Environmentally friendly and efficient propane heat pump

September 4, 2018, Fraunhofer-Gesellschaft
Modeled on nature: The newly developed heat pump distributor takes its inspiration from the branching limbs of a tree. Credit: Fraunhofer ISE

Heat pumps use environmental energy to provide us with heat. However, they generally require synthetic refrigerants, which contain environmentally harmful fluorinated greenhouse gases (F-gases). Fraunhofer researchers have now contributed to the development of a heat pump that uses propane instead. The pump is both more climate-friendly and more efficient.

"Heating and hot water account for around 40 percent of Germany's final energy consumption. Burning high-quality fossil fuels such as natural gas or crude oil not only makes little sense energetically, it also harms the climate. Each unit of electrical energy required to operate a heat pump, derived often from renewable resources, generates three to five units of CO2-neutral heat energy. This makes heat pumps an important element in implementing Germany's transition to a sustainable energy system," says Dr. Marek Miara, who coordinates work on heat pumps at the Fraunhofer Institute for Solar Energy Systems ISE in Freiburg.

A heat pump works in a similar fashion to a fridge. The refrigerant absorbs the heat inside the fridge and transports it outside. The difference is that heat left to escape freely from the back of a fridge is what a heat pump extracts – in this case from the ground, groundwater, or ambient air – to heat our homes or water.

To achieve this, the heated, vaporized refrigerant is compressed, which raises its temperature and pressure. The hot refrigerant gas releases its heat into water and condenses. The warm water flows into underfloor heating systems, radiators or hot water storage tanks, while the liquid refrigerant, now cool, flows back into a so-called evaporator, where it once again absorbs heat energy. The cycle then starts again from the beginning.

For the most part, refrigerants are composed of a mixture of synthetic substances containing environmentally harmful, fluorinated (F-gases). In June 2014, the European Commission announced that F-gases are to be phased out of the market. One environmentally friendly, natural alternative to synthetic refrigerants is propane, which is already gaining in popularity in air conditioning and refrigeration systems. But its use in heat pumps is still relatively new.

Because even though propane has excellent thermodynamic properties, it is highly flammable, and this poses a challenge when used in a heat cycle.

"If you want to use propane, you have to keep the volume of refrigerant as low as possible to minimize the risks involved," says Dr. Lena Schnabel, who heads the department for heating and cooling technologies at Fraunhofer ISE.

A bionic structure ensures even distribution

The solution of the ISE researchers, along with their European research partners, is to employ highly compact, brazed, finned heat exchangers that function well with small volumes of liquid. The thermal is transferred from one flowing substance to the other via heat exchangers. These are composed of numerous parallel channels containing the circulating refrigerant, which either absorb heat (known as "vaporizers") or radiate it ("condensers"). "The liquid should completely vaporize or recondense over the running length. To guarantee they operate efficiently, the vapor-liquid ratio must be identical in all the channels. Generally, that's not easy to achieve, and it becomes especially tricky if you're also trying to limit the volume of refrigerant."

To solve the problem, Schnabel and her team developed a distributor with a bionic structure: "Conventional Venturi distributors look like a pile of spaghetti made of many thin tubes that merge where they meet the vaporizer. Our distributor is different: it has a continuously branching structure like the branches and twigs of a tree, which ensure even distribution of the refrigerant into the individual evaporator channels, even with a small volume of ." This structure allows optimal use of the entire surface of the exchanger, which improves efficiency.

To reduce the risk of explosion when compressing the propane, Schnabel and her team used a specialized compressor in which all ignition sources were encapsulated. They took great care connecting the individual components of the pump to prevent propane from escaping. "We are currently modifying the technical design of the , testing the long-term behavior of its components, and developing sustainable safety strategies," says Schnabel.

Explore further: Natural refrigerant replacements could reduce energy costs and conserve the environment

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8 comments

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Isotherm7
5 / 5 (4) Sep 04, 2018
Title "Propane-burning" ??? The article describes the use of propane as a working fluid in heat pumps, not as the energy source for the cycle. It would be possible to additionally burn propane as the energy source, which is of course not CO2 neutral.
carbon_unit
5 / 5 (1) Sep 04, 2018
Propane? The phrase "what could possibly go wrong?" comes to mind...
The tree distributor seems like a neat idea, no matter what the refrigerant.
humy
5 / 5 (5) Sep 04, 2018
This is just one nice example of many of an environmental problem (involving fluorinated greenhouse gases in this case) being solved (via propane in this case) just by via some smart people thinking about it and researching it.
I have little doubt that one day we would have solutions to all these environmental problems and that we will stop burning fossil fuels because we will cease to have any need to.

Clearly the editor of this article didn't quite understand it as shown by inserting the erroneous "propane-burning" into the title when it doesn't actually burn the propane!
In theory (and eventually in practice), renewables (with perhaps just some occasional nuclear power) could power it without any combustion involved.
pntaylor
1 / 5 (2) Sep 05, 2018
+1 to carbon_unit. "What could Possibly go wrong?"

The idea of introducing a flammable and combustible material, into systems which will end up everywhere (residential, commercial, industrial) is both insane and stupid.
There will always, Always be leaks, from manufacturing defects, poor installations, fires, other local crises. Terrible dumb idea.
This is as much an environmental hazard as R-12 and R-22, just on a more local basis.
antialias_physorg
2.3 / 5 (3) Sep 05, 2018
Propane? The phrase "what could possibly go wrong?" comes to mind...

Not really.

1) We're dealing with very low volumes
2) Propane doesn't instantly burst into flames when it leaks. Like all flammable gases it must be within a certain concentration to even be ignitable. The lower explosive limit for propane is 2.1% and the upper explosive limit is 9.5-10.1%

For comparison: A HCF with a comparable autoignition temperature to Propane that was used in refrigerators before it got banned was Dichlorofluoromethane. This is flammable between 0.1% and 54% ... and we didn't hear about exploding refrigerators back then all the time, either.
humy
not rated yet Sep 05, 2018

The idea of introducing a flammable and combustible material, into systems which will end up everywhere (residential, commercial, industrial) is both insane and stupid..
pntaylor

Have you ever driven a car with a combustion engine or taken a cab or chosen to take a ride on a bus or a train or an airplane? Or what about using gas central heating or a gas cooker? Or matches and/or candles?
These things generally have in them flammable and combustible material (the fuel). Do you say that makes them "insane and stupid"? If so, why did you allow yourself to ever use any of them?
I am sure that eventually one day we have safer alternatives to all of them but for now we need them.
pntaylor
3 / 5 (2) Sep 05, 2018
"If you want to use propane, you have to keep the volume of refrigerant as low as possible to minimize the risks involved," says Dr. Lena Schnabel

I completely missed the above statement. I do stand corrected.
carbon_unit
not rated yet Sep 05, 2018
Propane? The phrase "what could possibly go wrong?" comes to mind...
Not really.

1) We're dealing with very low volumes
2) Propane doesn't instantly burst into flames when it leaks. Like all flammable gases it must be within a certain concentration to even be ignitable. The lower explosive limit for propane is 2.1% and the upper explosive limit is 9.5-10.1%

For comparison: A HCF with a comparable autoignition temperature to Propane that was used in refrigerators before it got banned was Dichlorofluoromethane. This is flammable between 0.1% and 54% ... and we didn't hear about exploding refrigerators back then all the time, either.
Nice write up, a_p. I kind of figured as much. A small leak will not be enough to cause any problem and propane will be less damaging to the environment than the F-gasses. One would have to be *really* unlucky to have a disaster. Such are tradeoffs. Still, I could no help thinking of the phrase.

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