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Energy Procedía 49 (2014) 438 - 446

SolarPACES 2013

Performance analysis of a novel air-based cavity receiver

P. Matarresea, A. Gaetanoa, S. Airaghia, D. Montorfanoa, M.C. Barbatoa*,

G. Ambrosettib, A. Pedrettib

a Department of Innovative Technologies, University of Applied Sciences of Southern Switzerland, 6928 Manno, Switzerland b Airlight Energy Manufacturing SA, 6710 Biasca, Switzerland

Abstract

In this paper a new design of a novel CSP cavity receiver for parabolic trough collector is analyzed by means of an analytical Matlab model. The receiver, designed by the Swiss company Airlight Energy Manufacturing SA, is 212 m long consisting essentially of a feed pipe, a run-back pipe and 4608 helically coiled heat exchangers designed to capture the incident solar energy concentrated by a parabolic trough. The heat transfer fluid is air heated to temperatures above 600°C.

The analytical Matlab model based on a pneumatic - electric circuit analogy was developed to assess the receiver performance in terms of mass flow rate distribution, pressure drop, air outlet temperature and thermal efficiency. A solution was proposed to approximately ensure the same mass flow rate for each cavity.

Different skew angles for the incoming solar radiation were considered and the receiver geometry was optimized minimizing the pressure drop and the thermal losses through the runback pipe. The main requirement was to achieve, at the outlet section of the receiver, an air temperature of 650°C; therefore, the total inlet mass flow rate was tuned accordingly.

The helically coiled heat exchanger and the receiver insulation sub-models were validated against accurate computational fluid dynamics simulations.

© 2013 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/3.0/).

Selectionandpeerreviewby thescientificconferencecommitteeof SolarPACES2013underresponsibilityofPSEAG. Final manuscript published as received without editorial corrections.

Keywords: Concentrated solar power (CSP); Parabolic trough receiver; Mathworks Matlab model; Receiving cavity; Helically coiled heat exchanger (HCHE); Heat transfer; Air; Receiver performance.

* Corresponding author. Tel.: +41-58-666-6639; fax: +41-58-666-6620. E-mail address: maurizio.barbato@supsi.ch (M.C. Barbato).

1876-6102 © 2013 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/ 3.0/).

Selection and peer review by the scientific conference committee of SolarPACES 2013 under responsibility of PSE AG.

Final manuscript published as received without editorial corrections.

doi:10.1016/j.egypro.2014.03.047

1. Introduction

Parabolic trough CSP systems with oil as heat transfer fluid (HTF) have since long demonstrated to be one of the most viable options for large-scale solar electricity production. However, conventional implementations are beset by several shortcomings, both technical and economical. These include a maximum operating temperature defined by the HTF (400°C of oil and 450°C for molten salt), a difficulty in creating rigid trough structures with large apertures, expensive vacuum insulated receivers, and costly thermal storage technologies (e.g. molten salt storages). Nevertheless, given their low active components requirement (e.g. tracking mechanisms), scalability, and absence of tall structures, trough systems remain potentially a very attractive choice with respect to other technologies such as solar towers.

The novel Airlight Energy's CSP collector tackles these shortcomings while leveraging the potential of troughs with four main innovations. A multi-arc pneumatic mirror system allows to achieve the focusing characteristics of a parabolic trough with high optical efficiency while the concrete sustaining structure provides a rigid frame that can be easily manufactured on-site, all leading to a very low collector cost per primary mirror aperture area. The receiver, which includes an additional concentration stage, in turn coupled to a string of cavity receivers, employs air as the HTF, which, besides being inexpensive and environmentally friendly, is optimally suited for working temperature above 600°C. Finally, the use of air allowed the development of low-cost rock packed bed thermal energy storage.

The first 3 MWth pilot plant is under construction in Ait Baha (Morocco) with all innovations of Airlight Energy CSP system [1].

2. Novel CSP receiver based on Airlight Energy Technology

2.1. Solar receiver

The solar receiver designed by Airlight Energy is composed by 3 principal parts (Fig. 1):

• an air supply system including a main feed pipe and 36 distribution modules

• 4608 helically coiled heat exchangers (HCHEs)

• an air runback pipe gathering the heated HTF

All the receiver's components are thermally insulated. The runback pipe is insulated by thermal radiation shields [2] (Fig. 1) while for the other components Microtherm® insulating material was considered. Some further geometrical parameters of the receiver are given in Table 1.

Table 1. Receiver main geometrical parameters

Geometrical Parameters Values Units

Number of modules 36 -

Number of HCHEs for each module 128 -

Receiver mirror length 212 m

Mirror aperture 9.7 m

Feed pipe diameter 400 mm

HCHE tube diameter 11 mm

HCHE tube length 2.5 m

Coil mean diameter 80 mm

Runback pipe diameter 453 mm

Number of insulation shields 17 -

Fig. 1 Schematic cross section of the novel receiver design - Courtesy of Airlight Energy Manufacturing SA.

2.2. Receiver working principle

In the Airlight Energy receiver air is distributed by the feeding pipe to the HCHEs. Those are irradiated by the solar radiation focused by the main mirror and further concentrated by hyperbolic secondary optics. As flowing into the coiled tube, air heats up to high temperature and is collected by the runback pipe. Each HCHE is able to heat a given mass flow rate of air up to a target temperature of 650°C.

Receiver performance were evaluated checking total mass flow rate, pressure drop, HTF temperature at the runback pipe outlet section, and the thermal efficiency.

3. Receiver modeling

3.1. Pneumatic-electric circuit analogy

Due to the size and complexity of the system the receiver steady state performance were analyzed by means of a zero dimensional model based on a pneumatic-electric circuit analogy assuming stationary air flow and energy flux conditions.

The feed and runback pipes and all the HCHEs are considered as part of an equivalent electrical circuit. The mass flow rates correspond to the currents of the circuit, the pressure drops correspond to the potential difference and an equivalent resistance Rflow relates pressure drop and mass flow rates, i. e.:

I; Ap ^ AV\ I

Figure 2 illustrates a schematic of the equivalent electrical circuit. In this example, there are 3 modules with 4 HCHEs each) with the corresponding flow resistance for each receiver component:

• Rf = Resistance given by the feed pipe between two adjacent linking pipes.

• Rm = Resistance given by the linking pipes.

• Rfm = Resistance given by the feed pipe of each module between two adjacent HCHEs

• Rc = HCHE's resistance

• Rrb = Resistance given by the runback pipe between two adjacent cavities

Fig. 2: Simplified equivalent circuit with resistances for the pressure drop calculation

Receiver pressure drop was computed according to the formulas reported in [3], [4] and [5]. The Kirchhoffs laws were used to solve the circuit evaluating the mass flow rates for the different components and the total pressure drop. The system symmetry was exploited to halve the number of equations needed.

3.2. Thermal modeling

The thermal behavior of the components was taken into account in order to predict the air temperature at the outlet section of each HCHE and at the receiver outlet section. HCHE's and runback pipe thermal losses were computed to evaluate the receiver thermal efficiency.

3.2.1. Helically coiled heat exchangers (HCHE) thermal model

The HCHE's performance was evaluated by means of a simplified thermal model described in [6]. The convective heat transfer inside the helically coiled tube was modeled according to the data reported in [7]. A schematic of the model is shown in Fig. 3.

Fig. 3: Simplified model of the helically coiled heat exchanger (HCHE)

The solar power (Qinitot) entering each HCHE was modeled according to a ray-tracing analysis [5] which takes into account the skew angle of the solar radiation collected by the primary mirror. The helically coiled tube was split into smaller elements which were analyzed considering: fluid inlet temperature (Tfluid(i-1)), fluid outlet temperature (Tflmd(i+1)), input power on each element (Qin(i)) and thermal losses by radiation (QradO)) and convection (Qconv(O). The overall power lost by the HCHE is computed taking into account the glass window which separates the cavity from the external environment. Radiation and convection losses (Qrad, Qconv) were computed as well as the temperature on the inner and the outer surface of the glass window (TglassIN and TglassOUT respectively).

The HCHE thermal model was solved using the software Matlab® and its results were validated by means of accurate 3D CFD simulation [6].

3.2.2. Runbackpipe thermal model

The runback pipe is insulated with a system of radiation shields. Its thermal losses were studied by means of an electrical analogy. Conduction through the shield as well as the heat transfer due to conduction, convection and radiation between two adjacent shields were taken into account.

The model results were validated comparing them with 2D CFD simulations [2]. Considering a temperature of 650°C for the runback pipe wall, Fig. 4 compares the radial temperature distribution predicted by the Matlab model and the one resulting from the 2D simulations performed with Ansys-Fluent v13.0.

■ ab

250 300 350 400 450

Radius [mm]

Fig. 4: Shields temperature comparison.

4. Matlab model parameters and results

The Matlab model for the whole system was created taking into account the different sub-models illustrated in the previous sections: pipes circuit, HCHE and thermal insulation system. The solution procedure of the equations system was optimized in order to reduce the computational time. The air flow equivalent electrical circuit model lead to a system of linear equations that can be expressed in the form of:

[Mtoti 'mHCHE — total (2)

Where [Mtot] is the "equivalent resistance matrix", mHCHE represents the vector of the mass flow rate for each HCHE and Ap tot is the pressure drop vector which is the same for each loop of the circuit going from the inlet

section to the outlet section (see Fig. 2). To speed up the solution process, the matrix [Mtot] was split into 5 different matrixes, with simpler structure, and then rebuild as schematically showed by Fig. 5:

[Mtot] = [MRf] + [MRm] + [MRfm] + [MRc] + [MRrb] (3)

Where:

• [MRf] = Feeding pipe equivalent resistance matrix

• [MRm] = Linking pipes equivalent resistance matrix

• [MRfm] = Modules' feeding pipe equivalent resistance matrix

• [MRc] = HCHEs' equivalent resistance matrix

• [MRrb] = Runback pipe equivalent resistance matrix.

The elements of each matrix are the equivalent resistances introduced in section 3.1.

Fig. 5: Schematic of the matrixes' structure.

4.1. Input parameters

The system performance were evaluated assuming the main input parameters reported in Table 2.

Table 2. Input parameters

Parameters Values Units

Total air mass flow 2.42 kg/s

DNI 1000 W/m2

Air inlet temperature 120 °C

A detailed list of all the other parameters can be found in [8].

4.2. Model results

The first simulation of the system performance showed a non uniform mass flow rate distribution among the different HCHEs. The mass flow rates through the first HCHEs closer to the inlet section of the receiver were much higher than that at the end of the feeding pipe. Since all the HCHEs receive the same solar power, this condition can be critical because HCHEs with lower mass flow rate are (see Fig 7) not effectively cooled down and the material can experience very high temperatures.

To overcome this problem, a solution based on "diaphragm valves" was proposed. A diaphragm was introduced inside each linking pipe (see Fig 6) and their diameters were optimized in order to get a more uniform distribution of the mass flow rate for the HCHEs.

module

Fig. 6: Diaphragm solution schematic

Figure 7 shows a comparison of the mass flow rate distribution with and without the diaphragm highlighting the effectiveness of the diaphragm solution.

Fig. 7: Mass flow rate distribution with and without diaphragm

Table 3 illustrates the system performance for three different skew angles of the solar radiation, 0° 18° and 40°, with the diaphragms' solution adopted.

Table 3: System performance at different skew angles

Parameters Skew 0° values Skew 18° values Skew 40° values Units

Air output temperature 655.51 626.36 502.54 °C

Total pressure drop 28.09 27.88 27.04 mbar

Pumping power 7.6617 7.6047 7.3759 kW

Power coming from sun 2056.1 2056.1 2056.1 kW

Optical efficiency 79.16 74.76 56.90 %

Power removed by the air flow 1388.2 1314.3 998.95 kW

Collector efficiency 67.52 63.92 48.58 %

Power block efficiency 30 30 30 %

Electrical power available 416.47 394.29 299.68 kW

Net power available 408.81 386.69 292.31 kW

Overall efficiency 19.88 18.81 14.22 %

As shown in Table 3 the thermal efficiency decreases at higher skew angles. The mass flow rate, constant for all the cases, was optimized for a 0° skew angle condition. During real working conditions, mass flow rates can be tuned in order to keep the same air outlet temperature. Figure 8 shows the air temperature inside the runback pipe and the runback pipe wall temperature versus the cavity position. Hot air flows in the opposite direction with respect to the one flowing in the feeding pipe, i.e., from the cavity position 2304 to the cavity position 1, accordingly to the model depicted in Fig. 2.

Cavity position

Fig. 8: Air temperature inside the runback pipe (blue line) and runback pipe wall temperatures (red line) vs cavity position.

5. Conclusion

A mathematical model based on a pneumatic - electrical circuit analogy was developed to analyze the performance of a novel solar receiver based on an array of absorbing cavities and using air as heat transfer fluid.

The zero dimensional model developed, implemented in a Matlab routine, is able of simulating the flow through the receiver and evaluates the pressure drop. To guarantee an even distribution of the mass flow rate through the 4608 HCHEs of the 212 meters long receiver, the model accounts for the possibility of controlling the system linking pipe cross sections. A cavity thermal model has been used to compute the air outlet temperature for each

HCHE and the temperature at the receiver outlet section. Heat losses along the runback pipe were taken into account and the different sub-model results were validated by means of CFD simulations.

The model developed in this work enable to calculating the collected thermal power as well as the required pumping power for different skew angles. An air outlet temperature of about 650°C was achieved for the 0° skew angle condition with a solar to thermal efficiency of approximately 67%.

Acknowledgements

This work was developed within the framework of SolAir-2 Project which involves SUPSI, ETHZ and Airlight Energy Manufacturing SA, funded by the Swiss Federal Office of Energy (SFOE - grant number SI/500091-01).

References

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[2] Montorfano D. Barbato M., Gaetano A., Pedretti A., Malnati F., Pusterla S., Thermal insulation based on radiation shields for CSP-HTF pipes: CFD modeling and experimental validation. Marrakech: SolarPACES 2012

[3] Qengel YA, Cimbala JM. Fluid mechanics: fundamentals and applications. 2nd ed. McGraw-Hill; 2009.

[4] Crane co. Flow of fluids: through valves, fittings, and pipe. Technical Paper 1982; 410 M.

[5] Manlapaz RL, Churchill SW. Fully developed laminar flow in a helically coiled tube of finite pitch. Chem Eng Commun 1980; 7:57-78.

[6] Barbato M, Gaetano A, Montorfano D, Zavattoni S, Di Stefano GM, Matarrese P, Good P, Steinfeld A, Zanganeh G, Ambrosetti G, Malnati F, Pedretti A, Pusterla S. Innovative solar collectors for efficient and cost-effective solar thermal power generation - 2. BFE report; July 2012

[7] Manlapaz RL, Churchill SW. Fully developed laminar convection from a helical coil. Chem Eng Commun 1981; 9:185-200.

[8] Matarrese P., Studio delle prestazioni del ricevitore di una centrale elettrica a concentrazione solare (CSP) (Italian), Bachelor thesis, SUPSI, 2012,C07926.