WO2013021091A1

WO2013021091A1 - Heat-storage module based on latent heat with high heat-transfer rates

Info

Publication number: WO2013021091A1
Application number: PCT/ES2012/070619
Authority: WO
Inventors: Esther RIVAS RAMOS; M. Esther ROJAS BRAVO; M. del Rocio BAYÓN CABEZA
Original assignee: Centro De Investigaciones Energéticas, Medioambientales Y Tecnológicas (Ciemat)
Priority date: 2011-08-11
Filing date: 2012-08-09
Publication date: 2013-02-14
Also published as: ES2414310A1; ES2414310B1

Abstract

Heat-storage module based on the transmission of heat from a heat-transfer fluid to a phase-exchange material in a channel delimited by metal plates in the form of a spiral which form an internal element of the module. The heat-transfer fluid is circulated via the channel outside the channel that contains the phase-change material. The internal element has a conical spiral longitudinal cross section in the upper and lower parts. This means that, once the fluid is running through the module from top to bottom, the condensate remains at the bottom, automatically separated out from the vapour, which may be recirculated.

Description

THERMAL HEAT-BASED THERMAL STORAGE MODULE

WITH HIGH HEAT TRANSFER RATES

D E S C R I P C I O N

FIELD OF THE INVENTION

The present invention relates to an energy storage module in the form of latent heat, which is designed to obtain high rates of heat transfer between a two-phase heat transfer fluid and a storage medium that changes from solid phase to liquid phase, when There is no direct contact between them.

BACKGROUND OF THE INVENTION There are currently a large number of industrial applications where it is necessary to take advantage of process heat and in which the use of storage systems that allow the use of heat at times after its generation is very common.

A clear example of the importance of thermal storage systems is in solar thermal plants (in English Concentrating Solar Power-CSP-

Plants), which generate electricity from concentrated solar radiation. In them, the presence of a storage system allows them to be kept in operation also during the hours of absence of solar radiation.

In general, the storage systems of commercial CSP plants are based on sensible heat since the heat transfer fluid (in English, Heat

Transfer Fluid or HTF) is a single phase fluid.

However, in plants with direct steam generation (in English Direct Steam Generation or DSG) the heat transfer fluid is a biphasic fluid, whose thermal potential is acquired mainly through evaporation. Therefore, to store and later recover this potential efficiently, it is It is advisable to use a substance that changes phase (Phase Change Material or PCM).

To develop a thermal storage system based on latent heat, it is essential that the PCM used is well suited to the working conditions of HTF, especially in terms of temperature. In addition, it is desirable that it presents a good behavior from the point of view of heat transfer and thermal storage under such working conditions, which are safe, durable after thermal and economic cycling.

In a DSG plant the working temperature range (ie steam / water saturation temperature of 210 ^e C-310 ^e C) is determined by the pressure range in which the turbine and the hydraulic circuit can operate (ie 20 bar -100 bar)

The solid-liquid phase change temperature of a large number of anhydrous salts and eutectic mixtures is within this range, in addition, in general, they all have a low cost, so they can be good candidates for storage systems. But, its main drawback is the low thermal conductivity (less than 1 W / mK), which greatly limits the power of the storage system.

For this reason, the development of a thermal storage system based on this type of phase change materials must focus on concepts that allow compensating for the limitation imposed by its low conductivity on heat transfer rates.

From this perspective, there are two main lines of action: either on the PCM itself trying to improve its intrinsic thermal conductivity, or on the design of the thermal storage system trying to maximize the effective area of heat transfer between HTF and PCM .

US Patent 2933885 describes a thermal storage system based on latent heat where the module is composed of a PCM container and a set of tubes embedded therein placed either vertically or in the form of downward spirals through which a heat transfer fluid circulates biphasic

Also, in the article published in Applied Thermal Engineering, 30 (2010) p. 2643-2651, a latent heat storage module consisting of horizontal tubes is described, through which a biphasic heat transfer fluid circulates, and a PCM mass (eutectic mixture NaN0 ₃ / KN0 ₃ ). In it, the thermal conductivity is improved by graphite fins attached to the tubes.

In both cases, during charging, the HTF is circulated through the tubes from the top of the module to the bottom, so that, during its course, most of the HTF condenses by transferring energy in the form of latent heat to the PCM With these configurations of tubes embedded in the PCM both the condensed and non-condensed HTF are forced to flow in the same direction, so that at the exit of said modules there is still a two-phase fluid. These types of fluids should not be recirculated as they could damage the circuit pumps due to cavitation phenomena. On the other hand, the vapor in suspension within the condensate at the exit of such modules cannot be reused, so its thermal potential is wasted. Therefore, these thermal storage modules that cannot separate the condensed HTF from the non-condensed require an additional separation system.

OBJECT OF THE INVENTION

The invention aims to alleviate the technical problems mentioned in the previous section. To this end, it proposes a thermal storage module comprising a substantially cylindrical container with a longitudinal axis, upper and lower lids at its ends, at least three inlet and outlet holes located in the lids and in the body of the container, a material of phase change and an internal element surrounded by said container. The internal element is formed by two spiral-shaped plates that together with the container delimit an outer channel and an inner channel and where said element, at its upper and lower ends, has a conical spiral geometry, the phase change material being found In the inner channel. Preferably, the module further comprises an interconnection piece provided with an opening adapted to cooperate with the internal element so that the phase change material does not invade the lower cover and the outer channel can pass through said piece. The lower cover will preferably be conical, that is, in the same way as the piece. The PCM is one or a eutectic mixture of the following compounds: nitrites, nitrates, chlorites, chlorates, fluorites, fluorates, hydroxides or alkali metal carbonates. Preferably, the module It comprises an insulating layer between the container and the outer channel and in the spaces between the upper and lower covers and the internal element. An additional plate can also be provided attached to the upper end of the internal element that extends the internal channel vertically to the upper cover and supports within the internal channel perpendicular to the internal element plates.

BRIEF DESCRIPTION OF THE FIGURES In order to help a better understanding of the features of the invention in accordance with a preferred example of practical implementation thereof, the following description of a set of drawings is attached where illustrative character has been represented the following: Figure 1: shows an external front view of the module.

Figure 2: is a top view of the container.

Figure 3: is a cross section of the container along line I.

Figure 4: shows a cross section along line II.

Figure 5: It is an external front view of the internal element that forms a double spiral channel.

Figure 6: shows a mid-section cross section of the double spiral channel.

Figure 7: It is a longitudinal section of the thermal storage module through the center.

Figure 8: shows a cross section of the thermal storage module along line III.

Figure 9: Detailed view of the scratched areas at the top and bottom of the module of Figure 7.

Figure 10: shows the double unwound spiral channel and the positions of the structural supports.

DETAILED DESCRIPTION OF THE INVENTION The thermal storage module is designed so that, during the charging process, the latent heat from the condensation of HTF is stored by the PCM in the form of latent heat of fusion and, during the discharge process, the latent heat of solidification released by the PCM is absorbed by the HTF in the form of latent heat of vaporization.

The thermal storage module (figure 1) is formed by a container 1, an upper lid 2, a lower lid 3, an internal element 4 (figure 5) formed by two spiral-shaped plates that delimit an outer channel 5 and a channel interior 6 and, finally, an interconnection piece 7 (figure 4) provided with a spiral opening 8.

The outer channel 5, through which the HTF circulates, is completely closed at its upper and lower ends, as shown in Figure 9, forming a cavity with conical spiral geometry, both at the top and bottom, to favor the drain of the condensed flow and the suction of the non-condensed fluid during the loading and the discharge of the steam flow generated during the discharge. In this way, the HTF is in indirect contact with the PCM (which is in the inner channel 6) always in the best possible conditions from the point of view of heat transfer.

The loading and unloading process, once the module of the invention comes into operation, is as follows: under load, a certain saturated steam flow (heat transfer fluid) is injected into the thermal storage module whose saturation temperature should be slightly higher than the PCM melting temperature; In discharge, a certain saturated liquid flow is injected whose saturation temperature must be slightly lower than the solidification temperature of the PCM.

In this way, the saturated steam that feeds the thermal storage module during charging condenses by giving latent heat to the PCM which, from solid to liquid state, stores energy in the form of latent heat of fusion. On the other hand, during the discharge, the heat transfer fluid, now in the form of a saturated liquid, evaporates, absorbing the latent heat that the PCM gives it when it passes from a liquid state to a solid state. It is therefore a module that works both ways. In figure 1 you can see the positions of the inlet and outlet ducts of the storage module 9, 10, 1 1. During the module loading process, the heat transfer fluid (saturated steam) is injected into the interior through the conduit 9 and the condensed flow resulting from the heat transfer of this saturated steam to the PCM exits through the lower conduit 10. The conduit upper 1 1 acts as an outlet for the uncondensed steam flow during this process, giving the thermal storage module the character of separator between steam and condensate.

During the module discharge process, the heat transfer fluid (saturated liquid) is injected into the interior through 10 and the steam flow resulting from the heat transfer of the PCM to this heat transfer fluid exits through 1 1 while maintaining 9 closed.

The internal element 4 must be accessible from the top and from the bottom to facilitate its assembly and future maintenance, hence the upper and lower covers 3 are preferably flanged.

Figure 3 indicates, in an indicative way, the way in which the thermal insulation 12 should be placed inside the module to minimize lateral thermal losses. Its asymmetric shape is due to the need to adapt it to the development of element 4 and to thermally reinforce the wall 13. On the other hand, in figure 7, it is shown how thermal insulation 12 also fills the upper dead spaces 13 and lower 14 to prevent thermal losses to the outside in these areas and to buffer thermal bridges that may exist in the lower part of the storage module. Likewise, the contours of 9, 10 and 1 1 must be adequately coated with thermal insulation 12 since they are crucial points from the point of view of heat losses.

Figure 4 represents a cross-section along the line II of Figure 1 of the interconnection piece 7 that is located between the lower cover 3 and the container 1. In this interconnection piece 7 a spiral opening 8 has been made, to allow the internal element 4 of Figure 5 to pass through it. The internal element 4 and the interconnection piece 7 must be welded so that the inner channel 6 is closed at its bottom.

Figure 5 shows the asymmetry of the internal element 4 with respect to the main vertical axis. Because the internal element 4 is asymmetric with respect to the axis main, the entry holes 10 and 1 1 are not in the exact center of the container.

Figure 5 also shows the channels 5 and 6 of the internal element 4 before being placed inside the container 1 and being welded to the part 7. In this case the channels 5 and 6 form two equally spaced spirals, but these They could also have configurations with variable spacing.

The outer channel 5, through which the HTF circulates, is closed at its upper and lower ends, which have a conical spiral shape. In this way, both the drainage of the resulting condensate flow in the load and the suction of the resulting steam flow in the discharge is favored. Thus, the closed channel 5 performs, during the loading process, the separator mission.

The inner channel 6, which houses the PCM 16, is closed in its lower part by the interconnection piece 7 which is welded to the internal element 4. This channel is delimited laterally by the plate that forms it, the outer channel plate 5 and by an additional plate 15 welded vertically to the latter, as shown in Figure 9. The internal channel height 6 must be such that it allows changes in the volume of the PCM during thermal cycling and, in turn, prevents The dilated PCM overflows.

It is important to note that the maximum height of the inner channel 6 is a design parameter of the thermal storage module that will depend on the value "e", the diameter "D" and the height Ή ", as well as the type of PCM used. PCM materials that can be used to fill the inner channel 9 are: nitrites, nitrates, chlorites, chlorates, fluorites, fluorates, hydroxides or carbonates of alkali metals as well as eutectic mixtures thereof.

It is important to mention that the internal element 4, preferably, must not be welded to the upper lid 2 of the container to allow free thermal expansion thereof and thus avoid possible torsion problems.

Figures 7 and 8 show the arrangement of the phase 16 change material inside the thermal storage module, noting how it occupies the interior of the inner channel 6, which is closed at the lower end thanks to the interconnection piece 7 of Figure 4. This optimizes the amount of PCM used, since the condensed fluid accumulates in the area between the interconnection piece 7 and the lower cover 3. The height that the PCM reaches in the thermal storage module before its expansion (ie in solid state) must coincide with the position of 9, that is, with the height at which saturated steam is injected in charge (as is indicated in figure 7).

Figure 8 also shows that PCM 16 is always surrounded by heat transfer fluid 17, except in dead spaces 13, where thermal insulation 12 is reinforced. In turn, the channel through which the heat transfer fluid, 5, will always be surrounded by thermal insulation 12 that is necessary to prevent thermal losses to the outside. In figure 9 the insulator 12 introduced in spaces 13 and 14 can be seen in more detail.

To avoid structural damage to the inner channel 6 when the working pressures of the heat transfer fluid are raised, optionally, the inlet 9 of the heat transfer fluid 17 can be tangential to the lateral surface of the container 1 instead of perpendicular as indicated in the figures.

For this reason, a certain number of supports 18 welded perpendicularly to the surface of the channel can also be placed inside the internal spiral channel 6 containing PCM 16, as shown by way of example in Figure 10, with a length equal to the spacing "e" and of the same material as element 4. The calculation of the number of supports per unit area and the thickness of the same must be done according to the working conditions of the heat transfer fluid, the nature of the PCM and the type of structural material of element 4.

Claims

one . Thermal storage module comprising a substantially cylindrical container (1) with a longitudinal axis, upper (2) and lower (3) lids at its ends, at least three inlet and outlet holes (9, 10, 1 1) located in the lids and in the body of the container and a phase change material (PCM), characterized in that it further comprises an internal element (4) surrounded by said container (1), where the internal element (4) is formed by two plates in spiral shape that together with the container (1) delimit an outer channel (5) and an inner channel (6) and where said element, at its upper and lower ends, has a conical spiral geometry, the phase change material being found in the inner channel (6).

2. Module according to claim 1 characterized in that it further comprises an interconnection piece (7) provided with an opening (8) adapted to cooperate with the internal element (4) so that the phase change material does not invade the lower cover ( 3) and the outer channel can pass through said part (7).

3. Module according to claims 1 or 2 characterized in that the lower cover (3) is conical.

4. Module according to any of the preceding claims, characterized in that the PCM is one or a eutectic mixture of the following compounds: nitrites, nitrates, chlorites, chlorates, fluorites, fluorates, hydroxides or alkali metal carbonates.

5. Module according to any of the preceding claims characterized in that it comprises an insulating layer (12) between the container (1) and the outer channel (5).

6. Module according to any of the preceding claims characterized in that the spaces (13, 14) between the upper and lower covers and the internal element (4) are provided with a thermal insulator (12).

7. Module according to any of the preceding claims characterized in that it is provided with an additional plate (15) attached to the upper end of the internal element (4) that extends the internal channel (6) vertically to the upper cover (2).

8. Module according to any of the preceding claims characterized in that it comprises supports (18) within the inner channel (6) perpendicular to the plates of the internal element (4).