WO1993022892A1

WO1993022892A1 - Method and apparatus for conversion of electrical energy to heat

Info

Publication number: WO1993022892A1
Application number: PCT/US1993/004266
Authority: WO
Inventors: Asoka K. Misra
Original assignee: Misra Asoka K
Priority date: 1992-05-07
Filing date: 1993-05-06
Publication date: 1993-11-11
Also published as: AU4235693A

Abstract

Heat can be generated using low current density electricity in an apparatus comprising: (a) first and second conductive electrodes; (b) a solid electrolyte body disposed between the first and second electrodes such that current flow between the electrodes passes through the solid electrolyte; (c) a power supply for imparting a current to the electrodes; (d) a heater for raising the temperature of the solid electrolyte; and (e) heat transfer means for removing heat from the solid electrolyte. The solid electrolyte body used in the apparatus of the invention is made from a material which is substantially non-conductive at ambient temperature but which becomes conductive at elevated temperature and which evolves heat in response to current flow through the material. A suitable material for use as the solid electrolyte body is Fe2O3.

Description

Descr 1iption

Method and Apparatus for Conversion of Electrical Energy to Heat

Background of the Invention

This application relates to a method and apparatus for the conversion of electrical energy into thermal energy.

Heat, particularly in the form of high quality steam, is a commonly used energy source in many industrial applications. The heat is usually generated on site by conversion of electrical energy into heat or by burning of fuels. Alternative energy sources such as solar power have made only minor in-roads in this area, however, because of the low efficiency with which electricity is generally converted into heat and the low current densities which these alternative energy sources are capable of providing. It would, however, be beneficial from both an environmental and an economic standpoint to be able to utilize solar and other alternative energy sources for the generation of heat for use in industrial applications.

It is an object of the present invention to provide a method and apparatus for low cost production of useful heat using low current density electricity.

Summary of the Invention

In accordance with the invention, heat can be generated using low current density electricity in an apparatus comprising: (a) first and second conductive electrodes;

(b) a solid electrolyte body disposed between the first and second electrodes such that current flow between the electrodes passes through the solid electrolyte; (c) a power supply for imparting a current to the electrodes; (d) a heater for raising the temperature of the solid electrolyte; and

(e) heat transfer means for removing heat from the solid electrolyte. The solid electrolyte body used in the apparatus of the invention is made from a material which is substantially non-conductive at ambient temperature but which becomes conductive at elevated temperature and which evolves heat in response to current flow through the material. A suitable material for use as the solid electrolyte body is Fβ_jOg. In use, the heater is initially activated to raise the temperature of the solid electrolyte to a point where it becomes conductive. Current flow between the electrodes and through the solid electrolyte is then commenced at which time the solid electrolyte begins to give off heat. The current density is increased in small increments. The heat generated is sufficient to maintain the solid electrolyte at a conductive temperature, thus allowing the heater to be shut off, while still allowing substantial amounts of heat to be removed for other purposes.

Brief Description of the Drawings

Fig. 1 shows an apparatus according to the invention; Fig. 2 shows an apparatus according to the invention;

Fig. 3 shows an apparatus according to the invention;

Fig. 4 shows an apparatus according to the invention;

Fig. 5 shows an apparatus according to the invention; and

Fig. 6 shows a test apparatus used in evaluating high temperature solid electrolytes. Detailed Description of the Invention

Fig. 1 shows a very basic apparatus according to the present invention. In Fig. 1, a solid electro¬ lyte 1 is disposed between two electrodes 2 which are in turn connected to a power source 3.

The solid electrolyte 1 is selected from materials which are at least substantially non-conductive at ambient temperature but which become conductive at elevated temperatures, and which evolve heat when electrical current is passed through them.

One material which meets these criteria is Fe₂0₃. Fe0₃ is an insulator at room temperature but becomes gradually conductive at temperatures in excess of 250°C. Furthermore, when current is passed through Fe₂0₃, heat is evolved in quantities in excess of that required to sustain conductivity.

Another material that meets these criteria is A1₂0₃. Al₂0₃ is also an insulator at room temperature, but become conductive at temperatures in excess of 1000°C. When current is passed through A1₂0₃, heat is evolved in quantities in excess of that required to sustain conductivity.

Other materials useful in the invention are other oxide such as Zr0₂, Si0₂, PbO, ZnO and Th0₂; halides, e.g. CaF₂ or, CuCl; and sulfides, e.g. CaS or MgS. Data for the onset of conductivity for some of these materials is listed in Table 1. Doped materials may also be used to lower the temperature at which the onset of conductivity occurs. The solid electrolyte body may be formed by compacting a powder of the material being used and then sintering it, for example, in air at temperatures over 1/2 the melting temperature of the material used. The electrodes 2 can be placed into the body during compaction and sintered in place to form a body with the electrodes affixed directly to it. Any other means of achieving electrical connection between the electrodes and the solid electrolyte body may be employed, however.

The electrodes 2 are formed from a conductive material and are suitably formed of known metal electrode materials such as platinum, copper, silver or nickel wire. The electrode material should be selected for stability and non-reactivity at the temperature of the system. The power source 3 is used to provide electrical current to solid electrolyte body. An important characteristic of the power supply 3 is the ability to provide a relatively constant current density of up to 2500-3500 mA/cm² at variable voltage from 10 to 1000 V. Further, the power supply advantageously includes means to gradually increase the current density because it has been observed that suddenly applying the maximum current densities used in this invention to the solid electrolyte bodies can result in unwanted electronic or thermal breakdown along with severe mechanical failure of the body. Subject to these considerations, suitable power supplies include CVCC de-Power Supplies, solar cells, nuclear cells and wind driven dynamos. The power source may also produce AC power or a pulsating current.

Fig. 2 shows an apparatus according to the invention disposed within a calorimeter. The Fe₂0₃ solid electrolyte 1 and platinum wire electrodes 2 are firmly joined to the end of an alumina tube 4 with high temperature cement 9. To test the heat output of the solid electrolyte, the assembly consisting of the solid electrolyte 1 and the alumina tube 4 were heated in a furnace to a temperature of about 650°C. A CVCC power supply 3 connected to electrodes 2 was then turned on with current and voltage set to zero and the maximum voltage set to 250 V respectively in a constant current supply mode. The current was then increased in increments of 25-50 mA, allowing time for equilibration to a constant voltage reading after each increase to a maximum current of 450 mA. In the furnace, this current level required only 30 V to maintain. The assembly was then removed from the furnace and equilibrated for 5 minutes in air while maintaining the same current level. During this time, the voltage required to maintain the 450 mA current level rose to 130 V due to the lower environmental temperature outside the furnace.

The assembly was then lowered slowly into the calorimeter shown in Fig. 2 which consists of a vacuum bottle 7 and a thin walled aluminum tube 5 filled with distilled water 8. As the assembly was placed in the calorimeter, a stop watch was started to record the time almost at the same time when the assembly was lowered. It was observed that the voltage required to maintain a current of 450 mA slowly increased to about 145 V and throughout the 17 minute experiment remained essentially constant at 144 V. A mechanical hand stirrer not shown in Fig. 2 was used for efficient heat removal from Al-tube wall.

At the end of 17 minutes the power supply was turned off and the solid electrolyte was rapidly withdrawn from the calorimeter. The temperature of the water within the vacuum bottle was then measured and found to be 58°C greater than the initial temperature of 32°C. This corresponds to a heat input of 17,400 cal (ΔT x wt of water (300 g) x sp. heat of water) in 17 minutes. A reference experiment with no current flow was performed to ascertain if the heat absorbed by the assembly body/mass contributed to the temperature rise in the exchanger fluid. In this experiment the assembly was brought out of the furnace while the current was on but during the 5 minutes holding period outside the furnace the current flow was switched off. Then the assembly was put into the calorimeter and held there for the same period of time, i.e. 17 minutes.

Finally, the heat exchanger fluid temperature was measured and it showed no measurable rise in temperature using a Hg- hermometer from that of 32°C initial temperature of the fluid.

The electrical energy used to generate this heat is given by the equation

Elec. Energy (cal) = -[₈ where V is the voltage, I is the current and t is the time in seconds. In this case 17,400 cal of heat were produced using 15,737 cal of electrical energy (144 V x

0.45 A x 1020 sec/4.18). Heat efficiency of the above experiment is 110.5 percent. Higher levels of excess heat, as high as 50% have been observed.

In an actual apparatus in accordance with the invention, the calorimeter shown in Fig. 2 is replaced with a means for recovering the heat generated. For example, a heat exchanger containing water or a heat exchanger fluid can be used. Alternatively, the solid electrolyte 1 can be embedded within a solid 53 with high thermal conductivity to form a plate heater as shown in Fig. 5. Several solid electrolyte bodies 1 can be arranged in series as shown in Fig. 3 and connected by connector wires 32. The solid electrolyte bodies may also be shaped to enhance heat transfer, for example into coils 41 as shown in Fig. 4. In the calorimetry experiment described above, a separate furnace was used to raise the temperature of the solid electrolyte body to a temperature where it is conductive. Alternatively, a heating means may be incorporated into the apparatus of the invention. Suitable heating means include resistance heaters wound around the solid electrolyte body, induction heating using a susceptor jacket (e.g. graphite) , and direct or indirect microwave heating. Example 1 A solid electrolyte body was prepared from 325 mesh chemical grade Fe₂0₃ powder. About 50 gm of this powder was compacted in a steel die-punch by a Universal Testing Machine. The green compact was 15 mm diameter and 20 mm high. On both ends Pt-wires were imbedded during compaction which served as the first and second electrodes as described in the text of the invention above. This green compact with the Pt- electrodes was sintered at 900°C for 2 hours in an air atmosphere. The in situ condensation ensured the good contact between the solid electrolyte body and the first and second Pt-electrodes during passage of electricity. The solid electrolyte was then used to make an assembly as described before using a double holed alumina tube as the support and high temperature cement to hold the alumina tube and the solid electrolyte firmly together. The Pt-electrodes were connected to the positive and negative terminals of a CVCC type Power Supply. The solid electrolyte assembly was placed in a furnace and the temperature was slowly raised to 600°C at which time the power source was turned on with constant current mode setting at 0.0 A and voltage setting at around 250 V. Slowly the current density was raised in increments until it attained 750 mA. The voltage requirement was 20 V. At this point the solid electrolyte was slowly withdrawn from the furnace and held in air at room temperature environment for about 5 to 7 minutes to equilibrate the temperature so that the system carries over no heat from the furnace excepting the heat it produces itself. After equilibration, the assembly was placed into the calorimeter chamber and a stop watch was started to record the time. The calorimeter contained 350 cc of distilled water as exchanger fluid at an initial temperature of 84°C. A hand driven stirrer is slowly operated in the heat exchanger fluid during the experiment for efficient heat removal from the Al-tube chamber wall. At the end of 2850 seconds (forty-seven and one half minutes) the power supply was switched off and simultaneously the solid electrolyte assembly withdrawn as soon as possible. This ensures that only heat produced by passage of current is recovered. Since the temperature of the assembly is the same before inser- tion and after withdrawal (as reflected in constant current/voltage levels) , the heat content of the assembly material is not transferred to the exchanger fluid-water. Throughout the experimental duration vol¬ tage requirement was 51 V in average. The final water temperature was 100°C, but it was observed that during the experiment water vapor was escaping from the mouth of the calorimeter. Therefore, when the volume of water was remeasured (294 cc) it showed 56 cc of water was vaporized. It is known that vapor of 1 cc water carries 536 calories of thermal energy with it. So,

Heat output (in cal.) = heat absorbed by water to boiling temperature

+ heat carried by vapor

= (Wt. of water x sp. gr x rise in temp-) + (volume of water vaporized x 536 cal)

= (350 x 1 x (100-84)) + (56 x 536)

= (350 x 16) + (30,016)

= 5,600 + 30,016 = 35,616

The electrical energy used to generate this heat is given by the equation:

Elec. Energy (in cal) = _{T TS}"

where, V = 51 I = 0.75 A t = 2850 sec Therefore ,

Heat input (in cal) = ^{51 x} ° ?18^{X 2850} = 26,080

So, heat efficiency of the process is given by:

Heat Efficiency = Heat Output _{x 100}

Heat Input - ³⁵r⁶¹⁶ x 100

26,080

=- 136.56%

Example 2 An A1₂0₃ crucible 61 was partially filled with molten silver metal 62 as shown in Fig. 6. The exterior crucible was then placed in contact with a pool of silver metal 63. Both the silver metal 62 and the pool of silver 63 were connected to a power supply via platinum wires 64. The entire assembly was then heated in a furnace to a temperature of 1200°C. At this time the power supply was turned on and the current gradually increased to a level of 1500 mA. The voltage required was 125 V. The furnace was then shut- off and the door half open while the current flow was maintained. The A1₂0₃ crucible was observed to continue to glow (an indication that the temperature remained above 1000°C during 60 minutes of current flow when the voltage requirement slowly increased to about 300 V.

Example 3 The test of example 2 was repeated using a Zr0₂ crucible.

Furnace Temperature 1300°C Current 1500 mA

Voltage 200 V The Zr0₂ crucible was observed to continue to glow during 60 minutes of current flow when voltage increased gradually to 425 V as the furnace atmosphere cooled.

Claims

1. An apparatus for conversion of electrical energy into heat comprising: (a) a solid electrolyte body; (b) first and second electrodes in electrical contact with the solid electrolyte body such that a current passed between said first and second electrodes will flow through the solid electrolyte; and (c) means for recovering heat generated by passage of electrical current through the solid electrolyte body, wherein the solid electrolyte body is formed from a material that is non- conductive at ambient temperatures but which becomes conductive at elevated temperature and which evolves heat upon passage of current.

2. An apparatus according to claim 1, further comprising means for heating the solid electrolyte body to render it conductive.

3. An apparatus according to claim 1, wherein the means for heating the solid electrolyte body is a resistance heater disposed around the solid electrolyte body.

4. An apparatus according to claim 1, further comprising means for passing an electrical current from the first electrode to the second electrode through the solid electrolyte body.

5. An apparatus according to claim 4, wherein the means for passing an electrical current is a solar energy cell body.

6. An apparatus according to claim 1, wherein the solid electrolyte body is an oxide.

7. An apparatus according to claim 6, wherein the solid electrolyte is formed from a material selected from the group consisting of Fe₂0₃, Si0₂, A1₂0₃, Zr0_2/ PbO, ZnO, Th0₂.

8. An apparatus according to claim 6, wherein the solid electrolyte is formed from Fe^.

9. An apparatus according to claim 6, wherein the solid electrolyte is formed from A1₂0₃.

10. An apparatus according to claim 6, wherein the solid electrolyte is formed from Si0₂.

11. A method of generation of heat comprising the steps of: (a) heating a body formed of a material that is non-conductive at room temperature but which becomes conductive at elevated temperatures, and which evolves heat upon passage of current; (b) passing an electrical current through the body whereby heat is evolved; and (c) collecting at least a portion of the heat evolved.

12. A method according to claim 10, wherein the body is formed from an oxide.

13. A method according to claim 11, wherein the semiconductive metal oxide is selected from the group consisting of FβjO-j, Si0₂, A1₂0₃, Zr0₂, ZnO, PbO and Th0₂.

14. A method according to claim 11, wherein the oxide is Fe₂0₃.

15. A method according to claim 11, wherein the oxide is Si0₂.

16. A method according to claim 11, wherein the oxide is A1₂0₃.

17. A method according to claim 10, wherein the electrical current passed through the body has a maximum current density of 2500 mA/cm².

18. A method according to claim 16, wherein the current density is increased gradually from about 10-50 mA/cm² to the maximum current density.