RO133284B1 - Solar concentrator with multiple focal points and stirling motor - Google Patents

Description

The present invention refers to a hybrid assembly of cogeneration of electric and thermal energy from mixed sources, consisting of a system of solar concentrators with multiple foci, elements for receiving and transferring thermal energy to a Stirling engine coupled with an electric generator, used in solar farms.

Since the mixed energy sources used include, in addition to solar energy, biogas or natural gas, the technical field of application of the invention can be the provision of electrical and thermal energy mainly in biogas producing farms but also in homes, SMEs, etc.

Finding the best solar concentrators to use concentrated solar energy in various applications is a long-studied field. One of the applications is solar concentrator-Stirling engine systems. Most of them use parabolic mirrors to concentrate the solar radiation, having a Stirling engine in the focus [1-4]. There are systems that use a very large Fresnel lens [5] or several small Fresnel lenses [6] that do not have the Stirling engine placed in the focus, and the transmission of concentrated solar energy to the Stirling engine is done through optical fiber. There are also systems that use, in addition to the solar concentrator, optical reflectors to redirect concentrated sunlight [3, 7, 8].

The big disadvantage of these alternative energy sources is that they are not permanent. The idea of hybrid systems that use an auxiliary energy source with the role of compensating for the moments when the primary energy source disappears or is reduced in intensity, thus appeared.

Most solar concentrator-Stirling engine systems use large parabolic mirrors to concentrate solar radiation. Making these large mirrors is not a negligible problem and the production prices are high. Even in the case of their modular construction, a failure in one of the components makes it impossible to replace only that component. Moreover, they present disadvantages due to the shading brought about by the positioning of the Stirling engine and by the length of its support arm which moves the center of gravity of the system away from the central axis. Using a secondary reflector in the focus of the main receiver [3] to be able to place the Stirling engine at the base of the main receiver (parabolic mirror) removes this inconvenience but does not solve the problem of shadowing and high costs due to the use of large parabolic mirrors.

US patent 6775982 B1 discloses a solar installation mounted on an automated tracking device for maximizing sun exposure, comprising a number of solar collectors with a Fresnel lens, in thermal connection with a Stirling engine having a generator mounted on the output shaft electric to produce electric current and by cooling it, domestic hot water. The solar collector includes flat Fresnel lenses, aligned horizontally and vertically in a grid, where each Fresnel lens concentrates solar radiation on a receiving surface formed by optical fiber ends that lead the radiation to a heat exchanger where, as a working fluid, hydrogen or the helium, or air with which the hot part of the Stirling engine is heated, the cooling being done with H2O water, and the resulting hot water is co-generated.

US 20080078435 A1 discloses a mechanical solar thermoelectric cogeneration plant mounted on an automated photocell tracker to maximize sun exposure and comprising a number of Fresnel lens solar collectors where each Fresnel lens focuses radiation solar and sends it through optical fibers to a heat exchanger where it heats the gases to be burned together with the hydrogen, and at the same time the hot part of the Stirling engine (fig. 4), the cold part being in the ambient air. The Stirling engine drives an electrical generator that feeds current into an automatic storage and distribution facility, which depending on needs directs the electrical energy 1 to storage or domestic consumption, or through a water electrolysis facility to produce hydrogen. The hydrogen, through a pump, can be stored in a tank, or burned in the 3 combustion chamber in close proximity to the heat exchanger, or through a fuel cell to be converted into electricity and stored in batteries. Hydrogen is used as the working fluid, and if the reserve is low, the burner can optionally be fed with methane gas, so that the cogeneration of electricity and domestic hot water is done continuously, without the influence of shading or the lack of sunlight.

The solar concentrator with multiple foci and Stirling engine, according to the invention, solves the technical problem of maintaining the use of the solar installation with the same efficiency, even during the period of shading or overcast skies. 11

The present invention removes the presented disadvantages and solves the proposed technical problem, in that it consists of a solar tracking support on which a number of solar concentrators are mounted frontally and facing the sun, each concentrator having been positioned in the focus by a system each a thermal receiver with the role of taking 15 concentrated solar energy and transferring it to the gaseous thermal agent, which is air from a compressor, air that then passes through a gas mixer where it is enriched with 17 oxygen from a electrolyser through a flexible hose, the mixer being provided with a flow regulator of the gaseous heating agent, which is then preheated in a flue gas heat recuperator 19, located above a Stirling engine combustion chamber, mounted behind the solar concentrators on the same solar tracking support, then the heated thermal agent 21 enters through a flexible connection into a system of insulated and branched pipes, where it is distributed to the thermal receivers and finally reaches the combustion chamber of the Stirling engine, 23 which is coupled mechanical with an electric generator that feeds a reversible AC/DC inverter that supplies the useful energy to the consumer, and the surplus energy is stored in a group of 25 accumulators, or used by an electrolyser for the production of oxygen and hydrogen, which feeds the gas mixer , respectively hydrogen, used to enrich the combustion gas 27, which enters through an elastic hose into a mixing system and then into the combustion chamber of the Stirling engine. 29

The solar concentrator presents on a frame where n*k solar concentrators are grouped in a matrix, on "n" rows and "k" columns, resulting in a system with "n*k" multiple foci 31, from which the concentrated solar energy is taken over by the agent gaseous thermal by means of "n*k" thermal receivers that are interconnected in parallel through a 33 distribution system formed by insulated and branched pipes, distribution system coupled at the output end with the combustion chamber of the Stirling engine. 35

Each receiver has, at the entrance and exit to the distribution system, valves to isolate the flow of thermal agent, valves that can be operated independently, and the thermal energy 37 resulting from the conversion of concentrated solar energy is stored in the gaseous thermal agent and transferred through thermally insulated pipes, under a pressure provided by a compressor, in the combustion chamber of the Stirling engine, where the agent is used as an oxidizer in the combustion process. 41

The thermal receptor has an internal lamellar structure, on three levels, and on the outside it is placed in a refractory brick protected by a stainless steel jacket. 43

Compared to the state of the art, the proposed solution presents the following cumulative and integrated advantages in the same system: 45

- uses a system of 2 solar concentrators with multiple foci, removing the classical concentrator with parabolic mirrors [1-4] or Fresnel lens [5] of large dimensions and focal length 47;

- thanks to the hybrid system (mixed sources) it can also work at low irradiance (cloudy sky) or at night;

- uses surplus electricity in the water electrolysis process, thus obtaining oxygen and hydrogen used to enrich air and gaseous fuel, respectively;

- produces both electrical and thermal energy;

- the thermal receivers 41 in the focus of the concentrators have isolation valves which, in case of a possible failure, allow the isolation of the route in question without the need to stop the entire assembly.

The invention will be further presented in connection with fig. 1-5 which represent:

- fig. 1a, schematic diagram of the solar concentrator assembly with multiple foci and Stirling engine;

- fig. 1b, perspective detail of the solar concentrator from fig.1a.

- fig. 2, front view of solar concentrator with “n” rows and “k” columns of Fresnel lenses;

- fig. 3, a perspective section through the thermal receiver system in the solar concentrator focus;

- fig. 4, block diagram of the energy process;

- fig. 5a, photograph of an embodiment with twelve solar concentrators and a Stirling engine with a 3 kW electric generator.

- fig. 5b, rear view, in perspective of the solar concentrator.

The solar concentrator with multiple foci and Stirling engine is a hybrid assembly of cogeneration of electricity and thermal energy from mixed sources, consisting of a system of 2 solar concentrators, with multiple foci, each concentrator 2 having a thermal receptor 41 positioned in the foci of taking concentrated solar energy 44 and transferring it to a gaseous thermal agent 43, which in turn is transferred via a flow distribution system 54 to a Stirling engine 31 coupled to an electric generator 33.

The assembly is mounted on a support assembly 1 of tracking type on two axes, which allows solar tracking during the whole day. The principle diagram is presented in fig. 1. The dimensions of the support 1 depend on the number of installed solar concentrators 2 and the dimensions of the Stirling engine 31 installed. The support 1 is composed of a support frame 11 that also allows the azimuthal movement of the whole assembly, to which a frame 12 is connected that allows the elevation movement and on which a Stirling engine assembly 3, the 2 solar concentrators, a distribution system are mounted 54 flow of the gaseous thermal agent 43 and a support 13 on which the thermal receivers 41 are mounted.

The support 13 also includes a position adjustment system, on 3 axes, which allows the positioning of the receivers 41 in the focus of the solar concentrators 2.

Support assembly 1 includes some motors and some reducers that ensure its movement on the two axes, some angular position readers, and a GPS system for identifying the geographic coordinates, date and time, necessary for positioning.

The solar energy is concentrated with the help of solar concentrators 2, of the Fresnel lens type, on the thermal receivers 41 which also act as heat exchangers, transferring the thermal energy, captured from the concentrated solar radiation, to the gaseous thermal agent 43.

The 2 solar concentrators are grouped in a matrix on "n" rows and "k" columns, resulting, as can be seen in fig. 2, a system with n*k multiple burners which are interconnected 54 in parallel and coupled to a combustion chamber 32 of the Stirling engine 31.

Using identical Fresnel lenses instead of parabolic mirrors brings a number of advantages that have been described above.

The thermal receptor 41 in the focus of the concentrator 2 is designed and optimized for the 3 type of lenses shown in the embodiment, so that it has a high efficiency in capturing and transferring heat to the thermal agent 43. The receiver 41 is placed in a refractory-insulating brick 42 to minimize thermal energy losses; the brick being covered by a stainless steel mantle that protects it from weather conditions. A section through receiver 41 is shown in fig. 3. The shape of the receiver 41 is of the layered lamellar type. The air enters the receiver 41 in the lower part, gradually heats up on the path 9 and leaves it in the upper area where it has the highest temperature, being positioned in the focus area of the solar concentrator 2. Before reaching the receiver 41, the thermal agent 11 43 is preheated in the heat recuperator 34, which is a heat exchanger with the help of which part of the thermal energy is recovered from the burnt gases of the Stirling engine 13 31 and transferred to the thermal agent 43. At the exit from the recuperator 34, the thermal agent 43 is distributed to the thermal receivers 41 in the focus of the concentrators 2 and then reaches the combustion chamber 15 with the help of the distribution system 54, thermally isolated from the surrounding environment to minimize energy losses. 17

The system 5 for driving, distributing and controlling the flow of thermal agent 43 includes an air flow source 51, which can be a compressor, fan, etc., which drives the air 19 to a mixing system 52 where the air is enriched with oxygen from from an electrolyser 10 through a flexible hose, thus forming the thermal agent 43 (air enriched with 21 oxygen). The mixer 52 is equipped with a flow regulator. The gaseous thermal agent 43 is directed to a heat recovery unit 34 from burnt gases, located above the combustion chamber 23 32 of the Stirling engine 31 where it is preheated then through a flexible connection it enters a distribution system 54 of insulated and branched pipes where it is distributed to the thermal receivers 41 from where it takes the thermal energy from the concentrated solar energy 44 and transfers it to the combustion chamber 32 of the Stirling engine 31. 27

System 5 also has some valves 53 for isolating thermal receivers 41. The valves 53 are arranged both on the input and on the output of each receiver 41. A schematization 29 of the entire energy process is shown in fig. 4.

The heat exchanger of the 31 Stirling engine, located in the 32 combustion chamber, must be 31 maintained at a constant temperature necessary for continuous operation. To compensate for periods of little or no sunshine, the combustion chamber 32 is provided with 33 gas mixture burners (biogas, methane gas, etc.) with hydrogen as an auxiliary energy source. The combustion process is controlled by an automated system 6 of proportional control 35 of the fuel flow and mixture.

The second thermal recuperator 35 on the combustion gas exhaust path 37 of the Stirling engine 31, is provided with some thermoelectric elements, cooled for the direct conversion of thermal energy into electrical energy. Their cooling system is 39 connected to that of the Stirling engine 31 and includes a heat exchanger 7 to recover thermal energy in the form of hot water. The assembly also includes an inverter 8 and some 41 batteries 9 for the conversion and storage of the electricity produced and an electrolyzer 10 that uses the surplus electricity in the electrolysis process of water, thus obtaining 43 the oxygen and hydrogen used to enrich the air and the fuel, respectively gaseous.

The assembly can be sized and configured in such a way as to correspond to both the local irradiance characteristics and the consumer's electricity and thermal energy needs. 47

An example putting the invention into practice is shown in fig. 5. It is composed of: twelve solar concentrators 2 Fresnel lens type in 5 mm thick PMMA, with an active surface of approximately 0.92 m ² , with 1.3 m focal length, and a transmittance of 90%. The 31 Stirling engine installed is a prototype, with a 3 kW electric generator 33. The solar tracking system 1 is dimensioned to be able to take over the loading forces of the entire system as well as external forces (wind, snow, etc.).

Most solar concentrator-Stirling engine systems use large parabolic mirrors to concentrate solar radiation. Making these large mirrors is not a negligible problem and the production prices are high. Even in the case of their modular construction, a failure in one of the components makes it impossible to replace only that component. Moreover, they present disadvantages due to the shading brought about by the positioning of the Stirling engine and by the length of its support arm which moves the center of gravity of the system away from the central axis. The use of a secondary reflector in the focus of the main receiver, [3] to be able to place the Stirling engine at the base of the main receiver (parabolic mirror), removes this inconvenience but does not solve the problem of shadowing and high costs due to the use of large parabolic mirrors. On the other hand, the use of several Fresnel lenses, instead of the large parabolic mirror, as main concentrators, coupled with secondary reflectors to concentrate the solar energy in the same focal point [7], solves part of the problems presented above, but reduces system efficiency due to losses induced by multiple reflections/refractions.

Bibliography

[1] . Roelf J. Meijer, Solar powered Stirling engine, US 4707990 A.

[2] . Glendon M. Benson, Solar powered free-piston Stirling engine, US 4642988 A.

[3] . Cristina Naranjo Sosa, Felix Gilabert Munoz, Isaac Garaway, Erez Harel, David Klein, Stirling engine solar concentrator system, WO 2011/053895 A1.

[4] . Mark Mehos, Kenneth Anselmo, James Moreno, Charles Andraka, K. Rawlinson, John Corey, Mark Bohn, Dish/stirling hybrid-receiver, US 20020059798 A1.

[5] . Kouzou Kitamura, Kiyohiko Tsukumo, Solar heat utilization Stirling engine power generation plant, US 6775982 B1.

[6]. D. Alan E. Johnson, Mechanical/Thermo-voltaic solar power system, US 2008/0078435 A1.

[7]. Gilbert Cohen, Roland Winston, Multiple reflector solar concentrators and systems, US 20040140000 A1.

[8]. Multiple-focus coincident Fresnel lens system, CN 103885165 A.

[9] . John F. W. Parry, Hot air solar engine, US 4414812 A.

[10]. Edgar English, Jr., Solar energy conversion apparatus provided with an automatic cut-in heat-supplying standby apparatus, US 4398391 A.

[11]. Worth H. Percival, David N. Wells, Hybrid solar/combustion powered receiver, US 4602614 A.

[12]. Yilong Chen, Qingping Yang, Yanfeng Zhang, Disc-type solar stirling engine power generation device capable of operating continuously day and night, EP 2716910A4.

Claims (5)

1. Solar concentrator with multiple foci and Stirling engine mechanically coupled with a 3 electrical generator (33) with inverter (8), consists of a solar tracking support (1) on which are frontally mounted and facing the sun a number of solar concentrators (2), 5 characterized by the fact that each concentrator (2) is positioned in the focus by a system (13), each with a thermal receiver (41) with the role of receiving concentrated solar energy (44) and of 7 a transfer of a gaseous thermal agent (43), which is air from a compressor (51), air that then passes through a gas mixer (52) where it is enriched with oxygen from an electrolyzer (10) through a flexible hose, the mixer (52) being provided with a flow regulator of the gaseous thermal agent (43), which is then preheated in a flue gas heat recovery unit 11 (34), located above a combustion chamber (32) of the Stirling engine (31) mounted behind the solar concentrators (2), on the same support (1) of the solar tracker 13, heated thermal agent (43) enters through a flexible connection, in a system of insulated pipes (54) and branched, where it is distributed to the thermal receivers (41) and finally reaches the 15 combustion chamber (32) of the Stirling engine (31) mechanically coupled with the electric generator (33) that supplies the AC/DC reversible inverter (8) that supplies the energy useful to the consumer, 17 and the surplus energy is stored in a group of accumulators (9), or used by an electrolyser (10) intended for the production of oxygen and hydrogen, which supplies the gas mixer 19 (52), respectively the hydrogen, used for enrichment of the combustion gas, enters through an elastic hose into a mixing system (6) and then into the combustion chamber (32) of the Stirling engine (31). 2. Solar concentrator, according to claim 1, characterized in that, on a 23 frame (12), n*k solar concentrators (2) are grouped in a matrix, on "n" rows and "k" columns, resulting in a system with n* k multiple foci, from which the concentrated solar energy (44) is25 taken over by the gaseous thermal agent (43) by means of n*k thermal receivers (41) which are interconnected in parallel through a distribution system (54) of insulated pipes and branched,27 distribution system (54) coupled at the outlet end with the combustion chamber (32) of the Stirling engine (31).29 3. Solar concentrator, according to claims 1 and 2, characterized in that each receiver (41) has, at the entrance and exit to the distribution system (54), some 31 isolation valves (53) of the agent flow ( 43) thermal, valves that can be operated independently. 4. Solar concentrator, according to claims 1-3, characterized in that the thermal energy 33 resulting from the conversion of concentrated solar energy (44) is stored in the gaseous thermal agent (43) and transferred through thermally insulated pipes (54), under the pressure 35 provided by a compressor (51) in the combustion chamber (32) of the Stirling engine (31) where the agent (43) is used as an oxidizer in the combustion process. 37 5. Solar concentrator, according to claims 1-4, characterized in that the thermal receptor (41) has an internal lamellar structure, layered on three levels, and on the outside 39 is placed in a refractory brick (42) protected by a mantle of stainless steel.