When plasma electricity generators become reality

Almost everyone who was interested in energy heard about the prospects of MHD generators. But the fact that these generators have been in the status of promising for more than 50 years is known to few. The problems associated with plasma MHD generators are described in the article.

Story with plasma, or magnetohydrodynamic (MHD) generators surprisingly similar to the situation with fusion. It seems that you need to take only one step or make a little effort, and the direct conversion of heat into electrical energy will become a familiar reality. But another problem pushes this reality indefinitely.

First of all, about the terminology. Plasma generators are one of the varieties of MHD generators. And those, in turn, got their name by the effect of the appearance of an electric current when electrically conductive liquids (electrolytes) move in a magnetic field. These phenomena are described and studied in one of the branches of physics - magnetohydrodynamics. From here the generators got their name.

Historically, the first experiments to create generators were carried out with electrolytes. But the results showed that it is very difficult to accelerate the flow of electrolytes to supersonic speeds, and without this, the efficiency (efficiency) of the generators is extremely low.

Further studies were carried out with high-speed ionized gas flows, or plasma. Therefore, today, speaking about the prospects for use MHD generators, you must keep in mind that we are talking exclusively about their plasma variety.

Physically, the effect of the appearance of a potential difference and an electric current when the charges move in a magnetic field is similar Hall effect. Those who worked with Hall sensors know that when a current passes through a semiconductor placed in a magnetic field, a potential difference appears on the crystal plates perpendicular to the lines of the magnetic field. Only in MHD generators a conductive working fluid is passed instead of current.

The power of MHD generators directly depends on the conductivity of the substance passing through its channel, the square of its speed and the square of the magnetic field. From these relationships it is clear that the greater the conductivity, temperature and field strength, the higher the power taken.

All theoretical studies on the practical conversion of heat into electricity were carried out as far back as the 50s of the last century. And a decade later, pilot plants “Mark-V” appeared in the USA with a capacity of 32 MW and “U-25” in the USSR with a capacity of 25 MW. Since then, various designs and effective operating modes of generators have been tested, and various types of working fluids and structural materials have been tested. But plasma generators have not reached widespread industrial use.

What do we have today? On the one hand, a combined power unit with an MHD generator with a capacity of 300 MW at the Ryazan State District Power Plant is already operating. The efficiency of the generator itself exceeds 45%, while the efficiency of conventional thermal stations rarely reaches 35%. The generator uses a plasma with a temperature of 2800 degrees, obtained by the combustion of natural gas, and powerful superconducting magnet.

It would seem that plasma energy has become a reality. But similar MHD generators in the world can be counted on the fingers, and they were created in the second half of the last century.

The first reason is obvious: heat-resistant structural materials are required for the operation of generators. Some of the materials have been developed as part of the implementation of thermonuclear fusion programs. Others are used in rocket science and are classified.In any case, these materials are extremely expensive.

Another reason is the peculiarities of the operation of MHD generators: they produce exclusively direct current. Therefore, powerful and economical inverters are required. Even today, despite the achievements of semiconductor technology, such a problem has not been completely solved. And without this, it is impossible to transfer huge capacities to consumers.

The problem of creating superstrong magnetic fields has not been completely solved either. Even the use of superconducting magnets does not solve the problem. All known superconducting materials have a critical magnetic field strength above which superconductivity simply disappears.

One can only guess what might happen when the conductors suddenly transition to the normal state, in which the current density exceeds 1000 A / mm2. Explosion of windings in close proximity to plasma heated to almost 3000 degrees will not cause a global catastrophe, but an expensive MHD generator will fail for sure.

The problems of plasma heating to higher temperatures remain: at 2500 degrees and additives of alkali metals (potassium), the plasma conductivity, however, remains very low, incommensurable with the conductivity of copper. But an increase in temperature will again require new heat-resistant materials. The circle closes.

Therefore, all power units with MHD generators created today demonstrate the level of technology achieved rather than economic feasibility. The country's prestige is an important factor, but building massively expensive and capricious MHD generators today is very expensive. Therefore, even the most powerful MHD generators remain in the status of pilot plants. On them, engineers and scientists are working out future designs, testing new materials.

When this work ends, it’s hard to say. The abundance of various designs of MHD generators suggests that the optimal solution is still far away. And the information that the thermonuclear fusion plasma is an ideal working medium for MHD generators pushes their widespread use until the middle of our century.