High capital costs are first a function of the cold water pipe, which due to its size requires large surface infrastucture to support it. Luis Vega point out in "First Generation 50 MW OTEC Plantship for the Production of Electricity and Desalinated Water" to produce 50 MW with the cold water pipe 138.6 m3/s (142,300 kg/s) of cold water is required. Only 2,750 kg/s of anhydrous ammonia as the working fluid is required however and since the density of ammonia is 682 kg/m3 this equates to 4 m3. You can move as much heat in this 4 m3 of the working fluid as you can in 138.6 m3 of water and thus you need much less pipe and surface infrastructure. Since the vapor condenses in cold water at a depth of 1000 meters there is little to no fouling of the surfaces. As to the evaporator the fouling can be addressed with either chloronation or ozonation. The design shown at http://www3.telus.net/gwmitigationmethod/100MWPlant.htm uses the byproduct of sea water electrolysis for this purpose. The output of the original prototypes was small, though Vega claims they did obtain a positive return, but this is mainly due to the size of the plants. Most consider 100MW is required to maximize the thermodynamics and noone has ever demonstrated a system anywhere near that size.
As Paul Curto, former chief technologist with NASA puts it, "the parasitic losses (using the heat pipe design) are cut in half. The costs for the cold water pipe are eliminated, along with the cold water return pipe and condenser pumps, the cleaning system for the condenser, and the overall plant efficiency approaches 85% of Carnot vs. about 70% with a cold water pipe.
The parasitic losses could be reduced as much as 50% and the complexity, mass (and cost) of the system reduced by at least 30%. The vast reduction in operating costs and environmental impacts would be worth investigation alone."
In a nutshell, that is what is different this time, yet not one cent has ever gone towards the investigaiton that is required to genuinely solve the climate problem.