## QMor4182

where e = the emissivity. an optical property that is characteristic of the surface, i.e. a dull black surface has a high emissivity whereas a polished gold plale surface has a low emissivity. A = area of the body for heat transfer (this is the projected area looking out to space—it does not help much to have a convoluted or irregular surface with a large surface area), ir-a number, called the Stefan-Boltzmann constant. T=t'ne absolute temperature of the body.

The most important aspect of the above equation is to note that the quantity of heat transferred from a body by radiation depends on the fourth power of the temperature. Doubling the temperature increases the heat lost by sixteen times!

On spacecraft, radiators are used to ellcet the heat transfer to space. If lluids are used to transfer heat from the power-plant lo the radiators it is necessary to armour the radiators so as to prevent punctures by particles in space. Radiators therefore tend to be large, heavy units which provide a large area for heat transfer.

However, the rate of heat transfer increases only linearly with the radiator area whereas it increases as the fourth power with temperature. There is. therefore, great attraction in operating the radiator at high temperatures since it can be made smaller and lighter.

Unfortunately this trend is exactly opposite to the requirement for high efficiency. It will be recalled that the efficiency Tf = C(Tmux—Tmin)/7'maK. The maximum temperature Tmnx is limited by metallurgical considerations in the hot parts of the system and should be as high as possible. The minimum temperature Tmin is the temperature of heat rejection from the cycle, the radiator temperature and should be as low as possible.

In the design of space power systems there is, therefore, a dichotomy of interest in the minimum cycle temperature between a low value on the one hand and a high value on the other. Considerations of system weight are dominant. Frequently the radiator is the principal system mass, perhaps as much as 50 to 60 percent of the total system, but can be limited in size by the use of a high minimum cycle temperature which in turn results in a low cycle efficiency. There is great advantage in increasing the maximum cycle temperature and this may lead it) the use of relatively exotic materials for the hot parts that are too expensive or difficult to fabricate except for highly specialized applications.

## Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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