Hybridisation of biomass and concentrated solar power systems in transcritical organic Rankine cycles: A micro combined heat and power application
Graphical abstract
Introduction
The combined heat and power (CHP) generation represents an interesting alternative to traditional systems characterised by separate electric and thermal production, with an increase in energy efficiency and saving capability and a reduction in operational costs and emissions [1]. Furthermore, the use of renewable sources upsurges the sustainability of the CHP systems and reduces the carbon dioxide emissions. In this framework, Organic Rankine Cycles (ORCs) appear an interesting and flexible technology for small CHP applications, especially for low-grade energy sources [2]. In particular, the systems are characterised by lower maintenance costs, enhanced partial load performances, faster start-up and stop procedures, and greater safety with respect to conventional installations [3]. In this context, the hybridisation of intermittent renewable sources with stable ones increases the system flexibility, operation stability, and allows to meet energy demands uninterruptedly [4].
Different fully biomass-based ORC applications have been studied in the literature for cogeneration [5] and trigeneration [6] purposes. There are also several integrated technologies: biomass-based combined Brayton-Rankine cycles [7], biomass gasification systems coupled with fuel cells [8], gas turbines [9] or internal combustion engines [10] integrated with concentrated solar power units. At the same time, several full solar ORC plants have been studied in the literature [11] with a size from 20 kWel to 5 MWel [12]. On the contrary, very few studies have been carried out on hybrid ORC configurations that couple solar exploitation with programmable energy inputs capable to overcome solar intermittency (waste heat or biomass) [13]. Bellos and Tzivianidis studied both stand-alone solar ORCs [14] and hybrid ORC [15] coupled with parabolic trough collectors in order to exploit low-grade waste heat (150–300 °C) and maximise the electric energy production. The electric nominal power of the proposed hybrid system, operating at nominal conditions, is higher than 500 kW. Srinivas and Reddy [13] numerically investigated the effect of boiler operating parameters and solar share on a solar/biomass hybrid system performance. Moreover, Soares and Oliveira [4] investigated a 60 kWel hybrid solar-biomass configuration to guarantee a constant electric production. Industrial applications are also limited [16]. In fact, even though the earliest theoretical studies were carried out in 1986 [17], the first CSP/biomass hybrid installation dates 2014 [18] with a 22.5 MW steam turbine power system. According to Soares and Oliveira, a 60 kWel hybrid prototype will be installed in Tunis, in the framework of the REELCOOP project [4], whereas the first CSP/biomass power plant (Pel = 3.8 MWel) has been recently installed in Denmark [19].
To the authors best knowledge, hybrid installations with solar-biomass ORC units are not available in the range of micro-CHP generation (Pel < 50 kWel). For this reason, the present work focuses the attention on a micro-hybrid biomass/solar energy system for domestic applications with hourly variable operating conditions considering full and partial load operations.
Another novelty of the work is the evaluation of the additional solar energy that can be exploited with respect to a full-solar power unit due to the hybridisation. In fact, when the solar source is not sufficient to drive the ORC, the low-grade solar energy would be lost in a solar only system. Conversely, the biomass integration permits also the exploitation of low solar radiations.
The proposed apparatus is based on a micro-scale transcritical ORC owing to its higher performance with respect to subcritical configurations [20]. Furthermore, the supercritical cycle guarantees a more efficient heat exchange between the thermal oil and the organic fluid and a decrease in the irreversibility and energy destruction due to the heat transfer and losses [21]. Another advantage in transcritical arrangements is the component downsizing owing to the high density of fluid in the high pressure T-s region.
A biomass burner and a concentrated solar power (CSP) system have been arranged in series to feed the ORC unit and the design thermal power input has been set equal to 250 kWth. Parabolic trough collector (PTC) technology has been chosen for the solar energy exploitation, as a better alternative to Fresnel mirrors, according to Cau and Cocco [22] while the biomass combustion has been selected due to its low cost and high efficiency in small-scale applications [11]. The performance of hybrid configurations have been compared with the traditional single-source ORCs, based on biomass-fired or solar-driven units. In order to show the advantages of the hybridisation, thermal and electric energy generated by hybrid, full biomass and full solar plants have been evaluated for different operating strategies. Furthermore, biomass and solar shares, biomass consumption, global electric and cogeneration efficiency, energy utilisation factor, primary energy saving index and surplus thermal power have been calculated. Possible additional users in a smart-grid community have been considered to exploit the thermal surplus.
Section snippets
Materials and methods
The hybrid CHP scheme is visible in Fig. 1. A concentrated solar power (CSP) system and a conventional biomass burner are integrated in series and are used to feed the organic Rankine cycle through a thermal oil circuit to prevent organic fluid chemical instability and overheating [2]. The ORC consists of a feed pump system, a heat generator, a turbine, and a condenser while the CSP system is based on parabolic trough collectors (PTC).
The organic fluid is pumped to the heat generator, where the
Results and discussion
The energy performance of a hybrid combined heat and power (CHP) system has been modelled and analysed. The apparatus is fuelled by biomass and concentrating solar energy sources to drive a transcritical organic Rankine cycle (ORC) unit and it is capable to operate at partial loads to meet the energy request of typical domestic users. The design ORC thermal power input has been fixed to 250 kWth. The hybrid arrangement (HYB) has been also compared with full solar-driven (SD) and biomass-fired
Conclusions
A fully renewable hybrid biomass/solar transcritical ORC system for combined heat and power (CHP) application has been modelled and compared with equivalent biomass only and solar only units. A concentrating solar power system (CSP), consisting of parabolic trough collectors, and a conventional biomass boiler operate in series to assure the thermal input to the hybrid ORC apparatus. The solar source has the priority while the biomass burner operates when the solar radiation is low or absent.
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