Cooler Design

In principle Stirling engines may be air-cooled or water-cooled just as internal combustion engines are. However we have seen earlier that because the exhaust stack loss must be low. the cooling system of a Stirling engine must handle up to twice the load imposed on the cooling system of an internal combustion engine of similar power output. In addition to this, the efficiency of a Stirling engine falls markedly as the cooler temperature increases and is accompanied by deterioration in the mechanical properties of the polymer materials commonly used for sliding seals. Therefore it is desirable to have the cooler temperature at the minimum possible value.

This combination of factors makes direct air cooling of Stirling engines virtually impossible except in small model engines or larger but unpres-surized low power, slow-running engines intended for long unattended operation.

Indirect air cooling is therefore obligatory for engines of moderate to high specific output using an intermediate liquid cooling loop connecting the engine and air/liquid 'radiator (actually an atmospheric forced-convective heat exchanger). There is no particular advantage to be had by using liquids other than water except in cold climes where a mixture of water and ethylene glycol or alcohol may be necessary.

Water has excellent heat transfer properties and unbelievably high rates of heat transfer can be achieved with very moderate temperature potentials between the fluid and the wall. As a conscquence the limiting heat transfer process will most likely prove to be the internal convective heat transfer between the workinu fluid and the cooler walls of the tubes or fins. Just as much careful attention must be given to the design of the cooler as to the heater despite the initial appearance of the heater as the challenging unit using exotic materials and operating at the technological limit. As an example Kitzner (1977b). in discussing the design of the Philips/Ford Type A 98 double acting engine with four cylinders and a swashplate drive, indicated Lhat 72 heater tubes were sufficient but that 2440 tubes of 0.9mm bore were considered necessary in the cooler. Of course the tact that the cooler operates near ambient temperature allows the use of lower cost metals (e.g. aluminium) and joining techniques (adhesives) not appropriate for high-temperature use.

REGENERATIVE HEAT EXCHANGERS Ideal regenerator

Ideal regeneration was assumed in our previous discussion of both the Stirling cycle and Schmidt cycle of operation. Ideal regeneration is achieved when the fluid entering and leaving the matrix does so at one of two constant temperatures, Tn at the expansion end and Tc ai the compression end of the matrix. This is possible only if operations are carried out infinitely slowly, if the heat-transfer coefficient or the area for heat transfer is infinite, or if the heat capacity of the fluid or matrix is zero or infinite, respectively.

In both the Stirling and Schmidt cycles there is no difference in the instantaneous pressure across the matrix, so that the ideal regenerator has no fluid friction. Further, in the case of the Stirling cycle, the void volume of the matrix is zero. In the Schmidt cycle, the void volume is an independently-chosen parameter, and is considered part of the total void volume of the system.

The form of the temperature field in the regenerative matrix is not significant for either the Stirling or Schmidt cycle, but is usually represented as a linear, or transitional, function along the length of the matrix, it is important in tire Schmidt cycle, because the effective temperature of the dead space 7'D is always taken as the arithmetic mean of the constant temperatures 7'H and Tc.

Practical regenerator

The regenerator in a practical engine operates under conditions far removed from those assumed for the ideal case, discussed above. The temperatures of the working fluid at the inlet to the matrix are not constant, but vary with cyclic periodicity, because the processes of compression and expansion are not isothermal. The temperatures at the exit from the matrix are also variable, not only because of the inlet periodic-

Solar Stirling Engine Basics Explained

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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