Heat transport and electrically conductive working fluid

TECHNICAL FIELD

This invention relates to a heat transport and electrically conductive working medium or fluid which is particularly useful for magnetohydrodynamic (MHD) genera¬ tors, also including such generators operating at low temperatures,below 100 C, for instance, thus making possible the utilization of solar energy. However, the working fluid according to the invention is also useful in other applications where the combination of a good heat transport ability and electrical conductivity is desirable.

TECHNICAL BACKGROUND

Heretofore, one have used as a working fluid for MHD generators a plasma as a flow and operating fluid. This means very high temperatures and high flow velocities, resulting in a number of disadvantages, not the least from a material point of view.

In an attempt to overcome these disadvantages and to make it possible to utilize solar energy as a power source, there has been proposed to use a sodiu -potassium- eutecticum as a working fluid; cf. for instance US-A-

-4191901 and Modern Power Systems Review, 1. (1981): April, Conway, A, "Small cool MHD generators could power the industrial and the third world", page 16 + 18, reference being had thereto with regard to the design and operation of MHD generators.

A working fluid of said last-mentioned kind is, however, difficult to handle, not the least from a security point of view (i.a. danger of fire) .

OMPI

^«

OBJECT OF THE INVENTION

It is an object of this invention to provide a novel working fluid of the kind mentioned in the intro¬ ductory paragraph which on one hand is an advantageous alternative to the sodium-potassium-eutecticum for

MHD applications, while giving a number of advantages, and on the other hand can be used in a broader sense due to its properties.

DESCRIPTION OF THE INVENTION

The above-mentioned object is achieved by a working fluid having the features defined in the appended claims. Thus, the working fluid of the invention is essen¬ tially characterized in that it is based upon a solution of ammonia NH, and one or more alkalic metals, such as lithium Li, sodium Na and potassium K, particularly lithium. The solution is preferably saturated or supersaturated. In the latter case, there is thus finely dispersed free alkalic component in the solution that can serve as a kind of "reserve supply".

The choice of alkalic component or components, in each particular case, can be made based upon the operating temper ture in- question and other conditions for the system in whic the working fluid is to be used. The working fluid of the invention has been found to be highly liquid or viscous, is a good heat transfer fluid and is a good electrical conductor, as will be clear from the following disclosure of a number of advantages of the working fluid. The specific heat (per kg) of the working fluid is about the same as for water, that is, very good _*

The working fluid has a low specific gravity (typicall

3 about 0.5 grams per cm ) and a low viscosity (typically about 0.1-0.-2 centipois at -30°C) . This means that the fluid will flow easily and with extremely low flowing losses, that

is, low "pumping power" is required.

*

The working fluid has very good electrical con- * ductivity, typically almost as good as that of mercury. The coefficient of temperature of the conductivity is 5 zero or close thereto. The working fluid has a very high electrone density which is advantageous, because in the present context current is passed not by ion migration but by electron migration.

The working fluid has been found to make possible 10 a high power density in MHD ;systerns, for instance, due to the fact that the sonic speed in the fluid is high.

The working fluid is a liquid both at operating temperature and at much lower temperatures, that is, also when the system is shut off but still pressurized, the 15 system being a MHD generator for instance. The working fluid easily can be drained from the system when opened. Remaining working fluid in the system, if any, will evapo¬ rate in a controlled way at the lower pressure prevailing at such a time, and the vapour produced can be taken care 20 of easily for instance by condensation. In other words, it is possible to remove the fluid completely from the system, for instance in connection with service or main¬ tenance.

According to an embodiment of the invention, the 25 working fluid can comprise also granulate or the like, such as grains or balls, of a foamed metal having good electrical conductivity, preferably better electrical conductivity than the rest of the solution. The metal ^ should not be attacked by ammonia. A typical size of the 30 grains or balls could be one or a few millimeter.

The foamed metal will be given a specific gravity (can be adapted by means of closed pores in the foamed metal) which is at least substantially the same as that of the remaining solution. 35 A working fluid according to the invention can be

used quite simply both in one .phase and in two-phase MHD generators _* Presently preferred examples of the working fluid according to the invention are a saturated ammonia-lithium solution of about 20 MPM and saturated ammonia-sodium solution of about 16 MPM, respectively, (MPM = moles percent metal) .

However, as mentioned, the working fluid of the invention is useful in other applications than MHD-gene¬ rators. Thus, to advantage it can be used as the heat transfer fluid of heating or cooling systems, especially sealed and pressurized systems, in particular when the working fluid is pumped by means of a pumping action achieved in an electromagnetic way.