Material and Design Requirements for Advanced Concentrators

Transcription

1 Advances in Science and Technology Online: ISSN: , Vol. 74, pp doi: / Trans Tech Publications, Switzerland Material and Design Requirements for Advanced Concentrators ROBERT PITZ-PAAL a, ECKHARD LÜPFERT b, DLR Institute of Technical Thermodynamics, Linder Höhe, D Köln, Germany a robert.pitz-paal@dlr.de; b eckhard.luepfert@dlr.de Keywords: concentrating solar collector, parabolic mirror, solar-weighted direct reflectance, focus deviation, QUARZ-Centre, Abstract. Concentrating solar collectors direct the sunlight towards a focus point or focus line. Relevant parameters are the fidelity of the concentrator with respect to its ideal parabolic shape, its stiffness under wind and gravitational loads, the angular accuracy of the tracking and the solar weighted specular reflectance of the reflector. Additional aspects refer to the long term durability and ease of cleaning of the reflector surface. Solar concentrators require lower geometrical precision than astronomic apparatus. Therefore, more cost effective designs are possible by using up the overall acceptable error budget to a level that collection efficiency of the reflected sun rays is still very efficient. Understanding the impact of the different parameters describing the quality of the concentrator with respect to system performance and cost is necessary for an advanced and efficient concentrator design. DLR has recently developed guidelines to measure the most relevant concentrator characteristics in its qualification center QUARZ. This paper presents the relevant parameters of mirrors for concentrating solar collectors and discusses their economic impact. Introduction The concentration of sunlight is limited in practice by a number effects that are related on the one hand side to the fact that the sun is not an ideal point source, and on the other hand to non-ideal properties of the concentrator surface and its inaccurate orientation. Real surface reflection properties are typically described by the reflectance. As shown in Figure 1, only the reflected energy that is directed to the absorber, influenced by the specularity of the reflector is of relevance for the absorption in the collector system. How to define and measure a meaningful reflectance value for solar energy applications is discussed in this paper. Figure 1: a) differences between diffuse, direct and specular reflection; b) angular deviation of reflected ray is twice as large as deviation of surface normal vector A second important effect is the possible deviation of the reflected rays from their ideal path, which may lead to the consequence that they are not intercepted by the absorber surface. The quality of the reflected beam is typically considered by the standard deviation total of the angular All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-18/02/16,20:13:35)

2 238 5th FORUM ON NEW MATERIALS PART C distribution function of the reflected rays. Figure 2 shows the impact of this number on the fraction of rays that finally hit the absorber tube for a typical parabolic trough collector design. In this case a standard deviation of more than 6-8 mrad leads to significant intercept losses. The deviation of the reflected rays from their ideal path results from the superposition of various effects that in many cases can each be described by a Gaussian distribution function. This assumption simplifies the calculation of the superposition effect to the sum of squares of individual standard deviations of distribution functions describing the effects of a finite sunshape size sun, non ideal contour orientation contour (fig. 2b), microscopic specular reflections properties spec (fig. 2a), tracking accuracy track and potentially others as shown in eqn. 1 and 2. Although the approximation of the sun s angular distribution by a Gaussian distribution (typical sun 4 mrad) is not very precise, it is still justified as along as other sources of error are in the same order of magnitude or larger. Contour inaccuracies are particularly relevant as the angular deviations of the reflected rays are twice as large as the deviation of the surface normal vector, as shown in Figure 1. Thus, a meaningful definition for the quality of the concentrator shape is discussed in this paper. Intercept factor total = 2 sun + Σ i 2 optical (1) 2 optical = 4 2 contour + 2 spec+ 2 track +. (2) total in mrad Figure 2: Typical curve showing the fraction of i ntercepted rays of a parabolic trough collector as t he function of t he standard deviation of t he overall angular distribution function In principle it is possible to design concentrators with a very small optical (e.g. for astronomical purposes), however for solar energy application more cost effective designs are possible by using up the overall acceptable error budget total to a level where high intercept factors can be still achieved. In order to asses different design options it is important to analyze the impact of the different parameters describing the quality of the concentrator on the system performance and cost. An example using today s parabolic trough technology under Spanish market conditions is discussed below. Essential Parameters for Solar Concentrators Solar Reflectance In order to asses the reflected solar energy that hits the absorber correctly, the fraction of reflected energy within a cone of 25 mrad over the wavelength range of the sunlight and for a typical incident angle needs to be measured. The used method must be able to deal with the curved mirror surface. No measurement device capable to perform the task is widely available. In order to overcome this constraint, and allow to asses the reflectance of different concentrator materials, two available measurement devices are used and their results are combined. The hemispherical spectral reflectance hem, i is measured using a spectrometer in combination with an integrating sphere (Figure 3a). The result is integrated over a standard solar spectrum (Figure 3c) to calculate the solar weighted hemispherical reflectance SWH according to eq. 3. The spectrum used for concentrating systems is no longer taken from ISO 9050 nor ASTM E 891, but from ASTM

3 Advances in Science and Technology Vol G173 including a data set for beam irradiance spectral distribution. A second device (Devices & Services reflectometer D&S 15R) is used to measure the direct reflectance direct, 660 at a specific wavelength (660 nm) and a constant incident angle (15 ) into a certain aperture (typcially 25 mrad) as shown in Figure 3b. As for many reflectors the ratio of direct and hemispherical reflectance at a constant wavelength is quite constant over the solar spectrum, the obtained value can be divide by the hemispherical value at the same wavelength, and multiplied with the solar weighted hemispherical reflectance (see eqn 4) to arrive at the solar weighted direct reflectance SWD. This value is the selected figure of merit to characterize the reflectance of solar concentrator mirrors. It still depends on the opening aperture of the direct reflectance measurement that is typically selected at 25 mrad, and must be stated when comparing reflectors. a) b) relative intensity 0,01 0,008 0,006 0,004 0,002 solar irradiance AM 1.5 ASTM G ISO 9050 SWH SWD N hem, i E i1 E0 direct,660 hem,660 i SWH i (3) (4) wavelength [nm] c) Figure 3: a) Spectral hemispherical reflectance measurement principle b) Specular reflectance measurement at constant angle of incidence and wavelength c) Solar reference spectra according to ASTM G and ISO 9050 Concentrator Shape The shape of the concentrator depends on the specific design concept and assembly procedure. Parabolic troughs, for example, consist typically of a steel support structure on which curved glass mirrors are attached. The shape is therefore impacted by the form and stiffness of the steel structure, that of the mirror and the assembly precision. Also the concentrator orientation plays a role as it impacts gravitational and wind loads also affecting the shape. The knowledge of the distribution of the surface normal vectors for each collector orientation is required to calculate the fraction of intercepted rays. However, this normal vector distribution alone does not describe the quality of the concentrator, since the intercept factor for each reflected ray also depends on the distance to the focal point which typically varies over the concentrator surface. In order to come up with a meaningful averaged figure of merit describing the integral quality of the concentrator it is more convenient to calculate the deviation of the reflected rays from their theoretical focal point in mm for all surface points. For line focus applications it make sense to differentiate the deviation in the longitudinal direction (y-axis parallel to the focal line) and the transversal direction (x-axis, perpendicular to the focal line), as only the latter have a strong influence on intercept factor. A typical distribution of the transversal focus deviation of a Eurotrough mirror panels is shown in figure 4. Eqn. 5 defines the average transversal focus deviation FDx that can be calculated from the distribution of the deviation values over the whole effective reflector area.

4 240 5th FORUM ON NEW MATERIALS PART C FDx n i 1 FDx 2 i a A i ges (5) Inner Outer 0.2 y c n e u q fre d0.15 e liz a rm o n 0.1 N Figure 4: Typical distribution function for the transversal focus deviation for two Eurotrough mirror panels This number has been suggested as figure of merit for mirror shape as it highly correlated to the tot al collector intercept as can been seen in Figure 4 for an EuroTrough type parabolic trough collector geometry. The total intercept factor is however not defined by this figure alone but depends from all the other error types mentioned in eqn. 2 and originating from all components of the collector. 100% 98% 96% inner mirror f1.84m Intercept Factor 94% 92% 90% 88% 86% outer mirror f2.48m 84% 82% 80% Focus Deviation FDx in mm Figure 5 Intercept factor derived form ray tracing calculations as a function of average Focus deviation FDx as defined in eq. 5 using additional assumptions on other error sources like sunshape, tracking, etc.

5 Advances in Science and Technology Vol Deflectometry is one very suitable method to measure this quantity. It uses the distorted images of a set of regular patterns that are reflected by the concentrator to determine the surface normal vector distribution with a high spatial density [1] This method can be used to qualify a complete concentrator, but is also used to separate the impact of mirror, support structure, assembly procedure and other effects from each other. In particular this is useful when comparing different mirror products used in the same collector design. The separation of the different effects also allows deciding on most cost effective solutions in improving the overall beam quality budget total to a more precise mirror, or a better support structure. Other methods like the assessment of the reflection points of a laser beam directed parallel to the optical axis and scanned over the surface also leads directly to the focus deviation distribution function. Impact of concentrator performance on cost Today back silvered glass mirrors are most widely used in solar concentrators. They have proven long term outdoor stability over more than 20 years and solar weighted direct reflectance between 93% and almost 96% depending on the thickness of the glass (1 4 mm). Advanced concentrator developments are often based on new reflector materials. Glas Polymer Alu1 Alu2 ρ SWH ASTM G ρ SWH ISO ρ SWD,25 ASTM G ρ SWD,25 ISO Table 1: Examples of solar weighted hemispherical and direct reflectivity for different reflector samples based on two different standard solar spectra Examples presented in Table 1 show that some new materials provide high solar weighted hemispherical reflectance, however their direct reflectance that is relevant to asses the performance of these materials in solar concentrators, is significantly lower in the presented cases. Other materials not presented here may however show higher values. To asses the competitiveness of such new materials it is relevant to evaluate the impact of reflectance properties of the mirrors on the annual performance of a solar power plant. A one percent point lower reflectance value would require increasing the solar field size by 2% (assuming a typical annual solar to thermal field efficiency of 50%) to end up with the same thermal output. In the case of a parabolic trough collector we can assume collector field costs (including HTF fluid) of approx 300 /m² that would justify a reflector price difference of about 6 /m² per percentage point of reflectivity. Today s glass mirrors with 93% clean reflectivity costs about 30 /m², so that a new reflector with 5% lower reflectance can t compete as it need to be available for free to offset the additional solar collector field cost. Also the degradation of the mirror is important to be considered. A 0.1% reflectance degradation rate per year reduces the average annual output over a twenty year lifetime by 1% which results in an average revenue decrease of about 1.3 /m²-year for a 50 MW ANDASOL-type parabolic trough power plant under the Spanish revenue scheme RD 661/2006 (based on detailed performance calculations using the GREENIUS code). Depending on the assumed interest rate (5-10%) this would offset a reflector price difference of /m² compared to a non degrading mirror which is more than a third of today s glass mirror price. A similar sensitivity analysis with respect to the concentrator shape is presented in Figure 6. Today s bent thick glass mirrors used in parabolic trough collectors typically achieve FDx values better than about 10 mm. An increase of 1 mm in FDx results in a change of annual energy production of about 1%, requiring a 1% larger solar field size to offset the additional intercept

6 242 5th FORUM ON NEW MATERIALS PART C losses. Under the assumptions mentioned above this equals about 3 /m² difference in the mirror price. 106% 104% 102% 100% annual electricity production tion Energy Produc 98% 96% 94% 92% 90% annual income 50MW Spain M incom e 88% % % % Focus Deviation FDx in mm Figure 6: Impact of the average focus deviation of a concentrator on annual electricity production and revenues for a 50 MW parabolic trough power plant of the ANDASOL type under the Spanish revenue scheme of 2008 based on GREENIUS calculations Summary The solar weighted direct reflectance SWD and the focus deviation FDx are the most important parameters to describe the quality of a solar concentrator mirror. Under today s commercial conditions with high feed-in tariff, even small improvements of these parameters may justify significant cost differences of concentrators. For the development of a competitive market for components for solar concentrators, it is essential that such quantities can be measured in the laboratory with high accuracy. Therefore, measurement guidelines and later standards need to be established. This is also the objective of the DLR QUARZ Centre with its independent measurement service to the industry. Measurement guidelines proposed from this work in QUARZ has been proposed at international level and leads towards preparation of technical standards for these components. References [1] E. Lüpfert, S. Ulmer: Solar Trough Mirror Shape Specifications, Solarpaces Conference, Berlin, September 2009 [2] Stephanie Meyen, Eckhard Lüpfert, Johannes Pernpeintner and Thomas Fend: Optical Characterisation of Reflector Material for Concentrating Solar Power Technology, Solarpaces Conference, Berlin, September 2009 [3] B. Schiricke, R. Pitz-Paal, E. Lüpfert, A. Neumann, K. Pottler, M. Pfänder, K.-J. Riffelmann, Experimental Verification of Optical Modeling of Parabolic Trough Collectors by Flux Measurement, J. Sol. En. Eng. Vol. 131 (1), 2009, pp ff. [4] Lüpfert E., Pottler K., Riffelmann K.-J., Ulmer S., Schiricke B., Neumann A.: Parabolic Trough Analysis Techniques for Optical Performance. J. Sol. En. Eng. 2007, Vol 129, 2

7 5th FORUM ON NEW MATERIALS PART C / Material and Design Requirements for Advanced Concentrators /