And Now for Something Completely Different: Rotation Heat Pump!

Heat pumps for space heating are all very similar: Refrigerant evaporates, pressure is increased by a scroll compressor, refrigerant condenses, pressure is reduced in an expansion value. *yawn*

The question is:

Can a compression heat pump be built in a completely different way?

Austrian start-up ECOP did it: They  invented the so-called Rotation Heat Pump.

It does not have a classical compressor, and the ‘refrigerant’ does not undergo a phase transition. A pressure gradient is created by centrifugal forces: The whole system rotates, including the high-pressure (heat sink) and low-pressure (source) heat exchanger. The low pressure part of the system is positioned closer to the center of the rotation axis, and heat sink and heat source are connected at the axis (using heating water). The system rotates at up to 1800 rounds per minute.

A mixture of noble gases is used in a Joule (Brayton) process, driven in a cycle by a ventilator. Gas is compressed and thus heated up; then it is cooled at constant pressure and energy is released to the heat sink. After expanding the gas, it is heated up again at low pressure by the heat source.

In the textbook Joule cycle, a turbine and a compressor share a common axis: The energy released by the turbine is used to drive the compressor. This is essential, as compression and expansion energies are of the same order of magnitude, and both are considerably larger than the net energy difference – the actual input energy.

In contrast to that, a classical compression heat pump uses a refrigerant that is condensed while releasing heat and then evaporated again at low pressure. There is no mini-turbine to reduce the pressure but only an expansion valve, as there is not much energy to gain.

This explains why the Rotation Heat Pumps absolutely have to have compression efficiencies of nearly 100%, compared to, say, 85% efficiency of a scroll compressor in heat pump used for space heating:

Some numbers for a Joule process (from this German ECOP paper): On expansion of the gas 1200kW are gained, but 1300kW are needed for compression – if there would be no losses at all. So the net input power is 100kW. But if the efficiency of the compression is reduced from 100% to 80% about 1600kW are needed and thus a net input power of 500kW – five times the power compared to the ideal compressor! The coefficient of performance would plummet from 10 to 2,3.

I believe these challenging requirements are why Rotation Heat Pumps are ‘large’ and built for industrial processes. In addition to the high COP, this heat pump is also very versatile: Since there are no phase transitions, you can pick your favorite corner of the thermodynamic state diagram at will: This heat pump works for very different combinations temperatures of the hot target and the cold source.

8 thoughts on “And Now for Something Completely Different: Rotation Heat Pump!

  1. Hi Elke,
    Please join us on G+ Advanced Thermodynamics…..A lot of what we discuss is applied thermo and engineering. It would be valuble to have you contributing.

    Thanks,
    Erik

  2. Love it! And we can better… Combine this with the Tesla turbine concept and the system would auto-rotate and probably be suitable for low power needs. I need to figure out the fluid and heat flow but I have feeling that the pressure vector is the deciding factor. The tangential component drives the turbine and the radial component the heat exchange. Food for creative thought.

    • Let’s check if I got the gist of your idea 😉 Do you want to 1) drive the rotation by a separate Tesla turbine, so a flow of another fluid is going to provide for the input energy – possible the fluid exchanging heat with the system (water in this case), or 2) do you want to modify the flow of the refrigerant itself?

      Let me add some stream of consciousness of mine for the fun of it, and I am taking the risk of making stupid mistakes 🙂 …

      1) As I understand the Tesla concept the fluid enters the system at the perimeter, is gradually slowed down via its viscosity and thus energy is released to the rotating system while the fluid ‘falls’ down to the center. But I suppose all the energy conversions involved would make the process less efficient … compared to just connecting a motor driven by AC power?

      2) Re tangential versus radial: Heat is exchanged when the pressure is constant, thus when fluid flows parallel to the axis; when the fluid flows radially and pressure changes, (ideally) no heat would be exchanged (adiabatic). So a turbine-like process already happens when the fluid flows radially.

      But my main point is: If you try to extract additional ‘kinetic’ energy from the refrigerant for providing for more ‘input power’ – that power would finally be missing in output power(?) It would might be a zero sum game, as the small ventilator pumping round the refrigerant (just to overcome friction, in the way a water pump does not need to provide for ‘height’ in a closed cycle) would then have to work harder if you ‘milk’ the fluid’s mechanical energy?

      One advantage of the process the inventors mention is the decoupling of the refrigerant’s flow (using the ventilator) from the rate of ‘heat throughput’ – compared to a classical heat pump where the compressor does both – the ‘thermodynamically relevant’ thing and moving the refrigerant.
      I am thinking about the process in this way: Pressure, temperature, and density of the gas follow the thermodynamics of the Joule cycle … which is just a statement about thermodynamic states, not about the rate in which the cycle is traversed. So p, V, mass and T are roughly related by the ideal gas law. Heat energy density transported with the fluid varies throughtout the cycle (‘locally’) as heat is constantly fed in at the source and removed at the target. Adding the refrigerant’s cycling motion to the picture means putting time derivative dots on the volume and the mass in the gas law, one on each sides, turning this into a statement about energy flow. The ‘kinetic’ flow energy I mean is ‘contained’ in that added rate component, basically in the time the fluid needs to complete one cycle – I am using quotes as I mean in contrast to the internal kinetic energy of the gas which is captured by the thermodynamic variables.
      Using the fluid itself also to ‘drive the thermodynamics of the process’ would mix these two things, so ‘kinetic’ energy might be transferred to heat energy. So energy transported could be higher but the overall output per cycle could stil be the same as the rate of the energy exchange slows down.

      … or, is there an option 3) ?

        • Thanks – but why would the efficiency be higher compared to a Rotation Heat Pump running directly on electrical power?

          They have managed to achieve efficiencies of nearly 100% for the compression of large machine, so the industrial process can hardly be improved. I think this might be more difficult to achieve on a smaller scale as mechanical imperfections and losses usually don’t scale (‘Micro-turbines’ are less efficient than larger ones).

          But even if the efficiency is lower for smaller devices – why would feeding in power via a fluid be more efficient than directly rotating on AC power? I think the mechanical imperfection argument would also apply to the Tesla turbine in the same ways as it applies to the heat pump itself.
          Of course it is finally an economical argument … how much would it cost to make the smaller heat pump as efficient as the large one. But I believe adding something to the design would make it more complex and thus rather more expensive?

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