Heat of evaporation based heat transfer for tubeless heat storage

Abstract

Disclosed is a thermal storage solution which can operate without any internal tubing or mechanical pumping in the heat reservoir, and features a heat transfer technology based on evaporation and condensation of heat transfer fluids that will prevent hot and cold zones in the thermal storage reservoir. The main advantage is that the reservoir will have a much lower cost, have more degrees of freedom regarding the interplay between storage capacity, input and output power, and can operate without any mechanical or pressurized parts.

Claims

1. A thermal storage, comprising at least the following parts: a heat storage reservoir comprising of a solid, non-porous, granular material, an input system comprising a heat source and a system to generate a vapor phase of a heat transfer fluid or mixtures or multitude thereof and to pass the vapor phase heat transfer fluid or mixtures or multitudes thereof to contact the granular material in the heat storage reservoir, an output system comprising a heat exchanger, a system to inject a liquid fluid into the heat storage reservoir, and a system to collect an evaporated fluid generated by contact of the liquid fluid with the granular material in the heat storage reservoir and to transfer the evaporated fluid to the heat exchanger to release thermal energy therein, and characterized by having a liquid recovery system that recovers a liquid from the heat storage reservoir to be supplied to the input system or the output system, wherein the recovered, liquid supplied to the input system is generated by contact of the vapor phase of the heat transfer fluid or mixtures or multitudes thereof with the granular material in the heat storage reservoir, or wherein the recovered liquid supplied to the output system is a non-evaporated liquid fluid from the output system that contacts the granular material without evaporating; and wherein the heat transfer fluid used in the input system or the output system has a pressure dependent boiling point and the pressure is variable to set the boiling point of the said heat transfer fluid according to the temperature state of the said thermal storage.

2. A thermal storage according to claim 1 where the heat storage reservoir granular material comprises stones with a diameter between 10 and 300 mm with a convex shape and a filling ratio between 0.5 and 0.9.

3. A thermal storage according to claim 1, wherein the granular material comprises a phase change material, wherein heat transfer occurs in the heat storage reservoir to and from the granular material, characterized by the fraction of heat transfer to and from said granular material that takes place through phase change of the heat transfer fluid is at least 50%.

4. A thermal storage according to claim 1, which does not comprise mechanical pumps to move the evaporated heat transfer fluids between the non-porous granular material and the input and output systems, respectively.

5. A thermal storage according to claim 1 where the granular material has a receding contact angle of at least 45 degrees.

6. A thermal storage according to claim 1 characterized by the said heat storage reservoir being maximally pressurized at less than 1 bar overpressure.

7. A thermal storage according to claim 1 where the operating temperature in the heat storage reservoir ranges from ambient temperature to 250° C.

8. A thermal storage according to claim 1 without any gas-phase mechanical pumps.

9. A thermal storage according to claim 1, wherein the operating temperature in the heat storage reservoir ranges from ambient temperature to at least 400° C.

10. A thermal storage, comprising: a) a heat storage reservoir comprising of a solid, non-porous, granular material, b) an input system comprising a heat source and a system to generate vapor phases of a multitude of heat transfer fluids and to pass the vapor phases of the heat transfer fluids to contact the granular material in the heat storage reservoir, c) an output system comprising a heat exchanger, a system to inject a liquid fluid into the heat storage reservoir, and a system to collect an evaporated fluid generated by contact of the liquid fluid with the granular material in the heat storage reservoir and to transfer the evaporated fluid to the heat exchanger to release thermal energy therein, and characterized by having a liquid recovery system that recovers a liquid from the heat storage reservoir to be supplied to the input system or the output system, wherein the recovered, liquid supplied to the input system is generated by contact of the vapor phase of the heat transfer fluid or mixtures or multitudes thereof with the granular material in the heat storage reservoir, or wherein the recovered liquid supplied to the output system is a non-evaporated liquid fluid from the output system that contacts the granular material without evaporating, and wherein the multitude of heat transfer fluids used have different boiling points and are used sequentially during charging and discharging of the thermal storage.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The method and apparatus according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

(2) FIG. 1 shows a flow chart of one embodiment of the invention. A heat source (1) provides a flow of hot fluid (2), which enters a heat exchanger (3) where it delivers part of its thermal energy, returning to the heat source as a cold return flow (4). The thermal energy is delivered to a flow of liquid heat transfer fluid (5), which upon receipt of the thermal energy evaporates to form a gaseous heat transfer fluid (6). The gaseous heat transfer fluid is led into the heat storage reservoir (7), where it condenses and thereby delivers thermal energy to the reservoir. After condensation, the now liquid heat transfer fluid is assembled, preferably by means of gravity in the bottom of the reservoir, and moved through the heat exchanger (3) again. Any non-condensed heat transfer fluid will be collected in a condenser (9), and the condensate will be stored in a storage (10).

(3) When the energy in the heat reservoir (7) is to be used, a liquid heat transfer fluid (11) is dispensed into the heat reservoir, where it evaporates forming a gaseous heat transfer fluid (12), which is transferred to a heat exchanger (13), where it condensates, thus releasing thermal energy. The released energy can be used to evaporate a condensed working fluid (14) to form an evaporated working fluid (15) which can drive a turbine (16).

(4) FIG. 2 shows a cross section of one embodiment of the granular heat storage, comprised of an air-tight shell (21) and randomly stacked granular material (22) with voids (23) in between. Furthermore, there will be external connections to the input and output system (24) and a recovery system for condensed heat transfer liquid (25).

DETAILED DESCRIPTION OF AN EMBODIMENT

(5) In one embodiment, a concentrated solar power plant delivering thermal oil at 350° C. is used as a heat source. The thermal oil is passed through a counter flow heat exchanger heating and evaporating a series of heat transfer fluids with boiling points of 100, 150, 200, 250, 300 and 345° C., respectively, while the heat reservoir is heat in the temperature intervals 50-100, 100-150, 150-200, 200-250, 250-300, and 300-345° C., respectively. During the evaporation of these fluids, the return temperature of the thermal oil to the concentrated solar power plant is 50, 100, 150, 200, 250, 300 and 345° C., respectively, ensuring a moderate thermodynamical efficiency with an average thermal gradient of 25° C. between the return temperature of the thermal oil and the heat reservoir.

(6) The heat reservoir consists of a stone reservoir contained in an air tight metal container having dimensions of 12 m (length)×2.35 m (width)×2.6 m (height) and being insulated using ceramic stone wool on the outside. The stones have an average diameter of 150 mm and a size distribution (spread) of 50 mm. The shape of the stones are rounded, thus forming an interconnected network of air in between with an average width of 10-30 mm, allowing for relatively unhindered flow of heat transfer fluid. The bottom of the container is made slightly sloped, so a small area is defining the lowest point of the container, where a mechanical extraction mechanism is placed in the form of a pump. At the top of the container, spray nozzles are placed with a distance of 1 m in a 11×2 layout, each capable of delivering a liquid flow of 0.3 kg/s. With an average heat of evaporation of 300 kJ/kg for the heat transfer fluids, this corresponds to a maximum extraction rate of 2 MW. The filling ratio of the stones in the container is 75% giving a total specific heat capacity of 44.5 kWh/K. (specific heat of the used stone 0.84 kJ/(kg*K), density of the stone is 2600 kg/m3). For a fully charged container (345° C.) this corresponds to a usable energy content of approximately 13 MWh (when discharging to a temperature of 50° C.). The output system collect the hot evaporated heat transfer fluids through piping to the container. The evaporated heat transfer fluid is passed through a heat exchanger, where the heat is transferred to the working gas in an ORC generator, thus producing electricity. The condensed heat transfer fluid is then re-injected into the container. The series of fluids being used for the energy extraction have a boiling point of 300, 250, 200, 150, 100, and 50° C., respectively, through the temperature intervals of the storage of 345-300, 300-250, 250-200, 200-150, 150-100 and 100-50° C., respectively, resulting in an average heat gradient (loss) between storage and evaporated heat transfer fluid of 25° C.

(7) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms “comprising” or “comprises” do not exclude other possible elements or steps. Also, the mentioning of references such as “a” or “an” etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

(8) All patent and non-patent references cited in the present application are also hereby incorporated by reference in their entirety.