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F. Calise, et al. Energy Conversion and Management 220 (2020) 113043
produced by PV or PVT panels is used to drive the RO process, in other performance predicted through dynamic simulations, supported by
system configurations. appropriate economic models, was excellent. This work is particularly
Several technologies can be combined in polygeneration systems interesting because it presents a comprehensive assessment on combi-
producing electricity, heating, DHW and freshwater [9]. Buonomano nation of different kinds of technologies (SHC, CPVT, geothermal wells
et al. [10] investigated Concentrating Photovoltaic Thermal (CPVT) and MED). Several additional islands in the Italian Mediterranean Sea,
collectors coupled with a gas turbine, Absorption Chillers (ACHs) and Ischia and Aeolian Islands, showing a similar potential in terms of
suitable thermal storage tanks to supply energy to a hospital. Accep- availability of geothermal and solar energy, are also investigated. For
table pay-back periods (from 4 to 6 years) are obtained in case of feed- all the investigated locations, a parametric analysis aiming at evalu-
in tariff scenarios on the produced thermal and electric energy. Linear ating the variation of Profit Index as a function of the ratio of DHW
Fresnel reflector (LFR) and evacuated tube solar collectors (ETC) cou- produced by the system and demanded by the user is performed. It
pled with a MED system, an auxiliary biomass-fired heater and ab- resulted that Profit Index dramatically decreases in case of scarce DHW
sorption technology are evaluated in reference [3]. In this case a single- demand. Other studies concerning the dynamic simulation of a novel
effect LiBr-H 2 O Absorption Chiller (ACH) is considered when ETCs are hybrid solar and geothermal polygeneration system capable to produce
included, whereas a double-effect ACH is included if LFR collectors are electric energy, fresh water and space heating and cooling, for the
used. For the analyzed case study of Naples (South of Italy) char- Pantelleria Island was presented and discussed in references [16] and
acterized by a medium beam-to-total radiation ratio value, ETCs re- [16]. Here, with respect to the previous layout, an Organic Rankine
sulted more profitable than LFRs, achieving SPB periods of about Cycle (ORC) supplied by geothermal and solar energy is considered. In
4–5 years, in the case of feed-in tariff incentives. The economic and particular, in reference [1], a low-temperature geothermal well (85 °C)
energy feasibility of a solar-assisted system in different Italian weather is used, mainly for the desalinization process, whereas in references
zones for school buildings was assessed in reference [11]. Here, a [16] and [16] a medium-enthalpy geothermal source (160 °C) is con-
single-stage LiBr/H 2 O ACH, ETCs and a conventional electric-driven sidered. In addition, in this work, CPVT collectors are replaced by
reversible heat pump (HP) are hybridized in a polygeneration plant for Parabolic Trough Collectors field coupled to a thermal storage tank.
heating, cooling and DHW production. Such a system allows one to From the energy analysis it resulted that the solar energy input is much
obtain significant energy savings: 52, 4%, 61.4% and 63.2% for Milan, lower than the geothermal one, as shown by the low value found for the
Trapani and Naples, respectively. As it happens for the great majority of solar fraction, 9.60%. During the year the MED unit is able to produce
renewable energy systems, the feasibility of the plant is positive only in fresh water, equal to 54% of the total seawater flowrate. Using an ac-
case of public funding policies with Simple Pay Back (SPBs) ranging curate energy, economic, exergy and exergoeconomic analysis of the
from 12 to 16 years for Naples and Milan, respectively. Low tempera- system, it was found out that the global exergy efficiency varies from
ture PVT collectors can be suitably coupled with solar-assisted heat 40% to 50% during the thermal mode and from 16% to 20% during the
pumps [12] and adsorption chillers [13] in novel solar-based poly- cooling one; besides, the exergoeconomic costs of electricity, cooling
generation systems. In particular, during the winter, thermal energy of water, chilled water and desalinated water resulted very interesting,
the PVT collectors mainly supplies the heat pump evaporator, whereas respectively in the ranges 0.1475–0.1722 €/kWh, 0.01612–0.01702 €/
in summer, it supplies the adsorption chiller (usually activated for kWhex, 0.1863–0.1888 €/kWhex and 0.5695–0.6023 €/kWhex. The
lower hot fluid temperatures) providing energy for space cooling. Such previous works [16] are developed by considering that all the useful
a system is not profitable without public incentive (simple payback products are consumed by the user. Instead, in a further study [17],
period, SPB, higher than 16 years) and becomes profitable with capital based almost on the same plant configuration, the system is supposed to
investment subsidy of 50%. A solar heating and cooling (SHC) system is be connected to a district electric, heating and cooling network. In this
coupled with concentrating photovoltaic/thermal (CPVT) collectors case, system energy production must match the real time-dependent
(parabolic trough collectors, PTC, with triple-junction solar PV cells) demands of electricity and space heating and cooling of typical build-
and an electrolyzer [14]. The electrolyzer is driven with electricity ings of Pantelleria Island (assumed as a case study). The system
produced by the CPVT and generates hydrogen, used in the fuel cell achieved a SPB equal to 8.50, with a potential primary source saving of
(FC), and oxygen, which is sold. Another hybrid polygeneration layout, 3039 t and a potential CO 2 avoided emission of 9451 t. The system can
combining a polymer electrolyte membrane fuel cells (PEMFC), a PEM cover the energy demands of 800 examined buildings. Moreover, the
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electrolyzer, a lead acid battery, a wind turbine, monocrystalline PV plant produces 1006 10 m of desalinated water and it is capable to
panels, a metal hydride tank, a RO desalination unit, using energy re- cover the fresh water global demand. Average ORC efficiency amounts
covery, and a hydrogen (H 2 ) vehicle is presented in reference [15]. to 15.30%. The system is mainly powered by geothermal energy, in
Results show a technically feasible polygeneration microgrid adapted to fact, the average solar fraction of the system is only 14.6%.
small islands financially profitable with a probability of 90% for the A 100% RESs system for Astypalaia island in Greek based on wind
present and 100% in the medium term. turbines, concentrating solar power (CSP) plant and desalination units
In literature, special attention is paid to the issue of the stability of is proposed in the work of Wang et al. [18]. The flexible scheduling of
the supply in island systems, which are strongly dependent on renew- CSP plants is used to complement to the wind power generation, and
able energy sources (RESs). The challenge is their energy self-suffi- the thermal storage system reduces the battery energy storage config-
ciency taking into account the limited availability of local fossil sources. uration for islands. By appropriately planning the capacity of MED and
In fact, islands are featured by a large availability of seawater, abun- RO units, these can meet the freshwater demand by effectively utilizing
dant RESs, and limited availability of conventional fossil fuels and fresh surplus electric power and low-grade waste heat from the CSP plant.
water. The goal of many works is the development of future 100% RES Main findings of the works regard the improved thermal energy utili-
islands. For example, this issue is addressed in the work reported in zation efficiency of the system and the cogeneration of water and
reference [1], where a hybrid solar and geothermal polygeneration electricity, when the latent heat of the low-grade waste heat from the
system combining CPVT collectors, a single-stage LiBr/H 2 O ACH and a CSP plant, although the coupling of the CSP plants and MED units re-
MED unit was studied. The plant supplies electric, thermal and cooling duce the power generation efficiency. The proposed system is more
energy, DHW and a certain amount of desalinated water able to cover flexible in scheduling, and it reduces the investment for energy storage
the whole request of the Pantelleria Island, assumed as case study. In systems. Karavas et al. [19] presented a techno-economic study of
particular, solar thermal energy, at a maximum temperature of about freshwater production by PV panels and RO technology in the Aegean
100 °C, in combination with the thermal energy produced by low-en- islands. The scenario of the proposed technologies is supposed to re-
thalpy (about 80 °C) geothermal wells, is used to supply the MED place the current water supply systems of these islands, based on the
system. Geothermal energy is also used to produce DHW at 45 °C. The water ships. The operating modes of the RO plant at nominal or variable

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