Many of our tech-savvy friends — the kind of folks who nurse along the beowulf clusters our climate models run on — are scratching their heads over some cheeky shrieking that recently appeared in a WIRED magazine article on Rethinking What it Means to be Green . Crank up the A/C! Kill the Spotted Owl! Keep the SUV! What’s all that supposed to be about?
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.
And that’s still not the end of the story. A house in need of air conditioning has other heat inputs besides the heat leaking in from outside, and all that extra heat needs to be gotten rid of as well. For example, heat is a waste-product of all energy use going on in the house. Four people produce 400W that needs to be gotten rid of, and then there’s the heat from hot water, lighting, the TV, cooking and what have you — all the energy usage within the house, plus 100W of biological heat per person needs to be gotten rid of. On top of that, you’ve got direct radiative heating from the sun, both from the sunllight getting through windows and solar heating of the exterior surfaces of the house, some of which will leak in through the insulation. Energy must be expended to remove all this heat. In contrast, in the heating season waste heat is subtracted from the energy needed for home heating.
So, WIRED got the story egregiously wrong, and not just because they did the arithmetic wrong. In their rush to be cute, they didn’t even make a half-baked attempt to do the arithmetic. But what if they had been right and air conditioning really were intrinsically more efficient than heating. Would that justify their conclusion that you can just "crank up the A/C?" without worry? No, of course not, because cranking up the A/C would still use additional energy and still lead to the emission of additional carbon. For the conclusion to be justified, it wouldn’t be enough for A/C to be more efficient than heating; it would have to be so much more efficient that the incremental energy usage from cranking it up were trivial. WIRED didn’t even try to make that case. If they had, they might have spotted their errors.
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.
What’s more, for a magazine that purports to be written by and for tech geeks, WIRED missed the biggest and most interesting part of the story: the same intrinsic efficiences of heat pumps can be run in reverse to give you the same economies for home heating as you get for air conditioning. To do this effectively, you’d have to run the heat pump off of natural gas rather than electricity (or perhaps run it off of locally generated solar power or wind). You’d also have to deal with the fact that heat pumps become less efficient when working across large temperature gradients, but that’s where geothermal heat storage systems come in, making use of the fact that the deep subsurface temperature remains near a nice 55F all year around. Now that would have been a nice story for a tech magazine to cover. And by the way, the decrease in efficiency of heat pumps as the temperature differential increases has another implication that WIRED missed: not only does global warming increase the basic demand for air conditioning, with all the attendant pressures on electricity demand, but it exacerbates the situation by decreasing the efficiency of the entire installed base of air conditioners.
Now about that spotted owl. This refers to a claim that industrial tree plantations take up carbon faster than old growth forests; Since spotted owls require the large trees found only in old-growth, the supposed implication is that if we want to soak up carbon we ought to damn the spotted owl and cut down all the old growth. WIRED really committed serial stupidities on this one. First of all, the article they cited in support of their claim was about carbon emissions from Canada’s managed forests, not from old growth. Now, it’s true that a rapidly growing young tree takes carbon out of the atmosphere more rapidly than a mature forest which more slowly transfers carbon to long term storage in soil. However, to figure out how much net carbon sequestration you get out of that young tree once it’s chopped down, you need to figure what happens to it. Lots of trees wind up in paper, carboard boxes, shipping palettes and other things that rapidly sit around decomposing or get burned off (or worse, turn into methane in landfills). Even the part that turns into houses has a relatively short residence time before being oxidized. Anybody who has maintained an old Victorian house knows about the constant battle against rot, and the amount of wood that needs to be replaced even if (knock wood) the thing doesn’t burn down or turn into a tear-down. So, WIRED is totally off the mark there, unless, to use the colorful language of my colleague Dave Archer, they can get trees to "drop diamonds instead of leaves."
Worse, they ignore the abundant literature indicating that old growth forests can be a net sink of carbon even in equilibrium, whereas the soil disturbance of clear cutting and industrial forestry can lead to large soil carbon releases. A classic article in the genre is "Effects on carbon storage of conversion of old-growth forests to young forests" (Harmon et al. Science 1990) . They state "Simulations of carbon storage suggest that conversion of old-growth forests to young fast-growing forests will not decrease atmospheric carbon dioxide (CO2) in general, as has been suggested recently.". For more recent work, take a look at what Leighty et al. (ECOSYSTEMS Volume: 9 Issue: 7 Pages: 1051-1065. 2006 ) have to say about the Tongass:.
- "The Tongass National Forest (Tongass) is the largest national forest and largest area of old-growth forest in the United States. Spatial geographic information system data for the Tongass were combined with forest inventory data to estimate and map total carbon stock in the Tongass; the result was 2.8 +/- 0.5 Pg C, or 8% of the total carbon in the forests of the conterminous USA and 0.25% of the carbon in global forest vegetation and soils. Cumulative net carbon loss from the Tongass due to management of the forest for the period 1900-95 was estimated at 6.4-17.2 Tg C. Using our spatially explicit data for carbon stock and net flux, we modeled the potential effect of five management regimes on future net carbon flux. Estimates of net carbon flux were sensitive to projections of the rate of carbon accumulation in second-growth forests and to the amount of carbon left in standing biomass after harvest. Projections of net carbon flux in the Tongass range from 0.33 Tg C annual sequestration to 2.3 Tg C annual emission for the period 1995-2095. For the period 1995-2195, net flux estimates range from 0.19 Tg C annual sequestration to 1.6 Tg C annual emission. If all timber harvesting in the Tongass were halted from 1995 to 2095, the economic value of the net carbon sequestered during the 100-year hiatus, assuming $20/Mg C, would be $4 to $7 million/y (1995 US dollars). If a prohibition on logging were extended to 2195, the annual economic value of the carbon sequestered would be largely unaffected ($3 to $6 million/y). The potential annual economic value of carbon sequestration with management maximizing carbon storage in the Tongass is comparable to revenue from annual timber sales historically authorized for the forest."
So, it looks like that old Spotted Owl and its kindred old-growth denizens are in fact sitting not just on a nest, but on a treasure trove of carbon credits worth potentially more than the timber harvest.
And should you keep that SUV? This blurb in fact contains some useful advice, buried amidst some fuzzy reasoning and published over a witless tag line stating that "pound for pound" a Prius takes more energy to manufacture than a Hummer. The apparent implication of that tag line is rebutted in the article itself, but why give the reader that as a 32-point type take-home point when the WIRED editors don’t even themselves believe it’s an important statistic? This factoid refers to the energy used in the nickel component of Prius batteries, but it’s irrelevant because "pound for pound" doesn’t count if your point is moving 4 people from point A to point B. What transport value do you get from transporting four people plus the weight of the Hummer? Now, the rest of the fuzziness in the logic is a bit more subtle. The author notes quite rightly that there is a very significant carbon emission from manufacturing a car, which is indeed more for a Prius (at least for the moment) than it is for comparable sized non-hybrids.. Thus, if you are faced with ditching your existing car (whatever it may be) and buying a Prius, you need to consider how much you drive per year and see how long it takes to "pay back" the carbon emission from manufacturing the Prius. So far so good. But this is more a statement about the transition to more efficient cars, and how to deal with mistakes of the past, rather than a statement about what is intrinsically desirable in the fleet. As far as carbon emissions go, we’d still be better off if everybody who needed a car were in a Prius, except maybe for people who drive very little per year — who should then be into shared hybrids via iGO or ZipCars, Maybe if you drive very little and live out in a rural area where there are not going to be any shared cars, getting a compact non-Hybrid might make sense. There must be at least a dozen or two people out there in that category, I guess.
The rest of the advice WIRED gives makes even less sense. They say that if you want to be green, you ought to buy a used Civic or something like that, not a Prius. That’s because the used car already has the manufacturing carbon emissions "written down" (or, I guess at least the carbon guilt accrues to the original owner, not that the atmospheric radiative forcing is going to care much about that). However, this advice, sensible-sounding though it is — ignores the fact that to make that used car available to you, the original owner almost certainly had to buy something else, and probably that was a new car, or at least a newer one. So, for the scheme to work, you’d have to buy your used Civic from somebody who was giving up driving altogether. I no longer own a car myself, but I’m sorry I wasn’t able to participate in a scheme like this; by the time I gave up our remaining car ten years ago, it was suitable only for the crusher, and in fact had to be towed there.
The real implication is that manufacturing costs count, so most people should buy a small, efficient hybrid and keep it until it runs into the ground. The implication is also that durability of cars counts for nearly as much as gas mileage, since an efficient car that needs to be replaced every five years isn’t really all that efficient.
Along with all the nonsense is a certain amount of true (if by now commonplace) advice. Among this is the basic truth that urban living is inherently green, and if more people lived in cities (and if more cities were kept livable so people would want to move there). then per capita carbon emissions would go down. Even there, the Economist managed to be both more informative and more iconoclastic with its surprising analysis of the pattern of urbanism in Los Angeles. The other truism in WIRED is that nuclear power deserves a second look, and probably has an important role to play in a decarbonized energy future. Still, if you compare the cost of making all those chilly New England homes efficient with the total true cost of building more nuclear plants, well, let’s just say I’m buying stock in argon-filled low-e window manufacturers rather than Areva, much as I like their track record on nuclear electricity.
Pardon the digression but — here’s a link for bicycle transport; the “Work Bikes” links in particular are innovative and practical:
http://www.sfbike.org/?racks
and trailers: http://www.sfbike.org/?trailer
This in particular is tempting us: http://www.xtracycle.com/
Recommended reading: a good article on concentrated solar power (CSP), also known as solar electric thermal:
With centralized utility-scale CSP and wind power, combined with distributed rooftop photovoltaics and small-scale wind power, and the improvements in efficiency that are fairly low-hanging fruit, we can phase out both coal and nuclear for electricity generation.
There’s another assymetry between heating and A/C.
Heating:
If it’s cold, heat losses from buildings (slightly) warm other buildings, so on can be grateful for a nearby building with bad insulation. I.e., a group of close buildings are at worst neutral, and may even help each other.
Cooling:
Running air conditioners heats the surrounding air, which makes everybody work harder, added on top of any other UHI effects. I.e., neighboring buildings work against each other.
We don’t have A/C in our house [don’t need it], but drive 5 miles down the hill to Palo Alto, and it is noticeably hotter. Although it’s not that dense, one can still feel the effects of the massed air conditioners along the more commercial streets.
Good passive building design [and roofs, and more reflective streets] are really big wins in sunny areas. We recently put in reflective insulating blinds on all the floor-to-ceiling glass, and they really help.
RE: 100
I think I understand the point you are making in 85. To paraphrase what I am getting from the most recent statement is, the combination of heat content and heat potential with cooling you have heat potential and cooling. Meaning, that cooling somehow offsets one of the heating content values. My issue is that I do not think the heat content is offset. What I see is that the heat is not being “removed” rather only “moved”.
As to 89 to have a radiant energy of 20 Degrees would be dependent on the black body having a certain heat content/unit of volume. For a black body to to have a 22 Degree emission it would have to have a higher heat content per volume. If the heat content were constant and the volume changed I expect the radiant value would change. The point I am trying to address is if you increase the heat content and the volume were constant then the intensity of radiant of rate of emission/decay of heat content would likely increase. However, if the volume or emission surface were to increase with the increase in heat content would that not suggest a constant radiant emission/decay rate?
I guess I am having a hard time with the concept that you are suggesting that the path between your house and space is immediate. Simple observation suggests that even if there were no GHG that there is water vapor and it could delay the release of radiant heat from your house. In the case of the wet/dry adiabatic transition zone if the altitude increased due to added heat content, would that not be similar to the volume of the black body increasing in volume
and hence moderating the radiant decay/emission rate?
(Note: Correction to my earlier post (96?), the correct terms apparently should have been exothermic and endothermic in that order, my error.)
Cheers!
Dave Cooke
Re Andrew @ 97: “However, has anybody ever considered that to purify the silicon for the typical PV panel requires significant electrical energy that comes for the most part from coal fired plants?”
For now. Not to mention transported and installed using petroleum fuels. And so it will remain until we get serious about replacing fossil fuels for power and transport with renewables and non-fossil sources, and thereby reduce the CO2 impact of the manufacture, transport and installation of PVs, windmills, and everything else that we manufacture.
In other words, the solution to the situation you point out is to install more, not fewer PVs. More, not fewer windmills. More, not fewer geo-thermal.
In the meantime, have you ever considered that as both petroleum and coal become ever more expensive, so too will the cost of manufacturing and installing their renewable replacements? And that if we follow the advice of those urging that we wait before acting, we simply may not have the means to manufacture the replacement to fossil fuels?
Re 91: “Some musings on geothermal (aka earth energy) heating/cooling systems. One could conceivably do even better than using the 55F deep ground as a heat source/sink by storing the winter cold and summer heat.”
These systems effectively do that to the extent that is possible. The issue is that that for these systems you normally have a large reservoir of that 55 degree heat that you can’t store much of a change to that for very long. So you won’t be storing heat from summer to help in winter. You might get a little diurnal effect, but this comes down to the particulars of the well or ground loop design.
RE: 98
The vehicle below could solve your parking problems…
http://www.allwebscooters.com/0536.asp
As for parking it is only 29 inches wide so you could actually drive it right in your front door, unless you have steps or the elevator is not a minimum of 72 inches deep…. Of course would have to modify it with a larger battery/motor so you can go faster then the 7 mph it is rated for.
Something more along the lines of a velocar might be an option. The Corbin Sparrow may still be in production if you were interested. This could be parked in the neighborhood Bike rack possibly…
http://www.microcarmuseum.com/tour/corbin-sparrow.html
( Other examples: http://www.microcarmuseum.com/tourindex.html )
Point being the Rhoades Car or the International Surrey Cars are both pedal cars and can be fitted out for an electric assist motor if desired; but, as you say they are not as small as the microcars above and the parking for the “bikes” would be expensive.
http://www.rhoadescar.com/jumplobb.htm
http://worksmancycles.com/shopsite_sc/store/html/page37.html
The main point if these cars had been produced so long then why are they not here today? The same goes for the old Citreon CV-2.
Primarily, the infrastructure of a large manufacturer is not there to support them. In short, the whole of the issue is economics. As long as it is not cost effective for a large durable goods manufacturer to produce many low margin products they will push for low volume, high margin vehicles like the Hummer.
Cheers!
Dave Cooke
Nylo, think about this–if your house is losing energy to space, that means you are using more energy to keep it at the same temperature, right? Since at least some of the energy lost by your house will be absorbed by greenhouse gas molecules, the net energy is still on the + side. With air con. it takes energy to cool a house–you are transferring energy against entropy, so the net energy will be more than the cooling achieved. Now if your energy source is fossil fuels, your little gift to the atmosphere will keep on giving for hundreds of years–and if you integrate over that time, the energies invovled will be dwarfed by the greenhouse effect of the CO2 emitted.
Re: Comment moderation. I’ve also been very frustrated by this because it slows the conservation to a crawl and makes it impossible to respond quickly to other comments. You should allow all comments from regulars to post immediately and only moderate first timers. This should effectively delete all spam and trolling *and* lower your moderation time by 95%.
In North America it is a serious mistake to claim that natural gas is better than heating with electrical power. Natural gas in NA is in terminal decline, of about 3% a year http://canada.theoildrum.com/node/4073, http://www.theoildrum.com/story/2006/11/27/61031/618, http://www.theoildrum.com/tag/natural_gas_peak.
The best way to heat and cool your home is with a ground source heat pump. It will cut your heating costs by as much as 75% and your cooling cost by 50%. They use electricity, but your over all energy consumption is much less. Plus you get off a fuel, natural gas, that will soon be in very short supply and very expensive (up 100% so far this past year).
Am I right, that the major weakness with these is that it still takes a lot of energy from some source to compress the air ? Or is there some other problem ? They were invented years ago, so I wonder why they aren’t common. Cheaper than a Prius.
http://www.youtube.com/watch?v=QmqpGZv0YT4
Re: Talking about heating/cooling and assymetric equations.
It takes approximately 1000 litres of water to grow the firewood equaivalent of 1 kg heating oil(2 g carbon/kg water), but only the evaporation of 20 litres of water to tie up the heat energy equaivalent of 1 kg oil(latent heat of vap 2.27(MJ/kg)/heat of comb. 44(MJ/kg)).
It ought in other woords be 50 times less water expensive to cool an house in the desert in a carbon neutral fashion, than heating an house in an cold, wet boreal forest.
They missed one. Accelerating faster is more efficient than accelerating slowly.
According to Swedish fuel efficiency experts in the NY Times:
Re Richard Wakefield @110: “The best way to heat and cool your home is with a ground source heat pump.”
I will agree with Richard on this one, and add to it the use of a solar-thermal water heating system, supplemented by an efficient on-demand water heater if needed. Together, ground-source geothermal and solar-thermal hot water can reduce household carbon footprint by around 70% if gas is currently used for both heating and hot water. Unfortunately, ground-source is fairly expensive to install and can be difficult, though not impossible, and thus even more expensive to install in built-up urban areas.
And gas prices are probably causing more consumption, not less.
Looking at the Energy Information Admistration data on consumption (http://tonto.eia.doe.gov/dnav/pet/xls/pet_cons_psup_dc_nus_mbbl_m.xls), fuel consumption is down less than 1% versus driving being down 4.3%.
That is a huge decrease in efficiecy which cannot be explained by people eliminating their most efficient driving (the efficient driving cut would need to be several times more efficient than average driving).
In addition to people acting on bad information, Giffen Behavior may be to blame.
[Response: Huh? How can consumption be down and yet have increased because of high gas prices? Doesn’t quite meet the logic standard. Consumption does appear to be sensitive to price signals – they go up, it goes down. – gavin]
RE: 110
The closer you are to the energy source and the less conversion involved the better. You have to consider where that electricity will come from today, it will not be solar and nuclear will remain a limited option. This leaves a choice of coal or natural gas.
As to the future limitation of natural gas supplies, you are correct in relation to gas wells, as far as known current US reserves. However, when you consider the opportunity for Bio Gas or possibly non-food cellulose pellet gasification option the possibility of developing renewable methane sources as a production support for the natural gas industry fits like a hand in glove option. Methane is methane regardless the source, the more renewable, the less carbon from earlier Epochs are required to maintain the current world population and the less fossil fuels being introduced to the current Epoch.
Until such time that NASA and the DOE develop a relatively safe highly efficient Radioisotope Thermoelectric Generator out of retired nuclear fuel pellets we really do not have a good current technology alternative. As much as I like your Geothermal option, the electricity required to drive a water to air heat pump is not trivial. Even with an artesian well it still requires a minimum of 30 amps/1000 sq feet or 12 amps/ton of hvac.
Though an Absorption refrigeration system would use as much as 500 cu feet of methane per day to cool 1000 sq feet 12 Deg. F. It takes about 720 cu feet per day to heat the average 1000 sq foot home 38 Deg F. The equivalent Heat Pump would require the equivalent of around 680 cu feet of methane to both heat and cool the same structure. The problem is that the conversion of the Natural Gas to Electricity means you would have to double the combustion of natural gas at the source to go from chemical to mechanical and then a tripling to make up for the conversion from mechanical to electrical with another doubling to make up for the line losses before it reaches the heat pump. (This equates to roughly 8200 cu feet of natural gas to power your heat pump for one day…)
As this is a back of the post card estimate it might be more appropriate for a professional to work out the figures. However, I figure I am within the ballpark, (though there might be a question of which city….)
Cheers!
Dave Cooke
Consupmtion is down far less than driving is. Consumption doesn’t look to be down significantly. We’re getting far less out of our fuel. See more here.
Positive feedbacks, unintended consequences and all.
RE: 116
Ooopps, grabbed the wrong value for the Natural Gas direct heating. I should have used roughly 70% (or 74 Deg. F) of the average 2400 cu required to maintain a well insulated 8500 cu foot home, with @30% RH, at 90 Deg. F when it is 35 Deg F outside or @ 1680 cu feet and not the 30% (720 cu feet) I used.
(0.7(max temp – min temp)+ min temp)= @ 73.5 Deg.
0.7(est. of 2400 cu feet to keep a 1000 home at 90 Deg. F on 35 Deg. F day)= 1680
Best guess is the 720 cu feet would keep a 1000 sq foot home at @ 51 Deg. F on a 35 Deg. day) Sorry, I try to get it right, most times I catch it before it goes out…
Dave
re the compressed air car (comment 111): Of course the energy needs to come from somewhere, but the real problem with powering a car on compressed air is that the energy density of compressed air is very low. At 30MPa, the energy density is 0.17MJ/liter, assuming isothermal compression. Liquid hydrocarbon fuels have an energy density of about 30MJ/liter. So even if your air car is very efficient, it will have no where near the range of a standard car. The only way they get them to look good in the glossy promotional material is by making the car very light, and driving the car slowly on flat roads. You can apply the same strategy to petrol driven cars to get extremely good mileage. For an extreme case, Google “shell eco-marathon” – gasoline powered vehicles can do better than 10000mpg.
For comparison, lead acid batteries have about the same energy density as compressed air, which is the major reason battery powered cars have never taken off.
If there is room for any more nails in WIRED’s coffin, it sounds like their argument also skips completely over the real world issue of latent heat (to be honest, I didn’t read it, I’m assuming this from the above comments and summary). Although the cooling load in Arizona would be dominated by the sensible heat load, a quick jaunt over to the southeastern US (or any other humid region of the world) would result in a cooling load that is dominated by the latent heat.
A simple analysis of the summer heat loads in the southeast US will show that in this region about 2-3 times more electricity is required to remove moisture than is required for the sensible cooling on a typical summer day. There is no counterpart to this latent heat in the winter heating load (although very cold, dry regions of the world do have to humidify in winter. Someone else will have to post with regards to the efficiencies on this and the percentage of heating load that is consumed this way).
In humid climates (assuming conventional HVAC equipment), the first step in controlling the humidity is to subcool the air so that the excess water vapor condenses out (without this step, your final air will be saturated–not fun to work in and devastating to sensitive electronics.) After the excess water is condensed, guess what the next step is? Reheat to final setpoint. Thus, to cool our saturated summer air by 5C we first have to cool it about 10C and then reheat it 5C (actual numbers vary with air properties, but you get the idea). To heat it 5C (in this region) we simply heat it 5C.
Thus, trying to compare a theoretical 5C sensible heating vs 5C sensible cooling load may be an okay place to start if we are talking Arizona, you would find that such a simplified analysis would miss your actual energy consumption (and CO2 footprint) badly in a more humid region of the world.
113 has a valid, and largely misunderstood point. Moderately fast acceleration is best. The tradeoff (aside from time), is getting into your highest gear quickly (much of the internal losses are in the engine), but slow down gradually. In a conventional vehicle, slow deceleration, means you are dumping less energy into your brakes, on a hybrid, the slower charging wastes less energy due to resistive losses which scales as current squared.
But their is a very strong case to be made, that the total world oil consumption is supply limited. This implies it will all be used, whatever an individual or even a country does to conserve. The real purpose for oil conservation is to maximize economic utility of a scare, and vanishing resource. A similar argument can probably be made for natural gas. Coal, at least the lower grades, will probably not be totally consumed, that is the emissions battleground that we have a chance to effect.
[Response: The sooner the conventional oil is used up the sooner people will turn to tar sands and coal-to-oil in a big way. But I have indeed heard economists make the argument that conservation of oil makes little difference since the worldwide demand is so huge that reductions in usage here just lower the price and lead to increases in usage elsewhere. I’d like to see some numbers put on that argument, though. –raypierre]
Pete Best has commented twice in this thread to the effect that there isn’t much use using a Prius on the highway. First off, his comment as written is a bit vague, but let’s assume he means that it loses most or all of its advantage over a median-mileage car when driven for hours at, say 65 mph. This statement, or anything close to it, is so wrong as to be cuckoo. You can go all day on the interstate, using heat or AC if needed, at about 50 mpg in your Prius, or anyway in my Prius. Most other vehicles on the road don’t come close to that. Of course, increasing to 75 mph loses mpg, but the other vehicles have a corresponding loss from a lower base.
Yes, it’s possible to do even better than that around town, but the average figure covers a huge variation from various causes, by far the most important of which is getting the engine to full operating temperature. That is, the first 5 minutes are bad, winter is bad, and the first 5 minutes in winter are terrible. (Terrible for the Prius being probably around the long-term average for the median car in USA.) This means the long-term average for a given Prius has a huge dependence on usage, notably length of commute. (Short commute of course is still lower fuel *usage*, and bike/walk is still none.) Operation as a taxi is ideal (warm all day).
Long trips on the highway fall short of the ideal mpg, but benefit from the warm engine in a way impossible for short commutes and other errands not bunched together.
#78:
The optimal rate should be close to the rate that old (too old to drive) cars get scrapped. In fact, if all cars are worth driving for as long as possible, the two rates should be almost the same (more new cars will be made, because the total number of cars is increasing). Other factors could affect the optimal rate–if most people cannot afford the more efficient car, then the optimal rate would decrease, because otherwise the people would not be able to pay for the things they need. Also, if the price of fuel increases enough, the optimal rate would be huge.
Andrew (#97) You must not have read the earlier posts. A large solar PV manufacturing facility can easily be powered using solar and wind power systems – meaning no carbon footprint at all.
In terms of reducing fossil carbon footprint, you always have to look at how to eliminate fossil fuels in each sector of the energy industry. Obviously, coal has the lowest energy delivery per ton of CO2 emitted (many carbon-carbon bonds, which store less energy than carbon-hydrogen bonds), and methane has the highest (four C-H bonds per carbon atom) – thus, coal should be the first fossil fuel to be phased out and replaced with solar and wind sources.
The main problem with nuclear is really the economics. Everyone knows that fossil fuel prices are skyrocketing, but so are uranium prices (ten fold increase over the past few years.) In addition, investments in nuclear power will not generate a steady increase in energy supply – but investments in solar PV manufacturing and wind turbine manufacturing will.
As far as energy demand in a house or other building, the design elements are critical. Passive solar, solar water heating, and solar PV all work best when incorporated into the design at the very beginning, rather than being added as an afterthought.
Re #122. I believe that crusing around all day in a diesel car gives around 60 MPG at 50 to 60 MPH because I do it all the time. The prius us a pollution saver and not a miracle cure for climate change. You can compare many European cars driven sanely against cars or vehicles in the USA where 40 MPG is an amazing feet whilst 50 MPG over here is more common as we are more likely to be driving efficient cars. Maybe Americans will now to.
I believe that the prius does around 500 miles on a tank whilst my car does around 600 I believe. Sure Diesels have a nasty little secret in black carbon and pollutants that the prius avoids but open highway driving lugging around a battery as opposed to driving around town (so called urban cycle) aint gonna save us.
The prius costs £18,000 (UK sterling) which is not cheap either but maybe the price would come down once millions were made. Lets hope so because the Prius and its type of car will be the future I have no doubt but small petrol and diesel cars will have their place to.
The USA could halve its gasoline consumption on a few short years but taking to smaller cars with 50 to 60 MPG no matter what they run on.
(posted again because of problems with “<” symbols)
Re 104 David Cooke:
Sorry, but I cannot agree with something I believe you are saying. Although maybe I am understanding you wrong.
Redistributing the heat content in a black body without modifying the total heat content DOES affect the ammount of energy radiated to the exterior, and therefore will slowly reduce the total heat content of the black body more quickly. For the same temperature average, the more extreme temperature differences at the surface of the black body at a given moment, the more energy it will radiate. You only have to do the maths. T^4 + T^4 < (T+t)^4 + (T-t)^4, for any positive T you choose and any positive t < T.
Re 108 Ray Ladbury:
Yes, I agree. CO2 effect is more important than any direct heating because the CO2 effect, even if it means fractions of miliwatts of heating in a reduced area, will do that for a very long time so that the total energy introduced is big. In the short term the warming created by the heting systems may be bigger, but that’s not a problem, since it’s the desired effect at that moment and place. The warming is only a problem when or where you don’t want it.
“Short commute of course is still lower fuel *usage*, and bike/walk is still none.”
Not quite. I have to eat more when I cycle to work. ;-)
pete best #22:
“The toyota prius only really makes a difference on the urban cycle because as soon as you go out on the open road (freeway/motorway) etc it switches to its petrol engine and you end up dragging a battery around for no good reason”
This argument is heard very often and it is wrong.
Firstly: with a constant speed on the highway, the weight of the battery only adds a negligable amount to the consumption.
Secondly: the Prius has a small engine (1.5 l, 57 kW). Because it is so small, this engine can be operated closer to peak efficiency than in the case of an ordinary car. The reason why it can get away with such a small engine is the fact that the electric motor provides extra power in case of acceleration.
So a normal car is dragging along an extra litre of engine displacement, the Prius is dragging along a battery and electric motor. The latter is more fuel efficient.
You can not simply compare the Prius to a Prius without the electric stuff. You will need to compensate for the loss of power with a bigger engine.
Nick Gotts writes:
The repeated “Americans are only 5% of the world’s population but uses 25% of the resources!” ignores the fact that America PRODUCES 25% of the world’s resources (as actually used, not as background stores).
pete best (again) #125:
“European cars driven sanely against cars or vehicles in the USA where 40 MPG is an amazing feet whilst 50 MPG over here is more common as we are more likely to be driving efficient cars.”
This 50 mpg, is that diesel or petrol? Aren’t you confusing American gallons and imperial gallons?
I know of no petrol car apart from a few very small ones (eg. Daihatsu Cuore, Suzuki Alto) or hybrids (Prius, Insight, Civic) that manage 50 American mpg’s.
In your enthusiasm of comparing mileages you seem to be mixing up diesel and petrol. Diesel is not petrol. Diesel is a denser fuel. Per litre it contains more energy and produces more CO2 emissions. In both cases around 12% more. So a 50 MPG diesel is comparable in energy use and CO2 emissions to a ~44 MPG petrol car. To beat a 50 MPG Prius, you would need a ~56 MPG diesel.
One final remark: I am completely ignoring financial aspects. This is purely about efficiency and the environment.
#129 [BPL] Nice of you to truncate my text so as to remove the main point: “including its ability to absorb pollution”. What proportion of the world’s GHG production is down to US consumption is difficult to calculate, but I’d be surprised if it is less than 25%, given the amount of flying, driving, air-conditioning and meat-and-dairy eating involved, and the recent export of much of US manufacturing industry to energy-inefficient, largely coal-powered China. Of course, western European, Australasian and Japanese consumption styles are only somewhat less unsustainable.
“ignores the fact that America PRODUCES 25% of the world’s resources (as actually used, not as background stores).”
Hmmm. Look at your stuff.
Made in China.
Which has always naffed me off about the “first world” complaining about China creating so many new coal power stations. We in the west outsourced all the manufacturing to China and got a nearly zero change in power requirement growth. China hasn’t yet managed to get to our level when we were doing the manufacturing. So why are we complaining about their increased use? We’ve placed demand for power on them!
A further point about A/C versus heat pumps.
The COP under ideal conditions for cooling and heating using the same heat pump are related as follows:
COP(heating) – COP(cooling) = 1
(see: http://en.wikipedia.org/wiki/Coefficient_of_performance)
For the same amount of work a heat pump will deliver more heat than it will remove in reverse as the work it does must be added to the heat flow – in or out.
This means the SEER(heating) will under ideal conditions exceed the SEER(cooling) by 3.43 units, given the same temperature differential. If anything is intrinsically more efficient it is using a heat pump.
Re 114: I’m getting one put in my place, getting off NG. 10 verticle holes 100ft down. The system will cost about $25K, but at current prices pay back can be as low as 7-10 years. Priceless when NG supply drops. The system also suppliments the electric hot water system, and it will also supply hot water to heat my greenhouse allowing me to grow food all year.
Re 116: Your math is not correct. Geothermal systems use far less energy that convensional heating systems, up to 75% less. http://www.waterfurnace.com/.
Second, where I live in Ontario most of our electrical supply comes from Niagara (renewable) and nuclear, and the province just announced the construction of new nuke reactors. So we are set.
I had a post either get eaten, or rejected.
My comment back in #42 was alluding to the fact that what I see happen when someone proposes something, such as with the WIRED article, is that any “inside the box” thinking — old school behaviors and the like — are strictly limited to “inside the box” solutions. A/C is less energy intensive than heating? Well, it’s true. AND with green power solutions it’s also less difficult to deploy. Driving old fuel efficient cars less energy intensive than scrapping them and buying new? True as well — especially if the scrapped cars are recycled for parts to keep them running, rather than ground up and turned into new metal for new vehicles. The hardest part in keeping an old car going is parts, and the largest supply of parts is the cars themselves.
But if someone suggests distributed generation, well, OF COURSE we can build a high voltage DC distribution system, completely re-engineer the grid, tens of millions of homes with solar power (forgetting about trees …), hundreds of millions of cars plugged into the grid, sucking down 20 or 30KWH every night for a recharge.
It’s the “outside the box” solutions that have the lowest probability of being implemented. There’s wide-spread deploying of CFL lightbulbs and more businesses are doing demand-response to reduce peak loads. None of these “mostly inside the box” solutions require new technology. Higher efficiency air conditioners, such as the Lennox I recently installed, can be legislated into use, if consumers don’t get religion on their own. Higher fuel economy cars are available — SUVs only sneak in because they are “trucks”.
We have “inside the box” today, and we aren’t make much use of it. Businesses still pollute the night sky like crazy for no particularly useful purpose, rich people drive gas guzzling cars just because, fossil fuels are used to produce packaging that will be thrown away and never recycled, reusable grocery sacks are $0.99 but the grocery doesn’t charge for the throwaway. There is a lot that can be accomplished “inside the box” that doesn’t require technology that doesn’t even exist yet.
I’ve been scolded in the past for mentioning that gas powered heat pumps exist: http://www.columbiagaspamd.com/products_services/natural_gas_heat_pump.htm
They don’t do all that well since a local gas engine does not get the same efficiency as a combined cycle turbine. Heat pumps will probably soon be the cheapest form of heating unless you have your own woodlot. One way to start converting (or diversifying) is to consider zone pumps. This company gets some good recomendations from tiime to time: http://www.fujitsugeneral.com/wallmountediaq.htm
Working in the car industry myself, I really don’t get the American excitement about the Prius. Not only are there a whole bunch of cars emitting less CO2 (yes,most of them are Diesels) in real world driving, but everything else you may expect from a car (value for money, active savety, comfort, transport capacity, use of recycled material) rates the Prius nowhere near the top. Even on this website, the term “Prius” is almost used like an overly simplistic symbol rather than what it is: a car burning fossil fuel to get you from a to b – probably a little less than some others and probably a little more than the best. Toyota really must have done a great job in terms of marketing.
With regards to using solar and wind to be “carbon neutral”: First it takes energy to make these products, but more important is that they are extremely limited and intermittent. With solar you not only have to have enough panels to operate your home, but double that many to charge the batteries. I’ve looked into this for myself, and just for my 1000 sqr ft home it would require 20 panels and some 60 batteries. And that would give me just 20% of my juice demand. Cost: $25K-30K. The other limitation is during the winter we have periods of WEEKS without any sun, just cloud, not including days that are only 8 hours long (so you need even more batteries and more panels to charge them.) The other limiting factor is batteries last only 7 years and have to be completely replaced.
As for wind, there is a huge misconception with their output. The output you hear about is name place capacity. That is a 1.5Mw turbine is only that output at maximum wind speed (50-55km/h). Output drops as the cube of windspeed, hence at 25km/h output is one eighth. Below 15km/h they produice nothing. In a two year period all of Ontario’s wind turbines spent 50% of their time below 13% output, 5% of the time producing nothing and never produced name plate output. This report to the Ontario Government http://www.ieso.ca/imoweb/pubs/marketreports/OPA-Report-200610-1.pdf claims that when wind power is needed the most, when demand is within 10% of peak, wind turbines only produced 13-17% of name plate. Thus for Ontario to get 15% of its electrical supply from wind turbines would require the construction of some 77,000 of them and take more than 100 years to build.
This looks like a good starting point for further discussion around economic models. A challenge appears to be that the economics of combating global warming are always portrayed as a negative (i.e. cost). Maybe a RealClimateEconomics that focuses on economic impacts of global warming and counter a lot of the media hype around $45 trillion costs with no mention of benefit. Maybe some suggestions for states and governments to adopt to improve their economic status.
For example, make it mandatory to perform all road construction at night. So instead of sitting in miles of traffic while some road crew fiddles with a median, make them do it at night and let the traffic flow. It would likely increase productivity by a significant amount, save on fuel and lower emissions.
Focus on the easy win-win scenarios first. Transportation might be a good one. A national power grid might be another with many suppliers of many different fuels sources. The grid is probably more important first than developing some radical new energy source. Further, tie the grid into transportation and start moving towards electric cars, trains and trucks.
I’m all for capitalism, but it’s a balance between long term planning vs short term gains. I bought a Prius because I’m voting with my dollars to invest in a longer term strategy and I’d like my grandchildren to have the freedom to drive (50 mpg is nice too and by the way I get 50 mpg going on long highway trips for several hours at a time with no compromise in pace and comfort). Toyota is number one in the world because they look 25 years down the road not 5. If I’m an oil company now is the time to invest in the future technology, while my profits are peaking not later on when I’m desperately scrambling for oil and I don’t have the money to take any risks.
Ric Merritt wrote: “You can go all day on the interstate, using heat or AC if needed, at about 50 mpg in your Prius, or anyway in my Prius. Most other vehicles on the road don’t come close to that.”
I can go all day on the interstate in my 17-year-old 1991 Ford Festiva at 50 MPG, using heat if needed … it doesn’t have air conditioning, but I’ve never felt the need for AC at highway speeds with the windows opened a little.
The fact is, for 20 years the automakers have had the technology to build 50 MPG or better conventional gasoline-fueled cars that are inexpensive, durable, reliable, safe and comfortable. They chose not to do so because building and selling such cars is not very profitable. There is a lot more profit in a $50,000 eight-mile-per-gallon Ford Extinction than in a $5,000 fifty-mile-per-gallon Ford Festiva. Hence the massive advertising campaign, using the most powerful bainwashing techniques ever invented by Madison Avenue, to convince Americans that they “want” huge, expensive, gas-guzzling SUVs.
Barton Paul Levenson wrote: “… America PRODUCES 25% of the world’s resources …”
Exactly what are these “resources” that America “produces” ?
FWIW, I had to replace my 20-year old gas furnace and central AC last year. I could not find anyone in the Washington, DC area who could even give me a price quote on a ground source heat pump. So, I got a conventional, high-efficiency above-ground heat pump and now have an all-electric HVAC system. I replaced the gas water heater with an electric water heater at the same time. I still have a gas clothes dryer and stove which I plan to replace with electric eventually. Combined cost of gas and electricity has been about the same since this conversion — somewhat lower during summer simply because the new system is a much more efficient AC than the old one. But the winter heating bills are no more than they were with gas. I expect that gas prices will rise faster than electricity prices in the coming years, perhaps dramatically so. The local (PEPCO) electric utility’s “standard mix” is about 55 percent coal-fired and 35 percent nuclear, with natural gas-fired electricity making up most of the rest. However, they let you choose your own electricity supplier, and I have chosen 100 percent wind power (not “offsets” as I understand it, but electricity purchased from actual wind farms in the mid-Atlantic region) which is somewhat more expensive than the conventional mix.
Conservation of oil in the US means less sucking up to foreign oil potentates. If only marginally so. Less likelihood of hideously wasteful wars.
Worldwide demand will always be out of our control.
With solar panels, it all depends on efficiency of conversion and energy storage. Top-performing and expensive satellite cells are currently at something like 40% conversion (meaning at the equator, with 1000 watts/square meter at noon, they would deliver 400 watts per square meter).
Cheaper multicrystalline silicon panels operate at 12% or so, though Kyocera’s new prototyoes perform at 18.5% efficiency. In any case, silicon panels are the way to go. The other thin film cells (i.e. the cadmium selenide variety, etc.) use highly toxic materials, have shorter lifetimes, and lower efficiency as well.
http://www.solardaily.com/reports/Kyocera_Reduces_Solar_Cell_Thickness_999.html
The key approach in using renewables is to have multiple energy sources. In the absence of fuels like uranium and fossil fuels, the only energy sources are sunlight, wind, geothermal and biofuels. All those energy sources have to work together to manage energy supply and demand, and that takes an integrated grid that can deliver solar energy during the day, biomass or wind energy at night, as well as store any excess generation for later use.
Remote, stand-alone solar installations also work fine – but that’s a smaller market. However, this is how remote microwave repeater stations work, that’s how Coast Guard and other marine buoys communicate, that’s how all the satellites are powered, and it is also a perfect approach for providing local power for water pumps and lighting for isolated Third World villagers .
For a good discussion of how to do this, look at what Germany is doing: phasing out coal and nuclear and bringing online a threefold approach using solar, wind and biogas:
http://www.youtube.com/watch?v=tR8gEMpzos4
P.S. Realclimate’s comment approach shouldn’t be changed. “Rapid response” or “preferred commentators” are very bad ideas. If anything, you might want to consider raising the standards a bit higher – for example, require references for unsubstantiated claims.
The following website has a lot of interesting and useful data on energy consumption in the U.S. and worlwide:
Energy Information Administration (EIA) Official Energy Statistics from the U.S. Government
http://www.eia.doe.gov
I would be curious to know if any of the information published on this site is known to be incorrect or misleading. I raise this concern because I just read in the June 6 issue of Science that the NASA Inspector General has confirmed that ‘a few key senior employees’ at the agency’s press office suppressed discussions of climate change research by James Hansen and others (and confirmed that Hansen was barred from speaking to National Public Radio). The IG found no evidence that research activities at NASA research centers were suppressed, though.
RE: 135
It appears you are correct. I attempted to correct a portion of my calculations and did not go far enough. However, I fear you may have missed the point I was attempting.
Though the individual hvac unit has a higher efficiency then the equivalent air-air heat pump, a water-air heat pump, (Such as I used for 18 years in Florida.) unit still requires electrical energy to run the circulator and heat exchanger. As to the direct use of thermal energy from chemical energy, such as in Absorption and Pulsed Combustion is a far more efficient conversion of fossil fuel to the desired function.
Keep in mind that other then the air handler (common to all modes of hvac) the electrical demand is next to nothing for these technologies. (Only a few 10s of watts for the controls.) Water to Air systems and even a geothermal system requires several hundred watts for pumps and the heat exchanger.
The point I was attempting to make was the total amount of CNG that would need to be consumed to deliver the same equivalent energy for a small home is less with direct heating/absorption cooling versus the electrical energy spent to run the water-air heat exchanger.
As to your power plant energy sources, bully for you, most of the rest of us do not have this opportunity and rely on primarily coal fired and CNG augmented power plants. The Nuclear option is only about 12% of the electrical resources here in the US. If we moved into the breeder design and modified the extraction technique it is likely Nuclear could make up to 50% without much issue, other then the economic requirements to build enough systems meet the demand and deal with the wastes.
One of the prime issues with implementing Nuclear is the inability to throttle up and down significantly. The lower the operating rate the lower the efficiency. If you used Nuclear to provide a base and augmented it with CNG to meet peak demand would actually be the most effective systemic design. The use of Nuclear augmented by CNG/Bio-Gas or cellulose gasification could offset the economics and reduce the Fossil Carbon combustion by greater then 60%.
As to the use of the Niagara, since the Falls are there are so damming the river it is not an issue; however, damming most rivers is considered bad environmentally. Anytime you place a barrage across a body of water you are isolating the species on either side. Our recommendation would be vertical axis devices lining the banks, with a clear channel along the tributary for wildlife and transportation. (it may not be as efficient; however, you are not limited to bottlenecks and can distribute the power generation to match the load.
Cheers!
Dave Cooke
Re: 145
Hey Ike,
Just a quick question, I had been doing a bit of research as to cooling photo voltaic cells by mounting them on a heat conductive plate tied into a heat exchanger coil. The issue is I can not get a specific answer from the manufactures as to the optimum temperature for operation of the solar cells. Do you have access to this data?
Cheers!
Dave Cooke
I guess my post is a bit off topic at this point of the discussion, but skipping through the comments left so far, I couldn’t find any objections to the claim by Edward Greisch (#8) that:
“Nuclear power is the safest source of electricity, counting Chernobyl, which killed a total of 52 people.”
I hope this was a bad joke?!!! 52 might be the official number referring to the accident itself but that does not count the (tens of) thousands of people eventually dieing from cancer because they were exposed to radiation!
“…Nuclear power plants cannot have nuclear explosions. Western built reactors cannot do what Chernobyl did.”
I’m not a nuclear physicist but as far as I know there can be other (severe) hazardous incidents. (Also – if reactors were so save – why should we figure out systems to prevent terrorist strikes at nuclear power plants? I guess it’s not for the sake of the people working there?!)
Furthermore, my point of view (as a layman) is that nuclear waste IS a problem which should not be ignored when discussing nuclear energy.
I won’t say anything to the claim that nuclear energy did not cause CO2 emissions because John Armour (#66) and others already dealt with it…
ps:
I think that the potential from saving energy (including small changes in our everyday behaviour / at home) is forgotten quite to often in the discussion about how to reduce GHG emissions…
Nuclear power plants — especially those with boiling-water reactors, i.e., where boiling occurs next to the fuel rather than in a separate steam generator — have no large difficulty throttling up and down, according to this discussion by people in the business.
This climate blogger says the heat trapped by the atmospheric CO2 from gasoline combustion exceeds the heat directly yielded by that combustion 40-million-fold. For methane, with less carbon per unit oxidation energy yield, maybe only 25 million.
So if nuclear power plants really weren’t throttleable, it would be atmospherically better to run them always at 100 percent, and shunt their unwanted electricity through large on-site resistors, than to turn them off and burn mined methane. It would also reduce the fuel-mining cost and environmental impact, as long as the time-averaged fraction of the electricity that found buyers exceeded ~2.5 percent.