Page 12 - Water-energy_2020

F. Calise, et al. Energy Conversion and Management 220 (2020) 113043
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Table 5 rural areas: C1 = 151 €/m and C2 = 1378 €/m [35]. The investment
Proposed systems 2 (PS2), Santa Maria di Salina. in this case is found to be approximately double that of the case of Santa
Marina Salina, mainly due to the more scattered buildings over the
Component Parameter Description Value Unit
surface area of the island.
PV (PS2) P max Maximum power 260 W p The economic feasibility of PS2 is good, mainly due to the present
Open-circuit voltage 37.7 V
V oc
low capital cost of PV panels, compared to the extremely high capital
Short-circuit current 9.01 A
I sc cost of CPVT collectors (Table 6). This is a remarkable result, con-
Voltage at point of MPP 30.5 V
V mpp
Current at point of MPP 8.51 A sidering the high cost of the piping network. On the other hand, the
I mpp
N s Number modules in series 2 – economic feasibility of PS1, is poor (Table 9) due to the huge capital
Number modules in 2900
N p costs for the district heating network. Similar economic results were
parallel
A PV module area 1.6 m 2 obtained for PS1 with layouts including CPVT collectors, with SPB
N cell Number cells in series 15 – equal to 12.5 [14] and 13.6 [25] without any economic incentive,
Module efficiency 15.8 where the costs of the piping network were not considered. The PES and
η PV
PV panel rated power 1.40 MW
P PV the avoided emissions ΔCO 2 of PS1 are also lower than those reached
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for PS2, 32.2% and 27.8%, respectively. This is due to the high elec-
A tot PV field area 9333 m
HP heat and cool P th,HP,heat Rated heating power 1.8 MW th tricity demand in the proposed system, mainly covered by the grid with
(PS2) P th,HP,cool Rated cooling power 1.7 MW th a low efficiency and to the high thermal energy demand covered
COP Coefficient of prestation 4.14 – principally by the auxiliary heater during winter.
EER Energy Efficiency Ratio 4.85
Tsw Temperature sea wataer 15 °C
5.2. Weekly results
HP DHW (PS2) P th,HPDWH Rated heating power 1.0 MW th
COP Coefficient of prestation 3.25 –
Tsw Temperature sea wataer 15 °C In order to better comprehend the trends of the main energy fluxes
over the seasons of year, the aggregated results of PS1 and PS2 are
RO (PS2) N membranes Number of membranes for 24 –
single train reported in Figs. 6 and 7 on a weekly basis. In PS1, the thermal energy
N vessels Number of vessels for 3 demand for DHW is covered from the solar tank TKdhw, for most of the
single train year. In fact, as shown in Fig. 6, approximately between the 13th and
N Trains Number of trains 1 the 18th week, the auxiliary heater is switched off. This occurs because
Active area of single 41 m 2
A membranes
membrane during these weeks the thermal energy demand for space heating and
Salinity feed seawater 39 g/l cooling of the district is null and all of the produced thermal energy
c f
Operation seawater 12–29 °C
T seawater with the CPVT collectors is used for DHW purposes. This circumstance
temperature also justifies the results reported in the previous sections, dealing with
Efficiency of low- pressure 0.85 –
η LPP
the yearly integrated results. In fact, in those weeks solar energy can be
pump
Efficiency of high- 0.85 used mainly for DHW purposes and desalination. Conversely, the
η HPP
pressure pump thermal energy of the AH is significant during summer, because the
Efficiency of pressure 0.95
η PX thermal energy produced with the CPVT collectors is used to supply the
exchanger ACH. In that period of the year the CPVT efficiency and solar avail-
Pr ,lim,min Limit pressure value 42 bar
(P el,limit condition) ability is high, resulting also simultaneous with the space cooling de-
Pr ,rated Rated pressure value 58 bar mand. The tank TK2, E th,TK2 , is not able to cover the energy demand of
(rated condition) the MED unit, E th,MED , the first and the last weeks of the year, corre-
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W ̇ rated Rated flowrate RO unit 350 m /h sponding to the winter season: therefore, the auxiliary heater is acti-
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W tot Total yearly capacity 131,400 m /year
vated to match the MED energy demand. In fact, in winter solar energy
is mainly used for space heating purposes since this demand is sig-
nificantly higher than solar availability, as mentioned in the previous
As for the calculation of the cost of the network, it is found that the
building density of the region of Santa Marina Salina is 1521.4 build- section. During winter, the space heating energy demand, E th,heat,dem ,is
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ings/km calculated for 852 buildings of mean surface area of 138 m in quite high, reaching about 125 MWh the first weeks of the year. Con-
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a built area of 0.56 km , where the buildings are concentrated. The heat versely, solar thermal production is low to the low beam radiation
available in winter. Then, it decreases until the end of the heating
and cooling generation of the district heating and cooling network is a
season, March 31st, due to the mild climate of the island of Favignana.
total of 1903 MWh/year, while that of the hot water district system
generation is 1552 MWh/year. The two district networks are identical. The solar thermal energy, E th,TKheat , of the tank TKheat covers only a
small part of the district demand. In particular, it reaches its peak on
The length of the tube per building is calculated to be 16 m with a pipe
diameter of 35 mm, and a total pipe length of the network of almost the 12th week of the year (March), due to the increase of the radiation
availability and, thus, the solar thermal energy production. Space
14 km. The capital expenditure of the piping network is estimated at
3.7 M€ [45], using the constants for outer city areas: C1 = 214 €/m and cooling energy demand, E th,cool,dem ,(Fig. 6) is required intensely be-
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C2 = 1725 €/m [35]. The island of Favignana includes 898 buildings tween weeks 25 and 40 and its peak is reached at the end of August.
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of mean surface area of 119 m . The built area of the island is estimated E th,TKcool of the the tank TKcool can cover the cooling energy demand at
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at around 5 km , while it is assumed that 80% of the buildings of the the beginning and at the end of the cooling season, whereas in the
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island are concentrated in an area of 1.5 km in the center of the island. middle of the season, the demand is partially covered by the electrical
chiller ECH (E th,ECH ).
The total heat and cooling demand of the island, covered by the district
Fig. 7 shows the weekly electric energy production of the PV panels,
heating and cooling network, is 2322 MWh/year, while the thermal
E el,PV , the total electric energy supplied to the RO unit, Salina district,
energy of the hot water district network for supplying the required hot
and auxiliary components of PS2 E el,RO+District+Aux , the electric energy
water demand is 1440 MWh/year. The piping systems of the two dis-
trict heating networks are identical. Under these calculations, the withdrawn from the grid, E el,fromGRID , and supplied to the grid, E el,toGRID ,
and the total electricity supplied to the HP for DHW, space heating, and
length of the tube per building is estimated at 27.9 m, resulting in a
total network pipe length of 25 km. In this case, the capital expenditure cooling, E el,HPheat&cool+DHW. E el,PV reaches maximum values during
summer. However, in these same summer weeks, a greater electricity
of the piping network was found to be 8 M€, using the constants for
demand of the district is noted, due to the increase of the tourists.
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