A Solar Simulator for the Measurement of Heat Collection Efficiency of Parabolic Trough Receivers

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 69 (2015) 1911 – 1920

International Conference on Concentrating Solar Power and Chemical Energy Systems, SolarPACES 2014

A solar simulator for the measurement of heat collection efficiency ofparabolic trough receivers Y. Okuharaa *, T. Kuroyamaa, T. Tsutsuib, K. Noritakeb and T. Aoshimac a Japan Fine Ceramics Center, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, Aichi 456-8587, Japan. Toyota Industries Corporation, Kyowa Plant, 8, Chaya, Kyowa-cho, Obu-shi, Aichi 474-9180, Japan. c EKO Instruments, 1-21-8, Hatagaya, Shibuya-ku, Tokyo 151-0072, Japan

b

Abstract A solar simulator consisting of linear focus units and a fluid circulation system has been constructed, and a basic scheme for evaluating heat collection efficiency of parabolic trough receivers has been demonstrated. The linear focal area along the 4 m long receiver was formed by an array of twenty linear focus units each with a 5 kW Xenon-short arc ramp. The flux distribution was measured by scanning a heat flux sensor, and the average flux of 24.3 kW/m2 was achieved along the half-round surface of the receiver tube. A parabolic trough receiver with the optical efficiency of 79.3 % has been fabricated and applied to the demonstration. During irradiating the linear focal flux to the receiver, the collected thermal output was measured from the increase in temperature of water running through the receiver. The absolute heat collection efficiency of 73.9 %, defined as the ratio of the collected thermal output to the irradiated optical input, was obtained under the thermal equilibrium condition. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG. Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG Keywords: Absolute solar-thermal conversion efficiency; Linear focus artificial solar; Tubular receivers; Qualification;

1. Introduction Parabolic trough concentrating solar power systems are one of the most prevalent and encouraging candidates for solar thermal power generation and industrial process heat systems. A key component in their systems is the

* Corresponding author. Tel.: +81-52-871-3500; fax: +81-52-871-3599. E-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.03.185

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receiver tubes for converting solar radiation to thermal energy. Accurate heat collection efficiency of the receivers provides a fundamental parameter for developing efficient receivers and for designing optimum solar plants. Performance of solar collectors and receivers has been evaluated by field tests at CIEMAT, Sandia [1] or PSA [2] where suitable conditions prevail. On the other hand, solar simulators for indoor tests have been developed for providing artificial point focal or linear focal flux [3-5], and the latter simulator known as ElliREC in DLR has been operating to evaluate parabolic trough receivers. Although it is difficult to completely emulate the focused natural sunlight, the solar simulators are promising candidates to evaluate the heat collection performance of receivers under various conditions with high stability, reproducibility and reliability. This paper describes a solar simulator for evaluating absolute heat collection efficiency of linear focus receivers by quantitative measurements of optical flux input and collected thermal output. The flux intensity, the spectrum distribution and the angle of light incidence were taken into consideration for designing the linear focus system in order to be closer to the actual focusing behavior in a solar field. Because wide angle of incident light reduces absorption efficiency of spectrally selective coatings depending on their layered structures, incidence angle to the receiver surfaces has to be controlled and normal incidence is desirable for the standard test. A heat transfer fluid (HTF) circulation system has been combined with the linear focus system to demonstrate the solar-thermal energy conversion behavior. 2. Construction of the solar simulator 2.1. Linear Focus System A schematic diagram of the designed linear focus unit is depicted in Fig.1 (a). In order to achieve high flux intensity and to simulate spectrum distribution of natural sunlight, a 5 kW Xenon (Xe) short-arc lamp (WACOM Electric, KXL-500HFW) was chosen as the light source, and the radiation spectrum was adjusted by an air-mass filter. The radiation from the Xe lamp is focused at the first focal point using an ellipsoidal mirror. After homogenizing the irradiance through a fly’s eye lens, the radiated flux was linearly focused by a Fresnel lens of 370 mm×390 mm made of polymethyl methacrylate (PMMA). The spectrum distribution of the flux coming out of the Fresnel lens shown in Fig. 1 (b) well matched with solar spectrum AM 1.5G (IEC 60904-3), which meets class A (±25 %) according to IEC 60904-9. 㻌

Xenon short-arc lamp

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Fig. 1. (a) A linear focus unit based on a Xe short-arc lamp and (b) the spectrum distribution of the focused flux.

Each Xe lamp is individually driven by a thyristor power supply, and the irradiance can be adjusted by the lamp current from 50 to 100 %. The irradiance can be attenuated to lower than 50 % by inserting a punched metal plate in the light path. Each focus unit equipped with an illuminometer for feedback control to keep the flux intensity constant. As is well known, the Xe lamps have a limited lifetime and the irradiance declines with operating time. Thus, the operating current for the standard tests was set at 75 % of the maximum in order to guarantee the same irradiance during their lifetime over 500 hours. Both sides of the Fresnel lens have asymmetric patterns; the incident surface toward the fly’s eye lens has a spherical Fresnel lens, and the outer surface toward the receiver has a cylindrical Fresnel lens. The light radiated from the fly’s eye lens transformed to parallel light by the multiple spherical arcs. The parallel light through the

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lens was output toward receiver tube and focused on a line by multiple cylindrical arcs. As a result, the angle of incidence irradiating on the receiver surface can be specified; the angle along the longitudinal direction is almost normal to the receiver surface within the uncertainty of ±3°, while the angle along the circumferential direction is in the range of ±10°, which was decided from the size (370 mm) and the focal length (1100 mm) of the Fresnel lens.

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Fig. 3. Linear focus systems for (a) parabolic trough receivers and (b) for central receiver tubes.

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The linear focal area formed by the optical unit had the flux distributions as shown in Fig.2. In Fig. 2 (a), the flux distribution along the longitudinal position covered ±200 mm corresponding to the width of the Fresnel lens. Even though the distribution profile was not uniform completely, the flux undulation was within ±10 % along the longitudinal position of ±150 mm. Because the plural focus units are arrayed in the solar simulator, the drop in the flux at the boundary around ±200 mm is expected to be raised by superposition of flux from both sides of adjacent focus units. Along the radial position, over 82 % of flux intensity was focused within ±35 mm as shown in Fig.2 (b), suggesting that the focus unit can well linearly confine the light on the receiver tube with a diameter of 70 mmI. As shown in Fig.3 (a), the linear focus system for irradiating to receiver tubes of 4000 mm in length has been constructed, which consists of 2 rows of 10 linear focus units. The central irradiance angle between the two rows is ±30°, which is the widest angle within the acceptable tilt angle of the Xe lamps. The receiver tube is located at the focal line, 1100 mm away from each Fresnel lens. The solar simulator has another arrangement of 5 units × 4 rows as shown in Fig.3 (b), in which each row has difference in tilt angle of 20°. Although the irradiance length is limited within 2000 mm, higher density of flux can be applied to the tubular specimens such as central receivers. Heat flux sensor

Cold plate

Linear / rotational scanning stage Fig. 4. Scanning mechanism of a heat flux sensor.

The optical flux power as energy input is fundamental for the calculation of the absolute heat collection efficiency. Before installing a receiver tube in the focal line, the irradiance distribution was measured by scanning of a Gardon heat flux sensor (MEDTHERM 64-20) with the detection area of 10 mm in diameter. Linear and rotational scanning the gauge surface as shown in Fig. 4 provided the flux map corresponding to the half-round absorber surface on the metal tubes. During the measurement, the irradiated light other than the sensing area was caught by a water-circulated cold plate at the backside of the sensor. Although the flux gauge has been calibrated with black-body source and absolute cavity radiometers, calibration with the artificial solar spectrum is required to confirm and correct the error. The flux gauge will be calibrated with pyrometers guaranteed by the World Radiometric Reference as the future work. Figure 5 (a) and (b) showed the measured flux distributions for both arrangements of 10 units × 2 rows for parabolic trough receivers and 5 units × 4 rows for central receiver tubes, respectively. In Fig.5 (a), although the longitudinal flux distribution had the undulation of about ±10 %, the radiation from 20 Xe ramps was well confined in the narrow linear area of 4000 mm×70 mmI. The average flux on the irradiated half-round surface was 18.0 kW/m2, and the total optical power of 8.0 kW is irradiated to the parabolic trough receiver. Assuming that the solar radiation (DNI) of 0.9 kW/m2 including concentration efficiency of 90 % is collected by EuroTrough systems with the aperture width of 5770 mm [6], the actual average flux of 47.2 kW/m2 is achieved on the half-round surface of the receivers of 70 mmI in diameter. Therefore, the average flux in our simulator was about 38 % of the actual one for the EuroTrough concentrators. An increase in the Xe lamp current to 100 % enhanced the average flux to 24.3 kW/m2, which was about 51 % of that for the EuroTrough. The angular distribution of the flux has one peak of Gaussian profile with the maximum flux of about 37.7 kW/m2, suggesting the optical paths from 2 rows merged and provided the one peak at the center of the angle 0°. The actual angular distribution on the receiver tubes has two peaks in parabolic trough collectors [4] and one peak in linear Fresnel collectors. The present angular distribution of our linear focus system is close to that for linear Fresnel collectors. When a thermocouple was set at the position of the central angle, it indicated 420 °C at the average flux of 18.0 kW/m2 and 470 °C at the average flux of 24.3 kW/m2.

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In general, central receiver tubes has wide variety of shapes and sizes. In our linear focus system, the size of 2000 mm×30 mmI was assumed for the central receiver tubes, and the radiated flux was well confined in the narrow linear area as shown in Fig. 5 (b). The average flux in the irradiated half-round surface was 51.8 kW/m2 at 75 % of the maximum lamp current, which was increased to 69.4 kW/m2 when the current increased to 100 %. The undulation of irradiation along the longitudinal axis was within ±12 %, which was similar to that for the arrangements of 10 units × 2 rows. The temperature measured at the highest flux area was 580 °C at the average flux of 51.8 kW/m2 and 640 °C at the average flux of 69.4 kW/m2. Because high concentration factor for central tower receivers diminishes the importance of solar selective absorption, the high flux intensity is more desirable than accurate spectrum distribution to evaluate the central receivers in the solar simulator. Removing the air mass filter from the linear focus units increased the average flux intensity from 69.4 kW/m2 to 86.6 kW/m2 on the halfround surface of the receiver tubes.

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2.2. HTF (Heat Transfer Fluids) Circulation System To investigate heat collection behavior, HTF circulation systems have been integrated with the linear focus system. Water and synthetic oil were chosen as the HTF in order to cover the temperature range from room temperature (RT) to 300°C. Schematic diagrams for the circulation systems are shown in Fig. 6. The water at room temperature was circulated through the loop in Fig. 6 (a), in which a reserve tank (300 L), an inverter-controlled pump (IWAKI, MD-55R), a flowmeter (SIEMENS, MAG5000+MAG5100W) and resistance temperature sensors (CHINO, Pt-100) were assembled. The water passing through the irradiated receiver collects the thermal energy, and continuous heating of the water circulated in the “closed loop” accumulates the enthalpy and keeps on increasing the water temperature. In order to keep the inlet temperature constant, the water passes once through the irradiated receiver in the “open-loop” configuration, in which the heated water is drained while tap water is continuously supplied in the tank. The flow rate over 12.5 L/min corresponds to the turbulent Reynolds number of 4000 or more. The oil circulation system consists of a heat resistant pump (Sanwa Hydrotec, MAGPAC-MH), a high temperature flowmeter (Endless+Hauser, PROMASS 80F), an electric heater unit (CHINO, 15 kW), a heat exchanger (CHINO, 20 kW) and the temperature sensors as shown in Fig. 6 (b). The synthetic oil “Barrel Therm 400” was selected as the HTF in terms of the low viscosity, the low vapor pressure, and the small heat capacity. This system is based on the “closed-loop circulation”. The HTF temperature can be increased to 280°C; the upper limit comes from the allowable maximum temperature of the pump. During irradiating the focused flux to the receiver, the inlet temperature can be kept constant by removing the increased enthalpy of the HTF using the heat exchanger. The turbulence flow is achieved by the flow rate of higher than 55 L/min. Temp. sensor㻌 se

Tout㻌

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(b) Heated oil circulation system Fig. 6. Schematic diagrams and photographs of HTF circulation systems.

The collected thermal power 'Q of the HFT is calculated from the following equation, (1) οܳ = οܶ ‫ܥ‬௣ ߩ ߥ where, 'T is the increase in temperature of the HTF through the receiver, and Cp, U, X is heat capacity, density and flow rate of the HTF, respectively. In addition to accurate measurement of 'T and X, precise Cp and U should be assigned for the calculation. It is known that the degradation of oil induces change in the thermal properties [7]. The initial heat capacity Cp as a function of temperature was measured by Calvet calorimetry (Setaram, C80), and

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had a good agreement with that in the specification as shown in Fig. 7. The long term change of Cp with operating time will be checked intermittently. 㻌

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Fig. 7. Temperature dependence of specific heat for the synthetic oil “Barrel Therm 400”.

3. Preparation of receiver tubes Parabolic trough receivers have been prepared for demonstration of the solar simulator as shown in Fig. 8 (a). The stainless tubes and glass envelopes for the receivers have the dimensions of 4060 mm×70 mmI and 3950 mm×125 mmI, respectively. The outer surface of the stainless tube has an absorber coating, while the glass envelope has no anti-reflection coatings. The absorber coating was composed of SiO2 and molybdenum layers for absorbing the artificial solar light on a silver layer for lowering the infrared emissivity. 㻌

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The solar absorbance D and the solar transmittance T were calculated from the following formulas, ߙ=

ಮ

‫׬‬బ { ூೞ೚೗ೌೝ(ఒ)×஺೎೚ೌ೟ (ఒ)} ௗఒ ಮ ‫׬‬బ ூೞ೚೗ೌೝ(ఒ)ௗఒ

,

ܶ=

ಮ

‫׬‬బ ൛ ூೞ೚೗ೌೝ(ఒ)×்೒೗ೌೞೞ (ఒ)ൟ ௗఒ ಮ

‫׬‬బ ூೞ೚೗ೌೝ(ఒ)ௗఒ

(2)

where, Isolar (O), Acoat (O) and Tglass (O) are the solar spectrum, the absorbance spectrum in Fig. 8 (b) and the transmittance spectrum in Fig. 8 (c), respectively. If the natural solar spectrum AM 1.5G was used for the Isolar (O),

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D of 84.2 % and T of 92.3 % were obtained. Multiplying the D by the T provides the optical efficiency of 77.7 % for the receiver. In contrast, the calculation using the artificial solar spectrum in Fig. 1 (b) for the Isolar (O) provided D of 85.8 %, T of 92.4 %, and hence the optical efficiency of 79.3 %, which was slightly higher than that calculated using the natural solar spectrum. This result suggests that the measurements of D and T are valuable for correcting the solar - thermal conversion efficiencies evaluated by the solar simulator to those in an actual solar field. 4. Demonstration of solar - thermal energy conversion After installing the receiver at the focal line in the solar simulator with the arrangement of 10 units × 2 rows shown in Fig. 4 (a), the water circulation system in Fig. 8 (a) was joined to the receiver.

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Fig. 9. Photo-thermal conversion behaviors of water through the receiver tube in (a) the closed- or (b) the open-loop configuration.

During continuously circulating water in the closed loop, the average flux of 18.0 kW/m2 shown in Fig. 5 (a) was irradiated on the receiver. The temperature difference 'T of about 2 °C was obviously detected at the flow rate of 42 L/min (0.22 m/sec), which increased with decreasing flow rate as shown in Fig. 9 (a). Because the inlet temperature of water was continuously increased by circulating through the heated receiver, the time required for the water to go through the receiver tube ('t) ought to be taken into account in the calculation of 'T = Tout (t+'t)- Tin (t). The collected thermal power 'Q was derived according to the equation (1), and the 'Q of around 6 kW was found to be stable at each flow rate as shown in Fig.9 (a). It should be noted that the stable 'Q was achieved by

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homogenization of outlet water temperature using a static mixer between the receiver exit and the outlet temperature sensor. Dividing the collected thermal output 'Q by the irradiated optical input provided the absolute heat collection efficiency. The efficiencies slightly increased with increasing flow rate as indicated by the blue line in Fig. 10. The absolute heat collection efficiency saturated at about 72.1 % at the flow rate of 25 L/min or more. The equal average flux was irradiated on the same receiver in the open-loop configuration, in which the inlet temperature of water was almost constant as shown in Fig. 9 (b). The 'T responded by decreasing the flow rate in a similar way to Fig. 9 (a), and the stable 'Q was obtained at each flow rate. The saturated efficiency of 73.9 % was achieved at the flow rate over 20 L/min, which was slightly higher than that in the closed-loop circulation. ideal optical efficiency

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It is reasonable that the evaluated efficiencies are under the ideal optical efficiency of 79.3 % since the thermal loss from the heated surface and other losses decrease the efficiency. The temperature dependence of the thermal loss was measured using a heater rod inserted in the receiver tube similar to the “ThermoREC” in DLR [1, 5], and the relationship for the receiver had a good agreement with the previous reported data [1]. On the assumption that the maximum flux heated the absorber surface to 420 °C as described above and the temperature on the half-round surface had similar distribution to Fig. 5 (a), the total heat loss of the 4 m length receiver was calculated to be 282 W. The possible thermal loss corresponds to about 3.7 % of the optical input of 7.9 kW in Fig. 9, and hence the possible range of the efficiency is expected to be from 75.6 % to 79.3 %. Although the exact thermal loss can’t be estimated because of the unknown actual temperature of the absorber surface, the measured efficiency of 73.9 % for the openloop system was close to the range of the expected efficiency. In contrast, the measured efficiency of 72.1 % for the closed-loop circulation was further from the expected range, which should be coincident with that for the open-loop configuration independently of the HTF circular route. A possible reason for the lower efficiency is a delay in the increase of water temperature Tout around the receiver outlet. The HTF temperature has always higher than that of the circulation pipeline under the non-equilibrium condition, causing unbalanced thermal energy flow from the water to the pipeline with various heat capacity Cp and temperature. The higher thermal energy flow is expected around the receiver outlet because the static mixer has higher Cp and temperature than those for the inlet, which delays the increase in Tout and reduces 'T and the efficiency. Therefore, the heat collection efficiency has to be evaluated at thermal equilibrium condition, and the efficiency of 73.9 % measured at the constant inlet and outlet temperature has higher reliability. The decline in the efficiencies at the lower flow rate may be attributed to the increase in thermal loss resulting from the higher temperature of the absorber surface. Another reason is a bending behavior of the receiver tube. Due to the thermal expansion of the irradiated half-round surface, the center of the receiver tube curved towards the light source at the high flow rate. In contrast, it was observed that upward bending perpendicular to the light incident direction was added at the low flow rate. Because the decrease in the flow rate reduced the Reynolds number toward the transition region, more peculiar distribution of temperature induced the bending of the tube. Therefore, a partial deviation of the bended receiver from the linear focal area seemed to reduce the irradiated optical input and the efficiency at the low flow rate as shown in Fig. 10.

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As a result, the most reliable efficiency of 73.9 % was obtained under the turbulent and the thermal equilibrium condition, but it was slightly lower than expected efficiency range. This result suggests that there are other thermal or optical losses for the receiver, for instance, reflectance losses by non-uniformity of absorber coating or at some joints of glass envelop and so on. Although these losses are difficult to be discriminated, this is the reason why the solar-thermal conversion efficiency including them has to be evaluated by the solar simulator. As the future work, it is important to accumulate the evaluation data for many receivers with different optical efficiencies under various operating conditions. Accurate correlations between the heat collection efficiencies and the optical efficiencies will be the evidence to support the validity of the evaluation process. Moreover, advanced tests using the heated oil circulation system as shown in Fig. 6 (b) will provide valuable data closer to the practical field conditions, such as temperature dependence of the photo-thermal conversion efficiency. It is also significant to confirm the validity of the evaluation scheme by comparing the measured efficiencies with those from other methods such as field tests. 5. Conclusion This paper proposed a basic scheme for evaluating absolute heat collection efficiency of parabolic trough receivers based on measuring energy input and output. As the first step, the solar-thermal conversion behavior has been demonstrated by a combination of the linear focus units and the HTF circulation system. The irradiance distribution of the focal line along the receivers of 4000 mm×70 mmI or 2000 mm×30 mmI achieved the maximum average flux of 24.3 kW/m2 or 86.6 kW/m2, respectively. The water or oil circulation systems have been constructed, and the former system was applied to the linear focus system for evaluating the parabolic trough receiver. The absolute efficiencies evaluated in closed- or open-loop configuration were 72.1 % or 73.9 %, respectively. The former low efficiency included a temperature measurement error under the non-equilibrium condition in the closed-loop circulation. The efficiency in the open-loop configuration was closer to the expected efficiency range from 75.6 to 79.3 %, which was estimated from the ideal optical efficiency and the possible thermal loss. In addition to the precise measurement of the irradiated optical input, the correct measurement of thermal output under the thermal equilibrium and spatially homogenized HTF flow was important to evaluate the accurate heat collection efficiency of the receivers.

Acknowledgements This work was carried out under the “Program for the Promotion of New Energy Infrastructure Development”, supported by Mitsubishi Research Institute (MRI) / the Ministry of Economy, Trade and Industry (METI), Japan.

References [1] Lüpfert E, Riffelmann K-J, Price H, Burkholder F, Moss T. Experimental analysis of overall thermal properties of parabolic trough receivers. J Solar Energy Engineering 2008:130: 021007-1-5. [2] Heller P, Meyer-Grünefeldt M, Ebert M, Janotte N, Nouri B, Pottler K, Prahl C, Reinalter W, Zarza E, KONTAS – A rotary test bench for standardized qualification of parabolic trough components. SolarPACES 2011, 20-23. Sept. 2011, Granada, Spain. [3] Petrasch J, Coray P, Meier A, Brack M, Häberling P, Wuillemin D, Steinfeld A, A novel 50 kW 11,000 suns high-flux solar simulator based on an array of Xenon arc lamps, J Solar Energy Engineering 2007:129: 405-411. [4] Pernpeintner J, Schiricke B, Lüpfert E, Lichtenthäler N, Macke A, Wiesemeyer K, Combined measurement of thermal and optical properties of receivers for parabolic trough collectors. SolarPACES 2009, 15-18. Sept. 2009, Berlin, Germany. [5] Pernpeintner J, Schiricke B, Lüpfert E, Lichtenthäler N, Anger M, Ant P, Weinhausen J. Thermal and optical characterization of parabolic trough receivers at DLR's QUARZ Center - Recent advances. SolarPACES 2011, 20-23. Sept. 2011, Granada, Spain. [6] Geyer M, Lüpfert E, Osuna R, Esteban A, Schiel W, Schweitzer A, Zarza E, Nava P, Langenkamp J, Mandelberg E, EUROTROUGH Parabolic trough collector developed for cost efficient solar power generation. SolarPACES 2002, 4-6. Sept. 2002, Zurich, Switzerland. [7] Marchã J, Osório T, Collares Pereira M, Horta P, Development and test results of a calorimetric technique for solar thermal testing loops, enabling mass flow and Cp measurements independent from fluid properties of the HTF used, Energy Procedia, 2014:49: 2125-2134.