Page 2 - Water-energy_2020

F. Calise, et al. Energy Conversion and Management 220 (2020) 113043

Nomenclature η el electrical efficiency
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A area, (m ) Subscripts and superscripts
AF annuity factor, (–)
C cost, (€) a Ambient
c specific Heat [J/kg/K] AH auxiliary heater
CPVT concentrating PhotoVoltaic Thermal solar collectors ap Aperture
concentration ratio back back surface
C PVT
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I b beam radiation [W/m ] cell photovoltaic cell
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total radiation [W/m ] cool Cooling
I tot
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h c convective heat transfer coefficient [W/m /K] conc Concentrator
fluid specific Enthalpy [J/kg] conv Convective
h f
ṁ f fluid mass flow rate [kg/s] dhw domestic hot water
P ̇ electric power, (kW) el Electric
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r area specific thermal resistance [m K/W] f Fluid
T temperature, (°C or K) heat Heating
v voltage, (V) in Inlet
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V volume, (m ) ins Insulation
E energy, (kWh) MED multi effect distillation
I current, (A) mp max power
J capital cost, (€) N Number
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j specific cost-price, (€/m ) net Net
M maintenance, (€/year) oc open-circuit
NOCT nominal operating cell temp, (°C) out Outlet
PI profit index, (–) ref reference conditions
rec Receiver
Greek symbols RO reverse osmosis
sky referred to sky
α Absorptance sub Substrate
Emittance th Thermal
ε R
ε heat transfer effectiveness TK Tank
Δ Difference th Thermal
ρ PVT PVT Reflectance top top surface
σ Stephan-Botzmann constant u Unit
η th thermal efficiency
Solar energy is considered one of the most promising renewable Thermal and/or Mechanical Vapor Compression (TVC, MVC), Reverse
energy sources [2], available almost all over the world with different Osmosis, (RO), and Electrodialysis (ED). Some of these technologies
intensities. It can be used as an energy input of polygeneration plants, (MSF, MED, TVC) use thermal energy to convert seawater into fresh-
including solar thermal collectors and photovoltaic (PV) panels. In fact, water. Other technologies (RO, ED) are driven by electricity. Therefore,
in solar-powered polygeneration plants, different solar technologies can desalination units can also be easily coupled to a plurality of renewable
be included for the direct conversion of the solar irradiation into heat energy sources and/or energy cascades. In MED systems, seawater is
available at different temperatures, according to the adopted solar desalinated by a series of heating and evaporation processes, until
thermal technology. For example, in case of low or medium tempera- freshwater is produced [7]. This technique is quite flexible in both
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ture levels, the produced heat is employed in solar heating and cooling operating temperature and capacity range (also higher than 20.000 m /
(SHC) systems and/or domestic hot water (DHW) applications [3]. SHC day of freshwater). This makes this technology especially attractive for
technology, converting the solar irradiation into space heating and small-medium scale communities in remote zones such as islands. RO is
cooling, is particularly interesting in summer, when the cooling energy an electrically-driven desalination technology. In RO, fresh water,
demand is often simultaneous to the availability of solar radiation. PV under high pressure, passes through semi-permeable membranes,
panels are used for the direct electric energy production and they are leaving behind the high-concentrated brine solution. More than 50% of
commonly coupled with electric vapour compression chillers. Hybrid the worldwide installed desalination plants are based on RO technology
PV-Thermal (PVT) collectors [4,5] for combined heat and power pro- due to its simplicity and relatively low energy cost and energy con-
duction are nowadays one of the most interesting applications for the sumption [8]. As mentioned before, desalination technologies are also
simultaneous production of useful thermal and electrical energy. In case often coupled with renewable energy sources and a number of theo-
of low-temperature heat (residential applications), thermal energy is retical and experimental studies are available in this field. However,
usually exploited for domestic hot water (DHW) preparation or space there is no consensus regarding the most profitable desalination tech-
heating purposes. In addition, due to the lower PVT operating tem- nology, since this selection also depends on a number of additional
perature - with respect to PV panels - the power production occurs at external parameters. Therefore, one of the aims of this work is the
higher electrical efficiencies. In solar trigeneration plants, PVT solar comparative analysis of different desalination technologies, depending
collectors can be combined with single-stage Lithium Bromide / Water on different system layout configurations. In particular, in this work,
(LiBr/H 2 O) absorption chillers for the production of cooling, heating, the most promising thermal and electrical desalination technologies
DHW and electric energy [6]. (MED and RO, respectively) are analysed considering different poly-
For freshwater production, several desalination techniques are generation plants. Here, when the MED process is included into the
available: Multi Stage Flash (MSF), Multi Effect Distillation (MED), plant, solar thermal collectors are implemented, while electricity

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