“In simple terms, it takes less energy to cool a room down by one degree than it does to heat it up by one degree.”
The media release is below.
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, research suggests
Living in colder climates in the US is more energy demanding than living in warmer climates.
This is according to Dr Michael Sivak at the University of Michigan, who has published new research today, 28 March, in IOP Publishing’s journal Environmental Research Letters.
Dr Sivak has calculated that climate control in the coldest large metropolitan area in the country – Minneapolis – is about three-and-a-half times more energy demanding than in the warmest large metropolitan area – Miami.
Dr Sivak calculated this difference in energy demand using three parameters: the number of heating or cooling degree days in each area; the efficiencies of heating and cooling appliances; and the efficiencies of power-generating plants.
Not included in the analysis were the energy used to extract fuels from the ground, the losses during energy transmission, and energy costs.
“It has been taken for a fact that living in the warm regions of the US is less sustainable than living in the cold regions, based partly on the perceived energy needs for climate control; however, the present findings suggest a re-examination of the relative sustainability of living in warm versus cold climates.”
Heating degree days (HDDs) and cooling degree days (CDDs) are climatological measures that are designed to reflect the demand for energy needed to heat or cool a building. They are calculated by comparing the mean daily outdoor temperature with 18°C.
A day with a mean temperature of 10°C would have 8 HDDs and no CDDs, as the temperature is 8°C below 18°C. Analogously, a day with a mean temperature of 23°C would have 5 CDDs and no HDDs.
Based on a previous study, Dr Sivak showed that Minneapolis has 4376 heating degree days a year compared to 2423 cooling degree days in Miami.
In the study, Dr Sivak used a single measure for the efficiency of heating and cooling appliances, as most are currently rated using different measures so they cannot be directly compared. His calculations showed that a typical air conditioner is about four times more energy efficient than a typical furnace.
“In simple terms, it takes less energy to cool a room down by one degree than it does to heat it up by one degree,” said Dr Sivak.
Grouping together climatology, the efficiency of heating and cooling appliances, and the efficiency of power-generating plants, Dr Sivak showed that Minneapolis was substantially more energy demanding than Miami.
“In the US, the energy consumption for air conditioning is of general concern but the required energy to heat is often taken for granted. Focus should also be turned to the opposite end of the scale – living in cold climates such as in Minneapolis, Milwaukee, Rochester, Buffalo and Chicago is more energy demanding, and therefore less sustainable from this point of view, than living in warm climates such as in Miami, Phoenix, Tampa, Orlando and Las Vegas,” Dr Sivak concluded.
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Sorry, I was wrong to translate the EER of 9.7 to 10%. I forget they measure output in BTUs. It is more like 30%, and it is a very rough estimate because the industry ratings for cooling systems are complex enough that direct comparison to heaters is not possible. Here’s a summary of how they do it:
http://www.cwlp.com/energy_services/efficiency_ratings/appliance_efficiency_ratings.htm
All true, but the claim that gave my BS gauge a jolt was “it takes less energy to cool a room down by one degree than it does to heat it up by one degree”.
That, if true, would apply equally to Houston and Minneapolis, no matter what season, because one degree is the same unit an all these situations. What he says, then, is that the cost of maintaining of the same one-degree thermal gradient across the house wall depends on whether said gradient is directed inside or outside.
Indeed, the cost does depend on direction in those situations where the inbound and outbound thermal fluxes are maintained using different technologies. Where the same technology is used (a reversible heat pump) the cost per degree will be the same, so that relatively uncommon case invalidates the claim (“same” is not “less”). In a more common situation, an evaporator/condenser system (10% efficiency) to take the heat out and a combustion heater to bring the heat in (50 .. 90%). That case also invalidates the claim. The only situation where it will hold (maybe) is if the heater is an open fire under a hole in the roof. That is probably going to be as inefficient as a typical AC system.
Where do I get the data? I confess I have not done my own measurements (although I have had an energy audit done on my home and have a good itea of the correspondence between the stated and measured efficiencies). I tend to believe the published ratings like these:
http://www.homedepot.com/p/t/203147368?catalogId=10053
Look at the EER value (9.7 in this case, which I think is typical). What it means is that the energy the device consumes per unit of time is 9.7 times the thermal flux against set temperature gradient used in the cerctification procedure (which I belive is chosen to represent typical operating conditions).
A combustion heater is at best 98% efficient. That means 98% of energy comes out as heat.
Code air conditioners have 13 EER. That is Energy Efficiency ratio, or the amount of energy that can be moved by energy. In other words, 1300% efficiency.
Everything I ever learned said that degree for degree it takes more energy to cool than it does to heat. Cooling requires energy to be converted to work which in turn removes the heat. Heating takes the energy, combustion or resistance, and uses most of it to create heat directly. Except for the circulation fans and controls, there is little going into work. Which implies the process is less wasteful.
But I suppose most of it has to due with the indoor/outdoor temperature differential. I don’t have enough information at my fingertips to do the math, but it might be reasonable to assume it takes more energy to heat the same amount air from 20 degrees to 68 degrees (48 degrees) than it would to cool air from 95 degrees to 72 degrees (23 degrees).
I have not read the study; it does not appear in the current issue nor does it show up in the “Forthcoming Articles” on the web site, so I base this on the content of the press release only. (end of caveat)
Dr. Sivak’s analysis sounds too simplistic to me. An analysis done on the basis of total degree-days per year doe not do justice to the complexity of the situation. Heat pumps do not create energy they merely move energy from one location to another; from inside to outside in the summer and from outside to inside in the winter. At the ideal indoor temperature, apparently 18 degrees (that is 64 degrees F and I much prefer 70, but whatever) a heat pump will use the same amount of energy moving heat if the outdoor temperature is 10 degrees as it will if the outdoor temperature is 26 degrees. The mountain up which the device must pump heat is 8 degrees high and the equipment does not care about the direction of the heat flow; 8 degrees is 8 degrees. At that temperature gradient, most modern heat pumps will move 3 to 5 btu of energy per btu of energy consumed by the equipment. That is their coefficient of performance will be between 3 and 5.
The problem occurs when the temperature mountain is too high. If the outdoor temperature falls below about 28 degrees F, heat pumps lose efficiency and their coefficient of performance can fall below 1, i.e. they will move less energy than they consume. At that outdoor temperature the mountain is 36 degrees high. The comparable outdoor temperature is 100 degrees F, rarer, even in Miami than a temperature below 28 degrees is in Minneapolis. That is why the geniuses at EPA devised the SEER, the seasonal energy efficiency rating (I think that is correct, I know that it conveys the concept) Heat pumps will have a different SEER in different climates.
Now, a rational person living in Minneapolis does not try to heat his house with a heat pump, he purchases a modern, gas or oil fired condensing furnace. Those furnaces extract the heat of condensation from the exhaust gases as well as the sensible heat of combustion. Burning methane creates 2 molecules of water per molecule of methane. The exhaust gas from these furnaces is warm to the touch. They extract about 95% of the heat energy from the fuel. You have to add a little energy to move the warm air around your house and an even smaller amount to force the exhaust gases to leave your home, but that total is not much. So, you cannot do a simple degree-day analysis and expect to get a result that has meaning.
In addition, he appears to have neglected humidity control, which you will always get from your air conditioner in Miami, but perhaps not in Phoenix.
Any air that infiltrates a house in Miami will contain a lot of water vapor, humidity. During most of the summer the dew point outside will be higher than 64 degrees, I believe that the average summer time dew point in Miami is probably in the mid to high 80s. That means that your air conditioner condenses a lot of water, at a cost of 1000 btu per pound of water. The kids do not have to leave the door open for very long each day for the cost of condensing water to approach 30% of your air conditioning costs. Again, that cost is less in Phoenix as well as it is in Minneapolis.
Based solely on the content of the press release, I think that this study is meaningless.
Well there are a couple of factors.
1: First in say Houston, it is warm in Winter, and hot in Summer, so you don’t use energy in winter but you use Air in Summer. In MN you are really cold in winter so you use heat and really hot in summer so you use air as well.
2: The winter difference is pretty extreme, if you compare heating from 0 – 70 in Winter to cooling from 95 – 70 in summer – and lets assume the heating area only needs climate conditioning in winter and the cooling area only uses climate conditioning in summer, If you look the differential assuming the same efficiency it should take considerably more energy to heat the home than it does to cool. (The difference creates an exponential increase in energy need. )
3: I don’t know where you get your figures. Modern heating is either 80% efficient or 92% efficient depending on whether you have return ducts. I am not sure about cooling but I suspect the 10 – 13 seer models that are typically out there do better than you state as well.
4: Even per the guidlines by my (former) utility, It should take 30% less energy to cool 25 degrees than it does to heat 70 degrees.
Sivak’s PhD is in experimental psychology, and this topic is not even in his self-identified Area of Expertise. His references and defns are non-standard to the HVAC industry, so easy comparison is hampered.
http://www.umtri.umich.edu/people.php?personID=40
I wondered the same thing.
The energy-per-degree doesn’t seem right. Air-conditioning is naturally less efficient than most heating methods.
Far more likely that the work of heating/cooling is greater in winter. Here in Spokane, for instance, the typical summer day runs from 55F to 85F, requiring 15 degrees of ‘work’ for part of the day to reach 70. The typical winter day runs from 20 to 25F, requiring 45 degrees of ‘work’ all day to reach 70.
My electricity bill is $100 per month in winter, $35 in summer, and $30 in May and Oct when neither heat nor A/C is active. Thus the heating adds $70 to the baseline and the A/C adds only $5.
Another point about climate is that will the US South can be miserable without air conditioning in the summer (very hot and humid in many places) one is not likely to freeze to death because the AC is out of commission. But if the heat is out of commission on a cold day hypothermia could be a real possibility and it is harder to prevent than heat stroke. So in a cold climate heating is absolutely mandatory while in a hot climate it is not an absolute necessity.
The upper US Midwest can get very cold in the winter months and if you have no heat source you have a real chance of hypothermia while on the warmest day in the US South staying properly hydrated and in the shade is more easier to do.
Does this mean Yankees should be FOR global warming?
Looks like the good PHD has forgotten about the simple relationship between building volume and building surface area.
Large buildings (filled with people, each equal to about a 100 watt light bulb) are much harder to cool than to heat. Add in all the modern computers (a nice liitle toaster all by itself) and you end up with todays energy intensive companies (with large computer databases) looking to build in cooler places. Recently one of these search engine firms looked to put a data center near Buffalo NY simply because the cooling requirements could be accomplished largely with the local ambient outside air (little additional AC required).
There is a reason elephants have large ears and (most) mice have small ears, the large ears act as a radiator and overcome the HUGE difference in mouse surface area to volume ratio versus the elephants surface area to volume ratio.
Sounds like mother nature figured this out long before this PHD.
Cheers, Kevin.
I guess this explains why the north is dominated by blue states. The Liberals are planning to destroy all of the cities because of global warming, Detroit is first.
“In simple terms . . .” Does that mean the 2nd law of thermodynamics has been rediscovered by the geniuses? Here’s a simple solution: force everyone to move to Hawaii, no questions asked, no birth certificate needed.
Sounds counter-intuitive. Coolers have huge heat losses, resulting in the overall efficiency of about 10-15%. Non-electric heaters can deliver 50-70% of the energy of their fuel. Electric heaters can be more than 90% efficient, but the higer cost of electric power covers combustion losses at the generating plant.
Better live in England, where neither heating nor cooling is necessary during most of the year.