heat source with other candidates is shown in Table 1 . 



Magnesium reacts with seawater according to Equation 1: 



Mg 



+ 2H 2 



Mg(OH), + H„ (gas) + Ah 



(1) 



where Ah is the heat of formation of the reaction. 



The theoretical energy density of this reaction is 1,885 W-hr/lb of 

 magnesium (14.9 MJ/kg). The reaction ordinarily proceeds slowly in 

 seawater, and heat is not released at a usable rate. But, by electrically 

 connecting the magnesium to a cathodic material, such as iron (forming 

 a galvanic couple), the reaction proceeds much more rapidly and liberates 

 heat at a usable rate. Similar systems have been developed as seawater 

 batteries. 



The CEL heater uses the same basic principle as the seawater 

 battery, but the battery's external load is replaced by a short circuit 

 to maximize the reaction rate. A simplified schematic model of the 

 reaction process is shown in Figure 1. Major steps of the process are: 



1 . Current flows from anode to cathode via the short circuit 

 because of the potential difference. 



2. Water is reduced at the cathode. 



3. Magnesium ions are formed at the anode. 



4. Hydroxide and magnesium ions migrate to a point where they 

 combine to form magnesium hydroxide. 



Chemical energy is converted into thermal energy by means of a 

 highly efficient electrochemical reaction. The energy given off by this 

 reaction heats the surrounding electrolyte. A more detailed discussion 

 of the reaction process is contained in Reference 12. 



The basic heat-producing element of the electrochemical reaction is 

 the dual-plate cell shown in Figure 2. The spacer washer provides both 

 an electrode gap and a short circuit current path. The electrode gap 

 provides for free passage of the electrolyte and removal of the reaction 

 products [H2 and Mg(0H)2]. As the magnesium is reacted, the anode 

 becomes thinner and the electrode gap increases. 



It is important for the spacer washer to provide a very low resis- 

 tance path (less than 10~3 ohms) for current flow. With a high resis- 

 tance path, part of the energy goes into inefficient electrical Jouie 

 heating, and the reaction rate* is reduced to unusable values in terms 

 of diver heating. 



Reaction rate is defined as power output per unit- 

 surface-area of anode. 



I 



J 



