7 Heat Exchangers In Stirling Engines

introduction

Heat exchangers arc key components in Stirling engines: their significance cannot he overemphasized. No engine can work properly with poor heat exchangers although, conversely, the best heat exchangers will not, of themselves, make good an otherwise poor engine.

There may be four separate heat exchangers in a Stirling engine system. These are illustrated in Fig. 7.1 and include, for the prime mover converting heat to work: (a) heater fb) regenerator (c) cooler td) exhaust/inlet-air preheater.

For the refrigerator, utilizing work to produce cold at low temperatures, a different terminology must be used. The heater becomes the •freezer' and the exhaust/inlet-air preheater becomes the 'precooler'.

For a Stirling engine operating as a heat pump, a yet dilferent terminology is appropriate. The heat pump utilizes work to elevate the temperature of heat supplied at near-atmosphere temperatures. In this application the heater of the prime mover becomes the 'absorber' of the heat pump and the cooler of the prime mover becomes the 'heater' of the heat pump.

In all cases whether the engine operates as prime mover, refrigerator or heat pump the direction of heat flow is the same: to the expansion space and from the compression space. Only the temperatures arc different: (a) In the engine the heater operates at a high temperature, near the metallurgical limit, and the cooler operates at ambient temperature. (b> In the refrigerator the freezer is at the low refrigerating temperature and the cooler operates at ambient temperature, (c) In the heat pump the absorber operates at ambient or heat supply temperature and the 'heater' delivers heat at an elevated temperature.

In principle, therefore, the primary considerations for the heat exchangers are similar for all applications of Stirling engines. With this preamble we shall hereafter confine our attention to the prime mover application.

function of the heat exchangers

The function of the heat exchangers is vcrv simplv to exchange heat.

Combustion chamber

Combustion chamber

American Combustion Technology Handbook
Fig. 7.1. Location of heat exchangers in Stirling engines

and supplies heat to the engine working lluid contained within the working space of the engine adjacent to the expansion space.

The cooler does just the reverse. It absorbs heat from the working fluid adjacent to the compression space and rejects this to the engine cooling medium (air or water) at the sink temperature.

The regenerator functions as a thermodynamic sponge alternately accepting heal from or rejecting heat to the working lluid. li can be visualized as a kind of 'storage lagoon' of energy but of a rather special kind, for the temperature ranges progressively over the whole range from cooler to heater temperature. Energy flows in the regenerator are very high, usually three or four times the energy flow in the heater.

The e.v/iausf- ga,v/inlet-air preheater serves to increase the temperature of fresh air entering the engine en route to the combustion space. This can be achieved utilizing the thermal energy contained in the combustion products leaving the combustion space at a high temperature. In this way fuel is saved. The amount of fuel necessary to raise the inlet-air to combustion temperature is reduced and so the engine efficiency is increased.

The exhaust-gas/inlet-air preheater is not essential to the operation of the engine in the same way the heater, cooler, and regenerator are. The

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It the engine's thermal source is a radioisotope, or solar or stored heat, instead of fossil fuel combustion, no inlet-air is necessary and so of course the preheater can be eliminated. Presently most Stirling engine applications use heat derived from the combustion of fuel in air and in most cases a preheater is essential.

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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