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General description of the operation

The choice of this thermal cycle for the conversion of the thermal energy in mechanical energy involves to my opinion advantages either regarding the Rankine cycle (steam engine) or regarding the Stirling motor; regarding the steam engine it has a greater efficiency and better conditions than emergency; regarding the Stirling motor a smaller weight and a greater picking up surface the solar energy, that can be absorbed through all the exposed surface of the coil tube, to arrange inside of one thermally isolated cavity at the height of the plan of focalization of the solar concentrator.

Moreover this thermal engine works with the same principle of the heat pumps and the air conditioners; therefore it can easy be converted for these uses.

Also the last version of the refrigerators to high efficiency uses the same principle of operation and the same fluid (the air) in place of freon and of other compounds that to short or in the long term damage wrap it of ozone in the stratosphere that ultraviolet us protegge from the dangerous ones.

Description of the operation: the fresh air to ambient temperature Ta enters from the valve To (that it can be set in action, to the par of the others two valves C and D by means of a system of auctions and rockers, making head everyone to one opportune fixed cam on the same drive shaft) and subsequently compressed; in this phase it increases its temperature to the Tc value; therefore it passes in the tube to coil passing through the B valve that is one simple valve not return (like those of the tires of the cars or the bicycles).

Passing through the coil it comes heated to the Tmax temperature and increases therefore to its thermal energy and its volume; therefore, when the piston of the seconnd cylinder is to the top dead center, it comes introduced in the cylinder through the D valve in the such amount to have, to the end of the successive adiabatic expansion, in correspondence of the bottom dead center of the same cylinder, approximately the equal pressure to that atmospheric one; to that point its temperature will have the value of Te inferior to that Tmax caught up in the coil but also better than the original tenperature of the atmosphere Ta.

It could to this point to think to recover part of the thermal energy that the air emitted to the point E still possesses partially heating the air as soon as compressed to the beginning of the coiled tube by an opportune heat exchanger; however it can be demonstrated that the maximum power, to parity of number of turns of the motor in the time unit, is had just when the temperature caught up in jam phase Tc  is equal to the temperature of the air of drainage to the term of the phase of expansion Te; in these conditions a good rendering is obtained however, without to resort to the heat exchanger; such air in escape instead could be immessa in an other tube to coil dipped in the water obtaining warm water to send to a thermally isolated tank for a successive one uses.

The efficiency of conversion of the thermal energy in mechanical energy can be expressed either by the expression:

                                                                         η  = 1 - (Te - Ta)/(Tmax - Tc)

or by the more practical expression:

                                                       η = 1 - 1/(rp ^ 0,286)

being rp the ratio between the pressure of the air to the inside of the tube, after the jam and the pressure of the external air (1 ata).

Just in order to control such compression ratio the valve A will not closed after the phase of aspiration but after a certain angle of spin beyond the bottom dead center of the first cylinder, reintroducing to the external part of the inhaled air; this is justified holding account that during the next heating the air increases of volume; therefore for one Tmax temperature around to the 350 °, that door to a doubling of the volume of the air, or better of the product Pressure * Volume, the valve To will go sluice nearly to half of the race of the first cylinder, in order to obtain an optimal rendering.

As an example, with a ratio between  pressures of  4:1 , the theoretical efficiency of such thermal cycle turns out:  η = 0,327 that is of 32,7%; remaining 67% of the acquired thermal energy from the sun can instead be used for heating the water.

Choice of the capacity of the air motor warm

Wanting to realize such motor with utensils and purchasable utensils near shops for hobby and bricolage is worthwhile to refer to a capacity around the 125 cm³s typical of the motors of the motorbike, in how much in the shops of exchanges for motorcycles they are easily available pistons for this capacity.

Besides the "shirts", the cylinders within which you/they stir the pistons, are that is easily obtainable from a pipe for plumbings gas without weldings, directly gotten that is for extrusion, of the diameter of 2 thumbs, from which the two long necessary cylinders 12 or 12,5 cms must be drawn through a disk troncatrice.

Such cylinders have that is a diameter inside note of 2 thumbs of 2,54 * 2 = 5,08 cms; in reality a pò maggiore,intorno to the 53 mms., and a form already almost perfectly circular.

The thickness of the metal, greater of 3 mms and the external diameter of 60 mms allow a rectification on you also return of small dimensions bringing the inside diameter to 54 mms.

In such way it will be enough to slightly ask to the supplier of exchanges for two pistons of inferior diameter, for instance of 53,75 mms, leaving a game of 0,25 mms.

The estate of the pressure will be guaranteed from the special rings of estate (1 of it or 2 are enough for piston).

In the case they were not available also the pistons you/they can be realized with the same lathe; the finishes will be less "refined" or sought after of those in commerce but surely more strong, departing from an aluminum cylinder of the diameter of 60 mms.

In such case from the retailer of exchanges the alone rings of estate must be purchased, even more easily available and after all difficult to realize.

The superior mass of the realized pistons artigianalmente won't be a problem because the speed of operation of such motor is inferior to those mottos borne by the motors for motorcycles, that is around the 3000 giri/mins, if joined to a dynamo to directly get the tension 220 Vs alternated or few greater if joined to an alternator for auto to get a tension of 12 Vs it continues fit to load one or more connected batteries eventually to a converter from 12 Vs to 220 alternated Vs, system by itself already usable as static converter of electricity for personal computer and as autonomous generator of emergency.

With the described dimensions the section of every cylinder is of 22,9 cm²s and the capacity it results equal to 137,4 cm³s with a run of 60 mms and 160 cm³s for a run of 70 mms.

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Approximate calculation of the power disbursed by the motor

To calculate the disbursed power needs to appraise the quantity of heat Qi introduced by the outside after the initial compression.

The formula that expresses her/it is:

Qi = m * Cp * (Tmax - Tc)

being m the mass of air inhaled by the motor in the unity of time and Cp the specific heat of the air.

To the level of the sea the atmospheric pressure is worth P = 1 ata or 0,0981 MegaPascals, that is 98100 Nw/m²s.

Under such conditions the mass of the dry air is worth around 1,29 grammi/litros to the temperature of 20°C.

The specific heat is worth 1 Joule/grammo/°C instead, that is an energy is necessary of 1,29 jouleses to increase of 1 centigrade degree the temperature of 1 liter air under conditions "normal."

The mass of air inhaled in the unity of time by the motor depends from the capacity and from the conditions of operation.

We hypothesize a capacity of 160 cm³s for every cylinder, a speed of rotation of 3000 giri/minutos, Tc = 180°C

(for a relationship of the pressures of 4:1) Tmax = 430°C, and therefore Tmax - Tc = 250°C.

For the considerations on the optimal operation of the motor the valve Á. will be closed after 1/3 some run back and therefore the air introduced to every turn will be of around 105 cm³s and the general volume of the air inhaled of 5,25 litri/secondos.

The energy absorbed by the sun for every according to it will be therefore:

Qi = 1,29 * 5,25 * 1 * (430 - 180) = 1693 joules

The energy gotten in the unity of time is gotten multiplying Qi for the output η

L = η* Qi = 0,327 * 1693 = 554 joules

correspondent to a power of 554 Watts.

You/he/she can seem a non elevated power but it is already proper when the solar radiation has an intensity of 500-600 W/m²s, that is verified when a pò of haze is present or with clear sky but in the first hours of the forenoon or in the first afternoon.

The power of 1 Kw could be gotten containing the preceding motor in a container that withstands the pressure of some atmosphere, realizing a closed circuit with inferior pressure of 2 atm and superior pressure of 8 atm, working with the same relationship of pressures 4:1.

It deals with absolute pressures, for which, using a manometer, that instead measures the pressure Pr related to that atmospheric, keeping in mind that:

Pr = Pa - 1

the inferior relative pressure, pointed out by the manometer, it will have to result = 1 atm while that superior will have to result = 7 atm.

In general the aforesaid power can be increased of turned n, working among an inferior relative pressure Prmin = n - 1 atm and a superior relative pressure Prmax = 4n - 1 atm.

In this case it is possible to use, to the place of the air, a mixture of carbonic anhydride and nitrogen, obtainable consuming the oxygen of the contained air in a container, that, also having her same thermodynamic characteristics of the air, it avoids every possibility of usury and inside corrosion of the motor, also using for the shirts the sweet steel of the pipings gas mentioned termicamente to the place of the treated steel and cemented of the shirts of the normal motors to burst.

Greater powers can be gotten using pipes from 3 or 3,5 thumbs, with pistons of correspondent diameter and opportune run that it brings to increase of 3 times or anymore the capacity and the performances of the motor.

In general it is worthwhile to operate with low speeds of rotation, as the consequential attritions from the inertial solicitations (due to the mass of the connecting rods and the pistons) they increase (or they decrease) with the square of the speed of rotation; on the other hand also the necessary joining to raise the speed of rotation to reach that application from an alternator for tension of net (to es 3000 giri/mins) or for autoveicolo (to es 4000 giri/mins) through cogwheels or pulleys and trapezoidal strap it dissipates a certain energy; therefore the solution must be optimized in operation of the plant that is wanted to realize.

Clipboard and dispensations:

For further details on the construction of this motor to write to:

zpavanel@libero.it

Italian version

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