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Suppose I live in Poway, CA (weather), where temperatures are roughly between 50 and 90 F throughout the year and relative humidity is generally between 40 and 60%. With climate change the highs have tended to be higher and we also get Santa Ana winds every fall which bring warm, dry air (relative humidity might drop into the 20% range, for example.)

Would it be more energy efficient to use a heat pump dryer that recirculates air while extracting moisture or a conventional gas or electric dryer that draws air in, heats it, and then discharges it?

I'm interested in this question because in this area, the air is normally warm and relatively dry, so I'm not sure if a heat pump dryer would even theoretically be more efficient. I would like to know the theoretical answer (i.e., ignoring the cost of gas and electrical power) but also, would be interested to get any takes anyone has. For example, I've seen data indicating that a Whirlpool heat pump dryer actually uses the same amount of energy to dry a load as its conventional electrical cousin.

In case it's relevant, our dryer discharges directly to the outside, so, ejecting heat into our home is (fortunately) not an issue.

Edit: Our dryer is in the garage, so it pulls in air that is usually warmer than outside air and about the same relative humidity.

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  • You've tagged this natural gas, but I'm guessing you mean a conventional electric dryer? Or are you actually considering a natural gas dryer?
    – LShaver
    Oct 18 at 14:21
  • I'm open to gas, electric, and electric heat-pump, whatever is most efficient. We currently have a conventional electric dryer and only after we got it did I realize it was probably the least efficient of the three. SDG&E uses natural gas to generate electricity, so, just burning natural gas in the dryer to generate heat would've been more efficient. We have solar, though, so, some of this is offset by what we produce.
    – matmat
    Oct 18 at 17:00
  • Remember, the entire thing that makes heat pumps a big win is that they are more than 100% efficient: So the power you put into an electric resistance dryer is easily 3-4X they power you put into a heat pump dryer for the same result - which is plenty to make it more (energy) efficient than a gas dryer. Cost efficiency will depend on energy prices, now and in the future, as well as using or not using your solar input, and the cost and lifetime of the appliance.
    – Ecnerwal
    Oct 18 at 17:48
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    I think there is also the consideration of power generation and transmission losses. Back of the envelope, I'd expect a natural gas plant to be about 55-60% efficient (assuming combined cycle), then a 5% loss during transmission, so ~52-57% at the house. A heat pump dryer could now move 300% more heat than it input, let's call it ~175% of the original energy input. Meanwhile, about 8% of energy is lost to transmission in gas pipelines, so, a gas dryer would come in around 92% efficient. The conventional electric dryer at around 52-57% efficient.
    – matmat
    Oct 18 at 18:49
  • When you say "dryer", do you mean clothes dryer that corresponds to a washer or something that dehumidifies the air in your house?
    – amphibient
    Oct 18 at 21:58
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You mention the outside air temperature. But most dryers are inside a house with controlled air temperatures.

If this is sitting in an open shed consuming outside air, and temps are high, you might not be spending much on energy for a conventional dryer.

But if it's in a house where the air conditioner is running, it's constantly sucking in the air you paid to cool, heating it up and discarding it. All of the air in the exhaust has to be replenished. Whereas for a pure heat pump dryer the air is reused and doesn't end up moving conditioned air out of the building.

I can't find raw energy cost numbers for the whirlpool, but the heat pump models have greater efficiency numbers on the energy star comparison site (which does not take home heating/cooling into account). So they are expected to be less per load on top of any benefits you get from less exhaust. I found a home study from 2013 where besides the population benefits, every single home in the study had lower energy costs after the introduction. But there were a couple of reasons (climate, confounding factors) why that wouldn't necessarily map to your location. I couldn't find any similar studies that addressed your climate, nor any that suggested there is a climate/humidity/temperature case where the heat pump would lose to a conventional in efficiency.

If you really want to be efficient in that climate, you'd try to take advantage of line drying or no-heat airflow as much as possible. Pure efficiency there.

If you're trying to save money, even the best places for it are going to have a hard time to recoup the premium purchase price. Current models make it very difficult to make that up even at a discount of 20 cents a load, and that is unlikely in most locations. Also, probably don't get them if you're impatient. In the most economical mode, drying takes longer.

I guess the thing that was tripping me up was that I don't understand the thermodynamic cycles of the heat pump dryer vs the conventional dryers. The heat pump dryer heats air, passes it through damp clothes, some of the heat evaporates water (water vapor concentration goes up), cools the air (condensing water and recapturing heat energy?), and repeat?

That's right. Air conditioners were first used as dehumidifiers. That same is happening here. The cooling fluid is compressed (releasing heat which warms the clothes in the tumbler). The fluid is allowed to expand which cools a coil or plate to condense the liquid and collect it.

The evaporation process takes energy, but the condensation process returns it. So if everything is engineered well, much of the temperature of the system is maintained without having to use a heater. We're taking liquid water from the clothes and turning it to liquid water in the condensate trap. (An entropy change but not an required energy change). The energy usage in such a system are going to be dominated by the efficiencies and losses in the design of the compressor, the blower, etc. We can't really look at the raw physics and say that 1kg of water removed from the clothes requires X joules of energy. You can drive that down with better (read expensive) engineering.

Conventional dryer is essentially the same without heat recapture?

So there the water is evaporated. Without the capture, that means you have a raw energy figure that is required for the water to evaporate. But since the air can supply some (usually a small amount) of that energy directly, it doesn't tell you what what your running costs are.

I must be missing something because the relative humidity of the air doesn't show up in these models but would seem to matter, e.g. one could dry clothes using just unheated air.

For the average dryer in a US home, it probably doesn't matter much because the air is heated well above ambient to reduce the drying time. Yes you can let the ambient air dry your clothes, but for a tub full of wet laundry, that's going to take a long time if you're not out in a hot desert. So the dryer heats the air to somewhere between 120 and 160F. Let's say your dryer is average and heats the air to 140F. That air has a moisture capacity of 130g/m^3. If you were running it outside in Florida on a 100% humdity, 86F day, it would have had a max of 30g/m^3. (while in your location it might be carrying only 3 or 4 g/m^3).

So yes, the humidity does affect the efficiency. But from very dry to very humid you still have 80% of the drying capacity. The energy required to heat the air also matters little. 100% humid air to dry air has a heat capacity within 3% of each other. So the dryer doesn't work significantly harder to bring the air to temp.

This would be a much bigger deal if the dryers tried to be more energy efficient and run at a cooler temperature. Your clothes would still dry at 90F, but it would take a lot longer. At that temp, the differences in cost would be a lot more significant on a percentage basis.

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    it's constantly sucking in the air you paid to cool, heating it up and discarding it. To be clear, that's what a conventional dryer does. A heat pump dryer extracts the heat from the air, and generally doesn't require exhaust -- just a drain for the condensate.
    – LShaver
    Oct 18 at 14:16
  • Thanks for your research and your thoughts, BowlOfRed. I should've included in the question that our dryer is in the garage, so it inputs generally warmer air than what is outside. Still, I would be interested to see if there there's an expression we can write down for the amount of energy necessary to evaporate a certain amount of water (maybe at a given temperature, maybe at a given relative humidity.)
    – matmat
    Oct 18 at 17:08
  • I think it would be difficult to create such a model from first principles that would be accurate. Would be nice to have an empirical model based on actual draw at various environmental conditions, but demand for such is tiny. I don't know who would publish one.
    – BowlOfRed
    Oct 18 at 17:16
  • Dead simple, really, but it does not directly address the problem since "how you get it" is variable. Latent heat of vaporization of water. But the heat pump will be second-most efficient, after line drying. en.wikipedia.org/wiki/Enthalpy_of_vaporization or link.springer.com/referenceworkentry/…
    – Ecnerwal
    Oct 18 at 17:41
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    I tried to add a bit more about the differences in the cycles themselves. Not sure if that's more of what you're asking about or not.
    – BowlOfRed
    Oct 18 at 20:10
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There are 3 main problems with a conventional dryer, which heat pump dryers aim to solve.

They are creating heat rather than moving it.

To boil away 1 pound of water out of your wet clothing, a dryer must bring 1100 BTU* of heat. Traditional dryers (gas or electric) create 1100 BTU of heat. That means either a) burning 1100 BTU of gas heat locally, or b) burning 2750 BTU of gas heat at the electric power plant.

They are sucking unconditioned outside air into your house.

The basic energy-efficiency requirement of modern appliances is they don't do that. If they need process air, they should bring it in from outdoors via a separate stack -- like a direct-vent furnace or a 2-hose portable air conditioner.

The reason is we are trying to make buildings relatively airtight, to control not only the temperature but also the moisture content of the air inside. Moisture content is very expensive to add or remove - again it takes 1100 BTU to add or remove 1 pound of water (because it must be boiled or condensed).

So what do conventional dryers do? Same thing as 1-hose A/Cs or 70% furnaces -- they steal their process air from conditioned air already in the room. You can't remove air from a room and just have less air. The pressure differential will cause outside air to leak in via every leak - which means that conditioned air it ejected is being replaced by outside air at Nature's choice of humidity and temperature. So now you must spend energy to re-condition that air.

And if your house is "nice and tight" like we like modern homes to be, it can actually draw negative pressure in the room, causing it to suck outside air in from any other parasitic-vented appliance. So now combustion air is moving backwards through your 70% gas furnace, dragging hot air into places inside that 70% furnace hot air was never meant to go, and filling the house with combustion products.

They have an exhaust vent in the first place

Which itself is a building penetration that invites all sorts of trouble, from maintenance hassles to sources of "cold draft" to vermin entry paths. In California, the #1 threat isn't earthquakes, it's wildfire. And often, you see burnt houses right next to unburnt ones in the same field of devastation. Why? Burning embers getting sucked in building penetrations is one common reason. Or for that matter, plastic fittings (e.g. dryer vent exhausts). A fire-resistant house doesn't want any vents it can avoid.

How do heat pumps solve the "creating heat" problem?

The key to understanding heat pumping is that the "temperature floor" is not zero degrees C, or even zero degrees F. The temperature floor is zero degrees Kelvin. That is 273 centigrade, meaning 293K (20C) is a comfortable house interior and water boils at 373K **. So you see, it's not such a big job to "pump" heat from 293 to 373.

So... rather than create heat, the heat pumps simply pump the heat that's already there.

The heat pump dryer takes airstream #1 and cools it to 278K (40F/5C). It takes the heat it stole, and puts it into airstream #2, which it warms, say, 393K (250F/120C, over boiling). It then runs airstream #2 through your clothing, causing water to boil out of it and causes this hot airstream to become saturated with water (100% humidity at that temperature; hotter air can hold a lot more water than cold).

So pumping 1100 BTU can happen at 300-800% efficiency, depending on temperature of the intake air to airstream #1. (the less "uphill" you're pumping heat, the more efficient it is). And that's more efficient than the gas power plant! So yes, it's more efficient to burn the gas at the power plant than locally, even if we ignored the "air ejection" problem.

Now what do we do with this hot, wet air?

Remember airstream #1? It wants hot air to steal heat from. Match made in heaven: the evaporator here gets nice hot air to steal heat from, which makes it yet more efficient still. This chills the air greatly. Cold air can't hold nearly as much water as hot air, so as a bonus side-effect, it also condenses the water in the airstream. The water is collected and pumped into the washing machine's drain.

How do heat pumps avoid exhaust vent / sucking in outside air?

Well, what air is left to vent outside? The exhaust of the condenser (heater) goes straight into the clothing, and then into the evaporator. The exhaust of the evaporator (dry air) goes back into the condenser.

Thus the whole system is a "closed loop". Add a lint filter and we're done. Air never leaves the building - it doesn't need to. So, no exhaust vent needs to exist.

And since air never leaves the building, it's not sucking air in through every orifice in the building.

An environmental aspect is versatility

Although this is true of any electric dryer.

Anytime you are using electric instead of gas, you are no longer making CO2 yourself. You are drawing electric, which is an intermediate form of energy, which can be sourced from a huge variety of sources. As your energy mix shifts from gas toward renewables, your electric dryer will automatically do so - whereas a gas dryer will be gas for its life.

Or for instance, there is a glut of solar in the mornings when the sun is out in full force, yet it hasn't warmed houses enough to need A/C yet. Running a dryer at that time shifts your energy mix toward solar.

Electric can benefit from pumped storage and if you look at the California aqueduct system, the whole system is built for pumped storage.




* Wait, isn't the latent heat of boiling only 970 BTU? Only if the liquid water is at boiling temperature. If it's at room temperature you'll need to add 142 BTUs to get it to/from boiling temp also.

** Or in Fahrenheit, you add 460F, so "comfortable" is 530 Rankine and boiling is 672 Rankine. Rankine is like Kelvin, but with Fahrenheit intervals (so 1.8x deg K).

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