WO2013005192A1

WO2013005192A1 - Thermal power plant with regenerator and method of producing same

Info

Publication number: WO2013005192A1
Application number: PCT/IB2012/053477
Authority: WO
Inventors: Benoît WATREMETZ
Original assignee: Saint-Gobain Centre De Recherches Et D'etudes Europeen
Priority date: 2011-07-07
Filing date: 2012-07-06
Publication date: 2013-01-10
Also published as: EP2729754A1; FR2977661A1; FR2977661B1; US20140144603A1

Abstract

The present invention relates to a thermal power plant comprising: -a unit producing heat energy, -a regenerator comprising a bed of energy-storage elements, -a consumer of heat energy, and -a circulation device which: -during a charging phase, circulates a charging heat-transfer fluid from the unit producing heat energy to the regenerator, then through the said regenerator, the said charging heat-transfer fluid entering the said regenerator at a charging temperature Tc of less than 1000°C, and -during a discharging phase, circulates a discharging heat-transfer fluid through the said regenerator, the discharging heat-transfer fluid entering the said regenerator at a discharging temperature Td, the energy-storage elements being made of a material which has a melting point higher than Tc + 50°C and lower than 2000°C, the concentration of all the elements leached from the said material, as measured in accordance with standard EN 12457-2 of December 2002, being less than or equal to 0.5 g/l, the thermal power plant being characterized in that the said material of the energy- storage elements has a characteristic ratio A higher than 0.3, with: A = (Cp(Tc) –Cp(Td)) / Cp(Td) where -Cp(Tc) is the specific heat capacity of the said material at the charging temperature, and -Cp(Td) is the specific heat capacity of the said material at the discharging temperature.

Description

Thermal power plant with regenerator and method of producing same Technical field

The invention relates to a thermal power plant comprising a regenerator for storing heat, and to a method for producing such a plant.

Technical background

A thermal power plant according to the invention conventionally comprises a unit producing heat energy, a consumer of heat energy and a regenerator for storing this heat energy so that there can be an offset over time between its production and its consumption.

The ability to store heat energy is also of use for harnessing soft energy, such as solar energy, which is renewable but the production of which is intermittent. Energy storage may also be of use in order to take advantage of differences in electricity prices between the "off-peak" times during which the electricity tariffs are at their lowest, and the "peak" times during which the tariffs are at their highest. For example, when energy is stored by compressing air, generating heat energy which is stored in a thermal regenerator, the electricity-consuming compression phases are advantageously performed at low cost during off-peak times, while the expansion phases that produce electricity are carried out during peak times in order to supply electricity that can be injected into the grid, according to the demand, at an advantageous tariff.

The heat energy is conventionally stored in a bed (packed bed) of energy-storage elements (media) of a regenerator, for example formed of a bed of rocks.

The storage operation, which employs an exchange of heat between a stream of heat- transfer fluid and the regenerator, is conventionally known as "charging", the heat-transfer fluid entering the regenerator during charging being known as the "charging heat-transfer fluid".

The transfer of heat energy may lead to an increase in the temperature of these energy- storage elements (in which case it is "sensible" heat that is stored) and/or to a change in state of these elements (in which case it is "latent" heat that is stored).

The stored heat energy can then be restored, by heat exchange between a stream of heat-transfer fluid and the energy storage elements. This operation is conventionally known as "discharging", the heat-transfer fluid entering the regenerator during discharge being known as the "discharging heat-transfer fluid".

The work entitled "A review on packed bed solar energy storage systems", Renewable and Sustainable Energy Reviews, 14 (2010), pp 1059-1069, describes the state of the art in the field of regenerators and, in particular, the influence that certain parameters have on the efficiency of the said regenerators.

The efficiency of a regenerator is very much dependent on its geometry and on the material of the energy-storage elements used to accumulate and restore the heat energy. There is a constant need to increase the temperature of the heat-transfer fluid leaving the regenerator during discharging, for the same volume of energy-storage elements. This need is all the more keenly felt as environmental regulations and a desire to keep expenditure down are encouraging industry always to seek further energy savings.

It is an object of the invention to satisfy this need, at least in part.

Summary of the invention This object is achieved by means of a thermal power plant comprising:

a unit producing heat energy, for example a furnace, a solar tower, a compressor, and

a regenerator comprising a bed of energy-storage elements, particularly a sensible-heat regenerator, and

- a circulation device which

during a charging phase, circulates a charging heat-transfer fluid from the unit producing heat energy to the regenerator, then through the said regenerator, the said charging heat-transfer fluid entering the said regenerator at a charging temperature Tc, and

- during a discharging phase, circulates a discharging heat-transfer fluid through the said regenerator, the discharging heat-transfer fluid entering the said regenerator at a discharging temperature Td,

the energy-storage elements being made of a material which has a melting point higher than Tc + 50°C and lower than 2000°C, and

the concentration of all the elements leached from the said material, in response to the test described in standard EN 12457-2 of December 2002, being less than or equal to 0.5 g/l. According to the invention, the thermal power plant is notable in that the energy-storage elements are made of a material having a characteristic ratio A higher than 0.3, with:

A = (Cp(Tc) - Cp(Td)) / Cp(Td)

where

- Cp(Tc) is the specific heat capacity of the said material at the charging temperature Tc, and

Cp(Td) is the specific heat capacity of the said material at the discharging temperature Td.

In the prior art, the material of the energy-storage elements is conventionally determined in such a way as to maximize the product p.Cp(25°C) of the density multiplied by the specific heat capacity at 25°C. The material is not therefore specifically suited to the particular conditions of use of the regenerator.

Rather than seeking materials having a maximum p.Cp(25°C) product, the inventors sought to check whether this parameter, although widely used, was actually the most suitable one. They therefore looked for alternative parameters and, to their particular surprise, discovered that the efficiency of the regenerator is also dependent on the characteristic ratio identified above. As will be seen in greater detail in the remainder of the description, the use of a material having a characteristic ratio higher than 0.3 makes it possible to achieve performance that maximizing the p.Cp(25°C) product alone is incapable of achieving.

A thermal power plant according to the invention may also have one or more of the following optional features:

- The characteristic ratio is higher than 0.35, preferably higher than 0.40, preferably higher than 0.45;

- The characteristic ratio is lower than 0.60;

- The charging temperature is lower than 1000°C, or even lower than 800°C and higher than 350°C, or even higher than 500°C;

- The discharging temperature is lower than the charging temperature, for example by more than 100°C, by more than 200°C or by more than 300°C, and by less than 1000°C, or even by less than 800°C;

- The discharging temperature is lower than the temperature at which the heat-transfer fluid leaves the regenerator at the end of the charging phase by less than 200°C, or even by less than 100°C, or even by less than 50°C; - The discharging temperature is higher than 20°C, or even higher than 50°C, preferably higher than 100°C and/or is lower than 700°C, or even lower than 600°C.

- The material has a melting point more than 100°C , more than 150°C, or even more than 200°C higher than the charging temperature;

- The material has a melting point higher than Tc + 100°C and/or lower than 1950°C, or even lower than 1900°C, or even lower than 1800°C;

- The material has a concentration of all elements leached from the said material, measured in accordance with standard EN 12457-2, that is less than or equal to 0.1 g/l, preferably less than or equal to 0.05 g/l;

- The material has a specific heat capacity at 25°C higher than 600 J.°C^"1.kg^"1, or even higher than 650 J.°C^"1.kg^"1, or even higher than 700 J.°C^"1.kg^"1;

- The material contains over 50 wt%, preferably over 60 wt%, preferably over 70 wt%, or even over 80 wt%, or even over 90 wt% of aluminium magnesium spinels, for example MgAI₂0₄, and/or of steatite, and/or of forsterite Mg₂Si0₄, and/or of ilmenite FeTi0₃, and/or of iron oxides. For preference, of the balance to 100 wt%, over 90 wt% or even 100 wt% of the balance material contains at least one oxide chosen from boron oxide, sodium oxide, copper oxides, iron oxides, silica, alumina and mixtures thereof. For preference, the oxide is chosen from silica, iron oxides and mixtures thereof;

- The material has a pyrometric cone equivalent temperature, measured in accordance with standard ISO 528 (1983), higher than Tc + 50°C, or even higher than Tc + 100°C, or even higher than Tc + 150°C and less than 1900°C, or even less than 1800°C, or even less than 1700°C, or even less than 1650°C. Advantageously, the density of the bed of energy-storage elements is thus kept substantially constant while the thermal power plant is in operation (no collapse).

In one embodiment, the energy-storage elements are in permanent or temporary contact with an acid liquid of pH lower than 6, or even lower than 5.5, or even lower than 5, or even lower than 4.5, or even lower than 4, this notably being an aqueous liquid.

For preference, a thermal power plant according to the invention comprises a consumer of heat energy, the said circulation device, during the discharging phase, causing the discharging heat-transfer fluid to circulate through the said regenerator, then from the said regenerator to the consumer of heat energy.

In one embodiment, the unit producing heat energy comprises, or even consists of, a compressor that is mechanically or electrically powered by an incineration plant or by an electricity power station, particularly a thermal power station, a solar power station, a wind energy power station, a hydroelectric power station, or a tidal energy power station. The unit producing heat energy and/or the consumer of heat energy may comprise a heat exchanger designed to perform direct or indirect heat exchange with the regenerator.

The invention also relates to a method of producing a thermal power plant according to the invention, in which the material chosen for the energy-storage elements, from a set of several materials, is the material that has the highest characteristic ratio A.

The invention also relates to a method for designing, producing and operating a thermal power plant, in which:

(a) there is designed and produced a thermal power plant that comprises:

a unit producing heat energy, for example a furnace, a solar tower or a compressor, and

a circulation device,

and (b) the said thermal power plant is operated by using the said circulation device such that:

- during a charging phase, it circulates a charging heat-transfer fluid from the unit producing heat energy to the regenerator, then through the said regenerator, the said charging heat-transfer fluid entering the said regenerator at a charging temperature Tc, and

- during a discharging phase, it circulates a discharging heat-transfer fluid through the said regenerator, the discharging heat-transfer fluid entering the said regenerator at a discharging temperature Td.

This method is notable in that, in step a), the material of the energy-storage elements has a melting point higher than Tc + 50°C and lower than 2000°C, the concentration of all the elements leached from the said material, measured in accordance with standard EN 12457-2, being less than or equal to 0.5 g/l, the material being chosen to have a characteristic ratio A higher than 0.3, with:

A = (Cp(Tc) - Cp(Td)) / Cp(Td)

where

- Cp(Tc) is the specific heat capacity of the said material at the charging temperature Tc, and - Cp(Td) is the specific heat capacity of the said material at the discharging temperature Td.

The thermal power plant used may also have one or more of the optional features described hereinabove or in the remainder of the description. Brief description of the figures

Other objects, aspects, specifics and advantages of the present invention will become more apparent in the light of the description and of the examples which follow and from studying the attached drawing in which:

- Figure 1 depicts curves of the change in temperature of the charging heat-transfer fluid along its path through a regenerator, with respect to the length of the regenerator.

These curves are considered to be substantially identical to the temperature of the storage elements along the said length of the regenerator. Curve C, is the curve obtained at the start of charging and curve C_f is the curve obtained at the end of charging. The length of the regenerator, in metres, is shown along the abscissa axis and the temperature of the charging heat-transfer fluid, in this instance air, is shown on the ordinate axis, in Kelvin;

- Figure 2 depicts curves of the change in temperature of the discharging heat-transfer fluid along its path through a regenerator, with respect to the length of the regenerator. These curves are considered to be substantially identical to the temperature of the storage elements along the said length of the regenerator. Curve D, is the curve obtained at the start of discharging and curve D_f is the curve obtained at the end of discharging. The length of the regenerator, in metres, is shown along the abscissa axis and the temperature of the discharging heat-transfer fluid, in this instance air, is shown on the ordinate axis, in Kelvin;

- Figures 3a and 3b, 4a and 4b, 5a and 5b schematically depict thermal power plants according to the invention;

- Figure 6 schematically depicts a regenerator;

- Figures 7a and 7b depict the change in temperature of the storage elements made of a material according to Example 1 and according to Example 2 respectively, arranged along the axis of the cylinder of the regenerator, in the steady state, as a function of the position ("axial position") along the said axis. The axial position, in metres, is shown on the abscissa axis and the temperature of the charging and discharging heat-transfer fluid, in this instance air, is shown on the ordinate axis, in Kelvin. For Figures 1 and 2, the calculations have been performed for a regenerator 30 m long and with a diameter of 5 m, the charging phase and the discharging phase lasting 10 800 seconds.

Figures 3a, 4a and 5a correspond to charging phases. Figures 3b, 4b and 5b correspond to discharging phases. The piping through which a fluid is passing is depicted in heavier line. The valves needed to alter the circulation in the various circuits have not been depicted.

In the various figures, identical references are used to denote components that are identical or analogous. Definitions

A "unit producing heat energy" is intended to mean not only units which are specifically designed to generate heat energy, such as a solar tower, but also units which, as a result of their operation, generate heat energy, for example a compressor.

The expression "thermal power plant" is also to be understood in the broadest sense, as indicating any plant containing a unit that produces heat energy.

The expression "consumer of heat energy" refers to an element capable of receiving heat energy. This may notably result in an increase in the temperature of the consumer (for example in the case of the heating of a building) and/or in conversion into mechanical energy (for example in the case of a gas turbine).

In this description, for the sake of clarity, "charging heat-transfer fluid" and "discharging heat-transfer fluid" are the names given to the heat-transfer fluid that flows through the regenerator during charging and during discharging, respectively. The charging heat- transfer fluid is said to be "cooled" when it leaves the regenerator. The discharging heat- transfer fluid is said to be "heated" when it leaves the regenerator.

A "bed" of energy-storage elements means a set of such elements at least partially stacked on one another.

A "ceramic material" means a material which is neither organic nor metallic.

The oxides contents relate to the overall contents for each of the corresponding chemical elements, expressed in the form of the most stable oxide, according to the standard convention used in industry.

Conventionally, the melting point is measured at atmospheric pressure, for example using differential scanning calorimetry (DSC). Unless indicated otherwise, all the percentages are percentages by weight.

"Containing a/an/one", "comprising a/an/one" or "including a/an/one" means "including at least a/an/one" unless indicated otherwise. In particular, a thermal power plant according to the invention may comprise several regenerators immediately in series or incorporated into compression stages, as described in F 2 947 015.

Detailed description

A thermal power plant according to the invention comprises a unit producing heat energy, a regenerator, a circulation device. It may also comprise a consumer of heat energy and/or a cavity. Unit producing heat energy

The unit producing heat energy may be designed to produce heat energy, for example may be a furnace or a solar tower.

In one embodiment, the unit producing heat energy is a compressor. Compressing a gaseous fluid, preferably an adiabatic fluid, causes energy to be stored in that fluid as a result of the increase in its pressure and temperature.

The energy resulting from the increase in pressure can be stored by keeping the fluid under pressure. This energy is then restored as the result of an expansion, for example through a turbine.

The energy resulting from the increase in temperature can be stored in a regenerator. This energy is then restored by exchange of heat with the regenerator.

The heat energy may be a by-product of production, which means to say that it is not sought-after as such.

For preference, the unit producing heat energy produces over 50 kW, or over 100 kW, or even over 300 kW, or even over 1 MW, or even over 5 MW of heat energy. The invention is in fact particularly intended for high-power industrial plants.

Consumer of heat energy

The consumer of heat energy may be a building or a collection of buildings, a reservoir, a basin, a turbine coupled to an alternator for generating electricity, an industrial plant that consumes steam, such as the paper pulping industry for example. Regenerator

The regenerator is formed in the conventional way of a bed of energy-storage elements which are made from a material chosen according to the invention. The bed may be an organized one, for example if the energy-storage elements are structured, or may be disorganized ("random"). For example, the bed may take the form of a mass of crushed bits (of no particular shape, such as a mass of rocks).

The shapes and dimensions of the energy-storage elements are nonlimiting. For preference, however, the smallest dimension of an energy-storage element is greater than 0.5 mm, or even greater than 1 mm, or even greater than 5 mm, or even greater than 1 cm and/or for preference smaller than 50 cm, preferably smaller than 25 cm, preferably smaller than 20 cm, preferably smaller than 15 cm. For preference, the longest dimension of a storage element is less than 10 metres, preferably less than 5 metres, preferably less than 1 metre.

The energy-storage elements may notably adopt the form of balls and/or granules and/or solid bricks and/or perforated bricks and/or cruciform elements and/or double cruciform elements and/or solid elements and/or perforated elements like those described in US 6 889 963 and/or described in US 6 699 562.

The height of the bed is preferably greater than 5 m, preferably greater than 15 m, preferably greater than 25 m, or even greater than 35 m, or even greater than 50 m.

The mass of the bed is preferably greater than 700 T, preferably greater than 2000 T, preferably greater than 4000 T, preferably greater than 5000 T, preferably greater than 7000 T.

For preference, the energy-storage elements are grouped together in an enclosure comprising first and second openings for introducing a heat-transfer fluid into and extracting same from the said enclosure, respectively.

The material of which the energy-storage elements are made preferably has an apparent porosity greater than 5% and/or less than 30%, preferably less than 25%, or even less than 20%, or even less than 15%, or even less than 10%, or even less than 6%.

For preference also, the energy-storage elements are sintered products. All the known methods of producing sintered products can be used. Sintering at a temperature of between 1000°C and 1500°C, preferably with a residence time at this temperature lasting longer than 0.5 hours and preferably less than 12 hours, may be very suitable.

According to the invention, the material of which the energy-storage elements are made is chosen according to its characteristic ratio, and therefore according to the conditions of operation of the thermal power plant. The specific heat capacity can be measured in accordance with standard ASTM E1269, for example using a Netzsch STA 409 CD differential scanning calorimetry (DSC) device.

Conventionally, the charging and discharging temperatures are more or less constant throughout the charging and discharging phases respectively.

If the charging and discharging temperatures vary throughout the charging and discharging phases, the charging and discharging temperatures may be calculated as the mean of the extreme temperatures during the said charging and discharging phases, namely Tc = (Tc_min + Tc_max)/2 and Td = (Td_min + Td_max)/2, Tc_mi„ and Td_min denoting the minimum temperatures of the charging and discharging heat-transfer fluid respectively, during the charging and discharging phases respectively, and Tc_max and Td_max denoting the maximum temperatures of the charging and discharging heat-transfer fluid respectively, during the charging and discharging phases respectively.

In one application, a number of materials may prove suitable. For preference, it is the material that has the highest characteristic ratio A that is then chosen.

However, this material has to have a melting point higher than Tc + 50°C in order not to liquefy during the regenerator charging phase. When Tc varies during the charging phase, this material preferably has a melting point higher than Tc_max + 50°C. The melting point is, however, lower than 2000°C. This is because materials that have melting points above 2000°C are poorly suited to the target applications, notably because of their production cost.

This material has also to be insoluble, namely, in a test performed in accordance with standard EN 12547-2 of December 2002, to lead to a concentration of all the leached elements that is less than or equal to 0.5 g/l, preferably less than or equal to 0.1 g/l, more preferably still, less than 0.05. Failing that, its life would be considerably shortened should it come into contact with water vapour, notably if it is used as charging or discharging heat-transfer fluid, or with water, notably resulting from the condensation of the water vapour contained in the charging or discharging heat-transfer fluid, particularly if the latter is air.

The material of the storage elements may be a natural material, in its native form. For example, the energy-storage elements may be pieces of rock.

For preference, the material of the energy-storage elements is a ceramic material.

For preference, oxides account for more than 90 wt%, preferably more than 95 wt%, preferably more than 99 wt%, or even more or less 100 wt% thereof. This material may in particular have one or more of the following optional features:

the material contains an alumina Al₂0₃ content of less than 40 wt%, preferably of less than 35 wt%, preferably of less than 30 wt%, preferably of less than 25 wt%, preferably of less than 20 wt%, preferably of less than 15 wt% or even of less than 10 wt%, or even of less than 5 wt%;

the said material has an Fe₂0_3, content preferably of more than 30%, preferably of more than 35%, preferably of more than 40%, or even of more than 45%, or even of more than 50% and/or preferably of less than 65%, or even of less than 60%; the said material has a CaO content preferably of more than 3%, or even of more than 5%, or even of more than 10%;

the said material has a Ti0₂ content preferably of more than 5%, or even of more than 10%, and/or preferably of less than 20%, preferably of less than 15%;

the said material has an Si0₂ content preferably of more than 5%, or even of more than 8%, and/or of less than 40%, preferably of less than 30%, preferably of less than 20%, preferably of less than 15%;

the said material has an Na₂0 content preferably of less than 5%;

- the said material has Fe₂0₃ + Al₂0₃ + CaO + Ti0₂ + Si0₂ + Na₂0 preferably of more than 85%, or even of more than 90%;

- the sum of the Fe₂0₃, Al₂0₃ and Si0₂, contents of the said material, Fe₂0₃ + Al₂0₃ + Si0₂, is preferably higher than 50 wt%, preferably higher than 60 wt%, or even higher than 70 wt%, or even higher than 75 wt%;

- the sum of the Fe₂0₃ and Al₂0₃, contents of the said material, Fe₂0₃ + Al₂0₃, is preferably higher than 40 wt%, preferably higher than 50 wt%, or even higher than 60 wt% or even higher than 70 wt%;

- the Fe₂0₃ content of the said material, based on the sum of the contents by weight of Fe₂0₃, Al₂0₃ and Si0₂, Fe₂0₃ + Al₂0₃ + Si0₂, is preferably higher than 45%, preferably higher than 50%, or even higher than 60%;

the material has the following chemical analysis, the percentages being percentages by weight:

- 25% < Fe₂0₃ < 70%, and

5% < Al₂0₃ < 30%, and

CaO < 20%, and

Ti0₂ < 25%, and

3% < Si0₂ < 50%, and

- Na₂O < 10%, and

Fe₂0₃ + Al₂0₃ + CaO + Ti0₂ + Si0₂ + Na₂0 > 80%, and other compounds: balance to 100%.

for preference, oxides constitute over 90%, preferably over 95%, of the "other compounds";

for preference, MgO, K₂0, P₂0₅ and mixtures thereof represent over 90%, over 95%, or even more or less 100% of the other compounds.

For preference, the said material incorporates residues from the production of alumina, notably red mud from the Bayer process, this process being notably described in "Les techniques de I'ingenieur [Engineering techniques]", in the article entitled "metallurgie extractive de I'aluminium [aluminium extractive metallurgy]", reference M2340 editions T.I., date of publication 10 January 1992 (particularly chapter 6 beginning on page M2340- 13 and Figure 7 on page M2340-15).

The said red mud may possibly be converted prior to use, for example during scrubbing and/or drying stages.

For preference, the energy-storage elements are grouped together into an enclosure having first and second openings for respectively introducing a heat-transfer fluid into and extracting same from the said enclosure.

In one embodiment, the opening of the regenerator via which charging heat-transfer fluid enters the regenerator during a charging phase is the opening via which heated discharging heat-transfer fluid leaves the regenerator during a discharging phase. By reciprocal arrangement, the opening of the regenerator via which discharging heat- transfer fluid to be heated enters the regenerator during a discharging phase is the opening via which cooled charging heat-transfer fluid leaves the regenerator during a charging phase.

For preference, the opening of the regenerator via which the heated discharging heat- transfer fluid bound for a furnace leaves the regenerator is in the upper part of the regenerator.

For preference, the opening of the regenerator via which the discharging heat-transfer fluid to be heated enters the regenerator is in the lower part of the regenerator.

Circulation device

The circulation device in the conventional way comprises a collection of pipes, valves and pumps/blowers/extractors all controlled in such a way as to allow the regenerator to be placed selectively in communication: - with the unit producing heat energy so that it can receive a charging heat-transfer fluid leaving the said unit, during the charging phases, and

- with the consumer of heat energy so that the heated discharging heat-transfer fluid leaving the regenerator can heat up the said consumer or, more generally, transfer heat energy to the said consumer, during the discharging phases,

and so as to be able to force the charging heat-transfer fluid and/or the discharging heat- transfer fluid to circulate through the regenerator.

For preference, a thermal power plant comprises a computerized system that is programmed to control the circulation device, in particular to ensure the said selective communication and the circulation through the regenerator.

Heat-transfer fluids

The charging and discharging heat-transfer fluids may or may not be of the same kind.

For preference, the charging heat-transfer fluid and the discharging heat-transfer fluid have the same composition. The heat-transfer fluid used for charging and/or discharging the regenerator may be a gas, for example air, steam, or a heat-transfer gas, or may be a liquid, for example water or a thermal oil.

For preference, when the heat-transfer fluid is a gas, the carbon dioxide CO₂ and/or carbon monoxide CO content, by volume, in the heat-transfer fluid is less than 50%, or even less than 10%, or even less than 1 %, or better still, substantially zero. Cavity

For preference, particularly when the charging and discharging heat-transfer fluids are of the same nature and when the heat-transfer fluid has experienced an increase in pressure, such as air compressed for example to 50 bar, or even 100 bar, or even 150 bar, the thermal power plant may comprise an enclosure known as a "cavity" for the temporary storage of the cooled charging heat-transfer fluid leaving the regenerator. The volume of the cavity is typically greater than 20 000 m³, or even greater than 100 000 m³.

The cavity is preferably not very permeable, or even completely fluidtight to the heat- transfer fluid.

For preference, the thermal power plant is configured to be able to operate according to at least some, and preferably all, of the rules described below. Operation

During charging, the charging heat-transfer fluid enters the regenerator at a temperature Tc, preferably substantially constant, generally via the upper part of the regenerator. Conventionally, in the steady state, the difference between the temperature of the heat- transfer fluid Tc and the temperature of the energy-storage elements with which it then comes into contact (T^ is 15% to 20% of Tc (namely around 90°C to 120°C) and the heat- transfer fluid rapidly cools down at the latter temperature.

For preference, the temperature Tc at which the charging heat-transfer fluid enters the regenerator during its charging is below 1000°C, or even below 800°C and/or preferably above 350°C, or even above 500°C.

The charging heat-transfer fluid then continues its path through the regenerator, heating up the energy-storage elements with which it is in contact. Its temperature therefore gradually drops, as represented in curve C, of Figure 1 , down to the temperature Tc,'. For preference, the temperature Tc,' at which the charging heat-transfer fluid leaves the regenerator, at the start of charging, is close to the discharging temperature from the previous cycle.

The curve representing the change in temperature of the charging heat-transfer fluid along its path through the regenerator is notably dependent on the material of the energy- storage elements and on the geometry of the regenerator. It changes over time, during the charging phase, as a result of the heating-up of the energy-storage elements (this is the shift from curve C, to curve C_f).

In the steady state, the curves C, and C_f are substantially identical from one charging phase to the next.

When the charging heat-transfer fluid is a gas, its cooling may lead to condensation on the surface of the energy-storage elements, particularly in sensible-heat regenerators.

At high temperature, such as the temperatures envisaged hereinabove in particular, the condensates may be highly corrosive. As shown by the examples below, the energy- storage elements of a regenerator according to the invention are advantageously highly resistant to corrosion from these condensates. During discharging, the discharging heat-transfer fluid enters the regenerator at a temperature Td preferably substantially constant, generally via the bottom part of the regenerator. Conventionally, in the steady state, the temperature Td is close to the temperature of the energy-storage elements with which it then comes into contact (T₂) and the heat-transfer fluid rapidly heats up at this latter temperature.

For preference, the temperature of the energy-storage elements with which the heat- transfer fluid comes into contact (T₂) is higher than 50°C, or even higher than 100°C, higher than 200°C or higher than 300°C and/or lower than 600°C.

The heat-transfer fluid then continues its path through the regenerator, cooling the energy- storage elements with which it is in contact. Its temperature therefore increases progressively, as depicted in curve D, of Figure 2, up to the temperature Td,'.

The curve representing the change in temperature of the discharging heat-transfer fluid along its path through the regenerator is also notably dependent on the material of the energy-storage elements and on the geometry of the regenerator. It changes over time because of the cooling of the energy-storage elements (represented by the shift from curve D, to curve D_f).

In the steady state, the curves D, and D_f are substantially identical from one discharging phase to the next.

The regenerator therefore experiences a succession of "cycles", each cycle comprising a charging phase, possibly a standby phase, then a discharging phase.

The cycle may be regular or irregular. For preference, it is regular, the duration of the first phases being the same as that of the second phases.

The duration of a regular cycle is generally longer than 0.5 hours, or even longer than two hours and/or shorter than 48 hours, or even shorter than 24 hours.

For preference, the regenerator is a sensible-heat regenerator, which means that the material of the energy-storage elements and the charging and discharging temperatures are determined in such a way that the energy-storage elements remain solid throughout the operation of the thermal power plant. Indeed it is in a sensible-heat regenerator that the probabilities of the heat-transfer fluid condensing are the highest. Specific embodiments

Figures 3a and 3b, 4a and 4b, 5a and 5b depict various advantageous embodiments. In all of these embodiments, a thermal power plant 10 according to the invention comprises a unit producing heat energy 12, a regenerator 14, a consumer of heat energy 16 and a circulation device 18. It may also include a natural or artificial cavity 20.

The circulation device 18 comprises a charging circuit 22 and a discharging circuit 24 through which a charging heat-transfer fluid and a discharging heat-transfer fluid respectively pass. This charging circuit 22 and this discharging circuit 24 allow the unit producing heat energy 12 to be placed in a heat-exchange relationship with the regenerator 14 during the charging phase and allow the regenerator 14 to be placed in a heat-exchange relationship with the consumer of heat energy 16 during the discharging phase, respectively.

Figures 3a and 3b depict a first specific embodiment in which the consumer of heat energy 16 comprises a heat exchanger 26 designed to perform an exchange of heat between discharging heat-transfer fluid from the regenerator 14 (Figure 3b) and a secondary heat-transfer fluid circulating in a secondary circuit 28. The secondary circuit 28 is configured to allow the heat exchanger 26 to be placed in a heat-exchange relationship with, for example, a building 30.

The thermal power plant 10 also comprises a direct heating circuit 32 allowing the unit producing heat energy 12, for example a solar tower, to be placed in a direct heat-exchange relationship with the consumer of heat energy 16 during the charging phase (Figure 3a).

In this embodiment, the regenerator 14 is preferably near to the unit producing heat energy, for example less than 500 metres, or even less than 250 metres away from this unit.

Figures 4a and 4b depict a second particular embodiment in which the unit producing heat energy 12 comprises a compressor 34 driven by the energy, for example mechanical or electrical energy, produced by a set 36.

The charging heat-transfer fluid, conventionally air, is therefore compressed and heats up as it passes through the compressor 34 before arriving, via the charging circuit 22, in the regenerator 14. The regenerator need not be in the close vicinity of the plant that generates the electricity needed to compress the air or of the compressor 34.

On leaving the regenerator, the compressed cooled charging heat-transfer fluid is stored in the cavity 20.

During discharging, the compressed discharging heat-transfer fluid (which means the charging heat-transfer fluid that was stored in the cavity) leaves the cavity 20, is heated up as it passes through the regenerator then passes through a gas turbine 38. The gas turbine 38 may drive an alternator (not depicted) with a view to generating electricity, for example sent to the domestic grid.

The heating allows the discharging heat-transfer fluid to accumulate heat energy. This energy, which is restored at the time of expansion, limits condensation and improves the efficiency of the gas turbine 38.

The gas turbine 38 therefore acts simultaneously as a consumer of heat energy (by reducing the temperature) and as a consumer of mechanical energy (by reducing the pressure).

The embodiment of Figures 4a and 4b is particularly well suited to plants that are not designed to generate heat energy, such as a wind farm or an electricity power station of the hydroelectric or tidal power type.

Such a plant is conventionally known as a "plant that stores energy by adiabatic compression". FR 2 947 015 describes a plant of this type.

Figures 5a and 5b depict an alternative form of the second particular embodiment. The thermal power plant 10 comprises, in addition to the elements of the second embodiment, a second regenerator 14' and,

- in a second charging circuit 22' for the second regenerator 14', upstream of the second regenerator 14' (when considering the direction in which the charging heat-transfer flu id flows), a second compressor 34', and

- in a second discharging circuit 24', downstream of the second regenerator 14' (considering the direction in which the discharging heat-transfer fluid flows), a second gas turbine 38'.

The second regenerator 14', the second charging circuit 22', the second discharging circuit 24', the second compressor 34' and the second gas turbine 38' operate like the regenerator 14, the charging circuit 22, the charging circuit 24, the compressor 34 and the gas turbine 38. With the regenerator 14 acting as a unit producing heat energy, they constitute a thermal power plant according to the invention.

For preference, the compressor 34 is a medium-pressure compressor and the compressor 34' is a high-pressure compressor.

Several thermal power plants according to the invention may thus be arranged in series.

Figure 6 depicts one example of a regenerator 14. This regenerator comprises a bed of energy-storage elements 40, an upper opening 42 and a lower opening 44 via which openings the charging and discharging heat-transfer fluids respectively enter the regenerator. The charging and discharging heat-transfer fluids leave the regenerator 14 via the lower opening 42 and the upper opening 44, respectively.

Examples

The following examples are provided by way of nonlimiting illustration.

The shape of the energy-storage elements is similar for examples 1 and 2.

The energy-storage elements according to example 2 were produced as follows:

In step a), the starting charge consisted entirely of red mud, with the following chemical analysis with respect to the dry matter of the said red mud: Fe₂0₃ = 55%, Al₂0₃ = 16%, CaO = 5%, Ti0₂ = 11 %, Si0₂ = 8%, Na₂0 = 4%, other = 1 %, and in which over 60 wt% of the particles have a particle size smaller than 10 μπι. The said starting charge contains no additive.

The shaping of the said starting charge, in such a way as to obtain preforms 1 1 mm long with a diameter of 16 mm, is performed by single-axis compression at a pressure of 125 MPa.

The preforms are then dried for 12 hours at 120°C.

The preforms are then sintered in air, in the following cycle:

- increased to 1200°C at a rate of 100°C/h,

- constant level of 3 hours at 1200°C,

- decrease at a rate of 100°C/h.

The apparent density and the apparent porosity were measured in accordance with standard ISO5017, after sintering in the case of example 2.

The chemical analyses were performed by X-ray fluorescence. The concentration of all the elements leached from the said material, into water, was measured in accordance with standard EN 12457-2, at a temperature of 22°C.

The following assumptions were used for calculating the heat energy restored by the regenerator and the temperature of the air at the end of discharge at the outlet from the regenerator:

- one-dimensional model:

^■ flow and heat transfer by forced convection in the porous medium, the void fraction being taken to be equal to 40%, and the effect of gravity being considered to be negligible,

■ fluid temperature and velocity in a section of the regenerator both constant,

^■ heat losses and the influence of the wall on the flow both considered to be negligible,

^■ uniform distribution of fluid flow and temperature over the upper surface (during charging) and over the lower surface (during discharging) of the regenerator,

- regenerator of cylindrical shape, constant cross section, diameter of 5 m and length L of 20 m,

- heat-transfer fluid: dry air,

- volume of storage elements constant,

- no radial heat loss,

- charging temperature 800°C, namely 1073 K,

- discharging temperature 400°C, namely 673 K,

- internal pressure equal to 20 bar,

- fluid flow rate constant and equal to 35 kg/s during charging and during discharging,

- charging time: 4 hours,

- discharging time: 4 hours. The following formula gives the amount of heat energy restored by the regenerator:

L Tf

J J p.S.Cp(T).dT.dx

0 Ti

In this formula:

Ti: temperature at the start of charging in the section of width dx, located at the axial position x, in Kelvin, Tf: temperature at the end of discharging in the section of width dx, located at the axial position x, in Kelvin,

p: apparent density of the bed, in kg/m³,

S: circular section of the regenerator, in m²,

L: length of the regenerator in m,

Cp(T): specific heat capacity of the storage material at the temperature T.

The analyses performed on the storage elements and the results of the calculations made are collated in table 1 below:

Table 1

As shown by the results indicated in table 1 , the regenerator containing storage elements of example 2 according to the invention has a temperature at the end of discharging of 71 1 °C, which is higher than the temperature at the end of discharging of the regenerator containing storage elements of example 1 outside of the invention (657°C). The performance of a turbine supplied with the air leaving the regenerator containing storage elements of example 2 according to the invention will be improved as a result.

Of course, the present invention is not restricted to the embodiments described and depicted, which have been given by way of examples. In particular, combinations of the various embodiments described or depicted also fall within the scope of the invention.

Neither is the invention restricted by the shape or dimensions of the regenerator.

Claims

Thermal power plant comprising:

a unit producing heat energy,

a regenerator comprising a bed of energy-storage elements,

a consumer of heat energy, and

a circulation device which:

during a charging phase, circulates a charging heat-transfer fluid from the unit producing heat energy to the regenerator, then through the said regenerator, the said charging heat-transfer fluid entering the said regenerator at a charging temperature Tc of less than 1000°C, and during a discharging phase, circulates a discharging heat-transfer fluid through the said regenerator, the discharging heat-transfer fluid entering the said regenerator at a discharging temperature Td,

the energy-storage elements being made of a material which has a melting point higher than Tc + 50°C and lower than 2000°C,

the concentration of all the elements leached from the said material, as measured in accordance with standard EN 12457-2 of December 2002, being less than or equal to 0.5 g/l, and

the thermal power plant being characterized in that the said material of the energy- storage elements has a characteristic ratio A higher than 0.3, with:

A = (Cp(Tc) - Cp(Td)) / Cp(Td)

where

Cp(Tc) is the specific heat capacity of the said material at the charging temperature, and

Cp(Td) is the specific heat capacity of the said material at the discharging temperature.

Plant according to the preceding claim, in which the characteristic ratio A is higher than 0.45.

Plant according to either of the preceding claims, in which the unit producing heat energy produces more than 50 kW of heat energy.

Plant according to any one of the preceding claims, in which the charging temperature is higher than 350°C.

5. Plant according to the preceding claim, in which the charging temperature is higher than 500°C.

6. Plant according to any one of the preceding claims, in which the discharging temperature is lower than the charging temperature.

7. Plant according to any one of the preceding claims, in which over 90% of the mass of the said material consists of oxides.

8. Plant according to any one of the preceding claims, in which the material contains over 50 wt% of aluminium-magnesium spinels and/or of steatite, of forsterite Mg₂Si0₄, and/or of ilmenite FeTi0₃, and/or of iron oxides.

9. Plant according to the preceding claim, in which, of the balance to 100 wt%, over 90 wt% of the balance material consists of at least one oxide chosen from boron oxide, sodium oxide, copper oxides, iron oxides, silica, alumina and mixtures thereof.

10. Plant according to any one of the preceding claims, in which the said concentration of all of the elements leached from the said material, measured in accordance with standard EN 12457-2 of December 2002, is less than or equal to 0.1 g/l.

11. Plant according to any one of the preceding claims, in which the said regenerator is a sensible-heat regenerator.

12. Plant according to any one of the preceding claims, comprising a consumer of heat energy, the said circulation device, during the discharging phase, causing the discharging heat-transfer fluid to circulate through the said regenerator, then from the said regenerator to the consumer of heat energy.

13. Plant according to any one of the preceding claims, in which the unit producing heat energy consists of a compressor mechanically or electrically powered by an incineration plant or by an electricity power station.

14. Plant according to any one of the preceding claims, comprising a heat exchanger designed to perform direct or indirect heat exchange with the regenerator.

15. Plant according to any one of the preceding claims, the discharging temperature being lower than the temperature at which the heat-transfer fluid leaves the regenerator at the end of the charging phase by less than 200°C.

16. Plant according to any one of the preceding claims, the discharging temperature being higher than 50°C.

17. Method for producing a plant according to any one of the preceding claims, in which the material chosen for the energy-storage elements, from a set of several materials, is the material that has the highest characteristic ratio A.

18. Method for designing, producing and operating a thermal power plant, in which:

(a) a thermal power plant according to any one of the preceding claims is designed and produced,

and

(b) the said thermal power plant is operated by using the said circulation device such that:

during a charging phase, it circulates a charging heat-transfer fluid from the unit producing heat energy to the regenerator, then through the said regenerator, the said charging heat-transfer fluid entering the said regenerator at a charging temperature Tc below 1000°C, and

during a discharging phase, it circulates a discharging heat-transfer fluid through the said regenerator, the discharging heat-transfer fluid entering the said regenerator at a discharging temperature Td.