Principle
and advantages of the regenerator
The
regenerator
(RGN) is fundamental to recycle inside the device (1) the thermal
transfers of the isochoric stages (2-3 et 4-1) and thus
to stretch toward the limit of Carnot.
The technical difficulties it currently
raises are a major brake to the improvement of
Stirling's engines. The current regenerators are often
victims of the alternate flux of
the fluid, not encouraging any good thermal exchanges, unless to
use fine grids that pose two new problems then : their cost
and
especially the losses of loads by lamination of the fluid. Otherwise,
they are often cumbersome and difficult to isolate.
Thus, the present invention makes the
choice of
unidirectional out-flows of fluid between every couple of PRATL
machines, one hot (2C) and the other cold (2F), what permits to
constitute a thermal exchanger with 4 pipes, of
preference rolled
up in helical. These pipes are browsed by the fluid: two among them
constantly bring the fluid from (2F) to (2C), and the two other in the
inverse sense: from (2C) to (2F). Besides, they are mutually in thermal
contact and constitute an optimal
exchanger of temperatures because :
- the length of the pipes can be strongly
raised
while keeping a good compactness because they are rolled up in helical,
- the section of the pipes can be
sufficiently big to make negligible the load losses by lamination, of
preference
gaseous,
- it is easy to isolate such a
regenerator by using a
cylindrical muff (ISO) with reflecting faces (to prevent the
radiative losses) and having at least one empty cylindrical cavity
(to block the thermal conduction from the regenerator toward
the
outstide).
Optimization
of Stirling's cycle with SPRATL engines
That the cycle is motor or receptor, one
sees that
the working is optimized with an effective regenerator, but also
when its area is maximized. So that the area is maximized, the
transformations must perfectly follow the previously described
thermodynamic trajectories; the figures 1A to 1F describe indeed
the normalized cycles, with a perfect regenerator with ideal
thermodynamic evolutions of the fluid. Actually, the cycle followed by
the fluid moves away meaningfully of the one of Stirling as drawn on
figures 1G and 1H. Four shortcomings are in general present :
- DTC : defect of homogenization of the
fluid to the hot temperature,
- DTF : defect of homogenization of the
fluid to the cold temperature
- DVMAX : volumic defect at maximum
volume Vmax,
- DVMIN : volumic defect at minimum
volumet Vmin.
- defect of homogeneization of
temperature due to :
o the
imperfect recovering of calories or frigories in the regenerator,
o the
slowness of the thermal
diffusion in the fluid when it is put to the contact of the
hot or
cold sources,
- Volumic defect due to :
o
imperfect kinematics of Stirling's machines,
o possible
flights of fluid.
The present invention settles completely
the
volumic problems thanks to a kinematics which perfectly respects the
isochoric stages. It also limits the flights of fluid well thanks to
all surfacic contacts and the possible use of
many segments of tightness. Moreover, it allow intense thermal
transfers :
- by convection :
o in the
cold and hot machines
where the fluid is injected in the rooms, then transported within the
cold and hot machines
,
while being entirely surrounded
by metal walls transmitting it their temperature (cold or hot),
o in the regenerator allowing a
setting in temperature much better just
before the isochoric stages.
- and by diffusion nearly the contact
walls/fluid.
The approach by convection is essential
because it
is a mean of a lot faster homogenization than the only diffusion. Thus,
as shown in strong features
on figures 1I
and 1J, thanks to the present invention, the cycles are a lot nearer of
the ideal Stirling's cycle (in thin features) and bigger than the
present cycles (hatched feature). The isotherms will be as much better
respected that :
- the machine will turn at low speed :
the strong
volumes by round of piston (PRA) with the machines (2,2F,2C) is in
itself an asset, permitting to make work a lot of fluid in spite of a
relatively weak rotation speed,
- the fluid will have an elevated thermal
conductivity : one will be able to use some fluids under more elevated
pressure and/or specific gases (Hydrogen,
Helium) already met with success in the
industry of the Stirling's motors.
Thermal working of the
SPRATL regenerator
The
regenerator (RGN) is
fundamental to recycle within the device (1) the thermal exchanges of
the isochoric stages and thus to
stretch toward the limit of Carnot. In the device (1), the regenerators
assure unidirectional out-flows of fluid (inside a given pipe) between
the hot machines (2C,2C1,2C2.) and the cold machines (2F,2F1,2F2.),
what permits to create thermal exchangers with pipes, of
preference rolled
up in helical. The half of these pipes constantly drives the fluid from
(2F,2F1,2F2.) to (2C,2C1,2C2.), and the other half in the inverse
sense: from (2C,2C1,2C2.) to (2F,2F1,2F2.).
Besides, they are mutually in thermal
contact and constitute an almost-perfect exchanger of temperatures.
As it has already been evoked, it is possible to pair some couples of
pipes in only one : then the
out-flow becomes unidirectional and continuous. Indeed, the fluxes of a
category of rooms (big: GC) (small: PC) of a machine (2, 2F,2F1,2F2,
2C,2C1,2C2.) are intermittent, identical and in opposition of phase ;
so that while connecting only one pipe on the 2 exits of identical
rooms of a same machine, the flux, in addition to be unidirectional,
becomes continuous.
The thermal aspect of the regenerator is entirely
explained in SPRATL patent
(in french) or in this
extract from the patent (in french) by using the figures
5A, 5B et 5C and by calling the equations of the
thermal diffusion. Some regenerating outputs of more than 99% are
reachable with very acceptable sizing and usual material like steel..
