On the selection of working fluids for OTEC power plants

This paper analyzes the effect of three different working fluids (ammonia, propane, and freon-114) on the size of OTEC heat exchangesrs and system performance. Seven different combinations of shell-and-tube heat exchangers are considered. For each combination, a simple computer model of the OTEC power system is used to compare the three fluids. The comparison is made on the basis of A/W_net, where A is the total heat transfer area (evaporator plus condenser) and W_net is the net power output of the plant. Overall, ammonia is shown to be the best fluid (i.e. it yields the lowest value of A/W_net), although in some cases only by a small margin. The thermophysical property that gives ammonia its general superiority is its relatively high thermal conductivity. The paper also discusses heat exchanger design problems associated with liquid entrainment and boiling liquid superheat.