Where they get the water from is another matter though. I always wondered how much you could scale up Air Well technology, which condenses water out of the air. I suspect the places that have lots of sunlight also don't have a lot of humidity. https://en.wikipedia.org/wiki/Air_well_(condenser)
I guess that's less than I had imagined. And apparently it's not just because northern houses don't have AC, since apparently [2] 87% of houses do.
Edit: Then again a lot of energy is used by industry, so maybe 6% used by residential air conditioning is a lot. [3] suggest 17% of residential energy is AC.
[1]: https://www.energy.gov/energysaver/home-cooling-systems/air-... [2]: https://www.eia.gov/consumption/residential/reports/2009/air... [3]: https://www.eia.gov/energyexplained/use-of-energy/electricit...
It's far more efficient to use a heat pump in reverse to heat a space than it is to use a traditional electric heater, which is "only" 100% efficient.
(Perhaps I should have said joules, not Wh? Units are confusing.)
Alec from Technology Connections recently made some videos about heat pumps on YouTube that touch on this sort of thing.
I wish I could tie the AC units into that, put all the rads in bypass during the summer, and use the hydronic water to preheat shower water before it hits the gas-powered on demand system.
My house actually does that!
I had never heard of the idea before moving here, but we have a geothermal system and in addition to the ground loop, it also dumps heat into the hot water heater when cooling the house. (Until the water reaches some maximum temperature.)
That comment about swimming pools nearly made me leap out of my chair. I was already convinced to get a heat-pump, but I was just planning on putting excess heat under the frost-line to save it for winter. To think, if it was insulated and disinfected properly, you'd have both a cheaply heated pool, and a huge thermal-battery.
This difference only matters for A/C, when cost is not much larger than benefit.
1 Wh = 3600 J, so you're not wrong.
It's the same thing as talking about using tonnes instead of kilograms, only the conversion factor here is 3600 instead of 1000.
This can be an unexpected problem with electric vehicles in cold climates, which have to waste a lot of power on heating the interior. Only recently have EVs started to use heat pumps for this purpose rather than resistive heating.
A lot of it seems to stem from the fact that most heat pumps operate poorly or inefficiently below about -10C (some higher than that). The fallacy that lies behind the inertia seems to be that "if you can't use it exclusively, you shouldn't bother at all". Since there's a non-trivial number of days below -10C where I live (about 1078 hours spread across 89 days here last year), it's considered frivolous.
Of course, because we build our houses to hold in as much heat as possible for winter, now that it's getting over 20 and 30 a lot more a lot more people are buying AC, which is all of a heat pump other than a reversing valve, so to me it seems silly to NOT get a heat pump for the like 200+ days in between peak heat and peak cool seasons.
Further, in many cases, the heat pump wouldn't keep your house warm during the coldest period of the year, which meant you still needed a heater. Not a huge deal since you probably have an AC for the summer anyway.
The issue as I understand it, was that the thermostats were not smart enough to know when to quit and so a heat pump would run until it literally could not keep up with demand, when it should have instead switched over to gas heat some time earlier than that when efficiency dropped.
Modern heat pumps deal with all these factors to the point that I expect them to become standard fare as current ACs get upgraded over the next decade or two. I know that if we ever need to upgrade our AC it will be replaced with a heat pump.
And there still is. Even now I'm talking to an hvac guy who seems generally knowledgeable and even he said something to the effect that "there isn't any heat in the air below -10" which is just not true at all. It's less efficient to pull it out for sure, but there's always energy to suck out of air.
Anyways, like I said, even up here where people barely know heat pumps exist there's still over 200 days of the year where even a shitty heat pump would do a decent job. Still people argue it won't get used.
(all of this is ignoring the fact that we should be doing even moderately expensive things to get off using natural gas if we can)
I live in the Pacific Northwest and when I wanted to swap out my furnace for a heat pump, a few vendors aid that they don't install heat pumps, they could do a furnace or A/C, but not a heat pump. I have no idea why that is, I figured that heat pump installation is nearly identical to A/C. (I still use a furnace for emergency heat as I wanted to be able to run from a small generator if needed)
The heat pump + furnace replacement cost 2 - 3X more than just a simple replacement furnace, so I can see why someone wouldn't want to spend the money if they didn't need A/C.
That might still not be enough if the local 1% design temp is +5°F and, as a result, your unit is sized to maintain temp at +5°F, so doesn’t keep up for the 88-ish hours per year that your temps are below the design temp.
In a moderately hot climate you may want to lower the temp 10-20 deg c, for example, from from 38 to 23c (~100 to 73f).
In a moderately cold climate you may want to raise the 25-35 deg c, for example from -10 to 22c (~14 to 72f).
The latter requires about twice as much energy as the former.
There's a common meme in the northern latitudes that people "down south" consume unreasonable amounts of energy on AC, but in many cases northern cities consume more energy in winter than southern cities do in summer. The truth is that just about every city uses large amounts of energy for climate control, and the actual expense varies more by microclimate than by latitude. There are only a few places on earth where people consume very little climate control, and those places tend to already be densely populated because "they're nice."
That is a good point, I think you're exactly right. I just tried to figure out how many kWh is equivalent to a therm of natural gas, and the answer of course is 'it depends.' In round numbers it seems like 1 therm is about 30 kWh.
Using myself as an example, during a cold month I might use about 90 therms. In a hot month I may use 1900 kWh for cooling. Gas is significantly cheaper indeed, though it's noticeably more energy (and that's after converted to hypothetical electricity, not the energy content of the raw gas).
What a PITA to compare that. And it's a pretty rough comparison, indeed, because gas is predominantly going for heat in the winter, but I have a gas stove and a gas water heater too. And I have an electric car, which pushes the kWh total up a bit. I'd have to do a much more detailed analysis if I actually wanted to compare the energy usage.
Because heat pumps are now so efficient, all the new apartment construction I see in NYC uses them. (While older buildings use dirty oil to heat water for radiators.)
My money is on "gas won't be cheaper for much longer, even where it is now" personally, though.
Ah yes, this is a very good point, that does make it much easier to conceptualize.
Depending on where "down south" you are, the AC could well be on eleven months of the year.
I'm in Florida now, and you can usually turn it off for parts of November - February but never every day. I'd average it at 10 months of ac usage in a year
When I was in NYC, the ac would be used for about 1.5 months and the heat for ~ 3. By being temperate in spring, early summer and fall, little or no external energy needed.
Winters, though, need a lot of heating to keep the place comfortable and most homes due to lack of said AC use the highly inefficient baseboard heaters to warm their entire house 6 months or more out of the year.
That's actually not correct, though I would have figured the same thing myself before the recent heat wave made it a discussion point. Seattle is among the lowest in the nation, but it's still just under half of all homes. Portland is about three quarters.
The 110°+ felt a lot like Florida when it's 92 and humid
I lived in the greater Seattle area in the last 4 years. Let's look at the top temps for the last couple years in Seattle - the bounds of the city, which is literally bordering the bay.
98 August 16, 2020
95 June 12, 2019
94 August 08, 2018
96 June 25, 2017
95 August 19, 2016
95 July 19, 2015
Not counting EXACTLY in Seattle you routinely break 100F in the greater Seattle Area and Washington State.
Let's look at Spokane
102 July 31, 2020
98 August 07, 2019
103 August 09, 2018
99 July 07, 2017
These are hottest days, not the only days of over 90. Remember the forest fires of the last couple years, it generated the WA memes about deciding if you want to cool your house by opening a window or avoid the billowing smoke that comes in.
Not for cooling; cooling also requires condensing excess water vapor out of the air (since the dewpoint of the cooled air is much lower than the dewpoint of the air before you started cooling it). Most of the energy expenditure for A/C is actually condensing water vapor, not cooling air. Air has a very low heat capacity compared to water (particularly water during the phase change from vapor to liquid).
Off the top of my head I think costs about 6 times as much energy to condense water vapor as to cool the air for typical A/C conditions; if that number is roughly correct, then for the temperature differences you give, heating actually uses about 2/7 (about 30%) of the energy of cooling (if we let the energy required to cool the air be 1, then the energy required to heat the air is 2, but the energy required to condense water vapor is 6, so the total cooling energy is 7.)
First, water vapor in air is usually less than 2% of air by mass, and contrary to what you say there's absolutely no phase change involved -- water vapor isn't steam, it's far far below boiling. And AC usually doesn't condense more than about 25% of humidity, so it's really a tiny, tiny amount of water being cooled relative to air being cooled. So I'm missing what could possible be so energy-intensive about cooling water vapor, or why condensation would be relevant to energy usage at all? (Cool condensation still helps "chill" the room.)
And second, even if that were the case, wouldn't it be the identical energy loss when heating? A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors. In fact, my heat pump runs in reverse for ~15 min once or twice a day on really cold days (suddenly it blows cold air inside and warm air outside) in order to melt the accumulated ice from condensation, to continue operating safely.
So I don't see at all how how cooling somehow uses 3.5x more energy than heating. My personal electric bill with my heat pump is also about 2x as expensive in the coldest winter months (heating from 25°F to 70°F = +45°F) as in the warmest summer months (cooling from 90°F to 70°F = -20°F). If your math were correct, it would be roughly the opposite.
As far as I can tell, the cost of running a heat pump is basically the same no matter which direction you're running it.
If you just cool the air without removing water vapor, you will drive up the relative humidity and often decrease occupant comfort.
And you often can't cool air without removing water vapor, the water condenses whether you want it to or not.
That’s why I gave you the term, in an effort to show you politely that you were wrong enough that the HVAC industry has an entire term specific to this concept and that suggests that it’s more than a rounding error.
It appears that you at least now agree that water in gas form is condensing into liquid form (which requires the removal of the heat of vaporization [what science calls it] or the latent heat [what HVAC calls it]).
But in any case, I finally found them and ran the numbers. The latent heat of water is ~4x that of air, but even on a really hot humid summer day water is only 3% of air by mass, which means in a worst-case common scenario, 11% of energy is going to condensing the water, while 89% is simply cooling the air and (non-condensing) humidity.
So if we round, we can say that a heat pump cooling humid summer air rather than drier winter air uses up to around 10% more energy (since winter air has a bit of humidity too), which means heating/cooling are still roughly the same for most purposes.
Whereas the commenter I was originally replying to seemed to be suggesting cooling takes 3.5x as much energy, which doesn't seem to match summer/winter energy bills at all.
Does that cover it, or is there something else my math is missing? Thanks again.
[1] https://en.wikipedia.org/wiki/Latent_heat#Specific_latent_he...
(Sorry for the use of “stupid units” here. It’s a lot easier to find the non-SI units for the HVAC applications of these and being an American, it’s easier for me to think in BTUs than calories, kWh, or Joules for HVAC.)
It takes (negative) 970 BTU to condense one pound of water from gas to liquid. (That’s the latent heat of vaporization.) It takes -0.24 BTU to cool one pound of mixed air by one degree Fahrenheit.
At 90°F, air can hold almost 5% of water vapor in suspension. If the task is to condense out about half the water (from 90% to 45% RH) and cool it from 90°F to 70°F, I think it’s:
For every 40 pounds of 90°F/90% RH air that you want to make into comfortable inside air:
You condense out 1 pound of water at a cost of sinking 970BTU. It’s slightly less than 1 pound, so generously call it 900BTU of latent heat removal.
You then cool 39 pounds of mixed air by delta-T of 20°F: 39 * 20 * .24: 195BTU of cooling required for sensible heat removal.
That makes it look to me like over 80% of the heat removal is latent and under 20% is sensible. (Phase change is expensive, but you’re glad for that when you’re sweating in air that’s not 100% RH.)
This is tapped out on a phone while traveling, so I’m not staking my life on it, but I think it’s right.
That's a worst case scenario, though.
For one, 90/90 would translate to a 87F dew point -- which is not a record, but it's pretty close, and there's a good chance it would be lethal to unprotected humans. So it does not happen too often, e.g. normal in a place like Phoenix Arizona might be 110F but 20% RH.
Second, air conditioning doesn't cool outside air, it cools inside air. So on average it isn't going to have nearly that much water to pull out of the air.
I’ll assume you were kidding about a dew point of 87°F being lethal to unprotected humans.
Make the assumptions such that you take 1 pound of water out of 200 pounds of air and (0.5% reduction in water content) and the latent and sensible heat are about equal. Dehumidifying is expensive energy-wise is the point.
That's a heat index of 122°F. You're literally able to die because you're unable to evaporate away body heat quickly enough if you don't take precautions.
On the NWS's heat index chart, that's color-coded "danger" while a single tick further up in either humidity or temperature is coded "extreme danger".
So I don't see this as kidding... it's some pretty extreme conditions you chose. :)
If you stay out in it without taking any precautions for long enough, it could be. But that doesn't mean that as soon as you step out of your front door in the summer in, say, Houston, Texas, you fall over and die. Millions of people in the US (quite possibly billions if you count the rest of the world) live in places where the outside conditions are 90/90 (or worse) frequently in the summer.
> it's some pretty extreme conditions you chose
As I have already posted elsewhere in this thread, 90/90 is a condition that is commonly tested for HVAC systems; it's not at all "extreme".
That would come as a great surprise to the millions of people who live in places like Houston, Texas or Miami, Florida, where 90/90 is common. (And that's only places in the US, since those are what I'm familiar with from doing HVAC testing years ago. Around the world there are many places that are even worse.)
> normal in a place like Phoenix Arizona might be 110F but 20% RH.
Yes, but Phoenix is a desert climate, not a hot, humid climate. There are many places that have hot, humid climates.
I can see it basically depends on a lot of factors.
I'm presenting temp/humidity numbers that are reasonable for where I live (NYC where the summers still are pretty hot and humid), but your numbers are much more extreme (and god forbid they should be daily averages!). And then, you're dehumidifying down to 45% RH -- my consumer-level heat pump doesn't even have that option. In the summer (like today) it never goes below 65% RH indoors on hot humid summer days, even if I put the indoor temperature as low as 68°F -- I can't control the humidity. I'm sure for office HVAC systems there's more control.
Also as another commenter mentioned, I'm also mostly cooling indoor air that's already had the humidity removed. You commented that typical houses have 0.5-2 air changes per hour, but my new-construction NYC apartment is nothing like that. It's maybe 0.5 air changes per day, it's so tight -- I know because I have a CO2 meter so I know when to open the windows and run a fan to force fresh air when levels double. Again, professional office buildings might bring in a lot more fresh air -- my apartment is new-construction so pretty airtight, and with a split heat pump so it can't intentionally bring in air from anywhere. It's not some massive HVAC.
Anyways, it seems like we're both right -- in a regular new-construction home in a climate where it makes sense to use a heat pump both for heating and cooling, heating and cooling costs are roughly the same for a temperature difference in either direction (as reflected by my electric bill). While in a humidity-controlled office building in an extremely humid tropical climate with tons of airflow where you can control the humidity level independently, the energy usage for cooling and removing humidity could significantly exceed the energy used for cooling air.
NYC, while it does get fairly hot in the summer, is actually not very humid compared to South Texas or South Florida.
> you're dehumidifying down to 45% RH -- my consumer-level heat pump doesn't even have that option
A typical home A/C system (not heat pump, just straight A/C) in South Florida or South Texas will output a dewpoint of around 50 to 55 degrees F. (I used 12 C = 53.6 F in my heat transfer calculations upthread.) That's significantly lower than the typical dewpoint output of a heat pump (different working fluids, hence different temperatures at the indoor heat exchanger).
There is absolutely phase change (and associated heat of vaporization energy) involved in humidity/water vapor (which is a gas) condensing into liquid water.
My experience as an HVAC engineer, which was some time ago, hence why I said "off the top of my head". However, it's easy to check by running some numbers.
The cases to be compared are:
A/C -- 38 C x 80% relative humidity (typical for a hot, humid climate) to 23 C x 50% RH (a dewpoint of about 12 C, which is typical as an output for air conditioners).
Heating -- -10 C to 22 C, no change in dewpoint.
We'll need some physical properties of air and water; approximate values will be good enough (i.e., we'll ignore that these values change with temperature and treat them as constant):
Air -- density about 1 kg/m^3, specific heat about 1000 J/kg per degree C
Water -- density about 1000 kg/m^3, specific heat about 4000 J/kg per degree C, latent heat of vaporization about 2.3 x 10^6 J/kg (note how huge that value is--that's key to the comparison we'll be making)
We'll also need to make some assumptions about the volume of air being cooled or heated; I'm going to use a round figure of 300 cubic meters, which I think is fairly average for a home (about 200 square meters of floor area times 1.5 meters of height).
Finally, we'll need the moisture content of air at the two A/C points we gave. We can get that by taking saturation values from the Engineering Toolbox [1] and multiplying by the relative humidity (since that is just the percentage of the saturation value that is actually present in the air). We'll have to interpolate because the table only has values for 20, 30, and 40 C. The values I get are:
38 C -- 0.047 kg/m^3 x .8 = 0.038 kg/m^3
23 C -- 0.021 kg/m^3 x .5 = 0.011 kg/m^3
So the difference in moisture content is 0.027 kg/m^3; that's what has to be condensed out of the air.
Now we can calculate the heat transfer required:
A/C cooling air -- 15 C x 1000 J / kg-C x 1 kg/m^3 x 300 m^3 = 4.5 x 10^6 J
A/C condensing water vapor -- 2.3 x 10^6 J/kg x 0.027 kg/m^3 x 300 m^3 = 18.6 x 10^6 J
So my memory was somewhat off: the condensing heat transfer is only about 4 times the cooling heat transfer, not 6. Still big, though.
Heating air -- 32 C x 1000 J / kg-C x 1 kg/m^3 x 300 m^3 = 9.6 x 10^6 J
So heating for these temperatures is a little more than twice the air cooling heat transfer, but only about half of the condensing heat transfer, so about 2/5 of the total A/C heat transfer.
Note that, as the condensing calculation above makes clear, the relative humidity at the higher temperature is a huge factor. In desert conditions (for example, 38 C x 25% RH, typical for, say, Phoenix, Arizona as opposed to Houston, Texas), the moisture content of the air is only 0.012 kg/m^3, i.e., only a little higher than at the 23 C x 50% RH target point, so the condensation heat transfer goes way down. So if you're used to hot, dry conditions as opposed to hot, wet conditions, then your intuition that the condensation heat transfer isn't that large is correct. But many locations where A/C is used are are hot and wet.
[1] https://www.engineeringtoolbox.com/maximum-moisture-content-...
> contrary to what you say there's absolutely no phase change involved
There most certainly is: water vapor is vapor, and the condensate that comes out of your air conditioner is liquid. Go look at your home air conditioner: there will be a line coming off of it that drains condensate (liquid water) somewhere, usually into your home's sump. Or look under your car after you've been running the A/C for a while: you will see a puddle of water, which is liquid condensed from water vapor in the air.
It produces condensation, yes, but the total heat transfer spent on that condensation is still pretty small, because now the condensation isn't removing moisture from the inside air, it's removing moisture from the outside air that is blowing past the heat pump's outside heat exchanger. So the amount of water vapor being condensed is only based on the difference between the ambient outdoor air temperature and the temperature of the outdoor air just after it's blown past the outdoor heat exchanger, which might only be 5 degrees C or so--say -10 C (ambient) to -15 C. From the Engineering Toolbox [1], that's 0.00231 - 0.00158 = 0.00073 kg/m^3 of air. With a latent heat of 2.6 x 10^6 J/kg (since we now have to count the latent heat of both condensation of vapor to liquid and freezing of liquid to ice), that's about 1900 J per cubic meter of air. So it would take about 5000 cubic meters of air being blown past the outside heat exchanger for the heat transfer due to ice formation to equal the heat transfer to heat up the inside of the house from -10 C to 22 C (which I calculated in my previous post a few minutes ago). I don't know the exact outside airflow of a heat pump, but that seems like a lot more outside air than would be needed for the necessary run time of the heat pump for that amount of heating.
[1] https://www.engineeringtoolbox.com/maximum-moisture-content-...
> As far as I can tell, the cost of running a heat pump is basically the same no matter which direction you're running it.
Not in terms of heat transfer, no; comparing the calculations in my previous post with the above, it should be evident that it makes a big difference which heat exchanger is doing the condensation of water vapor (inside or outside).
What is the typical humidity at 90 F where you live? As my previous calculations showed, that makes a huge difference to the heat transfer required. From the costs you give, I suspect your area is not very humid (say 50% RH at 90 F, vs. 80% or 90%, which is more typical in a hot, humid climate).
For a home, there are also other significant variables, such as how much sunlight vs. shade your home gets, how well insulated it is, how often you open the doors and windows, etc. Those variables can affect heating and cooling differently, which makes any comparison harder.
Here is a great chart on home energy use [1].
[1] https://www.eia.gov/energyexplained/use-of-energy/homes.php
A similar temperature differential exists in Minnesota and Wisconsin (and elsewhere in the US) for months in the wintertime.
Unlike air-conditioning which can be powered by solar and wind produced electricity, most heating is done by directly burning fossil fuels on-site.
I think we should all be very cautious with our energy scorekeeping ...
Solar can be more easily used for AC in desert environs because peak demand coincides exactly with peak production. And, it actually is. At least in places with access to capital.
The situation is sadly exactly the opposite up north. For everything except geothermal the peak production (both in time of day and season) doesn't match up well with the peak demand.
Also, AC requires a lot less energy than heating, regardless of generation method.
But deserts have other sustainability issues. Water.
There's no reason why it shouldn't, though. It makes logical sense.
(to be fair, by this definition, unhabitable is in fact a word)
Yes, though "habitable" is the normal word and "inhabitable" would be a nonce construction derived from "inhabit".
> It's like[] inflammable and flammable.
Not really; those words mean that something can catch fire. That's all well and good for "flammable", but it's bizarre for "inflammable", since "inflame" has no surviving meaning related to fire. If you wanted to express that meaning, you'd have to say "set fire" or "ignite".
The joys of english.
Actually warming up cold places in winter is more energy intensive than cooling down hot places. Nordic countries should not exist by that logic.
Furthermore, heat is related to entropy, which in a closed system can only increase. We have many devices that can generate heat where there was none previously (e.g. resistive heating elements) but none that can destroy heat, only shuffle it around somewhere else.
It's quite the stunt to make a habitable vacation spot in the hottest place on Earth. There's only one Death Valley, and people want to see it. The golf course is... kinda pushing it, yeah, but I have respect for the sheer human ingenuity it takes to do something like that.
Phoenix is more of a case of putting a bunch of people where a bunch of people probably shouldn't be. It can probably be made energy-independent, there is a lot of sun to be harvested, but water is a different story.
tl;dr scale matters.
[0] https://www.greenlodgingnews.com/how-xanterras-furnace-creek....
Not to mention that this is in a National Park; a place whose ostensible purpose is to protect and preserve nature! If you try camping outside of an official campground there (something totally fine and legal on the vast majority of public land) you'll get kicked out with a ticket. Meanwhile that same park is making a hard life even harder for local endangered plants and animals.
> Bobby: "111 degrees? Phoenix can't really be that hot, can it? Oh my god, it's like standing on the sun!"
> Peggy: "This city should not exist — it is a monument to man's arrogance."