Comment by knolan | Hacker News Reader

Comment by knolan | Hacker News Reader

Your refrigerator is a heat pump. If you put something warm in there its heat is drawn out to the environment heating your kitchen. A refrigerant is used to achieve this. When it’s pressure is reduced it will evaporate absorbing thermal energy from its surroundings to change phase. When pumped around it’s circuit and compressed and accordingly condenses releasing all that energy to its surroundings. The electrical energy required pumps the fluid and drives the compressor. Of course there will be some heating due to heating of the mechanical components.

A lot of homes here in Ireland (and I presume elsewhere in Europe) must be built to an ‘A level’ build energy rating. This typically means installing PV Solar or heat pumps. Rarely both.

My modest solar installation on my three year old house only provides about 40% of my electricity needs (typical daily import is 8 kWh and generation is 5 kWh). Winter is a different story.

Most heat pumps here are air to water systems. Homes don’t require AC here but you can get air to air systems that can heat or cool your home as needed. The benefit of a heat pump is it’s coefficient of performance (CoP). This is the amount of thermal energy you get out for electrical energy put in. A typical system can sustain a CoP of between 3 and 4. However this drops off as the outside temperature drops and you push up the heating demanded.

They should be as reliable and performant as your fridge or AC unit. So you many need to replace the pump or refill the refrigerant.

Another interesting technology is a so called solid state heat pump or thermoelectric module. These have an awful CoP usually much less than unity. They are interesting for thermal management due to their size however. You can aggressively cook a small high-flux component and then deal with the thermal load form the hot side of the TEC with a big cooler. You can even stack them. Their efficiency increases as the temperature difference goes to zero so a stack with a small temperature jump between layers can be better. They also work in reverse and can therefore be used to scavenge electrical power when you have hot and cold reservoirs. For example put one on the exhaust of your car and you can harvest enough energy to charge your phone.

> Your refrigerator is a heat pump. If you put something warm in there its heat is drawn out to the environment heating your kitchen.

I’ve tried to explain to people that putting something warm outside to chill in winter before refrigerating it costs them more in electricity and they look at me like I’m insane.

I don't understand how that conclusion follows from the quote.

I still think "putting something warm outside to chill in winter before refrigerating" cost less electricity than refrigerating immediately

By outside, I mean outdoors.

By bringing "the cold" from outdoors inside, you're going to need an energy source to heat it back up when your home is in heating-mode. Even if you put it inside a small island of cold inside your home.

On a net basis, you've made your home colder.

If you heat your home using a heat pump, it makes no difference. If you heat with gas, you will indeed use more electricity but less gas when refrigerating immediately.

Because what you said doesn’t make sense

Running the refrigerator requires power. Is that power usage more efficient than not running it and relying on another heat source?

> Is that power usage more efficient than not running it and relying on another heat source?

I can't get whether to answer yes or no to that question, but the fridge's heatpump is more cost-efficient that most people's heat sources. That's the entire premise of the article.

Parent is right, with some assumptions to be made explicit. Considering the reactions this deserves more explanations ;)

Let's say that the heat energy of the warm thing is "E" and the outside and fridge are at same temp for simplicity. Let's also assume that the house is not heated using a heat pump.

Consider the two cases:

1) The warm thing is put into a fridge. It's a heat pump, with coefficient of performance C. To cool the thing the fridge will consume E/C, and E will be released as heat inside your room. So you paid E/C of electricity to get E Joules of warming in your home;

2) The warm thing is put outside. The energy E is wasted outside, and then the piece put into the fridge with no extra cooling there. No energy spent by the fridge, but no warming of the house either. To do a fair comparison with the same final state, we need to add E joules of energy to heat the house, which will require E Joule of electricity or primary energy (because no heat pump to heat the house).

So to reach the same point, in case (1) we spent E/C of heating energy, and E in the second case. It is indeed more efficient to put the warm thing directly into the fridge, as this will contribute to heating the house with the efficiency of a heat pump.

And if you heat your house with a heat pump of same efficient C, you don't care either way: it's equivalent.

You’re going to have to explain your reasoning to me there.

If your fridge is at 4°C and you put something in there at 21°C it will have to transfer an amount of Joules of thermal energy to the internal environment and then the fridge will have to transport that energy out to the kitchen. The amount of energy depends of the specific heat capacity of the item (J/Kg/K). So to drop the temperature of 1kg of stuff from 21°C to 4°C your heat pump needs to work to remove however many Joules the specific heat capacity determines.

If on the other hand if your item is at 0°C then it’s going to heat up via heat transfer from the other items in the fridge. This means the internal temperature of the fridge will drop. But this means the heat pump will have to work less to maintain the internal temperature of 4°C. It’s not that the fridge attempts to heat the fridge up. The fridge simply works less because no fridge is perfectly insulated so it’s easy to let the temperature rise.

Because I'm trying to heat my home in winter.

Outside = outside_outside = outdoors.

Not, uhhh, inside outside like the kitchen counter.

> However this drops off as the outside temperature drops and you push up the heating demanded.

With Irish climate and modern heat pumps this should be a non-issue. Maybe in the colder areas a day or two in the average year. Newer heat pumps can maintain very good efficiency under 0°C.

> Newer heat pumps can maintain very good efficiency under 0°C

Well under 0℃, in fact. The Mitsubishi ones are still at 200% at -18℃ (0℉), down from 300% at 0℃ (32℉). They don't fall to 100% until around -26℃ (-15℉). Fujitsu is similar.

Their capacity also falls at lower temperatures going from 100% at -5℃ (23℉) to 76% at -25℃ (-13℉) so you might need to go with a bigger system in some areas.

Heatpumps are commonly used all over scandinavia. They work fine even with extreme temperatures. Temperatures stay well above 0 Kelvin. So there's still plenty of energy to pump around. Of course with extreme temperatures you need to produce more heat. Which simply means you need a bigger setup.

What’s the advantage of an air-to-water heat pump as opposed to an air-to-air system? Why are they used for new buildings, instead of a standard A/C indoor unit which can also offer cooling?

Water can carry heat around a building more efficiently (in power and space terms) than air.

In a heating dominated climate, this can make sense in a new building. (It’s also a good match temperature-wise to in-floor radiant heat.)

In a retrofit situation (such as my house), it also makes sense and is cheaper than running supply and return ducts everywhere.

It’s not as good a fit in mixed or cooling-dominated climates (though they can cool as well, but then you have to insulate the pipes much better to prevent condensation).

> Water can carry heat around a building more efficiently (in power and space terms) than air.

Heat is carried by the refrigerant though? It only gets "converted" to air in the room itself by the indoor unit.

I am talking about European-style systems where you have A/C units in every room, not the American-style with air ducts distributing air from a central unit in the utility room.

> European-style systems where you have A/C units in every room

Which European countries are you talking about? There is no European standard regarding A/C or heating.

I didn't mean a formal standard - I checked the definitions and what I'm referring as is that US generally prefers central or "packaged" systems (single big unit in the utility room, air is piped from there into the living space through various ducts) where as Europe prefers "split" systems where you have units in each room with refrigerant pipes going back to the outdoor unit.

Most of the US uses split units for central air (split meaning the condenser and evaporator are in separate units).

What you’re calling a “split” is more precisely called a “ductless (mini-)split” as even an evaporator in the mechanical room and linesets carrying refrigerant to an outdoor condenser is still a (conventional) split setup.

In our climate there is simply no need for AC as it never gets uncomfortably warm for more than a few days a year. Air to water is used to heat radiators and supply hot water in lieu of gas or oil fired heating. In other climates air to air is more popular and it can be less complex as you don’t need all that extra plumbing.

With new builds air to water makes sense as it’s no different to installing any type of central heating system.

As a retrofit option an air to air system would seem attractive as the installation is much simpler. Our homes tend to be smaller and there wouldn’t be any HVAC ducting so you would probably only have a since source of warm air with an air to air system.

I feel like installing a heat pump which cannot cool is a mistake at this point.

Heat waves are already getting more intense and more frequent. While AC was a luxury a decade or two ago, it is rapidly becoming a necessity.

It is absolutely bizarre seeing houses designed like -20C winters are still a common thing, while completely ignoring summer heat.

> With new builds air to water makes sense as it’s no different to installing any type of central heating system.

But it's the same level of effort than installing indoor A/C units, so why not install those instead? Plus refrigerant pipes can potentially be thinner depending on the max "load" of that room.

In a heating-dominated climate, there may well be no air conditioning installed. In a hydronic-only heating building (underfloor and/or baseboard/radiators), there can be less work to run heating pipes only (much faster, cheaper, and more forgiving than running refrigerant lineset and managing the condensation drain line).

For heat-only, I find hydronic to be nearly ideal: low noise, no dust, and high comfort (no air movement/drafts).

Because it can also heat a house at the same energy efficiency, that is, better than electrical resistance or gas heating.

Not an expert, but the much higher heat capacity (and heat conductivity) of water compared to air is likely one reason. But also, when it’s really cold outside, an air to air heat pump is much less efficient. If you have access to liquid water, that is of course much warmer than outside air.

Water is much more efficient at thermal transfer than air. All of those little brittle fins on the outside, less compressor run time.

It's also better in cold weather for heating as a water mass rarely gets below freezing.

The issues are a water source and corrosion.

Heating with air is less efficient in transporting heat with water. So you either need to move more air or heat it up to a higher temperature. The first one is obvious why it’s a bad idea, the second is a bad idea as at a certain point the dust in the air is burning up leaving a bad smell. So practically you can heat a house with air heating alone only when they are extremely well insulated or in a mild climate. The passivhaus-Standard in fact is defined by heating through air being enough.

Other posters are correct that water can transfer heat more efficiently than air, but they're missing or burying the lede:

The temperature is very stable once you dig down a couple of meters. Winter or summer, it's probably an even 10-20 C (depending on your specific location). The ground has tremendous thermal mass and can act as an enormous heat sink. Your little house-sized heat pump isn't going to vary the temperature very much. You can optimize your heat pump to the specific temp range and dump/draw as much heat as you like year round.

There is also question what is the water in picture. Heat pump efficiency decreases when the temperature delta increases. As such it really does depend is the heating done via radiators or underfloor. As later can be used with significantly lower temperature. Where as specially during cold times radiators need rather hot water. In such cases air-to-air might actually be more efficient.

Seems odd not to encourage both as PV solar and a heat pump go pretty well together. Is it more of a question of legislative particulars vs technology?

It’s down to cost. The energy rating is subdivided into thirds. An A1 home would have both but an A2 or an A3 will have just one. As long as it’s any type of A rated it’s acceptable.

For a ne whole adding both wouldn’t increase the cost to the buyer - they are already willing to pay X for the house. It would increase the cost to the builder, who would then have less money to pay the land owner for the land, who had received a massive planning gain when the 50k field was given to build 100 houses and increased in value to 3m.

I dont see the problem, PV and heat pumps should be mandated.

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