Simulation and economic analysis of a CPV/thermal system coupled with an organic Rankine cycle for increased power generation

Abstract

In concentrating photovoltaic (CPV) systems the incident solar radiation is multiplied by a factor equal to the concentration ratio, with the use of lenses or reflectors. This is implemented, in order to increase the electric power production, since this value has a linear dependence from the incident radiation. Therefore, the specific energy production of the cells (kWh/m²) radically increases, but due to this high intensity CPVs consequently operate at elevated temperatures, because heat dissipation to the environment is not so intense and heat produced cannot naturally convected. This temperature increase not only leads to a reduction of their electric efficiency, but also it must be dissipated, since issues regarding their degradation and reduction of their lifetime might arise. There are many reported ways of removing this heat, either by adding a cooling unit on the back side of the CPV module, or by recovering with possible uses in buildings, industry, additional power production or even desalination of seawater.

The current work is actually a feasibility study, concerning a concentrating photovoltaic/thermal (CPV/T) system, where the heat produced is recovered by an organic Rankine cycle (ORC) for additional power production. A pump drives the organic fluid of the cycle, which is evaporated in the tubes of the CPV/T and driven to an expander for mechanical power production. For the condensation of the organic fluid several possible alternatives can be applied. That way, the PV cells can be cooled effectively and increase their electrical efficiency, while the recovered heat is designated to produce additional electric energy through the organic Rankine process, when the expander of the Rankine engine is coupled to a generator.

The scope of the present work is to investigate an alternative application of concentrating PV modules through exploiting the generated heat by the ORC process and combining both technologies into an integrated system. The design of the system is presented in details, along with an optimization of some main parameters. The performance of the system will also be examined and compared with an equivalent conventional CPV system, referring to their design points. Finally, the annual and daily performance will be studied, which is a more realistic indicator, concerning the increased efficiency this integrated system is expected to have, followed by a cost analysis, in order to examine its economic feasibility as well.

Introduction

CPV/T systems currently constitute a major research subject. Lately, demonstration units have been constructed, in order to experimentally estimate their performance and some of their operational characteristics (Kribus et al., 2006, Mittelman et al., 2007, Rosell et al., 2005, Solanki et al., 2008). Nevertheless, very few commercial units have been realized. The variety of the concentration ratios is evident, since it strongly depends on the design, the orientation of the module, the possibility for incorporation of a sun-tracking system, which increases the complexity and control of the system (Chow, 2010), and the area availability, starting from values of 2 (Solanki et al., 2008) and reaching even 500 or more (Kribus et al., 2006). Nevertheless, concentration ratios for small-scale systems vary in the range from 10 to 40 (Coventry, 2005, Rosell et al., 2005).

A crucial aspect for their commercialization is new solar cells to be designed, constructed and long-term tested at elevated temperatures (Meneses-Rodrıguez et al., 2005). There is a demand for solar cells that their efficiency does not decrease substantially, when increasing their operational temperature (Kalogirou and Tripanagnostopoulos, 2007). In other words, the temperature coefficient in the expression of the electric efficiency of a solar cell (Skoplaki and Palyvos, 2009) should be as low as possible. To this direction many researchers deploy their experimental and theoretical studies, concerning new materials that can be used and also reducing the cost of manufacturing the next generation’s cells (Andreev et al., 2004, Green, 2001, Green, 2003, Nishioka et al., 2006, Yamaguchi et al., 2005, Yamaguchi et al., 2006). Most of these studies focus on CPVs, since their operation is much more demanding than the conventional flat plate collectors’ one.

An important design parameter that can influence the cells’ performance is the efficiency of the heat dissipation equipment and the accurate calculation of the heat transfer coefficient, in order for the PV cells to be cooled adequately. This issue has been treated by many researchers, who have conducted simulation and experimental studies concerning this feature (Ji et al., 2006, Tripanagnostopoulos et al., 2002, Tripanagnostopoulos et al., 2005, Tripanagnostopoulos, 2007).

As far as the organic Rankine cycle is concerned, its performance depends mainly on the evaporation/condensation temperatures of the organic fluid and their selection is a vital aspect, influencing drastically the overall system’s efficiency (Kosmadakis et al., 2009a). There are also many reported works in the literature, dealing with the proper selection of an organic fluid to be used in an ORC for applications of low-temperature (60–80 °C) heat recovery to high (120–150 °C) (Borsukiewicz-Gozdur and Nowak, 2007, Hung et al., 1997, Maizza and Maizza, 2001, Saleh et al., 2007, Tchanche et al., 2009, Tchanche et al., 2010, Wang and Zhao, 2009). Therefore, this aspect will not be elaborated in the present study and the results and conclusions extracted from the relevant works of the current authors (Kosmadakis et al., 2009a, Tchanche et al., 2009, Tchanche et al., 2010) will be applied here too.

The design and simulation of an organic Rankine cycle is of importance as well. The various components should be selected properly, in order for the integrated system to operate efficiently not only in the design point, but also in partial loads. Most of ORC components are available in the market, except from the expander, which is the most ambiguous component. It should be able to handle organic fluids and moreover it should meet the efficiency expectations addressed. Its lifetime and cost is another parameter considered, together with its efficiency at partial loads. Recently, the works of some researchers revealed that for small-scale systems, as the one investigated in the present study, the most suitable expander is a scroll compressor in reverse operation (Badr et al., 1984, Lemort et al., 2009, Manolakos et al., 2007, Manolakos et al., 2009a, Manolakos et al., 2009b, Quoilin et al., 2010), and this is the one considered in this study. The simulation studies concern the identification of the thermodynamic cycle and the calculation of the organic fluid properties at state changing locations of the Rankine engine. By introducing also the efficiency of every component, the mechanical power production of the expander and the pump power consumption (Kosmadakis et al., 2009b, Kosmadakis et al., 2010a, Kosmadakis et al., 2010b, Wei et al., 2007) can be calculated. The final goal is to estimate the efficiency, varying in the range of 6–12%, for the ORC and depends on the operational temperatures and the total number of thermodynamic stages (Kosmadakis et al., 2009b).

The concept of the present study can be considered a step forward in the theoretical research activities described in (Kribus and Mittelman, 2008, Vorobiev et al., 2006a, Vorobiev et al., 2006b). The bottoming heat engine, mentioned in the aforementioned works, has been replaced by an ORC engine, which has proved its reliability and the variety of applications used. Only a relevant approach is reported by NASA in the 1980s for incorporation of organic Rankine cycles in photovoltaic modules for applications in their space stations (Chubb, 1987, Hallinan, 1987), but this research did not reach concrete results, and later was abandoned, since the installation of such components in zero-gravity environment is prohibitive (Macauleya and Davis, 2002).

The design of the system is described in the following sections, along with a preliminary optimization study, in order to specify the most efficient design point, by further investigating some parameters, such as the concentration ratio and the organic fluid’s mass flow rate. The latter has an important contribution, since it is correlated directly to the evaporation temperature of the organic fluid and the operational temperature of the PV cells. The system’s simulation is also presented, calculating the combined performance for two characteristic time-periods, a winter one and a summer one. In addition, the annual simulation of the optimized system takes place, pointing out the overall performance at mixed full and partial loads too and comparing it with the performance of an equivalent system without the incorporation of the ORC. Finally, an economic analysis is implemented, in order to reveal, if the suggested integrated system performs better in economic terms than the same one without the ORC.