Methods of pumping heat transfer fluid in thermal energy storage systems

1. A method of pumping a heat transfer fluid in a thermal energy storage system, the thermal energy storage system comprising a first thermal energy storage tank connected to a second thermal energy storage tank via a bi-directional flow member, the first and second thermal energy storage tanks being connected via fluidic conduits with a pressure vessel system comprising a first and a second pressure vessel each first and second pressure vessel being partially fillable with an actuating liquid, wherein, the method for pumping comprises: oscillating the actuating liquid via a pump provided in the fluidic connection between the first pressure vessel and the second pressure vessel, thereby creating a pressure difference in the first thermal energy storage tank with respect to the second thermal energy storage tank, and therein oscillating the heat transfer fluid via the bi-directional flow member.

2. The method according to claim 1 , wherein the first and second pressure vessels and the first and second thermal energy storage tanks form a closed system having a substantially constant volume during operation.

3. The method according to claim 1 , wherein the actuating liquid has a temperature substantially lower than the temperature of the heat transfer fluid during operation.

4. The method according to claim 1 , wherein the method comprises oscillating the flow of actuating liquid from the second pressure vessel to the first pressure vessel and therein oscillating the flow of heat transfer fluid.

5. The method according to claim 1 , wherein a solar receiver is provided at a point along the bi-directional flow member and wherein the heat transfer fluid passes through the solar receiver.

6. The method according to claim 5 , wherein the heat transfer fluid is oscillated between the first thermal energy storage tank and the second thermal energy storage tank via the solar receiver.

7. The method according to claim 1 , wherein, the first pressure vessel is connected to the first thermal energy storage tank via a fluidic conduit having a first end provided at an upper region of the first pressure vessel and a second end provided at an upper region of the first thermal energy storage tank.

8. The method according to claim 1 , wherein the second pressure vessel is connected to the second thermal energy storage tank via fluidic conduit having a first end provided at an upper region of the second pressure vessel and a second end provided at an upper region of the second thermal energy storage tank.

9. The method according to claim 1 , wherein the bi-directional flow member has a first end provided at a lower region of the first thermal energy storage tank and a second end provided at a lower region of the second thermal energy storage tank.

10. The method according to claim 1 , wherein the heat transfer fluid has a melting point from about 200° C. to about 500° C.

11. The method according to claim 1 , wherein the temperature of contents of the first and second thermal energy storage tanks during operation is from about 500° C. to about 900° C.

12. A thermal energy storage system comprising a first thermal energy storage tank connected to a second thermal energy storage tank via a bi-directional flow member, the first and second thermal energy storage tanks being arranged to receive a heat transfer fluid, said first and second thermal energy storage tanks further being connected via fluidic conduits with a pressure vessel system comprising a first and second pressure vessel each first and second pressure vessel being partially fillable with an actuating liquid to be displaced between the pressure vessels via a pump provided in fluidic connection between the first pressure vessel and the second pressure vessel to create a pressure difference in the first thermal energy storage tank with respect to the second thermal energy storage tank indirectly displacing the heat transfer fluid.