage". Because sand is more expensive than water, harder to heat than water and has a lower specific heat capacity. Then I realised that a sand battery has a "Fully charged" temperature of up to 600*C.600*C is VERY high grade heat. Matched only by fossil fuels.The question is... how do you get the 600*C heat in teh first place.

author: Marco
author_rank: Super Contributor
timestamp: May 16, 2023, 02:39:54 pm
content: Resistive heating, the bigger problem is ratcheting slowly destroying the container over time (the packing at the bottom layer gets more and more efficient, until it doesn't flow any more and just thermally expands as a solid mass).

author: Siwastaja
author_rank: Super Contributor
timestamp: May 16, 2023, 04:13:19 pm
content: Quote from: paulca on May 16, 2023, 02:25:22 pmCan be stacked (cascaded) but the fundamental COP limit is still there. You get a small improvement on "mid grade" say 100degC dT, such cascades are used in large scale systems and maybe you can get COP=2 out of something which would be COP=1.5 with a single-stage system, I don't know. That's roughly the ballpark.Maximum theoretical COP is Th / (Th-Tc) where Th and Tc are absolute temperatures (in Kelvin). Going from +5degC to +60degC, maximum is 6.05, and to 600degC 1.47. Good actual heatpumps are maybe 40% of the theoretical maximum. Industrial-scale state of art a bit more than that.QuoteYeah. Use water instead of sand, and thermal capacity goes up by 5x or so. So instead of 600degC, you are fine with maybe 150. Still too much for a heatpump. Make it double the size of comparable sand thermal storage, and now we are talking. Remember to calculate the COP over the whole heating cycle. If you start low, you get excellent COP at first. Even if you need to finish the last drops of storage with resistive heating, the average COP over the whole cycle can be way over 1.

author: Someone
author_rank: Super Contributor
timestamp: May 16, 2023, 10:34:56 pm
content: Quote from: tszaboo on May 16, 2023, 02:09:30 pmThere are likely similar/identical devices available locally to you, relatively cheap/easy to install for a new build and not impossible for a building with accessible pipes (not cast into the slab). Its the ROI thats the problem as it only profits for larger dwellings, even 3-4 person houses would have a hard time justifying the cost if they were already using a heat pump hot water storage (or adding the heat pump would not longer be attractive if you already had the heat exchanger).

author: tszaboo
author_rank: Super Contributor
timestamp: May 17, 2023, 01:15:28 pm
content: Quote from: Someone on May 16, 2023, 10:34:56 pmWell, most of Europe everything built since the 19th century will be "brick and mortar" unlike the 4x2 wood constructions used in the US for example. So I always see that people have no issues retrofitting wires or airco in those houses with just a hammer or a hole saw. If I want to do the same, for starters the entire rooms needs to be painted again. Just placing an extra water pipe would be a mayor undertaking in these homes. In NL majority of the people for example have their bathroom on the first floor, the pipes going in the wall. This might is installable in the shower's tray, so retrofit without paying 1000s could work.