Thermal energy storage plant

Abstract

Provided is a thermal energy storage plant including a charging circuit where a first working fluid is circulated, the charging circuit includes a first fluid transporting machine for generating a flow of the first working fluid in charging circuit, a heating device electrically powered for transferring heat to the first working fluid, a heat accumulator for storing the thermal energy of the first working fluid, the heat accumulator including a hot end for receiving the first working fluid at a first temperature and a cold end for letting the first working fluid exit the heat accumulator at a second temperature lower than the first temperature, the heat accumulator includes a plurality of heat storage units connected in series between the hot end and the cold end, which may be separated by valves.

Claims

1. A thermal energy storage plant comprising: a charging circuit where a first working fluid is circulated, the charging circuit including: a first fluid transporting machine for generating a flow of the first working fluid in the charging circuit, a heating device for transferring heat to the first working fluid, and a heat accumulator for storing a thermal energy of the first working fluid, the heat accumulator including a hot end for receiving the first working fluid at a first temperature and a cold end for letting the first working fluid exit the heat accumulator at a second temperature lower than the first temperature, wherein the heat accumulator is oriented in such a way that the first working fluid is circulated through heat storage elements within the heat storage units from the hot end to the cold end along a horizontal direction; wherein the heat accumulator comprises: a plurality of heat storage units connected in series between the hot end and the cold end; and at least one valve interposed between two heat storage units of the plurality of heat storage units, wherein a closing of the at least one valve disconnects the plurality of heat storage units from each other during idling operations between charging and discharging phases to prevent a mass flow between the plurality of heat storage units initiated by natural convection.

2. The thermal energy storage plant according to claim 1, further comprising a discharging circuit, the discharging circuit including: the heat accumulator, a second fluid transporting machine for generating a flow of a second working fluid in the discharging circuit, the flow being oriented from the cold end to the hot end of the heat accumulator, a heat exchanger included in a thermal cycle for transferring a thermal energy from the second working fluid to a working fluid of the thermal cycle.

3. The thermal energy storage plant according to claim 1, wherein at least one of the heat storage units of the plurality of heat storage units comprises a housing for a plurality of heat storing elements having high thermal capacity.

4. The thermal energy storage plant according to claim 2, wherein the thermal cycle is a water-steam cycle including a thermal machine and the heat exchanger is a steam generator for transferring the thermal energy from the second working fluid to a mass of water to generate steam to be fed to the thermal machine.

5. The thermal energy storage plant according to claim 3, wherein the first working fluid and the second working fluid are a same fluid.

6. The thermal energy storage plant according to claim 1, wherein the heating device is powered from a renewable energy source.

7. A method for operating the thermal energy storage plant according to claim 1, the method comprising the steps of: heating the first working fluid in the heating device; generating a flow of the first working fluid in the charging circuit with the first fluid transporting machine, for charging the plurality of heat storage units in series from the hot end to the cold end; and stopping the heating and the flow of the first working fluid after at least one heat storage unit has been charged.

8. The method according to claim 7, further comprising the step of isolating the charged heat storage unit from the other heat storage units by means of at least one valve.

9. The method according to claim 8, wherein all heat storage units are isolated from each other, by means of at least one valve provided between the plurality of heat storage units.

10. A method for operating the thermal energy storage plant according to claim 8, the method comprising the steps of: opening the at least one valve; generating a flow of the second working fluid in the discharging circuit, from the cold end to the hot end, for transferring heat from the plurality of heat storage units to the second working fluid; and stopping the heating and the flow of the second working fluid after an inlet of a heat storage unit which is closest to the hot end has reached a temperature lower than the first temperature.

Description

BRIEF DESCRIPTION

(1) Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

(2) FIG. 1 shows a schematic diagram of a thermal energy storage plant, in accordance with embodiments of the present invention;

(3) FIG. 2 shows a partial schematic view of the diagram of FIG. 1, showing with more details some components of the thermal energy storage plant, in accordance with embodiments of the present invention;

(4) FIG. 3 shows a version of FIG. 2, in a first operative condition, in accordance with embodiments of the present invention;

(5) FIG. 4 shows a version of FIG. 2, in a second operative condition, in accordance with embodiments of the present invention;

(6) FIG. 5 shows a version of FIG. 2, in a third operative condition, in accordance with embodiments of the present invention; and

(7) FIG. 6 shows a schematic section view of a component of the thermal energy storage plant of FIG. 1, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

(8) The illustration in the drawing is schematically. It is noted that in different figures, similar or identical elements or features are provided with the same reference signs. In order to avoid unnecessary repetitions elements or features which have already been elucidated with respect to a previously described embodiment are not elucidated again at a later position of the description.

(9) FIGS. 1 and 2 schematically show a thermal energy storage plant 10 comprising a charging circuit 100 and a discharging circuit 200, where, respectively, a first working fluid and a second working fluid are circulated.

(10) The first and the second working fluid may be in particular the same, for example being both constituted by hot air.

(11) According to other possible embodiments, the first and second working fluids may be different gaseous or liquid or steam media.

(12) The charging circuit 100 includes, in a closed loop: a first fluid transporting machine 110 for generating a flow of the first working fluid in the charging circuit 100, a heating device 120 electrically powered for transferring heat to the first working fluid, a heat accumulator 130 for storing the thermal energy of the first working fluid, the heat accumulator 130 including a hot end 131 for receiving the first working fluid at a first high temperature T1 and a cold end 132 for letting the first working fluid exit the heat accumulator 130 at a second low temperature T2 lower than the first high temperature T1.
In the attached figures, the first fluid transporting machine 110 is immediately downstream the cold end 132 of the heat accumulator 130. According to another possible embodiment of the present invention (not shown), the first fluid transporting machine 110 is immediately upstream the hot end 131 of the heat accumulator 130.

(13) When the first working fluid is air, the first fluid transporting machine 110 may be constituted by a fan or blower.

(14) The heating device 120 may be a resistant or inductive heater or a heat pump fed by the electrical power generated by a renewable energy source, for example the wind speed generating power by means of a wind turbine or solar radiation generating power by means of photovoltaic cells, or from the electricity grid.

(15) The heating device 120 permits the first hot temperature T1 and the second low temperature T2 to be established between the hot end 131 and cold end 132 of the heat accumulator 130. According to possible embodiments of the present invention, typical values are T1=600 C. and T2=120 C. In other possible embodiments, values of T2 may be close to ambient temperature or 300 C.

(16) In general, other values are possible, the value of hot temperature T1 depends on the operating temperature of a thermal cycle 220 comprised in the discharging circuit 200 for transforming the thermal energy from the second working fluid into mechanical power, as detailed in the following. The low temperature T2 is typically kept higher than ambient temperature in order to reduce the heat load required in the heating device 120 for raising the first working fluid temperature up to the high temperature T1.

(17) According to possible embodiments of the present invention, the heat accumulator 130 is oriented in such a way that the first working fluid is circulated from the hot end 131 to the cold end 132 along a horizontal direction. In such type of installations, the temperature front which forms between the hot end 131 and the cold 132 of the heat accumulator 130 travels horizontally, from the hot end 131 to the cold end 132. The temperature front so oriented typically tends to tilt, in particular during idle periods, for the effect of natural convection.

(18) The discharging circuit 200 includes, in closed loop: the heat accumulator 130, a heat exchanger 227 included in a thermal cycle 220 for transforming the thermal energy from the second working fluid into mechanical power, a second fluid transporting machine 210 for generating a flow of the second working fluid in the discharging circuit 200, oriented from the cold end 132 to the hot end 131 of the heat accumulator 130. The working fluid of the discharging circuit 200 flows therefore in opposite direction with respect to the flow of the working fluid in the charging circuit 100.

(19) According to a possible embodiment of the present invention, the thermal cycle 220 is a cycle including a thermal machine 225 and wherein the heat exchanger 227 is a steam generator for transferring thermal energy from the second working fluid to a mass of water in order to generate steam to be fed to the thermal machine 225. The thermal machine 225 may be a steam turbine having an output shaft connected to an electrical generator 226 to produce electricity to be fed in a electricity grid. According to another possible embodiment, the thermal cycle 220 may include, instead of the steam generator 227, a boiler or an evaporator or other type of heat exchanger for transferring heat from the second working fluid to the thermal cycle 220.

(20) The thermal cycle 220 further includes a condenser 228, connected to the outlet of the steam turbine 225 and a pump 229, between the condenser 228 and steam generator 227. Other types of thermal cycles may be used instead of the described thermal cycle 220, provided that, in general, they are able to transform the thermal energy from the discharging circuit 200 into mechanical power for powering the electrical generator 226.

(21) With reference to FIGS. 2 to 5, the heat accumulator 130 comprises a plurality of heat storage units (three heat storage units 135a, 135b, 135c in the non-limiting embodiment of the attached figures) connected in series between the hot end 131 and the cold end 132. With reference to FIG. 6, each heat storage unit 135a, 135b, 135c comprises respectively a housing 150a, 150b, 150c extending from an inlet 138a, 138b, 138c to an outlet 139a, 139b, 139c. Each housing 150a, 150b, 150c contains a plurality of heat storing elements 160 having high thermal capacity, for example solid or bulk materials like stones, bricks, ceramics and other solid materials, which have the ability to be heated up and to keep their temperature over a long period of time in order to store the thermal energy which has been transferred to them. Each heat storage unit 135a, 135b, 135c is configured for working with the same working fluid to be used for the charging circuit 100 and for the discharging circuit. When the charging circuit 100 is operated, the working fluid, for example air, flows from the inlet 138a, 138b, 138c to the outlet 139a, 139b, 139c, transferring heat to storing elements 160. When the discharging circuit 200 is operated, the same working fluid flows from the outlet 139a, 139b, 139c to the inlet 138a, 138b, 138c, receiving heat from the storing elements 160.

(22) According to other possible embodiments of the present invention, other types of heat storage unit may be used, in particular being configured for the use with two working fluids, one for the charging circuit 100, the other for the discharging circuit 200. This may be achieved, for example, providing the each heat storage unit 135a, 135b, 135c with a first inlet and a first outlet for the first working fluid and with a second inlet and a second outlet for the second working fluid.

(23) The heat accumulator 130 further includes one or more valves (two valves 137a, 137b, in the non-limiting embodiment of the attached figures) interposed between respective couples of consecutive heat storage units 135a, 135b, 135c. In the embodiment of the attached figures, a first valve 137a is placed on a pipe connecting the outlet 139a, 139b, 139c of the first (i.e. closest to the hot end 131) heat storage unit 135a with the inlet 138a, 138b, 138c of the intermediate heat storage unit 135b while a second valve 137b is placed on a pipe connecting the outlet 139a, 139b, 139c of the intermediate heat storage unit 135b with the inlet 138a, 138b, 138c of the last (i.e. closest to the cold end 132) heat storage unit 135c.

(24) According to the present invention, a method for operating the thermal energy storage plant 10, during charging of the heat accumulator 130, comprises the steps of: heating the first working fluid by means of the heating device 120 of the charging cycle 100, using the first fluid transporting machine 110 for generating a flow of the first working fluid in the charging circuit 100, for charging the heat storage units 135a, 135b, 135c of the heat accumulator 130, in series from the hot end 131 to the cold end 132. During charging, a temperature front moves from the first heat storage unit 135a to the last heat storage unit 135c (an intermediate operative condition with the temperature front in the intermediate heat storage unit 135b is shown in FIG. 3), stopping the heating and the flow of the first working fluid after the last heat storage unit 135c has been charged, i.e. when the temperature front reaches the outlet 139c of the last heat storage unit 135c (FIG. 4) and the temperature T2 at the cold end 132 rises, isolating the heat storage units 135a, 135b 135c from each other by means of the valves 137a, 137b.

(25) After the charging of the heat accumulator 130 has been completed, in the first and intermediate heat storage units 135a, 135b the temperature profile is constant from the respective inlet 138a, 138b to the respective outlet 139a, 139b. This will prevent the occurrence of natural convection in the first and intermediate heat storage units 135a, 135b. This condition can be maintained easily by closing the valves 137a, 137b between the heat storage units 135a, 135b, 135c, in particular the second valve 137b between the intermediate and last heat storage units 135b, 135c. In such a way, natural convection which may occur in the last heat storage unit 135c will not influence the other heat storage units 135a, 135b.

(26) The same result can be obtained, according to other embodiments of the present invention, using a different number of heat storage units, for example two or more than three heat storage units. The same result cannot be obtained with only one heat storage unit, because in this case it will not be possible to isolate the portion of the heat accumulator 130 having a constant temperature profile from the portion of the heat accumulator 130 containing the temperature front.

(27) According to a possible alternative embodiment, only the second valve 137b is closed for isolating only the last heat storage unit 135c, i.e. the heat storage unit which includes the temperature front after the charging has been completed.

(28) With reference to the embodiment of the attached figures, this leaves the first and intermediate heat storage units 135a, 135b, which have a constant temperature profile, in communication with one another, but isolated from the last heat storage unit 135c.

(29) According to possible alternative embodiments, intermediate charging states may be possible, e.g. when wind conditions do not allow for full charging of the storage. In that case, it is not the last heat storage unit 135c, which contains the temperature gradient but, for example, the intermediate heat storage unit 135b.

(30) According to embodiments of the present invention, a method for operating the thermal energy storage plant 10, during discharging of the heat accumulator 130, comprises the steps of: opening the valves 137a, 137b, generating a flow of the second working fluid in the discharging circuit 200, from the cold end 132 to the hot end 131, for transferring heat from the storing elements 160 of the heat storage units 135a, 135b, 135c to the second working fluid, stopping the second fluid transporting machine 210 and the flow of the second working fluid after an inlet 138a of the first heat storage unit 135a has reached a temperature lower than the first temperature T1 (FIG. 5).

(31) In the heat exchanger 227, the heat received from the storing elements 160 is then transferred from the second working fluid to the thermal cycle 220.

(32) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

(33) For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements.