METHOD FOR TRANSFERRING THE HEAT CONTAINED IN A GAS AND CORRESPONDING HEAT EXCHANGER

Description

[0001] The present invention relates to a method according to the preamble of claim 1, a heat exchanger according to the preamble of claim 10 and a method for operating a furnace according to the preamble of claim 16.

[0002] Solar power plants, such as solar tower power plants, operate on an industrial scale. For example, the Ivanpah solar power plant in California, with a nominal output of 392 MW, directly generates steam at temperatures above 750 K and 170 bar by focusing concentrated solar radiation onto pipes of a heat exchanger located within the tower, where the steam is produced. Salt thermal storage allows steam to be used beyond periods of good solar radiation. For higher efficiency, solar tower power plants are increasingly being designed that use a volumetric receiver to heat air to temperatures above 1000 K, up to around 1500 K, and, currently experimentally, to around 2000 K and above. The heated air is then used via heat exchangers to generate steam or process heat.It is further foreseeable that, for example, such solar tower power plants will also be designed for a power range similar to that of Ivanpah.

[0003] On the other hand, it is known that small, local stand-alone systems with a dish receiver (which can also be combined into clusters) can achieve temperatures above 1000 K, even above 2500 K, with a power output in the kilowatt-hour range. Utilizing such heat sources is also interesting. By its very nature, the radiant heat generated in the receiver of solar thermal systems is carried away from the receiver as heated air at temperatures above 1000 K. Utilizing this transported heat, in turn, requires downstream heat exchangers. The efficient heat transfer to the medium being heated, which is crucial for a heat exchanger, necessitates considerable design complexity, usually accompanied by significant flow resistance in the medium being heated.

[0004] Accordingly, the object of the present invention is to provide an improved method for transferring the heat contained in a gas and an improved heat exchanger.

[0005] Document WO2015181594 A1 represents the nearest prior art for the subject matter of claim 1. Document US 3,604,400 A represents the nearest prior art for the subject matter of claims 10 and 17.

[0006] This problem is solved by a method according to the characterizing features of claim 1, by a heat exchanger having the characterizing features of claim 10 and an operating method having the characterizing features of claim 16.

[0007] By using an infrared-emitting gas that is fed into the absorber chamber of a heat exchanger, its heat can be transferred radiatively, i.e., via its infrared radiation, with sufficient power to an absorbing surface of the heat exchanger. This eliminates process steps and arrangements for optimized heat transfer by convection, which are complex to manufacture due to the need for a large ratio of convective surface area to the volume of the heat-emitting gas and generate significant flow resistance.

[0008] By introducing a gas that emits heat through infrared radiation into the absorber chamber of a heat exchanger, the desired heat transfer can occur predominantly through absorption, allowing for a correspondingly simple design of the heat exchanger. By equipping a furnace with a supply line for indirectly heated, infrared-radiating gas, it can be operated with this indirectly heated gas instead of combustion products that emit heat through radiation (gases and especially soot, which also radiates in the visible frequency range and thus makes the flame visible). This allows a furnace that can be converted with minimal effort to operate emission-free, or eliminates the need for emission control systems and enables the continued use of existing infrastructure with minor modifications.

[0009] If the ratio ψ of the heat flux through absorption to the total heat flux flowing through the absorbing surface is ≥ 0.6, the advantages shown above are particularly evident.

[0010] Infrared-emitting gases can be heated, in particular, by a receiver according to the as yet unpublished Swiss patent application 00627 / 17, which, unlike known volumetric receivers, heats a gas absorptively. Such gases, which can be heated by radiation in the manner of blackbody radiation, themselves emit radiation in the infrared range and are then, alongside conventionally heated infrared-emitting gases, ideally available for use according to the invention.

[0011] Preferred embodiments have the features of the dependent claims.

[0012] The invention is explained in more detail below with reference to the figures.

[0013] It shows: Fig. 1 schematically shows a cross-section of a heat exchanger according to the invention, Fig. 1b schematically shows an external view of the absorber chamber of a heat exchanger of the type described by Fig. 1a , Fig. 1c schematically shows a modified embodiment of the heat exchanger of Fig. 1a and b in the view of Fig. 1b , Fig. 2 a scheme of a system with a heat storage unit and a heat exchanger according to the invention, Fig. 2b the steps of the inventive method for transferring the heat contained in a gas, Fig. 3 a schematic view of the absorber chamber of a conventional reformer operated according to the invention, Fig. 3b the steps of the inventive operating method, and Fig. 4 a receiver according to for the production of indirectly heated, infrared-radiating gas.

[0014] Figure 1aFigure 1 schematically shows a cross-section through an embodiment of a heat exchanger 1 according to the present invention. A heat-emitting, indirectly heated (see below) gas enters an absorber chamber 3 via a supply line 2 in the direction of the arrow shown, flows through this chamber to a discharge 4, through which the gas leaves the heat exchanger 1 again, wherein in the embodiment shown, the supply line 2, the absorber chamber 3 and the discharge 4 form an arrangement for transporting gas through the heat exchanger 1.

[0015] An indirectly heated gas according to the present invention has been heated in front of the heat exchanger 1, wherein its heat to be exchanged in the absorber space does not originate from its own combustion - in contrast to a flue gas generated in the absorber space, i.e. a combustion product whose heat to be exchanged has been generated by a combustion process acting in the absorber space.

[0016] Between the supply line 2 and the absorber chamber 3, there is a distributor 5 for the gas flowing through the absorber chamber 3, such that the gas flows through the entire diameter of the absorber chamber 3. A collector 6 for the gas carries it from the absorber chamber 3 to the outlet 4. The distributor 5 and the collector 6 are also components of the arrangement for transporting gas through the heat exchanger 1. Distributor 5 and collector 6 can be appropriately designed by a person skilled in the art in this specific case.

[0017] In the embodiment shown, the absorber chamber 3 is cylindrical in shape; the gas passed through is a gas that radiates in the infrared range or a gas mixture containing gases that radiate in the infrared range.

[0018] In the absorber chamber 3, axially extending lines, here designed as tubes 7, are arranged. These lines are fed by a distributor 8 for the fluids to be heated and lead into a collector 9 for the heated fluids. Again, the distributor 8 and the collector 9 can be suitably designed by a person skilled in the art in the specific case. The surfaces of the tubes 7 form infrared radiation-absorbing surfaces. If only one tube 7 is provided, at least one absorbing surface is present; otherwise, several are present, particularly in the present case with a bundle of tubes 7 (see figure). Figure 1b and 1c ).

[0019] Warm, infrared-radiating gas flowing through heat exchanger 1 and thus absorber chamber 3 radiates onto the surfaces of the tubes 7, heating them. This results in heat exchange between the infrared-radiating gas and the absorbing surfaces of the tubes 7, which in turn heat the fluid flowing through them. The tubes thus form a surface designed for heat exchange via infrared radiation – in contrast, for example, to the surface of the wall 10 of absorber chamber 3, which, despite insulation, causes an undesirable heat loss from the gas that does not serve the intended heat exchange. Furthermore, the tubes are designed to absorb infrared radiation, meaning they have a surface that is not designed to act as a reflector.

[0020] Examples of gases that emit infrared radiation include CO2, water vapor, CH4, NH3, CO, SO2, HCl, NO, and NO2.

[0021] Generally, gases emit electromagnetic radiation in their characteristic frequency bands, with the radiation intensity or the power of the radiation per area in W / m² depending on the temperature.

[0022] For solid bodies, the model of the (ideal) radiating black body ("black body radiation") applies, whose radiation intensity in W / m² depends on its temperature across the entire frequency spectrum, with the intensity maximum shifting towards higher frequencies with increasing temperature of the black body and simultaneously increasing with the fourth power of the temperature.

[0023] Even real solid bodies exhibit dips in their radiation frequency spectrum compared to the frequency spectrum of (ideal) black bodies - for a given real solid body, the radiation intensity drops considerably in frequencies characteristic of that body.

[0024] Gases that radiate only in specific frequency bands, which can lie in the ultraviolet, visible, or infrared range, possess more extreme properties in this regard. If a gas radiates in the ultraviolet range at a comparatively low temperature, for example, its radiation intensity, i.e., the emitted power, is low; if it radiates in the infrared range at the same temperature, its radiation intensity is high. According to the invention, this property of gases is utilized by employing infrared-radiating gases in currently technically manageable temperature ranges of up to 2500 K or above (these temperature ranges will shift upwards in the coming years), whereby their radiation intensity becomes considerable and thus efficiently usable for heat exchange.

[0025] This allows for the design of a very simple yet efficient heat exchanger, in addition to the advantages listed below, which transfers heat from an infrared-radiating gas to another medium. Among other things (see below), the usual design complexity for convective heat transfer, which occurs through contact and consequently requires a large heat-exchanging surface area relative to the volume of the heat-emitting gas, is eliminated.

[0026] The applicant found that the design advantages of heat exchange through emission / absorption are particularly evident when the ratio ψ of the heat flow exchanged by absorption to the total heat flow exchanged (by absorption and convection) is ≥ 0.6, i.e., when the proportion of absorption predominates over the proportion of convection. Even with the simplest heat exchanger design, some heat exchange through convection is unavoidable and, in terms of heat exchange, quite desirable. However, the lower the proportion of convection, the greater the design simplification and the lower the effort required for manufacturing and, if applicable, maintaining the heat exchanger itself. In this specific case, the person skilled in the art can determine the determining parameters (the infrared-radiating gas or...)the gas mixture, its temperature, pressure and flow velocity, the geometry of the absorber chamber and the at least one absorbing surface, etc.) are designed to suit the ratio ψ ≥ 0.6 and to realize the advantages according to the invention.

[0027] It should be noted here that the illustrated embodiment with five pipes 7 in the diameter of the cylindrical absorber chamber 3 is purely exemplary, intended to illustrate the relationships. For example, it is possible to provide any number of pipes leading through the absorber chamber in any configuration and with any direction (i.e., not just axially), provided that the proportion of absorptive heat absorption still predominates, i.e., the ratio ψ ≥ 0.6 is achieved.

[0028] According to the invention, a method for heat exchange of heat contained in a fluid is thus obtained, wherein an indirectly heated, infrared radiation-emitting gas is used as the fluid, which is guided via a supply line (2) to the heat exchanger (1) and in this through an absorber chamber (3), wherein at least one surface absorbing the infrared radiation of the gas is provided in the absorber chamber (3) for the purpose of utilizing the heat of the gas, and wherein the mass flow rate and the temperature of the gas are set and the at least one surface absorbing for heat exchange is designed such that in operation the ratio ψ of the heat flow through it by absorption to the total heat flow through it is ≥ 0.6.

[0029] A heat exchanger for carrying out this process has an arrangement for transporting gas emitting heat in the heat exchanger by means of infrared radiation, which has an absorber chamber with at least one surface provided for heat exchange via infrared radiation, wherein the heat exchanger is designed with a supply line for indirectly heated, heat-emitting gas and the absorber chamber and the at least one absorbing surface are designed such that, in operation at a predetermined mass flow rate and a predetermined temperature of the heat-emitting gas in the absorbing surface, the ratio ψ of the heat flow passing through it by absorption to the total heat flow passing through it is ≥ 0.6.The heat exchanger is preferably designed with an arrangement for transporting heat-emitting gas in the heat exchanger by means of infrared radiation, which has an absorber chamber (3) with at least one surface provided for heat exchange via infrared radiation, and an indirectly heated gas flowing through the arrangement that emits heat by infrared radiation, wherein the heat exchanger is designed with a supply line for indirectly heated, heat-emitting gas and the absorber chamber and the at least one absorbing surface are designed and the mass flow rate and the temperature of the heat-emitting gas are set such that the ratio ψ of the heat flow through the absorbing surface by absorption to the total heat flow through the absorbing surface is ≥ 0.6.

[0030] For example, the heat exchanger can be equipped with a suitable control system that regulates the mass flow through the absorber chamber. Preferably, a temperature sensor is provided for the temperature of the heat-releasing gas flowing into the heat exchanger. This sensor generates a corresponding temperature signal for the control system, which then adjusts or regulates the mass flow. The control system is not necessarily integrated into the heat exchanger but can be located at a suitable location within the system in which the heat exchanger is used. A person skilled in the art can design the control system appropriately for the specific application, including the sensors for the data to be processed by the control system, such that the ratio ψ ≥ 0.6 is achieved.

[0031] Figure 1b shows an external view of the absorber chamber 3 of heat exchanger 1. Figure 1aVisible are the cylindrical wall 10 of the absorber chamber 3, as well as the pipes 7, which are arranged side by side in a spatial bundle and thus fill the cylindrical absorber chamber 3 approximately uniformly in the illustrated embodiment. For process engineering reasons, a different arrangement can also be provided. The pipes 7 are used in the operation of the heat exchanger 1 ( Figure 1a The pipes are heated primarily by absorption along their length by the axially flowing, infrared-radiating gas (e.g., water vapor), symbolized by arrows 15. Within the pipes themselves, components for the (catalytic) production of syngas or other chemically reactive substances, or simply a fluid that can carry the exchanged heat to a suitable location, perhaps in the form of a secondary circuit, as might be the case with process heat, flow in the same direction as indicated by arrows 15.

[0032] In the case of syngas production, the Figure 1a and 1bThe arrangement shown represents a reformer with a heat exchanger implemented according to the invention. High temperatures are required for syngas production, for example, an average pipe temperature of 1200 K. For the purposes of this simplified embodiment, an outer pipe diameter of 130 mm, an inner pipe diameter of 100 mm, and a pipe length of 12 m (i.e., a height of the absorber chamber 3 of 12 m) are assumed, whereby the heat flow q into a pipe 7 is assumed to be 100 kW / m² of the pipe surface (which naturally depends on the specific process taking place in the pipe). Furthermore, it is assumed that the distance between the centers of the pipes is six times the outer diameter of the pipes, i.e., 780 mm, and the distance of an outermost pipe to the wall 10 of the absorber chamber 3 is half that distance, i.e., 390 mm. Thus, the absorber chamber 3 has a diameter of 3.9 m.The partial pressure of the infrared-radiating gas is 1 bar, whereby the water vapor flowing through in the assumed example has an ambient pressure of 1 bar (i.e., no non-radiating gases are mixed in).

[0033] A rough calculation by the applicant, based on the above assumptions, yields the values ​​for the ratio ψ shown in the table below, using two examples with different temperatures of the available indirectly heated gas: T in (Gas inlet temperature) 1700 0< K 1580 0< K T out (Gas outlet temperature) 1340 0< K 1460 0< K v (flow velocity) 7.1 m / s 21.1 m / s ψ (ratio of absorption / absorption + convection) 0,97 0,94 T Gas Average temperature of the gas for the rough calculation 1520 0< K 1520 0< K

[0034] Thus, the heat exchange according to the invention is achieved by the heat exchanger 1 shown ( Figure 1a) feasible. It should be noted that the preliminary calculation assumed uniform conditions in the absorber chamber 3, thus neglecting the effects on the wall 10 and the actual temperature profile from the inlet to the outlet of the pipes 7 (i.e., along the axis of the cylindrical absorber chamber 3). However, this does not change the fundamental feasibility.

[0035] Depending on the reaction (as mentioned above: for example, a syngas reaction) or the requirements of the heat-absorbing fluid, an overpressure of, for example, 40 bar prevails in the pipes (tubes 7), although a higher overpressure of, for example, 100 bar may also be desirable. In this case, the wall thicknesses of the correspondingly pressure-resistant pipes (tubes 7) increase accordingly, which in turn is detrimental to the heat flow into the pipe. Therefore, for example, the temperature of the infrared-radiating gas must be increased to maintain the desired heat flow (in this example, 100 kW / m²).

[0036] According to the inventive concept of the absorber chamber 3 (or the heat exchanger 1, Fig. 1aThe pipes 7 running through the absorber chamber 3 can be bundled compactly, for example, by forming a tubular wall 10 that defines the absorber chamber 3. This, in turn, allows the absorber chamber 3 to be designed relatively easily for higher or high pressures, since pressure-related tangential stresses in a tubular wall area are essentially easier to control from a design perspective. If there is positive pressure in the absorber chamber 3, the pressure difference to the internal pressure of the pipes 7 decreases. Accordingly, the wall thickness of the pipes can be reduced, thus improving the heat flow into the pipes.

[0037] The invention provides a heat exchanger with an arrangement for transporting heat-emitting gas in the heat exchanger by means of infrared radiation, which has an absorber chamber with at least one surface provided for heat exchange via infrared radiation, characterized in that the heat exchanger is designed with a supply line for indirectly heated, heat-emitting gas and the absorber chamber and the at least one absorbing surface are designed such that, during operation at a predetermined mass flow rate and a predetermined temperature of the heat-emitting gas in the absorbing surface, the ratio ψ of the heat flow passing through it by absorption to the total heat flow passing through it is ≥ 0.6.

[0038] It further follows that preferably the at least one absorbing surface is formed by a number of lines, preferably pipes, and wherein the lines are arranged as a bundle extending through the absorber space, and space is provided between the lines for operatively flowing, infrared-radiating emitting gas.

[0039] It further follows that the absorber chamber is preferably pressure-resistant, designed for a pressure of the heat-emitting gas of >_ 5 bar, preferably >_ 10 bar, particularly preferably >_ 20 bar and most preferably ≥ 50 bar.

[0040] A rough calculation under the above assumptions for the dimensions of the pipes 7 and the heat flow of 100 kW / m² yields an operating pressure of 10 bar in the absorber chamber 3 when water vapor is used as the infrared radiating gas: T in (Gas inlet temperature) 1620 0< K T out (Gas outlet temperature) 1340 0< K v (flow velocity) 3.8 m / s ψ (ratio of absorption / absorption + convection) 0,9 T Gas Average temperature of the gas for the rough calculation 1480 0< K

[0041] The emissivity ε of a gas increases with its pressure p and the thickness L of the radiating gas layer (path length), as given by ε = f(p, L). The dimensionless emissivity curve has the shape of an inverted Hokeystock curve, with a steep slope at short path lengths (L) and a shallow slope at a certain distance from the radiation-absorbing wall, i.e., at longer path lengths. Therefore, with increasing pressure, the distances between pipes 7 and 7 can be reduced, in this example to 390 mm. At an operating pressure of 10 bar in absorber chamber 3, this results in a diameter of 1.95 m – half the diameter of absorber chamber 3 compared to the case with an operating pressure of 1 bar. These smaller dimensions support the pressure-resistant design of the absorber chamber, which in turn facilitates even higher pressures within it.In a specific case, the expert can, for example, determine an operating pressure in the absorber chamber 3 based on the optimal internal pressure in the pipes 7 and their wall thickness (heat transport), taking into account the effort required for pressure safety and the heat transport into the pipes 7.

[0042] It should be noted at this point that in the quantitatively described examples above, the ratio ψ is in the range of 0.9 and above – partly due to the high temperatures of the indirectly heated gas of over 1500 K. However, according to the applicant's findings, the ratio ψ ≥ 0.6 according to the invention can also be achieved with temperatures of the indirectly heated gas of ≥ 1000 K, for example 1300 K, or, as in the examples above, of more than 1500 K. It follows that, according to the invention, the indirectly heated gas preferably has a temperature of ≥ 1000 K, preferably ≥ 1300 K, and particularly preferably ≥ 1500 K.

[0043] Fig. 1c shows an external view of the absorber chamber 3 of another embodiment of the heat exchanger 1. Figure 1a It is evident, as with Fig. 1b the cylindrical wall 20 of the absorber chamber 21, as well as the tubes 7, which are arranged in a spatial bundle running next to each other and thus fill the cylindrical absorber space 21.

[0044] Compare to the embodiment of Fig. 1bAdditional walls 22 are provided, which are arranged between the pipes 7. These walls 22, or rather their surfaces 23, are heated during operation by the infrared-radiating gas through its infrared radiation until a temperature equilibrium is reached, since the walls 22 themselves lose energy through (real) blackbody radiation. This blackbody radiation then advantageously heats the conduits, or in this case, pipes 7. It should be noted that the wall 20 of the absorber chamber already acts in this way, but the walls 22 are additional, separate walls with additional surfaces 23 that are separate from the wall of the absorber chamber 20.

[0045] It is mentioned above that the emissivity curve ε exhibits an initial region with a steep rise and a second region with a gradual rise, although the transition is not sharp. This means that the energy radiated into a wall initially increases sharply with increasing thickness (path length L) of the radiating gas layer, and then, after a thickness that depends, among other things, on the gas itself, it gradually decreases (but still continues to increase). This can be interpreted as the gas isolating itself; in other words, a region of the gas located further away from the wall is less able to transfer energy to the wall via its radiation.If another wall is now placed in a suitable area, which also heats up and then emits (real) blackbody radiation, this radiation will reach the wall to be heated practically unimpeded, at least in the frequency ranges where the infrared-radiating gas does not absorb (the insulating effect is absent for these frequencies), and thus heat it without attenuation. As a result, the heat flux q flowing through the absorbing surfaces of the pipes increases.In this specific case, the person skilled in the art can arrange such a further wall at a distance from the absorbing surfaces (here: the series of tubes 7) that lies in the range of the weak increase in the emissivity ε of the infrared-radiating gas, and further such that the intensity of the blackbody radiation of the further wall at the absorbing surfaces is greater than the intensity of the infrared-radiating gas layer cut off by the further wall (additional intensity due to the cut-off path length L) would be.

[0046] It should be noted here that the conduits or pipes 7 also emit blackbody radiation, but at a comparatively low temperature, since they act as heat sinks due to the fluid flowing within them, which needs to be heated. The aforementioned additional walls reach an equilibrium temperature above the temperature of the conduits 7 or the at least one absorbing surface in the absorber chamber, so that, with the suitable arrangement mentioned, the heat radiation onto the at least one absorbing surface can be improved by these walls.

[0047] In summary, a number of pipes, each with an absorbing surface, are arranged in the absorber chamber and at least one further surface is provided which can be heated by the heat-emitting gas during operation and which irradiates the absorbing surfaces during operation by blackbody radiation, such that the heat flux q is increased by absorption by the absorbing surfaces.

[0048] Figure 2a Figure 3 schematically shows the arrangement of a system 30 for the use of solar energy with a heat exchanger 1 according to the invention, a heat storage unit 31, a receiver 32 for (concentrated) solar radiation and a photovoltaic receiver 37.

[0049] In receiver 32, a suitable fluid is heated by incident solar radiation 33, for example to a temperature above 1000 K or up to above 2000 K. It then flows via line 34 into a heat storage unit 35 and back to receiver 32 via line 36, so that the heat storage unit 35 can be charged with heat via the corresponding circuit through lines 34 and 36. A photovoltaic system or a photovoltaic receiver 37 generates electricity by incident solar radiation 33. This electricity is supplied via power lines 37 and an electric heater 38 to provide heat of the desired temperature in the heat storage unit 31 as needed, and is particularly capable of maintaining a uniform upper temperature in the heat storage unit at, for example, 2000 K. The heat from the heat storage unit 31 then heats an infrared-radiating gas, which is circulated through the heat exchanger 1 via lines 39 and 40.

[0050] Preferably, however, an infrared radiating gas is already heated by the receiver 32, which is sent in a circuit through the heat exchanger 1 via lines 41,42, i.e., is optionally supplied directly to it, depending on whether the heat storage unit 38 is to be charged or not.

[0051] In heat exchanger 1, a fluid is heated that flows through it via lines 43 and 44. As mentioned above, this fluid can generally be process heat of any kind, reactants / products for a chemical reaction, or steam to drive a steam turbine (electricity). One advantage of the system 30 shown is, for example, the simple temperature control of the infrared-radiating gas supplied to heat exchanger 1: for cooling, such unheated gas can be added to the circuit (directly via receiver 32 or via the heat storage unit 35); for heating, the electric heater 38 is available, which, in principle, can generate very high temperatures even if the solar radiation intensity is below the threshold that receiver 32 would require for the desired very high temperature.Another advantage of the system shown is that the infrared-radiating gases can be guided under pressure in the cycle shown with comparatively simple design effort, and heat transfer in the heat exchanger 1 can also be achieved with minimal effort.

[0052] The system shown can therefore be operated with only one heat source for the indirectly heated gas (solar receiver 32 or heat storage unit 38) or in combination with these heat sources, if desired additionally with the heater 38.

[0053] In the illustrated embodiment, the arrangement for transporting the heat-emitting gas is operationally connected (e.g., in a circuit) to a heat source that indirectly heats the gas, which preferably includes a heat storage device. Furthermore, the heat source comprises at least one of the heat-producing means of a solar receiver, a heat storage device, or a preferably electric, and particularly preferably photovoltaic, heater. For the method according to the invention, it is preferably the case that the infrared-emitting gas is guided in a circuit including the heat exchanger, in which the gas is indirectly heated, and wherein a heat storage device is more preferably provided in this circuit upstream of the heat exchanger.Furthermore, it follows that for indirect heating at least one of the heat sources receiver, heat storage or heating, preferably an electric heating element, which is particularly preferably photovoltaically powered, is used.

[0054] The Receiver 32 can be configured as a receiver for a solar tower power plant; see the description for details. Figure 4 below. The heat storage unit 35 can be designed according to WO 2012 / 027854, wherein the bulk material can consist of Al₂O₃ (the currently common salt heat storage units are not suitable for temperatures according to the present invention, as the salt would decompose). Photovoltaic solar systems are known to those skilled in the art.

[0055] Figure 2bFigure 45 shows the steps of the heat exchange process according to the invention. In step 45, a heat-emitting gas is selected whose heat to be exchanged did not originate from its combustion and which has been heated before the heat exchanger, i.e., it is an indirectly heated gas, and which emits infrared radiation to exchange heat via this radiation. Heteropolar gases are preferred. Suitable gases include CO₂, water vapor, CH₄, NH₃, CO, SO₂, HCl, NO, and NO₂, or a mixture of these gases. Non-radiating gases may be added in specific cases if this is indicated for process engineering reasons, but within the limits indicated by the ratio ψ.

[0056] In step 46, the gas (or gas mixture) is selected for the specific case, and the mass flow rate (flow velocity and pressure) and its temperature are mutually matched with the respective configuration of the absorbing surfaces in the absorber chamber such that the ratio ψ of the heat flow used for exchange by absorption to the total exchanged heat flow is ≥ 0.6, i.e., absorption predominates over convection. Preferably, the absorbing surfaces are formed by the surfaces of pipes 7 that run through the absorber chamber 3 ( Figures 1a to 1cThe heat exchanger is designed as follows: a bundle of pipes is particularly preferred, as the gas can easily flow around them. The fluid to be heated is guided through these pipes. The absorbing surface can also be formed by a section of the absorber wall or by one or more pipes (preferably tubes) for the fluid to be heated by the heat exchange. The higher the ratio ψ is chosen, e.g., ≥ 0.7, preferably > 0.8, and particularly preferably > 0.9, the simpler the heat exchanger design becomes. If the gas is guided through the absorber under overpressure (preferably ≥ 5 bar, preferably ≥ 10 bar, particularly preferably ≥ 20 bar, and most preferably ≥ 50 bar), the heat exchanger according to the invention can be designed to be particularly compact.

[0057] In step 47, the prepared heat exchange is carried out by passing the selected gas or gas mixture through the absorber chamber with the appropriately designed absorber surfaces under the predetermined operating parameters.

[0058] It follows that the gas emitting infrared radiation is preferably circulated in a circuit encompassing the heat exchanger, in which the gas is indirectly heated, and wherein a heat storage device is preferably provided in this circuit upstream of the heat exchanger. In the case of solar heating of the gas, this allows it to be made available even outside of periods of intense solar radiation. Furthermore, at least one of the following heat sources is preferably used for indirect heating: a receiver, a heat storage device, or a heater, preferably an electric heater, which is particularly preferably photovoltaically powered, thus resulting in advantageous independence from the intensity of the prevailing solar radiation.

[0059] Figure 3aFigure 50 schematically shows the absorber chamber (or heat chamber) of a conventional reformer operated according to the invention. Reforming is used for chemical conversion in process engineering and is often carried out catalytically as an endothermic process. Particularly well-known are "tubular reformers," i.e., reforming plants in which the substances to be processed are passed through a heat chamber in tubes, so that the heat required for the chemical process can be supplied to the substances through the tubes. The heat is generated by burners; the combustion products often contain soot and gases that radiate in the infrared range.

[0060] The heating chamber, which functions as a furnace, is rectangular and elongated. Along the center line of the heating chamber, the tubes are arranged in a row, creating elongated compartments on either side of the tubes. The flames generated by the burners extend into these compartments. In the "top-fired" design, the flames enter the compartments from above; in the "terraced wall" design, the burners are positioned on steps along the outer walls of the compartments; and in the "side-fired" design, the burners are distributed along the outer walls. A disadvantage of this design is that the temperature is difficult to control along the length of the tubes (high temperatures at the point of the flame, followed by a sharp temperature drop), and sufficient space must be provided for the unimpeded development of the flame.Finally, this principle also implies that in a heat chamber the tubes are arranged in rows to create an efficient ratio between the area of ​​the flames and the area of ​​the absorbing surfaces created by the tubes.

[0061] According to the invention, a conventional reformer with a heating chamber 50, in which tubes 51 are arranged one behind the other in a series, is provided with a supply line for indirectly heated, infrared-radiating gas, and the combustion arrangement is not used, i.e., it is deactivated or removed for the intended operation of the furnace, and thus of the reformer. The gas flows, as indicated by arrows 52, through the sub-chambers 53, 54 on both sides of the series of tubes 51 (and, of course, also between the tubes) and heats them by its infrared radiation.

[0062] For a rough calculation, it is again assumed that syngas is to be produced in the reformer, for which, for example, an average pipe temperature of 1200 < K is required. How to Figure 1b An outer pipe diameter of 130 mm, an inner pipe diameter of 100 mm, and a pipe length of 12 m (i.e., a height of the heat chamber 50 of 12 m) are assumed, with the heat flux q into pipe 7 being 100 kW / m² of the pipe surface. The temperature of the outer wall is 350 K, and the convective heat transfer coefficient h is 10 W / (m² K), which, at an ambient temperature of 300 K, results in a total heat flux q (through convection and thermal radiation) of 0.89 kW / m² through the insulation.

[0063] If, for example, methane is burned in air at ambient pressure in the furnace of the conventional reformer (CH 4 + 2 O 2 + 8 N 2 → CO 2 + 2 H2O + 8 N 2 ) and this conventional operation is compared to the operation according to the invention with indirectly heated gas, here steam at ambient pressure, a rough calculation by the applicant yields the following: parameter conventional oven indirectly heated gas T in (Flame temperature / Gas inlet temperature) 2050 0< K 1580 0< K T out (Gas outlet temperature) 1330 0< K 1340 0< K v (flow velocity) 3.6 m / s 11.2 m / s T Gas Average temperature of the gas for the rough calculation 1550 0< K 1460 0< K p (Partial pressure of the infrared-emitting gas - N2 does not emit infrared radiation) 0.27 bar 1 bar ψ (ratio of absorption / absorption + convection) 0, 95 0,97

[0064] A conventional reformer, which generates the required heat via a furnace, can thus be operated using the inventive operating method with indirectly heated gas radiating in the infrared range. It is advantageous that the indirectly heated gas is circulated (with or without heat storage, see below). Figure 2a) can be carried out, unlike in a furnace where the flue gases cannot be reused. Furthermore, if indirect heating is achieved through a solar receiver, the environmental footprint can be significantly reduced without requiring major investments on the part of the reformer. Finally, it also applies here that, in the absence of a necessarily very hot flame along the length of the pipes, a more uniform and therefore more efficient heat distribution can be expected.

[0065] Figure 3b Figure 55 shows the steps of the inventive method for operating a furnace. In step 55, a furnace or reformer is selected to be operated with indirectly heated, infrared-radiating gas instead of substances to be combusted.

[0066] In step 56, the combustion arrangement is shut down or removed and a supply line for indirectly heated, infrared radiating gas is set up.

[0067] In step 57, analogous to step 46 of Figure 2b , the infrared radiating gas or gas mixture is selected and its operating parameters are set to achieve a ratio of ψ ≥ 0.6.

[0068] In step 58, the furnace is put into operation as intended by introducing the gas. Again, heteropolar gases are preferred. As mentioned above, CO₂, steam, CH₄, NH₃, CO, SO₂, HCl, NO, and NO₂, or a mixture of these gases, are suitable. Non-radiative gases may be added by a person skilled in the art if this is indicated for process engineering reasons, but within the limits indicated by the ratio ψ.

[0069] According to the invention, a method for operating a furnace is provided which has a flow path for gases to be burned to generate useful heat, wherein an absorber chamber is provided in the flow path which has at least one surface provided for receiving heat exchange via infrared radiation, and wherein the combustion arrangement of the furnace is not used and the furnace is provided with a supply line for indirectly heated gas radiating in the infrared range, such that this gas can be operatively guided into and along the flow path for gases to be burned, and wherein indirectly heated gas radiating in the infrared range is supplied to the furnace during operation with a mass flow rate and a temperature such that in the at least one absorbing surface the ratio ψ of the heat flow by absorption to the total heat flow is ≥ 0.6.Preferably, the furnace is connected to a circuit of indirectly heated gas, which preferably includes a heat storage unit.

[0070] In an embodiment not shown in the figures, heat exchange in the heat exchanger according to the invention does not take place via lines carrying a fluid that absorbs the exchanged heat, but directly onto a material present in the absorber chamber, which heats up by absorbing the infrared radiation. Examples include metals for heat treatment or for producing a metal melt, or for glass production by generating a melt from the starting materials. This results in a process in which an absorbing surface is preferably formed by a material to be heated.

[0071] Figure 4Figure 60 schematically depicts a receiver 60 according to the still unpublished Swiss patent application CH 00627 / 17 for the indirect heating of a gas or gas mixture emitting infrared radiation. The receiver 60 has a heating area 61, which includes an optical aperture 62, for example, a quartz window, and an absorber 63. An absorption chamber 65 is provided between the quartz window 64 and the absorber 63. The infrared-emitting gas to be heated flows through this absorption chamber 65 from right to left, i.e., towards the absorber 63, in the direction indicated by the arrows. For this purpose, the transport device 66 of the receiver 60 has inlet nozzles 67 for the heat-transporting medium arranged around the quartz window 64, leading into the absorption chamber 65, and a central outlet nozzle 68 behind the absorber 63.

[0072] The absorber 63 is designed as a blackbody radiation arrangement with reduced convection, i.e. it has a surface 70 arranged in the path of the incident solar radiation 69, which absorbs this radiation and is designed in such a way that it heats up to operational efficiency due to the solar radiation 69 incident on the surface 70 and then emits corresponding infrared radiation 71 into the absorber space 65 via its surface 70.

[0073] Thus, the absorber 63 releases a significant portion of its heat output in the form of infrared radiation into the absorber chamber 65, where the gas flowing towards it is already heated absorptively before it reaches it.

[0074] As mentioned above, real structures only radiate approximately in the same way as an ideal blackbody. In this context, a "blackbody radiation arrangement" is understood to mean that the incident solar radiation 69 is absorbed as much as possible at the surface of the absorber (i.e., only a small portion penetrates the absorber, unlike a volumetric absorber), so that this surface heats up considerably and thus radiates into the absorber chamber 65 at the corresponding high temperature, similar to a blackbody. At absorber temperatures 63 up to 2000 K (or even higher), the majority of the radiation emitted into the absorber chamber 65 lies in the infrared range, i.e., at frequencies lower than those of visible light.

[0075] This eliminates the need for a complex, multi-layered absorber structure, particularly for volumetric receivers, which absorbs incident radiation across its depth by at least partially scattering it internally and then increasingly absorbing it after multiple reflections. Consequently, the complex thermal problems often associated with such absorber structures are also eliminated.

[0076] The absorber 63 is designed with low convection, meaning it allows for easy flow, without requiring enhanced convective properties for heat exchange. This also eliminates the need for a structure designed to maximize convection of the flowing medium, i.e., a structure with a large surface area relative to the flowing volume of the heat-exchanging medium, which is necessary for a highly efficient heat exchanger. Such a structure is complex to manufacture for high efficiency and results in a significant pressure drop of the flowing medium during operation.

[0077] It should be noted at this point that a certain amount of convective heat transfer at the absorber 63 is of course unavoidable, especially in the embodiment shown. Figure 4, since it forms a wall section of the absorption chamber 65. The corresponding convective heat transfer to the heat-transporting gas is, of course, desirable in terms of heat transfer – however, the outlet temperature Tout of the heated gas should be based to a significant extent (see below) on absorption, thus enabling a simplified design of the receiver 60. The simplified design of the absorber 63, as mentioned above, opens up the possibility of more stable operation, for example, with regard to temperature (temperature distribution across the absorber 63), which leads to improved industrial suitability of the receiver.

[0078] This results in a receiver with a heating region for heating a heat-transporting medium, which has an optical aperture for sunlight, and an absorber arranged in the path of the incident sunlight that absorbs sunlight, with a transport arrangement for transporting the medium through the heating region, wherein the absorber is designed as a blackbody radiation arrangement with reduced convection and the transport arrangement is designed for transporting a gas as the heat-transporting medium.

[0079] Preferably, the absorber, designed as a blackbody radiation arrangement with reduced convection, is configured for the flow of the heat-transporting gas and is further preferably located opposite the optical aperture 64.

[0080] An infrared-absorbing gas or gas mixture is used as the heat-transporting medium, absorbing in frequency bands of the infrared range. Such gases include, for example, heteropolar gases, preferably CO₂, water vapor, CH₄, NH₃, CO, SO₂, HCl, NO, and NO₂. The use of such gases ultimately results in a greenhouse effect that can be utilized by the receiver 60, since these gases are highly transparent to visible light, which thus essentially reaches the absorber 63, but are only slightly or almost completely transparent to the infrared radiation of the absorber, so that they heat up absorptively in front of the absorber 63. It should be noted here that real gases do not absorb visible light or infrared radiation uniformly across all frequencies, nor are they transparent, but rather exhibit varying degrees of absorption, particularly in frequency bands specific to each gas. Additionally, absorption decreases with distance from the radiation source.This leads to the use of terms like "highly transparent" and "low to almost no transparency" above with regard to the absorption or transparency of radiation.

[0081] The absorber 63 can be designed as a perforated plate, preferably as a double perforated plate, or as a simple, planar grid structure. In the case of the perforated plate, a hole pattern is distributed across its surface, allowing the heat-transporting gas to flow easily, while still providing sufficient surface area for the absorption of incident solar radiation and infrared back-radiation into the absorber chamber. A person skilled in the art can easily determine the optimal hole pattern for a given application. The same applies to the case of a grid structure or a double perforated plate with two parallel plates, where the holes of one plate are offset from those of the other plate, such that, despite the low convection of the heat-exchanging gas, a continuous, radiating surface of the absorber is presented to the absorption chamber. Silicon carbide (SiC) is a suitable material for the absorber.

[0082] Since gases that absorb infrared radiation also emit infrared radiation, the gases mentioned above can be used for solar heating in the receiver (preferably, but not exclusively) of a solar tower power plant and then for the heat exchange according to the invention. It is also possible that a receiver according to Figure 2 is used in dish concentrators, since temperatures up to 2000 K and above are achievable there as well due to the two-dimensional concentration of solar radiation.

Claims

1. Method for heat exchange of heat contained in a fluid, characterized in that an indirectly heated gas emitting infrared radiation is used as the fluid, which gas is located in an absorber chamber (3) of the heat exchanger (1), at least one surface absorbing the infrared radiation of the gas being provided in the absorber chamber (3) for utilizing the heat of the gas, characterized in in that the fluid is guided to the heat exchanger via a feed line (2) and in the latter through the absorber space (3), and in that the mass flow and the temperature of the gas are set in such a way and the at least one surface absorbing for the heat exchange is configured in such a way that, in operation, the ratio ψ of the heat flow passing through it by absorption to the total heat flow passing through it is ≥ 0.6.

2. The method of claim 1, wherein the ratio ψ is ≥ 0.7, preferably ≥ 0.8, and particularly preferably >_ 0.9.

3. The method of claim 1, wherein the gas comprises a heteropolar gas, preferably CO2, water vapor, CH4, NH3, CO, SO2, HCl, NO, and NO2 or a mixture of these gases.

4. The method according to claim 1, wherein the at least one surface is formed by conduits for a medium to be heated, preferably pipelines (7) or by at least one wall section of the absorber chamber.

5. The method according to claim 1, wherein an absorbing surface is formed by a material to be heated.

6. The method according to claim 1, wherein the gas emitting infrared rays is guided in a circuit including the heat exchanger (1), in which the gas is indirectly heated, and wherein preferably a heat accumulator (31) is provided in this circuit upstream of the heat exchanger (1).

7. The method according to claim 1 or 6, wherein for the indirect heating at least one of the heat sources receiver (32), heat accumulator (35) or heater, preferably an electric heater (38), which is particularly preferably photovoltaically fed, is used.

8. The method according to claim 1, wherein a gas indirectly heated to a temperature of ≥ 1000 0K, preferably ≥ 1300 0K and particularly preferably ≥ 1500 0K is used as fluid.

9. The method according to claim 1, wherein the gas emitting infrared radiation is passed through the absorber chamber (3) at a pressure greater than the ambient pressure, preferably ≥ 5 bar, preferably ≥ 10 bar, particularly preferably ≥ 20 bar and most preferably ≥ 50 bar.

10. Heat exchanger for carrying out the method according to claim 1, having an arrangement for transporting gas which gives off heat in the heat exchanger by infrared radiation, which arrangement has an absorber space (3) with at least one surface provided for heat exchange via infrared radiation, the heat exchanger (1) being configured with a feed line (2) for indirectly heated, heat-absorbing gas, characterized in that the absorber space (3) and the at least one absorbing surface are configured in such a way that in operation, at a predetermined mass flow and a predetermined temperature of the heat-emitting gas in the absorbing surface, the ratio ψ of the heat flux flowing through it by absorption to the total heat flux flowing through it is ≥ 0.6, wherein the at least one absorbing surface is formed by a number of conduits, preferably pipe conduits (7), and wherein the conduits (7) are arranged as a bundle extending through the absorber space (3), and between the conduits space is provided for operationally flowing infrared radiating emitting gas.

11. Heat exchanger according to claim 10, comprising an arrangement for transporting in the heat exchanger infrared radiation heat emitting gas comprising an absorber space (3) with at least one surface provided for heat exchange via infrared radiation, and an indirectly heated infrared radiation heat emitting gas flowing through the arrangement, wherein the heat exchanger (1) is formed with a supply conduit (2) for the indirectly heated infrared radiation heat emitting gas, heat-emitting gas, and the absorber space (3) and the at least one absorbing surface are formed and the mass flow and the temperature of the heat-emitting gas are set in such a way that in the absorbing surface the ratio ψ of the heat flow passing through it by absorption to the total heat flow passing through it is ≥ 0.6.

12. Heat exchanger according to claim 10 or 11, wherein wherein the conduits are configured as pipes (7).

13. Heat exchanger according to claim 10 or 11, wherein in the absorber space (3) a number of conduits distributed therein are arranged, each with an absorbing surface, and at least one further surface (23) is provided which in turn can be heated in operation by the heat-emitting gas and which in operation irradiates the absorbing surfaces by blackbody radiation in such a way that the heat flow is increased by absorption by the absorbing surfaces.

14. Heat exchanger according to claim 10 or 11, wherein the absorber chamber (3) is pressure-resistant, designed for a pressure of the heat-emitting gas ≥ 5 bar, preferably ≥ 10 bar, particularly preferably ≥ 20 bar and most preferably ≥ 50 bar.

15. Heat exchanger according to claim 10 or 11, wherein the arrangement for transporting the heat-emitting gas is operatively connected to a heat source indirectly heating the gas, which preferably comprises a heat accumulator (31).

16. Heat exchanger according to claim 15, wherein the heat source comprises at least one of the heat producing means solar receiver (32), heat accumulator (31) or a preferably electric heater (38), particularly preferably fed by photovoltaic current.

17. Method of operating a furnace which has a flow path for gases to be burned to produce useful heat, an absorber space (50) being present in the flow path, which absorber space (50) has at least one surface intended to receive heat exchange via infrared radiation, characterized in that the combustion arrangement of the furnace is not used and the furnace is provided with a supply line for indirectly heated gas radiating in the infrared range, such that this gas can be operably guided into and along the flow path for gases to be burned, and that indirectly heated gas radiating in the infrared range is supplied to the furnace during operation is supplied to the furnace with a mass flow and a temperature such that in the at least one absorbing surface the ratio ψ of the heat flow by absorption to the total heat flow is ≥ 0.6.

18. Method according to claim 17, wherein the furnace is connected in a circuit of indirectly heated gas, preferably including a heat accumulator (31).

19. Method according to claim 18, wherein the indirectly heated gas has a temperature of ≥ 1000 0K, preferably ≥ 1300 0K and particularly preferably ≥ 1500 0K.