CHARTOUNI, Daniel (Haselstrasse 31, Baden, CH-5400, CH)
WESTOVER, Lindsey (Martinsbergstrasse, Baden, CH-5400, CH)
OHLER, Christian (Burghaldenstrasse 9, Baden, CH-5400, CH)
CHARTOUNI, Daniel (Haselstrasse 31, Baden, CH-5400, CH)
WESTOVER, Lindsey (Martinsbergstrasse, Baden, CH-5400, CH)
CLAIMS
1. Thermoelectric energy storage system (20, 30) for providing thermal energy to a thermodynamic machine (27) for generating electricity, comprising
- a first thermal energy storage (25) at a first temperature level (T3), which first thermal energy storage (25) comprises a first heat storage medium, a first heat generating means (24) for generating heat to be stored in the first heat storage medium, and a first heat transfer means (26) for transferring heat stored in the first heat storage medium to a working fluid of the thermodynamic machine (27), characterized in that the system comprises
- a second thermal energy storage (21) at a second temperature level (T1) below said first temperature level (T3), which second thermal energy storage (21) comprises a second heat storage medium, a second heat generating means (18) for generating heat to be stored in the second heat storage medium, and a second heat transfer means (22) for transferring heat stored in the second heat storage medium to said working fluid for pre-heating the working fluid, and
- a working fluid circuit (28), connectable to the thermodynamic machine (27), for circulating the pre-heated working fluid from the second thermal energy storage (21 ) to the first thermal energy storage (25).
2. The system according to claim 1 , characterized in that the first and second heat storage media are distinct.
3. The system according to claim 1 , characterized in that the first and second heat generating means (24,18) are different.
4. The system according to claim 1 , characterised in that it comprises an intermittent renewable energy source such as wind power, or low-cost base-load electricity from a power grid, as a source of electrical power for both the first and second heat generating means (24, 18).
5. The system according to claim 3, characterised in that the second heat generating means (18) is a heat pump that takes thermal energy (Q1 th ) from a heat reservoir (17) at ambient temperature.
6. The system according to claim 1 , characterised in that the working fluid circuit (28) is a water/steam circuit and in that the thermodynamic machine is a steam turbine, and in that the first and second heat transfer means (26, 22) are arranged in series within the water/steam circuit.
7. The system according to one of the claims 1 to 3, characterized in that it comprises a third thermal energy storage (21') at a third temperature level (T2) below said first temperature level (T3) and above said second temperature level (T1), which third thermal energy storage (21') comprises a third heat storage medium, a third heat generating means (18') for generating heat to be stored in the third heat storage medium, and a third heat transfer means (22') for transferring heat stored in the third heat storage medium to said working fluid, and in that the working fluid circuit (28) circulates the working fluid from the second thermal energy storage (21) to the third thermal energy storage (21') and further to the first thermal energy storage (25).
8. A method for storing thermoelectric energy, comprising
- converting electric energy into thermal energy by means of a first heat generating means (24) and storing said thermal energy at a first temperature level (T3) in a first heat storage medium of a first thermal energy storage (25), characterized in that the method comprises
- converting electric energy into thermal energy by means of a second heat generating means (18) and storing said thermal energy at a second temperature level (T1) below said first temperature level (T3) in a second heat storage medium of a second thermal energy storage (21), and - converting the stored thermal energy back into electric energy by transferring thermal energy from the second thermal energy storage (21) to a working fluid, circulating the working fluid to the first thermal energy storage (25), transferring thermal energy from the first thermal energy storage (25) to the working fluid, and circulating the working fluid to a thermodynamic machine (27) for generating electricity.
9. The method according to claim 8, characterised in that it comprises converting, by both the first and second heat generating means (24, 18), electric energy from an intermittent renewable energy source such as wind power, or low- cost base-load electricity from a power grid.
10. The method according to claim 8, characterised in that it comprises converting electric energy into thermal energy by means of a heat pump taking thermal energy (Q1 th ) from a heat reservoir (17) at ambient temperature as the second heat generating means (18). |
DESCRIPTION
THERMOELECTRIC ENERGY STORAGE SYSTEM AND METHOD FOR
STORING THERMOELECTRIC ENERGY
FIELD OF THE INVENTION
The present invention relates generally to the storage of electric energy. It relates in particular to a system and method for storing electric energy in the form of thermal energy in a thermal energy storage.
BACKGROUND OF THE INVENTION
In an earlier patent application (WO-A1 -2005/088122) the applicant has described the idea of a thermoelectric energy storage system (TEES). It converts electricity to heat, stores the heat, and converts the heat back to electricity, when necessary. Such an energy storage system is robust, compact, site independent, and has energy capacity costs, which are about ten times smaller than for batteries. Thus it suits the storage of electrical energy in large amounts. For example it can store wind energy for later use during calm periods.
Because Carnot's law limits the conversion of heat to work, thermoelectric energy storage has a modest round-trip efficiency. If the heat being stored is provided
through resistor heaters, it may be 40%. Generally, a high efficiency for the conversion of the heat back to electricity is only possible with a high input temperature of the stored energy.
It is known in the art - though largely forgotten by the scientific community - that heat can be provided to the storage unit through a heat pump, e.g. a Stirling machine (for reference, see M. Werdich, Stirlingmaschinen, and US 3O80706, col.2, lines 22-30). A scheme similar to the one disclosed in the Werdich reference is shown in Fig. 1 : The thermal energy storage unit 10 of Fig. 1 comprises a thermal energy storage 11 and a Stirling machine 15, which either converts thermal energy taken from the thermal energy storage 11 into mechanical energy (mechanical drive 16), or works as a heat pump driven by the mechanical drive 16 to pump heat into the thermal energy storage 11. The thermal energy storage 11 is used to store energy that comes from an electrical heater 12 and/or a solar heater 13 and/or a combustion device 14.
A heat pump transports heat from a cold side to a hot side. The work needed to do that is less than the heat that is transported, thus a heat pump will "multiply" the heat as compared to resistive heat generation. The ratio of heat output to work input is called coefficient of performance (COP), and it is a value larger than one. In this way, the use of a heat pump will increase the round-trip efficiency of a thermoelectric energy storage system. The round-trip efficiency is the amount of electricity provided from the storage divided by the amount of electricity provided to the storage, and - with everything else kept equal - the latter is smaller using a heat pump.
However, the efficiency of the heat pump deteriorates if it has to "pump" the heat to high temperatures. The ideal COP of the heat pump is the inverse of the Carnot factor, (Thot/Thot-Tcoid), and reduces to one for high temperature differences T ho t- T C oid- For this reason, no heat pumps for large temperature differences are commercially available.
DESCRIPTION OF THE INVENTION
It is an objective of the invention to provide a thermoelectric energy storage system for converting electric energy into thermal energy to be stored and converted back to electric energy with an improved round-trip efficiency. This objective is achieved by a thermoelectric energy storage system according to claim 1 and a method according to claim 8. Preferred embodiments are evident from the dependent claims.
The thermoelectric energy storage system according to the invention has at least two separate thermal energy storages that work at different temperature levels comprised in distinct temperature ranges. The storage at the lower temperature is used for pre-heating a working fluid that is subsequently heated by the storage at the higher temperature to an input temperature for a thermodynamic machine. The fact that the proposed configuration allows to adjust or tune the temperature differences between the two thermal energy storages and the working fluid ultimately results in an increased efficiency and/or in reduced total storage costs.
In preferred embodiments of the invention, the two or more thermal energy storages are also distinct in regard of at least one of their main constituents. The latter comprise, among others, the heat storage medium, the volume of the heat storage medium, the heat generating means for generating heat to be stored in the heat storage medium, and a heat transfer means for transferring heat stored in the heat storage medium to a working fluid. This allows to tailor each one of the thermal energy storages to its respective temperature level by using optimized constituents.
In particular, the heat storage media of the thermal energy storages may be distinct, with e.g. the lower temperature storage being made of a less costly material that can not stand higher temperatures. Likewise, one of the thermal energy storages may store heat in sensible form in a solid refractory, while the other stores heat in latent form, i.e. in the melting energy of a salt or a metal.
Furthermore, the heat generating means may be different, with e.g. the thermal energy storage with the higher temperature level receiving thermal energy by means of a resistor heater and the thermoelectric storage unit with the lower temperature level receiving thermal energy by means of a first heat pump. This configuration avoids the drawbacks implied by using a heat pump for the total heat content of the thermoelectric storage system.
According to an embodiment of the invention the first heat pump takes thermal energy from a heat reservoir at ambient temperature.
According to another embodiment of the invention a working fluid circuit provided for circulating the working fluid from the lower to the higher temperature thermal energy storage and further to a thermodynamic machine comprises a water/steam circuit including a steam turbine, with heat exchangers in the thermal energy storages being arranged in series within the water/steam circuit.
Another embodiment of the invention is characterized in that a third thermal energy storage is provided, which works at a third temperature level lower than said first temperature level and higher than said second temperature level, and which receives thermal energy by means of a second heat pump and likewise transfers thermal energy to the working fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention will be explained in more detail in the following text with reference to preferred exemplary embodiments, which are illustrated in the attached drawings, in which:
Fig. 1 shows a scheme for a prior art energy storage unit;
Fig. 2 shows a simplified diagram of a thermoelectric energy storage unit with one heat pump in a sub-unit according to an embodiment of the invention;
Fig. 3 shows the heat flow scheme of the thermoelectric energy storage unit according to Fig. 2;
Fig. 4 shows the dependency on the upper temperature of the round-trip efficiency of the heat pump in the sub-unit of Fig. 2;
Fig. 5 shows a simplified diagram of a thermoelectric energy storage unit with two heat pumps in two sub-units according to another embodiment of the invention;
Fig.6 shows the heat flow scheme of the thermoelectric energy storage unit according to Fig. 5; and
Fig.7 shows the dependency on the upper temperature of the round-trip efficiency of the second heat pump in the sub-unit of Fig. 2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The central idea of the present invention is to split the thermoelectric storage unit in two sub-units, a low temperature part and a high temperature part, and use the heat pump only for the low temperature part. An embodiment of such a split thermoelectric energy storage unit is shown in Fig. 2. Fig. 3 illustrates the heat flow of the unit of Fig. 2.
The thermoelectric energy storage unit 20 of Fig. 2 comprises two sub-units 17,..,22 and 23,..,26. The second sub-unit 17,..,22, which works at a lower temperature level (T1), has a second thermal energy storage 21 with a second
heat exchanger 22 and a first heat pump 18. The first heat pump 18 (= second heat generating means) pumps heat Q1 into the first thermal energy storage 21 from a lower temperature heat reservoir 17, which may be the ambient air, surface water, or the ground (heat Q1t h )- The first heat pump 18 is supplied with electrical energy through a second electrical energy input 19, which adds heat Q1 e ι to the pumped heat Q1.
The first sub-unit 23,..,26, which works at a higher temperature level (T3), has a first thermal energy storage 25 with a first heat exchanger 26. Heat Q2 (= Q2 e ι) is added to the contents of the first thermal energy storage 25 by means of a resistor heater 24, which is supplied with electrical energy (Q2 e ι) through a first electrical energy input 23.
The heat exchanger 22 and 26 of the two sub-units are part of a water/steam circuit 28 of a water/steam cycle, wherein water at low temperature is fed through a water input 29, is heated up and finally vaporized by flowing through the heat exchangers 22 and 26, and is used to drive a steam turbine 27 (and a generator not shown). Other parts of the water/steam circuit like condensers, feed water pumps, etc., are not shown in Fig. 2.
The splitting of the thermoelectric energy storage unit 20 into the two sub-units 17,..,22 and 23,..,26 is possible because the working fluid of the water/steam cycle (the water and the steam) will start from ambient temperature when it is heated up to the high temperatures at the inlet of the steam turbine 27. Thus there is some heat needed at modest temperature levels, and it will be a waste of energy to supply this heat from the high temperature part 23,..,26.
Fig. 4 shows the round-trip efficiency (in %) as a function of the chosen hot side temperature of the first heat pump 18. The round-trip efficiency without the first heat pump 18 was set to 40%. The COP of the first heat pump 18 was set to 60% of 1 /efficiency (Carnot). It is assumed that the first heat pump 18 would store this heat in latent form. Thus the thermal energy storage 21 would stay at the same
temperature as the heat content increases or decreases, and the first heat pump 18 would always work against the same (maximum) temperature level. However, the inventive idea is independent from whether sub-units 17,.., 22 and/or 23,.., 26 comprise sensible or latent heat thermal energy storages 21 and 25.
With a single first heat pump 18 operating at a single, fixed hot side temperature level, the round-trip efficiency of the thermoelectric energy storage unit 20 can be raised from 40% to 43%. The jump at 311 0 C in Fig. 4 is real. It is a consequence of the latent heat of evaporation that occurs at this temperature (assuming 100 bar pressure in the water/steam circuit 28) and brings this efficiency to 45%. But the total cost optimum is rather the left one of the two optima.
Starting with a thermoelectric energy storage unit according to Fig. 2 one more degree of freedom can be added to the system by splitting the unit into three sub- units 17,..,22, 17',..,22' and 23,..,26, as shown in Fig. 5 and 6. Now, there are two heat pumps 18 and 18', operating at different temperature levels (T1 und T2), and pumping heat Q1 (= Q1 t h+Q1 e ι) and Q1' (= Q1 Vi + QI 'ei) into respective thermal energy storages 21 and 21'. The graph in Fig. 7 shows the round-trip efficiency (in %) as a function of the chosen hot side temperature of the hotter of the two heat pumps (18'). The other heat pump (18) operates at the best corresponding temperature level (usually at about half way from ambient to the first heat pump temperature). The second heat pump 18' (= third heat generating means) can raise the efficiency to 45% or even 47%.
LIST OF REFERENCE NUMERALS
10 thermal energy storage unit
11 thermal energy storage 12 electrical heater
13 solar heater
14 combustion device
15 Stirling machine
16 mechanical drive
17,17' heat reservoir (e.g. ambient air)
18 second heat generating means / heat pump 18' third heat generating means / heat pump
19,19' electrical energy input
20,30 thermoelectric energy storage system
21 second thermal energy storage
21 ' third thermal energy storage 22 second heat transfer means / heat exchanger
22' third heat transfer means / heat exchanger
23 electrical energy input
24 first heat generating means / resistor heater
25 first thermal energy storage 26 first heat transfer means / heat exchanger
27 thermodynamic machine / steam turbine
28 working fluid circuit / water/steam circuit
29 water input
T1 ,T2,T3 temperature level
Next Patent: PROCESS AND APPARATUS FOR PRODUCING A SHAPED BUILDING MATERIAL