It is probably far more cost effective to buy a new air source heatpump. They’re so efficient and work well at low temps now. I have infloor radiant heat at a rural house in upper Midwest, have thought a lot about using solar heat pipes and adding a storage tank, and it’s all so fiddly, pipes and pumps and manifolds and glycol and maintenance, all custom, all expensive. I’m almost certainly better off with PV panels and off the shelf heat pumps.
Even a modern air source heat pump typically doesn't beat gas for $/kwh. Even in europe where gas prices are sky high, electricity still costs more when you look at the seasonal efficiency of heat pumps, which is typically advertised around 4.4, but various studies show that in most real world scenarios you typically won't reach the quoted lab numbers, so expect to get more like 2.5-3.0.
From that paper, the average COP measured was 3.06 (averaged between 2 and 7 degree outdoor temperatures, typical of the UK).
The UK electricity price is currently fixed at 34p/kwh for electricity and 10.3p for gas.
A new gas boiler has a COP(efficiency) of 1.054 (it can manage efficiency higher than 100% because gas is metered by the 'lower heating value', which assumes the exhaust gas escapes as steam, but the boilers actually condense most of that steam to water, getting additional energy out).
So. Total price is: Gas: 10.3/1.054 = 9.77p/kwh in your home.
Total ASHP price is: 34p/3.06 = 11.1p/kwh in your home.
And this analysis ignores the fact that ASHP's typically have much worse efficiency making hot shower water (which a gas boiler doesn't), and obviously also have considerably higher upfront costs too.
However, the ASHP can also be used to cool your home in the summer. In the past this has been a dubious benefit for most of northern Europe, but heat waves have been getting stronger, last longer, and happen more frequently as time goes on. This is turning into a serious consideration.
Also, you can theoretically power a ASHP with renewable energy, while there are few if any carbon neutral replacements for natural gas.
Most boilers purchased today allow use with a Hydrogen mix, and some under development allow 100% hydrogen. Before the widespread extraction of natural gas, towns were powered with 'town gas', which is ~50% Hydrogen, so this is very much proven tech.
There are a bunch of potential ways to make green hydrogen too.
So, there very much is a path to green with a gas boiler.
Green Hydrogen has not worked out so far. It is not clear that it even has a path forward. There are a lot of people hopeful that it will be a solution in the future, but as of today it is so uneconomical that even people willing to spend more to be green don't use it.
But making green hydrogen has pretty bad efficiency. It could never compete with a heat pump using the same electricity source, even if it had an efficiency of 100%, since the heat pump has an efficiency of 200-400%.
Yeah, these can make ASHP a good idea anyway, but the grandparent was nevertheless correct saying that they don’t beat gas for heat in terms of cost in normal (I.e. not current) circumstances.
Thus the solar panels. So long as the installation proves to make financial sense, the PV production offsets the heat pump efficiency, even in winter months.
Exactly. Also, the top commenter noted that only electricity was available and currently was relying on massively inefficient baseboard heaters. Solar panels and heat pumps also have great rebate programs and eventually gas will be phased out. Some US cities are now banning gas on new construction in the near future.
The point is that custom solar/geothermal installs seem neat and efficient —- waste not, want not — but probably have a hard time competing with the economies of scale and low maintenance of PV panels and air source heat pumps.
This may work in mild climates, but the areas where winter heating is most needed will have the solar panels under a bed of snow during the winter months.
Additionally, winter months are usually the cloudy ones. It's not very uncommon to have just a couple of sunny days per month in European winters, driving down solar gains even if there's no snow cover.
In climates where you will have a bed of snow during the winter months, your optimum tilt angle for a fixed solar panel is something like 30° off of vertical or steeper. I find that on mine the snow falls off since it is a south facing, steep, dark surface.
You engineer to make sure you have enough energy captured each month to meet that months needs. That might lead you to a tilting mount, or just a fixed angle and having surplus energy most months.
In my case, panels were much more expensive when I designed my system, I initially roof mounted them, but when replacing the roof under them I moved them to a pole and went with a tilting mount and manually move the panels twice a year. My load is much higher in the summer, but I still need some power in the winter. And northern winters can be cloudy a lot and have limited sun even on a clear day. At my location there is about a factor of 5 difference in solar energy per square meter per month. So you design for each month and then pick a solution that is best. In my case the summer load is so high that even though I get 5 times the energy, it is still the driving force on sizing the system.
Interesting. Last winter my parents' rooftop solar had zero output until April, as it produces nothing until all the panels are clear of snow. Granted their installation is at a fixed angle that matches the roof, and this is in the Nordics.
During summer months the production mostly covers and partly exceeds their use, but the sell price is so much lower that it doesn't even begin to pay for the rest of the year. But that of course then relates to the installed capacity.
I wonder if it makes sense to build some simple resisting heating elements into the panels, to allow the snow to slide off. Shouldn’t use all that much energy to run it occasionally.
You can actually push a bit if current backwards through the cell to heat it up. Some people have tried this but not sure about what prevents this from being more wide spread
So much of this discussion depends on where you are. In the US, a typical solar installation is on the pitched roof of a house. When the sun comes out, the dark roof and PV panels heat up, and the snow slides off. Rowhomes or apartment buildings in a city might have flat roofs with panels mounted on racks, but most US cities won't get enough snow often enough for this to be a problem.
My parents do have a traditional pitched roof. Winter sun is not warm enough to warm up anything, even if dark, until late winter. Plus the color of course is white if it's covered in snow :) Yeah very much location dependant.
I moved them to the ground. The roof pitch is about 1:1 and I’m not feeling like trying to stick to it any more. The pole puts everything at a comfortable working height.
Selling PV electricity back to the grid is almost always many times worse, depending on state incentives, than consuming the electricity. For example, my utility sells electricity to me at 9 cents/KWh, but only buys from me at 2.5 cents, and charged me a fixed monthly fee for the meter to boot.
If the commenter’s house doesn’t have gas, then it doesn’t seem to make sense to install it, and the price differential in the US isn’t as great as in the UK (how’s that Brexit thing working out?).
Considering solar owners are all pushing electricity onto the grid at the same time, when it is needed the least, such a system doesn't make sense. There are real costs associated with getting rid of all that unwanted energy.
I don’t think this is true in many areas. In our area, during the summer the neighborhood is running A/C when the sun is at its peak. During the winter, heating. Also the transport loss from a power station is nothing to sneeze at - my understanding is that locally produced power often just results in reduced demand on the larger grid.
I think solar makes sense for most who use air conditioning. That's a load matched pretty closely to the timing of PV generation.
Here in the Sierra foothills, it's been a blazing hot summer. About 90% of our PV generation powered our home air conditioning. We shipped very little energy to the grid on hot days. And it was awesome to have a comfortable environment without sucking grid power on the days when it hit 113°F.
This is an enormous subsidy to residential PV. Electricity wholesalers would love to be able to sell it to the grid at retail prices, like you are. Instead, they sell it for something like a quarter of retail price. This huge subsidy is only sustainable for so long as the PV penetration is low in residential market.
Yes. Even just being able to connect to the grid and only pay for the energy consumed is a subsidy. There is a large fixed cost to provide the connection and guarantee power will be available on it when demanded.
It would make even more sense to sell that power back to the grid, and then spend the $$$ earned on gas, which would work out cheaper overall.
Obviously in many parts of the world, market distortion means the buy price and sell price for electricity is very different, and in that case a heat pump might make sense to combine with PV.
That's highly dependent on your local electricity and gas prices. A quick google search tells me residential electricity in Germany costs about 2-3x what it does where I live in the northwest US. We're on mostly government owned hydro power and electric prices have been stable for a long time, meanwhile gas keeps going up.
EDIT: Did some quick math using my last power bill, at current prices a heat pump just needs to be about 2.9 average COP to beat gas in cost for me, if gas keeps going up that'll keep dropping!
There should be no economic way that a high efficiency turbine produces and distributes electricity to heat homes at a greater than 1:1 ratio compared to storing, pressurizing, and delivering, then burning in irregularly maintained consumer homes.
Likely the error is that gas pipes to the house are subsidized (albeit the electrical likely is too, with heat pumps and induction stoves, the gas lines are unnecessarily redundant)
A furnace causes a very large second law loss, converting chemical energy to low grade heat. If that gas is used to drive turbines, and the work produced used to drive heat pumps, much of this entropy generation is avoided.
A home gas-driven heat pump could be a better option from an efficiency standpoint, but those are not widely available, probably for cost and reliability reasons.
"high efficiency" turbines are not all that efficient. Burning gas releases 100% of the available energy as heat. Converting that gas to electricity is << 50% efficient. You also lose ~5% just transmitting the electricity to homes.
Also, electricity from gas is relatively expensive. Other sources like coal or hydro are cheaper, which lowers the average cost of electricity.
If you combine gas turbines with district heating you can recover almost all of the heat energy. If you use the produced electricity for heat pumps you come out ahead. For climate change purposes it is way better since gas pipes to homes leak way more methane than those to central power stations
There's not really a "should be" in thermodynamics. All heat engine cycles have upper bounds of theoretical efficiency, and burning gas in a gas turbine to generate electricity to create resistive heat is never ever going to be more efficient than burning that gas at the point you need the heat. It's simply not possible. There's always going to be losses - the exhaust gas will contain energy, there will be mechanical and transmission losses. There is no physical way we can change that.
The best thermal power plants - combined cycle gas turbines - get about 60% efficiency. Most are closer to 45%.
What we can do is either use the excess heat from the gas turbine in district heating (which combined with resistive electrical heat probably approaches the same efficiency as a local gas boiler), or use that electricity to drive a heat pump, which gives a greater than 1x return on heat where you want it. An efficient combined cycle gas turbine driving a heat pump is going to give you more heating than the same gas being burned in a boiler. - 0.5 * 4 = 200% efficient.
You can also get (although they're much less common) gas powered heat pumps (eg propane fridges in RVs). They might have a "primary energy ratio" of 1.5-2, bringing the total system efficiency of a local gas powered heating system back up to pretty close to that of a remote generation + electrical heat pump system. The electrical system has the benefit that you can slot renewables into the mix as well.
That's kind of disappointing news. I had a skim through the article and was surprised to find that it's all based on models. There wasn't a single empirical measurement of an installation of either kind. Not that I don't believe in modeling and its usages, but I am going to radically discount the findings of this paper because there was no actual experiment performed here, just fiddling with models.
Give it a re-read. They collected data from actual houses (although granted only 6 boiler-years worth of hourly data), then fitted a best fit model to that data, then used the model for their conclusions.
They did that because they needed to compare the manufacturers datasheet lab figures to the real world figures, but there are 10+ variables that affect efficiency, and a direct comparison isn't possible unless all the variables match - hence using a model to act as the 'convertor'.
> They’re so efficient and work well at low temps now.
Can someone provide me model numbers for these heat pumps that “work well” at low temperatures? I’ve been through more than a few over the decades and currently have a Trane from last year and yep, still doesn’t hold up very well once the temperature hits freezing. Don’t get me wrong, I like heat pumps, but my eyes roll into the back of my skull when I hear over and over again that they’re “so efficient now” at low temperatures.
If you actually live in an area where temperatures drop below -30C regularly, you might want to consider a ground source heat pump. They are a bit more expensive than the typical air source heat pumps since there's some digging involved, but for a new house they aren't that much more expensive since there's already digging for electricity, plumbing, etc. For example ground source heat pumps are getting quite popular in Finland.
Who’s talking about -30C? I’m talking about 32F, or 0C. I’ve yet to be impressed by a heat pump once the temperature gets at or especially below freezing.
Geothermal heat pumps are great but at least in the US they are wildly expensive compared to traditional ones. You either need a large lot for horizontal laying of the pipe or a DEEP hole if going vertical ($$$). Plus they’re uncommon enough that contractors are happy to charge a serious premium.
interesting. i have a PV system. i want to use the half installed central heating system (only piping laid down) (no boiler or FCU/room unit) and i have this crazy idea.
use solar water heater to heat water, that feeds into a water storage tank, that water gets heated MORE by an air source water heater and a small circulation pump uses that water storage to heat the house.
i am hoping, since we currently have 0 whole house heating, this would be better than nothing and the problem of not having consistent electricity to run the air source heat pump all times would be alleviated somehow by using solar water.
Yeah, sadly, it’s crazy. Solar heaters are expensive and require a large area to get decent BTUs out of. If you can find free solar heaters that someone’s giving away, that can make the difference, but they’re often a few thousand dollars and that’s about equivalent to a Fujitsu or Mitsubishi heat pump base install, especially when you factor in all the extra plumbing and pumps to/from the solar thermal array.
WORSE, there’s maintenance. You’ve got the potential for leaks (probability rises to 1.0 over the years), and you have to worry about the water freezing. That can be mitigated by using a glycol mix, but then you’ve got glycol and that lowers the efficiency.
And then you have to think about climate. Is it reliably sunny in the winter? Or is it only sunny 1 out of 3 days, with long stretches of gray days? You are probably better off plowing that money into efficiency/insulation improvements to lower the overall heat load (and those improvements often have zero maintenance).
I’d suggest looking into an air source heatpump that can produce domestic hot water, paired with a large storage tank. Check out Artic Heat Pumps: https://www.arcticheatpumps.com/
I am currently looking to do something similar. I have zero heating in my house at the moment and have a 300L (65 gallon) thermosiphon solar water heater [1] (tank plus panels all in one system) that gives me plenty hot sanitary water.
I am looking to install an hydronic fan coil [2] radiator in my rooms and need water to heat them - I thought to use the excess heat from the solar heater to supply hot water to the fan coil units but I am not sure how do do that. If you have an idea, please share.
This is not so complicated, you just need tanks with heat exchanger loops. The solar water heater input comes into your tank at the bottom and goes through a copper coil, giving you domestic hot water.
_Some_ tanks will have a similar coil in the middle/top of the tank. What you can do is then run the cold return from your heating units through this coil. It will heat the radiator water before going to the fan coil unit. This is often done as a precursor "boost" before sending the water through a boiler/heater -- the solar heat is applied to cut the temperature differential and reduce the energy expended by the boiler/heater for the radiant heat, but you're not relying solely on solar heat.
You can get heat pumps that use a water storage tank with extra heat exchanger loops like this, precisely for providing domestic hot water. The DHW and radiator water never mix. There aren't any moving parts, so it's simple.
The only question is what the max temp is from the thermosiphon tank, and whether that exceeds either the max radiator water temp or, if you use some kind of modulating boiler/heater for the radiator, the max input temp.
The math for solar storage gets daunting, however. If your typical max temp for the thermosiphon tank is 120F on a sunny day and the cold return from the FCU comes back at 70F, then with 65 gallons you can store about 26K BTU in your tank. That sounds like a "2 ton"/24K heater, but it isn't because the BTU rating of heaters is _per hour_. So, if your house needs a ~24K BTU system for heating, a sunny day storing heat in your tank will get you about an hour's worth of free heating. 24K BTU/hour is a pretty small heat load for a typical house... (though plenty doable if you invest in insulation, windows, etc.)
So, the thermosiphon is maybe adequate to provide you with domestic hot water, but in a cold climate you'll need 3–5 more of them and a repurposed and well-insulated 500gal milk tank in a shed near your house with insulated below-ground pipes running to it.
Thanks for your input. So I understand that heating the house requires much more hot water than what my solar heater can probably provide. In that case, connecting it to the radiator system is not so beneficial, and probably better to rely on a heat pump for the radiator water (the solar heater will not provide much help to justify connecting to it)
Basically. There are all sorts of neat DIY examples of people using large solarthermal arrays and big storage tanks to provide adequate heat, but they’re major projects and custom.
For your system, what I’d potentially recommend is looking at a heat pump that can provide both domestic hot water -and- hot water for your radiators. They exist! Then, explain to your vendor/installer that you want a second heat exchanger in the tank for input from the thermosiphon. Then you would be providing solarthermal to both house heating and DHW— it won’t be enough, but it is theoretically better to apply that input across both loads, in a shared manner.
However, the question is cost. You’ve got a DHW system right now that works. To install a new system with a tank that can accommodate the thermosiphon input and provide DHW and radiant heat may be too expensive to be worth it.
A second thing for you to consider is the heating area of the radiators. Old radiators are designed for very hot water from boilers, ~180F. Heat pumps for radiant heating operate between about 95F-120F (lower is more efficient, but less responsive). Because of the lower temperature, they need a larger heating surface to deliver the same heat to the house. So, typically there’s a mix of larger European-style radiator panels and in-floor heat. In-floor heat is pretty great! A room at 65F feels like 72F when the floor is warm.
Thanks a lot for the detailed reply. Old radiators operate in high temperature but from what I understand, the fan coil heaters operate in lower temperature more equivalent to radiant floor heating.
For cost reasons, installing undefloor heating is not an option, that's why I opted for low temp radiators on top of floor. However, it sounds like the heat pump system will not be cheap either. According to BTU calculator I found online, seems like I need 28K BTUs to heat the home. I thought about pellet burners as well, but here in Europe, the cost of pellets has sky rocketed this year. I don't want to be dependent on gas either, also electricity is not cheap... So I am not sure exactly what i'll go with. I'll do the calculation, but your input on the effectiveness of the theromosiphon system is valuable, thank you.
A "variable speed compressor" is what you want in a heat pump, as these modulate their speed (and electricity usage) to produce a constant amount of heat. You might be okay with a 24K BTU system, especially when coupled with the thermosiphon and then some effort spent on insulation & air-sealing?
Here in the US the base system would be about $3K USD, but with installation costs on top of that.
this is interesting. i have been told about using 319 steel insulated tank to store hot water, i was also told that this is a "pretty big deal in terms of cost" but i am willing to make that investment.
here is my idea.
300Litre solar water heater+200Litre air source heat pump + storage + FCU.
I don’t have intuition for how large a 300L solar water heater would be — you mean the solar array on the roof, right?
If you have infloor radiant heat with concrete floors, then you might not need much water storage — the concrete provides heat mass. I would look more at deploying PV arrays for offsetting the electricity usage of the heat pump than adding more water storage and solarthermal. Solarthermal is technically more efficient in providing heat alone than equivalent electrical energy, but a heat pump uses that electricity to gather heat from outside, operating at 200-400% efficiency. Electricity is also more useful across the whole house, and of course is a great help in the summer for providing air conditioning.
yeah, the problem with infloor is that the floor is already laid down so i would have to redo the flooring for all rooms.
my use case is winter nov-feb which occasionally has snow, some year there is 0 snow, some year there is 12 inches, usually inbetween.
electricity from the grid is often down during this time period but when it does come, we could use the heat pump to heat up the "storage". 300L water heater in this case means 300L/day of water heater (yeah, rooftop solar water)
the storage can be anything, 500L, 1000L, 2000L, that is cost dependent only. if we feel more storage is needed, we just add another tank
Ok, you're in Kashmir and don't have a reliable grid. I'm not sure it would make sense to spend the money on a heat pump if you didn't have reliable electricity to operate it (if you had sufficient PV panels, then it might be worth it).
Build It Solar has a ton of DIY projects for improving efficiency and using solar energy in various ways. You might find inspiration there: https://www.builditsolar.com/. In particular, they have plans for a solar collector that's a wood frame box with clear plastic sheeting and window screen material — extremely cheap and simple to build. It provides hot air, and it's best if you can fit a few cheap fans (like 120mm 5v PC fans) into it, powered by a small PV panel. I want to build one of these to heat my garage.
Some of the nice things about air-based solar collectors is that they're cheap, low maintenance, can't leak, and can't freeze. The downside is that storage is harder. If you have enough heat mass inside the house it might not matter, though.
Oh, also: an air source water heater would need an external heat source—otherwise you’ve just made a closed system and won’t be able to heat the house beyond what solar thermal provides, plus heat loss of the house.
