CHEMISTRY OF SOLUTIONS
FAVOURS ENERGETICS
Modern power engineering is mostly thermal and based on nonrenewable sources, hydrocarbons and uranium. The part of energy based on renewable sources is little and the increase of the part can’t be foreseen for many reasons which we are not going to consider in this article. Promising thermonuclear sources of thermal energy are still at the development stage and their cost will be quite high.
Existing systems of conversion of thermal energy into electric one are based
on vapour-water thermodynamic cycles and their efficiency is limited to 30 %
for atomic, 52 % for supercritical coal and 63 % for combined turbo-boiler
kettle cycles working on natural gas.
However
there exist real promising ways of considerable efficiency increase of energy
conversion cycles along with operating temperature reduction and
determinative factor here will belong to chemistry of solutions and membrane technologies.
Relatively
small heat efficiency is the peculiarity of solution of crystalline materials
and other substances in solvents. In addition osmotic pressure is frequently
large and can be calculated with the help of Van’t Hoff law. Besides osmotic
pressure depends on solution temperature. At that it’s necessary to mention
that in process of dissolving values of heat efficiency are greatly less than
thermal costs required for vaporization of solute. Heat efficiency of solute
release out of solutions are the same as we have at dissolving but with
reversed sign. So the use of osmotic pressure in energy generation cycles is
much more profitable owing to small loss of heat which should be drop into
the environment on the low-temperature part of the cycle in order to get a parent soluble substance and a solvent out of solution.
Let’s
briefly explain the principle of working of such a cycle even for a layman to
understand it. For the purposes of
illustration we’ll take usual sodium chloride NaCl. A fair amount of heat is
necessary to evaporate a mole of its crystal but in water it will evaporate
with extremely low heat consumption: water breaks off combinations in a crystal
and the total heat efficiency of dissolution is rather low. According to Van’t Hoff law the osmotic
pressure of the solution will be as though vaporous salt is placed in empty
volume which is equivalent to the volume of water in which the salt was
dissolved. We’ll need two selective membranes which let in only water but not
salt. So let’s take two capacities coupled by tubes; the capacities are
separated by membranes. On the one side of the membranes there is salt in
water, on the other side there is pure water; compartments with salt in
capacities are coupled by tubes, compartments with water are coupled by tubes
too. There is no stir at all as each of the saturated solutions tries to suck
in water through the membrane on its side with equal forces (the osmotic
pressures are equal). Now let’s place one of the capacities on ice and the
other one we’ll heat. We’ll see that the solution and water shift along the
assembled contour and the salt will gradually shift from the capacity with a high
temperature to the capacity with a low one.
The
reason for this is that the osmotic pressure of the solution is higher at the high
temperature than at the low one and water will be pressed out through the membrane
out of the solution in the low-temperature capacity as local osmotic pressure
of the saturated solution is lower here. As a result a flow of solution is
formed from the high-temperature capacity to the low-temperature one. There’s
one more thing to be done – to organize the conveying of salt from the
low-temperature capacity to the high-temperature one (or simply change their
roles), to put hydraulic motor into the tube with water, the tubes themselves
must be placed into oncoming heat exchanger where water will become warm and
the solution will get cool in the
approaching movement. We’ll pay our readers’ attention to the fact that steam
is not used for work of such conversion energy cycle and consequently we do not
need expensive and complicated in manufacturing steam turbine; hydraulic motor or
water turbine will be quite enough.
However
there is a good possibility to improve essentially even this scheme. The matter
is that different substances dissolve with different thermal effects and generated
osmotic pressures are often quite different from theoretically predicted by
Van’t Hoff law. There are substances which require fairly large expenses of
thermal energy for their dissolving and generate lower osmotic pressure;
and there are substances which require small expenses of thermal energy for
their dissolving but they generate higher osmotic pressure. It depends on the
peculiarities of dissociation of substances in a concrete solvent. There is one
more possibility: at one and the same thermal effect of dissolving the
solutions can generate different osmotic pressures. So if
we use such two substances for one solvent and bring them together into one
common contour then we can get a system converting thermal energy into electric
one with efficiency exceeding 85%. It’s
necessary to say that nothing prevents us from making an assembling on
different contours with different solvents connected by heat exchangers and
mechanic energy transmission.
Let’s
briefly explain work of such a scheme. So we have two substances with different
thermal effects and similar osmotic pressures. If we combine such two solutions
with the help of the selective membrane, letting in only solvent, the solvent
will not shift in these solutions. Now we couple such two capacities with
membranes by tubes in this way: capacities with the solution of one type will
be joined with the help of such a tube, capacities themselves will be at
different temperatures. There won’t be any shift of the solvent and we’ll have
to use a pump. But let’s pay attention to the fact that the energy consumed by
this pump is quite small in comparison with the energy which this pump will be
able to lift at a high level. The matter is that this pump will simply have
to overcome membrane resistance and drop in several atmospheres is necessary
for this. It means that heating factor will be very large and temperature drop
may account to 100 -150° C or even more. Such drop will help to organize the
thermodynamic cycle with the application of generative osmotic contour as was
mentioned above. But it’s not compulsory for capacities with solutions to be
joined to each other with the help of one membrane; they may be joined with the
help of two membranes and tubes passing the solvent and this will allow differentiate capacities
according to their temperatures both at the high and low temperature level.
This in its turn will compensate small differences in osmotic pressures or get
rid of additional generative contour, if we put hydraulic motor directly
into the contour.
It’s
quite evident that great variety is possible in realization of such
thermodynamic cycles.
All
advantages in application of such energy conversion cycles based on solutions
are quite obvious. For example, 80 % energy in
September 12th Pelipenko Andrey,
engineer
2006 Kolisnichenko Nikolay, engineer
Energy-of-membranes@yandex.ru
Energy-of-membranes@ya.ru
Energy-of-membranes@narod.ru