Design and simulation of a prototype of a small-scale solar CHP system based on evacuated flat-plate solar collectors and Organic Rankine Cycle
Introduction
In the last few years, the interest towards the Organic Rankine Cycle (ORC) technology is considerably increased, due to its potential application in distributed generation systems and to its favorable characteristics in the exploitation of low-temperature heat sources [1]. ORC systems are especially attractive when coupled with solar thermal collectors in a solar power system [2]. Usually, small-scale solar power systems are based on concentrating solar thermal collectors and volumetric expanders [3]. Solar systems can get different temperature levels: the appropriate selection of the ORC working fluid allows one to use even the low-temperature solar thermal heat. The selection of the working fluid plays a key role in ORC design. Many studies on the criteria for the selection of the organic working fluid are available in the scientific literature. Rayegan and Tao presented a thermodynamic study using Refprop database, comparing 117 organic fluids. They selected 11 possible candidates for low or medium temperature ORC systems (for example powered by solar collectors), showing that the best performance was achieved by R-245fa, R-265mfc and R-245ca [4]. Madhawa Hettiarachchi et al. investigated the use of ammonia, HCFC123, n-pentane and PF5050 in ORC powered by low-temperature geothermal resources. They concluded that HCFC 123 and n-pentane showed the best performance, while PF 5050 has the most preferable physical and chemical characteristics [5]. HCFC 123 was also investigated by Yamamoto et al. [6]. The authors developed a numerical code and performed an experimental analysis in order to test the possible vantages of this fluid. Both numerical and experimental results showed a dramatic increase of the system performance with respect to other conventional fluids [6]. In case of low-grade thermal sources, a good performance of the ORC is achieved in case of wet fluids showing very steep saturated vapor curve [7]. From the exergetic point of view, the best performance is obtained by R236EA, as shown by Dai et al. presenting a parametric optimization of ORC systems [8]. When the ORC is coupled to a vapor compression cycle for ice production, R123 shows the best performance in both cycles [9]. Calise et al. [10] investigated the performance of the ORC system object of this work by using different working fluids and varying the heat source temperature level from 120 °C to 300 °C. The authors stated that two organic mediums are suitable for the exploitation of low to high temperature heat sources, namely, n-butane and isobutane, while the R245fa can be used when the heat source temperature reaches up to 170 °C. Many authors simulated and modeled the ORC with various methods. Li and Sun [11] reported an analysis of an ORC recovery heat power plant using R134a as a working fluid. The authors developed a new mathematical model of the ORC system; the ROSENB algorithm with penalty function method was used to search the optimal set of controlled variables: relative working fluid mass flow rate, condenser air fan mass flow rate and expander inlet pressure. Lemort et al. [12] also presented both numerical and experimental analyses of an ORC system using R-123 as a working fluid; the experimental study demonstrated the importance of using adapted positive displacement compressor as expander in small scale ORC system. Wang et al. [13] presented a simulation of a solar-driven regenerative ORC; they analyzed the variations of thermodynamic parameters on the system performance in steady-state conditions and employed a genetic algorithm to perform the parameter optimization using the daily average efficiency as objective function. Quoilin et al. [14] investigated the design of a small-scale solar ORC for rural electrification purposes; the model – developed in Engineering Equation Solver (EES) environment [15] – was used to design the components of the cycle and to calculate the ORC performance under different working fluids and expansion machine configurations, single and double stage. Dincer and El-Emam [16] showed an exergy analysis of a trigenerative ORC system fed by solar energy; in order to identify the main exergy destruction sources and to improve the global system exergy efficiency, authors made reference to three different operation modes; again, a mathematical model of the system in EES environment was developed to this scope; authors assess that the ORC evaporator is one of the main sources of exergy destruction while the variation of turbine inlet pressure does not significantly affect the exergy performance of cycle.
Solar power plants are commonly based on concentrating solar collectors. In particular, parabolic trough collectors (PTCs), Fresnel (particularly suitable for solar ORCs, since they require a lower investment cost, but work at lower temperature [17]) and others. In fact, for conventional systems, solar concentrating collectors can achieve the temperatures required to drive a heat engine. In conventional stationary non-concentrating solar thermal collectors, outlet temperature is too low for such kind of applications. Regarding PTCs, the NREL (National Renewable Energy Laboratory) analyzed a small scale MTPP (Modular Trough Power Plant, from 500 to 1500 kWe) consisting in a Concentrating Solar Parabolic using diathermic oil as a heat transfer fluid, and an ORC turbine with pentane as a working fluid [18]. Different cycles are analyzed: conventional, superheated and regenerated. The maximum temperature of the cycle is 304 °C. Results show that the best configuration from a technical point of view is achieved in case of regenerated cycles. The results of the economic analysis show that the system is scarcely competitive without incentives. Gang et al. [19] investigated the design, analysis and optimization of a system based on a compound parabolic concentrator. Two-stage collectors and heat storage units with PCM (Phase Change Material) are adopted to improve the efficiency of the system. The thermal storage tank ensures a stable and continuous operation in case of scarce solar radiation. Storage is a key issue in solar ORC systems, since the implementation of thermal energy storage is arguably a key advantage over systems based on photovoltaic (PV) technologies, as also shown in [20], where a configuration with a regenerated ORC is presented; here, the organic fluid is preheated, saving energy and increasing the thermodynamic efficiency, but this increases the inlet average temperature of the solar collectors, reducing their efficiency; an optimal temperature value of 55 °C is found. Kosmadakis et al. [21] replaced the PTC included in conventional solar ORC power plants with Concentrating Photovoltaic/Thermal (CPVT) solar collectors, in order to increase the electrical production and the system efficiency. Such configuration shows an electrical efficiency of 11.83%, higher than that achieved by the CPVT collectors alone (9.81%). Ksayer [22] investigated the thermal recovery from the condenser of a solar ORC cycle. The author pointed out that the advantages of the ORC system are the electricity generation, the hot water production and the low cost compared to photovoltaic electric power generation [22]. A model for a typical parabolic trough solar thermal power generation system with Organic Rankine Cycle (PT-SEGS–ORC) was built within the transient energy simulation package TRNSYS by He et al. [23] analyzing the effects of several key parameters. The study shows that the variation of heat collecting efficiency with oil flow rate increases sharply and then approaches a constant value. In addition, the optimal volume of the thermal storage system was found sensitively dependent on the solar radiation intensity [23].
The studies cited above are based on concentrating high-temperature solar collectors, in order to achieve a high driving temperature of the ORC and reasonable efficiencies for the system as a whole. In fact, when the ORC is coupled with low or medium-temperature solar collectors, the overall efficiency may significantly decrease. However, the use of concentrating solar collectors in small-scale systems has many disadvantages. They are very difficult to integrate in buildings, especially in urban areas, and are very sensitive to dust deposition and to the efficiency of the tracking system, so that a continuous and accurate maintenance is mandatory.
The present paper proposes an alternative technology with respect to the concentrating one, in order to remove the above cited disadvantages. In particular, as discussed in detail later on, the layout presented in this paper is based on innovative stationary non-concentrating solar thermal collectors. In authors’ knowledge the attempt to the design such an innovative has never been successfully (i.e. at reasonable ORC efficiency) performed in literature. In fact, the use of flat-plate solar collectors in solar power applications is scarcely investigated. Zhao et al. [24] designed, built and tested a low-temperature solar ORC, using R245fa as a working fluid, coupled with evacuated solar collectors. The results showed an overall power generation of 4.2%. Wang et al. [25] presented a regenerative ORC coupled with flat-plate solar collectors. The results indicate that increasing turbine inlet pressure and temperature or lowering the turbine back-pressure could improve the system performance. The parametric optimization also shows that system performance is improved when the turbine is supplied by high temperature saturated vapor. Compared with other working fluids, R245fa and R123 show the highest system performance and lowest operation pressures. The calculated electrical efficiency ranges between 4% and 6% [25].
Another significant innovation included in this paper is the development of a complete model for the ORC, using the software EES, with a variable temperature at the evaporator inlet, with subsequent implementation in a TRNSYS dynamic simulation model. In fact, the majority of the models available in literature only consider a constant hot stream temperature for the ORC and they do not consider a dynamic 1 year operation of the system. Tempesti et al. [26] investigated a hybrid solar-geothermal ORC plant, using EES; the evacuated solar collectors produce heat at 150 °C, whereas the geothermic fluid is available at 80–100 °C; the ORC efficiency (7–11%) is higher using R245fa, reducing the solar field capital costs due to lower flow rate value. Farrokhi et al. [27] developed and tested a natural gas-fired ORC-based micro-scale Combined Heat and Power system. Isopentane was used as the ORC working fluid. Experiments were conducted at different heat source temperatures, ranging from 85 °C to 65 °C. The maximum electrical power output of 77.4 W was generated with a inlet temperature of 84.1 °C, corresponding to net cycle electrical efficiency of 1.66%.
In the authors’ knowledge, the design of a highly efficient solar ORC system has never been presented, in which an ORC, operating at variable inlet temperature, is fed by stationary non-concentrating flat-plate evacuated solar collector. Such collector shows thermal efficiencies even higher than those of compact concentrating devices. In addition, it is scarcely sensitive to dust, does not use any tracking system and can be easily integrated in buildings like any conventional flat-plate solar thermal collector. In this work, the ORC model is developed in EES by zero-dimensional energy and mass balances and it allows one to evaluate the off-design performance of the system, setting the geometrical parameters of all heat exchangers of the ORC and the design conditions of the turbine. The EES ORC model was integrated into a broader dynamic model, developed in TRNSYS, in which a small-scale solar power plant is analyzed, including the ORC system and many other components, such as the novel stationary flat-plate evacuated solar collectors mentioned above. Therefore, the aim of this paper is to design, simulate and analyze a novel small-scale solar CHP system, based on stationary innovative solar thermal collectors, which can be easily integrated in urban areas, presenting a simple and cheap maintenance similar to the one of PV systems. The solar ORC power plant presented in this paper is especially suitable for small-scale applications. The results of this analysis will be used for the design of an experimental set-up to be installed in the next months, aiming at proving the feasibility of the system proposed in this paper. The paper presents a dynamic simulation model of this novel design which will be experimented in Naples (Italy) in the next future.
Section snippets
System layout
The layout of the system under investigation is shown in Fig. 1. This is the typical layout of conventional solar ORC power plants, including some novel control strategies as discussed in this section. However, as discussed in detail in the previous section, conventional concentrating solar thermal collectors are replaced by the innovative flat-plate evacuated solar thermal collectors presented in this paper. The system includes three main circuits are shown:
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SCF: Solar Collector Fluid, i.e.:
System model
The solar power system described in the previous section was dynamically simulated in TRNSYS, which is a well-known software diffusely adopted for both commercial and academic purposes. The software includes a large library of built-in components, often validated by experimental data [28]. Such methodology was also used by some of the authors in previous works (e.g. [29]), where the models of both built-in and user-developed components are discussed in detail. Here, for sake of brevity, the
Results and discussion
On the basis of the simulation model discussed in the previous section, a case study has been developed for the city of Naples, Southern Italy. The simulation is performed for one year, using the weather data included in Meteonorm package. The tool also allows one to integrate the results for different integration periods, such as: days, weeks and months. The case study regards a residential building located in Naples, South of Italy, but other locations were also investigated. The thermal
Conclusion
The results of the dynamic simulations in TRNSYS environment show the significant potential of energy savings of the system under investigation, consisting in a field of flat plate evacuated solar collectors, TVP Solar, a variable inlet temperature Organic Rankine Cycle, supplied by diathermic oil from a storage tank, and a gas-fired auxiliary heater. The dynamic simulations performed for a case study developed for Naples (Southern Italy) show that the system is capable to produce electricity
Aknowledgments
This work has been financially supported by the Regione Campania, within the framework of the POR Regione Campania program, FESR 2007–2013 (RISE project).
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