Patent Application
20200400384
In this invention is presented a new device that enables storing heat in a vessel, by enclosing a fluid at high temperature, using a design consisting of a concave lower head and intermediate body supported from the top, which enables free radial and axial dilatation, contains a minimum volume of hot fluid in a central extension outwards as a tank heel, have very low manufacturing costs and weight, having a second upper head designed to close the ullage gas.
The main object of this invention submitted for an invention patent is a device for storing heat with high reliability at high temperature and low manufacturing cost.
More specifically, this invention consists of a high reliable device for storing heat by enclosing a fluid at high temperature, with an inexpensive construction that enables, with low investment cost, its use for storing energy as a molten salt at high temperature and atmospheric pressure.
For many years fluids have been stored in vessels and tanks at ambient pressure and temperature, but recently has been and increasing interest in storing thermal energy as a molten salts at high temperature. As analyzed by the World Energy Council in the year 2016, when big amounts of energy, in the order of tenths to hundredths of megawatts per hour, are currently required to be stored for many hours with high efficiency, only the thermal energy storage by means of molten salts is available, since the other large-scale alternative option, i.e. the use of pumping hydroelectric facilities, only can used under very specific and limited geographic and hydrological conditions, with limited efficiencies.
As explained by Xavier Py et al. in the year 2017, the fact that sun and wind availability is intermittent and unmatched to consumers' needs has strongly increased interest in concentrated solar power technologies with central receivers, since in these plants it is significantly easy, inexpensive and efficient to directly store in a “two-tank system” the heat derived from the concentrated solar power receiver, so that the heat can be used for electric power generation by means of a steam turbine according to the operator's decision and to the heat that plant was able to store during the diurnal period. In these plants, the molten salt enters, from a first tank of molten salt at low temperature, into a receiver that gets sunlight as concentrated with mirrors that are oriented in a controlled way, so that the molten salt gets out at high temperature and comes to be stored in a second tank of hot molten salt. Under the same reference is explained that in the recently inaugurated Gemasolar (Spain, 2011) and Crescent Dunes (USA, 2015) plants up to 20 and 110 megawatts of electric power can be produced, respectively, with 15 to 6 hour storage headacity, which enables to significantly reduce intermittence and deliver the stored thermal energy according to the electric power demand.
For this reason, as explained by Bauer et al. in the year 2012, the two-tank molten salt system is the only system commercially available in large thermal storage headacities for concentrated solar power plants. The success and consolidation of this system resulted from developing and building over a number of years different technology prototype-plants to store the generated solar power in the central receiver.
As described by Falcone in a design handbook for this type of solar power plants published in the year 1986, several methods for storing large-scale thermal solar power were tested in the '80s. In the Themis plant, France, and in the CESA-I plant, Spain, were developed the first prototype plants of 2.5 and 1 electric megawatt/hour, respectively, with a storage headability of 12 and 3 electric megawatt/hour each, respectively, using the two-tank molten salt design for the first time. In the same time period, the first large-scale test was performed in the USA for two-tank molten salt storage, with a storage headacity of 7 thermal megawatt hour, in the CRTF facility.
The feasibility and competitiveness of storing large volumes of molten salt at very high temperature depend, to a large extent, on the feasibility of being able to build tanks headable of holding the whole of that volume with low heat loss, implying an investment compatible with electric power generation, and with a reliability that enables failure-free operation over long periods of time.
As detailed by Winter et al. in the year 1991, that Themis and CESA-I plants were using horizontal cylindrical tanks or vessels with spherical heads, while the CRTF facility used vertical cylindrical tanks with a flat bottom that is supported on a refrigerated concrete-made foundation.
According to Falcone's discussion, the two hot salt storage tanks can be designed in three alternative designs, i.e. flat-bottomed vertical cylindrical tanks with external thermal insulation, flat-bottomed vertical cylindrical tanks with internal thermal insulation for the hot tank and external thermal insulation for the cold tank, or several horizontal cylinders for both fluids. In the same discussion, Falcone mentions that the spherical tanks were considered in some early designs, but they were subsequently discarded as more expensive than the alternative options above mentioned. A similar discarding of spherical tanks, as more expensive, was also mentioned in 1987 by Rothrock, in a letter patent for reducing structural solicitation affecting vertical cylindrical tanks.
Both in Rothrock's letter patent and in the Falcone's design handbook, it is explained that the vertical cylindrical tanks undergo strong thermal-mechanic solicitation caused by the creep and stress exerted on the joining of the flat bottom with the lateral cylindrical walls, and the efforts and degradation undergone by the lower base as said lower base has to slide with regard to its supporting base due to the periodical tension and dilatation cycles. These problems are more serious for the tanks subject to higher temperature. In the current commercial facilities, the tank in which the liquid salt is stored at lower temperature before getting into the receiver, also called “cold tank”, is kept at a temperature of around 290° Celsius, while the tank in which the salt is stored after its having been heated in the solar receiver, also called “hot tank”, is kept at a temperature of around 565° Celsius.
Due to these problems encountered with the vertical cylindrical tanks that are supported from their lower base, Falcone et al. mention the alternative option of flat-bottomed tanks supported from their top or by means of lateral columns, as Winter et al. mention the possibility of conical tanks with very narrow angle that are supported from their external perimeter.
After several tests and configurations performed in the '90s and at the beginning of year 2000, it was established that the best system, on account of its renderings, efficiency and costs, was the molten salt storage directly in two flat-bottomed vertical cylindrical tanks. Therefore, as detailed by Tyner et al. in the year 1995, the storage system of the Solar One's prototype plant, which used heat exchangers and a single rock and sand tank, was replaced. In this plant, which was renamed as “Solar Two”, heat was being stored directly by means of a system consisting of two flat-bottomed vertical cylindrical tanks, supported by/on an insulating base and refrigerated concrete.
As from its successful operation, for more than two years, of Solar Two prototype plant in the USA at the end of the '90s, the plant was closed and dismantled, having set the Design Bases for the current concentrated solar power plants, with the more successful and currently operating central receivers, as the aforesaid Gemasolar plants.
These Design Bases were set forth in the year 2001 by Zavoico, in the Design Basis Report published by Sandia National Laboratories, USA. This report includes the Design Bases for heat storage at high temperature by means of molten salts, using the design of two flat-bottomed vertical cylindrical tanks with external thermal insulation, as supported on an insulating sliding lower base and on refrigerated concrete. Similar thereto is the design described by Moore et al.'s subsequent report in 2010, published by the same laboratories.
Current plants follows the Sandia National Laboratories' Design Basis guidelines for the heat storage systems, through the horizontal two-tank system, as available in the descriptions provided by Taylor and Burgaleta in the years 2016 and 2012, respectively.
A more detailed analysis, however, shows that the successful operation of the tanks in the Solar Two plant does not match in such a direct way with the Crescent Dunes and Gemasolar tanks operation. As described by Brean in the year 2016, Crescent Dunes had to stop operating for a number of months in order to repair a hot tank failure upon approximately one year's operation, while, as outlined by Sener in the year 2017, the Gemasolar plant had to fully replace a hot salt tank before 6 years operation. These times of operation between failures should be compared with the classical purposes of a commercial plant, in which, for amortizing significant headital costs it is usually required that the plant systems and components can operate for long periods of time, typically of 20 to 25 years.
As detailed by Pacheco in the year 2002, the Solar Two tanks were of 11.6 meter diameter and 8.4 meter height, could store 106.5 thermal megawatt hour, and were tested for an approximate period of 15,000 hours each tank. In comparison with those tanks and that operation background, the Gemasolar tanks are of 23 diameter and 11 meter height, and can store 670 thermal megawatt hour, and their hot tank was replaced upon approximately 50,000 operation hours, while the Crescent Dunes tanks are of 43 meter diameter, 12 meter height, and can store 2700 thermal megawatt hour, and major repairs had be made on the hot tank upon approximately 10,000 operation hours.
It clearly results, therefore, that extrapolation to larger dimensions and longer operation times, as compared with the values tested on the Solar Two tanks, required for a commercial plant has generated a major problem to the commercial development and the reliable operation of concentrated solar plants fitted up with heat-storage central receivers.
As these problems are not fully unexpected, proposed devices can be found, e.g. in Rothrock's letters patent of year 1987, Bell et al.'s letters patent of years 2011 and 2014, and Cutts' letters patent of year 2013, which intend to do away with the tension and deformation challenges implied by the flat-bottomed support with a hot fluid at such a high temperature, with daily changes in level and temperatures. Thus it can also be understood that at the start of this technology other alternative options have been evaluated as compared with the bottom-supported vertical cylindrical tanks, such as the proposals above mentioned, i.e. Falcone's and Winter's in the years 1986 and 1991 respectively.
A strong deviation that can explain tank failure and replacement in the commercial plants is that, as mentioned in the Zavoico's Design Bases, the hot salt storage tanks are specified under standard API 650, and this standard sets an operation limit of up to 260° Celsius for refrigerated tanks, which is much lower than the 565° Celsius for hot salt tank. At a temperature significantly higher than the standard limit value, failure risk is higher and reliability is low.
Additionally another problem is encountered, overlooked in the costs of concentrated solar plants with central receivers, as directly incorporated in the Design Bases without an impact critical analysis, i.e. flat-bottomed vertical cylindrical tanks are assumed to be the only feasible option for heat storage.
The electric engines of the molten salt pumps are fitted outside and above each tank, while the pump driver is submerged near the bottom of the tank by means of a long shaft and vertical pipe that drives and carry the salt towards an output nozzle located near the electrical engine, above and outside the tank. The cost of the molten salt pumping system is thus simplified, made reliable, and reduced, making use of the advantage that the tanks are under atmospheric pressure.
The submerged pumps require a minimum submergence or fluid level above their suction inlet, so that they do not generate vorticity that could drag air as the salt enters into the pump inlet, and a minimum clearance below their suction inlet so that they do not generate any restriction for salt circulation at said pump inlet. According to the value recommended in the Design Bases published by Sandia National Laboratories, the minimum salt height for each tank should be equal to 1 meter of molten salt or higher. This minimum depth of molten salt, or molten salt heel or tank bottom, generates two economic effects that significantly penalize the plant cost.
In a first effect, to the salt volume that each tank should be headable of storing to contain the heat that is required to store, is added the salt volume from the heel that is required to operate this tank plus the salt heel volume from the other tank, because each tank should be able to store whole volume of the molten salt in both tanks, in the event of drying the other tank for maintenance. As the volume of this tank heel neither can nor should be driven by the pump, it is not an integral part of the molten salt stock that is utilized for heat storage purposes. Considering that each tank should thus be headable of storing a salt volume that is twice the salt heel of each tank, and that the height of a tank can typically range from 10 to 14 meters, these two meters approximately represent an increase in the tank volume and weight of approximately 20% as compared with the volume strictly required for heat storage purposes, which in principle is consistent with a similar percentage in the increase in the cost of each tank.
In a second effect, the total salt inventory necessary to operate the plant also requires, besides the salt mass required to contain the heat required to store, the operating stock of the piping and components of the molten salt circuits, and the inactive mass of both heels of the tank. This implies an increase in the total salt inventory, also in the order of 20%. If it is considered that one of the main headital costs for this type of plants is the cost of the salt inventory, this main cost is also increased in approximately 20%.
Taking as reference the cost ratios of the different components of a commercial plant shown by Kelly in the year 2010, the total direct costs of a commercial plant increase by 1.5% due to both effects, with 0.4% thereof relating to the higher cost of the tanks and 1.1% thereof relating to the higher cost of the salt. Another way to represent on an economic basis the penalty implied by both effects is also obtained by comparing the additional cost generated by the cost of the tanks. In this case, it results that the increase in costs is equivalent to 90% of the cost of the two tanks.
The flat-bottomed vertical cylindrical tanks, which reduce the manufacturing cost by transfer the weight to the foundations and using single curvature metal sheets, has the disadvantage that requires more material weight and welding thickness, increasing the manufacturing costs, for those part of the tank with less height by the increase of the hydrostatic pressure, compared with curved casks which requires the less material weight and welding thickness.
There thus exist the actual requirement to have an inexpensive device, and competitive with horizontal cylindrical tanks, headable of storing a molten salt heat with lower failure rate and higher reliability, significantly reducing the direct additional costs generated by the tank heel demands from the molten salt pumps and significantly reducing the manufacturing costs of the tanks. ACA
Considering the problems described with regard to the current devices for storing heat in molten salts at high temperature, this invention has been conceived to solve those problems.
In other words, this invention is intended to have a device that enables to store heat by enclosing a fluid at high temperature, with inexpensive construction, allowing its application for energy accumulation in the form of molten salt at high temperature and atmospheric pressure, with high reliability and low risk, significantly reducing the costs implied by the tank heel demands from the molten salt pumps.
The device can be applied to store other fluids at high temperature, as well as those fluids that can require storage at higher working pressures.
For accomplishing the purpose proposed, the device consists of a tank or vessel made-up of the joining two curved shells, the axis of revolution of which is vertically oriented, i.e. the lower shell is a curved head welded to an upper shell build by rings of conical sections, in which the two curved shells contains the hot fluid and is supported with ground clearance from its upper portion, so that the vessel is free from thermally expand to the supporting plane.
The vessel ends on the top by an upper head to contains the ullage gas.
For lowering the tank weight and manufacturing cost, the device could use a concave lower head of ellipsoidal shape, in which the ellipsoid height to diameter ratio is higher than or equal to two, and it is designed according to the design pressure generated by the weight of the hydrostatic column of the stored fluid and the height of the supporting upper zone. The device can also use other concave shapes, such as a torispherical head, according to the production costs of one or another head.
In order to lower the manufacturing cost, the vessel could ends on the top in the central zone by a flat vessel head connected to a conical section build by facetted planes or by a ring of cylindrical truncated cone.
In order to lower the tank weight and the manufacturing cost, the device uses an upper head with height to diameter ratio higher than or equal to four, which can be of torispherical or ellipsoidal shape, designed to withstand its own weight and the pressure generated by the slight overpressure exerted by the ullage gas, plus the expectable overpressure exerted during transitory events and the opening pressure of the ullage gas relief valves.
For minimizing the economic penalty involved in using fluid impelling pumps with their engine fitted outside and above the tank and their driver submerged near the bottom, the lower head could have a central downward extension in its center, which ends with a concave lower head, the depth with regard to the beginning of the extension being similar to the height required as heel for pumping the coolant, and an internal diameter that enables suction without generating restrictions to the fluid getting into the pump. This extension exerts the effect of the so-called pumping wells and for its small size, weight and simple construction it is of low cost body, that also requires a much smaller quantity of molten salt than the molten salt quantity required for heat storing.
The union between the lower head and the central downward extension could be made with a smooth transition revolution body to reduce the local stress.
As there is an technological limit with regard to the reachable highest depth for the molten salt pumps currently available, in which the engine is fitted outside and above the tank and the driver is submerged near the bottom, the device uses an ellipsoidal shape for the lower head, of height to diameter ratio equal to two, in the case the pump required by said tank is compatible with the depth technologic limit of the available pumps, i.e. in that case, the device uses a semispherical head, cylindrical truncated cone, flat head or faceted cylindrical truncated cone.
When the pump required by a tank is within the depth technologic limit of the pumps currently available, the device uses an upper head of inverted type to minimize the distance the distance between the molten salt upper limit level and the center of the upper head. The inverted type upper head could have a concave curve head or flat head at the center, with a smooth transition revolution body to the external zone of the upper head which had a higher height than the central height. The external zone of the upper head could combine toroidals, conical sections, planes or faceted conical sections shapes.
Comparing the amount of steel required by a sphere to contain a given volume of fluid at atmospheric pressure, it is noticed that a sphere requires approximately four times the mass for two times the volume of a hemisphere. Even though they would seem geometrically similar, with the same diameter, the upper portion increases by two times the design pressure, by two times the weight of the hydrostatic column, which increases by four times the weight of the metal sphere, containing only two times the volume stored with regard to the hemisphere.
Therefore, this lower efficiency on material use of the sphere could be the reason that explain why in the past several authors discard spherical tanks compared with vertical cylindrical tanks. Nevertheless, as derived from the foregoing explanation, this discarding does not apply to a device containing hot fluid with the shape proposed in this invention.
Considering that, besides, the proposed device significantly reduces the salt weight and tank volume penalty generated in the pumping heel demand for operating the tank when it is at minimum level for flat-bottomed vertical cylindrical tanks, advantage of this device is even clearer.
From the functional and economic viewpoint, the proposed device requires less weight of vessel than the horizontal cylindrical tanks
The horizontal cylindrical tanks, requiring higher thickness at the bottom, requires higher costs in materials, transport, construction, joints preparation, and welding compared with the proposed device.
Besides, on account of its geometry, supporting, materials, loads, corrosion, and temperature, this device is fully admissible with the design under ASME VIII Code, as all of these parameters are included in those available in said Code. For this reason, and considering the high reliability and low failure rate of the vessels designed and constructed according to the ASME Code, failure risk and low reliability are significantly reduced in the proposed device.
In addition, the vessel support and loads transfer from to the foundation could be performed with well known supports design alternatives for ASME Vessels, either using static or sliding commercial available supports technologies, at loads and temperatures required for thermal heat storage.
To better understand this invention, we provide below a detailed description on the basis of the following figures, which serve the sole purpose of illustrating the preferred way to execute this patent and not to represent a limit to the invention:
In this invention a new device is presented that enables to store heat by enclosing a fluid at high temperature, with an inexpensive construction, enabling its application for energy accumulation in the form of molten salt at high temperature.
Following, 117 claims are provided on pages 21 to 29:
| Number | Date | Country | Kind |
|---|---|---|---|
| P190101717 | Jun 2019 | AR | national |
| P200101702 | Jun 2020 | KR | national |
