A novel solar trigeneration system integrating PVT (photovoltaic/thermal collectors) and SW (seawater) desalination: Dynamic simulation and economic assessment
Introduction
During the last few years, the worldwide energy consumption is rapidly increased, basically due to the dramatic growth of the emerging Countries. Unfortunately, the majority of such energy is obtained by fossil or non-renewable fuels (such as: gas, oil, carbon, nuclear), whose future availability and environmental impact are becoming a severe issue. In addition, the consumption of fresh water is dramatically increasing, so that several scientists consider such resource crucial for the future, even more than energy. As a consequence, a more sustainable energy supply scheme should be considered in order to achieve a sustainable and environmental friendly worldwide development [1]. In this framework, renewable energies are considered one of the most promising technologies. In particular, the present work focuses on a combination of solar energy and biomass, used to produce simultaneously thermal energy for heating and cooling, electrical energy and desalinated water. The following technologies are simultaneously included in the system: SHC (Solar Heating and Cooling), CPVT (Concentrating Photovoltaic/Thermal collectors) and MED (Multiple-Effect Distillation) for SW (seawater) desalination.
SHC systems can convert the solar irradiation incident on a solar collector field in both heating and cooling energy. Thus, the SHC technology is particularly attractive in summer, when the demand for cooling is often simultaneous to the availability of solar radiation [2]. Unfortunately, the number of operating SHC systems is very low, mainly due to their high installation cost. In the last decade, several design tools have been developed in order to support the implementation of SHC technologies and reducing their costs. For this purpose, advanced modeling and simulation tools of SHC plants play an important role for analyzing and optimizing the system layout, the control strategy and the components operation [3], [4], [5], [6], [7], [8], [9], [10].
PVT (photovoltaic/thermal collectors) are solar devices which simultaneously provide electricity and heat. The basic principle of a PVT collector is simple, since it can be obtained by a conventional thermal collector whose absorber is covered by a suitable PV layer [11]. The absorbed thermal energy is distributed to a fluid (typically air or water), whereas the PV produces electricity [12], [13]. The final result of this arrangement is the combined production of electricity and heat and a possible improvement of PV efficiency. In fact, the PV electrical efficiency is strongly dependent on the system operating temperature, linearly decreasing with high values of such parameter [14]. Although the basic idea of the PVT was developed about 40 years ago, such technology is still far from a mature commercialization [15]. Thus, several novel PVT arrangements were recently investigated [15], [16], [17], [18], [19]. CPVT are considered as one of the most promising PVT arrangement. CPVT are simple PVT collectors placed in the focus of some reflectors (Fresnel, parabolic, dish, etc.) [12], [13], [20], [21]. In theory, the specific cost of such system is dramatically lower than that of a flat plate PVT, due to the lower amount of PV needed per unit area. CPVT collectors are especially attractive when equipped with novel PV materials, such as multi-junction solar cells, since they are able to achieve ultra-high electrical efficiencies up to 100 °C [20], [22]. Unfortunately, CPVT commercial availability and the related academic research are scarce. Some significant works were presented by Mittelman et al. [20], [23], [24] performed some experimental and theoretical works dealing with a dish concentrator CPVT system used for SHC and desalination. Parabolic Trough CPVT collectors were investigated by Coventry [25], Li et al. [26], [27], [28] and Bernardo et al. [29]. In particular, Li et al. [26], [27], [28] concluded that GeAs cells increase electrical efficiency with respect to silicon cells, whereas thermal efficiency is higher for silicon cells. The authors also pointed out that the cost of unit area of the GeAs is 3067.16 $/m2, versus the 131.34 $/m2 of the silicon cell. Finally, Rosell et al. [30] investigated a low concentrating PVT system based on a linear Fresnel receiver: the rated thermal efficiency was about 60%.
Several technologies have been used to face drinkable water scarcity by desalting SW or brackish water. Thermal systems, based on either MSF (Multi-Stage Flash) [31], [32] and MED [33] schemes, have been mainly devoted to large scale fresh water production, with average capacities usually exceeding 10–20 thousands m3/day; main limits of thermal desalination systems consist in their low energetic efficiency, which induces the consumption of 50–60 kWhthermal/m3 of water and increases the production cost.
Then, the relevant improvements observed in membrane technologies [34] have led RO (Reverse Osmosis) systems gradually to cover an increasing share of the worldwide installed desalination capacity. Although RO systems consume a more precious energetic resource, i.e. mechanical energy or electricity, to pump water at high pressures, their lower energy consumption (in the order of 4–6 kWhe/m3) and their modular design have been favoring the spread of these systems especially in middle-small scale applications.
Of course, this advantage of mechanical systems compared to thermally fed units disappears whenever the low grade heat consumed by an MSF or a MED system is supplied either by a heat recovery by power plants operated in cogeneration model [35] or by renewable energy sources [36]. In particular, the interest for the use of the solar source has been growing in the last few years, due to the large availability of this source in many arid areas and the possibility to work with mature technologies. Several different direct and indirect thermal desalination systems have been designed, as accurately reviewed in Ref. [37]:
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Single-effect solar stills (even in greenhouse combination or externally activated configuration), single- and multiple-effect basin stills [38], wick stills and diffusion stills. These systems, often adopted for small production capacities (below 100 m3/day) are based on trapping solar energy and favoring the evaporation of water from a basin, finally obtaining an approximately salts-free water from the successive condensation of the distillate;
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HD (Humidification–Dehumidification) plants operate on a similar principles, but they avoid the direct contact between the solar collector and the saline water, thus overcoming the problems of scale formation and corrosion [39]. The process utilizes dry air to evaporate saline water, thus humidifying the air;
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MSF and MED systems may consume thermal energy supplied by flat plate, evacuated tube or parabolic through collectors, eventually in the form of hot or superheated water. Since these thermal desalination systems are conceived for a 24 h/day operation at constant conditions, special designs have been developed to handle a variable heat source [40];
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Plants based on separation by freezing have been also developed, where the solar source has been used to supply the thermodynamic cycle of a power plant, the electricity being used to drive the compressor of a refrigeration unit. This technology, however, has been mainly applied in pilot systems [41].
The focus in this paper is posed on MED plants integrated with PVT, because MED plants offer a large flexibility as concerns the temperature level and the fresh water capacity to install: in case of integration with large solar fields, in fact, this technology allows the installation of large fresh water production capacities, with the aim to supply small-medium scale communities in remote areas such as islands.
Among the small scale solar MED plants available in literature, the “Sol-14” plant built-in Almeria represents a best example, for the high conversion efficiency induced by the use of a complex fourteen-effects design in spite of the moderate capacity of the plant. Accurate energy and exergy analyses of the plants have been presented [42]; more recently, a layout integrated with a double-effect absorption heat pump has been proposed to allow for an effective integration of the solar source during hours with moderate irradiation [43].
As discussed above, literature review showed a large number of papers investigating separately CPVT, SHC and MED systems. However, the analysis of these three technologies in a single polygeneration system has never been investigated. Some few works are available investigating only the possibility of integrating CPVT and SHC technologies. In particular, Vokas et al. [44] analyzed the theoretical feasibility of flat plate PVT, producing both space heating and cooling (by an ACH (absorption chiller)). Mittelman et al. [20] investigated the theoretical feasibility of integrating CPVT, based on triple-junction cells, with SHC based on a single-effect LiBr–H2O ACH. The electrical efficiency ranged approximately between 19% and 23% (as a function of the PVT operating temperature, varying between 65 °C and 120 °C). On the other hand, the thermal efficiency of the PVT was stably slightly lower than 60%. The combination of PVT and CPVT collectors with SHC was also investigated by the authors in previous works [6], [45], [46], [47]. All the studies showed that the considered CPVT–SHC systems suffered of a large amount of heat, that cannot be supplied to the user for space heating and cooling purposes.
Therefore, based on the results of the previous research [6], [45], [46], [47], the idea of this paper is to use this exceeding solar thermal energy (not used for space heating or cooling) to drive a MED desalination unit, producing fresh water from SW. The system considered in this work includes all the above mentioned technologies (SHC, CPVT and MED) and it produces simultaneously electrical energy, thermal energy for heating and cooling, and fresh water.
As mentioned before, this study is a continuation of an on-going research developed by the authors in the past few years, aiming at designing and evaluating the thermo-economic performance of novel polygeneration systems based on renewable energy sources. In such research, authors diffusely investigated several configurations of SHC systems [3], [7], [8], [48], [49], [50], [51]. Simultaneously, some of the authors also developed new models for CPVT collectors [52], [53], also evaluating the possibility of their integration in SHC system for the simultaneous production of heat, cool and electricity [6], [45], [46], [47]. However, none of these studies evaluated the possibility to use solar thermal energy in excess to drive a desalination unit. This improvement has been performed in the present paper. To this scope, several innovations have been included: i) a new detailed model of a MED; ii) a new system layout integrating the MED unit and a biomass boiler; iii) a new control strategy including a double storage system, one for space heating and cooling and one for the MED, and a complex control system, for solar heat management.
Section snippets
System layout
As mentioned in the previous section, the system investigated in this paper combines SHC, CPVT, Biomass Heater and MED technologies. So, it can be considered as a polygeneration system, providing as an output different energy (electricity, cool and heat) and mass flows (demineralized water). For sake of brevity, the system will be named RPS (Renewable Polygeneration System). A simplified layout of the system under investigation is shown in Fig. 1, where only the main components are displayed.
Simulation model
The RPS polygeneration system described in the previous section was dynamically simulated by TRNSYS, which is a well-known software diffusely adopted for both commercial and academic purposes, including a large library of built-in components, often validated by experimental data [54]. As mentioned above, the RPS layout investigated in this paper was originated from that developed in previous works [48], [49], [50], [55], [56], where the models of both built-in and user-developed components are
Discussion and results
A case study was developed using the weather data of Naples, South of Italy. The main design/operational parameters are summarized in Table 1. Further details about the values of some specific design parameters can be found in Ref. [53] for the CPVT collector and Refs. [48], [49], [50], [55], [56] for other components.
Note that some of the design parameters shown in the previous table are closely related to each other. In particular, a variation of the number of solar collectors determines a
Conclusion
A polygeneration system based on the integration of three technologies (CPVT, SHC and MED) was presented and dynamically simulated in TRNSYS. A numerical case study was developed and widely discussed, putting in evidence the significant potential of energy savings achievable by such system, also due to the opportunity of maximizing the utilization factor of the thermal energy produced by the CPVT, especially during the summer. On the other hand, the winter performance was by far less
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