Compared to river dams, tidal-power projects are very expensive, since massive structures must be built in a difficult saltwater environment. As an example of the cost of setting up, a proposed 8000 megawatt tidal power plant and barrage system on the Severn Estuary in the UK has been estimated to cost US $15 billion, while another in the San Bernadino strait which would produce 2,200 megawatts as a tidal fence in the Philippines will cost an estimated US $3 billion. The relatively low head of water above the turbines restricts the capacity of individual generators to about 25 to 50 megawatts; therefore, many machines are needed to produce a significant block of power. The machinery also has to withstand the rigors of saltwater operation. For all this investment, the average electric power output is severely limited by the twice-daily ebb and flow of tides: average output of tidal electricity is less than 40% of the installed generating capacity; whereas production of power from river dams typically averages 70-100% of installed capacity. The lunar cycle of 24 hours 50 minutes means the raw production of tidal energy moves in and out of phase with the normal, solar-oriented daily pattern of electrical consumption. Unlike the energy from river dams, the daily, monthly and annual availability of tidal energy is fully predictable, but it must be integrated with other sources of generation that can be adjusted to accommodate the fluctuations of tidal generation. Finally, optimum tidal-power locations are site-specific, which often means that lengthy power transmission lines must be provided to move the power from the tidal-power site to the end user. At present the untapped world-wide potential of tidal power is thought to be about 1 Terawatt (1 x 10^12 watts).
How many 1,000,000 watt (1 mW) tidal turbines would be required to utilize the 1,000,000,000,000 watts (1 TW) potentially available from the tides every hour here on Earth, assuming that the tidal turbines are operating at 35% (0.35) efficiency? And how much would it cost for that many tidal turbines?
1,000,000,000,000 watts/(1,000,000 watts per turbine x 0.35) = 2,860,000 turbines
Each turbine costs about $2,270,000 installed.
$2,270,000/turbine x 2,860,000 turbines = $6,500,000,000,000
Excuse me? $6.5 TRILLION dollars, batteries NOT included! ...and you WOULD need batteries - LOTS of batteries - or some other system for smoothing out the periodic nature of the tides.
It's a pretty scary number, but just for future comparison purposes, to eventually put this number into perspective, let's remember that this also happens to be the cost per Terawatt: $6.5 Trillion/Terawatt (remember this number!)
Energy produced from fossil fuels in 2008 was 15 times the total potential energy that will ever be produced from tidal power.
Thus, the development of tidal power could make a substantial contribution in replacing the total energy produced from the burning of fossil fuels, but the the site-specific nature of tidal power and its irregular patterns of power production suggest that many other resources will need to be exploited as well.