General > General Technical Chat
What is the real story around heat pumps?
zilp:
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---Going from what I've read, humidity can both increase, as well as decrease the COP. If the temperature is above 6°C, high humidity generally increases the COP. At temperatures below 6°C, high humidity deceases the COP, which is worse around freezing, then becomes less of an issue below freezing, since freezing air holds less moisture.
--- End quote ---
The point is that COP doesn't get magically worse around 6°C, COP gets worse due to freezing. Or more accurately, COP gets worse with lower outdoor temperature, but the dip somewhere around 6°C is due to freezing and the required defrost.
Which also means that this is not somehow specifically at a magic 6°C, but rather at whatever outdoor temperature that happens to cause the evaporator surface temperature to dip below the freezing point of water. Which depends on, among other things, the thermal power that you are extracting from the evaporator. And on the amount of air being blown through the evaporator. And on the geometry of the evaporator. And who knows what.
So, it is possible that you might be seeing defrost cycles at 6°C with some heat pumps in some setups. But really, you mostly shouldn't, because the heat pump will generally be dimensioned for sufficient thermal power to keep your house warm (without resistive heating) at -15°C or whatever (depends on your location, obviously), so the thermal power you are drawing from the evaporator at +6°C (assuming a modulating/inverter heat pump) should be relatively low and should not be sufficient to pull it to below 0°C, especially not with humid air that provides significantly more heat per volume than dry air would.
Here, we had rainy wheather and temperatures in the 4 to 8°C range the last few days, but effectively no defrost. I say "effectively" because the controller does a defrost cycle shortly after startup at outdoor temperatures around ~ 5°C or lower (no clue what the exact decision criteria are), but then it keeps going 8 hours or whatever without any further defrosting, until it shuts off because the target temperature was reached, and the ~ 250 Wh of heat it dumps into the evaporator for that don't matter all that much overall (especially given that it probably recovers a significant part of that). (And also, there really is no ice forming, so it's not like COP is dropping but the controller just fails to do a defrost cycle.)
Around 3°C with humid air is where things get into terrible territory here, i.e., where you get ~ hourly defrosting.
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---In my specific situation, I expect the heat pump to mostly run when the temperature is around 0°C to 6°C, hence why I believe the humid climate where I live is a hindrance, rather than a help. If I were going to use it for hot water, say to heat a pool in summer, then it would be different.
--- End quote ---
I mean, it's certainly not ideal, but I suspect it wouldn't be quite as bad as you are imagining it. But if you were to ever go that route, it certainly would make sense to see whether you can set it up such that the heat pump can extract the heat for defrosting from your house. As mentioned, mine pulls from the DHW temperature storage tank, and even defaults to resistive heat for defrost, which is kinda bad. But at higher heating water temperatures, it could potentially also pull from under-floor heating, for example. Some other heat pumps can also do so at lower temperatures. There will always be some cut-off, because a source with insufficient thermal power would risk freezing up and thus damaging the refrigerant/water heat exchanger, but if you somehow can arrange for defrost energy to be pulled from some low-ish temperature thermal mass in your heating system, then that is the most efficient way to go.
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---Yes, your standby consumption does sound high. I would have hoped that minimising it would have been an important design goal.
--- End quote ---
Well, yeah, the thing has two separate transformer-based (i.e., mains frequency transformers) power supplies. That seems to be the result of the thing being a bit of Frankenstein's monster built from the heating controller of an old German manufacturer of heating systems and the actual heat pump from Daikin. Certainly doesn't help with efficiency ...
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---I thought reading objective studies about heat pumps in areas with a similar winter climate, to my location would give me a reasonable figure. I accept that it will be different, depending on how I use the heat pump, but I would have thought it would by closer, than a general ballpark of 350%, which doesn't even take into account the local climate.
--- End quote ---
Well, the problem is that the required heating water temperatures have a significant effect on efficiency. And the 350% does take "local" climate into account. Being more accurate wrt local climate doesn't really help much if the details of the heating system remain a variable with a lot of potential variability.
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---I'm not sure how much I trust COP figures given by manufactures, given there are numerous different factors and they are likely to be skewed in the manufacture's favour. I've seen this with component data sheets, audio amplifiers and vehicle mileage, although I've also had instances when it's better than expected, such as my motorcycle doing more miles per tank, than expected. I suppose I'm more careful, because it would be a big investment, it's not something I'm overly familiar with and then there's the political side to it, which doesn't help.
--- End quote ---
What I am talking about here are detailed tables or graphs that specify COP at specific outdoor and heating water temperatures, not something like yearly averages. Just as with specs in component datasheets: They tend not to be outright lies, they just tend to be measured under ideal circumstances, or typical rather than worst case, that sort of stuff. So, a Vf of 0.2 V on the title page of a data sheet might be for 10 mA for a 2 A diode, say. But the Vf vs. I graph won't be completely made-up bullshit. And the same seems to apply for heat pump specs.
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---I'm aware of the fact that gas stoves aren't very efficient, I've read 40% is reasonable, but don't forget electricity costs four times as much for me, as gas, so an induction hob will still be more expensive for me to run. I doubt it will make much of a difference since, only a tiny part of my energy usage is for cooking, but it's something to consider.
--- End quote ---
Yeah, my point was more that it's not worth the base fees to keep cheap gas for cooking, and that the advantage probably isn't huge. And 40% might be a bit optimistic, but it's the right ball park.
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---I agree. That doesn't sound expensive for the heat pump itself. It's the rest of the installation and associated labour which is expensive. Subsidies are only given if you use an approved contractor and it has to meet certain criteria. I don't even know if it's legal for you to do it yourself. I would expect a licenced refrigeration tech will be required, or at the very least an electrician with the appropriate qualifications.
--- End quote ---
IIRC, Siwastaja was talking about a monoblock system, so the refrigerant loop is a closed system within the outdoor unit and only electricity and water need to be hooked up, which is why you in most places don't need a licenced refrigeration tech, which is why they are popular with DIYers.
Siwastaja:
--- Quote from: Marco on February 25, 2024, 08:34:22 pm ---Wood is clearly the cheapest per kwh if you have the space to have ton's worth delivered by truck.
--- End quote ---
Maybe / possibly somewhere, but I'd say usually not.
Fun fact: two years ago due to events that did not actually affect Finland that much in any meaningful way, people got hysteric and got an impression that cost of electricity would skyrocket to 2x, 3x, 5x, 10x, 20x, whatever insane value. Mass media poured buckets of gasoline into the fire. A few were gullible enough to get colossally expensive 2-year fixed contracts in irrational fears of increasing prices which never happened, but this con did not affect the majority of people.
In reality electricity prices rose by 25% (typical cost; energy 0.05€ --> 0.08€ /kWh, transmission+tax 0.07€ --> 0.07€, total 0.012€ --> 0.015€), but those selling wood, mainly small individuals who own some forest, in a cascading effect took advantage of the impression of expensive electricity so that m^3 of wood (around 1000 kWh including typical losses) went from like 30-40€ to well over 100€. So the individuals who criticize large corporations from causing greedflation by increasing prices by 10% for no reason themselves increased prices by 200% almost overnight without blink of an eye.
But it's free market, sure. In some cases, burning wood became more expensive than resistive electric heating with COP=1. Wood transitioned from the "cheapest" form of heating to the most expensive one!
In mid-European climate though, heatpumps easily get COP between 3.0 and 5.0, so electricity needs to be specially expensive, or wood specially cheap for wood to be "clearly the cheapest".
pcprogrammer:
--- Quote from: Marco on February 25, 2024, 08:34:22 pm ---Wood is clearly the cheapest per kwh if you have the space to have ton's worth delivered by truck.
--- End quote ---
Only if you can source it cheaply, and in the Netherlands you most likely can't.
It also involves a lot of work. The truck load is usually dumped somewhere accessible and not necessarily near the space where it is stocked. So wheelbarrow to the stocking is needed and when the time comes you have to bring it inside to the burner. For sure it keeps you warm multiple times. >:D
I know someone who bought a truck load of of-cuts from a barrel maker down here and transported it to the Netherlands to sell it with quite a bit of profit. :-DD
tszaboo:
--- Quote from: Zero999 on February 25, 2024, 07:30:56 pm ---
--- Quote from: Siwastaja on February 25, 2024, 07:08:49 pm ---
--- Quote from: tszaboo on February 25, 2024, 02:14:13 pm ---I've quickly looked up air to hot water pumps, they start at 3500. You get a subsidy, but only if they install it.
--- End quote ---
But 3500 is not that much at all IMHO. Sure I'd like to see them start at 2000, but :-//. 3500EUR is quite easily recovered already during halfway of the lifetime of the product. It gets nasty when the heatpump is 7000 (some sort of premium brand name which isn't that much better in reality) and then the install cost another 7000, or even more.
--- End quote ---
I agree. That doesn't sound expensive for the heat pump itself. It's the rest of the installation and associated labour which is expensive. Subsidies are only given if you use an approved contractor and it has to meet certain criteria. I don't even know if it's legal for you to do it yourself. I would expect a licenced refrigeration tech will be required, or at the very least an electrician with the appropriate qualifications.
--- End quote ---
Keep in mind this is the cheapest I was to find in a particular Dutch webshop. Let's assume the installation costs and the subsidies cancel each other out. The installation is relatively painless compared to exchanging the entire heating system. Then we are at 3500 EUR investment which is only for hot water, not for heating. Upon double checking the shop, it's not on stock, might be one of those things that is never on stock to just get your attention with good price. Anyway, let's move on.
I take my summer hot water costs, it's 0.5GJ/month/person. Which is billed for me at 22EUR, excluding the fixed costs. And that's 140KWh per month energy. The same would cost me 56 EUR with direct electric heating (0.4EUR/kWh). District heating is incredibly expensive because of bad regulations here, and the monopoly, but that's just a sidenote.
Let's assume SCOP of 5 which would put this at 12 EUR/month, saving of 10 EUR per month. That would place it at 30 year ROI. For 4 person, 7.5 year ROI. This is, if I have to buy the electricity.
If I can use my own electricity to heat it up, the story changes. I can oversize my solar system by 1500KWh per year, which is about 1000 EUR investment here. About 2-3 panels will do that much. We have to assume yearly net metering for this to work. Then The investment is 4500 EUR, and the monthly saving is 22 EUR. ROI is 17 year for one person, 10 year for 2, 6 years for 4 person. That also assumes that you can place an extra 6000KWh/year solar capacity on your roof, that's about 11 panels here. That's assuming that the cheapest heatpump will even suffice for a family of 4, looking at the supplied tank size, I don't think so, it's tiny compared to what I've seen. And it's capacity is only 5kW. Net metering is also being phased out. The regulation around district heating is changing, so I expect much lower prices next years.
So I only see this to be valuable for large families.
--- Quote from: pcprogrammer on February 26, 2024, 07:43:33 am ---
--- Quote from: Marco on February 25, 2024, 08:34:22 pm ---Wood is clearly the cheapest per kwh if you have the space to have ton's worth delivered by truck.
--- End quote ---
Only if you can source it cheaply, and in the Netherlands you most likely can't.
It also involves a lot of work. The truck load is usually dumped somewhere accessible and not necessarily near the space where it is stocked. So wheelbarrow to the stocking is needed and when the time comes you have to bring it inside to the burner. For sure it keeps you warm multiple times. >:D
I know someone who bought a truck load of of-cuts from a barrel maker down here and transported it to the Netherlands to sell it with quite a bit of profit. :-DD
--- End quote ---
Yeah someone in the neighborhood was buying wood for cooking in the garden and that sort of things, and I was seriously questioning his sanity when he told me how much it cost. I think there is an issue when people are just uninformed how much something should cost, and companies are praying on us with extra profit. The good prices are not easily accessible. That's why we have internet at 70 EUR a month, while the same speed and accessibility, costs less than 20 in other countries.
tom66:
A monobloc can be installed by someone without a refrigerated gas (F-Gas) license in the UK because the system is 'pre-gassed' -- only hot water passes out of the unit. There are other types of air conditioners which can be installed by people without F-Gas certification, like propane based mini-splits as propane has a GWP of 3 and ODP of 0 so its release into the atmosphere is not considered particularly hazardous.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version