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What is a heat pump?

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from How Stuff Works

Canto: So We’ve been reading, inter alia, Saul Griffith’s practical and informative little renewable energy tome, The big switch, in which he mentions, inter alia, heat pumps and their relative efficiency. He may have explained heat pumps somewhere, but I was adrift at the time. Can you inform me?

Jacinta: Well, instead of rifling through the book, which unfortunately doesn’t have an index, let’s google it. Australia’s largest electricity generator has a piece boosting heat pumps, which may sound biased, but Griffith boosts heat pumps too, so I’m presuming it can be trusted. The humble heat pump, as it’s described, isn’t a new technology, and is a proven winner in terms of energy efficiency, for example in reverse-cycle air-conditioning. Particularly so for heating. It ‘gathers heat from outside and warms it to a higher temperature, then moves it from one place to another’.

Canto: It gathers heat from outside. Easier said than done, surely. Presumably it sucks?

Jacinta: Well apparently there’s more than one type of heat pump technology, and this article mentions two – air sourced, and ground sourced (geothermal). And as they point out, it may seem counter-intuitive to think you can extract heat from outside, whether from the air or from under the ground, in the depths of winter. But heat is energy, and even in the winter there’s a lot of that energy around. Anything above absolute zero contains energy, and there’s surprisingly little difference between winter and summer heat energy.

Canto: Well I notice they highlight this statement about your standard heat pump:

It uses a compressor and liquid or gas refrigerant (the stuff that’s in your fridge) – a substance that absorbs heat from the environment – to concentrate heat and move it around to warm your house.

Now that’s something that needs explaining.

Jacinta: Well, off-hand, I understand that heat tends to disperse, when it can, from a compressed high-energy state to a low-energy one, something to do with a law of thermodynamics…

Canto: I suppose heat pumps come under the umbrella of HVAC systems – heating, ventilation and air-conditioning. But do all air-conditioning systems work with heat pumps?

Jacinta: Well I don’t think so – there’s evaporative air-conditioning, which I think’s quite different.

Canto: So a heat pump just is a reverse-cycle aircon? How, exactly, does it do the cooling? Or the heating for that matter.

Jacinta: Just off the top of my head, the cooling works just as the heating does, but in reverse. It sucks the hot air from the room and chucks it outside. And then, uhhh, brings colder air in somehow, or cools it in that big box, somehow like a refrigerator…

Canto: Brilliant. So now I’m reading How Stuff Works on heat pumps. And of course you’re on the right track, it’s all about heat transfer, so it doesn’t require energy for burning..

Jacinta: Or freezing?

Canto: Uhhh, it requires energy, of course, but it’s pretty energy-efficient. They’re particularly good in moderate climates like ours (In Adelaide), because you don’t have to transfer large amounts of heat from outside or inside to make life comfortable.

Jacinta: It’s pulling and pumping. You pull heat from the outside, or inside depending on the season, and pump  it into the inside, or outside.

Canto: Yeah, I get that, but what’s the mechanism?

Jacinta; Well, sucking air in is pretty simple, our mouths do it all the time, and think of a vacuum cleaner, which does it more powerfully, but generally for shorter periods. Aircons just require a gentle, longer-term suck.

Canto: Very technical.

Jacinta: That’s probably why the emphasis is on the pumping. Now, I’m talking about air-source heat pumps in particular, because that’s what we have in our home – on the wall, with another box outside. What they call a split system. So, I’m going to quote at some length from How Stuff Works, and then we’ll try to break it down, if necessary:

One of the most common types of heat pumps is the air-source heat pump. These take heat from the air outside your home and pump it inside through refrigerant-filled coils, not too different from what’s on the back of your fridge. The air source variety is pretty basic, and you’ll find two fans, the refrigerator coils, a reversing valve and a compressor inside to make it work.

The key to allowing the air-source heat pump to also cool is the reversing valve. This versatile part changes the flow of the refrigerant so the system can operate in the opposite direction. So instead of pumping heat inside your home, the heat pump releases it, just like your air conditioner does. When the refrigerant is reversed, it absorbs heat on the indoor side of the unit and flows to the outside. It’s here that the heat is released, allowing the refrigerant to cool down again and flow back inside to pick up more heat. This process repeats itself until you’re nice and cool.

Canto: Hmmm, that sort of makes sense, the hot air inside is made to flow through these ‘refrigerant filled coils’, but what exactly is this refrigerant? Didn’t they use to use freon or something, which damaged the ozone layer? And this doesn’t exactly sound like a split system. It sounds more like an all-in-one.

Jacinta: Yeah so not all reverse cycle aircons are split systems. Apparently split systems are easy to install and work pretty efficiently. I can’t easily find how they compare with all-in-ones but I suspect they’re pretty similar, efficiency-wise. Another type of reverse cycle aircon is ducted., which can warm or cool several rooms at once through various ducts. The refrigerant most used nowadays is puron, a HFC (hydrofluorocarbon) known to be non-damaging to the ozone layer. It’s become the standard refrigerant since 2015, but our aircon is surely older than that. Freon, the culprit you mentioned, hasn’t been used in new air-conditioning systems, at least in the USA, since 2010. Freon is a HCFC (hydrochlorofluorocarbon), apparently much naughtier than HFCs. Of course, I’ve no idea what our system uses.

Canto: Okay, last question. I don’t think we’ve answered how the pump thing works.

Jacinta: Well I suppose it does so just as mysteriously and simply as it sucks. We suck air in with our mouths and noses – or we suck in oxygen, and how we separate that gas from all the others is another story – and we pump out carbon dioxide. All without the slightest effort, apparently.

Canto: So we’re actually heat pumps? And we’re pumping out CO2 at an alarming rate, eight billion of us. We’re the principal cause of global warming!

Jacinta: Haha, I think you’ll find it’s a bit more complex…

Written by stewart henderson

July 13, 2022 at 8:23 pm

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