About a sixth of the world’s electricity is produced by hydro-power and about seventy percent of its renewable energy. This renewable energy source exploits the energy potential of gravity, which is overcome by evaporation that converts water on a surface of a body of water into a vapor that rises to a higher elevation where it is precipitated and gains new gravitational potential.
A mass of water (in kgs) raised to a height (in meters) times gravity (10 m/s²) produces a potential energy expressed in Joules. Hoover Dam, for example, at a height of 221 meters, holding back a single kilogram of water, has an energy potential of 2210 Joules. Since the efficiency of the dam is about ninety percent this mass produces 1989 Watts of power given that a watt is one Joule per second.
The benefits of hydroelectricity are it is renewable, non-polluting for the most part, reliable, flexible and generally safe.
As with every energy source it has drawbacks; including environmental consequences like the destruction of arable land and consequences to fish populations, including methylmercury poisoning.
Hydro-electricity is expensive. The estimate for the Site C Dam project in British Columbia, rated at 1.1GW, is $8.3 billion, which is comparable to North American costs for nuclear power plants of similar capacity.
Electricity generation and energy prices are dependent on the availability of water, which as in the case of the Hoover Dam can be problematic.
Although roughly twenty major plants are under or near completion around the world in anticipation of the increased need for renewable energy, we are starting to run out of prospective sites for large scale facilities and as a consequence the Eurelectric group has put the global technical and economic potential of hydroelectricity at between 1.0–1.4 terawatts.
This is about one tenth the primary energy currently derived from fossil fuels and as a consequence Tom Murphy of the University of California, San Diego, in his blog “Do the Math – Using physics and estimation to assess energy, growth, options, puts “Dam Energy” in the “niche” category.
In a 2012 podcast, “Time to Be Honest With Ourselves About Our Looming Energy Risks – Simply not enough BTUs to meet rising global demand, Murphy “likens the earth to a battery having spent eons soaking up solar energy and storing it (in the form of hydrocarbons). Humans finally figured out how to tap into this battery ~200 years ago, and we’ve been drawing it down so quickly and violently within such a short period of time (an instant, geologically-speaking) that it’s akin to a short circuit. The big question is: What will life be like once this once-in-a-species planetary gift is gone?”
What he and most have overlooked is the fact that the earth’s solar battery continues to continues to store energy in the oceans, to excess, and is in fact is overcharging.
A temperature difference is analogous to the hydraulic head warehoused behind a hydroelectric dam. Each degree centigrade corresponds to a 427-meter head as is evident from the following diagram published by Robert Kreiger in The Journal of Nutrition.
My late colleague Dominic Michaelis once said of ocean thermal energy conversion, OTEC, which has at least ten times the global potential of hydro power; “It is as if much of the world’s ocean water were captured behind invisible “thermal dams” of significant heights.”
A difference of at least 21°C between surface temperatures and ocean depths of 1000 meters can be found in all tropical latitudes 30 degrees either side of the equator. These 21 degrees corresponds to hydraulic head of 8967 meters if 100% conversion efficiency could be achieved, which is not possible.
The Carnot efficiency for temperatures in this range is 1-277/298 or 7 percent, but for practical purposes this is usually halved even though my colleague Vicente Fachina has proposed a concept with an exergy efficiency of around 78% or about 5.5 percent of Carnot.
For argument sake, assuming only 2.5% efficiency, this means tropical waters have the equivalent to a head of 224 meters which is 3 meters higher than the Hoover Dam, which is 379 meters long, hold backs the largest reservoir in the United States, Lake Mead, which has a maximum depth of 162 meters, holds back 32.2 cubic kilometers of water and has a potential to produce 2 gigawatts of power.
The average annual solar radiation arriving at the top of the Earth’s atmosphere is roughly 1366 W/m2 but the atmosphere attenuates about 27 percent of this radiation leaving about 1000 W /m2 reaching the surface on a clear day. Oceans cover about two-third of earth surface, capturing about 70% of the solar energy; about 4000 times current human energy consumption.
Recent research concludes that between 7 to 30 terawatts of electric potential energy is available from the one third of the ocean with the appropriate surface temperatures without having any adverse impact on natural thermal currents and ocean temperatures and this is constant and reliable energy because the surface temperature barely fluctuates day or night or regardless of the season. This constancy of temperature is true of the ocean depths as well, which have about 300 times the volume of the surface and where the 97.5 to 95 percent of the surface heat not converted by the ocean thermal energy conversion process is relocated.
As noted in a recent AGU presentation, Negative-CO2-Emissions Marine Energy With Direct Mitigation of Global Warming, Sea-Level Rise and Ocean Acidification and a current Scientific American article this removal of heat from the surface counters ocean/atmosphere warming and sets up the conditions for a chemical reaction that sucks carbon dioxide from the air, generates hydrogen and produces ocean alkalinity that mitigates the chemical and biological effects of ocean acidification.
Professor Murphy claims “Anything whose growth is dependent on something that can’t grow will stop growing. Like our global economy. The way it is set up today, it must grow in order to function properly. But that growth is dependent upon ever more energy output each year to power it.” (Dominic Michaelis and I once entered GE’s Ecomagination challenge with an entry “The more energy produced the more the ocean’s surface is cooled”.) . . “We must either choose to transition now to new economic models that do not depend on growth, or be forced to do so later, once our current model stops working. Either way, our relentless demand for growth will end,” Murphy said.
Respectfully, the global economy can continue to grow and prosper with a process that removes heat from the ocean’s surface, sucks carbon dioxide from the air, generates hydrogen, produces ocean alkalinity and mitigates the chemical and biological effects of ocean acidification in one fell swoop. That in turn can be had for no more than the cost of nuclear or hydro power, with equal or better capacity yet with better sea level, storm surge and aquatic life outcomes.
Taking these challenges on discretely, in less than a holistic fashion, will be too time consuming, costly and ultimately will be an effort ultimately doomed to failure; to say nothing of dooming our grandchildren.