Eco Cute and the Green-House-Gas Gangsters

Marketing has clearly gone awry in the world of CO2 air supply heat pumps. It would be a painfully damaging stretch of the imagination to believe that there was anything cute[1] about the Eco Cute. Perhaps painting large, sympathetic eyes and covering it in fluff might work?

Setting aesthetics aside (although why environmentally-friendly products often look butt-ugly is perhaps a discussion for another time), does the Eco Cute deliver on the Eco part of its name?

I’ve discussed how heat pump technology can heat water and homes using less energy and emitting less CO2. There are claims that the Eco Cute can reduce CO2 emissions by up to 50% under certain conditions. Whether the Eco Cute is really this effective is debatable, but it is certain that heat pumps generate more heat than the amount of electrical energy added.

But the Eco Cute is a CO2 air supply heat pump. That name seems to suggest that it uses CO2 to emit less CO2. Sounds like devilry.

To break through the confusion, let’s explore why CO2 got such a bum rap.

CO2: Philanthropist or Racketeer?

Carbon Dioxide has a nasty reputation. It’s considered an environmental criminal, going on a rampage warming up the globe.

Poor Carbon Di. It never meant for things to get like this. It started out with good intentions. It was, after all, always busy helping keep the planet warm by stopping the radiation from the sun from all bouncing back out into space.

It didn’t work alone, either. It worked as part of team with some other gases in the atmosphere. Big-name gases like Water and Methane. These guys, and a few (but highly influential) others formed the GHG Gang.

Without the work of the GHG Gang, the Earth would be a frozen inhospitable place. People would be begging for global warming, assuming we’d had a chance to evolve at all.

But Carbon Di just worked too hard and did its job a little too well. Instead of keeping the Earth at a pleasantly comfortable temperature, Carbon Di just kept pumping out there into the atmosphere. Carbon Di kept landing all the Total Combustion deals, spreading its wealth around, heating the Earth.

Now the Earth is getting too hot. Carbon Di went too far, and so the IPCC issued an arrest warrant.

The minor players in the GHG Gang are also fugitives from the law, but the real kingpin—Water—is keeping a mysteriously low profile. Perhaps Water has better biological contacts or something?

Gas Gansters can be rehabilitated though. Perhaps not the most morally bankrupt ones, like Mustard Gas, but CO2 started out a good guy. Some would argue Carbon Di still is a good guy, and has been unjustly persecuted. Regardless of your viewpoint on whether or not it’s doing anything wrong, it is obvious that Carbon Di is an overachieving Earth-Heating workaholic.

All that’s needed is to get our workaholic warming gas interested in something else. Something like refrigeration.

Putting Carbon Dioxide to Work

It might seem a little insane to think of CO2 as a refrigerant. CO2 causes global warming and refrigerants make things cold—but that’s the heat pump magic for you.

Phase diagram for CO2

Under normal circumstances (and by normal I mean conditions under which life will not be crushed, incinerated, or freeze-dried) CO2 is a gas. By increasing the temperature and pressure, it is possible to turn it into a super-critical fluid. It’s neither a gas or a liquid, but it still flows. The diagram shows the sort of temperature and pressure that the CO2 needs to be put under to make it supercritical.

Turns out that this super-critical fluid has some very handy physical properties, making it an excellent refrigerant for use in heat pumps.

Since the supercritical CO2 is cycled around the closed pipes in the heat pump, none of it is released to the atmosphere. That’s not to say that some might not leak out, but none of the CO2 is intentionally emitted from the heat pump. Thus the CO2 in the heat pump contributes very little to the global warming problem during its lifetime.

Competitors to the Eco Cute heat pumps use other refrigerants, such as HFCs. Although HFCs have an engineering advantage in that they can operate at much lower pressures than super-critical CO2, they have an environmental disadvantage. They have a significant Global Warming Potential (GWP). The most popular refrigerant, HFC-134a, has a GWP of around 1300 times that of CO2.

Since heat pumps don’t intentionally discharge their refrigerant to the environment, a refrigerant with a higher GWP isn’t a huge problem until the end of the heat pump’s life. When the heat pump is disposed of, that refrigerant has got to go somewhere. Even attempts to reclaim and recycle it will result in some losses to the environment. HFCs are clearly going to have a bigger impact than CO2.

These days engineers need to take the treehuggers a lot more seriously, so have found ways to overcome the challenges posed by the higher pressure operation of CO2 refrigerants. The solution could be the Eco Cute.

Now if they could just get it to actually look cute…

[1] Cute is apparently phonetically similar to the phrase meaning “supply hot water” in Japanese.

Image credits: EcoCute by User:Uri, via Wikipedia (CC-BY-SA); Green Leaf of a Bio Plant in Nature by epSos.de, on Flickr (CC-BY); I love my ball… by FlyNutAA, on Flickr (CC-BY)

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Heat Pump Magic

Thermodynamic Heat Crime

“Setting fire to chemicals like gas should be made a thermodynamic crime. If people want heat they should be forced to get it from heat pumps. That would be a sensible piece of legislation.” — Professor David MacKay (source)

Burning things is now a heatcrime. Doubleplus ungood!

Never fear. An upcoming alternative in order to heat water and rooms in households in colder climates is the Heat Pump. It’s not that heat pumps are a particularly new technology, they just haven’t ever been particularly popular. People prefer to burn things in order to heat their homes, because this makes sense. Humans have been burning things since the stone-age in order to keep warm, so it’s something that is easily understood and everyone is comfortable with.

Air source heat pumps take heat from cold air outside and use it to warm the inside of your house.

Read that again. It makes no sense. Especially considering that certain heat pumps can operate at outside temperatures as low as -20°C, and heat water inside to 90°C. We are used to hot things heating cooler things. If the room is cold, we turn on a radiator. The radiator is hotter than the air around it, and it is this difference in temperature that slowly heats the room. Heat pumps use something cold to make something warm. Insane science fiction! Surely?

Heat Pump Magic Demystified

  • You already have a heat pump in your house. You call it a fridge.

Refrigerate!

It’s in your kitchen, purring away as it keeps your food fresh and cool. When you first switch on your fridge the temperature inside the fridge is the same as the temperature inside the kitchen. As the fridge runs the temperature inside the fridge starts to drop. The heat is pumped out of the fridge into the kitchen.

Have you felt the back of a fridge? It is quite a warm place. Although the back of the fridge is warm, the temperature in the kitchen doesn’t increase noticeably. The heat flows into the kitchen, but because the kitchen is so much bigger than the inside of the fridge, it doesn’t make a significant difference.

We are comfortable with fridges. Not because humankind has had fridges since the stone-age, but because they’ve been around as long as anyone has been alive. We don’t question how they work, but actually fridges don’t make any sense either. They are taking heat from air inside the fridge to cool the inside the fridge which also warms the inside of your kitchen (but not enough to notice).

The secret to what is going on with fridges is explained with this thought experiment.

Imagine an electric kettle filled with water. Using electricity, the element in the kettle adds heat to the water, raising the temperature. After enough heat is added the water boils. At sea-level the atmospheric pressure is 101.3 kPa and the boiling temperature is 100°C.

Now imagine if you could somehow boil the water without heating it. You don’t turn on the kettle. It just starts boiling the water, seemingly spontaneously. What would happen then? Instead of heat being added to the water, it would leave the water, cooling it down and dropping the temperature. The kettle wouldn’t get hot, it would get cold.

Can we make something boil without heating it? Absolutely. It doesn’t happen spontaneously though. It requires a drop in pressure. A drop in pressure causes the liquid to boil and the temperature to drop. This is demonstrated in this video.

That’s basically how the fridge gets cold. Instead of water, the refrigerant inside the pipework of the fridge evaporated, cooling itself and its surroundings inside the fridge.

Unlike in the video above, the fridge doesn’t throw the gas away. It sends it through the pipework to the compressor. The compressor takes the gas and squashes it, increasing the pressure. Like blocking the outlet of a bicycle pump while pushing down on the plunger, the increase in pressure raises the refrigerant’s temperature.  It gets hot.

The hot refrigerant gas is then condensed into a liquid, causing the liquid to lose its heat. The heat has to go somewhere, so it passes into the kitchen via the pipes at the back of the fridge.

The refrigerant is now back where it started, as a warm liquid, ready to be evaporated again to cool the inside of the fridge.

  • Get a really big fridge. Heat your home with it and call it a heat pump.

With a fridge, the goal is to cool the inside of the fridge. It doesn’t heat the kitchen much because it isn’t designed to do that.

With a heat pump, the goal is to heat the inside of the house. By carefully selecting the refrigerant, operating pressure and temperatures, it is possible to design the heat pump to sufficiently warm a home during winter. It doesn’t cool the outside much (it does a little), because it isn’t designed to do that.

For more on principles and equipment used in refrigeration, see HowStuffWorks or my own explanation.

Isn’t CO2 bad?

You might be wondering about CO2 and greenhouse gas emissions. The Eco Cute CO2 air supply heat pump [pdf] claims to reduce CO2 emissions, but how is that possible if it uses CO2?

I’ll explore this issue in next Thursday’s edition.