WO2012123853A1

WO2012123853A1 - Thermal storage facility especially suitable for concentrating solar power installations

Info

Publication number: WO2012123853A1
Application number: PCT/IB2012/051038
Authority: WO
Inventors: Johannes Paulus KOTZE
Original assignee: Stellenbosch University
Priority date: 2011-03-11
Filing date: 2012-03-06
Publication date: 2012-09-20
Also published as: AP2013007169A0; US20140000583A1; EP2683983A1; ZA201307450B; JP2014513260A

Abstract

A thermal storage facility is provided for storing thermal energy at elevated temperature. The facility comprises an enclosure for retaining a thermal storage medium; primary heat transfer surfaces for transferring heat from a circulating heat transfer fluid; and secondary heat transfer surfaces for transferring heat from the thermal storage medium to steam pipes associated with a power generation facility. The thermal storage medium is a metallic phase change material, preferably having a melting temperature above 500 °C, interposed between the primary heat transfer surfaces and the secondary heat transfer surfaces. The metallic heat transfer fluid is preferably liquid over an extended range of temperatures from ambient temperature through to a maximum operating temperature in excess of the melting point of the metallic phase change material. The metallic heat transfer fluid is preferably selected from a molten alkali metal or alloys of alkali metals. The invention extends to solar power installations incorporating one or more such thermal storage facilities.

Description

THERMAL STORAGE FACILITY ESPECIALLY SUITABLE FOR CONCENTRATING SOLAR POWER INSTALLATIONS

FIELD OF THE INVENTION

This invention relates to a thermal storage facility that is especially suitable for use in storing thermal energy and especially, although not exclusively, thermal energy that has been derived from the sun using a concentrating solar power installation.

BACKGROUND TO THE INVENTION

One of the advantages of concentrating solar power (CSP) with a solar receiver mounted on a tower, is that thermal energy storage is made possible. Thermal storage is required in order to buffer the power generating cycle of sunny, clear days so that a solar power plant is enabled to supply electrical energy at night time and during bad weather. Current thermal storage systems mostly operate in the sensible heat mode, and either store thermal energy in a molten salt of some kind, or store the heat in solid bodies, like concrete. The use of molten salt has the disadvantages that solidification of the salt may occur in the receiver loop at night; trace heating is therefore needed to keep salts in a liquid form during non operational times such as night time and in auxiliary piping; and this may lead to maintenance problems.

Other phase change concepts have been noted but they are all based on low conductivity salts, and require extensive heat transfer modification to improve their thermal conductivity. Solid storage materials, such as concrete bodies, also have limited power output and, furthermore, may suffer from thermal stress that limits the useful life of such media. Modifications to the solid storage elements makes them expensive.

In other storage systems, oil may be used as a storage medium and heat transfer fluid. One of the primary problems with this is the fact that the maximum operating temperature of oil is about 400 °C. This seriously limits the maximum receiver temperature, and curbs the maximum efficiency of the power plant.

As far as applicant is aware all of the curent modes of thermal storage have limitations. Generally there are two limitations to thermal energy storage, one is a temperature limitation, and the other is high melting point heat transfer fluids that causes a inherent blockage problem.

There is accordingly a need for an alternative thermal energy storage system that eliminates the need for trace heating of the heat transfer fluid, and offers high power thermal energy storage at higher temperatures than currently practiced.

SUMMARY OF THE INVENTION

In accordance with a first aspect of this invention there is provided a thermal storage facility for storing thermal energy at elevated temperature, the facility comprising an enclosure for retaining a thermal storage medium, primary heat transfer surfaces for transferring heat from a circulating heat transfer fluid, and secondary heat transfer surfaces for transferring heat from the thermal storage medium to steam pipes associated with a power generation facility, the thermal storage facility being characterized in that the thermal storage medium is a metallic phase change material (PCM) that is interposed between the primary heat transfer surfaces and the secondary heat transfer surfaces and in that the heat transfer fluid is a liquid metal.

Further features of the invention provide for the metallic phase change material to have high thermal conductivity, high latent heat of fusion with an high melting temperature, preferably above about 500 °C, and high operating temperatures. One candidate material is a metallic phase change material that is an alloy of aluminium and silicon, preferably a commercially available alloy such as one containing 87.76% Al and 12.24% Si (AISM 2 which is also known as LM6 casting alloy).

The invention further provides for the metallic heat transfer fluid to be liquid over an extended range of temperatures from ambient temperature through to a maximum operating temperature in excess of the melting point of the metallic phase change material, with the metallic heat transfer fluid typically being molten alkali metals or alloys of alkali metals and especially an alloy of sodium and potassium such as that known as NaK; and for the heat transfer surfaces to be those of generally parallel pipes extending through or adjacent the enclosure.

The invention also provides a thermal storage facility as defined above in which the thermal storage facility forms part of a unit selected from a pre- heater, a boiler, a super heater, a re-heater and a combination unit designed to perform the function of any two or more of such units.

In one variation of the invention a solar power installation that is typically of a relatively small capacity, may have a single heliostat field and associated solar receiver mounted on a tower forming part of the solar power installation in which instance the thermal storage facility as defined above may be built into the tower. In another variation of the invention a solar power installation may comprise multiple heliostat fields with associated solar receivers carried on towers with each of the solar receivers being connected to one or more common thermal storage facilities as defined above. In such an instance a thermal storage facility may be embodied in any one or more units selected from a pre- heater, a boiler, a super-heater, and a reheater, typically all of these, with the flow of heat transfer fluid to each of the facilities being controlled to achieve the relevant objective of the relevant unit It will be understood that with the construction of a thermal storage facility as defined above, suitable molten alkali metal alloys can be employed as heat transfer fluid whilst the presence of the metallic phase change material between the molten hot alkali metal alloy and water or steam prevents contact from being made between the alkali metal and the water or steam, which could otherwise have disastrous results. The arrangement enables the particular properties of materials not heretofore used in the relevant situations to be usefully employed and advantage to be taken of their special properties. The invention therefore enables the water or steam heater or generator to be combined with the thermal storage unit.

In use therefore, heat may be transferred from a receiver atop a tower by molten heat transfer fluid, typically in the form of molten alkali metal alloys such as NaK (eutectic or hypoeutectic), to the metallic phase change material that melts to thereby absorb thermal energy by way of the material's latent heat of fusion with the heat being stored and transferred to the secondary heat exchange surfaces to generate steam from water and superheat it for use in a power generation plant, as may be required.

In order that the above and other features of the invention may be more fully understood, two different proposed embodiments thereof will now be discussed in further detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:- Figure 1 is a schematic illustration of one variation of concentrating solar power installation embodying the facility of the invention wherein a single solar receiver is located atop a tower that carries it; Figure 2 is a similar illustration of a another variation of concentrating power installation embodying the facility of the invention in which multiple solar receivers are located atop multiple towers that carry them and heat transfer fluid is fed to multiple thermal storage facilities that utilise the heat to heat water or steam as may be required; and,

Figure 3 is a schematic cross-section taken through one form of a storage vessel according to the invention showing the primary and secondary heat transfer pipes distributed through the metallic phase change material.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

In the embodiment of the invention illustrated in Figure 1 the concentrating solar power installation includes a substantially conventional field of heliostat (1 ) with a solar receiver (2) mounted on a tower (3). The solar receiver on the tower is to be used to heat a metallic heat transfer fluid in the form of the alkali metal NaK.

In this particular embodiment of the invention, the thermal storage unit, steam generator and circulation system of the heat transfer fluid, are all embodied in the tower. It is envisaged that such an arrangement will only be applicable to certain sizes of concentrating solar power installation and not to larger ones that will most likely be more like the embodiment of the invention described with reference to Figure 2. Reverting to the embodiment of the invention illustrated in Figure 1 , the heat transfer fluid is circulated by means of a pump (4) through the receiver and through metallic phase change material (5) contained within the enclosure (6) substantially filling the tower by way of a series of heat transfer pipes (7). The heat transfer fluid, namely the molten NaK heat transfer fluid, is thus heated in the thermal receiver and circulated through the series of heat transfer pipes (7) that define the primary heat transfer surfaces.

The steam pipes (8) that define the secondary heat transfer surfaces are also in contact with the metallic phase change material but are spaced apart from the heat transfer pipes (7). The steam pipes extend between a lower reservoir (9) and an upper steam chamber (1 0) or there may be an external steam drum.

The remainder of the concentrating solar power installation is substantially conventional and includes the usual steam treatment equipment, high pressure, medium pressure, and low pressure turbines (1 1 , 1 2, 1 3), and condensation recycling equipment generically indicated by numeral (14).

It is to be mentioned that in this instance a re-heater (1 5) for reheating steam leaving the high-pressure turbine (1 1 ) and preparatory to it entering the medium pressure turbine (1 2), can be constructed in a similar manner and have its own heliostat field (1 6), receiver, tower and thermal storage facility.

Turning now to the embodiment of the invention illustrated in Figure 2, in a larger concentrating solar power installation, a plurality, in this instance four, heliostat fields (21 ) each have their own receiver (22) mounted on a tower (23) with their heat transfer fluid outputs being connected together so that the entire solar power receiver assembly can supply a separate arrangement of thermal storage units that form part of water and steam heating and generating units. In this particular instance the combination of thermal storage and water and steam heating and generating units include a pre-heater (24), a boiler (25), a super-heater (26) and a re-heater (27) that is interposed between the high- pressure turbine (28) and medium pressure turbine (29). In each instance the relevant unit has its own enclosure that is composed of heat transfer pipes and steam pipes that have not been separately shown as they are arranged in substantially the same way as those described below with reference to Figure 3.

As illustrated in Figure 3, in each instance in which a thermal storage arrangement according to the invention is used, a preferred general type of arrangement of steam pipes (31 ) and heat transfer fluid pipes (32) is achieved by distributing the pipes throughout the interior of a containment enclosure (33) that is filled with metallic phase change material (34) in a suitable pattern, such as on a series of concentric circles.

The desirable properties of the metallic heat transfer fluid include low melting point (preferably below room temperature) ; high thermal conductivity; high density; and high specific heat capacity. A low melting point is very important for a reliable concentrating solar power plant, as the receivers will be cold for at least half of their life cycle. A high melting point heat transfer fluid in the receivers would reduce the reliability of the installation since solidification of the heat transfer fluid may pose significant problems.

NaK is a eutectic mixture of sodium and potassium and its eutectic composition is 78% potassium and 22% sodium (by mass). It is a liquid between -12.6°C and 785 °C. Compositions between 40% and 90% potassium (by mass) are liquid at room temperatures. Mixtures with more sodium are preferred, since the specific heat capacity of sodium is more than that of potassium. The properties of NaK include high reactivity with water; if it is stored in air, it forms a potassium super oxide that can ignite and is highly reactive with organics which makes it dangerous to store in organic solvents and mineral oil. Eutectic NaK has a very high surface tension ; a density of 0.855 g/mL at 100°C; and a thermal conductivity of 23.2 W.m^"1.K^"1 @ 100°C

Years of research in the nuclear industry established a standard NaK design and handling protocol so that NaK can be safely used in a concentrating solar power plant.

Regarding the metallic phase change material, two good contenders are an alloy composed of 87.76% Al and 12.24% Si that has a melting temperature of 557 °C and a heat of fusion of 498 J/g; and an alloy composed of 83.14% Al, 1 1 .7%Si and 5.16% Mg that has a melting temperature of 555 °C and a heat of fusion of 485 J/g. The former was chosen because it is a common and low cost casting alloy with high thermal conductivity. It is non toxic, readily available, and there are prospects of improving its latent heat of fusion through further alloying.

The upper receiver temperature of over 770 °C, and the melting temperature of the AISi alloy at 557 °C makes the use of a steam cycle possible. Superheated steam can therefore be produced directly from heat stored in the metallic phase change material.

There are a number of ways the heat transfer concept can be implemented, but the final geometry of the concept will be dictated by the heat transfer requirements of the specific design. The containment enclosure does not need to be cylindrical, but in the instance of a tower, a cylindrical form makes sense. A regards possible pipe layouts associated with the containment enclosure, more specific geometries may be suitable for particular applications. One factor that needs to be considered is the heat to be transferred by way of the primary heat transfer surfaces being those of the heat transfer fluid pipes. Another factor is the distance between those heat transfer fluid pipes and the secondary heat transfer surfaces being those of the steam pipes in order to provide for the required heat flux. Another factor is the volume of the metallic phase change heat storage material present. Naturally the heat transfer requirements are size dependant.

It will be understood that numerous possibilities exist and that the embodiments of the invention described above are simply illustrative of possible arrangements. The invention therefore provides an effective thermal storage facility that is especially suitable for use in conjunction with heliostat field receiver plants.

Claims

A thermal storage facility for storing thermal energy at elevated temperature, the facility comprising an enclosure for retaining a thermal storage medium, primary heat transfer surfaces for transferring heat from a circulating heat transfer fluid, and secondary heat transfer surfaces for transferring heat from the thermal storage medium to steam pipes associated with a power generation facility, the thermal storage facility being characterized in that the thermal storage medium is a metallic phase change material that is interposed between the primary heat transfer surfaces and the secondary heat transfer surfaces and in that the heat transfer fluid is a liquid metal.

A thermal storage facility as claimed in claim 1 in which the metallic phase change material has high thermal conductivity, high latent heat of fusion with a high melting temperature.

A thermal storage facility as claimed in claim 2 in which the melting temperature of the metallic phase change material is above 500 °C.

A thermal storage facility as claimed in claim 3 in which metallic phase change material is selected from an alloy of aluminium and silicon containing 87.76% Al and 12.24% Si and an alloy composed of 83.14% Al, 1 1 .7%Si and 5.16% Mg.

A thermal storage facility as claimed in any one of the preceding claims in which the metallic heat transfer fluid is liquid over an extended range of temperatures from ambient temperature through to a maximum operating temperature in excess of the melting point of the metallic phase change material.

6. A thermal storage facility as claimed in claim 5 in which the metallic heat transfer fluid is selected from a molten alkali metal or alloys of alkali metals. 7. A thermal storage facility as claimed in claim 6 in which the metallic heat transfer fluid is an alloy of sodium and potassium.

8. A thermal storage facility as claimed in any one of the preceding claims in which the heat transfer surfaces are those of generally parallel pipes extending through or adjacent the enclosure.

9. A thermal storage facility as claimed in any one of the preceding claims in which the thermal storage facility forms part of a unit selected from a pre-heater, a boiler, a super-heater, a re-heater and a combination unit designed to perform the function of any two or more of such units.

10. A solar power installation having a single heliostat field and associated solar receiver mounted on a tower wherein a thermal storage facility as defined in any one of the preceding claims is built into the tower.

1 1 . A solar power installation comprising multiple heliostat fields with associated solar receivers carried on towers with each of the solar receivers being connected to one or more common thermal storage facilities as defined in any one of claims 1 to 9 with the thermal storage facility being embodied in any one or more units selected from a pre-heater, a boiler, a super-heater, and a re-heater, with the flow of heat transfer fluid to each of the facilities being controlled to achieve the relevant objective of the relevant unit.