Half a croissant, on a plate, with a sign in front of it saying '50c'
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Clip-On Thermoelectric Shower Booster

Add energy to the cold side of the shower.
  [vote for,

The most common type of shower is a head fed by a simple mixer-tap that variably combines hot & cold water. The hot water is usually supplied by the main hot water heating system in the building. These can be gas, solar, geothermal etc. but in the US as least the most common is a bloody great (slightly) insulated tank heated by a resistive electrical element. Heating things is energy intensive, water especially since it has a high specific heat capacity. This leads to big electricity bills, which are annoying.

Thermoelectric elements, or Peltier devices are small solid state chip-like things that get hot on one side and cool on the other when you apply the right kind of electricity. You can use them to make horrendously inefficient mini- fridges, they can be quite useful in temperature controlling small electronics etc. But they're still a sort of solution looking for a problem. Their simple construction and the wonders of mass production mean that these things are now dirt cheap - ~$5 or so for a 40-50W device.

Instead of using them as coolers, when used as heaters they get quite interesting. As an example, I considered replacing my aquarium heater with a Peltier. The tank in question needs about 30W to stay at 77F/25C. To achieve this, my 50W heater is active ~3/5 of the time, consuming 30W. Because the temp gradient between the room and tank is only ~5C, calculations suggest a Peltier could pump heat from the room with only a net electricity cost of 2- 5W*. That's a big relative saving. For 25W, it's probably not worth it, but it illustrates the point that Peltier elements are good at moving heat across modest gradients.

To boost a shower with Peltier element, you could heat the hot water. But, the delta of 50C or so would mean they're operating <link> so inefficiently, that there's no real gain. However, under the bath/shower tray/behind the wall, there is also the cold feed to the shower. Temperatures vary, but a range of 8-18C seems reasonable. No-one in their right mind uses a shower in pure cold mode, nor does anyone pop into the shower for a fresh glass of water, so we're free to manipulate the temperature of the cold side, where the temperature gradient is perfect.

So, we build our device. The market is the casual sunday hardware store browser. They've been looking at heat pump water heaters, but the $1000's more than a conventional unit stings. Our target price is the $100-200 range. We start with an aluminium block, with a radius relief cut/cast into it to allow good contact with standard copper pipe. Then, ~10 or so 45mm Peltier devices attached in-line, simple screw clamps heatsinks and fans. Pg9 of <link> suggests we can pump 55W using only 20W, but we get to use that 20W as well.

So, our 10-element design gets us 750 extra Watts of heating, good for ~3.0C warming at 1 gpm. This assumes the cold is 1/2 the total flow. All for 200W input, it's 370% efficient, the remainder of the heat is extracted from the underbath environment, heat is reclaimed from the draining shower water. The other components are a non- contact flow meter for switching, Hall effect or even sound will work. A controller, some heatsinks/fans and a fairly beefy DC supply.

The result to the user will be that they simply turn the mixer tap slightly toward cold since the average temperature of the input water is now higher. This will reduce flow from the water heater, reducing workload. There will also be lower losses since less heat will be lost through the hot pipes to the air. In summer, the pumped heat ~550W can be subtracted from any air conditioning load, saving electricity there.

Cleverly, the power supply can route through the floor/wall so that only low voltage DC is in the bathroom environment.

*online calculators exist, but power supply/control losses also do.

bs0u0155, Oct 13 2020

Pg9 for a fairly beefy Peltier https://www.cuidevi...t/resource/cp85.pdf
[bs0u0155, Oct 13 2020]

Power Pipe https://www.homedep...ingads&locale=en-US
Off the shelf heat exchanger for waste water [scad mientist, Oct 16 2020]


       Haven't had a hot-tank supplied shower for decades. Showers here are almost always heat-on-demand, either gas or electric. Suggest living in a developed and/or civilised country/part of the world.
pocmloc, Oct 13 2020

       A friend of mine simply put a heat exchanger between the incoming cold water and outgoing shower drain. Not sure how well it works, but better than 0% (no recovery)...
neutrinos_shadow, Oct 13 2020

       Our gas hot water heater costs maybe $400 a year to run?
RayfordSteele, Oct 14 2020

       //put a heat exchanger between the incoming cold water and outgoing shower drain.//   

       I was halfway through building one for an electric clothes drier, until we moved and the prospect with messing about with a gas drier made me a lot more nervous. The idea of a heat exchanger in that application is reasonably sound. When you think it through however, there's a lot of downsides. The two have to be specially plumbed together, you either build it in in the first place - no housebuilder is adding cost and time for that kind of thing, or you re-plumb the waste and water line. How do you make it? Aluminium is out, too many drain cleaners would destroy it. Stainless will be expensive.   

       //water heater costs maybe $400 a year to run?//   

       Ours is double that. The way this is running, the 550W over a 15min shower saves about 135W/hrs. 4 People, 1 shower/person/day the payback time is a little over 10 years. If 2 of those people are teenage girls, it pays back by March. Depending on the time of year, you can add 20- 50% on top of that for saved HVAC electricity. Also the time to acceptable temperature would be less, so some water saved also.   

       The same device could also be added to the water heater inlet, hand washing sinks, clothes washers etc.
bs0u0155, Oct 14 2020

       // How do you make it? Aluminium is out, too many drain cleaners would destroy it. Stainless will be expensive. //   

       Copper. You can order a Power Pipe heat exchanger from Home Depot in the USA. See link.   

       I have one of these under my house between the main drain and the cold water going into my hot water tank and the cold water taps in the tubs/showers. It's a 3 foot long 3 inch copper pipe with smaller copper pipe wrapped around it. It is installed vertically. The theory is that the drain water tends to flow along the walls of the vertical pipe, so it transfers the heat more efficiently than it would from a pipe running mostly horizontally. Since it's a but it's a standard 3" copper drain pipe there's no problem with clogging or maintenance. The incoming water spirals around this from bottom to top, making it a counter flow heat exchanger. I forget the numbers, but I think it was supposed to be more than 50% efficient in good conditions. I don't see that mentioned in the current description. It just says it will save 26% on water heating costs. The longer pipes are more efficient, saving up to 39% for the 72 inch model. I only had enough elevation drop from my drains to the septic system for the shortest pipe.   

       The Power Pipe only works if you have >4 feet of drop in the drain line. My guess is that would not apply to the majority of houses. When I designed my house I made a few small changes to make this work, but I think in my case it pays for itself very quickly. Just counting energy cost, the payback could be a while, but in my case is also allows me to save money on the water heater.   

       The building department required that I install a heat pump water heater (or some other energy saving system that was even more expensive). Those are efficient, but not very powerful, so I needed a big tank. However by recovering the waste heat I was able to buy a 50 gallon tank rather than an 80 gallon tank. That saved almost as much as the cost of the heat exchanger, and the heat exchanger should last through many water heater replacements.   

       The heat exchanger seems to work fine. I have been able to feel the temperature difference of the pipe before and after, but unfortunately I don't have instrumentation installed to calculate how much it is actually saving me.   

       And [+] for the main idea. It seems somewhat plausible (and quieter than the compressor on our heat pump water tank). It seems to me you might also be able to integrate it with a heat exchanger. For my house, we don't have or need A/C, so pumping the heat from the house doesn't help much.
scad mientist, Oct 16 2020

       Apparently there is also a new system called EcoDrain that is horizontal. I'd also worry about it being more likely to clog and possibly problematic getting a snake through it, but their web site says it isn't a problem.
scad mientist, Oct 16 2020

       We initially misread the title as "Clip-On Thermonuclear Shower Booster", which would be awesome.   

       [+] for the concept.
8th of 7, Oct 16 2020

       40 seconds of hot water ( you can stay in the shower longer but you have to turn the water off when you're not using it, so the total run time is <40 secs)
pocmloc, Oct 16 2020


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