Why is it not used again and wouldn’t it be very efficient to combine with a heat pump for reuse?
Especially since water from shower or sink isn’t usually very dirty, couldn’t it be stored for a while in an isolated tank so that e.g. a heat pump can run more efficient on it than on usually colder air or colder ground water?
In: Engineering
Graywater tanks get into some messy plumbing code, not to mention the additional design considerations, maintenance and upfront cost. Maybe that can change if there’s enough of an audience for energy recapture.
The easier solution would be what someone else already posted with just a stack pipe radiator. Although, I have no idea how effective that would be.
It is in my area! There’s a conversion station nearby in a low elevation area. Warm local sewage flows down, and heat is extracted via a heat pump. The heat is then used to heat a smaller volume of water much hotter (~80C iirc, it’s been a while). This hot water then heats many of the buildings in the area!
This obviously has a pretty big overhead and requires a lot of waste water to be effective – as others have pointed out there isn’t much use to using personal waste water only, there’s just not enough energy. But it’s definitely something that can be done at the municipal level.
In our municipality of 300 000 people the sewage goes to treatment plant. From there it goes to heatpumps of district heating company. After that it goes to the ocean. They say the input and output temperature difference is about 5 celcius. Its enough to heat about 15000 households.
1) heat transfer is proportional to temperature difference
2) heat is not very easy to reuse
1) means that, while warm water contains alot of energy, its incredibly difficult to extract it as the temperature gradient is small. Notice all of out industrial power stations, they rely on huge temperature differences. The modern ones will superheat water to force water to go to even higher temperatures than normally possible.
2) the energy can’t be used efficiently. mildly warm water means it can only heat things to temperatures less than the water.
Because it’s inefficient. You would need much better insulation and an efficient system of doing this. All of this costs money and more than likely renovation. Why spend a shit ton of money to save dollars over the course of a year? For newer homes, why spend the investment on something that doesn’t pay for itself for decades. What you are describing is saving dollars for something that costs much more to invest.
By the time water comes out, it’s less than 100 degrees. It then hits colder air and a cold tile/wall/surface, goes down a cold pipe and then what? It circulates at a much cooler temperature USING ENERGY to maybe warm something’s a tiny amount.
I researched this a few years back while renovating our house.
Used hot water is called greywater.
1. It doesn’t pay off for a single household. The reason for that is the low efficiency (small amount of warm water and the relatively low temperature) and the content of fats/hair/etc from soap in the graywater forcing regular cleaning in order to maintain functionality/efficiency. To circulate two separate water circuits in order to transer heat energy also requires electricity (pumps).
2. Larger installations for multiple households can yield an overall industrial-like benefit that pays off in the long run, but it depends on several factors.
Usually it’s not economically viable. You can’t really use the dirty water directly due to particles and residue building up in heating equipment, and heat exchangers are quite expensive compared to simpler heating devices. Also, water used for heating is 1.5 to 2 times hotter than waste shower water – e.g. 60-80 C vs 40 C.
Latest Answers