Receivers

The receiver absorbs energy reflected by the concentrator and transfers it to the engine's working fluid. The absorbing surface is usually placed behind the focus of the concentrator to reduce the flux intensity incident on it. An apertur e is placed at the focus to reduce radiation and convection heat losses. Each engine has its own interface issues. Stirling engine receivers must efficiently transfer concentrated solar energy to a high-pressure oscillating gas, usually heliu m or hydrogen. In Brayton receivers the flow is steady, but at relatively low pressures.

There are two general types of Stirling receivers, direct-illumination receivers (DIR) and indirect receivers which us e an intermediate heat-transfer fluid. Directly-illuminated Stirling receivers adapt the heater tubes of the Stirling engine to absorb the concentrated solar flux. Because of the high heat transfer capability of high-velocity, high-pressur e helium or hydrogen, direct-illumination receivers are capable of absorbing high levels of solar flux (approximately 7 5 W/cm2). However, balancing the temperatures and heat addition between the cylinders of a multiple cylinder Stirlin g engine is an integration issue.

Liquid-metal, heat-pipe solar receivers help solve this issue. In a heat-pipe receiver, liquid sodium metal is vaporize d on the absorber surface of the receiver and condensed on the Stirling engine's heater tubes (Figure 3). This results in a uniform temperature on the heater tubes, thereby enabling a higher engine working temperature for a given material, and therefore higher engine efficiency. Longer-life receivers and engine heater heads are also theoretically possibl e by the use of a heat-pipe. The heat-pipe receiver isothermally transfers heat by evaporation of sodium on th e receiver/absorber and condensing it on the heater tubes of the engine. The sodium is passively returned to the absorber by gravity and distributed over the absorber by capillary forces in a wick. Receiver technology for Stirling engines i s discussed in Diver et al. [2]. Heat-pipe receiver technology has demonstrated significant performance enhancement s to an already efficient dish/Stirling power conversion module [3]. Stirling receivers are typically about 90% efficien t in transferring energy delivered by the concentrator to the engine.

Solar receivers for dish/Brayton systems are less developed. In addition, the heat transfer coefficients of relatively low-pressure air along with the need to minimize pressure drops in the receiver make receiver design a challenge. The most successful Brayton receivers have used "volumetric absorption" in which the concentrated solar radiation passes through a fused silica "quartz" window and is absorbed by a porous matrix. This approach provides significantl y greater heat transfer area than conventional heat exchangers that utilize conduction through a wall. Volumetric Brayton receivers using honeycombs and reticulated open-cell ceramic foam structures that have been successfull y demonstrated, but for only short term operation (tens of hours) [4,5]. Test time has been limited by the availability of a Brayton engine. Other designs involving conduction through a wall and the use of fins have also been considered . Brayton receiver efficiency is typically over 80% [4,5].

Sodium

Figure 3. Schematic which shows the operation of a heat-pipe solar receiver.

Sodium

Figure 3. Schematic which shows the operation of a heat-pipe solar receiver.

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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