Investigation of convective and radiative heat losses from modified cavity based solar dish steam generator using ANN

https://doi.org/10.1016/j.ijthermalsci.2014.08.005Get rights and content

Highlights

  • Estimation of heat losses from modified cavity receiver of solar dish.

  • Investigation of cavity receiver under saturated and superheated boiler conditions.

  • Development of Nusselt number correlation for total heat loss.

  • Artificial neural network simulations are carried out to predict heat loss.

Abstract

In this article, total heat losses from three configurations of modified cavity receiver of solar parabolic dish are investigated using 3-D numerical model. The effects of various parameters such as diameter ratio (d/D), angle of inclination (θ), operating temperature (T), insulation thickness (t) and emissivity (ε) of the cavity cover on the heat losses from the modified cavity receiver are investigated. The variable boundary conditions are considered for modified cavity receiver to produce sub-cooled hot water, saturated steam and superheated steam. An Artificial Neural Network (ANN) model is developed to predict the heat loss for a large set of influencing parameters. Based on ANN modelling, improved Nusselt number correlations are proposed for convective, radiative and total heat losses from the modified cavity receiver. The convective heat losses are greatly influenced by receiver inclination whereas the radiation heat losses are influenced by the cavity cover emissivity. The diameter ratio also plays a major role in heat losses from the cavity receiver. The total heat loss is estimated as 522 W for superheat steam condition for θ = 0 and d/D = 0.05; whereas it is 288 W for diameter ratio 1. The total heat loss varies 25% for inclinations 0 and 90°. For saturated and sub-cooled steam conditions, the total heat loss variation is found to be 28% and 21% respectively from 0 to 90° inclinations for diameter ratio of 0.05. The present method predicts the heat losses more accurately compared with the existing models.

Introduction

Solar energy provides a promising scope of usage in process heat and power generation applications. The solar energy can be utilized in many ways, out of which concentrating solar collectors plays a major role in production of high temperature heat. The solar thermal sources have a large amount of potential (5.6 EJ/y) for process heat in 2050 [1]. In various industrial sectors, like food, textile, paper and pulp, etc., the share of heat demand at low temperature (less than 100 °C) is about 30% and medium temperature (100–400 °C) is about 27% and high temperature applications (above 400 °C) is about 43% [2]. International Energy Agency's Solar Heating and Cooling programme (IEA SHC) has counted about 200 solar process heat systems worldwide till 2010, corresponding to a total capacity of 42 MWth. Among many countries, India leads the list of having about 10% solar process heat systems of total installed solar thermal systems in 2010 [3]. The usage of parabolic dish collector for process heat applications has been least analysed or utilized. Scheffler dishes are used to produce low temperature steam and are used for low temperature applications like cooking. Sharma et al. [4] performed field tests on paraboloid solar collectors that are used for process heat applications in milk pasteurization unit.

The solar parabolic dish collector is the most efficient energy conversion technology out of the four concentrated solar power technologies namely; linear Fresnel collector, parabolic trough collector, parabolic dish collector and power tower. The solar parabolic dish collector consists of a parabolic shaped reflector, and a cavity receiver placed at the focal point to convert concentrated solar energy into thermal energy (Fig. 1). The receiver plays a major role in conversion of the incident solar energy into thermal energy. It becomes necessary to study the heat losses from the cavity receiver to estimate the useful conversion of heat from solar energy.

Various researchers have carried out extensive research on heat loss analysis from the cavity receiver of different geometric shapes and design parameters. Wu et al. [5] presented a comprehensive review of convection heat loss mechanism, experimental and numerical studies on various shapes of cavity receivers for parabolic dish system. Various cavity designs related to electronic cooling, buildings, etc., are also presented in the work apart from those related to solar receiver design. Harris and Lenz [6] analysed the thermal performance of cavity receivers of different shapes considering heat loss mechanisms. It has been reported that the variations in concentrator rim angle and cavity geometry affects the power profiles. The system efficiencies of about 60–70% are attainable with state of art parabolic dish technology available during 1985 for working temperatures of 550 °C–900 °C. A comparison of convective heat losses from cavity receivers for a solar parabolic dish collector is carried out by Kumar and Reddy [7]. The thermal performance of three types of cavity receivers is analysed and modified cavity receiver with an optimum area ratio is found to be suitable for low cost solar parabolic dish collector [8].

A combined laminar convection and radiation heat loss analysis from a modified cavity receiver has been performed by Reddy and Kumar [9]. The inclination of the receiver causes wide variation in convective heat loss and emissivity affects the radiation heat loss. The influence of various parameters on total heat loss from the receiver has been investigated. Separate correlations have been proposed for convective and radiative Nusselt numbers. Kumar and Reddy [10] also used asymptotic computational fluid dynamics technique to investigate the combined natural convection and radiation heat losses from a modified cavity receiver, which requires fewer data points than usual techniques.

Kumar and Eswaran [11] carried out a numerical investigation of combined radiation and natural convection in a differentially heated cubic cavity with two opposite sidewalls maintained at a temperature ratio (Thot/Tcold) of 2 with cold wall temperature at 500 K, and all other walls insulated. A non-Boussinesq variable density approach has been used to incorporate variation in density due to temperature variation. The convective, radiative and total heat loss analysis from cavity walls has been performed for different flow phenomena with Ra = 105 and Pr = 0.71. Gonzalez et al. [12] numerically analysed the heat transfer from an open cavity by natural convection and surface radiation and concluded that for large temperature differences, radiative heat transfer predominates convective heat transfer. Balaji and Venkateshan [13] reported combined conductive, convective and radiative heat loss from a vertical slot. The parametric studies have been made to study the effect of various parameters with the conjugate heat transfer along with nature and coupling between different modes of heat transfer. Natarajan et al. [14] presented the combined heat loss characteristics of trapezoidal cavity for linear Fresnel reflector and presented Nusselt number correlation to study the effect of various parameters on heat loss.

Wu et al. [15] have proposed a heat pipe receiver for solar parabolic dish system and found that convective heat loss from receiver is influenced by aperture position and size, which in turn depends on tilt angle. The natural convection heat loss linearly varies with wall temperature for different aperture positions and sizes. Prakash et al. [16] performed experimental and numerical convective heat loss analysis from cylindrical cavity receiver for different inlet fluid temperatures and various inclinations considering wind and no-wind conditions. Prakash et al. [17] also numerically investigated natural convective heat loss from open cavities viz., cubical, spherical and hemispherical cavities for same heat transfer area and proposed a Nusselt number correlation considering the effect of cavity shape, inclination angle, opening ratio and Rayleigh number.

Artificial Neural Networks (ANN) have been used by the researchers for a variety of applications like solar resource estimation [18], simulation model for maximum power point tracking controller of PV system [19], optimization of high ash coal fired ultra-critical power plant [20]. An attempt has been made to estimate the combined natural convection and radiation heat losses from a modified cavity receiver for the wide range of operating parameters.

The isothermal cavity receiver surface and adiabatic cavity outer walls with or without wind conditions have considered in the past. However, in practical, the cavity wall surface temperature depends on the type of application it is used for and the cavity outer walls are not adiabatic. Varying temperature boundary condition along the inner surface walls of the modified cavity receiver is considered for three different receiver configurations say, superheated (500 °C, 45 bar), saturated (250 °C, 40 bar) and sub-cooled (180 °C, 10 bar) conditions. The temperatures considered in the present study are based on the steam generation in CSP applications. Lovegrove et al. [21] demonstrated the generation of superheated steam using parabolic dish collector for multi megawatt power generation. The saturated steam finds its applications in food-processing industries, textile industries and steam cooking purposes. The sub-cooled steam or hot water is used for air conditioning purposes, paper and pulp industries and desalination, etc., [22]. In this article, combined natural convection and surface radiation from modified cavity receiver of a solar parabolic dish collector is investigated for different operating temperatures, different inclinations of the receiver, insulation thickness, diameter ratio and cavity cover emissivities.

Section snippets

Mathematical model

The modified cavity receiver consists of tubes wound in a hemispherical shape and covered with insulation to minimize the heat loss from the receiver. The receiver diameter is considered as 0.305 m based on the size of the focal image. The tubes are considered as a strip equal to half the radius of the actual tube. The surface temperature of the receiver is assumed same as the temperature of the fluid flowing through the tubes. The ceramic insulation and a metal sheet of thickness of 2.5 mm has

Results and discussion

The effect of various influencing parameters such as receiver inclination, insulation thickness, diameter ratio, and cavity cover emissivity on the heat losses from the modified cavity receiver have been discussed below.

Conclusion

A 3-D numerical investigation was carried out to study the effect of various parameters on the heat loss from the modified cavity receiver of solar parabolic dish collector for three different steam generation conditions. It is found that the convective loss is greatly influenced by the orientation of the receiver whereas radiation heat loss remains constant for different inclination of the receiver. The convective heat loss is high at θ = 0°and found least for θ = 90°. The effect of variation

Acknowledgement

The High Performance Computing Facility (HPCF) of P. G. Senapathy Center for Computing Resources, IIT Madras is used for performing the simulations.

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