System and method for generating liquid ice and method for cooling
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SIEMENS ENERGY GLOBAL GMBH & CO KG
- Filing Date
- 2024-07-23
- Publication Date
- 2026-05-13
Smart Images
Figure EP2024070809_06032025_PF_FP_ABST
Abstract
Description
[0001] Description
[0002] Liquid ice production plant, method for producing liquid ice and method for cooling
[0003] The present invention relates to a liquid ice production plant comprising an insulated container suitable for generating a rough vacuum, which forms a liquid holding volume in the lower region and a vapor holding volume in the upper region, wherein the container is provided in the region of its liquid holding volume with a liquid inlet opening and a liquid ice discharge opening, to which a liquid supply line and a liquid ice discharge line can be connected, an agitator extending into the liquid holding space, which is designed to stir liquid present in the liquid holding volume, and a vacuum pump connected to the container in the region of the vapor holding volume, which is designed to reduce the internal pressure of the vapor holding volume at least to the triple point pressure of the liquid to be used, which is in particular water.Furthermore, the present invention relates to a method for producing liquid ice, in particular using a liquid ice production plant, and to a method for cooling the air sucked in by a compressor of a gas turbine and using liquid ice.
[0004] Thanks to advances in renewable energies, gas turbine-based power plants are already being used primarily to provide the residual load, and this will become even more so in the future. On the one hand, this leads to fewer operating hours and longer downtimes. On the other hand, the highest possible output should be achieved during the comparatively shorter operating times in order to improve economic efficiency. The output of gas turbines is, among other things, heavily dependent on the temperature of the air sucked into the gas turbine compressor. The colder this temperature is, the higher the output of the gas turbine. Depending on the location, the possible output of the gas turbine-based power plant is limited by high ambient temperatures. A targeted reduction in the intake air temperature enables a significant increase in output of several percentage points.Due to reduced operating hours and longer downtimes, the use of a cold storage facility can make economic sense, as it can be charged at a relatively low power level during the comparatively long downtimes using inexpensive and possibly surplus renewable energy, resulting in lower CAPEX. If sufficient energy cannot be provided by renewable energy sources and an electricity price spike occurs, the gas turbine-based power plant can be operated at increased power thanks to intake air cooling using cold from the storage facility. The cold storage facility is crucial to the economic viability of such an approach.
[0005] Until now, intake air cooling systems were generally implemented without cold storage. The most common are evaporative coolers, but these only function satisfactorily at comparatively low ambient air humidity levels. Also known is so-called "wet compression," in which extremely fine water droplets are injected, which also enter the compressor. It works even at high ambient air humidity levels. Both approaches have a significant water requirement in common; with "wet compression," even demineralized water must be used.
[0006] If no water is available for intake air cooling, another option is the use of heat exchangers cooled by chillers and placed in the intake area of the gas turbine. In general, intake air cooling using chillers is particularly beneficial in hot and dry locations, because falling below the water dew point has a significant negative impact on the cost-benefit ratio.
[0007] All of these solutions date back to a time when gas turbine-based power plants had many operating hours and few downtimes or off-peak periods, and when volatile renewable energies were available in much smaller quantities and at significantly higher costs. Therefore, cold storage systems were generally not implemented. Instead, the intake air cooling systems were designed to meet the direct cooling requirements. Due to economies of scale, the relative benefit of intake air cooling is greater for large gas turbines with an output of, say, 300 MW than for smaller gas turbines, for example, with a maximum output of around 30 MW.
[0008] Due to the change in market conditions due to now competitive, but still volatile renewable energies, cold storage is now an option to improve the economic efficiency of intake air cooling systems and ultimately of gas turbine-based power plants.
[0009] To date, there are primarily three options for cold storage. First, the use of a suitably cold stored liquid, such as glycol-water mixtures, which utilizes the sensible cold. The second option is the use of ice storage, which utilizes the latent cold during the phase change from ice to water. The third option is the creation of ice layers by sprinkling cold water onto appropriately cold heat exchanger surfaces, which are periodically heated. The ice layer is thereby melted and falls as a "block" into a cold water reservoir below.
[0010] Another new option is the use of liquid ice storage, in which a pumpable mixture of water and ice provides the cold, thus combining sensible and latent cold components. Liquid ice storage offers several advantages over other cold storage options.
[0011] Compared to cold liquid storage systems, liquid ice storage systems have the advantage, with regard to a specific cooling requirement, that the required storage volume of the liquid ice storage system, with a 50% ice content and considering a temperature increase from, for example, 0 °C to 15 °C, is only about a quarter of the storage volume of a cold liquid storage system with a 20% glycol-water mixture. The surface area of the storage system, and thus the cooling loss, is only about half.
[0012] A comparison of an ice storage system with a liquid ice storage system shows that the storage volume required for a liquid ice storage system with an ice content of around 50% is roughly the same as for a pure ice storage system for a specific cooling requirement. This is because very large heat exchanger surfaces with corresponding space requirements must be provided in the ice storage system in order to enable acceptable charging and discharging rates. This is negatively reflected in the costs of the pure ice storage system, as well as in the achievable heat transfer rates, for which the desired formation of ice layers or water on the cooled or heated tank heat exchanger is always detrimental to the heat transfer rate. In contrast to an ice storage system, the charging and discharging rates of the liquid ice storage system can be selected differently.This opens up the possibility, for example, of combining a smaller, more cost-effective chiller with 1.5 MW of cooling capacity, which charges the cold storage over a longer period, with a large cold discharge capacity of, say, more than 15 MW, as is often required for a large gas turbine. This case is particularly relevant for the intake air cooling of gas turbines under today's market conditions, since the plant has significantly more downtime and off-peak hours than full-load hours. However, the latter is where the money is earned.
[0013] When comparing liquid ice storage with block ice / cold water storage, the required storage volume of the block ice / cold water storage is higher for a given cooling requirement than with the liquid ice storage. This is because the arrangement of the heat exchanger surfaces for ice formation in the air space above the water / ice area and the shape of the ice blocks mean that the ice content in the storage volume is well below 50%. Added to this is the relative inefficiency of the approach, which is unavoidable due to the necessary periodic thawing of the heat exchangers to release the ice blocks.
[0014] In known liquid ice production plants, the production of liquid ice at the triple point of water, which is found at around 6 mbar and 0.01 °C, is achieved by means of a steam turbo compressor which compresses a very large volume flow of steam from around 6 mbar to at least 50 to 100 mbar. A disadvantage of this steam turbo compressor solution is that the water used as a refrigerant in the temperature range relevant for liquid ice production around 0 °C has unfavourable properties from a refrigeration machine perspective, which translates into comparatively high energy requirements. Compared to ammonia, for example, the volume flow is more than 300 times greater for the same cooling capacity and the required pressure ratio for a 25 K temperature difference is more than 2.25 times higher.The high volume flow rate also makes the steam-based turbo compressor approach very sensitive to pressure drops, which are unavoidable and therefore also contribute to the higher energy consumption. The required high pressure ratio could be achieved much better with a positive displacement compressor. However, the high volume flows require a turbo compressor. In this respect, the production of liquid ice using a steam turbo compressor is also a compromise solution in this respect. The entire process also takes place in a vacuum, which therefore requires the vacuum pump to be designed accordingly. This has to be very large because there is no special, specially cooled heat exchanger surface suitable for separating non-condensable gases, which is why water vapor is also extracted in addition to non-condensable gases.Furthermore, the steam turbocompressor is an expensive special component currently available from only a few suppliers. Due to the resulting large steam volume flows, its power will continue to be limited to less than 1 MW in the future. Higher power outputs, such as those required for intake air cooling of one or more large gas turbines, require the parallel arrangement of several steam turbocompressor systems, which entails correspondingly high costs.
[0015] Based on this prior art, it is an object of the present invention to provide an alternative liquid ice production plant, an alternative method for producing liquid ice and an alternative method for cooling the air sucked in by a compressor of a gas turbine using liquid ice, with which the problems or disadvantages associated with steam turbo compression are at least partially eliminated.
[0016] To achieve this object, the present invention provides a liquid ice production plant comprising an insulated container suitable for generating a rough vacuum, which forms a liquid receiving volume in the lower region and a vapor receiving volume in the upper region, wherein the container is provided in the region of its liquid receiving volume with a liquid inlet opening and a liquid ice discharge opening, to which a liquid supply line and a liquid ice discharge line can be connected, an agitator extending into the liquid receiving volume, which is designed to stir liquid ice present in the liquid receiving volume, and a vacuum pump connected to the container in the region of the vapor receiving volume, which is designed to reduce the internal pressure of the vapor receiving volume at least to the triple point pressure of the liquid to be used,wherein this is in particular water, characterized in that in the upper region of the steam receiving volume a heat exchanger surface of a heat exchanger is arranged, which is part of a refrigeration machine.,
[0017] One advantage of the liquid ice production plant according to the invention is that the storage costs and losses due to the liquid ice approach are significantly reduced compared to conventional cold storage systems, and the operational flexibility and project-specific adaptability are significantly increased. The use of a heat exchanger surface within the container as part of a refrigeration machine enables the selection of a refrigerant that is optimally suited to the temperature range, in contrast to the turbo compressor approach, in which water is also the refrigerant. This results in better performance and lower costs. The performance disadvantage resulting from the separation between ice production and the refrigerant circuit is thus more than compensated for. Furthermore, the vacuum pump can be dimensioned much smaller.With regard to the solution according to the invention, a further cost reduction results from the usability of cost-effective standard refrigeration system solutions already available from a large number of suppliers as well as the possibility of realising high individual refrigeration system outputs, with double-digit MW figures being possible.
[0018] Preferably, the heat exchanger surface arranged in the steam absorption volume and / or the inner surfaces of the container have a surface that prevents ice buildup, in particular a hydrophobic surface. Accordingly, downtimes for defrosting these surfaces can be prevented and efficient operation can be ensured.
[0019] Advantageously, the container in the liquid receiving volume has a barrier arranged between the liquid inlet opening and the liquid ice removal opening, which barrier is designed such that it prevents a direct flow of fresh water entering through the liquid inlet opening in the direction of the liquid ice removal opening, in particular in the form of one or more container walls projecting into the liquid receiving volume.
[0020] According to one embodiment of the present invention, the liquid ice production plant comprises a liquid ice storage unit connected to the liquid ice discharge opening. The use of such a liquid ice storage unit enables the decoupling of cold generation and cold utilization. Accordingly, the power of the electrically driven refrigeration machine does not need to be provided during full-load operation, so that, for example, the maximum power output of the power plant to the power grid is not reduced.
[0021] Furthermore, to achieve the object mentioned at the outset, the present invention provides a method for producing liquid ice, in particular using a liquid ice production plant according to the invention, which has the steps of: a) partially filling a container with a liquid, in particular with water, b) lowering the pressure prevailing inside the container until the liquid contained in the container begins to boil and partially evaporates, c) condensing the rising vapor using a heat exchanger surface of a heat exchanger positioned in the upper region of the container, which heat exchanger forms part of a refrigeration machine, d) pumping the resulting liquid ice into a liquid ice storage tank and e) replacing the pumped-out liquid ice with fresh liquid, in particular with fresh water, wherein the mixture of liquid and ice contained in the container is in particular constantly stirred.
[0022] When making liquid ice, in a first step a) a container is partially filled with a liquid, which is preferably water. In a further step b) the pressure inside the container is reduced using a vacuum pump by sucking out the air in the container. This also takes place during operation, since the non-condensable gas components in the water being fed in, such as oxygen, have to be constantly removed. If the pressure in the container is sufficiently low, the water in the container begins to boil. Steam is produced. The water cools down accordingly and the condition in the container approaches the desired triple point.The resulting steam rises due to its lower density and condenses in step c) on the heat exchanger surface of a heat exchanger positioned in the upper region of the tank, which forms part of a refrigeration machine. The temperature of the heat exchanger surface is chosen such that the condensing water does not freeze but is supercooled as a liquid and falls back down into the boiling water, where it is heated by the rising steam, in extreme cases even back to the boiling point. The falling drops thus increase the surface area available for heat exchange. Over time, the entire system reaches the triple point and all three states of aggregation of water exist simultaneously. From this point on, a mixture of water and ice can be pumped out of the tank in step d) and stored in a preferably well-insulated liquid ice storage facility.The water-ice mixture removed from the container is replaced in step e) with fresh water, which is generally at ambient temperature. A stirrer ensures thorough mixing of the solid and liquid phases during the process and, in particular, prevents the formation of a solid layer of ice in parts of the container.
[0023] The liquid used is advantageously water, preferably demineralized water. Freezing at the melting point of the liquid used requires the presence of crystallization nuclei, for example in the form of dirt particles or the like. As long as these are not present, the liquid used can be cooled well below the melting point before it turns to ice. The vapor condensing on the heat exchanger surface is basically free of such crystallization nuclei. According to one embodiment of the present invention, the pressure in step b) is reduced to the triple point pressure of the liquid, in the case of water to less than 10 mbar, preferably to about 6 mbar.
[0024] Preferably, the heat exchanger surface on which the vapor rising inside the container condenses in step c) has a temperature below the triple point of the liquid, in the case of water in particular a temperature slightly below 0 ° C, in particular approximately -2 ° C. Accordingly, freezing of the vapor condensing on the heat exchanger surface can be reliably counteracted.
[0025] It is advantageous to keep the ice content within the container at a maximum of 50-60% in order to ensure pumpability.
[0026] According to one embodiment of the invention, if a layer of ice forms on the heat exchanger surface, for example due to water droplets entrained in the steam flow, the heat exchanger is defrosted, in particular by temporarily switching the refrigerant circuit of the refrigeration machine.
[0027] In addition, to achieve the object mentioned above, the present invention provides a method for cooling the air sucked in by a compressor of a gas turbine using liquid ice, characterized in that a liquid ice production plant according to the invention is used and / or liquid ice produced by a method according to the invention is used. According to a first variant, the liquid ice is pumped directly through a heat exchanger arranged in an intake duct of the compressor.
[0028] According to a further variant, the liquid ice is pumped through a heat exchanger which recools an intake cooling circuit of an intake duct of the compressor, said intake cooling circuit containing a heat exchanger.
[0029] Advantageously, the water is used for further purposes after cooling the drawn-in air, possibly after intermediate storage. Further use could be for the production of liquid ice. The water can also be used by other power plant components. Further use of the water counteracts water waste.
[0030] Further advantages and features of the present invention will become clear from the following description with reference to the accompanying Figure 1, which shows a liquid ice production plant according to an embodiment of the invention.
[0031] Figure 1 shows a liquid ice production plant 1 according to one embodiment of the present invention. The liquid ice production plant 1 comprises an insulated container 2 suitable for generating a rough vacuum, which forms a liquid holding volume 3 in the lower region and a vapor holding volume 4 in the upper region. In the region of its liquid holding volume 3, the container 2 is provided with a liquid inlet opening 5 and a liquid ice discharge opening 6. A liquid supply line 8 provided with a pump 7 is connected to the liquid inlet opening 5 and connects the container 2 to a fresh water reservoir 9. A liquid ice discharge line provided with a pump 10 is connected to the liquid ice outlet opening 6 and connects the container 2 in this case to a liquid ice reservoir 12.The liquid ice production plant 1 further comprises a motor-driven agitator 13, which extends into the liquid receiving volume 3 and is designed to stir liquid ice present in the liquid receiving volume, and a vacuum pump 14, which is connected to the container 2 in the region of the vapor receiving volume 4 and is designed to reduce the internal pressure of the vapor receiving volume 4 at least to the triple point pressure of the liquid to be used, which in this case is demineralized water. In the upper region of the vapor receiving volume 4, a heat exchanger surface 15 of a heat exchanger, which is part of a refrigeration machine 16, is arranged.In this case, a liquid ice outlet line 18 provided with a pump 17 is connected to the liquid ice storage unit 12. The liquid ice outlet line leads to a heat exchanger 23 installed in the outlet region of the compressor 19 of a gas turbine 20 of a gas turbine-based power plant 21, which heat exchanger cools the ambient air sucked in by the compressor 19. The heat exchanger surface 15 and the inner surfaces of the container 2 preferably have a surface that prevents ice build-up, in particular a hydrophobic surface. Furthermore, in the embodiment shown, the container 2 has a barrier 22 arranged in the liquid receiving volume 3 between the liquid inlet opening 5 and the liquid ice removal opening 6, which barrier is designed such that it prevents a direct flow of fresh water entering through the liquid inlet opening 5 in the direction of the liquid ice removal opening 6.For example, the barrier 22 can be provided in the form of one or more container walls projecting into the liquid receiving volume 3.
[0032] To produce liquid ice using the liquid ice production plant 1, in a first step the liquid receiving volume 3 of the container 2 is filled with liquid, in this case with clean, preferably demineralized water.
[0033] In a further step, the pressure prevailing inside the container 2 is reduced towards the triple point pressure using the vacuum pump, which can be, for example, a water ring pump with an air ejector connected upstream and driven by it. The pressure reduction using the vacuum pump 14 takes place in the present case throughout the entire operation of the liquid ice production plant 1, since the non-condensable gas components contained in the water to be fed in must be constantly removed, for example in the form of the oxygen contained in the water. If the pressure prevailing inside the container is sufficiently low, the water contained in the container 2 begins to boil. Steam is produced. The water cools down accordingly and the state in the container approaches the desired conditions at the triple point of water.The resulting steam rises within the steam holding volume 4 due to its lower density and condenses on the heat exchanger surface 15 in the upper region of the container 2, which is recooled by the refrigeration machine 16. The temperature of the heat exchanger surface 15 is kept just below 0 °C, for example at around 2 °C. The water condensing from the steam does not freeze, but is supercooled as a liquid, falls downwards into the boiling water and is reheated by the rising steam. The falling drops thus increase the surface area available for heat exchange. Over time, the overall system approaches the conditions at the triple point and all three states of water exist simultaneously. From this point on, a mixture of water and ice can be drawn off from the container 2 and pumped through the liquid ice discharge line into the liquid ice storage tank 12.To maintain the pumping capacity of the pump 10, the ice content should be a maximum of 50 to 60%. This water-egg mixture is replaced by fresh water, which is fed to the liquid holding volume 3 from the fresh water reservoir 9 using the pump 7 via the liquid supply line 8. The fresh water contained in the fresh water reservoir 9 is currently at ambient temperature. The barrier 22 ensures that the supplied fresh water cannot flow directly towards the liquid ice outlet opening 6. The agitator 13 ensures thorough mixing of the solid and liquid phases of the water and, in particular, prevents the formation of a solid layer of ice in parts of the container 2.
[0034] The water condensing on the heat exchanger surface 15 does not icing over because the commonly known temperature of 0 °C is not the freezing point but the melting point of water. Freezing at this temperature requires the presence of crystallization nuclei, for example in the form of dirt particles. As long as these are not present, water can be supercooled to approximately 40K below 0 °C before it turns to ice. The water vapor condensing on the heat exchanger surface 15 is free of them. In addition, the heat exchanger surface 15 and other internal surfaces of the container 2 are provided with a special hydrophobic surface which prevents the build-up of ice. In principle, the heat exchanger surface 15 should be very thoroughly cleaned before the liquid ice production plant 1 is put into operation in order to prevent the formation of undesirable layers of ice due to crystallization nuclei that would then be present there.If, for example, a layer of ice has formed due to dirt particles that were not removed before commissioning or water droplets entrained in the steam flow and the crystallization nuclei contained therein, the heat exchanger can be defrosted as required by switching the refrigerant circuit of the refrigeration machine 16.
[0035] The cold can be provided by any type of chiller 16 as long as it is able to set and permanently maintain temperatures of just below 0 °C on its evaporator surface. A compression chiller will often be a suitable option as it is readily available in a wide range of capacities and is comparatively inexpensive, uses renewable electricity and can be easily integrated into existing power plants. An absorption chiller, for example with the working fluid pair ammonia and water, would also be conceivable and would potentially have the advantage that excess free waste heat from the power plant can be converted into cold. However, the retrofitting of such a solution is much more difficult and absorption chillers with all their ancillary systems also have a higher CAPEX. Added to this is the dependence of the drive heat availability on the operating times of the power plant.
[0036] If there is a need for cooling to cool the intake air of the compressor 19, the mixture of water and ice particles is taken from the liquid ice storage 12 via the liquid ice outlet line 18 and either pumped directly through a heat exchanger 23 in the intake duct of the gas turbine 20 or supplied with it to a separate heat exchanger which recools the intake duct cooling circuit. In this context, the ice content is melted and the water is heated according to the temperature of the heat exchanger and the temperature of the medium to be cooled. The heated water can then either be used directly in the power plant 21 or pumped back into suitable storage tanks, for example into existing demineralized water or raw water tanks in the power plant 21, and is also available for, among other things, the later production of further liquid ice.
[0037] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.
Claims
Patent claims 1. Liquid ice production plant (1) comprising an insulated container (2) suitable for generating a rough vacuum, which forms a liquid receiving volume (3) in the lower region and a vapor receiving volume (4) in the upper region, wherein the container (2) is provided in the region of its liquid receiving volume (3) with a liquid inlet opening (5) and a liquid ice discharge opening (6), to which a liquid supply line (8) and a liquid ice discharge line (11) can be connected, an agitator (13) extending into the liquid receiving volume (3) and designed to stir liquid ice present in the liquid receiving volume (3), and a vacuum pump (14) connected to the container (2) in the region of the vapor receiving volume (4) and designed to reduce the internal pressure of the vapor receiving volume (4) at least to the triple point pressure of the liquid to be used,wherein this is in particular water, characterized in that in the upper region of the steam receiving volume (4) a heat exchanger surface (15) of a heat exchanger is arranged, which is part of a refrigeration machine (16).
2. Liquid ice production plant (1) according to claim 1, characterized in that the heat exchanger surface (15) arranged in the steam receiving volume (4) and / or inner surfaces of the container (2) have a surface which prevents ice build-up and is in particular hydrophobic.
3. Liquid ice production plant (1) according to claim 1 or 2, characterized in that the container (2) in the liquid receiving volume (3) has a between the liquid liquid inlet opening (5) and the liquid ice removal opening (6) arranged barrier (22), which is designed such that it prevents a direct flow of fresh water entering through the liquid inlet opening (5) in the direction of the liquid ice removal opening (6), in particular in the form of one or more container walls projecting into the liquid receiving volume (3).
4. Liquid ice production plant (1) according to one of the preceding claims, characterized in that it has a liquid ice storage (12) which is connected to the liquid ice removal opening (6).
5. A method for producing liquid ice, in particular using a liquid ice production plant (1) according to one of the preceding claims, comprising the steps of: a) Partially filling a container (2) with a liquid, in particular with water, b) Lowering the pressure prevailing within the container (2) until the liquid contained in the container (2) begins to boil and partially evaporates, c) Condensing the rising vapor using a heat exchanger surface (15) of a heat exchanger positioned in the upper region of the container (2), which heat exchanger forms part of a refrigeration machine (16), d) Pumping the resulting liquid ice into a liquid ice storage device (12), and e) Replacing the pumped-out liquid ice with fresh liquid, in particular with fresh water, wherein the mixture of liquid and ice contained in the container (2) is in particular constantly stirred.
6. Method according to claim 5, characterized in that the liquid used is water, preferably demineralized water.
7. The method according to claim 5 or 6, characterized in that the pressure in step b) is reduced to the triple point pressure of the liquid, in the case of water to less than 10 mbar, preferably to about 6 mbar.
8. Method according to one of claims 5 to 7, characterized in that the heat exchanger surface (15) on which the vapor rising inside the container (2) condenses in step c) has a temperature below the triple point of the liquid, in the case of water to a temperature slightly below 0°C, in particular about -2°C.
9. Method according to one of claims 5 to 8, characterized in that the ice content within the container (2) is kept at a maximum of 50-60%.
10. Method according to one of claims 5 to 9, characterized in that, when a layer of ice forms on the heat exchanger surface (15), the heat exchanger is defrosted, in particular by temporarily switching over in the refrigerant circuit of the refrigeration machine (16).
11. A method for cooling the air sucked in by a compressor (19) of a gas turbine (20) using liquid ice, characterized in that a liquid ice production plant (1) according to one of claims 1 to 4 is used and / or liquid ice produced by a method according to one of claims 5 to 10 is used.
12. Method according to claim 11, characterized in that the liquid ice is pumped directly through a heat exchanger (23) arranged in an intake channel of the compressor (19).
13. Method according to claim 11, characterized in that the liquid ice is pumped through a heat exchanger which recools an intake cooling circuit of an intake duct of the compressor (19) containing a heat exchanger (23).
14. Method according to one of claims 11 to 13, characterized in that the water is supplied for further use after cooling the sucked-in air, if necessary after intermediate storage.