[McQueen]

Why burn the coal at all, why not just place it on the piston and see it do its work !

[alancalverd]

Hi Alancalverd,
Here is an update. I wrote to one of the vacuum elevator manufacturers asking how much time in seconds, it takes for the lift cage to start moving after the button is pressed, she replied that it was considerably less than a second. Does this change anything ?

No. The air brakes on a truck or the vacuum brakes on a train work instantaneously (we hope) when required to do so, but it takes time to charge (or evacuate) the reservoir between applications of the brakes.

Why burn the coal at all, why not just place it on the piston and see it do its work !

Because when the piston reaches the bottom of its stroke, you have to schlep it all up the hill again.

[evan_au]

That's a nice animation of the Newcomen engine. It's mode of operation is as I remember it; what the animation does not convey is how slowly it operated.

I might be a die-hard believer in the second law for the conservation of energy, but I am an even stronger believer in the rapid dissipation of energy by vortices in fluids. The Home Grid will need a continuous input of energy (provided in the form of burning coal, for the Newcomen engine) to overcome losses due to friction, viscosity and turbulence.

I am happy to promote sound engineering principles, such as used by bridge builders. These include applying the laws of physics and calculating forces. But bridge builders who ignore the complex behaviour of moving air sometimes come unstuck.

[McQueen]

That's a nice animation of the Newcomen engine. It's mode of operation is as I remember it; what the animation does not convey is how slowly it operated.

That's a funny thing to say because as it happens the Wartsila Sulzer diesel engine works at an rpm of about 22 - 100 rpm and has a piston stroke that is about 2.5 m in length, (10 ft.) yet it is quoted everywhere as being the most efficient engine in the world.

I am an even stronger believer in the rapid dissipation of energy by vortices in fluids. The Home Grid will need a continuous input of energy (provided in the form of burning coal, for the Newcomen engine) to overcome losses due to friction, viscosity and turbulence.

The kinetic energy provided by the descending counterweight is greater than what is needed to run the vacuum generating device used in the vacuum elevator to evacuate an area two to three times the volume of the Home Grid system.  It should not be a problem, as for dissipation by vortices, if modern vacuum pumps did not have adequate measures to prevent blow back, at least while generating , what is in modern terms, such a slight vacuum, then technology has been in vain.

No. The air brakes on a truck or the vacuum brakes on a train work instantaneously (we hope) when required to do so, but it takes time to charge (or evacuate) the reservoir between applications of the brakes.

Well modern sewage vacuum devices which work over several miles, work in real time, no spooling up.

[SimpleEngineer (OP)]

Ahhh the good ol two stroke diesel engine..

As for the vacuum drive... I understand the principle of removing air, (not as hard as many would wish to believe), find it hard to see how this could be achieved as efficiently as you describe, or sustainably.. the losses in your system (friction, resistance, etc) would suggest that, yes it would start and carry on for a bit, but the strokes would get shorter and shorter until they stop and the energy required to get it started would more than likely be more than you could possibly get out of it..

I would LOVE to see this tried in practice though.. but would HATE to have to specify the tolerances and talk about them to contractors.. lol..

Reminds me a little of a particular hydroelectric plant I used to visit.. two lakes at different heights.. drop the water from the top one when the power is required for peak loading, pump it back up when there is excess energy to be used.. obviously your system might be able to provide a longer (but lower) continuous output, but operated like this would spread household use over a longer period, evening out those nasty spikes.

[McQueen]

I feel that it will either work or it won't, I can't see it slowly running down, b'cos basically it's just like the Newcomen Engine, those worked for a long time. In the early days the Newcomen engines created laughable vacuums, with the piston being sealed by rags etc., but everything gradually improved, in any case creating a vacuum with steam can't be compared to trying to suck the air out.

[SimpleEngineer (OP)]

It should slowly run down, each stoke will take out energy due to friction, giving you less energy to generate the vacuum from, any leaks would reduce your vacuum, increasing energy losses meaning less energy returned from the counterweight, less vacuum etc.

I can see how your thinking goes, and in an ideal system it should go forever, but realities tell us something different. I dot know how your turbines would draw through air at a fixed rate regardless of energy input, but I could say a rotation of say, a few variable vane pumps (offset rubber impeller) would deliver a fixed value evacuation per movement, and this will slow down as the energy available to move the air will decrease due to friction and resistances.   

[McQueen]

It should slowly run down, each stoke will take out energy due to friction, giving you less energy to generate the vacuum from, any leaks would reduce your vacuum, increasing energy losses meaning less energy returned from the counterweight, less vacuum etc.
I have a feeling that you have paid only the most cursory glance at how the 'Home Grid' system works. So although I had decided not to, i will try once again to explain:
The scientific  definition of a perpetuum mobile machine is as follows:- "hypothetical machine which, once activated, would continue to function and produce work"  indefinitely with no input of energy.”
A more accurate description would be “perpetual motion machine - a machine that can continue to do work indefinitely without drawing energy from some external source;”
   Machines which extract energy from seemingly perpetual sources—such as ocean currents—are capable of moving "perpetually" (for as long as that energy source itself endures), but they are not considered to be perpetual motion machines because they are consuming energy from an external source and are not isolated systems. Similarly, machines which comply with both laws of thermodynamics but access energy from obscure sources are sometimes referred to as perpetual motion machines, although they also do not meet the criteria for the name.
   The starting point is to decide which of these definitions best describes the working of the ‘Home Grid’ system.  As you can see I have settled on the third definition which I have underlined. The source of external energy to begin with is therefore gravity, which means that at the start of the operation the counterweight has to be hoisted into place.  Once that is done however, we find that  the height at which the counterweight has been placed, determines if there is enough energy  available to run a vacuum machine for long enough to create a vacuum within the system. Fortunately the ‘vacuum elevator’ which works on a similar principle operates to a height of 10m.  All things being equal the total volume of the ‘Home Grid’ system to be evacuated  is smaller than the volume to be evacuated in the ‘Vacuum elevator’ system.  Let us take as an example a counterweight of 150 Kg suspended at a height of 10m, then as explained, since it is connected by cable to an open ended shuttle, the only forces opposing the descent of the counterweight is the friction present at the sides of the shuttle and at the point where the cable passes over the generator sheave. To all purposes the counterweight is almost in free fall and its K.E can be calculated using the equation: mgh.  This gives a kinetic energy of 14.7 KJ, since the time taken for the counterweight to descend is 1.4 seconds, more than 7 KJ is available to turn the 5KW generator. Again since ( we questioned the manufacturers) it takes less than 1 second after the button is pressed for the vacuum elevator cage to start ascending, we assume that the 1.4 seconds available to use, is more than adequate time to create the vacuum, (a look at commercially available vacuum lifters should bear this out). The shuttle is now sealed, it has a vacuum below it and atmospheric pressure above it. Even if a vacuum of only 500mbar , or less, has been achieved it is still enough energy  to  lift the counterweight back to its original height. This can be calculated using the equation:   a= (m1 – m2)g/(m1 + m2) to find the acceleration of the piston as it moves down. So enough energy is available to both lift the counterweight back to its original position AND produce about 5KW of electrical output !
Look at the picture below:
 [ Invalid Attachment ]
This is a genuine perpetual motion machine design that is perpetually hopeful. Here only a single source of energy, namely gravity is used, in opposition to itself. Therefore it will never work, the forces equalise. In the ‘Home Grid’ system two forces are used atmospheric pressure in the presence of a vacuum and gravity. Remember that 24 horses, straining with all their might could not separate the spheres at Magdeburg, designed by Otto Von Guericke.  Atmospheric pressure in the presence of a vacuum is a powerful force my friend, easily the equal of gravity.  Infact to lift a 1 metre square board against a vacuum of 1 torr would require 10 tons , 22,000 lbs !
P.S. I will try to post a clearer explanation of how the 'Home Grid' system works when I have more time. But here is a tip, the counterweight,cable and pulley  system that power the vacuum do not contribute useful electrical output!

[McQueen]

I would LOVE to see this tried in practice though.. but would HATE to have to specify the tolerances and talk about them to contractors.. lol..
This sounds a bit more like it !  The only problem is that I don’t see why it can’t be done. Money continues to be thrown at wind and solar pv and solar thermal and to a lesser extent at tidal and wave power, why not give new ideas like ‘Home Grid’ a chance, surely it would not cost too much ?
As for the vacuum drive... I understand the principle of removing air, (not as hard as many would wish to believe), find it hard to see how this could be achieved as efficiently as you describe, or sustainably.. the losses in your system (friction, resistance, etc) would suggest that, yes it would start and carry on for a bit, but the strokes would get shorter and shorter until they stop and the energy required to get it started would more than likely be more than you could possibly get out of it..
The first cylinders used by Thomas Savery, were just suitably caulked wooden casks, these soon gave way to iron cylinders that were so badly cast it was almost impossible to get an air tight fit, many of these first cylinders were more ovoid than circular, but they still worked. The reason that they worked in the Newcomen engine was because the surface area of the piston was calculated in such a manner that the amount of force acting on the  piston ensured that it descended almost as rapidly as air leaked into the cylinder and under the piston, even taking into account the lifting of the counterweight.  This of course is the million dollar question, can modern technology build up a sufficient vacuum in the time given, about 1.4 seconds, to ensure that the counterweight rises again. This is the only question that remains to be answered and even here the outlook is bright because not one but  two manufacturers of  ‘vacuum elevators’ informed us that the lift cage starts to rise within a second of the button being pressed.  The other concerns, such as the system running down because of friction and inertia, seem far fetched, especially considering that the Newcomen engines, performed pretty well for almost two hundred years. (i.e., given the existence of a sufficient vacuum there is no reason for the system not to work).  In any case a true engineer does not sit there imagining what might happen, he takes out his slide rule or electronic calculator and gets down to calculating data and then , if so indicated, puts the design to the test by building something.  Even a simple engineer should know that.

[SimpleEngineer (OP)]

There has not been enough information given to actually do ANY calculations. But I will give my ideas a go.

the system is 150kg cylindrical block (probably benefit from a different shape but for calc means a cylinder) at 8000kg/m3 this give a cylinder with a height of 1m (arbitrary) and a radius of 0.077m.

The energy generated would be (from mgh) 14,700J from the drop, (vented top) the air pressure would supply 5.4kN at max vacuum.. we only need 2.94kN for return journey (barring resistances) meaning we would only need to draw 0.54 of a vacuum (so reduction to 46kPa) (assuming the remainding air is free to remove due to evacuation by the shuttle)

The volume of air would be 0.18m3 we would need to remove 54% of this.. 0.0972m3 per movement, 1 a second will give a vacuum requirement of 175m3/hr which requires 9kW (Power=flow.pressure difference). That's a very basic calc.. another works on pump down time (S=v/t . ln(Po/Pi) ) which gives 468M3/hr.. this would need a 25kW vacuum pump..

So depending on which you look at its still not possible. you will gain 7.35kW in total from each unit and need minimum of 9kW to operate it. (based on this system of course)

Calculations can go towards, slower running, different shapes and volumes, mechanical linkages etc. I based this off electrical power (and even if you got the difference down to a minimum there are efficiency to worry about)

[McQueen]

There has not been enough information given to actually do ANY calculations. But I will give my ideas a go. The system is 150kg cylindrical block (probably benefit from a different shape but for calc means a cylinder) at 8000kg/m3 this give a cylinder with a height of 1m (arbitrary) and a radius of 0.077m.

This is more like it, how positive and optimistic it is to actually be doing some calculations. What is not so nice though, is the armchair cynicism, can we really afford to be so offhand about things, I think if there is the slightest chance of something working we should get our noses to the grindstone until the idea has been completely, exhausted.

The volume of air would be 0.18m3 we would need to remove 54% of this.. 0.0972m3 per movement, 1 a second will give a vacuum requirement of 175m3/hr which requires 9kW (Power=flow.pressure difference). That's a very basic calc.. another works on pump down time (S=v/t . ln(Po/Pi) ) which gives 468M3/hr.. this would need a 25kW vacuum pump.
So depending on which you look at its still not possible. you will gain 7.35kW in total from each unit and need minimum of 9kW to operate it. (based on this system of course)

Have a look at the SMB-C06 roots vacuum pump manufactured by Vacuum Pumps America, it has a throughput of 600 m^^3/hr and operates at 3500 rpm, the motor works on 2.2 KW and the final vacuum produced is on the order of 10^^-4  Torr.  The fantastic thing is that the ‘Home Grid’ application needs only one hundredth thousandth of that degree of vacuum !!! What do you think of that, one hundredth thousandth of that degree of vacuum, several orders of magnitude lower than what is available ! So yes, it lays the field wide open as to what is possible and what is not. I am pretty sure that it can be done.

[SimpleEngineer (OP)]

Of course, I used the simpleset of simple calcs for my run through, however it may have shown why it is a very difficult system to calculate..

That pump you found is to be used as a booster pump, meaning that there is another beefier pump that will take the main power for the draw.. but a direct mechanical linkage to a piston type evacuater built correctly MAY provide enough vacuum but I am no pneumatics expert.

[SimpleEngineer (OP)]

Actually.. I reran the calcs.. dropped a few clangers in there.. apologies. Will try to fix tomorrow.

[McQueen]

Actually.. I reran the calcs.. dropped a few clangers in there.. apologies. Will try to fix tomorrow.

While trolling the net I have come across several vacuum pumps that will fulfill the criteria of 600^^3m/hr, dry running and using just 0.4 KW, that will reach 1 Torr without any backing or booster pump involved. This still leaves a margin of a factor of ten, since the vacuum that is required is more like 100 Torr than 1 Torr.  With the pump drawing such low power, it is possible that a separate vacuum pump can be used for each paired tube system, increasing the efficiency and  making the continuous generation of 15 KW possible.

[SimpleEngineer (OP)]

That is pretty much what I am finding with the errors in my calcs.. I am about a factor of 10 off in a few areas.. *sigh* the eternal struggle with calculations using different units.. We noticed this when i passed my calcs in front of my colleague who has designed vacuum systems.. (He is resolute that the system will lose too much energy through friction yet cannot negate the principle)

[McQueen]

(He is resolute that the system will lose too much energy through friction yet cannot negate the principle)

Since you seem to be so obsessed with the idea that friction is going to clog up the works, I decided to investigate the problem. It seems that PTFE  ( polytetrafluoroethylene) has a sliding co-efficient of friction lower than ice, this applies to PTFE and steel and  almost any other material including itself.  The co-efficient of friction of PTFE (a type of teflon) is just 0.05, lower in fact than ice on ice, you can be sure that the pulley will also have a  PTFE component  in it, either as a coating for the bearings or as an integral material.  Could you kindly factor in the numbers and tell me how you think that friction will slow the whole system down and eventually bring it to a halt. Frankly the logic behind the statement continues to baffle me, given the large amount of K.E involved  both during descent and ascent of the counterweight. The descent being powered by gravity and the ascent by atmospheric pressure in the presence of a vacuum.
Here are a few more facts about PTFE
PTFE possesses the lowest friction coefficients of all solid materials; between 0.05 and 0.09:

    The static and dynamic friction coefficients are almost equal, so that there is no seizure or stick-slip action
    when increasing the load, the friction coefficient decreases until reaching a stable value
    The friction coefficient increases with the speed
    The friction coefficient remains constant at temperature variations.
Wear
The wear depends upon the condition of the other sliding surface and obviously depends upon the speed and loads. The wear is considerably reduced when adding suitable fillers to the PTFE (see filled PTFE).
 

[SimpleEngineer (OP)]

See work up..
Dimensions
Weight = 150kg
Density (steel) = 8000kg/m3
Volume = 0.018m3
h=1m
r= 0.075m
A= 0.018m2
movement = 10m
Volume air = 0.18m3
Forces
F=ma
Downwards force = 1470 N
Maximum upwards force
If pressure above is 0
Pressure below weight = 100kPa
Force acting on weight = 100000*0.018  = 1800 N
Maximum Pressure required for return = 18kPa
Acceleration and times
Downstroke
 a= 9.8m/s2
 t= 1.43s
Upstroke
amax = 2.2m/s2
tmin=3s
Vacuum Calcs
cycle time of 5s  (using vacuum pot to ensure vacuum is instantaneous)
so 0.18m3 of air in 5 secs
minimum increase in pressure.. say 1kPa, If vacuum pot is at 1 torr. (133pa) then 0.18m3 of atmospheric pressure air would occupy 555.5M3 to give a rise of 1kPa... This require a vacuum pump that can do 111.1M3/sec on outline calc (555.5/5)
Using S=V/t x ln(Po/Pi) = 238 M3/s (V= 555.5, t= 5secs , Po= 1133Pa, Pi=133Pa)
Using Power = S x dP/mech eff (typically 0.85)   
=   130kW or 280kW

Friction force with PTFE will still be 60N with the differential force on this example being 330N, this is fairly significant.. that does not take into account the friction of the air movements etc.

Work up may be wrong only had 15mins to write it (10 mins was drawing a picture that was too large to be posted )
Point out changes please.. As the power is large.

Something forgotten is the increase volume of air at lower pressures.. I have tried to take this into account

[McQueen]

Using Power = S x dP/mech eff (typically 0.85)   =   130kW or 280kW

All I can say is that using your calculations, the vacuum elevator would never have been built, would not have even been a pipe dream or a gleam in someone's eye. Don't forget that the vacuum pumps used in a vacuum elevator have to be pretty sophisticated to maintain a pressure differential that allows a constant velocity of 0.15m/sec, the final velocity for 'Home Grid' would be about 14m/sec! So I would surmise that the power needed for the vacuum pump would be far less than the 3 KW needed for the Vacuum elevator. I repeat a figure of about 0.5 KW, I could give links but that doesn't seem to bother you too much.

[SimpleEngineer (OP)]

oh it does bother me.. I have rerun the calcs you see above, and now get a solid solution.. This is for the system described.. a better designed system could obviously offer a massive reduction to power or other means of recouping enrgy.. but as for now I only have the calculation steps above to go with.