US 3884216 A
A compact, self-contained, closed circuit heater system for use by undersea divers to supply heated fluid through a diver's suit having internal fluid circulation passages therein. The heater system uses a magnesium/iron/seawater electrochemical energy source which is shortcircuited to produce heat by the reaction of magnesium with seawater.
Description (OCR text may contain errors)
United States Patent 1 1 McCartney 14 1 May 20, 1975 1 1 ELECTROCHEMICAL ENERGY SOURCE FOR DIVER SUIT HEATING  Inventor: Joseph F. McCartney, Solana Beach, Calif.
 Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.
 Filed: Sept. 19, 1974  Appl. No.: 507,645
 U.S. Cl. 126/204; 126/263', 128/402; 165/46  Int. Cl. A6lf 7/06  Field of Search 126/204, 263; 128/402; 165/46  References Cited UNITED STATES PATENTS 3,207,149 9/1965 Spindler 126/263 3,367,319 2/1968 Carter, Jr 126/204 3,385,286 5/1968 Jones 126/204 3,450,127 6/1969 Harwood, .Ir. 126/204 3,465,245 12/1969 Lahr et a1. 165/46 X 3,497,672 2/1970 Harter et al. 165/46 X 3,737,620 6/1973 Harvey 126/204 X 3,743,012 7/1973 Laxo 165/46 X Primary Examiner-Meyer Perlin Assistant ExaminerRonald C. Capossela Attorney, Agent, or FirmRichard S. Sciascia; Joseph M. St.Amand  ABSTRACT A compact, self-contained, closed circuit heater system for use by undersea divers to supply heated fluid through a divers suit having internal fluid circulation passages therein. The heater system uses a magnesium/iron/seawater electrochemical energy source which is shortcircuited to produce heat by the reaction of magnesium with seawater.
12 Claims, 3 Drawing Figures W ELECTRIFIED DIFFUSION LOCAL ACTION AT ANODE-CATHODIC AREA ELECTROCHEMICAL ENERGY SOURCE FOR DIVER SUIT HEATING BACKGROUND OF THE INVENTION This invention relates to an electrochemical heat source and particularly to a method and apparatus for supplying heated fluid to a divers suit.
Exposure to cold water for extended periods of time necessitates that a diver be protected from thermal losses to the environment. Thermal balance for divers can be expressed by the simplified equation:
Heat Replacement Respiratory Heat Loss Diving Suit Losses Metabolic Heat Generated Respiratory are to the involuntary heating of inspired gas 68.3) to body temperature prior to expiration. The amount may vary up to 500 watts depending upon the type of gas breathed, inspired gas temperature, depth of the dive and volume of gas insp red. Metabolic heat generated varies over a wide range for individuals and depends considerably on the degree of physical or mental activity. For example, a trained underwater swimmer can generate 115 watts for sustained periods of time.
Diving suit losses present one of the major problems in maintaining thermal balance for the diver. Recent tests have shown that diving suit losses vary considerably for different suit materials as a function of temperature and depth. For example, the standard Navy threeeighths inch thick neoprene suit (wet suit) with a skinto-water temperature difference of 50F at a depth of 1,000 feet may conduct as much as 3,000 watts. Experiments with more recent suit developments such as the dry suit in which a layer of helium gas is maintained between the skin and the inside of the suit may reduce these losses to about 1,000 watts at the same temperature difference and depth. Other suit materials such as incompressible suits can provide better thermal insulation but provide less flexibility and thus restrict diver manueverability. In either event, the heat losses by the diver exposed to cold water for extended periods of time must be minimized to ensure the safety and effectiveness of the diver.
One method to ensure adequate heat balance for the diver is to provide a heat source which can make up the losses to the environment. Such a system could be used to provide a protective warm layer between the diver and the suit material, thus maintaining skin temperature and reducing body heat losses. The same heat sources could be used to heat the inspired gas to the diver minimizing the respiratory heat losses. A portable, self-contained heat source is preferred because it allows the diver to work and swim free of umbilical connections.
Over the past years, several methods of providing heat for divers have been investigated. All selfcontained heat sources and previous developments have exhibited several disadvantages: batteries are usually bulky, expensive, and short-lived; nuclear heat sources produce insufficient heat and can be used by the diver for only short durations because of radiation exposure; in addition, the radiation shielding and thermal safety devices make the nuclear heater bulky and complicated to operate; thermochemical heat sources employ exotic reactants and involve high operating temperatures and complicated control systems. A compact, lightweight, high-energy-density, easily 'controlled, reliable and safe heat source that can be integrated with open or closed circuit hot water suits is vitally needed.
SUMMARY OF THE INVENTION The invention described herein is for a heat source which can provide up to 2,000 or more thermal watts of heat in the form of hot water for circulation through flexible tubes surrounding the divers body. Some of the advantages of the system of this invention are: (1) system operates at ambient pressure; (2) high overall energy density; (3) no toxic or radioactive materials, or high temperatures involved; (4) simple, safe, and reliable operation; (5) simple maintenance; (6) inexpensive, readily available fuel; and (7) indefinite shelf life.
The reaction of magnesium plates in seawater was found to provide sufficient heat when the magnesium anode is placed in close proximity to and electrically shorted to a cathodic material such as an iron plate. Controlled heat output is provided by mechanically varying the distance between anode and cathode plates. The single most important factor controlling the reaction rate has been found to be the pH of the electrolyte. The heat source is an expendable cartridge type refill unit of low cost. The refill is modular in size to permit integral nominal output values (e.g., 500 watts, 1000 watts, 1500 watts, 2000 watts, etc.) depending on the mission and the accompanying environmental conditions. The system is simple enough in operation so that diving personnel may be fully trained in theory, use and care of the heat source in just a few hours.
OBJECTS OF THE INVENTION An object of the invention is to provide a compact, portable diver heater capable of providing 2000 thermal watts of heat in the form of hot water delivered to a closed circuit tubing divers suit.
Another object of the invention is to provide a diver heater operational to 1000 foot depths for periods up to 8 hours without replenishment.
A further object is to provide a heat producing seawater/magnesium/iron electrochemical energy source having a controlled heat output for diver suit heating.
Still another object is to provide a method for supplying heated fluid to a divers suit.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a divers suit showing an example of a general arrangement of the electrochemical heat source and SCUBA pack in relation with the body of the diver.
FIG. 2 shows the reactions and ion composition of layers of a magnesium seawater cell.
FIG. 3 is a diagrammatic illustration of one embodiment of the electrochemical heat source of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT A=diver suited in a typical diving suit which has flexible tubes throughout the suit surrounding the divers body is shown in FIG. 1. Mounted on the front of the suit is a heat source 12 for heating water or other fluid which passes through the flexible tubing of .the suit.
A SCUBA back-pack 14 provides air or other suitable breathing gas mixture to the diver. Heat source 12 uses a magnesium/iron/seawater electrochemical energy source as hereinafter described for heating the water which flows throughout the diver suit 10.
The oxidation reaction of magnesium has been used for the heat source in the present system because of simplicity, reliability, compactness, low operational cost and comparatively high energy density. A comparison of the heat produced by the magnesium-seawater reaction with other possible sources is shown below in Table 1.
TABLE 1 ternal load replaced by a short circuit to maximize the reaction rate.
ELECTROCHEMICAL PROCESS A basic electrochemical cell considered in the development of the present system consists of anode and cathode rectangular plates of magnesium alloy and iron, respectively. The assembled diver heater consists of several cells stacked in series, with each cell shorted to the adjacent cell.
The electrochemical process of a seawatermagnesium cell as a low-rate discharge battery is as follows a. electron motion in the external circuit from anode to cathode with 2(OH) or 2 Cl- ENERGY VALUES OF VARIOUS REACT ANTS e electrical output, t thermal output, NEA no experimental values available.
Source Energy Produced (watt-hr/lb) Theoretical Actual Batteries: Magnesium-seawater reaction 1,850 1,800 Mg H O(Fe) 400,. Magnesium-silver chloride (torpedo 90 70. battery) Lead-acid (secondary stora e battery) 74 15. Silver oxide-zinc (silver eel?) 201 50 Manganese dioxide-zinc (dry cell) 130 Nickel-cadmium 9e 15': Mercury oxide-zinc (mercury cell) 104 40 Zinc-air (air batteries) 400 80 Sodium-oxygen (battery) 1,010 250 Magnesium bromide (experimental- 250 organic cathode) 0 Sodium sulphur (experimental) 350 150 Lithium chloride (experimental-high 1,000 250: temperature fused chloride) Lithium fluoride (experimental) 750 100,, Combustion: Fuel oil and liquid oxygen 1,370 1,370, Fuel oil and 90% hydrogen peroxide 970 970, Butane-O 2,100, Latent Heat: Lithium hydride (sensible heat and 250 NEA change of state) Lithium fluoride (sensible heat and 305 NBA change of state) Boron (sensible heat) 290 NEA Mono Fuel: Hydrazine hydrate (decomposition) 1,520 NEA Isotope: Plutonium 238 (8-hr exposure)(value 200,
includes weight of shielding and hardware Without contact with iron, magnesium reacts with seawater according to Equation 1.
Mg 2H O Mg (OH) H (gas) Ah (1) where Ah is the heat of formation of the reaction.
It is hereinafter shown that the theoretical energy density of this reaction is 1,850 watt-hr/lb of magnesium. This reaction proceeds slowly in seawater under normal conditions and the energy is not produced at a usable rate. By electrically connecting the magnesium to a cathode material, such as iron, the reaction can be made to proceed more rapidly thus liberating energy at a usable rate. Such systems have been developed as seawater batteries. The heater uses the same basic principle as the seawater battery but with the batterys exb. electrolysis of H 0 at the cathode c. (OH) migration towards the anode because of C1 depletion and Na attraction d. Na motion towards the cathode e. migration of Cl towards the anode f. migration of Mg from the anode to a point where they combine g. formation of Mg(OH) directly from Mg and (OH) (local anode reaction) and from reaction of intermediary products NaOH and MgCl The mechanism for cell reactions in a high-rate discharge system for the short-circuited diver heater cell has been analyzed. The rate of anodic corrosion of magnesium is controlled by the rate of formation of H ions at the surface and the migration of these ions through an anode surface film of MgO and/or Mg(OH) The concentration of ions is controlled by 3 8 84,2 1 6 5 6 the pH. FIG. 2 shows a schematic model of the cell and Substituting in Equation we have the various components of the reaction.
The primary concern for the diver heater is with heat produced by the reactions. The primary reactions are shown in FIG. 2 and are summarized below. 5
(8.314) (273) I (96,500) X 2.3 log k Assuming log k to be independent of temperature Mg- Mg 2e at anode (g) (8 314) (273) 21-1 0 2e Ml-l 2(OH at cathode 211 Ze H (gas) at cathode (4) 10 2 (96,500) (2'3) ("905) 244V Mg 2H O Mg(Ol-l) H sum of 2, 3, and 4 (1) (gas) (8.314) (350) some intermediate reactions which may also take Substituting these values in Equation 9, where 4.183 is place ar 15 the joule equivalent.
Mgcl2 in electrolyte (5) 2 0O 44 Mg ZNaCl 2H O MgCl 2Na(OH) in electrolyte (6) AFQO ,2 18); 13 Keal/mOl n M g(OH) NaCl Mg(OH)Cl Na(OH) in electrolyte 7 20 (2) (96 500') 67) zrfir T-IB Kcal/mole Fr m ana i of th h t e b l t 2 96,500 3.i3
o lys s e ea g nerated y ce ls ested AFN: 4'18); 144 (Cal/mole the major heat producing reaction appears to be that of the overall Equation 1. The Ah for this reaction is:
Thus it can be seen that temperature directly affects Mg 2 )2 2 Ah the available energy of the system.
(8) Pressure also can affect the energy due to its influence on the cathode reaction:
Ah 223.4 2( 68.3 86.8Kcal/mole 1850 V watt-hr/lb Where Ah heat of formation of reaction The effect of pressure can be calculated from Equa- Thus the theoretical estimate for the energy density tions 9 and Here k has the form of the system is 1,850 watt-hr/lb for standard temperature (20C) and pressure (1 atm). Energy density is deum fined as watt-hours per pound of magnesium anode re- T acted.
Calculation of the effect of temperature on the theo- 40 where H i l pressure H d denotes retical free energy of the oxidation reaction can be centration fi i l i made y the application of Equations 9 and 10 to Substituting in Equation 10 for atmospheric pressure perimental values for the variation of the electrode pod f 10 t h re a d ming a H f 9 tential of reaction (Equation 11 with temperature.
AF n FE E (8.314) 273) l lulm' 0 E u 3) lo 10 =4l4V where (2 (96,500) a g AF change in free energy n number of electrons from Equation 9 F faraday (constant) (coulomb/mole) E electrode potential (volts) (2) (96,500) P474) mm 22 Kcal/mole E %;2.3 log k (1 Am... 3 1 414) Keel/mole where Thus pressure has a comparatively small affect on re- R gas constant (joule/K-mole) 6 action rate. The reaction rate which can finally be real- T temperature (K) ized depends upon electrolyte pH, ion concentration, K equilibrium constant for the reaction conductivity of the electrolyte, temperature and pressure. A high value for pH will reduce the reaction rate by clogging the anode surface with Mg(OH) Under 1 l) these conditions, the anode surface becomes pitted and particles of Mg coated with MgO or Mg(Ol-l) sluff-off At 25C the experimental value for the reaction potenand their useful energy is lost. Sample cells run in a caltial is 2.67\/. orimeter with MgCl added to bay seawater showed a reduction in energy density. This may be explained by the fact that since the solubility of Mg(OH) is low it is essentially un-ionized thus retarding the migration of Mg ions into solution. The result is an excess Mg ion concentration which reduces available energy.
The quantity of free ions in the electrolyte must remain fairly constant throughout the reaction cycle in order to ensure a constant reaction rate. Variation in ion concentration causes fluctuation in the rate of heat output. This may partially explain why an initial power surge is realized with fresh seawater electrolyte as discussed later. Some of the lower concentration ions present in seawater such as Mg, Ca, S and K may react during the initial cell operation and produce heat. In general the cell proceeds under the direction of the Na and Cl ions present and the pH of the solution. The initial surge is probably a result of both the initial low pH of the fresh electrolyte and fresh anodes. When the clean anodes are placed in the electrolyte the reaction proceeds rapidly; as the pH increases the surface layer of MgO and Mg(OH) rapidly forms and the reaction thus slows to a rate which remains under control by the pH.
The driving force for the reaction is the electrical potential developed between the anode and cathode and the associated current through the short circuit. Replacing the short circuit with a load resistance greatly reduces the reaction rate. Since a short circuit is provided in the diver heater no method has been devised to measure the actual current flow. However, it has been estimated that the electrical joule heating of the electrolyte may provide as much as percent of the total heat output.
EXPERIMENTAL DETERMINATION OF CELL CHARACTERISTICS The present electrochemical heat-producing cell arrangement is shown in FIG. 3. Prior to use of this arrangement it was thought that the greater the cathode area in direct electrical contact with the anode the more rapidly the reaction would proceed. In addition, for a low-discharge-rate battery it was shown that wire screen cathodes reduced cathode polarization and improved cell voltage. Thus, the first cell was fabricated with iron screen cathodes stacked alternately and in direct contact with the anodes. A pressure plate on each end of the cell and a threaded rod was used to provide variable contact resistance between the anode and cathode. Fresh seawater electrolyte was forced through the bottom of the cell case and the byproducts removed from the top of the cell.
The reaction proceeded rapidly during the initial minutes of the operation then decayed after several minutes to negligible heat. Magnesium hydroxide sludge collected in the spaces of the screen cathode and blocked the passage of fresh electrolyte through the cell thus effectively blocking the reaction. The black color of the sludge was attributed to surface blocking of the anode causing unreacted magnesium particles coated with MgO and Mg(OH) to be sluffed off the plates.
It was found that corrosion of the anode does not take place in the area immediately adjacent to cathode contact but instead at some short distance from the contact area. These findings led to the development of the present plate cathode arrangement in FIG. 3 and electrode gap control as a solution to the sludging problem.
Plate spacing (electrode gap) is the distance between the anode and cathode which provides for free passage of electrolyte and reaction products. As plate spacing increases the total resistance of the electrolyte (internal resistance) between the plates increases. Since the reaction rate is partially controlled by electron motion, the rate (power density) should decrease as electrode gap increases. Power density is defined as the thermal power produced per unit area of anode surface and is a measure of the reaction rate. As the magnesium anode reacts with seawater the electrode gap increases. In order to provide a heater with constant power output the relationship between plate spacing and power density is especially significant.
To determine the effects of plate spacings on heater performance, cells fabricated from one-eighth-in.-thick magnesium alloy and steel shim stock were tested. Surface oil films on the anode and cathode were removed with synthetic paint thinner and subsequent washes with commercial grade acetone. Since it is essential that a low-maintenance cell be developed, no attempt was made to remove normal oxide layers from either the cathodes or anodes. Steel bolts at both ends of the rectangular plates provided the short circuit while steel washers between the plates at the bolts provided the desired electrode gap. Table 2 summarizes the average power density as a function of plate spacing.
Averaged Power Output of Magnesium Anode and Iron Plate Cathode as a Function of Plate Spacing Plate Spacing Average Power (in.) Density (watts/in?) The first three data points of Table 2 show increased power density for decreased plate spacing. Power density for 0.080 and 0.060-inch gaps decreased because of sludge accumulation between the plates. Accumulation of sludge reduces electrolyte circulation and in- ..creases electrolyte resistance which in turn reduces power density. It is later shown that with forced circulation plate spacings of 0.060 inch can be maintained without sludge accumulation.
The theoretical discussion showed that the electrolyte temperature should affect the reaction rate (power density) and total energy available per pound of magnesium consumed. These'calculations are based on the fact that thermodynamic equilibrium is maintained throughout the reaction. Since equilibrium conditions are not present during the reaction, experiments were performed to help gain insightinto the true nature of the reaction temperature dependence. action temperature dependence is important in selecting operational temperatures for the diver heater.
Increased temperature increases the reaction rate for fixed plate spacing. At low temperatures the cells tend to gather sludge which blocks the anode surface and eventually reducesthe reaction rate. At operational temperatures above F the reaction tends to be unstable in heat output resulting in control problems. In addition, a considerable quantity of water vapor is car- 9 ried away by the escaping hydrogen thus increasing system losses.
The resultant effects of temperature on cell performance are further compounded by the fact that the total energy available appears to be reduced as cell temperature increases. This phenomenon is attributed to sluffing of unreacted magnesium particles from the anodes. As the reaction rate increases it appears that a higher percentage of unreacted magnesium is sluffed from the anode surface.
The resultant effects of temperature on total cell performance indicate that 160F is an optimum operational point in terms of anode efficiency (energy density), reaction control, and reaction rate.
Analysis showed that high ambient pressure (from ocean operation) may adversely affect the cell operation. The effect was attributed to compression of hydrogen bubbles at the anode surface slowing down removal of the reaction products and causing increased surface blocking of the anode.
A simple test was run to determine the effects of pressure. Two cells with identical surface area and plate spacing were prepared. The anodes of each cell were cleaned and weighed prior to and following each test. One cell was placed in a pressure vessel at 1 atm in seawater heated to 1 16F. The cell was operated for 90 minutes. The other cell was placed in the same vessel at the same temperature for the same period of time. This cell was pressurized to 520 psi (1,170 ft) for 85 of the 90 minutes.
Test weights showed that approximately 14 percent by weight more anode was consumed during the pressurized cell operation. Thus this result indicates a higher reaction rate (more energy released) due to the effect of increased pressure. Some sludge accumulated between the plates of the pressurized cell substantiating the fact that hydrogen removal of reaction products is less effective under pressure. The anode surface of the pressurized plates were severely pitted indicating that anode surface blocking (sluffing of unreacted Mg) had increased.
The net result of the pressure tests appears to show that pressure increases the reaction rate but decreases the total energy available because of increased sluffing of magnesium.
A number of tests were performed to determine what effect anode composition had on cell performance. It has been reported that the anode composition and heat treatment affects the total energy available and sludge characteristics of battery systems.
Single cells of a known anode composition and weight were tested to determine the relative difference in their energy densities. The cells were placed in an insulated dewar flask and the temperature rise of a known volume of seawater was monitored. Following a known time interval the cells were removed from the flasks and the anodes were scraped, washed in distilled water and acetone, and reweighed. No significant difference in energy density was noted for the alloys tested. Since the plates were not chemically cleaned to remove all the reaction products, only relative values of energy density are presented.
Further tests were run on three different alloys to determine reaction rates and sludging characteristics. Cells were placed in insulated containers and the temperature rise of a known volume of seawater was recorded as a function of time.
High purity (99.9%) magnesium showed the highest reaction rate but sludge buildup between the plates stopped the reaction after two hours. The sludge of high-purity magnesium was pure white and tended to flocculate readily. AZ3 lB-O alloy (96% Mg, 3% Al, 1% Zn) reacted faster than AZ61 alloy (93% Mg, 6% Al, 1% Zn) during the first 30 minutes of operation after which the rates were approximately equivalent. The sludge formed from AZ61 alloy was whiter than that of AZ3 lB-O alloy, indicating that slightly less surface blocking was present and thus less free magnesium was sluffed from the plates.
Since salinity and conductivity of seawater are directly related, a number of tests were performed to determine what effect salinity would have on the reaction process. Commercial grade sodium chloride was added to known volumes of distilled water to produce salinity of 33.8 parts per thousand by weight and 41 parts per thousand by weight. Cells of a known surface area were placed in insulated beakers with the salinity solutions. A third cell was placed in natural seawater with a salinity of 34 parts per thousand by weight. Temperature rise of each beaker was recorded as a function of time.
The reaction rate for fresh seawater was higher than either of the two synthetic solutions. The sludge of the seawater was darker in color than either of the salinity solutions. Other similar tests performed with lower salinity solutions have shown that as the reaction rate increases the sludge becomes darker in color.
The higher reaction rate for the seawater cell is attributed to the fact that seawater is a good buffer, thus the pH of the solution remains lower for a longer period of time. Lower pH tends to decrease surface blocking of the anode and affects the surface diffusion layer to the extent that higher reaction rates are possible. The darker colored sludge suggests that more unreacted magnesium is sluffed from the anode surface, lowering the system energy density.
Increased conductivity appears to provide higher initial reaction rates which in turn results in decreased energy density. In addition, the buffering quality of seawater maintains the electrolyte pH at a lower value, sustaining the reaction rate for longer periods of time.
Previously discussed tests have shown that the magnesium reaction rate increases as cell electrolyte temperature increases. Depending upon the volume and temperature of the initial charge of electrolyte the full scale diver heater (sized to provide 2,000 watts for 8 hours) will require a definite time period to reach operational temperature. During this time period (start-up time) the full energy of the cell is required to bring the electrolyte to operational temperature. With an initial charge of 12 liters of fresh electrolyte at F, start-up time to reach l60F is approximately 30 minutes. In an effort to reduce the start-up time a series of tests were performed on cells with chemicals added to the seawater. The objective of the test was to increase the reaction rate of the cell during the start-up period. Only nonhazardous chemicals were tested in order to ensure a safe operational system for the diver. Of the additives tested, citric acid, citric acid/sodium citrate buffer, acetic and acetic acid/sodium buffer all yielded greater temperature increases with time, and thus greater rate of heat output. However, in all cases where rapid temperature rise occurred the cell electrolyte temperature reached a plateau where it either remained for a period of time or decreased. Also, the anode surfaces become coated with a sticky substance which effectively block the reaction.
It was concluded that while the additives tested increase the initial reaction rate the endresult was to reduce the final cell output to a below-normal level thus making the cell essentially inoperable.
A number of different materials were tested to determine the most effective cathode. For the diver heater, in order to conserve heat, the electrolyte is slowly added and removed from the cell, thus a considerable portion of the reaction products remain in suspension. With the iron screen cathode, magnesium hydroxide forms in the screen gaps and eventually blocks the cathode from electrolyte contact and the reaction decays.
Other cathodes tested included K Monel, titanium alloy (4% Al, 3% Mo, 1% V), Inconel, phosphorous bronze and 410 stainless steel. None of the materials tested proved to produce reaction rates exceeding 25 percent of the rate with theiron cathode. The iron cathode referred to throughout was fabricated from cold-rolled steel shim stock.
The theoretical presentation showed that the major reaction products are magnesium hydroxide and hydrogen gas. Since the solubility of magnesium hydroxide in seawater is very low the chemical remains in the electrolyte in solid form as the reaction proceeds. If no Mg(OH) is removed from the cell the electrolyte density increases. Another product is the unreacted magnesium which is sluffed from the plates and is coated with either MgO or Mg(OI-I) The magnesium hydroxide is generally soft in texture while the unreacted magnesium is coarse and granular in form. For a 2,200-watt cell operating for 8 hours duration, the estimated volumes of the reaction products based on 75 percent anode efficiency are shown below:
Mg 2H O Mg (particles) Mg(OI-I) H 3,630 5,250 9l0 2,725 2.5 X 10 (volume, cm
The hydrogen gas generated is vented continuously to the environment as the reaction proceeds. Fresh electrolyte must be added to accommodate both utilization of water (electrolysis) and change in volume between reactants and products. Tests have shown that approximately 80 cm of water per minute was required to maintain constant electrolyte level for the 2,200- watt heater. The need for a considerable portion of this water results from vapor carried off with the hydrogen.
To determine the results of increased electrolyte density (retaining reaction products) three cells were fabricated and run consecutively in the same electrolyte. A heat exchanger was used to remove power from the electrolyte. After 2 hours of operation the first cell was replaced by a second cell of identical configuration. A third cell replaced the second cell after another 1.75 hours. In each case the power output of the fresh cell was reduced from that of the previous cell. The results show a definite relationship between electrolyte condition and cell performance.
Decay of the power from a single cell has previously been shown to be a function of both increasing pH of the electrolyte and increasing plate spacing as magnesium is consumed. This accounts for most of the negative slope on a power curve. The difference in starting power between the cells is strictly a function of electrolyte condition. The major contributing factor for the decrease appears to be the pH increase. However, as the electrolyte density increases, movement of the fluid (velocity at anode surface) also decreases, thus slowing down removal of reaction products from the anode surface. The end effect is the same as that of increased pH which results in increased anode surface blocking and decreased power and efficiency.
Some portion of the electrolyte containing reaction products must be continually removed and replaced with fresh electrolyte in order to maintain constant electrolyte pH and density. In this way the upper limit of power can be maintained at acceptable levels.
The effect of pH level on reaction rate was further noted in the full-scale tests described later. Readings of pH were taken at regular intervals during an 8-hour period. It was found that with a plate spacing of from 0.09 to 0.1 1 inch, the power density can vary from less than 0.9 watt/in. to greater than 1.5 watts/in. depending upon the electrolyte pH.
It has been shown that a number of factors reduce the output of a given heater cell as the reaction proceeds to completion. In order to provide the diver with constant source of heat it is necessary to control these factors to acceptable levels. The actual heat needed by divers vary as a function of both individual requirements and environmental conditions. Since it is not practical to design the final heater for an individual or to be operational under all environmental conditions, it is necessary to design the heater to provide sufficient heat over a range of environmental conditions. Thus it is desired that heat cells capable of providing different amounts of power such as 500, 1,000, 1,500 and 2,000 watts be available. Excess heat developed can be rejected to the environment. The heat cell must accommodate for changes in power due to increasing plate spacing from magnesium consumption and degradation of the electrolyte from reaction products.
Most of the degradation of power due to electrolyte condition (increased pH etc.) can be corrected by the controlled addition of fresh electrolyte and removal of reaction products. The expelled products can be used to preheat the entering fresh electrolyte by utilizing an economizer heat exchanger. With this system, loss of heat carried by reaction products can be minimized.
Analyses of previously discussed tests have shown that plate spacing can be used as an effective means of controlling the reaction rate. Further, since anode depletion must be accommodated, variable plate spacing can be utilized to counteract this and, in addition, provide a controlled reaction rate.
The variable-plate-spacing method type cell arrangement is shown in FIG. 3. Within housing 30, beryllium copper springs 31 are used between the cathodes 33 and anodes 34 to maintain an even gap 35 between each of the plates. The backup plate 36, tension plate 37, and adjustable steel bolt 38 through the center of the cell are used to accommodate variable plate spacing. The short circuits are provided by brass shim stock 39 riveted to the magnesium plates and soldered to the steel plates. The brass short-circuit strips 39 are in a flexible form which adjust to the spacing between plates 34 and 35. Adjustment of the electrode gap is accomplished by rotating the knob 40 attached to bolt 38.
Beryllium copper conically wound compression springs and brass shim stock are used since both materials have a low susceptibility to hydrogen embrittlement. The large-diameter coils of the springs may be soldered to the cathode to keep them in place. The reaction at the anode/beryllium copper interface is very slight and full spacing adjustment can be accomplished throughout the cell life.
Another technique for control of cell output is to vary the contact resistance between the anode and cathode. An increase in resistance between cathode and anode reduces electrical current flow and thus reduces the reaction rate. For example, iron washers can be placed between cathode and anode at the center adjustment bolt. Contact resistance is varied by adjusting the torque on the adjustment knob. However, only limited control during the early hours of operation is available with this technique because the contact area becomes contaminated with reaction products which significantly reduce current flow.
Limited success with the variable-contact-resistance cells demonstrated the possibility of controlling the heat output of the cell by providing an adjustable external load resistance in place of the normal anodecathode short circuit. A number of experiments performed to demonstrate the feasibility of this approach showed that an external electrical load will not allow extraction of maximum internal power no matter how small the electrical resistance of the load. To extract maximum internal power for a given plate separation requires direct internal shorts which are in direct electrical contact with all the plates of the cell (assuming a cell consisting of alternatecathode and anode plates is used). If each short is in electrical contact with only the anode plates and the cathode plates, then, as a re-.
sult of relatively longer current paths, the internal power yield is only about 50 percent of the maximum attainable with the internally short-circuited cell. The springs, as illustrated in FIG. 3, gave superior results. Other suitable means for varying the plate spacing, however, can also be used.
Major parameters which determine the cell design are: operating temperature; plate spacing; anode composition; and, reaction products. All of these parameters interact and affect the power density (watts/in?) An operating temperature of 71C (160F) and a plate spacing of 0.1 inch, for example, were selected to optimize the energy density (watt-hr/lb) of the magnesium anode. To accommodate anode depletion, the variable plate-spacing system was devised, as shown in FIG. 3. Springs 31 placed between the anode and cathode provide even spacing throughout the cell, as previously indicated. The thin brass strips 39 attached to the plates provide the short circuit, while stiffeners 36 and 37 on the outer plates of the cell provide even load distribution over the cell. Threaded rod 38 projects through the center of each cell plate and adjustment is made by turning knob 40 attached to the opposite end of the threaded rod. This electrode assembly is made as a replaceable cartridge and can be replaced when expended by removing bottom plate 41 from housing 30 and knob 40.
Several anode alloys were tested and their reaction products examined. Of the anodes tried, only three produced the required power.
1. high-purity (99.9%) Mg 2. A261 alloy (93% Mg, 6% Al, 1% Zn) 3. AZ3lB-O alloy (96% Mg, 3% A1, 1% Zn) The products of the high-purity magnesium adhered to the cathode and slowed the reaction. The A261 alloy is good from the standpoint of energy density and reaction products, but is not commercially available in the plate thickness required for 8 hours of operation. AZ31B-0 alloy has a light reaction product which does not stick to the cathode but remains in suspension in the electrolyte. AZ3 lB-O alloy is the best commercially available anode material.
As shown in the embodiment of FIG. 3, seawater is made to enter the heater via inlet 43 where it is used as the electrolyte. Since the electrodes are short circuited, all the energy of the reaction including exothermic heat produced from the corrosion process is generated within the heater assembly. This heat is transferred to the seawater in spaces 35 between the electrodes and passes to the top of the unit where it warms liquid (e.g. water) that flows through heat exchanger 46 to the divers suit. A circulation pump 48 is used to pump the liquid through heat exchanger 46 and to divers suit 10 via tube 47. The expended seawater electrolyte, which includes reaction by products such as MgOl-l, is expelled via outlet 49. A separate pump within pump housing 48 can be used to control the flow of seawater electrolyte through the heater unit 12. Return water from the divers suit 10 re-enters heat exchanger 46 via tubing 50.
There are several methods that can be used to pump the fluid from the heat source to the divers dress. The circulation pump can be a centrifugal or diaphragm type and is primarily dependent on the source of energy for driving the pump. A low voltage battery could best power a solenoid actuated diaphragm pump. A centrifugal pump could be operated with a higher voltage source. Finally, a diaphragm type pump or turbine driven pump could be powered with the by-products H gas from the electrodes corrosion process.
Batteries for powering circulation pumps 48 can be located in the divers SCUBA pack 14 and connected to the pumps via an electrical conduit 52.
Seawater is allowed to enter the system by opening a valve at inlet 43 and one at outlet 49. When the seawater electrolyte within the heater reaches a temperature of 71C (F), for example, the circulation pumps are turned on and the electrolyte flow rate is adjusted to supply the heat energy needed by the diver.
As an alternative, where the MgOH is completely isolated from the divers skin, or by filtering the MgOI-l from the seawater electrolyte, the heated seawater can be pumped directly to the divers dress, thereby eliminating need for a heat exchanger and separate circulating fluid.
The reaction of magnesium and seawater in the present system can provide sufficient, reliable and safe heat to sustain divers in even the most demanding heating situations.
Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
1. Apparatus for heating an undersea diver comprismg:
a. a divers suit having passage means therein for circulation of a heated liquid throughout said suit adjacent to the divers body;
b. said passage means having an inlet and an outlet thereto;
c. a heating unit on said divers suit for heating and circulating said liquid;
(1. said heating unit including a chamber having a heated fluid outlet communicating with the inlet to said divers suit circulation passage means and a fluid inlet communicating with the outlet to said divers suit circulation passage means;
e. pump means for circulating heated fluid from said heating unit through said divers suit circulation passage means;
f. a seawater electrolyte inlet means to said heating unit chamber;
g. an electrochemical heat source assembly Within said heating unit chamber, said heat source assembly comprising:
1. a plurality of magnesium anodes and metal cathode plates alternately positioned in equally spaced apart relationship to allow free passage of seawater electrolyte and reaction products;
2. means for short-circuiting said anode plates to said cathode plates, where upon admission and circulation of seawater electrolyte into said heating unit heat is generated by the reaction of said magnesium anodes with said seawater;
3. said heat of reaction being imparted to said liquid which is circulated throughout said divers suit;
g. spacing control means for varying the electrode gap between said anode and cathode plates to thereby control the rate of reaction and heat output 2. Apparatus as in claim 1 wherein a heat exchanger is provided in said heating unit through which the fluid which is circulated throughout said divers suit passes for having said generated heat of reaction imparted thereto.
3. Apparatus as in claim 1 wherein pump means is provided for circulating the seawater electrolyte through said heating unit.
4. Apparatus as in claim 1 wherein an outlet is provided to eliminate expended electrolyte and reaction byproducts from said heating unit.
5. Apparatus as in claim 1 wherein the heated electrolyte is the liquid circulated throughout said divers suit.
6. Apparatus as in claim 1 wherein equal spacing between said anode and cathode plates is provided by means of conically wound beryllium copper springs.
7. Apparatus as in claim 1 wherein the short-circuits between said anode and cathode plates are provided by thin brass strips.
8. Apparatus as in claim 1 wherein said cathodes are iron.
9. Apparatus as in claim 1 wherein said magnesium anodes are made from a magnesium alloy consisting of 96 percent magnesium, 3 percent aluminum and l percent zinc.
10. Apparatus as in claim 1 wherein said magnesium anodes are made from a magnesium alloy consisting of at least 93 percent magnesium.
11. Apparatus as in claim 1 wherein said spacing control means comprises spring means between said anode and cathode plates and a threaded rod which passes through said anode and cathode plates which allows the plates to be drawn together as the anodes are consumed by reaction with the seawater electrolyte.
12. Apparatus as in claim 1 wherein said heat source assembly is replaceable within said heating unit chamber, whereby different capacity assemblies can be used and consumed assemblies can be removed and replaced.