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F. Calise, et al. Energy Conversion and Management 220 (2020) 113043
proposed. In particular, for the year 2035 the vehicle-to-grid concept is controllers (feedback, proportional and on/off) and schedulers (daily
implemented to use electric vehicles as potential storage systems for the and seasonal). Note that reference [25] did not consider any specific
variable energy production. The issue of the Islands interconnection to user demand since the authors assumed that all the produced energy
achieve 100% RESs is addressed also in the Azorre archipelago, the Pico and water could be consumed by the user. This simplification is not
and Faial Islands, by Alves et al. [24]. The mismatch between demand considered in the present paper and the hot- and cold-water loops are
and supply and the stability issue are also discussed in this paper. hydraulically connected with the respective systems of the buildings
included in the district. Therefore, in the presented study the produc-
1.1. Novelty and aim of study tion of energy and water must be compared with the time-dependent
user demand. As a consequence, a detailed control strategy is also im-
The above reported literature review clearly shows that it is im- plemented in order to manage the system operation and to match the
portant to achieve 100% renewable energy islands, implementing dif- heating, cooling and DHW demand of the district users. For this pur-
ference energy actions to guarantee the self-sufficiency energy supply pose, further auxiliary devices are included in the SHC network in order
and network stability. The literature review also shows that virtually to cover the district demand when the solar radiation is low and/or user
infinite polygeneration layouts can be conceived, due to the large demand is high. This additional equipment includes an electric chiller
number of combinations of the available renewable sources, energy (ECH) for space cooling and wood-chip auxiliary heaters (AH) for DHW
conversion and desalination technologies. However, the above reported and space heating. The set point temperature of the AH for space
literature review shows that most of the published studies about this heating T setAH,heat is 55 °C and the set point temperature of the AH for
topic only focus on a single specific polygeneration plant. There is no DHW T setAH,DHW is 45 °C. During winter, after the pump P1, the diverter
consensus regarding the most profitable system layout, nor regarding D2 diverts the hot water to the district network for space heating and
the integration of the different available renewable and desalination DHW, i.e. to the heat exchanger HE2; during summer the diverter D2
technologies. No comprehensive study is available in literature com- diverts the hot water to the set district network for space cooling. The
paring electrically and thermally driven desalination technologies on water of the secondary loop is heated in the heat exchanger HE2 and it
combination with renewables in polygeneration systems serving is- is further divided with the diverter D4 to the DHW and space heating
lands. networks by means of the pumps P5 and P6, respectively. Two hot
In fact, to the best of the authors’ knowledge, none of the papers water stratified storage tanks, TKdhw and TKheat, are expected to
available in literature compare different polygeneration plants produ- manage the fluctuations of the district demand. Pumps P8 and P9 col-
cing water and energy, simultaneously considering energy and eco- lect the hot water from the top of the tank to supply the users. A by-pass
nomic aspects, comparing different renewable and desalination tech- circuit consisted of diverters and mixers in both the DHW and heating
nologies. This work aims to address this gap, presenting a comparative networks is also included. Therefore, when the tank top temperature is
study of different polygeneration layouts including different solar and lower than T setAH,heat , the diverter D7 (or D6) directs the return water
desalination technologies. In particular, in the present work both PV from the user to the AH, which heats it up the selected set point tem-
and CPVT technologies are considered in different combinations with perature. When the tank top temperature is higher than T setAH,heat , the
other devices. In particular, solar subsystems were coupled with dif- diverter D7 directs a part of the return cold water from the user to the
ferent thermal energy activated technologies (TATs), such as single M7 to obtain the hot water at the selected set point. This occurs until
ACH, MED desalination units or with electric energy activated tech- the tank top temperature decreases to T setAH,heat . The bottom water of
nologies (EATs), such as electric vapor compression chillers and RO both tanks on the secondary circuit returns to the heat exchanger HE2
desalination units. The present study considers different combinations with the mixer M4 to get reheated and continue the loop. When no
of the above-mentioned technologies in order to detect the best com- heating is required, D2 diverts a part of the hot water to the HE2 to
bination from both energy and economic points of view. Note that one cover the demand for DHW. Therefore, D4 closes the exit in the di-
of the layouts considered in this work is based on the one reported in rection of the tank TKheat. During the cooling season, a certain amount
reference [25]. However, with respect to the cited previous study, the of the hot water supplies the ACH, which produces chilled water at
present work includes further improvements. In fact, in reference [25] 6.5 °C stored into a further stratified storage tank TKcool. To cover the
it is assumed that all the produced energy and water is consumed by the cooling demand, the chilled water is pumped by pump P7 from the
user, independently of the demands of specific districts. This simpli- bottom side of the tank to the user. The electric chiller is switched on
fying assumption allowed one to significantly simply the calculations when the bottom temperature of the tank is lower than 7 °C. In this
since the estimations of user energy and water time-dependent de- case, a by-pass circuit is also included. In order to cool ACH and ECH,
mands was not necessary. This simplifying assumption is removed in two heat exchangers supplied with seawater at a constant temperature
the present work, where the hot and cold water loops are hydraulically of 25 °C, (reasonable value for the Mediterranean Sea) are used.
connected with the selected district systems where heat, cool, elec- The second layout “PV-REV_HP-RO” includes PV panels coupled
tricity and water time-dependent demands are carefully estimated. To with a RO unit, which produces freshwater, and two different electric
this scope, detailed building clustering is implemented. Then, advanced heat pumps for the separated production of space heating, cooling and
building dynamic simulations are performed in order to evaluate dis- DHW. Therefore, two water-to-water reversible heat pumps (REV_HP),
trict energy and water demand in combination with measured data. In operating in parallel, are included to match the space heating and
particular, for each time step of the simulation, the heat, cool, elec- cooling energy demand. During the winter operation, the district supply
tricity and water production are compared with the related user de- hot water temperature is set at 45 °C, appropriate value to supply the
mands. Finally, the results of this study can be used in a more general fan coil units. To guarantee the correct operation of the heat pumps and
manner to design polygeneration systems, producing space heating and avoid continuous switching on/off, a suitable controller and a stratified
cooling, DHW and freshwater for small Mediterranean Islands. storage tank TKheat&cool are included. The controller reads the top
temperature of the tank TKheat&cool and when the tank top tempera-
2. System layout ture is lower than 44 °C, heat pumps are switched on. This circumstance
occurs until the tank top temperature is higher than 45 °C. During
Two layouts of the investigated plants are shown in Fig. 1. The first summer operation, it is assumed that the temperature of the chilled
layout “CPVT-ACH-MED” is mainly based on the layout reported in water on the fan coil units ranges between 13 and 15 °C. This occurs by
reference [25], combining SHC, CPVT collectors, a biomass auxiliary a further suitable controller reading the bottom temperature values of
heater, a LiBr–H 2 O single-effect ACH, and a MED unit. The same control the tank TKheat&cool and managing the activation of the heat pumps.
strategies reported in reference [25] are implemented using suitable In particular, heat pumps work when the bottom temperature is greater

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