Death Valley prides itself on being the hottest place on earth, based on the temperature record of 54.0 °C (air temp at 1.5m).
However, DV does not come out on top when looking at temperatures measured by satellites (EDIT: ground temperature, and less reliable.) From Wikipedia [1]: "The highest recorded temperature taken by a satellite is 66.8 °C (152.2 °F), which was measured in the Flaming Mountains of China in 2008. Other satellite measurements of ground temperature taken between 2003 and 2009, taken with the MODIS infrared spectroradiometer on the Aqua satellite, found a maximum temperature of 70.7 °C (159.3 °F), which was recorded in 2005 in the Lut Desert, Iran. The Lut Desert was also found to have the highest maximum temperature in 5 of the 7 years measured (2004, 2005, 2006, 2007, and 2009). These measurements reflect averages over a large region and so are lower than the maximum point surface temperature." According to [2], the record is 80.8 °C at Lut Desert.
You're right, it doesn't need to be an average. Meteorologists are interested in outdoor air temperature in the shade, away from local disturbances like buildings and the ground. That way it's not influenced by uninteresting local effects like the color of the walls, direct solar heating (affected by the properties of the thermometer itself) or somebody turning the oven on.
That 70.7 degrees from the satellite was ground temperature, not air temperature, so it can't be compared.
You're comparing apples to oranges though. The 54.0 reading in Death Valley was measuring the temperature of the air - in particular, 1.5 meters above the ground. Satellites measure ground temperature - as in, temperature at the ground. Lots of things can affect ground temperature.
> You're comparing apples to oranges though. The 54.0 reading in Death Valley was measuring the temperature of the air - in particular, 1.5 meters above the ground. Satellites measure ground temperature - as in, temperature at the ground. Lots of things can affect ground temperature.
Exactly. I have a radio thermometer in my attic whose readings max out at 140°F. During the summer it's regularly maxed out, so the actual air temp is hotter still (and the actual surface temp of the roof must be even hotter than that). Outside air temp is nowhere near any of those temps.
I found it absolutely shocking that Furnace Creek has a (well-watered) golf course and a whole bunch of air conditioned hotel rooms, despite being in a clearly inhabitable location. Exiting a fully air conditioned room at 20 degrees into 45+ degrees heat feels like literally like opening a furnace door. The sheer waste of energy to keep that hotel cooled down to those temperatures and golf-course playable year-round is just ridiculous.
It might be a "waste" but it doesn't have to be damaging. Air conditioners (a special case of heat pumps) are extremely efficient and could easily be powered from a solar array. The hottest places in the world tend not to be short of sunlight.
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)
According to [1] Air conditioners use about 6% of all the electricity produced in the United States.
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.
Heat pumps are amazing things, they are more than 100% efficient, since they use only n watt-hours to move >n watt-hours of energy from one place to another. Under ideal conditions they can move 3n while consuming only n.
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.)
When cooling a dwelling, the trick is figuring out where to move the heat. Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air.
Alec from Technology Connections recently made some videos about heat pumps on YouTube that touch on this sort of thing.
This has always made me super grouchy— I even have my house plumbed for hydronics!
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.
I'm sure this might happen somewhere, but I grew up in Florida and it never happened in Orlando. I would always have been happy for the pool to be warmer.
> Typically we dump it outside (waste) but we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat at the same time that we're removing it from the indoor air.
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.)
And the cool thing is that the spare "coolness" of the water is released into the air, providing free Air Conditioning while also cheaply heating your water.
> we should be heating our hot water tanks, swimming pools, or anything else that needs concentrated heat
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.
It's only >100% efficient because % efficiency is defined in a goofy way: benefit/cost, instead of benefit/(cost+benefit). (Moving 3n requires moving 4n and wasting n: 75% real efficiency).
This difference only matters for A/C, when cost is not much larger than benefit.
This doesn’t make sense to me. You’re saying a water heater that uses N joules to heat water by 0.5N joules is 33% efficient? That would be difficult for most people to translate: I use 100 wh to get 50 wh of heat, which should imply 50% efficiency. As the cost to gain the end state I want is what I care about.
But only n has to be low-entropy energy, while the 3n is ambient heat that you’re trying to get rid of. There’s no scarcity of ambient heat when you’re just trying to move it anywhere else but inside.
Unless you're using a heat pump for heating (we should be, but it's extremely rare), heating almost always uses more energy than cooling.
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.
There's a lot of inertia against heat pumps for reasons that are not at all clear. Where I live, in a pretty northern climate where until recently almost no one owned an AC but climate change is very visibly changing that, it's very difficult to get HVAC people to take you seriously if you say you want a heat pump even if you're getting an AC anyways.
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.
The reason heat pumps have traditionally been untenable is that they are prone to icing over when below freezing. If the coils are covered in ice, then air can't be blown past them to get new heat to pull in.
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.
Yes, these are all definitely issues. None of them should have been difficult to solve a long time ago though, there was just a lot of inertia behind it.
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 think it's cost, especially in an area where A/C isn't needed (or at least, hasn't been needed in the past).
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.
Cost probably explains a good chunk of it. Regardless of why it's more expensive. When our house was built in 2012, I looked into getting a heat pump. But it was a couple grand more than a high efficiency furnace with an air conditioner. Given that natural gas remains inexpensive compared to electricity, it wasn't an obvious win financially. And if we're being honest, for average people the hit to the pocketbook is going to take higher priority than considerations for the environment.
I believe heat pumps for heating tend to not work very well when the temperatures drop too low. (At least in the RV versions I've seen. Once the temp gets below freezing, you have to use the onboard propane heaters)
There are modern, “hyper heat” models that can heat at 90% of rated output down to -13°F.
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.
They don't work below a certain temperature because condensation collects on the cool side, creating frost, and makes them less efficient than baseboard radiative heat. The lower boundary is about -10c. Not to mention gas is just way cheaper than both of those options. But it does emit greenhouse gas. I don't know why this topic comes up so much recently. It's just not common at all where I am for a reason (Canada).
I don't have good aggregate data on energy spent, but the relative expense is actually easy to understand since it mostly just comes down to the delta in temperature you're trying to achieve.
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."
The difference is that most people heat their homes with gas or oil, which is significantly cheaper than electric heaters (heat pump or otherwise) and for some reason isn't considered as energy the same way electricity is. Air conditioners/heat pumps can only use electricity, and they use quite a lot of it.
> isn't considered as energy the same way electricity is
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.
Gas/oil is definitely cheaper than electric heaters, but I thought these days heat pumps were cheaper than gas/oil, at least until you got down to temperatures below ~10°F?
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.)
Heat pumps definitely shouldn't be lumped in with resistive heating (which is what I think of when I think of "electric heaters"). They are definitely more energy efficient than either resistive or gas/oil, but whether they're cheaper depends on the relative cost of local electricity and gas delivery. Gas is still often cheaper than electicity, so it really depends on your goal: energy efficiency/carbon output vs. cost efficiency.
My money is on "gas won't be cheaper for much longer, even where it is now" personally, though.
> There's a common meme in the northern latitudes that people "down south" consume unreasonable amounts of energy on AC, but
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.
I live in the PNW and heat dome notwithstanding very few homes have AC because it almost never gets above 90ish degrees in the summer. A good cross breeze is more than sufficient to keep cool in that case.
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.
> I live in the PNW and heat dome notwithstanding very few homes have AC because it almost never gets above 90ish degrees in the summer.
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.
> it mostly just comes down to the delta in temperature you're trying to achieve
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.)
Do you have a reputable source for that? I've never heard that before, it seems to defy intuition in a couple of ways, and it defies my own electricity bill.
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.
You appear to have previously to be laboring under the misconception that latent heat (the heat required to condense water in gaseous form into water in liquid form) didn’t exist or that humidity isn’t gaseous water. My comment was directly responsive to whichever of those misconceptions was underlying your comment above.
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]).
Ah ha, thank you! I was indeed under the misperception that latent heat was associated only with evaporation/condensation at high-energy boiling points, not at e.g. room temperature for water. Turns out it's fiendishly difficult to find anything on Google or educational materials about phase changes or latent heat that aren't at a boiling (or freezing) point -- no wonder I was never taught it. (Finally found [1].)
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.
(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.
> For every 40 pounds of 90°F/90% RH air that you want to make into comfortable inside air:
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.
Typical house has between 0.5 (quite tight) and 2 air changes per hour. It’s true that the HVAC return air is coming from conditioned space, but it’s also true that that air is frequently exchanged to outside air, meaning it’s going to need to be continually dehumidified.
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".
> 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
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.
> I'm presenting temp/humidity numbers that are reasonable for where I live (NYC where the summers still are pretty hot and humid)
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).
> contrary to what you say there's absolutely no phase change involved -- water vapor isn't steam
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.
> 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.
> A heat pump in the winter is literally just air-conditioning the outside, and produces similar condensation in the unit outdoors.
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.
> 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).
> 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).
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.
Heating takes a lot more energy generally than air conditioning (citation needed) in the US. Which makes sense when you think of the difference in temperatures from the inside of the house and the outside in winter compared to summer. Many people complain about the rise of air conditioning and how much energy it wastes, but actually it allows people to not live in the cold parts of the country, and is probably a net positive energy saver.
Solar arrays you install in such an area can power useful activity elsewhere - and its not like we're at 100% renewable energy everywhere to obviate this. So The waste of energy _is_ damaging.
Agree with your points. Also, your comment reminded me that habitable, unhabitable, inhabitable, uninhabitable, is one of the most confusing situations in the english language.
> Habitable and inhabitable mean the same, don't they?
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".
I have more respect for that than I do for, say, Phoenix AZ.
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.
to be fair; Phoenix has (had) quite a bit of its own water via the Salt River. And there was also quite a lot of ground water (which is nearing depletion now). The controversial bit is that Phoenix also ties in to the Colorado River system, via canals, and THAT is consuming far more water than can be sustained. Most of it for growing crops (not sustaining the massive population) - and I think that if the current drought situation continues, they're going to have to stop growing cotton and lettuce out here. Which . . . is fine.
I agree with you about scorekeeping, but you did a weird comparison in your sentence about heating/cooling. You compared the potential of one with the reality of another. As in, AC can be powered by [renewable] electricity, but that most heating is [powered] by burning fossil fuels. It might be reasonable to ask how much AC actually is powered by renewables vs fossil fuels. Heating can also be done electrically, and therefore could be done with renewables. We all know that it's hella expensive at present to heat with electricity compared to natural gas, propane, or even wood in some places (at present).
AC-heavy environs -- especially deserts -- are a better use-case for existing renewable solutions than colder environs. And, also, actual uptake is higher.
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.
The only problem I have is that its in a national park. I feel like parks should be for everyone but the renting a room is really expensive so you have to be mostly well off to get one. I guess it feels like an eyesore that richer people can use while everyone else is forced to camp.
This is false. Heat is an incredibly common type of waste energy, and far more common than waste "cold" (if such a thing exists at all), so most (if not all) heating devices are more efficient than cooling devices.
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.
This isn’t really true because pumping heat requires less energy to create the same differential than generating, and most homes are heated by generation.
According to this [0], the water comes from a natural spring. The electricity could be generated on site with solar panels. Doesn't seem that outlandish to me.
It's not readily apparent but pulling water out of the regular ecosystem in a desert environment like this is crushing to the local wildlife population. I spend a lot of time in the desert in this part of the country and it's tragic how many springs are dry these days due in large part to ground and spring water being taken for things like this golf course. I absolutely think that having a golf course like this is a terrible waste.
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.
I stayed for a night at Furnace Creek and it was "air conditioned" and the cold water was "cold". They're not cranking the industrial ACs here, it's just enough to be on the bearable end of the range for tourists that are not used to the heat. Air temp was 116 at the official station.
This is a good illustration of a principle that when your sample contains the most extreme events you will find that measurement errors make up a much larger fraction of them than you would expect.
I used to live just south of Death Valley. One summer, it was up around 115F every day for two weeks straight in July.
An occasional spike in temps to that level was not a big deal. The houses were built to some degree for that environment and had , for example, narrow windows with some kind of architectural detail I've never seen elsewhere that helped shade the windows.
But after days and days of such temperatures, the AC blowing cold air felt like a joke. The walls were hot. The floor was hot. The couch was hot.
You learn to eat different and live different in an environment like that. You make sure you get enough to drink. You teach the kids to stay hydrated. You teach everyone the importance of electrolytes.
I went for walks for exercise after dark when the temp dropped to 99F. There was precious little shade and I sunburned easily which is another serious health threat on top of the risk of heat prostration.
Desert cities of old were built to keep things tolerable at street level. City location was chosen for ability to mitigate the heat, typically a plateau a little above the surrounding terrain. Streets were oriented to maximize the cooling effect of prevailing winds. Buildings were designed to reflect heat rather than soak it up, among other things.
Humans used to work out in the weather much more consistently than we do these days and tended to primarily get around on foot. Now, we go from home to car to office to car to store to car, etc. We don't have the same relationship to the weather and we seem to have forgotten how to live with weather.
At least in the US. Maybe other places are a bit better about such things.
Houses - and especially high rise developments - are increasingly built according to global fashions or individualistic whims, divorced from regional styles which evolved with the local climate and available materials.
regional styles which evolved with the local climate and available materials.
That's often called vernacular architecture and it typically innately includes elements of passive solar design. Such building styles were well-suited to local climate with a minimum of energy-intensive cooling or heating because people simply couldn't afford it.
Additionally, we had cultural practices rooted in staying adequately warm or cool without counting on an HVAC system:
Tapestries on castle walls blocked drafts and provided insulation.
A "three dog night" was a night so cold, the upper classes let three of their large hunting dogs sleep with them to stay warm.
Siestas were naps taken during the hottest part of the day in place of working in the heat.
Buildings routinely had windows positioned to create a cross breeze if you opened them.
Windows routinely were positioned to provide adequate light for essential tasks without having to turn on any lights during the day.
Etc.
Now we build some awful little cardboard box and slap an HVAC system on it as if tech makes up for bad design. It doesn't.
> Now we build some awful little cardboard box and slap an HVAC system on it as if tech makes up for bad design.
This is really infuriating. I lived in a small town which was full of houses built decades ago for workers building hydro plant close by. They were supposed to be temporary and really deserve to be knocked down these days. But they still function... with multiple split system ACs heating them up.
Even if they're money/energy sinks, people don't have enough savings to invest in a more efficient rebuild.
Even if they're money/energy sinks, people don't have enough savings to invest in a more efficient rebuild.
Poverty is always expensive. That's what keeps people trapped in it.
If it were cheap to be poor, it wouldn't be poverty. It's the fact that you get bled for your time, money, energy and anything else you've got that makes people poor.
Sometimes, yes, but not generally. I know people who can't help themselves from spending whatever money they've got on random short term pleasures. Even when they receive a windfall, it's quickly gone with nothing to show for it. They certainly don't invest in a more energy efficient house or even bulk buy household items or other boring money-saving things when they have the opportunity. There's ways of thinking and behaving that make people get poor and stay poor. That's not even a bad thing. Some of these people seem quite happy. They're not trying to be rich.
What Doreen linked 100%, but also - you need to actually know how to save in a long term. If you didn't get that daily-things-that-improve-your-position continuous education as you grow up, it's not necessarily trivial to continuously apply when you get older. I've seen people do really stupid things because they were not aware of better ways which were obvious to me. (And I bet I did some stupid things which are obviously stupid to you)
I'm thinking here of "skills to survive on minimal pay" being very different from "skills of long-term financial management".
There are certainly all sorts of reasons for poverty, but I'm just pointing out that poor people aren't always poor because they're trapped in a vicious cycle. Sometimes they're impulsive or don't have a way of thinking about the future. Also that that way of life seems to be quite enjoyable. It may be not something that needs fixing. Who says accumulating wealth is the most important goal in life? One such person I know had a good upbringing with proper education and professional caring parents and all that but can't hold onto money. It's not about lack of opportunities but maybe mental illness or far-from-normal personality or somehow just doesn't care about long term planning.
Total tangent, but I've been struggling with in-home humidity recently so household mold has been a nagging concern in the back of my mind. I'll try searching around a bit, too. Anecdotally, quite a few people in my friend group are also dealing with high (> 70%) relative humidity in their homes, so it'd be interesting to find out there's a broader trend happening.
Well in a way houses are still built for the local climate, but the local climate has abundant cheap energy.
Building houses to be better insulated and able to tolerate extreme temperature events with just minor discomfort is possible[0], but it is also more expensive. If you spend $50,000 more to insulate a house to tolerate this, and it reduces the energy costs by $1,200/year, then financially it's not worth it [1]. Of course for the environment that's a big difference, but capitalism :-)
[0] I live in Northern Europe, in a house built to our pretty strict building standards, and we don't even turn on the heating until it goes below freezing outside. Yesterday we had highs of 32C (90F) - which is hot for here - and I was out so the AC was off. When I came home in the afternoon, our living room which directly faces south and has no shade other than decorative blinds was... 25C (77F).
[1] Average stock market returns over the last century are around 10% per year. Even if you conservatively half that, it still doesn't work out.
There have been 10 year periods with negative inflation-adjusted annualized return. E.g. for the S&P500 it was -3.4%/year in the 2000's and -1.4%/year in the 1970's [1]. That makes any investments with a guaranteed long term benefit look better.
“in houses that look toward the south, the sun penetrates the portico in winter, while in summer the path of the sun is right over our heads, and above the roof, so that there is shade.”
That is Socrates himself, explaining the advantageous way to orient houses. The Greeks paid special attention to this during their energy crisis, when they had burned the available forest and the citizens were eyeing the olive groves.
This reminds me of a story my dad used to tell. A friend of him was tasked by the wildlife protection agency to count the grouse population in the county every year.
What he did was to walk a given path through the mountains at the same time each year, and count how many grouse he saw along the way. This was then used by the agency to estimate the population.
One year he was unavailable, and as such he asked his friend to do it. He instructed him on the path, how to count and all that.
Later that year the local paper's headline stated "Dramatic increase in grouse population!"
Turned out his buddy didn't bother to do the long walk, and instead just came up with a number he submitted to the agency.
In high school we were supposed to do a similar project about squirrel populations. We got lazy, stopped doing the walks and just submitted numbers similar to the team that had done the same project the year before. I mention this to one of those older students at a party and he tells me they did the same thing.
We live in spaceships, hooked up to life support. Skyscrapers are nothing but sealed spaceships pumping in air and water and pumping out waste. Same for most homes.
I do like the idea of ground source heat pumps. If we all lived in caves hundreds of feet deep, the temperature would be near perfect year around. In a way, the heat pump “brings the cave to the surface”. There’s something very satisfying about this design, we’re not using technology to insulate ourselves from the Earth — we’re working directly with it.
I am uncomfortable with historical meteorological revision. There is enough problem with climate change non-believers that retroactive "adjustment" of historical weather records is only going to fuel conspiracy theories - especially when they are adjusted down. There is more than enough evidence of climate change without revising past records - so why do it?
Especially given that this record requires that an un-identified person may have possibly made an error reading a thermometer. The evidence that this value is wrong is lacking.
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[ 2.4 ms ] story [ 190 ms ] threadHowever, DV does not come out on top when looking at temperatures measured by satellites (EDIT: ground temperature, and less reliable.) From Wikipedia [1]: "The highest recorded temperature taken by a satellite is 66.8 °C (152.2 °F), which was measured in the Flaming Mountains of China in 2008. Other satellite measurements of ground temperature taken between 2003 and 2009, taken with the MODIS infrared spectroradiometer on the Aqua satellite, found a maximum temperature of 70.7 °C (159.3 °F), which was recorded in 2005 in the Lut Desert, Iran. The Lut Desert was also found to have the highest maximum temperature in 5 of the 7 years measured (2004, 2005, 2006, 2007, and 2009). These measurements reflect averages over a large region and so are lower than the maximum point surface temperature." According to [2], the record is 80.8 °C at Lut Desert.
[1] https://en.wikipedia.org/wiki/Highest_temperature_recorded_o... [2] https://journals.ametsoc.org/view/journals/bams/aop/BAMS-D-2...
I am confused how 66.8C can be "the highest recorded temperature taken by a satellite", while the Aqua satellite found a maximum temperature of 80.8C.
That 70.7 degrees from the satellite was ground temperature, not air temperature, so it can't be compared.
Exactly. I have a radio thermometer in my attic whose readings max out at 140°F. During the summer it's regularly maxed out, so the actual air temp is hotter still (and the actual surface temp of the roof must be even hotter than that). Outside air temp is nowhere near any of those temps.
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
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)
https://www.merriam-webster.com/dictionary/unhabitable
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.
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.
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.
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.
[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."
https://xkcd.com/852/
An occasional spike in temps to that level was not a big deal. The houses were built to some degree for that environment and had , for example, narrow windows with some kind of architectural detail I've never seen elsewhere that helped shade the windows.
But after days and days of such temperatures, the AC blowing cold air felt like a joke. The walls were hot. The floor was hot. The couch was hot.
You learn to eat different and live different in an environment like that. You make sure you get enough to drink. You teach the kids to stay hydrated. You teach everyone the importance of electrolytes.
I went for walks for exercise after dark when the temp dropped to 99F. There was precious little shade and I sunburned easily which is another serious health threat on top of the risk of heat prostration.
Desert cities of old were built to keep things tolerable at street level. City location was chosen for ability to mitigate the heat, typically a plateau a little above the surrounding terrain. Streets were oriented to maximize the cooling effect of prevailing winds. Buildings were designed to reflect heat rather than soak it up, among other things.
Humans used to work out in the weather much more consistently than we do these days and tended to primarily get around on foot. Now, we go from home to car to office to car to store to car, etc. We don't have the same relationship to the weather and we seem to have forgotten how to live with weather.
At least in the US. Maybe other places are a bit better about such things.
That's often called vernacular architecture and it typically innately includes elements of passive solar design. Such building styles were well-suited to local climate with a minimum of energy-intensive cooling or heating because people simply couldn't afford it.
Additionally, we had cultural practices rooted in staying adequately warm or cool without counting on an HVAC system:
Tapestries on castle walls blocked drafts and provided insulation.
A "three dog night" was a night so cold, the upper classes let three of their large hunting dogs sleep with them to stay warm.
Siestas were naps taken during the hottest part of the day in place of working in the heat.
Buildings routinely had windows positioned to create a cross breeze if you opened them.
Windows routinely were positioned to provide adequate light for essential tasks without having to turn on any lights during the day.
Etc.
Now we build some awful little cardboard box and slap an HVAC system on it as if tech makes up for bad design. It doesn't.
This is really infuriating. I lived in a small town which was full of houses built decades ago for workers building hydro plant close by. They were supposed to be temporary and really deserve to be knocked down these days. But they still function... with multiple split system ACs heating them up.
Even if they're money/energy sinks, people don't have enough savings to invest in a more efficient rebuild.
Poverty is always expensive. That's what keeps people trapped in it.
If it were cheap to be poor, it wouldn't be poverty. It's the fact that you get bled for your time, money, energy and anything else you've got that makes people poor.
I'm thinking here of "skills to survive on minimal pay" being very different from "skills of long-term financial management".
Building houses to be better insulated and able to tolerate extreme temperature events with just minor discomfort is possible[0], but it is also more expensive. If you spend $50,000 more to insulate a house to tolerate this, and it reduces the energy costs by $1,200/year, then financially it's not worth it [1]. Of course for the environment that's a big difference, but capitalism :-)
[0] I live in Northern Europe, in a house built to our pretty strict building standards, and we don't even turn on the heating until it goes below freezing outside. Yesterday we had highs of 32C (90F) - which is hot for here - and I was out so the AC was off. When I came home in the afternoon, our living room which directly faces south and has no shade other than decorative blinds was... 25C (77F).
[1] Average stock market returns over the last century are around 10% per year. Even if you conservatively half that, it still doesn't work out.
[1] See table at bottom of http://www.simplestockinvesting.com/SP500-historical-real-to...
That is Socrates himself, explaining the advantageous way to orient houses. The Greeks paid special attention to this during their energy crisis, when they had burned the available forest and the citizens were eyeing the olive groves.
What he did was to walk a given path through the mountains at the same time each year, and count how many grouse he saw along the way. This was then used by the agency to estimate the population.
One year he was unavailable, and as such he asked his friend to do it. He instructed him on the path, how to count and all that.
Later that year the local paper's headline stated "Dramatic increase in grouse population!"
Turned out his buddy didn't bother to do the long walk, and instead just came up with a number he submitted to the agency.
Especially given that this record requires that an un-identified person may have possibly made an error reading a thermometer. The evidence that this value is wrong is lacking.
Then maybe it's not a conspiracy theory?