Optimization and dynamic analysis of a novel polygeneration system producing heat, cool and fresh water
Introduction
In the last years, the future availability of non-renewable fuels (such as: natural gas, coal, oil) as well as the drinkable water scarcity particularly in costal/island and semi-arid areas, are becoming a severe issue. As a consequence, a more sustainable energy supply scheme, based on renewable energy sources as solar, geothermal, biomass, etc., should be considered, in order to reach a sustainable and environmental friendly worldwide development [1]. In this framework, the solar source is even more used, mainly in arid regions featured by a high solar radiation, seawater availability and which are hardly connected to the conventional production systems of drinking water (islands). For this kind of user, solar polygeneration systems, integrating Solar Heating and Cooling (SHC) [2] and water desalination systems, result to be a promising technology for water and energy supply. SHC technology, which convert the incident solar irradiation on a solar field in both heating and cooling energy, is beginning to be particularly attractive in summer, when the availability of solar radiation is often simultaneous to the cooling energy demand [3]. SHC technology is less efficient during the winter operation, due to the lower solar radiation, when auxiliary devices are always mandatory for matching the heating demand.
Fresh water may be obtained from the seawater using solar energy. This technique is well-known in literature as solar desalination [4]: in a solar desalination plant, the heat produced by a solar thermal collectors field or the electricity produced by a PhotoVoltaic (PV) panels field, may be utilized to drive a desalination process, basically removing dissolved minerals and salts from seawater, in order to produce fresh water.
Several solar desalination techniques are presently available [5]: Thermal or Mechanical Vapour Compression (TVC)/(MVC), Multi Effect Distillation (MED), Multi Stage Flash (MSF), Electrodialysis (ED), Reverse Osmosis (RO). In particular, in this paper the MED technology is analysed.
MED is a special case of the thermal desalination technology, where the process is driven by thermal energy. In particular, MED systems mostly rely on boiling; the temperature and pressure are lowered between subsequent stages to allow boiling at a lower temperature, using the condensation heat from the previous stage as heat source [6]. It is available for a wide range of drinkable water capacities. Therefore, MED technology is especially attractive for small-medium scale plants available in isolated zones (islands), where the solar irradiation is high and drinkable water availability is low. In fact, in such cases, MED systems may be coupled to large solar thermal collector fields in order to produce significant amounts of fresh water (also higher than 20,000 m3/day), On the other hand, the main problem of MED technology is the poor energy efficiency of the process, with a high specific thermal energy consumption, about to 50–60 kWhth/m3. Therefore, its application could be economically profitable if fed by renewable energy sources, such as solar, wind, geothermal and biomass, which are expected to have a flourishing future and an important role in the domain of brackish and seawater desalination [7].
Different solar thermal collector technologies may be considered in such systems. This selection dramatically affects the operation, the design and the fresh water producibility of MED systems, depending specifically on its number of effects (more efficient reuse of the heat [8]) as well as on the operating temperature at the MED first effect (it should vary between 75 °C and 70 °C [9]). In particular, both stationary - flat-plate collectors, evacuated tube collectors (ETC) - or concentrating solar thermal collectors - Linear Fresnel Reflector (LFR), Parabolic Trough Collectors (PTC), Concentrating PV and Thermal collectors CPVT - are generally considered for this kind of application. For example, in Ref. [10] an experimental characterization of a MED system is coupled to a flat plate solar collector field; in Ref. [9] PTCs are investigated by means exergy and thermo-economic analyses in a combined solar organic cycle with MED desalination process; LFR solar collectors are used as energy source for a Rankine Cycle integrating MED system in Ref. [11]; the water desalination based on MED process and CPVT systems is investigated in Ref. [12].
Several analyses are available in literature, studying solar MED systems, from both experimental and numerical points of view. An experimental study of a MED unit coupled to large-aperture flat plate collectors is carried out by Chorak et al. [10]. It is found that when the MED operates at 65 °C and by using a heat storage, the system operation may be extended up to 6 h, by obtaining a continue operation also in cases of cloudiness. Furthermore, in this work, a theoretical calculation is also performed, and the predicted yearly production of distillate is between 6,701 and 11,314 m3.
Askari et al. [13] performed both technical and economic analyses of different MED/TVC arrangements with 9,000 m3/day water production driven by LFR collectors, by considering the Kish Island (South Iran) weather condition. The system includes a suitable thermal storage and it achieves a 67.77% solar fraction. Unfortunately, the fresh water cost (FWC) obtained by the investigated system is very high (3.32 $/m3), especially with respect to the FWC achieved by the conventional MED plants supplied by fossil fuels (1.26 $/m3).
Askari et al. [14] also compared PTC and LFR technologies, involved to supply a Solar Rankine Cycle, for fresh water and electricity production. Here, the condenser of the Rankine Cycle is replaced by a MED unit for desalinated water production. Results show that the FWC and electricity production costs of the LFR/MED plant are 10% and 25% lower than PTC/MED plant ones. In addition, PTC solar field efficiency resulted higher than LFR one. Therefore, PTC/MED may achieve the same electricity and water production obtained by LFR/MED plant, only in case PTC capital cost would be decreased by 66% with respect to its initial cost assumption (420 $/m2).
A MED unit - without and with TVC - combined to a LFR solar field for Rankine Cycle is studied for different solar radiation levels and seawater temperatures by Askari et al. [15]. The comparison is carried out by analysing the FWC in various system arrangements: i) LFR-RC-MED; ii) LFR-RC-MED/TVC); iii) LFR-MED; iv) LFR-MED/TVC plants. For LFR-MED and LFR-MED configurations the direct steam production obtained by the LFR solar field is adopted. The configurations with higher FWCs, up to 3 $/m3, are LFR-MED and LFR-MED/TVC, without Rankine Cycle for electricity production. By considering fuel prices of 0.23 $/m3, all analysed configurations attain the same FWC. Results also show that the solar radiation magnitude dramatically affects FWC, much more than seawater temperature.
Talebbeydokhti et al. [16] examined a hybrid system consisting of a PTC field in a Concentrating Solar Power (CSP) plant with a Discrete Ericsson Cycle, with air as heat transfer fluid. The cycle also produces heat for MED processes at low temperature. The resulted daily specific consumption of the novel technology (low temperature -MED/CSP plant) is 0.062 kW/daym3, significantly lower than the one achieved by a conventional system, equal to 0.195 kW/daym3,
The PTC-MED plant performance, in terms of fresh water production by changing the solar tracking mode, is analysed by Gholinejad et al. [17]. Authors found that the system with a full tracking system, at the winter solstice, produces 341% more desalinated water than the system with stationary collectors.
A PTC field coupled to geothermal wells, a MED unit and Organic Rankine Cycle (ORC) into a renewable polygeneration plant is investigated in Refs. [18,19], by dynamic simulations. The plant is designed to produce electrical energy, thermal and cooling energy, fresh water and domestic hot water (DHW) for Pantelleria, a Sicilian island (South Italy). In particular, the produced solar thermal energy is combined with a medium-enthalpy geothermal source (160 °C) to supply heat to the ORC. The outlet geothermal brine drives the MED unit. The study presented in Ref. [18] assumed that all the useful energy outputs and the produced desalinated water are fully consumed by the user. Results of accurate thermoeconomic analyses are also presented. In particular, it is worth noting that: i) the solar fraction of the plant is very low, equal to 9.60%; ii) the ORC efficiency is equal to about 11.6%; and iii) the yearly production of fresh water is equal to 54% of the total seawater flowrate [18]. Conversely, in Ref. [19] although the same layout presented in Ref. [18] is investigated, the renewable polygeneration plant is designed to match the real time-dependent demands of space heating and cooling, electricity and fresh water of Pantelleria. The avoided CO2 emissions and the potential primary energy saving are 9451 tons/year and 37.5 GWh/year, respectively. The payback period is 8.50 years, the yearly fresh water production, equal to 1006 103 m3, covers the global fresh water demand.
The economic feasibility of a polygeneration plant based on the MED and PTC technology, for the weather conditions of northern Chile and Venezuela, is evaluated by TRNSYS dynamic simulations by Mata-Torres et al. [20]. It resulted that for both locations, the plant produces electricity and water for more than 85,000 inhabitants, by increasing of only by 6%–12% the overall yearly cost of the system.
In several research works, optimization studies are also developed. For example in Ref. [21], a two-step optimization procedure is implemented in order to find the optimal capacity of a polygeneration plant, which can provide fresh water from seawater, fuelled by natural gas, solar conventional thermal collectors and gasified biomass, located in a tourist resort in Spain. It is found out that renewable technologies, as solar and biomass, are the best options from an environmental point of view, since a significant CO2 emission reduction, equal to 314.1 ton/year, is achieved. The optimised polygeneration system achieves a primary energy saving equal to 18%.
Another optimization study concerning detailed exergy and energy analyses of a small scale PTC-MED plant in Almeria is presented by García-Rodríguez and Gómez [22]. Here, a complex 14-effects design is realized in order to achieve high conversion efficiency. Results show that the implementation of some important actions (aiming at maximizing the energy recovery) for this solar thermal desalination plant determine an increase of the global system and MED exergy efficiency from 1.4% to 4.7% and from 14.3% to 25.7%, respectively.
A complete model for a MED plant driven by a solar steam generation plant, consisting of a PTC collectors field is presented by Saldivia et al. [23]. The model is used to evaluate a prototype solar MED plant in Valparaiso (Chile), but also to analyse the distillate production in the case where the same prototype plant is installed at seven different locations. It is observed a linear trend between the average distillate production and yearly horizontal radiation of the localities: in particular, for each increment of 100 MJ/m2/year in the solar radiation, the average distillate production increases by 1 m3/day.
It is worth noting that the majority of the works cited above are developed using dynamic simulation models. In fact, such complex systems are dramatically dependent on the variations of the external conditions (weather, heating and cooling demands, etc). Therefore, any reasonable and reliable analysis must mandatory be performed using detailed dynamic calculations, taking into account the time-dependent variations of the main operating and design parameters. Any other approach based on steady state conditions would return significant errors and are commonly considered inappropriate for those systems. On the other hand, the analysis of the papers available in literature in this area showed that only a few papers address this topic from the experimental point of view. The majority of the authors implement simulation models rather than experimental analyses. The few experimental papers in literature, see the review work on the solar-thermal powered desalination [24], focus only on lab-scale systems [25]. In fact, an experimental analysis of such systems is a very complex and expensive task due to the large number of components included in these hybrid systems and due to large capacities of the components. This issue also affects the development of dynamic simulations, due to the scarcity of data to be used to validate the simulation model. Therefore, in a framework where the experimental data of the system as a whole are missing, authors usually prove the reliability of their model by validating the models of the components one by one, omitting the validation of the system as whole. This approach is commonly considered acceptable in cases where experimental data of the system as a whole are not available.
The above described literature review clearly shows that a significant number of researchers performed significant efforts in order to design and simulate hybrid systems integrating solar source and MED technology. This effort was also performed by the authors of this paper in the past few years, as reported in Refs. [18,19,26], analysing several novel layouts of polygeneration systems combining renewable energy (biomass, geothermal, solar) and desalination processes. Among these works, one of the most attractive layout is based on the combination of the CPVT collectors and biomass energy, SHC and MED desalination unit [26]. In this work, a new detailed model of the MED unit as well as a complex control strategy for the solar heat management is presented. The analysis shows that the designed system exhibited an excellent performance from an energy point of view, since it allows one to produce simultaneously electricity, cool, heat and desalinated water using only renewable energy. Therefore, such kind of system resulted extremely attractive for islands and/or isolated communities where the production of energy and fresh water is a critical issue, from energy, environmental and economic points of view. However, this novel layout suffered for the fact that it is based on CPVT collectors, using III-V cells. Such technology is extremely promising, but it is presently extremely expensive. Thus, such proposed layout will be economically feasible only in the next future, when CPVT cost is expected to decrease. Therefore, in the present work, authors present a modification of the above presented layout, aiming at obtaining an immediate profitability. The idea is to replace the advanced CPVT collectors with cheaper solar thermal collectors. Thus, the layout proposed in this paper does not produce electrical energy, but it is featured by a lower capital cost. The aim of the present paper is to compare this novel cheaper layout with the previous one based on CPVT collectors, using both energy and costs as objective functions. On this basis, this work is developed in order to enhance the thermoeconomic performance of the novel MED–SHC–solar-biomass polygeneration plant by means an optimization procedure. In other words, the previous study aimed at proposing a novel future technology whereas the present one is focused on a more practical approach proposing a novel polygeneration system based on cheap and well-established technologies. In fact, in the authors’ knowledge no study above reported aiming at analysing the energy and economic performance, achieved by dynamic simulations, of a novel arrangement of a solar polygeneration, also evaluating these performances with the ETC technology [27].
In this framework, this paper aims at furtherly improving the knowledge in this field, by presenting such solar polygeneration system producing thermal energy, cooling energy and desalinated water. In fact, in the present paper, the layout investigated includes several technologies, such as: ETC, SHC, single effect absorption chiller (ACH) and MED unit. Those subsystems are here rearranged in a novel polygeneration plant which is able to produce thermal energy for heating/cooling, DHW and fresh water. For fresh water production, the indirect configuration consisting of two separated subsystems, the solar collectors field and MED unit, instead of direct one, where both evaporation and condensation take place in the same device, is selected.
Section snippets
System description
The system layout of the investigated plant is conceived from the one used in Ref. [26], where CPVT collectors are investigated. As mentioned before, the investigated plant combines ETC and a MED unit for fresh water production, a SHC system for space heating/cooling and DHW purposes and Auxiliary biomass-fired Heater (AH). The main components and operating fluids, with their corresponding loops, are shown in the system layout in Fig. 1. Here, eight operating fluids are provided: (SCF) Solar
Simulation model
The investigated solar MED polygeneration plant, is modelled in TRNSYS environment, a dynamic software mainly adopted for academic and commercial purposes, performing dynamic simulations. TRNSYS is a general-purpose simulation environment. The library includes a large number of built-in components (HVAC, Hydronics, Electrical, Building, etc). The models of such components are based on unsteady algorithms validated by experimental data or based on manufacturers’ data [28]. The project can be
Results
The dynamic simulation model developed in this work provides the results for any time basis multiple of the selected time-step (seconds, days, weeks, months, year etc.). In this work, the time-step of the simulation is assumed equal to 0.02 h. The selection of a time step lower than 1 h is possible even though the tool adopts hourly weather data. This occurs because results (temperatures, heat fluxes, flowrates, etc) corresponding to a time basis lower than 1 h are achieved by considering the
Conclusion
In this work, the energy and economic evaluation of a solar MED polygeneration plant, based on ETCs, is carried out by a dynamic simulation model, developed in TRNSYS environment. The investigated plant supplies space heating and domestic hot water by heat exchangers, space cooling energy by a single effect ACH, and desalinated water by a MED process, using solar and biomass energy.
For such plant, a case study situated in Naples (South Italy) is analysed and extensively discussed, showing the
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