The different types of volumetric receivers that have been developed with major participation of DLR use ambient air as heat transfer medium. In commercial power plants the most commonly used heat transfer media are liquids (water, oil, molten salt) or solid particles.
The newly developed receiver types have already reached a maturity level that enables the technology transfer to industry partners for the deployment in commercial installations.
The objective of a receiver is to "capture" the concentrated solar irradiation and to pass the heat to a transfer medium air at temperatures as high as possible.
Because of the extreme temperatures from 700°C up to more than 1000°C, as well as the high radiation flux densities up to approximately 1 MW/m², there are high requirements on the durability of the materials used in the receiver. Therefore, high-temperature resistant ceramic is used as material for the components which are directly exposed to the highly concentrated solar radiation.
Open Volumetric Receiver
Several ceramic absorber modules absorb the concentrated solar radiation in the open volumetric receiver and are heated up to 1000°C. The ambient air that enters the open pores passes the ceramic structure and absorbs a large part of the heat. This hot air is used for firing the vessel in a water/steam circuit for electricity generation. In parallel, heat can be fed to a thermal storage.
The cooled down air from the vessel or storage is, together with residual heat at 100-150°C, recirculated to the receiver and led to the front of the irradiated receiver surface to be sucked in again. As not all of the air can be sucked in, it's a not closed air circuit.
As a simple scaling of different performances, the open volumetric receiver is constructed with few elements.
The absorber module is the central element of the receiver. It contains a porous absorber that is resistant to high temperatures and is embedded in a holding cup. The current technology consists of ceramic honeycomb structure made of silicon carbide captured in a cup of the same material. Also metallic absorbers made of suitable alloys may be installed. The solar irradiation penetrates the channels of the honeycomb structure, is absorbed at the inner surface and heats the absorber structure. The air flows through the channels in the irradiation direction and absorbs heat from the channel walls. The absorber reaches a temperature of more than 1000°C at its front side.
The single absorber modules are kept in a metallic carrier structure, which may be modularly expanded. The carrier structure is cooled down from the returning hot air before its being blown out in front of the receiver.
In order to enable operation of large receiver areas with inhomogeneous radiation, the airflow through the different receiver sections must be adjusted with throttling devices.
Pressurized Volumetric Receiver
The closed volumetric receiver, also called pressurized volumetric receiver, consists of an internally insulated pressure vessel that is closed by a dome-shaped quartz glass window.
There are ceramic or metallic highly porous "volumetric" absorbers inside the receiver. These absorbers operate in a similar way as the absorbers of the open volumetric
receiver: The concentrated solar radiation transmits through the quartz glass window and reaches the absorbing structure that is assembled behind the window in the vessel. The absorber structure is heated up by the radiation and passes its heat to the air that is flowing through it.
This receiver type is used in order to transfer solar energy to the heat transfer medium (air) in a high pressure module of a gas turbine process. The compressed air (10-15 bar) is led to the twin-wall pressure vessel of the receiver and passes by the quartz glass panel in front of the absorber. The air is heated with a metallic absorber material of ca. 600°C and a ceramic absorber material of above 1000°C. If needed, the air is heated up to the defined outlet temperature of the turbine in the downstream combustor.
The maximum size of these receivers is limited by its design. In order to achieve higher performance, several receivers have to be arranged next to each other and connected in parallel. Additionally these receivers are equipped with a secondary concentrator, which allows a gapless arrangement of several receivers in one tower.