30-03-2013, 04:46 PM
Blue Energy
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Introduction
Energy crises lead to technological breakthrough. Some of these breakthroughs survive. The charcoal crisis in England
centuries ago boosted coal mining and the development of the steam engine. More recently, the shortages of many
goods such as energy after the Second World War promoted nuclear energy supported by the ‘Atoms for Peace’ program.
The oil crisis in the seventies of the last century gave birth to modern wind energy technology. In all these periods of
energy crisis, attention was given to and development spent on a number of new energy technologies. Among them
we find coal gasification, nuclear fission, geothermal energy utilization, solar energy including concentrated solar
power (CSP), ocean thermal energy conversion (OTEC), tidal and wave energy and salinity gradient power.
Although world oil production is predicted to peak within the next five years, there is no direct energy crisis due to
shortage. Coal is still abundant throughout the world. Geopolitical instabilities in the OPEC countries in the seventies
and eighties were a second factor in promoting the search for alternatives. The same geopolitical argument has
continued in recent years and has reawakened interest in alternatives to oil.
But now a new incentive is contributing to the further development of other energy sources. After the initial „Limits
of Growth” document published by the Club of Rome 30 years ago, climate change is now the focus of political
attention. Reduction of greenhouse gases such as CO2, formed by the production of fossil-fired power generation, is
the main goal. A number of countries signed the Kyoto treaty in order to combat global warming. CO2 now has a price,
a negative price. Influenced by Al Gore’s movie „An inconvenience truth” the European Union is going even further
than Kyoto. In 2020, the „three 20%s” will be mandatory for all EU countries: 20% energy saving, 20% CO2 reduction
and 20% renewable energy. Each country can deviate slightly from the 20% renewable energy as part of the whole
energy portfolio, but this will also drive a third factor: the development of renewable – non-CO2 – energy sources.
A general characteristic of renewable energy sources is that they are derived from the sun and therefore intermittent,
rather diffuse and need large surfaces for harvesting. The largest surface of the Earth is covered by the sea and the
oceans, which capture most of the solar energy. Therefore we see that Ocean Power is one of the revived options of
renewable energy. Ocean Power includes OTEC, wave and tidal energy and salinity gradient power. Salinity gradient
power is also derived from the sun as can be seen when the hydrological cycle is considered. By solar irradiation and
heating of the salt water of the ocean a demixing takes place and fresh or sweet water is formed in clouds. A part of
this fresh water returns to the sea through rivers, where the fresh water is mixed again with the salt water and energy
is released.
Entropy of mixing
From thermodynamics and the formulas of Gibbs free energy, ΔG including the chemical potential μ, it is known that
a solution represents a lower chemical potential than the pure solvent. Nature tries to equalize the chemical potentials
of two different solutions in contact which each other in order to create maximum entropy. So the driving force for the
transport of a component, for example across a membrane between two solutions, is such a gradient in ΔG.
In order to obtain energy from salt water, two solutions of different concentration must be available. Such a salinity pair
might be formed from sea water, saline lakes or brines left from salt manufacture, coupled with a very low concentration
source such as river water. The energy that can be extracted from the two solutions is directly proportional to the
absolute temperature, T (K), and the logarithm of the ratio of their concentrations (activity ratios). By mixing 1 m3 fresh
water per second with an excess of seawater, considered as a salt solution with a salt concentration c=0.5 molar, the
maximum recoverable dissolution energy is ΔG = -2.35 MJ/s = -2.35 MW.
Vapor pressure
The vapor pressure of a salt solution is lower than that of pure water at the same temperature. This results in a higher
boiling point for salt water. People have known this for centuries.
If a dilute and a concentrated solution of brine are connected by a vacuum, the dilute solution will evaporate and
condense into the concentrated solution. In this way, the transport of vapor can be used to do work. However, the
process would rapidly lead to cooling of the evaporating solution, thus lowering its vapor pressure, and the evaporation
would anyway tend to equalize the concentrations and stop the process. A useable device should therefore return
the heat generated by the condensing vapor to the evaporating dilute liquid via a thin heat-conducting wall.
This would maintain the two solutions at almost the same temperature and, if the liquid in each compartment
is continuously changed, keep the concentration difference constant. Because the fluid leaving each compartment will
have a different concentration to that entering, it has been suggested that a multi-stage device could be developed.
Electricity would be generated by a turbine between the two compartments. This type of device is less developed
than the two membrane methods: reverse electrodialysis and osmotic techniques.
Osmotic pressure
In 1784 the French priest and physicist Jean-Antoine Nollet put a pig’s bladder filled with wine in a barrel of water.
To his surprise, the bladder swelled and finally burst. The osmotic energy was converted into an increase in pressure.
The Dutch Nobel Prize winner (1902) Van ‘t Hoff derived the formula for calculating the osmotic pressure П.
A device, see figure 1, that extracts energy based on the osmotic pressure uses a semi-permeable membrane through
which water can pass but not salt. Water from the compartment with the dilute solution enters the compartment
with the concentrated solution through the semi-permeable membrane and raises its level. The difference in height
achieved can then be used to drive a water turbine to produce electrical energy. In the fifties there was a growing
interest in producing potable water from sea water. A breakthrough was made by the American Sidney Loeb by
producing a semi-permeable membrane. Production of fresh water by reverse osmosis (RO) is now a major industry,
especially in the Middle East. The same membranes can be used in an installation for producing electricity by pressureretarded
osmosis (PRO). In Norway the PRO method is under investigation by Statkraft.
Reverse Electrodialysis
A second membrane method is based on reverse dialysis. It requires two types of membranes, namely one that
is selectively permeable for positive ions and one that is selectively permeable for negative ions, see figure 2. Salt water
separated from fresh water between two such membranes will lose both positive ions and negative ions. This charge
separation produces a potential difference that can be utilized directly as electrical energy. The voltage obtained
depends on the number of membranes in the stack, the absolute temperature and the ratio of the concentrations
of the solutions, the internal resistance and the electrode properties.