Maximizing Energy Savings in Showers: The Potential of Heat Exchangers

[gschjetne]

I actually got this idea while in the shower. I thought it was such a pity pouring all that hot water down the drain. I thought some absurd stuff like using a stirling engine to charge batteries and whatnot, but it quickly became obvious that this stuff had to be kept simple.

So, would it work piping the shower water through a heat exchanger on the cold water inlet on the water heater?
How much energy would you save? Why hasn't it been done before?

I don't consider myself cheap, but I don't like the idea of pouring perfectly good joules straight down the drain. After all, Earth's resources are definitely finite, and the rate of consumption in western countries are most significant.

 

[Danger]

I hear you. The thought of pouring perfectly good jewels down the drain is the only thing that bothers me about showering with the gf.

Yeah, I know... get back to GD.

 

[brewnog]

Dear oh dear!


Yes, theoretically you can make use of the waste energy going down your drain. However, basic thermodynamics gives rise to the fact that the efficiency of any heat engine you run off waste heat depends on the temperature gradient between your hot and cold reservoirs. Find a really really cold reservoir and you'll be winning, otherwise you'll have little hope of getting any useful energy out of your heat exchanger because the temperature differences are so small.

Remember that things like car engines are (at best) 40% efficient, think of all of that heat going out of your exhaust and radiator.

 

[Q_Goest]

What is that thing you call yourself? gschnetje? Ok, I'm going to have to inform the name police. Whatever you're calling yourself can't be pronounced, and that's illegal in 5 European nations, Japan, Mexico and the US.

Please ignore that silly Brew man , he seems to be thinking you want to utilize the thermal energy to produce work which will create electricity or some such thing. He's correct in assuming the energy won't be worthy but that's not what you're asking. What you want to do is simply remove the heat energy you added to the water by returning it to the shower. That's simple, and in fact I'm glad you asked. I've been tempted to do the same thing for a very long time, just never had the interest in calculating the benefit. Now that I have, I'll try and do something about it for sure!

Imagine the water coming into your home at some pressure and temperature. The water is then split, some of the cold going directly to the shower, and some going first to your hot water tank and then to your shower.

Here's what you need to do: While you're standing in your shower, measure the temperature of the water coming out of the shower head. Then measure the temperature of the water going down the drain. Also, stand there with a 1 gallon bucket and see how long it takes to fill. The last thing you need to do is figure out how much you're paying per unit of energy. If you have an electric water heater for example, and you pay 0.06$ per kilowatt hour, that is the only additional bit of information needed. You also need to figure out how long it takes you to do the shower thing and how many showers per month you & others in the home take.

I believe the most efficient way of recovering the heat from your shower is to simply pass the drain water through a heat exchanger and warm up the water coming into your home. That way, you're actually warming not just the cold water coming into your shower, but the cold water that will wind up in the hot water system as well. If we do this, the enthalpy difference of the drain water will equal the enthalpy difference of the cold water coming into the home. You save the amount of money equal to the cost of the energy which is equal to the enthalpy difference of the cold water coming into the home (equals the enthalpy difference of the water going down the drain). But to do this you need a few guesses as to those variables I gave you above.

Assuming:
Shower flow = 2 GPM (kinda slow actually)
Water temp into the house = 50 F
Water temp at shower nozzle = 100 F
Water temp at drain = 95 F
Approach temperature of heat exchanger = 5 F (pretty efficient heat exchanger!)
Cost of energy = $0.06 /kW-hr
Average shower length = 10 minutes

Then the cost savings per shower is roughly $1. US. If you have other variables you'd like to put into the mix, just post them and I'll put them in the spread sheet I created for this. It's pretty simple really, and hopefully I didn't make any dumb mistakes, though I must admit, I'm not idiot proof! LOL

One point: The "approach temperature" for your heat exchanger is a difference between the water temperature of the drain water leaving your heat exchanger and going into the sewer - and the temperature of the water coming into the home. Designing the heat exchanger is the next step, though that’s not much harder actually.

 

[russ_watters]

I've considered the idea of using a heat exchanger to simply preheat incoming water as well. I guess people just figured that there wasn't enough of a benefit to justify re-piping houses for it.

Theoretically, if you use 20 gallons of hot water at 120F and water comes into your house at 50F, that's 12,383kJ, which is 3.4kWh, or roughly $0.50 worth of electricity (someone check my math...) every time you shower.

Something I had kicked-around in my head is the idea of using the incoming water for your hot water heater for condenser cooling in your air conditioning. Since showering doesn't necessarily coincide with air conditioning use, you'd need an insulated holding-tank with a heat exchanger inside for the preheating. Same problem though - lot of effort for not much benefit, but at least it lowers both your air conditioning and hot water heating bills at the same time.

 

[russ_watters]

Hmm, Q-Goest, your price per kWh is about half what it should be, you used the same amount of water at a lower temp, and you got double the cost per shower that I did. But I'm too lazy to check your work and double-check mine to find the error... Plus, you beat me by 7 minutes, you bastard.

 

[brewnog]

Please ignore that silly Brew man , he seems to be thinking you want to utilize the thermal energy to produce work which will create electricity or some such thing.

Yeah, sorry. I saw 'Stirling engine' and my mind went off at a tangent. That'll teach me to ignore stuff in bold...

 

[Moonbear]

Hmm, Q-Goest, your price per kWh is about half what it should be, you used the same amount of water at a lower temp, and you got double the cost per shower that I did. But I'm too lazy to check your work and double-check mine to find the error... Plus, you beat me by 7 minutes, you bastard.

Well, without doing ANY math, I'd say your numbers are likely more accurate than Q-Goest's simply based on my monthly gas bill. On the months when I'm not running the furnace, just using gas for heating water for laundry, showers and dishwasher, plus the gas stove for cooking, the bill isn't high enough to account for $1/shower let alone to get a $1/shower savings.

 

[Integral]

Better solution.

It's called a Navy Shower.

Turn the water on and get wet. Shut the water off, soap up and wash. Turn the water on and rinse. This was the shower required in the navy back in the days when hot fresh water was at a premium aboard ships. You can manage a shower with just a few gallons of water.

 

[russ_watters]

Better solution.

It's called a Navy Shower.

Turn the water on and get wet. Shut the water off, soap up and wash. Turn the water on and rinse. This was the shower required in the navy back in the days when hot fresh water was at a premium aboard ships. You can manage a shower with just a few gallons of water.

I used to get yelled-at for taking too long of a shower. It's still an issue on smaller ships, anyway.

 

[Q_Goest]

Ok, found the problem with the calc (now that I'm sober and well rested :zzz: ) Hey, I can be excused - it is Thanksgiving holiday here you know!

The savings for the example I had comes out to $0.10

Looks like I used the ten minutes twice by accident...

 

[Moonbear]

Better solution.
It's called a Navy Shower.
Turn the water on and get wet. Shut the water off, soap up and wash. Turn the water on and rinse. This was the shower required in the navy back in the days when hot fresh water was at a premium aboard ships. You can manage a shower with just a few gallons of water.

My mom went through various stages of water conservation kicks when I was a kid. One of them was to set a timer for 5 min...when the bell rang, the water had to be turned off. Shutting the water off to soap up and lather up the shampoo was another one...or maybe that was my dad's contribution. It was the one and only time in my life I considered getting my hair cut short...I was getting tired of always having a little bit of conditioner still unrinsed out of my hair after every shower.

Back on topic...from a practical application perspective, how would you plumb this heat exchanger, assuming one determined it was worth doing (which it might not be), so that you would get proper flow of the water to not back-up the drain? As it is, there are houses where there are problems getting sufficient gravity flow of wastewater out to the sewer that require a pump be installed (such as from a basement bathroom), so you might lose any benefit if you need to install a pump to get the water back out to the sewer after it has passed through your heat exchanger.

Also, how would you handle things like soap and hair build-up in the wastewater? It's not like it's just clean water running out the drain.

Lastly, I think a 5 degree drop from shower to drain might be underestimating the cooling as the water passes through air, hits cold tile floors, or porcelain tubs, or whatever surface your shower is enclosed with, and travels through the drain to wherever it's going. You might have to take a pretty hot shower to get much benefit, in which case, you might save more by just taking a slightly cooler shower than by setting up a heat exchanger thingamabob.

It might only be something that provides a benefit in a very cold climate where the cold water coming in is particularly frigid. I doubt it's going to help in a place like Florida, where the "cold" water comes in fairly tepid.

 

[gschjetne]

The winter is coming here in Norway, so the temperature differences will be significant. A large area filter in an accessible location between the heat exchanger and the shower should catch the residue.
It should definitely not affect the comfort of having long showers or baths, so traditional schemes are out. But like I said, I hate to let good things go to waste. (If only things were as good for me as they are for you, Danger )
Having the water pass through the air-condition is an excellent idea, however here in Europe you only get an air conditioner when it's too hot to take hot showers, and when you need to take a lot of hot showers, you really don't need an air conditioner either. The latter is the case in Norway!
Also, I think the nature of my house (very old) makes big plumbing projects cumbersome. Just airing my idea, if I am to do a planning lab on thermodynamics, I might try to calculate it. If I'm still interested when I get a place of my own (not rented of course), or if they won't be doing this in the future, I might even give it a shot. If nothing else, for the fun of engineering things.

And by the way, Schjetne is my last name, doesn't seem to be illegal in Norway yet

 

[FredGarvin]

Time to be nit picky...

Q,
Given the infrequency of howering versus the total time in one day, I think one thing would need to be looked at. That is that if cold water entering your home is flowing through the exchanger all day, it would take some of the time the shower is running to bring the heat exchanger up to temperature, i.e. a lot of the energy from the drain water will go to bringing the heat exchanger components to temp and not go to the cold side.

I'm not saying it would be a huge amount, but considering that the excahnger can be essentially cold soaked for 23.9 hours and then expect to have a good efficiency from the get go might be a bit unreasonable. I would think it would take a couple of minutes to get to some kind of equillibrium point.

 

[Q_Goest]

Hey Moonbear, interesting story. My folks were much the same. They grew up in London during WW2, so they brought their thrifty habits with them when they moved to the US. My dad installed an on/off valve on the shower head which we were expected to use to conserve water. (yea, right).

Fred, just a quick calc shows that a pound of copper going from 55 to 90 F requires 3.24 Btu. So if the heat exchanger needs 10 pounds of copper, that's 32.4 Btu. The energy saved (transfered) is about 800 Btu/minute, so it shouldn't take long for the heat exchanger to come to equilibrium. Even 50 pounds of copper would take less than 15 seconds.

Hey gschjetne, how much is the cost of energy where you're at?

 

[FredGarvin]

I guess I should have just run through the numbers myself before posting that.

I wonder if soapy water has a better or worse Cp than plain water...time to consult Mark's Handbook.

 

[Clausius2]

I would say that the small thermal energy contained by the shower water once it goes through the hole of the bath would be almost entirely lost before reaching the entrance of the heater, due to heat transfer looses to the pipe system.

 

[Q_Goest]

Hey Fred. Regarding soapy water, I'd have to believe there's so little soap in the water (about 20 gallons of water versus a teaspoon of soap) the Cp difference wouldn't even be measurable.

Hi Clausius. I wouldn't worry about heat losses to the environment. Doing a quicky calc on a flow of 2 GPM of water going through a 1" pipe with a dT of 25 F shows the temperature drops only about 1 degree F after going 100 feet.

 

[plusaf]

alternative design?

Hey Fred. Regarding soapy water, I'd have to believe there's so little soap in the water (about 20 gallons of water versus a teaspoon of soap) the Cp difference wouldn't even be measurable.
Hi Clausius. I wouldn't worry about heat losses to the environment. Doing a quicky calc on a flow of 2 GPM of water going through a 1" pipe with a dT of 25 F shows the temperature drops only about 1 degree F after going 100 feet.

of the top of my little round head...

put the heat reclamation as close as possible to the shower. a coaxial heat exchanger, with the shower's cold-water feed running through ["next to, not commingling with] the hot drain water will probably do the best, fastest, and most efficient job of preheating the cold water side of the water you're going to shower in. that means you can use less hot water, as the heat exchanger swings into action. this would probably ALSO work best if you have a shower valve with a preset outlet temperature, else you'll be constantly readjusting the hot and cold valves as the cold water "gets hotter."

how's that sound?


my plan? solar-electric collectors, and an electric hot water heater upstream of the gas hot water heater, and set to a slightly higher temp than the gas unit. it's a nice, insulated, large thermal mass, and after the hot water "preheater" is topped off, that use of gas drops a lot, AND further "excess" electricity can be pumped back into the grid, saving even more "real money" as well as being very ecological.

a long time ago, i also conceived of the idea of using an engine to supply electricity to the house, and use its water-cooling system to preheat water for the home hot water system, AND use its exhaust, piped through a labrynth in the driveway, to melt snow and ice off the driveway.

lots of reasons why that's a mediocre solution, but maybe you can brainstorm it into something more plausible.

i love brainstorming.
+af

 

[brewnog]

a long time ago, i also conceived of the idea of using an engine to supply electricity to the house, and use its water-cooling system to preheat water for the home hot water system, AND use its exhaust, piped through a labrynth in the driveway, to melt snow and ice off the driveway.
lots of reasons why that's a mediocre solution, but maybe you can brainstorm it into something more plausible.
i love brainstorming.
+af

This isn't such a mediocre idea. I develop engines which do almost the same thing. In many of our installations, the heat from the charge air and jacket water coolers is used in a separate water circuit to heat housing blocks and greenhouses. The reason you probably wouldn't have one in your house (unless you're in a remote area with no reliable grid supply) is economical. Using such a system, you can effectively approach a 100% overall efficiency, since you're using all of your 'waste' heat.

 

[Clausius2]

Hey Fred. Regarding soapy water, I'd have to believe there's so little soap in the water (about 20 gallons of water versus a teaspoon of soap) the Cp difference wouldn't even be measurable.
Hi Clausius. I wouldn't worry about heat losses to the environment. Doing a quicky calc on a flow of 2 GPM of water going through a 1" pipe with a dT of 25 F shows the temperature drops only about 1 degree F after going 100 feet.

Ok. I'll trust on your numbers. I proceed to shut up.

 

[gschjetne]

Lots of great numbers and ideas! After mock exams I hope to get some time to fiddle with the problem myself...

And Q_Goest, energy here is currently about 40 ø/KWh, that is 5 €c/KWh, or 6 ¢/KWh. On cold winters following relatively dry summers and autumns it's usually much higher (when you need it most)

 

[Artman]

A few things to consider when dealing with a sewer system are:

1. The system flow velocity must remain above 2 ft/sec to allow for basic scouring to carry dirt, sand, grit, etc. off the piping.

2. Sewers rarely see full flow condition.

3. There will be "hydraulic jump" at the base of the stack. A point where the water is running very fast (following a fall in elevation) and shallow until it meets the slower deeper water in the horizontal (slightly graded) piping. This distance is usually about 10 pipe diameters (20" on a 2" pipe) away from the base of the stack. This, in my opinion, is the point where you should begin to apply your exchanger to take advantage of maximum pipe coverage.

 

[Artman]

More thoughts on this:

1. Your pipes should be made of metal. PVC does not transfer heat well.

2. If your pipes are cast iron, you should not use copper for the heat exchanger without a dialectric connection.

3. Chances are the only cold water moving while someone is showering will be the makeup to the hot water heater (maybe a sink here or there). So we are not talking about a lot of cold water movement, perhaps 2 or 3 gpm unless there is a recirculating system and a holding tank as Russ described.

With these items in mind, I would put in a section of copper drain, with dialectric fittings on either end if connecting to cast iron (not required if connecting to PVC), starting at 10 diameters from the drop if there is a stack (your echanger won't work in a vertical drop), wrap the copper drain section in copper tubing to form your exchanger and cover that in insulation. Run the ends of the copper tubing to the cold water makeup for the water heater (this is pretty much the only water moving anyway and you know it will be simultaneous with the water out.)

This sounds like a neat project, good luck with it.