WO2012016873A1

WO2012016873A1 - Solar radiation heat absorber for a stirling motor

Info

Publication number: WO2012016873A1
Application number: PCT/EP2011/062744
Authority: WO
Inventors: Salvatore Abate; Riccardo Barberi; Giovanni Desiderio; Giuseppe Lombardo
Original assignee: Innova Solar Energy SRL
Current assignee: Innova Solar Energy SRL
Priority date: 2010-07-30
Filing date: 2011-07-25
Publication date: 2012-02-09
Anticipated expiration: 2013-01-30
Also published as: CN103052788B; CN103052788A; ITRM20100428A1; IT1402134B1

Abstract

A solar radiation heat absorber (1) for a Stirling motor comprising a motor head (2) and a heat exchanger (3), the absorber comprising a cavity shaped to be engaged on the head (2) and to transfer heat to the heat exchanger (3). The absorber has an outer cylindrical symmetry shape tapered from a larger base (S), adapted to collect a light radiation, to a smaller base (SM) in which the cylindrical cavity that communicates and is concentric with the smaller base is formed.

Description

"Solar radiation heat absorber for a Stirling motor"

DESCRIPTION

Technical field

The present disclosure refers to an absorber and to the relative method of dimensioning for a Stirling motor, originally designed so as to operate by burning natural gas. In particular, the present disclosure concerns the design of an absorber, that, when receiving solar energy, is capable of efficiently accumulating and transferring the flow of heat coming from the solar radiation to the head of the Stirling motor, without using vector fluids.

State of the art

Stirling motors are external combustion heat engines, with a closed cycle that use a gas as a thermodynamic fluid, usually air, nitrogen or helium in the high performance versions: it proves to be very versatile in relation to different heat sources.

Stirling motors are made up of one or more cylinders in which one or more pistons slide following the expansion and the contraction of the thermodynamic fluid. The gas alternately flows from a hot exchanger and a cold exchanger after having passed through one heat regenerator: the corresponding harmonic movement of the pistons can produce electric energy, through a mechanical-electrical converter. The heat is generally supplied to the heat exchanger, which is at one end of the cylinder/s, and provides for heating the inner thermodynamic fluid that moves the piston/s.

Usually, the heat exchanger is formed by a plurality of fins of material having good thermal conductivity, or by a plurality of small ducts in which the working fluid flows.

The regenerator alternately absorbs and gives back heat from/to the working fluid and increases the transformation efficiency. The cold exchanger, which constitutes the low temperature source, is usually a cross flow exchanger with tube bundle in which the tubes are licked outside by cooling water, whereas the working fluid flows inside the tubes. The fact that the Stirling motor works by external combustion is advantageous since the heat can be supplied externally by a wide array of combustibles, even with low calorific power.

A very common application of the Stirling motor foresees making the hot source by burning natural gas.

Another application foresees using the Stirling motor in generating electric energy from renewable sources and in particular by means of the collection and the possible accumulation of heat obtained by solar radiation with the purpose of defining said hot spot inside the motor.

In such a field, the use of Stirling motors is divided between applications in which the heat is given up to a vector fluid and from this it is transported to the head of one or more Stirling motors and applications in which the motor is aligned along a focusing axis of a reflecting mirror, avoiding the use of vector fluids outside the motor. Examples of this last application are given in US6735946 and US7026722. In the field of combustion of natural gas, WO02/14671 describes an absorber formed by a cylindrical ring, to be mounted outside the head of the cylinder of the Stirling motor, and comprising fins of heat conducting material to efficiently collect the heat obtained by the combustion of natural gas and transfer it inside the motor by means of an exchanger.

In particular, WO2005/054654 describes a Stirling engine assembly comprising a heat exchanger defined by a ring that internally surrounds one end of the cylinder near to the head. Such a type of heat absorber is not suitable for solar power application, especially since its thermal capacity is so low that it cannot ensure continuous operation of the Stirling motor. On the other hand, by eliminating the absorber and directly irradiating the head of the cylinder there is the risk of damaging the motor.

Summary of the invention

The present invention proposes to provide a solar radiation heat absorber for Stirling motor, in particular for solar power application, that is capable of collecting the heat of solar radiation and efficiently transfer it to a heat exchanger inside the Stirling motor. The present invention proposes to supply, with a concentrated solar radiation, Stirling motors which need heat flows that are lower than 1000 Watt/m² in order to operate.

In particular, such types of motors are currently in production and are adapted to operate by means of the combustion of natural gas, ensuring excellent efficiency, but are not suitable for solar applications. One example of similar motors is given by the aforementioned patent WO2005/054654, which is enclosed here for reference.

Therefore, the object of the present invention is a solar radiation heat absorber for Stirling motors, according to claim 1 .

Advantageously, the absorber object of the present finding makes it possible to absorb concentrated solar radiation over a well-defined area, optimizing the operating temperatures of the motor and transferring the energy absorbed to the head of the Stirling motor in the form of heat minimizing the losses by radiation and convection. Moreover, the absorber has a thermal capacity such as to keep the motor switched on for a predetermined interval of time even when the solar radiation is lower than 1000Watt/m².

The absorber, that defines a sort of cap of the motor head, thus operates as an interface between a concentrated solar radiation and a heat exchanger inserted inside the head of a Stirling motor.

Another purpose of the present invention is that of providing a system for converting solar energy that is based on a Stirling motor comprising the aforementioned heat absorber.

In a preferred variant of the finding, the Stirling motor is of the type capable of producing both mains frequency electrical energy, and hot water.

A further purpose of the present invention is that of providing a method of dimensioning a heat absorber for a solar application of a Stirling motor comprising a heat exchanger inserted in a motor head.

Therefore, a method of dimensioning a heat absorber of a Stirling motor also forms the object of the present invention, according to claim 10.

According to the present finding, a head of a Stirling motor can be shared by two or more cylinders. Description of the drawings

Further characteristics and advantages of the invention shall become clearer from the detailed description of preferred, but not exclusive embodiments of a solar radiation heat absorber for Stirling motors, illustrated as an example and not for limiting purposes, with the help of the attached drawing tables, in which:

figure 1 represents a longitudinal section of an absorber inserted on a head of a Stirling motor;

figure 2 represents a trend of the temperatures of the section of figure 1 during the collection and heat exchange operations.

The same reference numerals and the same reference letters in the figures identify the same elements or components.

Detailed description of the invention

With reference to figure 1 , the absorber 1 (or solar cap) has the task of collecting a light radiation and of conveying the heat to a cylindrical area at the heat exchanger 3 of the motor head 2.

In figure 1 slits 4 are shown, in which thermocouples are inserted for monitoring the temperature at the interface between heat exchange 2r and absorber 1 .

Preferably, two thermocouples are used arranged in two opposite parts along the circumference of the motor head 2. The section represented in figure 1 , thus represents an axial section passing by the slits 4 intended to house respective thermocouples which are not represented.

In order to dimension the thermal capacity of the solar cap 1 correctly, it is necessary to know the minimum power necessary to start up the Stirling cycle of the motor and therefore also the power of steady operation necessary in order to keep the operating temperature of the motor constant. The example shown in the rest of the description refers to a motor originally designed for applications based on the combustion of natural gas and the head of the cylinder of which cannot be directly illuminated with a concentrated solar radiation without the motor becoming damaged.

The present description discloses how to: determine the thermal capacity and the mass of the solar cap 1 needed for the steady operation of the Stirling motor,

determine one or more shapes and/or geometries which optimise the operation of the solar cap 1 .

Determining the mass of the solar cap

In order to efficiently carry out the function of accumulating heat from suitably concentrated solar radiation it is necessary to identify the thermal capacity needed to keep the Stirling motor actuated. In order to do so it is necessary to preliminarily acquire the following data:

^■ minimum heat power P_sta_rt absorbed by the heat exchanger needed to start the Stirling cycle;

^■ temperature T_start reached by the heat exchanger when the power absorbed is P_start ;

^■ heat power absorbed in steady operation by the heat exchanger for the steady operation P_stead_y of the Stirling;

^■ temperature T_st_eady reached by the heat exchanger when the power absorbed is Psteady ;

From these parameters a range of values of thermal capacity of the solar cap 1 are calculated, from which its mass is calculated, so that it can absorb a quantity of heat necessary for the Stirling motor to start itself and then keep itself in steady operation even without solar radiation for a predetermined interval of time.

For the Microgen™ Stirling motor running on gas the following data has experimentally been obtained:

Pstart = 2.5 KW;

Tstart = 200 °C;

Psteady = 4.5 KW;

I steady- 550 °C.

It is presumed, for example, that copper is used as the material from which to make the solar cap, having specific heat c_Cop_Per = 380 J/Kg ^*K.

Then, the amount of energy that the solar cap 1 must absorb in order to keep the Stirling motor actuated for a predetermined interval of time 5t without the head being irradiated is calculated, i.e. by means of the single heat accumulated by the absorber. For example, it is presumed that said interval is 5t = 5 minutes. Therefore, the minimum energy and the steady operation energy are respectively:

Qmin = Pstart X 5t * 0.22 KWh

Qmax = Psteady X 5t * 0.38 KWh

From said energy values, the minimum mass m_min and then also the maximum mass m_max that the solar cap must have to achieve the aforementioned purposes, are calculated:

Qmin = m_min x Ccopper x (T_steady-T_start) => 0.22 [KWh] = m_min x 0.036 [KWh/Kg] so the minimum mass m_mi_n ^s 6 Kg.

Following the same procedure for Q_max the maximum mass m_max = 1 1 Kg of the solar cap is obtained.

Determining the shape and/or the geometry of the solar cap

With the purpose of determining an optimal shape for the absorber, the following objectives have been selected:

^■ optimizing the absorption of the concentrated radiation on one or more surfaces of the absorber;

^■ transferring the energy absorbed, in the form of heat, from the areas exposed to solar radiation towards the surfaces in direct contact with the heat exchanger, which is ring-shaped;

^■ minimizing the losses due to radiation and/or convection.

From the aforementioned objectives it is clearly advantageous to make the collection area of the solar radiation equal to the dimensions of the focusing area and to minimize the surfaces that are not directly exposed to solar radiation and that are involved in transferring heat towards the heat exchanger 3.

On the other hand it is necessary to ensure sufficient thickness of the material of the absorber so as to allow sufficient transmission of heat from the collection surface S to the surface of release of the heat to the exchanger, so that the Stirling motor can operate correctly. Moreover, the surface for the heat to pass through is greatly restricted by the width of the outer diameter of the head of the Stirling, so that it is not possible to reduce the parts of the cap which are not directly irradiated as desired with the purpose of minimizing losses, since there is the risk of not being able to transmit the energy needed for the operation of the motor to the heat exchanger.

Such dimensioning is further restricted by the mass of the absorber, which can vary in the range previously defined between m_min and m_max.

According to the present finding the general equation of heat conduction (Fourier's law) is used to calculate the outer diameter d_e of the part of cap that must be engaged on the head of a Stirling motor. In particular, the parameters to be replaced in said equation are accurately selected as shown hereafter:

W = λ x ^{!Γ int} ^ x S (Fourier's law) where:

- W is selected to be the power transmitted from the area exposed to solar radiation to the area in direct contact with the heat exchanger of the Stirling;

- λ is the thermal conductivity of the material of the solar cap;

- T_irr is selected to be the average temperature of the surface exposed to solar radiation;

- Tim is selected to be the average temperature of the interface with the heat exchanger;

- S is selected to be the collection surface of the solar radiation, and if the reflecting mirror is a portion of a sphere, then S has a circular shape that is smaller or equal to the focusing area with a diameter d_e-max ;

- L is the average thickness of the solar cap 1 ;

In particular, it is selected to set:

- L is at least equal to the height of the cylinder of the piston,

- Tint is equal to T_steady ,

- λ depends upon the material of the absorber, for example copper, and it is known

- T_irr is set to be equal to T_max which is the maximum temperature that the preselected material, for example copper, can withstand without becoming damaged.

The aforementioned parameters have the following values, with reference to the Microgen™ motor:

W = 4500 Watt, i.e. equal to the steady operation power P_stead_y λ= 400 W/m^*K

L = 80 mm,

Tma f copper is 1 100 °K (1373 °C) and

S = is the exchange surface, the inner diameter of which, that is

equal to the outer diameter of the motor cylinder, is equal to d,=1 16 mm.

From the aforementioned equation, replacing the aforementioned parameters, the minimum outer diameter d_e-min of the ring necessary to transmit steady operation power Psteady is calculated:

In figure 2, the trend of the temperatures is shown by means of colour mapping that goes from white (T= 720 °C) to black (T=590°C).

Therefore, in accordance with the present finding, once the parameters P_start, Tstart, steady. steady, L T_int of a Stirling motor have been identified, as well as knowing the dimension of the focusing area S and the relative diameter d_e-max, a material is selected with which the absorber is made and T_irr (equal to T_max) and λ are determined, then d_e-min is calculated by means of the following formula:

then Q_min = P_start x 5t and Q_max = Psteady x 5t is calculated from which m_min and m_max are calculated by means of the following formula

Q 1

m =

(^"^steady ^" Tstart )

where c is the specific heat of the material of the absorber. So that, once a mass of the absorber has been selected, the collection surface S that defines a larger base is joined with a smaller base SM defined by said diameter d_e-min, obtaining a solid with a cylindrical symmetry tapered towards the smaller base, i.e. the part for engaging on the head of the Stirling motor, in which the engagement cavity is formed from the smaller base and is concentric with it. According to an axial section of the absorber, said absorber 1 can thus have side surfaces 1 1 that are rectilinear, concave or convex in relation to the aforementioned joining operation.

The advantages obtained by applying the present invention are clear:

by means of the present finding it is possible to adapt a Stirling motor, originally designed to be supplied with gas, to operate by means of solar radiation, optimizing the operation of the motor with this type of heat source,

the absorber protects the head of the cylinder ensuring to keep the temperature of the head within the limits of motor operation,

the absorber collects solar radiation and optimizes the transfer of heat towards the heat exchanger inserted in the motor head,

since said motor is already optimized for generating mains frequency electrical energy and for producing hot water, it is not necessary to use further devices for converting energy in order to make a solar system based upon the aforementioned motor and comprising the described absorber operative,

an analytical method of dimensioning absorbers and a concrete example of application of the method are presented.

The elements and the characteristics illustrated in the different preferred embodiments can be combined with one another without for this reason departing from the scope of protection of the present application.

Claims

1 . A solar radiation heat absorber (1 ) for a Stirling motor, said Stirling motor comprising a motor head (2) comprising a heat exchanger (3), the absorber comprising a cavity shaped to be engaged on said motor head (2) and to transfer heat to said heat exchanger (3), characterized in that:

- said hat absorber (1 ) has an outer cylindrical symmetry shape comprising a larger and closed base (S), adapted to collect a light radiation, and a smaller base (SM); and in that

- said heat absorber (1 ) has said cavity communicating with the smaller base (SM), wherein said outer cylindrical symmetry shape is tapered from said larger base (S) to said smaller base (SM).

2. A solar radiation heat absorber (1 ) according to claim 1 , wherein said cylindrical symmetry is truncated conical in shape and wherein, according to an axial section, it defines at least partially rectilinear and/or at least partially concave and/or at least partially convex side walls (1 1 ).

3. A solar radiation heat absorber (1 ) according to claim 1 , wherein said inner cavity is cylindrical in shape and has an inner diameter (d,) equal to the outer diameter of said head (2).

4. A solar radiation heat absorber (1 ) according to claim 3, wherein an outer diameter (d_e) of said smaller base is obtained by the following equation:

where Psteady is a power offered to said heat exchanger (3) for a steady operation of the motor, λ is a thermal conductivity of the absorber, T_irr is an average temperature of an absorber surface exposed to a solar radiation (S), T_int is an average temperature at an interface between absorber (1 ) and heat exchanger (3), L is a height of a cylinder of the motor comprising said head (2), and d, is an outer diameter of the motor cylinder.

5. A solar radiation heat absorber (1 ) according to claim 1 , wherein a mass m of the absorber is calculated by means of the following formula:

i <^'T ' s!saiti - T ¹ start )■· where Q is a heat supplied by the absorber, c is the thermal capacity of the material of which the absorber is made, T_st_eady is a temperature reached by the heat exchanger (3) when it absorbs a power (Psteady) needed for a steady operation of the motor, and T_start is a temperature reached by the heat exchanger (3) when it absorbs a power (P_start) needed to start the motor.

6. A solar radiation heat absorber (1 ) according to claim 5, wherein said heat Q supplied by the absorber is equal to the product of the power (P_start) needed to start the motor and a predetermined interval of time (5t) of motor operation by means of a single heat accumulated by the absorber.

7. A solar radiation heat absorber (1 ) according to claim 5, wherein said supplied heat is equal to the product of the power (Psteady) needed for the steady operation of the motor and a predetermined interval of time (5t) of motor operation by means of the single heat accumulated by the absorber.

8. A solar radiation heat absorber (1 ) according to claims from 5 to 7, wherein said mass m of the absorber is in a range between a first mass value m_mi_n and a second mass value m_max calculated by means of said power (P_start) needed to start the motor and said power (Psteady) needed for steady operation of the motor, respectively.

9. A system for converting solar energy comprising a Stirling motor comprising a heat absorber (1 ) according to any one of claims 1 -8.

10. A method of dimensioning a solar radiation heat absorber (1 ) according to any one of claims 1 -8, the method comprising the following steps:

- identifying the following parameters of a Stirling motor:

- a power Psteady offered to said heat exchanger for a steady operation of the motor,

- a dimension L at least equal to a height of a motor cylinder comprising said head (2),

- outer diameter d, of the head,

- an average temperature T_int at an interface between absorber (1 ) and heat exchanger (3),

- determining the following parameters of a material defining said absorber:

- thermal conductivity λ of the absorber,

- an average temperature T_irr of an absorber surface exposed to a solar radiation,

- defining a collection surface S equal to a focusing surface of a reflecting mirror,

- calculating a dimension d_e-min of said smaller base (SM) by means of the following formula

- joining the side walls of the exchanger between said collection surface (S) and said smaller base (SM).

1 1 . A method according to claim 10, further comprising a further step of identifying the following further parameters of the Stirling motor:

- temperature T_steady reached by the heat exchanger (3) when absorbing said power P_stead_y needed for the steady operation of the motor,

- power Pgtart needed to start the motor,

- temperature T_start reached by the heat exchanger (3) when absorbing a power (Pstart) needed to start the motor;

and a subsequent step of defining a mass m of said absorber by means of the following formula

Q 1

m = — ——

^C ' ' sieasfcr ^" ' start *

where Q is a heat supplied by the absorber equal to P_start or Psteady multiplied by an interval of time (5t) of motor operation by means of the single heat accumulated by the absorber and c is a specific heat of the material defining the absorber.