Half a croissant, on a plate, with a sign in front of it saying '50c'
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Toilet Cistern Heatsink

Utilize the soon-to-be-flushed water as a source/sink of heat
  [vote for,

Toilet cisterns have about 3-4 gallons of water in them. So, let's give it an additional role. First, insulate the cistern. Then, fit it with a Peltier element with appropriate heatsink/fan arrangement.

In summer, you can heat the cistern water and cool the bathroom. You should be able to heat the water up to around 70C without and real risk of burning anyone, in the process dumping around 2-3 mJ into the water and eventually down the sewer. With a couple of flushes per hour, you should be able to get a kW of cooling. In winter, you have the Peltier cooling the cistern water and flooding the bathroom with lovely heat stolen from the water. There's much less temperature gradient to play with here, but it would probably be OK to form a little bit of ice as long as the chunks are small enough to transit the relevant pipe work... MUCH energy to be got there. In addition, the Peltier's inefficiency will work in our favor at this point.

This should scale nicely to the amount of people in the building, and 70C water is a reasonably effective sanitation aid.

bs0u0155, Mar 03 2014


       70C water probably adds a bit of preventative declogging, too.
the porpoise, Mar 03 2014

       umm... (here) water comes in from the mains @ 60'ish F. Why not just insulate both cistern and bowl, then leave the lid up in the summer and down in the winter. I don't think a fan that blows air around a bathroom is a good idea.
FlyingToaster, Mar 03 2014

       Good general idea, but I don't like the implementation too much.   

       1) Ditch the Peltier devices. For a stationary installation that doesn't need to be micro-sized, the compressor from a small fridge will be much better, but ideally you'd add a little plumbing to connect to your whole house heat pump.   

       2) Heating the water to 70C is a waste. As soon as you exceed the outside air temperature, you're better off using that as the cold source for your heat pump. Of course heating to 40C may result in faster algae growth. I wonder what would happen if a toilet normally operated at 40C, but had a "self-cleaning" mode where a couple tank-fulls of water could be heated to 95C. That ought to kill whatever is growing, but I'm not sure how much it would help make it look clean.   

       One alternative/addition to try is a Power-Pipe. Those are normally used to pre-warm water going to the water heater, but I don't see a reason that you couldn't use one with a heat pump. Depending on the climate and how much heating vs. AC is used, it might be good to keep the warm drain water and cold drain water separate and use two Power-Pipes to extract heat from one or cool from the other depending on the season.   

       Doing the heat exchange in the toilet tank has the advantage of allowing the heat pump to extract heat on its schedule rather than having to kick on to capture cool whenever the toilet flushes, but then you either need to retrofit the tank on an existing toilet or buy a toilet that has the special tank, which may not be as good in other aspects.
scad mientist, Mar 03 2014

       Hmm, if youre using this for heating, it seems like you'll definitely be better off extracting the heat after flushing. No matter how good your tank insulation is, if you extract the usable heat when the water first goes into the tank, but it sits there for hours, that cold will leak back into the room. Now, if the heater is manually controlled, and you turn it on a few minute before using the toilet, it would work well, but I'm not sure too may people want to plan that far ahead.
scad mientist, Mar 03 2014

       // Now, if the heater is manually controlled, and you turn it on a few minute before using the toilet, it would work well, but I'm not sure too may people want to plan that far ahead.//   

       Hence why the big, stationary, insulated tank is ideal. Even though post flush water may be warmer, it's zipping by quickly, and is therefore difficult to efficiently extract heat from.   

       It might be useful to take waste water from the sink, put that in the cistern as grey water and then extract heat from that.
bs0u0155, Mar 03 2014

       I like the general idea, but my house gets very cold water in the winter, and warmish in the summer. Groundwater is 55 degrees F, here, but the water tower and the mains really follow the sun.
baconbrain, Mar 04 2014

       Wonders what level of thermal shock your porcelain pot will tolerate before shattering...
lurch, Mar 04 2014


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