Thermal storage system

1. A heat storage system comprising a system gas inlet, a system gas outlet, and three or more thermal stores connected together in series between the system gas inlet and the system gas outlet, wherein each store comprises a chamber having a gas inlet, a gas outlet, and a gas-permeable thermal storage media disposed therebetween,

the system further comprising flow controllers operatively connected to bypass passageways and so configured that, during operation, a flow path of a gas flowing through the system and the storage media for transfer of thermal energy to or from the storage media is selectively altered in respect of which stores in the series are used in response to the progress of a thermal front through the storage media, and

wherein each store outlet apart from the last store outlet in the series is selectively connectable either to the system outlet or to the inlet of the next store in the series, and wherein the system is configured so that the gas flow path keeps being altered so as to bypass all stores in the series downstream of the store containing a leading edge of the thermal front.

2. A heat storage system according to claim 1 , wherein each store inlet apart from the last store inlet in the series is selectively connectable to the inlet of the next store in the series.

3. A heat storage system according to claim 1 , wherein the system is configured such that, in operation, the gas flow is capable of being stopped from passing through a store dependent upon the progress of a trailing edge of the thermal front.

4. A heat storage system according to claim 1 , wherein:

The system is configured for first redirection such that, in operation, the gas flow is capable of being directed to the next store in the series dependent upon the progress of a leading edge of the thermal front;

The system is configured for second redirection such that, in operation, the gas flow is capable of being stopped from passing through a store dependent upon the progress of the trailing edge of the thermal front; and

The first redirection and the second redirection of the gas flow are independent of one another.

5. A heat storage system according to claim 1 , wherein the three or more thermal stores are connected both in series and in parallel.

6. A heat storage system according to claim 1 , wherein the stores in the series are of at least two different types.

7. A heat storage system according to claim 1 , further comprising a control system for selectively altering the gas flow path that comprises control logic for controlling a leading edge of the thermal front and/or a trailing edge of the thermal front, the control system configured to switch the gas flow path between stores when a certain preset rise or fall in temperature is detected.

8. A heat storage system according to claim 1 , wherein the gas-permeable thermal storage media in at least one store in the series is particulate with a mean particle size of 20 mm or less.

9. A heat storage system according to claim 1 , wherein the system is configured so that the gas flow path keeps being altered so as to bypass all stores in the series upstream of the store containing a trailing edge of the thermal front.

10. A heat storage system according to claim 1 , wherein the system is configured such that, in operation, the gas flow is directed to the next store in the series dependent upon the progress of the leading edge of the thermal front, the gas flow being directed to the next store in the series when a gas exit temperature at the respective store gas outlet, or at the system gas outlet, has risen or fallen to a certain preset level.

11. A heat storage system according to claim 1 , wherein the gas-permeable thermal storage media comprises a porous or particulate solid.

12. A method of operating a heat storage system comprising a system gas inlet, a system gas outlet, and three or more thermal stores connected together in series between the system gas inlet and the system gas outlet, wherein each store comprises a chamber having a gas inlet, a gas outlet, and a gas-permeable thermal storage media disposed therebetween, the system further comprising flow controllers operatively connected to bypass passageways, the method comprising:

passing gas from the system gas inlet to the system gas outlet through the thermal storage media for transfer of thermal energy to or from the storage media, and selectively altering the gas flow path in respect of which stores in the series are used in response to the progress of a thermal front through the storage media,

wherein each store outlet apart from the last store outlet in the series is selectively connectable either to the system outlet or to the inlet of the next store in the series, and wherein the gas flow path keeps being altered so as to bypass all stores in the series downstream of the store containing a leading edge of the thermal front.

13. A method according to claim 12 , wherein the three or more thermal stores are connected both in series and in parallel, and wherein during the course of a charging phase, or during the course of a discharging phase, stores are used in series and in parallel.

14. Electricity storage system comprising a heat storage system according to claim 1 .

15. A method according to claim 12 , wherein the gas flow is stopped from passing through a store depending upon the progress of a trailing edge of the thermal front.

16. A method according to claim 12 , wherein the gas-permeable thermal storage media comprises a porous or particulate solid.