ReviewEfficiency determination of tubular solar receivers in central receiver systems
Introduction
Solar input power and thermal output power need to be known to determine the efficiency of solar tower receivers. While thermal power is calculated by measured mass flow and temperatures, solar input can be determined among others by indirect measurement. Employing a camera-target method, widely common in R&D applications (Lovegrove and Stein, 2012), a brightness distribution on a Lambertian surface (“target”) is measured using a CCD (Charge Coupled Device) camera and direct flux sensors for calibration (Neumann and Groer, 1996, Lüpfert et al., 2000, Ulmer, 2003). Incorporated targets are either uncooled but moving or fixed but actively cooled to avoid overheating and therefore requiring additional maintenance and certain installation effort. For large scale receivers and commercial application new approaches have been proposed to reduce cost and increase reliability (Röger et al., 2014).
One of the proposed methods is based on ray-tracing simulations supported by indirect measurement of the solar flux around the receiver. The flux around the receiver is easily measured on the radiation shield, while the flux in the aperture area of a cavity receiver is not. Thus, the spillage flux around the aperture and not the solar input power into the aperture is measured. Instead of a target passing in front of the receiver aperture (“moving bar”), the front protection around the aperture is used as reflecting surface. In parallel, the flux distribution is simulated with ray-tracing and results are compared with the taken image. The best fitting simulation is then used to calculate the input power into the receiver. The principle of the method is shown in Fig. 1. This method has been validated with moving bar measurements in a pre-study showing promising results (Röger et al., 2014).
This approach is especially of interest for the evaluation of cavity receivers where no measurement surface is available in the aperture. Measurement of the spillage flux helps to close this lack of information. Thus, the simulation supported by indirect measurement has been chosen to be applied on the SOLUGAS demonstration plant to determine the solar input power.
This paper describes the detailed work flow of the efficiency determination of the SOLUGAS receiver. Major focus is given on the solar input power determination and the achieved improvements of this method. Additionally, test plan and evaluation of measured data including detailed uncertainty propagation are presented and conclusions discussed.
Section snippets
System
The SOLUGAS system is a demonstration power plant of a solar hybrid gas turbine system (Korzynietz et al., 2012). The system is located at the Plataforma Solar Sanlúcar la Mayor close to Seville, Spain. It consists of a 77 m high tower and 69 heliostats of 120 m2 mirror surface each. Gas turbine and receiver are situated in the topmost part of the tower at 65 m height (Fig. 2 shows the tower with heliostats focussed on the receiver and on the calibration target below). The turbine is a commercial
Methodology to determine receiver efficiency
To determine the thermal efficiency ηth,rec of the receiver, several steps are conducted successively. These steps are visualized in Fig. 4 and described in detail in the following section where numbers and letters refer to the description in the figure. The given values in italic are results of an evaluated test day explained in Sections 4 Results, 5 Discussion of measurement results and uncertainties.
Three intermediate results are required to determine the thermal efficiency: (A) thermal
Results
The methodology described in Section 3 is applied on all evaluated tests. This paper highlights the methodology and demonstrates the steps of the evaluation on an exemplary test (June 20th 2013 between 13:15 h and 13:25 h local time). Results from all further evaluated tests are given in Section 4.4 and in the publication Solugas - Comprehensive Analysis of the Solar Hybrid Brayton Plant (Korzynietz, 2016).
Discussion of measurement results and uncertainties
Several factors influence the quality of the efficiency determination. Besides uncertainties from hardware and image acquisition mainly the quality of input data into simulation and the comparison with the image has high influence on the results. For all measured values, uncertainties are considered.
If not specifically mentioned, uncertainties of measurements are obtained by evaluating the data of June, 20th. For some sources more than one test day is taken into account to increase the data
Conclusions and outlook
A methodology for evaluation of thermal receiver efficiency for a central tower receiver system based on a new approach for determination of solar input power has been applied. In the period from March 2013 until March 2014 sixteen tests have been evaluated.
Acceptable uncertainty in solar input power of −1.3%…+6.3% (respectively −2.3%…+7.3% for days with high wind) and thermal receiver power of ±2.4% is achieved that leads to an overall uncertainty of the thermal efficiency of −2.8%…+7.7%
Acknowledgment
The authors would like to thank the European Commission for funding and supporting the project under the 7th Framework Program with grant agreement number: TREN/FP7EN/219110/“SOLUGAS”.
The authors also thank the whole operating team of the SOLUGAS plant, Antonio Abad Jimenez, Azucena del Río, Manuel Quero, Christoph Hilgert, Christoph Prahl, Bijan Nouri, Anne Schlierbach, Stefan Wilbert and Ralf Uhlig for their valuable contributions.
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