That's an interesting point. Not sure I understand the physics well enough to do this calculation correctly, but at a glance, the latent heat of vaporization for water is 0.628 kWh per kg of water absorbed. So I think you'd want to discount that from the 1.30 kWh (48% of 2.7 kWh) you could scavenge from a bath. I'm highly unsure of how many liters of water would realistically evaporate. One liter? More? Less?
At a minimum, it seems plausible that, say, 0.10 liters could evaporate out of a 80 liter bath which would mean a cost of .0628 which would still be something like 5%, so worth accounting for...
It also appears to depend strongly on the velocity of air in the room and the humidity of the air (as well as the air temp and altitude). So I think it's hard to make big generalizations, but from playing around with an online calculator, it doesn't seem difficult to have circumstances with multiple liters of water evaporating, meaning a negative effect as you suggest.
I think that not leaving the water overnight seems sensible. But maybe also this suggests that it would be a good idea to leave the bathroom door closed during the process (so that the room would reach 100% humidity and reduce this effect?) Or, alternatively, some people might want more humidity in air, and they are currently running a humidifier to achieve this, and they'd run that purifier less with more humidity, so this might be neutral or positive on net despite the fact that this cooling is real? Highly unsure—but in any case, I think it's a great point, thanks!
Not a lot, and less if the exposed surface area of the water is small (such as if it is stored in a pipe). Also, the amount of evaporation which would occur would decrease as the temperature of the water decreased, and for entropic reasons is necessarily less than the total amount of energy released by the water.
Modernity focuses a lot of its means on efficiency. It's a prime directive in energy and labor and market methodologies. But it seems to me (beyond your excellent analysis using scientific facts and data) "efficiency" is leveraged a tool to continue a system of living that always ends up increasing consumption and waste...a fancy way to hide the paradoxical pea, instead facing the consequences of an astoundingly destructive high energy lifestyle.
I don't mean to be critical of your work, I actually appreciate it. I'm just also curious what you think of the meta-role of efficiency in modern times. I mean, simply being more efficient at incessant exponential growth, which will always require more energy input seems absolutely bonkers.
The idea that civilizations advanced enough will eventually build structures around stars (Dyson's Sphere) to collect all its energy... How much energy would be required to build such a thing?
I guess my view is that we find ourselves in this universe where space has these astronomically large fusion reactors already sitting there shooting out ungodly amounts of energy. Sometimes I look at all the lights on in a city at night and think of all the waste involved in those lights. And then I think about the fact that the sun is orders of magnitude brighter, and it covers the _entire_ planet with light in daytime, and all the light that hits the earth is just 1 billionth of the light the sun puts out, and I think of the "waste" of the sun just sitting there and doing that for billions of years for no reason... And it becomes hard to avoid the conclusion that my instincts for "waste" just aren't very well adapted for thinking about things on a planetary scale.
More practically speaking, I guess my attitude is: Whatever I personally might think, the overwhelming majority of the population has little interest in sacrificing their lifestyles for the good of whatever. It's just not going to happen. So the only realistic option is technological progress: Finding ways to continue to advance our lifestyles while minimizing/reducing the destruction. I do wish that we were more intentional about the damage that's done to the planet, often for little real gain (e.g. things like long-chain carbon molecules in water repellent coatings)
I think this overlooks that the bathroom is not the place you want to heat (at least, we don’t heat ours). The amount of heat needed to keep e.g. the living room and kitchen warm will hardly be reduced and those are usually where the majority of heating energy goes.
I've heard this argument before, but I wonder: Is the temperature of your bathroom closer to the outside temperature or the indoor temperature? That energy has to be coming from somewhere. Imagine doing the reverse and opening a window in the bathroom...
Assume a simple model with two equally sized square rooms, sharing one wall and each sharing one wall with the outside. All other walls are perfectly insulating. We’re ignoring radiative cooling.
If one room is kept at 20 degrees and the outside is 0 degrees, then the other room will reach an equilibrium temperature of 10 degrees. The room that is kept warm loses twice as much energy directly to the outside as it does via the other room (because effectively the wall to the outside is twice as thick via that room). Call the total power needed to keep that one room at 20 degrees (and the other at 10 ‘for free’) 3X.
The amount of energy used for heating is ~4 times that used for heating tap water (at least, in my country). Assuming we can reclaim half of the energy in heated tap water in the room we’re heating, we could save 1/8th of our heating bill, so 12.5%.
Now suppose we instead reclaim that heat in the other room. Then the equilibrium temperature of that room will be ~12 degrees and we need to supply 2.8X in heating. So we would save ~7%.
Of course the geometry of a real house is very different. At the other extreme, when all the other walls border the outside as well, you start, without reclaiming tap water hear, at an equilibrium T of 5 degrees, requiring 7.5X in power. Reclaiming the tap water heat in the heated room would still save 12.5%. With a power source of 7.5/8X in the other room, it would reach an equilibrium temperature of ~7 degrees, requiring 7.3X in power, reducing savings to ~2.5%.
So because it’s the ‘wrong’ room, as a rough estimate you lose another 40-80% in efficiency.
> If one room is kept at 20 degrees and the outside is 0 degrees, then the other room will reach an equilibrium temperature of 10 degrees.
This is extremely unrealistic. For room B to reach an equilibrium temperature halfway between room A and the outdoors it is necessary that room B can exchange heat equally easily with room A and with the outdoors. But room B is connected to room A via an open door and insulated from the outdoors via an insulated wall.
I always allow the water to collect in my shower. I don't find it unpleasant at all to step in, in fact the warm water warms up my feet and makes the bottom of the shower less slippery when I wash my feet and then put the soapy foot back on the bottom of the tub. I also let the hot water collect so that it warms up the cold bathroom in the morning, but this is only important in the winter. I'm not sure I like your calculation. well first you only want to save the water heat ~1/2 of the year. Second some fraction of the water heating energy goes to just keeping the water warm in the tank. (I'm not sure what the fraction is here.) I'd do a more direct calculation, take your ~3kWh per shower and compare that to how much energy goes into heating your house, (For me I use ~700 gallons of heating oil/ yr. (I live in near Buffalo, NY USA) 1.4e5 BTU/ gallon * 3e-4 kWh/BTU is ~3e4 kWh, I heat maybe 200 days per year so ~150 kWh/ day. ) (oops made a mistake the first time.)
I am British and living in a London exurb. Having had a shower, I am now running a dehumidifier, yes a dehumidifier, for an hour to prevent my home from being damp. Typical ambient humidity here is more like Florida than California, so rarely below 80%, whilst the outside temperature is currently 5.5 degC. Consequently all British homes, ancient and modern, tend to have to battle mould caused by condensation on exterior-adjacent walls. Just last week a young child died from such mould going untreated (you can easily Google the headlines) Although I love the principle of heat reclamation from wastewater, in reality it would need to be captured in a heat exchanger linked to the waste pipe system to prevent extra evaporation - and consequently require re-plumbing of the house (and be completely impossible for flats) to separate the warm waste water from the unheated foul waste, I think. Also, since few houses here have basements or cellars, I'm not sure where we're going to locate such heat exchangers. Nice idea though :)
I think some sewerage plants are actually already experimenting with doing it on a grander collective scale, but whether it will prove feasible is as yet unknown.
An inquisitive mind is a wonderful place! I respect and commend your research and tenacity. However, I differ on ONE of those percentages; which starts the Domino Effect, which creates chaos!
You say 90% of the heat in the water delivered, goes down the drain, right?
As an example, a gram of 90° water, leaving the showerhead, will be 81° when it goes down the drain? Not to insult your intelligence (100% Delivered @90° - 10% heat loss (9°) = 81° of heated water is going down the drain?? I don't think so
A single gram of 90°, delivered shower water will lose Temp the moment it leaves the nozzle; exponentially, I'm sure! The farther & longer it travels the geosphere of the shower; the heat will release till saturation; MUCH MORE than 10% !!! You're a "DETAILS" thinker, too, I see! It can get in my way sometimes it was fun reading your article; looking forward to being here!
200 IQ: we can reduce the energy crisis by using bath and shower water to heat your home
300 IQ: We can further mitigate the energy crisis by having short and cold showers.
150000 IQ: If we stop showering we can end the energy crisis.
Evaporation subtracts heat, though, doesn't it?
I wonder how much heat is lost if you have a bunch of room-temperature water sitting in your apartment, slowly evaporating.
That's an interesting point. Not sure I understand the physics well enough to do this calculation correctly, but at a glance, the latent heat of vaporization for water is 0.628 kWh per kg of water absorbed. So I think you'd want to discount that from the 1.30 kWh (48% of 2.7 kWh) you could scavenge from a bath. I'm highly unsure of how many liters of water would realistically evaporate. One liter? More? Less?
At a minimum, it seems plausible that, say, 0.10 liters could evaporate out of a 80 liter bath which would mean a cost of .0628 which would still be something like 5%, so worth accounting for...
Well, it would depend on how long you let the water sit.
So, for example, if you shower before going to bed, "leave the water in the tub overnight" might be negatively helpful.
It also appears to depend strongly on the velocity of air in the room and the humidity of the air (as well as the air temp and altitude). So I think it's hard to make big generalizations, but from playing around with an online calculator, it doesn't seem difficult to have circumstances with multiple liters of water evaporating, meaning a negative effect as you suggest.
I think that not leaving the water overnight seems sensible. But maybe also this suggests that it would be a good idea to leave the bathroom door closed during the process (so that the room would reach 100% humidity and reduce this effect?) Or, alternatively, some people might want more humidity in air, and they are currently running a humidifier to achieve this, and they'd run that purifier less with more humidity, so this might be neutral or positive on net despite the fact that this cooling is real? Highly unsure—but in any case, I think it's a great point, thanks!
Just FYI, I updated the post to mention this as a concern. Let me know if you'd rather I don't use your name. Thanks again!
That's nice of you to ask about mentioning his name; even nicer that your actions show respect, integrity & inclusivity! Thank you ☺️
Not a lot, and less if the exposed surface area of the water is small (such as if it is stored in a pipe). Also, the amount of evaporation which would occur would decrease as the temperature of the water decreased, and for entropic reasons is necessarily less than the total amount of energy released by the water.
But this is a good point.
Typically heated winter air is dry and people also run humidifiers though.
Modernity focuses a lot of its means on efficiency. It's a prime directive in energy and labor and market methodologies. But it seems to me (beyond your excellent analysis using scientific facts and data) "efficiency" is leveraged a tool to continue a system of living that always ends up increasing consumption and waste...a fancy way to hide the paradoxical pea, instead facing the consequences of an astoundingly destructive high energy lifestyle.
I don't mean to be critical of your work, I actually appreciate it. I'm just also curious what you think of the meta-role of efficiency in modern times. I mean, simply being more efficient at incessant exponential growth, which will always require more energy input seems absolutely bonkers.
The idea that civilizations advanced enough will eventually build structures around stars (Dyson's Sphere) to collect all its energy... How much energy would be required to build such a thing?
Madness!
I guess my view is that we find ourselves in this universe where space has these astronomically large fusion reactors already sitting there shooting out ungodly amounts of energy. Sometimes I look at all the lights on in a city at night and think of all the waste involved in those lights. And then I think about the fact that the sun is orders of magnitude brighter, and it covers the _entire_ planet with light in daytime, and all the light that hits the earth is just 1 billionth of the light the sun puts out, and I think of the "waste" of the sun just sitting there and doing that for billions of years for no reason... And it becomes hard to avoid the conclusion that my instincts for "waste" just aren't very well adapted for thinking about things on a planetary scale.
More practically speaking, I guess my attitude is: Whatever I personally might think, the overwhelming majority of the population has little interest in sacrificing their lifestyles for the good of whatever. It's just not going to happen. So the only realistic option is technological progress: Finding ways to continue to advance our lifestyles while minimizing/reducing the destruction. I do wish that we were more intentional about the damage that's done to the planet, often for little real gain (e.g. things like long-chain carbon molecules in water repellent coatings)
I'm with you on the frustration of futility. " If ONLY people would just......___, we could save__! "
Doesn't work that way. If I can influence just one; I've done well! Knowing we've done what we can in treating our "Home" with respect.
Amazing thought process. I often wonder about this kind of stuff as well :).
I think this overlooks that the bathroom is not the place you want to heat (at least, we don’t heat ours). The amount of heat needed to keep e.g. the living room and kitchen warm will hardly be reduced and those are usually where the majority of heating energy goes.
I've heard this argument before, but I wonder: Is the temperature of your bathroom closer to the outside temperature or the indoor temperature? That energy has to be coming from somewhere. Imagine doing the reverse and opening a window in the bathroom...
Assume a simple model with two equally sized square rooms, sharing one wall and each sharing one wall with the outside. All other walls are perfectly insulating. We’re ignoring radiative cooling.
If one room is kept at 20 degrees and the outside is 0 degrees, then the other room will reach an equilibrium temperature of 10 degrees. The room that is kept warm loses twice as much energy directly to the outside as it does via the other room (because effectively the wall to the outside is twice as thick via that room). Call the total power needed to keep that one room at 20 degrees (and the other at 10 ‘for free’) 3X.
The amount of energy used for heating is ~4 times that used for heating tap water (at least, in my country). Assuming we can reclaim half of the energy in heated tap water in the room we’re heating, we could save 1/8th of our heating bill, so 12.5%.
Now suppose we instead reclaim that heat in the other room. Then the equilibrium temperature of that room will be ~12 degrees and we need to supply 2.8X in heating. So we would save ~7%.
Of course the geometry of a real house is very different. At the other extreme, when all the other walls border the outside as well, you start, without reclaiming tap water hear, at an equilibrium T of 5 degrees, requiring 7.5X in power. Reclaiming the tap water heat in the heated room would still save 12.5%. With a power source of 7.5/8X in the other room, it would reach an equilibrium temperature of ~7 degrees, requiring 7.3X in power, reducing savings to ~2.5%.
So because it’s the ‘wrong’ room, as a rough estimate you lose another 40-80% in efficiency.
> If one room is kept at 20 degrees and the outside is 0 degrees, then the other room will reach an equilibrium temperature of 10 degrees.
This is extremely unrealistic. For room B to reach an equilibrium temperature halfway between room A and the outdoors it is necessary that room B can exchange heat equally easily with room A and with the outdoors. But room B is connected to room A via an open door and insulated from the outdoors via an insulated wall.
That makes things worse.
Suppose there was no wall at all. You now need 4X in heating and the tap water supplies 1/8th, so you need 3.5X instead of the original 3X
I always allow the water to collect in my shower. I don't find it unpleasant at all to step in, in fact the warm water warms up my feet and makes the bottom of the shower less slippery when I wash my feet and then put the soapy foot back on the bottom of the tub. I also let the hot water collect so that it warms up the cold bathroom in the morning, but this is only important in the winter. I'm not sure I like your calculation. well first you only want to save the water heat ~1/2 of the year. Second some fraction of the water heating energy goes to just keeping the water warm in the tank. (I'm not sure what the fraction is here.) I'd do a more direct calculation, take your ~3kWh per shower and compare that to how much energy goes into heating your house, (For me I use ~700 gallons of heating oil/ yr. (I live in near Buffalo, NY USA) 1.4e5 BTU/ gallon * 3e-4 kWh/BTU is ~3e4 kWh, I heat maybe 200 days per year so ~150 kWh/ day. ) (oops made a mistake the first time.)
I am British and living in a London exurb. Having had a shower, I am now running a dehumidifier, yes a dehumidifier, for an hour to prevent my home from being damp. Typical ambient humidity here is more like Florida than California, so rarely below 80%, whilst the outside temperature is currently 5.5 degC. Consequently all British homes, ancient and modern, tend to have to battle mould caused by condensation on exterior-adjacent walls. Just last week a young child died from such mould going untreated (you can easily Google the headlines) Although I love the principle of heat reclamation from wastewater, in reality it would need to be captured in a heat exchanger linked to the waste pipe system to prevent extra evaporation - and consequently require re-plumbing of the house (and be completely impossible for flats) to separate the warm waste water from the unheated foul waste, I think. Also, since few houses here have basements or cellars, I'm not sure where we're going to locate such heat exchangers. Nice idea though :)
I think some sewerage plants are actually already experimenting with doing it on a grander collective scale, but whether it will prove feasible is as yet unknown.
An inquisitive mind is a wonderful place! I respect and commend your research and tenacity. However, I differ on ONE of those percentages; which starts the Domino Effect, which creates chaos!
You say 90% of the heat in the water delivered, goes down the drain, right?
As an example, a gram of 90° water, leaving the showerhead, will be 81° when it goes down the drain? Not to insult your intelligence (100% Delivered @90° - 10% heat loss (9°) = 81° of heated water is going down the drain?? I don't think so
A single gram of 90°, delivered shower water will lose Temp the moment it leaves the nozzle; exponentially, I'm sure! The farther & longer it travels the geosphere of the shower; the heat will release till saturation; MUCH MORE than 10% !!! You're a "DETAILS" thinker, too, I see! It can get in my way sometimes it was fun reading your article; looking forward to being here!