Energy storage via thermal reservoirs and air turbines

Abstract

The present invention relates to a process of storing energy through the conversion of thermal energy and subsequent power generation by means of a gas turbine set with compressor, expander and power generator, with at least one and with a second low-temperature reservoir, and a high-temperature reservoir with bulk material as the heat storage medium, the electric energy is stored in the form of high-temperature heat above the turbine outlet temperature in a thermal reservoir, that during the power generation phase a compressed gas from the compressor is heated in a low-temperature reservoir to a temperature near the turbine outlet temperature and subsequently heated in a high-temperature reservoir with stored heat from electric power to a temperature level of at least the turbine inlet temperature, and that the ratio between the bed height in flow direction and the mean particle diameter of the bulk material in the high-temperature reservoir is at least 10.

Claims

1. A process to store energy through conversion into thermal energy and subsequent power generation by means of: a gas turbine set with compressor, gas turbine and power generator, with at least a first and a second low-temperature reservoirs, and a high-temperature reservoir with bulk material as the heat storage medium; wherein the process comprising: electric energy is stored in the form of high-temperature heat, at a temperature higher than a gas turbine outlet temperature in a high-temperature reservoir, that during a power generation phase a compressed gas from the compressor is heated in the at least the first and the second low-temperature reservoirs to a temperature near the gas turbine outlet temperature and then heated in the high-temperature reservoir with stored heat from the gas turbine outlet temperature which is temperature of the gas at an inlet of the high temperature reservoir to a temperature level of at least a gas turbine inlet temperature of the gas and that a ratio between a bed height in flow direction and a mean particle diameter of the bulk material in the high-temperature reservoir is at least 10, and at end of a discharge phase, before the gas turbine comes to a standstill, a first pair of valves or second pair of valves, are closed in such a manner that the high-temperature reservoir, the at least the first and the second low-temperature reservoirs and the gas turbine are remaining near operating pressure.

2. The process according to claim 1, further comprising, discharge time t from the high-temperature reservoir corresponds to the following relationship:
0.5.Math.(Ms/mG).Math.(Cs/Cp)<t<0.99.Math.(Ms/mG).Math.(Cs/Cp).Math. where Ms is mass of the bulk material, m.sub.G is gas flow, c.sub.s is specific heat capacity of bulk material particles, C.sub.p is specific heat capacity of the gas, =(TPHE-TOT)/(TMAX-TOT) is relative temperature difference, where, TPHE is a mean temperature of the gas at an outlet of the high-temperature reservoir, TMAX is a maximum temperature of the stored high-temperature heat, and TOT is the turbine outlet temperature.

3. The process according to claim 1, further comprising, cooling in the high-temperature reservoir during a power generation phase is limited only to the gas turbine outlet temperature.

4. The process according to claim 1, wherein the compressed gas is fed to a heat exchanger to utilize waste heat that has developed as useful heat.

5. The process according to claim 1, wherein the compressed gas is cooled through injection of water.

6. The process according to claim 1, wherein the high-temperature reservoir is heated by the turbine outlet temperature to a temperature above the gas turbine inlet temperature.

7. The process according to claim 1, further comprising, conversion of electric energy to high-temperature heat for the high temperature reservoir takes place via current resistance or induction.

8. The process according to claim 1, further comprising, electric heating of the high-temperature reservoir takes place in at least two segments distributed throughout the overall height.

9. The process according to claim 1, further comprising, by means of a bypass line and a bypass valve the turbine inlet temperature and the gas turbine performance can be selectively controlled.

10. The process according to claim 1, wherein during the power generation phase, the compressed gas that serves as working medium is air or another oxygen containing gas.

11. The process according to claim 1, further comprising, adding a volume of natural gas or other gaseous or liquid fuel into a line between the high-temperature reservoir and the gas turbine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Below, the advantages of the inventions are explained and described as embodiments with reference to the figures, where:

(2) FIG. 1 shows a schematic block diagram with all main components of the system and their connections;

(3) FIG. 2a and FIG. 2b show the same block diagram as in FIG. 1, but with the flow paths of the gas shown during the power generation phases;

(4) FIG. 3a, 3b show a schematic block diagram of the process indicating the volume in which the operating medium is stored during the turbine standstill;

(5) FIG. 4 shows an advantageous temperature distribution during a discharge phase in a bulk material with a bed height/particle diameter ratio of 1000;

(6) FIG. 5 shows an advantageous embodiment of electrical heating in three different segments of the bulk material;

(7) FIG. 6 shows the heating elements in high-temperature reservoir 5 in the form of stacked and connected spirals;

(8) FIG. 7 shows the bypass line with bypass valve 9; and

(9) FIG. 8 shows the supply of natural gas NG or other gaseous or liquid fuels 10.

DETAILED DESCRIPTION

(10) FIG. 1 shows a schematic flow diagram of the system for the thermal storage of excess power and its regeneration when there is a lack of power in the grid. This system comprises a gas turbine set with compressor 1, turbine 6 and power generator 8, a high-temperature reservoir 5, two smaller low-temperature reservoirs 3, 4 with corresponding changeover means 31-34 and 41-44, a gas cooler 2 and a discharge stack 7.

(11) During a power storage phase, the high-temperature reservoir 5 is heated with power of turbine outlet temperature TOT to at least the turbine inlet temperature TIT. This conversion from electric to thermal energy can be achieved via current resistance or induction. This phase can last several minutes, hours or days, depending on demand in the grid and on the design of the components.

(12) FIG. 2a shows the flow paths of the gas in the power generation phase. In compressor 1 the ambient air is compressed to pressure PC and thus heated to temperature TC, which is clearly above the ambient temperature. To utilize this heat and at the same time to minimize losses through the stack, the compressed air is cooled in gas cooler 2, and heat generated is used for heating or other purposes. When the changeover means 33 and 34 are opened, the cooled air flows through a first low-temperature reservoir 3, where it is heated by stored heat close to the turbine outlet temperature TOT, but clearly higher than TC. The thus pre-heated air flows through the high-temperature reservoir 5, where its temperaturethanks to the stored high-temperature heat of electrical originrises to at least the turbine inlet temperature TIT. Compressed air at a temperature TIT enters turbine 6 where the expansion to ambient pressure occurs, causing the temperature to drop to TOT. Since the changeover means 41 and 42 are also opened, the relaxed air flows through a second low-temperature reservoir 4, gives off its heat to the stored mass, cools down to temperature TS and leaves the system through a stack 7.

(13) After a certain time, as a rule between 10 and 60 minutes, the changeover means 33, 34, 41 and 42 close and the changeover means 31, 32, 43 and 44 open, such that the low-temperature reservoirs 3 and 4 change roles, as shown in FIG. 2b.

(14) Instead of cooling the compressed air in a convective heat exchanger 2, water can be injected and cooled by means of water evaporation. With this, the possibility is lost to utilize the waste heat that develops, but at the same time, the mass flow through turbine 6 and thus the performance is increased, and especially the overall degree of effectiveness of the process.

(15) FIG. 3a and FIG. 3b show an advantageous operating mode of this system which results in a quick start-up ability. At the end of a discharge phase, when gas turbine 6 is in operation, the high-temperature reservoir 5 is under operating pressure, and while the low-temperature reservoir 3 is under pressure (FIG. 3a), the valves 41 and 33 are slowly closed in that order, such that the high-temperature reservoir 5, the low-temperature reservoir 3 and turbine 6 remain near operating pressure. Should at the end of a discharge phase the low-temperature reservoir 4 be under pressure (FIG. 3b), the same happens with valves 31 and 43, with the result that now the high-temperature reservoir 5, the low-temperature reservoir 3 and turbine 6 remain under near operating pressure.

(16) The control variable for valves 41 and 31 will be the maximum operating pressure of the turbine. One of the two valves is closed until that pressure is reached. If this pressure is reached or exceeded before the respective valve is fully closed, the valve will stay in position until the pressure falls below operating pressure again.

(17) In valves 33 and 43 the control variable is the pressure difference. One of the two valves is closed until the difference between the pressure after compressor 1 and the operating pressure of the turbines 6 is as small as possible. If the pressure after the compressor is greater than the operating pressure, the valves will remain in position until the pressure begins to drop again.

(18) When all these valves are closed, the operating medium or air is stored under operating pressure in the volume between. In FIGS. 3a and 3b this volume is shown as bold likes. If the heating elements 12, 13 and/or 14 (see FIG. 5) are turned on again, the mean temperature rises in this closed volume, and thus also the pressure. That is why it is necessary to install an additional safety valve 20. Safety valve 20 can also be installed at the outlet of turbine 6 to better maintain the temperature there and to facilitate a smoother new start-up.

(19) In a news discharge phase (see FIG. 3a) valve 41 will open slowly such that turbine 6 begins to rotate and to drive compressor 1. When compressor 1 delivers sufficiently high pressure, valve 33 will open as well, such that a nominal turbine operation can follow.

(20) In case of conditions as in FIG. 3b, valve 31 will open first followed by valve 43. In this way, the turbine start-upin both cases shown in the drawingswill also be in under 30 seconds, which is necessary for participation in primary standard performance.

(21) FIG. 4 shows the advantageous S-shaped temperature profiles during the discharge of a 1 high-temperature reservoir 12 m in height, with 12 mm aluminum oxide balls as a bulk material. Here, the H/d ratio is 1000. At maximum storage, there is an even temperature of 1100 C. (t=0 h) in the heat reservoir. The discharge phase begins when the air heated to 550 C. enters from below, is heated up to 1100 C. and leaves the heat reservoir from the top in the direction of a gas turbine. Outside the reservoir, it mixes with an approximately same volume of air pre-heated to 550 C. to reach the right turbine inlet temperature (TIT) of such as 830 C. After a discharge period of two hours, the temperature in the bulk material above 3 m remains constant at 1100 C. between a height of 1 m and 3 m, there is a very steep temperature profile. After 4, 6 and 8 hours it is very similar, but the incline of the profile drops slightly. After 10 hours of uninterrupted discharge, the outlet temperature drops slightly to below 1100 C., but it still remains above the inlet temperature, which is usually between 830 C. and 970 C. Without such a favourable temperature profile, which is achieved due to the above described characteristics of the bulk material bed, the outlet temperature drops much more quickly and finally forms a linear temperature profile. The discharge phase must be ended when the outlet temperature drops below the turbine inlet temperature although there is still high-temperature heat in the reservoir.

(22) If the discharge time is shorter than 10 hours, such as 2 or 4 hours, it is not useful to heat the upper segments of reservoir 5 since this can lead the overheating of these zones or damage the heating elements. It is then advantageous to distribute several heating elements operated independently of each other throughout the entire height. As an example, FIG. 5 shows 3 such heating elements 12, 13 and 14. After a discharge phase of less than 4 hours, it is then sufficient when only the bottom heating element 12 is turned on during a loading phase. After a discharge phase of up to 8 hours, the two lower heating elements 12, 13 are turned on, and only with a complete discharge of the storage medium 11 in reservoir 5 are all three heating elements 12, 13, 14 put in operation. For such an operating mode, the above mentioned S-shaped temperature profile is not only advantageous but essential.

(23) A possible advantageous embodiment of electrical heating elements in the form of stacked inter-connected spirals is shown in FIG. 6. This arrangement is particularly advantageous for the bulk material s heat storage mass because it can distribute freely and evenly all around the spirals. To increase the overall length of the heating lines, the spirals are connected with other in the middle or at the end. Here, as an example, four spirals are shown in three different perspectives to better illustrate the said connection sites.

(24) FIG. 7 shows a bypass line with a bypass valve 9 to bypass the high-temperature reservoir 5 with a partial stream, to obtain a turbine inlet temperature TIT that is lower than the outlet temperature from the high-temperature reservoir 5. In this manner, even higher temperatures can be stored in the high-temperature reservoir 5 and such increase its heat capacity. In addition, the performance of turbine 6 can be regulated with bypass valve 9.

(25) FIG. 8 shows the possibility to use a line 10 to add a volume of natural gas NG or other gaseous or liquid fuel into the line between the high-temperature reservoir 5 and turbine 6 to achieve a higher turbine inlet temperature TIT. This is of interest when the discharge time is longer than planned due to conditions in the power grid, and if the air temperature from the high-temperature reservoir drops below the nominal turbine inlet temperature TIT.

(26) All characteristics disclosed in the application documents are claimed as relevant for the invention if alone or in combination they are novel in relation to the state of the art.

REFERENCE NUMBERS

(27) 1 Compressor 2 Heat exchanger, gas cooler 3 First low-temperature reservoir 4 Second low-temperature reservoir 5 High-temperature reservoir, electrically heated 6 Turbine, Gas expander 7 Stack 8 Power generator 9 Bypass line with bypass valve 10 Supply of natural gas or another gaseous or liquid fuel 11 Heat storage medium for the high-temperature reservoir 12 First (bottom) heating element 13 Second (intermediate) heating element 14 Third (top) heating element 20 Safety valve (pressure relief valve) 31, 32, 33, 34 Changeover means in the first low-temperature reservoir 41, 42, 43, 44 Changeover means in the second low-temperature reservoir PH-E Electrically heated high-temperature reservoir PH Low-temperature reservoir PC Pressure after the compressor TC Temperature after the compressor TIT Turbine inlet temperature TOT Turbine outlet temperature TMAX Maximum temperature of the stored high-temperature heat TPHE Mean temperature of the gas at the outlet from the high-temperature reservoir TS Temperature in the stack NG Natural gas or another gaseous or liquid fuel