Abstract:
A thermoelectric generator having both a hot and a cold heat sink. Fast  r times, and stable output voltages for short duration, are accomplished using fluids at their melting points at the hot and cold junctions.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to thermoelectric generators. More particularly, this invention relates to a thermoelectric generator having both a hot and a cold heat sink. Still more particularly, but without limitation thereto, this invention relates to a thermoelectric generator having a fast rise time at high rates, with a relatively stable voltage and high reliability for short duration. 
     2. Description of the Prior Art 
     Presently used designs which require fast rise time and a high rate power source utilize a variety of methods which are neither reliable nor inexpensive. Thermal batteries which are capable of high voltages and high loads cannot give a fast rise time (≦1 second). Electromagnetic generators are unable to supply a stable voltage for any period over a few milliseconds. Conventional thermoelectric designs can provide only 50 to 100 milliseconds of power albeit with a fast rise time and high reliability. 
     SUMMARY OF THE INVENTION 
     According to this invention, a thermoelectric generator is designed to allow for a fast rise time at high rates with a stable voltage and high reliability for short duration, through the use of both hot and cold heat sinks, thereby creating a high temperature differential across the thermoelectric array. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE represents a cross-sectional view of a thermoelectric generator, the casing of which (shown in phantom lines) is standard in the art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention uses a thermoelectric element having inherent fast rise time and high reliability, that has both the hot side and the cold side of the thermoelectric array clamped at a fixed temperature for a period up to several seconds. 
     In the FIGURE, a high temperature differential across the SiGe thermoelectric array 10 is maintained by using heat paper 20 to heat the hot side material 30 in the hot side heat sink 40 adjacent to the hot side 5 of array 10. Heat paper 20 is ignited by means of electric match 50. 
     While heat paper (Zr/BaCrO 4 ) is preferred, pyrotechnic material or Fe/KClO 4  is also suitable for use with this invention. Heat paper 20 heats the hot side material 30 to its melting point, thereby maintaining the hot side 5 of array 10 at or near the melting point of hot side material 30. When all of material 30 is melted, hot side heat sink 40 acts as a thermal reservoir. The heat paper used must generate a minimum of 405 cal/g and must have a burn rate of at least 36.9 in/sec. 
     Hot side material 30 must be non-corrosive, chemically stable and have a melting point within the range of 700°-800° C. The preferred material is a low melting borate glass such as sodium tetraborate (Na 2  B 4  O 7 ). Other suitable materials include: alkali metal halides such as NaBr and the NaCl-KCl eutectic, borosilicate glasses, low melting phosphate glasses such as KPO 3  and NaPO 3  and carbonates such as K 2  CO 3 . 
     As the hot side material 30 is melting, the cold side material 60 in the cold side heat sink 70 adjacent to the cold side 15 of array 10, absorbs the entire heat of crystallization of material 30 in the heat of fusion of material 60 upon melting. In this manner the cold side 15 of array 10 is clamped at the melting point of the cold side material 60 until all of said material is molten. 
     Cold side material 60 must be chemically stable and have a melting point within the range of 50°-100° C. The preferred material is an aliphatic glycyl ester such as tristearin. Other suitable materials include: fatty acids such as stearic acid, organic waxes such as paraffin, silicone waxes such as silicone grease and low melting inorganic salts such as Ca(NO 3 ) 2 .4H 2  O. 
     Hot side heat sink 40 and cold side heat sink 70 are made of copper or steel. 
     By using both a hot side heat sink 40 and a cold side heat sink 70, a high temperature differential across the thermoelectric array 10 may be established. The preferred temperature difference should be within the range of 400°-700° C. Since the differential is proportional to output, a high differential provides a high, stable output voltage. 
     The output of the thermoelectric generator is also affected by the thermal array 10 which is comprised of a series of small elements formed by n-type and p-type junctions. Increasing the number of elements results in higher voltages while increasing the size of the elements yields more power. Therefore, the number and size of elements must be optimized given the allowed area constraints. 
     Table 1 provides calculated data for a thermoelectric generator having NaBr as the hot side material and tristearin as the cold side material. With a starting temperature differential of 695° C. (750°-55°), output voltages within the range of 36-42 volts are obtained. Table 1 further illustrates the effect of varying the number and size of elements in the thermoelectric array. 
     
                                           TABLE 1__________________________________________________________________________                       HEAT             NUMBERHOT SIDE  HOT SIDE         COLD SIDE                COLD SIDE                       PAPER,                            n-TYPE                                  p-TYPE                                        OF        OUTPUTTEMP, °C.  MAT&#39;L.sup.a, gm         TEMP, °C.                MAT&#39;L.sup.b, gm                       gm   SIDE, cm                                  SIDE, cm                                        ELEMENTS                                               ΔT.sup.c                                                  VOLTAGE,__________________________________________________________________________                                                  V750    66.2   55     64.3   12.6 0.32  0.29  184    430                                                  36750    62.5   55     60.0   12.0 0.29  0.27  214    444                                                  42__________________________________________________________________________ .sup.a NaBr .sup.b Tristearin .sup.c Temperature differential at the end of discharge 
    
     The thermoelectric generator described herein and in the FIGURE, allows for a fast rise time at high rates, with a stable voltage and high reliability for periods of several seconds. Discharge duration is limited by the design and efficiency of the heat paper 20 and the heat sinks 40 and 70. This invention, through the novel use of both hot and cold heat sinks widely expands the utility and capability of thermoelectric generators by providing a power source with applications wherever a large amount of dependable power is required rapidly for a short discharge period. 
     This invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.