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Clean, low running cost refrigerators

Patent application number GB 0907998.9

 

Stirling cycle refrigerators are famous because of their
low pollution, but disappoint because of their high
running costs.

 

What we are offering is a totally new Parallel Stirling refrigerator cycle that overcomes the low efficiency problem.

You don’t need to be familiar with the existing Stirling refrigerator cycle to follow our explanation, but just in case you are curious, we explain the conventional Stirling cycle in an appendix at the end of this page.

Cryocoolers
Our design can be used for very low temperature refrigerators or cryocoolers.

Here are some potential applications for a highly efficient parallel Stirling cryocooler:
(i) They could be used to cool power transmission cables to very low superconducting temperatures where there are no electrical resistance losses in the cables. (Currently, approximately 10% of all electricity generated is lost in the transmission process from supplier to consumer.)
As a bonus, burying superconducting cables underground is easier than burying conventional cables because there are no overheating problems to be solved.

(ii) Superconducting magnets would reduce heat losses in electric motors and generators.

(iii) They would reduce the running costs of magnetic traction rail transport systems. (Hyperlinked article.)

(iv) Cutting edge physics.
Large capacity cryocoolers suitable for cooling
nuclear particle accelerators are discussed in Section 4 below.

Throughout this article we will refer to our design as a “cryocooler” to emphasise these low temperature roles.

Looking for a research project?
Be the first to build a Parallel Stirling Cycle refrigerator!

In Section 6 we suggest a simplified version of the design for student project purposes.

 

 

1  The Parallel Stirling Cryocooler- How it works

Conventional refrigerators work by pumping heat out of a cold chamber,
raising it to a temperature above ambient, and then dumping it into the
environment as low grade heat.

Parallel Stirling cryocoolers and refrigerators are fundamentally different because they cool the working gas by forcing it to do external work.

We will start off with a diagram showing the main components, and then  elaborate on the details.

Fig. 1. Gas expansion and compression are happening simultaneously in different parts of the system.
In conventional Stirling refrigerator designs
expansion and compression take place in sequence.

 

Below we reproduce the same diagram with brief explanations of how the main components work together.

Fig. 2. Heat is extracted from the cold chamber and electricity is generated. 

To a first approximation,

Rate of heat extraction = rate of electricity generation - rate of working against drag

 

The turbine unit consists of a series of sets of axial rotor blades, each set separated from its predecessor by static blades attached to the side walls. The tapered shape intended to offset the increase in gas density as it cools has been exaggerated for clarity.

The heat exchange mechanism

The pipe work and turbine chamber walls are made from aluminium alloy or other good thermally conducting metal. This allows the gas travelling from the warm chamber to transfer heat to the cooler gas passing back to the warm chamber via the jacket.

Fig. 3. The design offers heat exchange features. The two half jackets can be replaced by a single jacket and two exit valves. But this reduces the temperature drop caused by expansion because the total volume of gas expanding at any given time increases.

The gas travelling from the warm chamber to the cold chamber is subject to two cooling and two warming mechanisms, before it enters the cold chamber.

Cooling   (i) Cooling due to external work being done, driving the turbine.
              (ii) Cooling as heat is transferred to gas returning to the warm chamber.

Warming (i) Warming as the gas is compressed ahead of the piston.
              (ii) Due to friction as work is done against drag.

Cooling caused by gas expansion is useful because it increases temperature gradients and hence rates of heat flow. It does not produce significant net cooling because expansion in those parts of the system behind the piston is offset by compression ahead of the piston.

Design notes
(i) For visual clarity, narrow bore pipes and sharp bends in the pipes have been drawn. In reality, a design aim is to reduce drag outside the turbine, especially on the return path to the warm chamber. This means bends in the pipe work and valves will be softer and the pipe diameters will be as wide as possible. 
(ii) If, in the steady state, the gas exits the compression chamber at a temperature above ambient, then the outflow pipes can be un-lagged and heat dissipation fins added.
 

2 Examples of suitable dual chamber pumps

2.1 A simple pump based on existing loudspeaker designs.

Fig. 3. An oscillating piston partitions the warm chamber. Two motors drive the piston, these being essentially powerful moving coil loudspeaker type motors. Armature coils mounted centrally, one on each side of the piston carry alternating currents. These act in harmony, causing the piston to oscillate to and fro, between two pot shaped permanent magnets.

 

2.2

Fig. 4. Two single chamber pumps operating in anti-phase can simulate a dual chamber pump.

2.3 For dual chamber pump designs offering lubricant free action suitable for cryogenic applications please visit our Lubricant free pump page.

 

3   The turbo-generator unit

3.1 The tapered turbine

Fig. 5. Conventional gas turbines flair out, but ours tapers very slightly in.
This unorthodox design works because the dual chamber pump is simultaneously pushing and pulling the gas through the turbine.

A second difference is that we use drag to our advantage because the pulling effect of the piston allows the gas to expand in the cold chamber.

 

 

 

3.2 A magnetic bearing design for cryogenic temperatures

Fig. 6. The coil draws its current from the turbo-generator, so the coil’s magnetic attraction for the underlying bar magnet varies in step with the rate at which gas is pumped through the turbine.

 

4 Large capacity coolers

The following design is proposed for cooling superconducting power grid cables and large superconducting electromagnets such as those that will be required for the next generation of nuclear particle accelerators.

 Essentially it consists of our Parallel Stirling cryocooler combined with an insulated Latent Power Turbine (LPT).

4.1  The low temperature end, a helium filled Parallel Stirling Cryocooler

We propose using a simplified version of the cryocooler described above.

Fig. 7. In order to reduced the number of moving parts in the coldest parts of the system, the turbo-generator has been replaced by by two long lengths of narrow bore metal tubing.

4.2  The "high temperature end, a hydrogen filled Latent Power Turbine

Here is a brief explanation of how an LPT works. If you require more details  switch to our LP Turbine theory page.

Fig. 8. In order to generate a net output of power the trick is to drive the gas through the turbine at a higher speed than it transits the pump so that its kinetic energy increases at the cost of Venturi cooling.
Latent Power Turbines produce a net output of electricity if the turbine-generator output is greater than the work done on the working gas by the pump. Drag limits the lower size of  LP Turbine units so they cannot compete with Parallel Stirling Cryocoolers for small refrigerators.

4.3 Combining the two coolers

Fig. 9. This diagram illustrates the principles of a two stage cryocooler for cooling cables to superconducting temperatures.

 

5  Key benefits of the Parallel Stirling cryocooler design

1. Existing Stirling refrigerators are inherently inefficient because their sinusoidal movement is only a crude approximation to the jerky stop-start movement demanded by ideal Stirling theory. The parallel design theory does not require stop-start movements.

2. The parallel design further improves efficiency by generating electricity as a by-product of the cooling process.

3. There are no solid moving parts in the cold chamber. Consequently, cold chamber abrasion and swarf accumulation problems are eliminated.

4.  Linear motors are employed in the warm chamber, so piston side slap wear is eliminated.

5. Any  gas leakage across the warm chamber piston/diaphragm stays within the warm chamber housing, so there will be reduced loss in  performance due to aging.

6. The cold chamber can be moulded to fit snugly around an electromagnet, circuit or sensor. The engineer also benefits from a similar flexibility in deciding on the Dewar flask shape.

7. The cold chamber has a minimum of connections to the warmer parts of the system, so heat leakage into the cold chamber is also minimal.

8.     At maximum power, the dual chamber design allows the total volume of the warm chamber to be used This offers a good power to size ratio compared with existing cryocoolers,

9. The use of linear motors eliminates energy losses due to the swirling of gases inside rotating flywheel crank case.

10.   The parallel design is more complex because valves and external work units are required, but the use o magnetic bearings means that lubricants are eliminated and  fewer precision engineered components are required.

 

6  The proof of concept model - A student project?

The inventor, Bill Courtney,  is looking for partners to build a proof-of-concept Parallel Stirling refrigerator. If you would like to take up this challenge, here is a basic refrigerator design which does not involve any novel components.

Fig. 10. The function of the obstacles is to create conditions for pressure drops inside the cold chamber as gas is drawn out behind the currently advancing piston.
Heat is generated inside the obstacles as work is done against viscous drag.
Standard design features can be incorporated to enhance the rate of heat dissipation from the obstacle surfaces. e.g., adding cooling fins, painting surfaces matt black placing the apparatus in a draught etc,.
 

This is only a proof of concept design
It is important to emphasise that
(i) The obstacles are not Joule-Kelvin plugs. (But
Joule-Kelvin cooling can occur as a bonus, if the gas exits the plugs below its inversion temperature.)
(ii) This design would be needlessly inefficient for practical purposes because we can do useful work instead of generating waste heat.

These cryocooler designs are protected by a patent application and copyright. Please contact Bill Courtney before embarking on any experimental work that you intend to publish.

Appendix

The current "series" Stirling refrigerator design

Fig. 11. The basic features of a Stirling refrigerator.

Stirling refrigerators include warm and cold chamber pumps. These are used for pumping a gas, typically helium, between the chambers.
The pistons move in a series of synchronised steps to cause heating in the warm chamber and cooling in the cold chamber.

The ideal series of steps is as follows:

Fig. 12 The warm chamber piston moves part way to right, while the cold chamber piston blocks the mouth of the cold chamber. This compresses the working gas in the warm chamber. Ideally, all of the heat of compression is dissipated into the environment.
 

Fig. 13 Both pistons move to the right so that pressurised gas is displaced into the cold chamber at approximately constant pressure.
 

Fig. 14 The cold chamber piston moves further to the right so that the gas expands and cools.

 

Fig. 15 Both pistons move to the left, so that the gas moves back into the warm chamber at low pressure. The first stage is then repeated.

In reality it’s not possible to stop and start the pistons when they are operating at a useful frequency. Instead the cycle is approximated by oscillating the pistons sinusoid ally, but 90o out of phase. This reduces efficiency because the compression and expansion phases partly overlap.

An even bigger problem is the wear and tear caused by operating a piston in the harsh environment of the cold chamber.

Engineers have developed variations on the Stirling concept that eliminate the need for a piston in the cold chamber but at the price of a further drop in efficiency caused by even larger deviations from the ideal stop-start Stirling cycle.