Systems with Concentrating Solar Radiation

Systems with Concentrating Solar Radiation

Saša R. Pavlović (University of Niš, Serbia) and Velimir P. Stefanović (University of Niš, Serbia)
DOI: 10.4018/978-1-4666-4450-2.ch031
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Abstract

In this chapter, description and working principles of the parabolic trough power plants, solar tower power plants, parabolic dish power plants, and power plants with Fresnel reflectors in the world and their potential use in Serbia are given. In addition, the examples and technical characteristics of some concetrating solar power plants in the world are given. The cases in which solar cells are used to generate electrical energy are very rare. Solar systems referred as mid temperature (100–400 oC) are considered suitable for integration with industrial processes, cooling, and polygeneration systems through use of concentrating solar collectors. The results of this research may be applied in the construction of small solar systems, but also in the design and construction of large polygeneration systems. Physical and mathematical model is presented, as well as numerical procedure for predicting thermal performances of the P2CC (Parabolic-and-Circular Collector) solar concentrator.
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Introduction

Global increase in the number of population and this same increase in fortunes bring about increased consumption thus creating lack of resources especially sources of energy and drinking water. It is estimated that the consumption of electrical energy will double in the next 15-20 years. Irrespective of the ecological arguments increase of the production of electrical energy generated by renewable sources is necessary so that countries diminish their dependance on the import and the necessity to provide for new resources. Renewable sources are in the long run a valuable alternative. Many areas in the world abound in free solar energy while in some other areas wind and other types of renewable sources of energy impose themselves as a logical choice (Marković et al., 2011). Having in mind that fossil fuels on Earth are limited and that their usage ensues the emission of CO2 that exerts negative influence on the environment, more and more renewable sources of energy are being used worldwide with the Sun as a primary source of energy. By means of the adequate equipment the energy of the sun irradiation can be converted into thermal and electrical energy. Depending on the degree of the heating of the working fluid we can differ between the low-temperature (T<100oC), middle-temperature (100oC<T<400oC) and high-temperature conversion (400oC<T<4000oC). For low-temperature solar energy conversion one uses flat collectors with water and air, for middle-temperature conversion one uses vacuum colletors and collectors with concentrators, and for high-temperature one uses solar furnaces and CSP plants (Tomislav, 2007) and (Fernández-García et al., 2010). Concentrated Solar Power (CSP) plants denote plants that generate electrical energy by means of concentrated sun irradiation. CSP plants are composed of the solar concentrator, steam turbine and electricity generator. Solar concentrators can be parabolic troughs, heliostats, parabolic dishes and Fresnel reflectors (Robert, 2007). For the continuous functioning of the CSP plant during the night and in overcast days a thermal energy from the heat tank or gas as an additional source of energy is used (Sharma, 2011; Kaygusuz, 2011). The chapter goes on to give a description of the P2CC (Parabolic-and-Circular Collector) solar concentrator, solar energy potential in Serbia, current and future solar energy activities in Serbia. This chapter gives the analysis of the energy system simulation software, with an energy system defined as any system capable to satisfy one or more energy demands. The scope of this paper is to give brief descriptions about the energy system software, their similarities, differences, features, capabilities, limitations, and help modelers to choose a most suitable package for different tasks. The survey focuses and tries to provide data regarding most important features of the software such as:

Key Terms in this Chapter

P2CC Solar Collector (Parabolic-and-Circular Collector): The investigated concentrator labelled as P2CC has a tubular receiver and two identical reflectors, one from each side of the tubular receiver. The tubular receiver is a concentric tube consisting of a metal tube (absorber) inside the glass tube. Each reflector curve is defined by using three curves where one curve is parabola and two curves are circular arcs. The prototype P2CC has high efficiency, low price, possibility of its production in small and medium companies, and wide distribution. Its application in a large part of year (or entire year) would mean significant savings of electric energy which is now used for preparation of warm water, and in central and long-distance heating systems.

Polygeneration: One of the most effective measures for increasing energy efficiency is the use of polygeneraton systems for simultaneous heating, cooling and electricity production. An integrated process which has three or more outputs, that include energy outputs, produced from one or more natural resources. In this way, a substantial increase in overall efficiency is achieved, and thus, indirectly, a reduction of pollution and green house gas emissions. Small scale polygeneration systems involve a combination of conventional and new technologies for heating cooling and electricity production, but also other products such as CO 2 , bio-fuels etc. Polygeneration system may be modeled as a combination of its components as substructures. The system is than built by connecting substructure models. The connections are made in node points where the flows cross the set control volumes. Generally, the behavior of a polygeneration system is represented by a set of nonlinear differential equations and a system of non linear algebraic equations.

Helios Software: A computer program for modeling the optical behavior of reflecting solar concentrators. HELIOS is a flexible computer code for evaluating designs for central-receiver, parabolic-dish, and other reflecting solar-energy collector systems; for safety calculations on the threat to personnel and to the facility itself; for determination of how various input parameters alter the power collected; for design trade-offs; and for heliostat evaluations. Input variables include atmospheric transmission effects; reflector shape, surface, and suntracking errors; focusing and alignment strategies; receiver design; placement positions of the tower and mirrors; time-of-day and day-of-year for the calculation.

Brayton Cycle: The solar thermal Brayton cycle uses the concentrated power of the sun as a heat source to generate mechanical power. Low operation and maintenance costs make the small-scale open and direct solar thermal Brayton cycle with recuperator attractive for power generation. A black solar receiver, mounted at the focus of a parabolic dish concentrator can be sized such that it absorbs the maximum amount of heat. The irreversibilities of the open and direct solar thermal Brayton cycle with recuperator are mainly due to heat transfer across a finite temperature difference and fluid friction, which limit the net power output of such a system. In this work, the method of total entropy generation minimisation is applied to optimise the geometries of the receiver and recuperator at various steady-state weather conditions.

Parabolic Dish Systems: Parabolic dish systems consist of a parabolic-shaped dish concentrator that reflects solar radiation onto a receiver mounted atthe focal point. These concentrators are mounted on active tracking systems to follow the sun. The heat collected by the receiver is typically used by a heat engine mounted on the receiver that moves with the dish structure, such a Stirling and Brayton cycle engines. To be most effective the parabolic-shaped concentrator needs to focus the sunlight on thereceiver and hence the shape of the parabola needs to be relatively precise. Large parabolic dish concentrator mirrors are an important component of many solar energy systems. They need to be relatively precise and are expensive to fabricate and to transport.

Tubular Receiver for P2CC Solar Collector: The tubular receiver is a concentric tube consisting of a metal tube (absorber) inside a glass tube. The cylindrical copper absorber is surrounded by a glass layer for lowering convective loss from the collector pipe. The area between the pipe absorber and the glass surrounding layer is evacuated. Conversion of solar energy into heat is conducted on the pipe collector. The pipe absorber is colored with a selective color of high absorbing properties and low emissivity ( e r ).

Monte Carlo Ray – Tracing Technique: Although the term Monte Carlo encompasses a broad range of numerical integration tools based on random sampling, in the case of concentrating collector analysis there is a convenient physical analogy: “bundles” of photons are ray-traced as they enter the collector from randomly-sampled locations over the aperture and undergo multiple randomized surface-interaction events in until they are either absorbed at a surface or escape from the collector aperture. It should be noted that, in order to simplify calculation throughout this thesis, geometry is treated as fully two-dimensional, although an extension to include 3D effects is straightforward. In addition, incoming radiation is assumed to be perfectly collimated, so that only the initial ray position must be sampled.

CIRCE 001: This is a computer code for analysis of point - focus concentrators wuth flat targets. CIRCE, an acronym for convolution of Incident Radiation with Concentrator Errors, was developed for the optical analysis of point-focus concentrating dish collector systems. The solution techniques of CIRCE, in which the concentrator errors are convolvedwith the solar intensity profile (sunshape) to produce the flux density distribution onan arbitrary target, are identical to those used in HELIOS. CIRCE may be used to analyze reflectors that are spherical, parabolic, or flat and are either a continuous or faceted surface. The curvature and aim point of each optical facet that comprises the reflector may be individually assigned. The current version of CIRCE, referred to as CIRCE.OO1, is restricted to analyses of reflector systems with flat rectangular or circular targets.

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