Life cycle energy of GSHP vs ASHP
norm_farwell
| Posted in Mechanicals on
I am curious if there is any research on the life cycle energy of heat pumps? A typical ground source heat pump has a higher COP than an ASHP, but I would guess at the cost of greater embodied energy due to trenching for a loop field, loop field piping etc. Does the GSHP’s greater efficiency eventually overcome the greater embodied energy? I understand there are a lot of variables here including climate, site, occupant behavior etc.
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Data seems to be thin on the ground (or at least closely held). From local observation, it seems that drilling the holes is rather "diesel heavy". Some data from Switzerland (in German):
http://www.eco-bau.ch/resources/uploads/Oekobilanzdaten/Flyer_Oekobilanzdaten_Oktober_2014.pdf
Look for "Erdsonde" -> about 475 MJ/m, 132 kWh primary energy per meter.
A recent development direction is to recharge the ground source in summer using solar thermal energy or free cooling, thus improving the COP of the heat pump, and avoiding "freezing up" of the loop field.
Norm,
You raise an interesting question. You also hint that performing these calculations is difficult -- perhaps impossible -- because there are so many variables. (For more information on these calculations, see All About Embodied Energy.)
Cost-effectiveness calculations are certainly simpler than embodied energy calculations, and for many materials and construction decisions, the cost graph tends to overlap the embodied energy graph fairly well. There is a good chance that a $30,000 investment involves more embodied energy than an $8,000 investment (although of course there are exceptions).
In most (but not all) cases, a residential air-source heat pump installation is more cost-effective than a residential ground-source heat pump installation.
There is one other type of calculation that is revealing: a comparison between (a) the cost of an air-source heat pump system and a PV array large enough to supply the system's electrical needs on an annual basis, and (b) the cost of a ground-source heat pump system and a PV array large enough to supply the system's electrical needs on an annual basis. Many energy consultants have performed this calculation on residential systems, and option (a) wins out.
Martin,
Thanks, using money as a proxy for embodied energy in evaluating technology seems like a good short cut. At a basic level it makes sense that money and energy would be somewhat interchangeable.
I am trying to think where the money=energy formula might break down. It seems to work less well for some materials, foam insulation for example, where an economy of scale and hidden subsidies probably skew things: cellulose has approximately 1/25 the embodied energy of foam but costs only a little less on a dollar per R value unit basis. On the other hand the embodied energy in cement is worse than it's cost would indicate. And I'd guess dollars spent on human labor probably have a smaller energy footprint than dollars spent solving the same problem with machines and technology.
All arguments for natural materials and keeping things as simple as possible.
(Risking a digression: There is an interesting argument on the progressive fringe of economics that the end of cheap fossil fuel might have a surprising effect on the economy. Instead of rising prices due to resource scarcity, we may actually see falling prices and falling demand. Fossil fuel creates money, so peak oil will cause peak capital and a deflationary spiral rather than inflation. Looked at from that perspective, there may a much tighter connection between money and energy than we usually think. Gail Tverberg writes about this at http://ourfiniteworld.com/ She's not an optimist, so I hope she's wrong.)
The embodied energy calculations are multi-layered, but not a huge factor in the GSHP/ASHP. The ground heat exchanger loops (in general) have a much longer lifecycle than the heat pumps themselves. Open-well systems have very little material in the loop relative to large slinky arrays of plastic pipe, but the embodied energy of even those system is inconsequential compared to the total lifecycle energy moving through the system. A greater part of the difference in cost is engineering / design time and implementation labor, not the source materials energy cost. The increased efficiency performance will pay off the difference in embodied in energy use well within the anticipated lifecycle, but it won't necessarily EVER pay off the difference in design & implementation costs (though it might, in some cases.)
But then again, not all GSHP systems will outperform ASHP systems. With GSHP there is a lot of system unknowns and design risks. The all-in pumping, air handler, standby power & back up heat strip power included seasonal average COP of GSHP systems measured in-situ by third party investigators is often in the 3s, even with heat pumps rated at 4.5-5. While it's true that ducted ASHP systems will often come in at seasonal averages in the low 2s in cold climates, best-in-class ductless mini-splits with optimal oversizing can now hit the mid-3s, even in a US climate zone 5 & 6 locations. There is much lower design risk with these systems as well. Averages are one thing, peak draws are another. At -10F outdoor temps even the best mini-splits struggle to hit a COP of 2, even if it has the capacity to fully cover the heat load at that temp. Then again, GSHP systems sized for the 95th percentile heat load with resistance heat strips to cover the shortfall at outdoor conditions below that temperature bin might only hit a COP of 2 at that temp too (another part of the design risk.)
"...cellulose has approximately 1/25 the embodied energy of foam but costs only a little less on a dollar per R value unit basis."
Not exactly true on either count- it depends on the accounting. If one discounts the source energy it took to create the paper for the cellulose to ZERO it might have 1/25 the embodied energy of expanded polystyrene, but it can have an even higher embodied energy if taking that fully into account. If using only post-consumer paper feedstocks it might be valid to count the paper-making energy as zero, but it's a very fuzzy thing when you start getting into printing over-runs, virgin-stock fiber, or multiply-recycled paper as feedstock.
In open blown low density applications the installed cost of cellulose is about 1/3 that (or less) of any foam product on an R/ft^2 basis. But in a dense-packed cavity fill application it's performance/$ can be substantially lower than continuous EPS sheathing, if comparable to open cell foam cavity fill, but damp sprayed cellulose cavity fill can be substantially better performance/$ than open cell cavity fill. It all depends on the application. As a practical matter can't exactly replace open-blown attic fill with any foam product anywhere near cost-effectively, nor can you replace rigid continuous sheathing with cellulose.
"And I'd guess dollars spent on human labor probably have a smaller energy footprint than dollars spent solving the same problem with machines and technology. "
Not necessarily--especially not if the labor is on-site and each individual commutes to the site in a large vehicle ... I'm guessing that the commuting energy for the labor for a passive house could in many cases exceed the lifetime energy consumption of the house.
Drilling wells might not be all that bad. I'm guessing that the ~50 mile drive that the drilling rig made to my house to install the GSHP burned more fuel than the process of drilling the wells did. And even if that was only at 5 mpg, that would be 20 gallons of diesel, which is less than a week's worth of heating oil.
If we expand to lifetime global warming impact instead of just energy, we also have to consider the global warming impact of accidental refrigerant releases. In theory, there are fewer place for that that happen in a non-split heat pump, but I've personally had refrigerant leaks in my GSHP and I haven't heard of many in mini-splits. But my GSHP is older than most mini-splits. I hope I can replace it with a CO2 based system before it leaks again.
Refrigerant leaks happen with mini-splits too. It's usually installer error, such as not using the correct flaring tools on the refrigerant lines, or re-using a flared connection that had been opened back up without the proper attention.
I keep hoping that lower impact HFO refrigerant or CO2 refrigerant heat pumps will eventually dominate this market, but for now it looks like an R410A dominated show.
Even a fairly big system would have 20-25lbs of R410A in it, and at a 100 year GWP of 1725 x CO2 that's worth about 17-22 tons of CO2, call it 20 tons. Lighting off heating oil in a boiler or furnace produces about 22.4 lbs of CO2 per gallon, so losing the ENTIRE CHARGE (rare, but it happens), is worth about 40,000 / 22.4= 1785 gallons of oil, or about 3 year's worth for a pretty-good 2500' code min house in New England.
A typical 3/4-1 ton mini-split has about 2.5-3lbs of refrigerant in it, and with 50' of refrigerant line (25' each way) another 5lbs. Rounding up, call it call it 10lbs total. Losing the complete refrigerant charge on one of those is the global warming equivalent of torching 8-900 gallons of heating oil. It's not nothing. But if it hangs onto it for at least 5-10 years it's doing way better than a fossil-burning heating solution (assuming relatively green grid sources, not 30% thermal efficiency sub-critical coal or something.)
And even if they leak, they don't typically lose the whole load. It's important to find and fix the leaks though.
Ideally, a leak would be caught before the whole charge is lost. My experience, was, tragically, that not only did the whole charge leak out (while we were out of town), but it took two additional tries for the refrigeration tech to fix all the leaks (some of which may have been introduced in repairing the first leak). I considered adding a permanently installed gauge so I can keep an eye on the charge and nip the next leak in the bud, but that also introduces another place where the system could leak, so I didn't.
20-25 lbs of R-410A sounded high, so I looked up a some numbers on the Waterfurnace site. Their 5 series water to air units, for example, use 2.6 lbs for a 1-ton, 5.24 lbs for a 3-ton, and 9.4 lbs for a 6 ton. Water-water units use less. So I think the comparison Dana gives is pretty skewed--the GSHP option actually uses substantially less R-410a per ton of capacity than mini-splits regardless of your assumptions about which sizes and numbers of units and you are looking at.
Perhaps that 20-25 lb number was for a direct expansion version that has refrigerant throughout the ground loops? That's certainly a bad idea from the perspective of quantity of refrigerant and potential for leaks!
My ducted ASHP is running a heating COP of at *least* 3 in its Boston
climate, and probably closer to 3.5 after fiddling with "reduced demand"
settings in the outdoor unit so it runs lower-n-slower. While the
air handler may be losing a wee bit more heat to the basement because
it happens to be down there, the fact that it's ducted really shouldn't
affect its performance.
_H*
Charlie: My clarity was definitely suffering by referring to "...a fairly big system...". I wasn't talking about GSHP having that much refrigerant, nor was I comparing mini-splits to GSHP- I wasn't talking about GSHP at all(!), but was addressing the refrigerant leaks & quantities in mini-splits and other heat pumps. The 20-25lbs was for large split-system air conditioners & ASHPs with long refrigerant line runs, or at least that's what I had in my mind's eye while writing that. I was looking for a worst-case scenario of something - the largest heat pump system that someone might actually have in their house, then scaling back to compare what a single mini-split would have. I apologize for the confusion. They all leak, some have more juice to leak than others. Very few would have more than 25lbs of refrigerant.
With self-contained GSHP units it would of course be much lower, as you correctly pointed out. With split-systems there's often (usually?) more refrigerant in the line-sets than in the unit itself, and it's not surprising at all a mini-split would have more refrigerant in the system than a GSHP twice it's size.
But nobody really cares about how much R410A is in a GSHP, 'cuz nobody owns one! :-) (At least from a total market point of view, that's the case.)
In the spirit of inventing statistics on the fly, all of the R410A in all of the residential GSHPs in the US probably adds up to less than 0.1% of the total amount of R410A in residential air source heat pumps & air conditioners, not counting window-shakers and dehumidifiers. There are plenty of 6-8 ton AC units out there with 50-100' of line between the compressor on the back patio and the air handler in the attic.
Hobbit: How are you measuring the COP of your system, and does that include the power burned in the pan-heater you rigged up?
Dana: Thanks for the clarification and for the humor. I'm pleased with my GSHP, but worrying about its lifecycle climate impact is starting to feel a little like Stanley Steamer owners discussing how to get sustainably harvested wood to burn in them.