Wireds mytavslöjanden

av jstenling

Wired körde för ett nummer sedan en artikel som syftar till att slå hål på några "myter" om hur man bäst lever miljövänligt. Jag tyckte att det var en ganska fånig artikel som verkade innehålla uppenbara sakfel och förenklingar. Den har dock kommenterats på ett flertal ställen. Den bästa vederläggningen hittade jag hos Realclimate.

Jag tycker det här är ett bra exempel på hur lätt det blir fel när man försöker förenkla komplexa frågor. Jag tror inte att Wired medvetet har försökt ljuga, men fel har det blivit i alla fall. Här är ett exempel, läs gärna hela artikeln i Wired och sedan Realclimate för deras svar.

Från Wired:

As a symbol of American profligacy, the air conditioner may rank second only to the automobile. Energy-sucking AC props up an unsustainable lifestyle in scorching desert cities like Phoenix, while the cheerful New Englander splitting wood and tending his potbelly stove is the epitome of ecological harmony MDASH so goes the green cant. But this stereotype gets it wrong. When it’s 0 degrees outside, you’ve got to raise the indoor thermometer to 70 degrees. In 110-degree weather, you need to change the temperature by only 40 degrees to achieve the same comfort level. Since air-conditioning is inherently more efficient than heating (that is, it takes less energy to cool a given space by 1 degree than to heat it by the same amount), the difference has big implications for greenhouse gases.

In the Northeast, a typical house heated by fuel oil emits 13,000 pounds of CO2 annually. Cooling a similar dwelling in Phoenix produces only 900 pounds of CO2 a year. Air-conditioning wins on a national scale as well. Salving the summer swelter in the US produces 110 million metric tons of CO2 annually. Heating the country releases nearly eight times more carbon over the same period. Meanwhile, chilly Northeasterners can at least take heart in one thing: With global warming you can turn the heat down.

Från Realclimate (förkortat, men ändå långt):

Let’s take air conditioning for starters. Basically WIRED took a look at the carbon footprint of New England heating vs. Arizona cooling and jumped to the conclusion that air conditioning was intrinsically more efficient than heating. To see where they were led astray let’s consider a house sitting where you need to cool it by 20 degrees to be comfortable. The heat leaks into the house at a rate that is approximately proportional to this temperature difference, and the heat leaking in needs to be removed. Now, in order to move that heat from inside to outside, energy has to be expended. Given a fixed electric power usage (in watts), a better air conditioner can remove more heat per day than a worse one, but every air conditioner needs to expend some energy to move the heat. That’s just thermodynamics.

Efficiency of air conditioners is measured by a SEER rating, which is the ratio of heat moved to the outside (in BTU/hr) to the electric power consumption (in Watts). A typical modern air conditioner has a SEER rating of 10, We can convert this into nicer units by converting BTU/hr into Watts, which means dividing the SEER rating by 3.413, which then gives us a Coefficient of Performance, in units of Watts of heat moved per Watt of electricity used. For the aforementioned efficiency, we move heat at a rate of 2.92 Watts if we expend 1 Watt of electric energy. An air conditioner is just a heat engine run in reverse: instead of making use of a temperature differential to use heat flow from hot to cold to do work, we expend mechanical work in order to move heat from a colder place to a hotter place. Thus, an efficient heat engine is an inefficient air conditioner. That’s basically why the Coefficient of Performance gets smaller when the temperature difference between indoors and outdoors is greater — with bigger temperature difference heat engine cycles tend to get more efficient, which means that air conditioner cycles tend to get less efficient. That’s also where the "S" in SEER comes from. It stands for "Seasonal," and reflects the fact that efficiency must be averaged over the range of actual temperature differentials experienced in a "typical" climate. Your mileage may vary.

This situation can be contrasted with heating. If that same house were in an environment that were too cold instead of too warm, so that it had to be kept 20 degrees warmer than the environment, then the amount of heat leaking out of the house each day would be about the same as the amount leaking into the house in the previous case. That heat loss needs to be replaced by burning fuel. Now, generating heat is the only thing that can be done with 100% efficiency. Old furnaces lose a lot of heat up the chimney, but modern sealed-combustion burners– the kind that can use PVC pipes instead of a chimney — lose virtually nothing. With a heat exchanger between the air intake and the exhaust, they could closely approach the ideal. But still, in this case we are generating heat rather than just moving it, so it takes 1 watt of heat power from fuel burning to make up 1 watt of heat loss. That would seem to make heating a factor of 2.92 less efficient than air conditioning.

But wait, the story doesn’t stop there. First, there’s the fact that air conditioning almost invariably runs off of electricity, and the increased electricity demand is a big source of the pressure to build more coal-fired power plants. A house can be heated by burning natural gas, and right there air conditioning becomes 1.8 times worse than heating, because natural gas emits only 55% of the carbon of coal, per unit of heat energy produced. And it gets even worse: Coal fired power plants are only 30% efficient at converting heat into electricity, on average, so there you get another factor of 3.3 in carbon emissions per unit of energy transferred between the house and its environment. Finally, figure in a typical electric line transmission loss of 7% and you get another factor 1.075. Put it all together with the energy efficiency of the air conditioner itself and air conditioning comes in at a whopping 2.19 times less efficient than heating. for a given amount of temperature difference between house and environment. That means that so far as carbon emissions go, heating a house to 70 degrees when the outside temperature is 40 degrees is like cooling the same house to 70 degrees when the outside temperature is 83.7 degrees.


Is there any real conclusion that could have been drawn from more clear thinking about the heating vs. air conditioning issues danced around in the article? Yes, in fact. The conclusion is that it makes a lot of sense to build houses in places where the environment requires neither much heating nor much cooling. This is in fact why Los Angeles scores pretty well in carbon footprint per capita, despite all the driving (as noted recently in The Economist.). Another conclusion to be drawn from the carbon footprint of New England heating is that there are probably a lot of leaky homes up there heated by inefficient oil-fired furnaces. Fixing that situation represents a huge untapped virtual energy source.

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Thank you wired…


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