Solar Integrated Anaerobic Digester: Energy Savings and Economics

energies

Article Solar Integrated Anaerobic Digester: Energy Savings and Economics

Lidia Lombardi 1,* , Barbara Mendecka 2 and Simone Fabrizi 1

1 Department of Engineering, Niccolò Cusano University, 00166 Rome, Italy; [email protected] 2 Department of Economics, Engineering, Society and Business Organization, University of Tuscia, 01100 Viterbo, Italy; [email protected] * Correspondence: [email protected]

Received: 23 July 2020; Accepted: 14 August 2020; Published: 19 August 2020

Abstract: Industrial anaerobic digestion requires low temperature thermal energy to heat the feedstock and maintain temperature conditions inside the reactor. In some cases, the thermal requirements are satisfied by burning part of the produced biogas in devoted boilers. However, part of the biogas can be saved by integrating thermal solar energy into the anaerobic digestion plant. We study the possibility of integrating solar thermal energy in biowaste mesophilic/thermophilic anaerobic digestion, with the aim of reducing the amount of biogas burnt for internal heating and increasing the amount of biogas, further upgraded to biomethane and injected into the natural gas grid. With respect to previously available studies that evaluated the possibility of integrating solar thermal energy in anaerobic digestion, we introduce the topic of economic sustainability by performing a preliminary and simplified economic analysis of the solar system, based only on the additional costs/revenues. The case of Italian economic incentives for biomethane injection into the natural gas grid—that are particularly favourable—is considered as reference case. The amount of saved biogas/biomethane, on an annual basis, is about 4–55% of the heat required by the gas boiler in the base case, without solar integration, depending on the different considered variables (mesophilic/thermophilic, solar field area, storage time, latitude, type of collector). Results of the economic analysis show that the economic sustainability can be reached only for some of the analysed conditions, using the less expensive collector, even if its efficiency allows lower biomethane savings. Future reduction of solar collector costs might improve the economic feasibility. However, when the payback time is calculated, excluding the Italian incentives and considering selling the biomethane at the natural gas price, its value is always higher than 10 years. Therefore, incentives mechanism is of great importance to support the economic sustainability of solar integration in biowaste anaerobic digestion producing biomethane.

Keywords: anaerobic digestion; biogas; biomethane; integration; solar thermal energy

1. Introduction Biogas is a renewable fuel produced by the anaerobic digestion (AD) of biodegradable organic substrates and its main components are CH4 and CO2. The main sources of biogas are municipal solid waste landfills, digesters of sludge from wastewater treatment plants and industrial anaerobic digestion plants using different feedstocks as biowaste, manure, and energy crops. With reference to 2017, the production of biogas as primary energy in the European Union (EU) was about 16,811,000.6 ton of oil equivalent (toe), mainly generated by AD processes of different substrates (excluding sewage sludge). The main biogas producers were Germany (46.7%), United Kingdom (16.2%), and Italy (11.3%) [1]. Biogas can be directly used in gas turbines, internal reciprocating combustion engines, fuel cells, or organic Rankine Cycles [2]. However, the possibility of upgrading biogas to biomethane, i.e., namely

Energies 2020, 13, 4292; doi:10.3390/en13174292 www.mdpi.com/journal/energies Energies 2020, 13, 4292 2 of 16

by CO2 removal, used for fuelling vehicles or for injection into the natural gas grid, is already quite frequent in Northern Europe and it is gaining increasing interest worldwide. Biomethane, being renewable, can replace natural gas: its expected future increased production and use can provide an important contribution in decreasing the heavy dependency on foreign supply of several EU countries. To this aim, several EU countries developed devoted types of subsidy systems, to boost the biomethane production and use: a summary of incentive scheme is reported in Lombardi and Francini [3]. Around 540 biogas plants with biomethane upgrading were active in Europe in 2017 [4]. In Italy, there were about 7 plants producing biomethane in 2018 [5], thanks also to the latest economic incentives promoting the production of biomethane to be injected into the natural gas grid and used for transportation purposes. Industrial AD requires thermal energy to heat the feedstock and maintain mesophilic or thermophilic process conditions inside the reactor (compensating for heat loss to the environment). It is quite common that the required thermal energy can be recovered from the exhausts of the internal combustion engine fuelled by biogas. However, in some cases the thermal requirements are satisfied by burning part of the biogas in devoted boilers. Some authors proposed to provide the AD required thermal energy by solar integration, with different purposes for various specific applications. Mahmudul and Rasul [6], in their study regarding opportunities for solar assisted biogas plant in subtropical climate in Australia, assessed the possible benefits and challenges of the process, highlighting that it may reach higher biogas yield by increasing digester temperature and hence improved possibilities for recovering energy from waste. Another study, on small fermentation system [7] integrated with solar collectors, showed that at Hothot (China) in October, an area of 2 m2 of solar collectors, running 8 h, can provide the heat required by 6 m3 digester. Ouhammou et al. [8] studied an up-flow anaerobic sludge blanket (UASB) digester coupled with solar thermal system, located in western North Morocco, and concluded that it allows saving 100% of energy consumption for almost ten months per year and 70% for two cold months per year. The coupling of anaerobic digestion and solar thermal energy can be viewed also as way to store the solar energy into biogas, as proposed by Zhong et al. [9] for the case of mesophilic digestion and co-digestion of manure, who concluded that the preferred system to store solar energy into methane biogas is the mesophilic case. Borrello et al. [10] proposed to integrate a solar collector system, equipped with thermal storage, into a mesophilic anaerobic digester with the aim of continuously providing the required thermal power. Specifically, the considered digester is a wet continuous flow stirred tank reactor (CSTR) with a volume of 320 m3, fed by an organic load of 5 kgVS/m3d. They concluded that digester needs a solar field equal to 560 m2. Thus, using solar energy as heat source for the AD is an interesting solution for maintaining the required temperature inside the reactor. Especially in the AD plants where biogas is upgraded to biomethane, where biogas boilers are generally used, the possibility of avoiding or at least reducing the biogas self-consumption is directly translated into higher amount of biomethane for being injected into the grid, and this can represent a significant increased revenue, especially when incentives for biomethane injection are acknowledged. However, the economic evaluations of solar thermal energy integration into AD reactors was not investigated in the previously mentioned studies. Thus, the present work aims to contribute at filling this gap, introducing a preliminary economic analysis. In particular, this study aims to investigate the possibility to integrate thermal solar collectors into a mesophilic or thermophilic biowaste anaerobic digestion plant, equipped with the upgrading section producing biomethane. The amount of thermal energy that can be supplied yearly by the solar source, using different collector types and for different geographical locations, is evaluated. The supplied solar thermal energy allows saving part of the biogas—generally used for heating the reactor—that can be additionally upgraded to biomethane. The amount of yearly saved biomethane is presented and a Energies 2020, 13, x FOR PEER REVIEW 3 of 17

95 is presented and a preliminary economic evaluation of the additional costs and additional revenues, 96 with respect to the reference case (mesophilic/thermophilic anaerobic digestion without solar 97 integration), is reported in term of payback time of the additional investment.

98 2. Materials and Methods

99 Energies2.1. 2020System, 13, 4292Description 3 of 16 100 The studied concept refers to the AD reactor integrated with solar collector system. Figure 1 101 preliminaryshows the economic layout of evaluationthe investigated of the additionalsystem. Two costs operating and additional conditions revenues, of the withanaerobic respect reactor to the are 102 referencestudied case, in particular (mesophilic the/thermophilic mesophilic and anaerobic thermophilic digestion condition withouts. solarAD plant integration), is fed by is about reported 9.800 in t/y 103 termof ofsource payback-sorted time organic of the f additionalraction of m investment.unicipal solid waste (SS-OFMSW), with 70% moisture content 104 for both operating conditions. Different digester types can be used for the AD of organic wastes. The 105 2. Materialsselected type and depends Methods on operational factors, including the nature of the waste to be treated, e.g. its 106 solid content [11]. In this study the reactor is assumed to be a conventional wet one, based on the 107 2.1.CSTR System model Description, working with total solids (TS) content equal to 12%, 365 days per year. Assuming for 108 theThe SS- studiedOFMSW concept a density refers equal to to the 1.12 AD t/m reactor3, the flow integrated rate entering with solarthe reactor collector resulted system. 157 Figuret/day (12%1 109 showsTS), theequal layout to 140 of m the3/day. investigated The expected system. biogas Two production operating rate conditions for the mesophilic of the anaerobic and thermophilic reactor 110 areconditions studied, in are particular 140 Nm3 the/h and mesophilic 168 Nm3 and/h, respectively thermophilic, with conditions. 60% volumetric AD plant content is fed of by CH about4. The 111 9.800typical t/y of h source-sortedydraulic retention organic time fraction (HRT) ofis municipal15 and 21 days solid in waste thermophilic (SS-OFMSW), and mesophilic with 70% moisture condition, 112 contentrespectively for both. A operating summaryconditions. of the work Diingﬀ erentparameters digester of the types anaerobic can be useddigester for for the both AD ofconditions organic is 113 wastes.presented The selectedin Table 1. type depends on operational factors, including the nature of the waste to be 114 treated,The e.g., reactor its solid is contentassumed [11 to]. be In built this in study reinforced the reactor concrete is assumed material to, with be a cylindrical conventional shape wet and one, flat 115 basedcover. on Its the volume CSTR model,is calculated working from with the inlet total volumetric solids (TS) flow content rate equal and the to 12%,HRT, 365considering days per an year. extra 116 Assumingvolume forof about the SS-OFMSW 17.5%. Technical a density and equal geometric to 1.12 data t/m3 ,of the th ﬂowe rea ratector, enteringfor both theoperating reactor conditions resulted , 117 157are t/day presented (12% TS), in Table equal 2. to 140 m3/day. The expected biogas production rate for the mesophilic and 118 thermophilicSeveral conditions biogas upgrading are 140 Nm technologies3/h and 168 Nm are3 / availableh, respectively, on the with market, 60% volumetric each one with content specific of 119 advantages and drawbacks. However, they are quite similar in terms of consumptions and yields [3], CH4. The typical hydraulic retention time (HRT) is 15 and 21 days in thermophilic and mesophilic 120 condition,being the respectively. high-pressure A summarywater scrubbing of the working(HPWS) the parameters most popular of the on anaerobic a commercial digester basis for [12] both. The 121 conditionsexpected is biomethane presented inoutput Table flow1. s, for the mesophilic and thermophilic conditions, are 84.00 Nm3/h and 122 100.80 Nm3/h, respectively.

Figure 1. Schematic layout of the solar-integrated anaerobic digester. .

Table 1. Operating conditions of the anaerobic digesters. 123 Figure 1. Schematic layout of the solar-integrated anaerobic digester. Conditions Parameter Description 124 Mesophilic Thermophilic T Temperature ( C) 37 55 ◦ HRT Hydraulic retention time (days) 21 15 3 Qbiogas Biogas production (Nm /h) 140 168 3 QCH4 Biomethane production (Nm /h) 84 100.8

The reactor is assumed to be built in reinforced concrete material, with cylindrical shape and ﬂat cover. Its volume is calculated from the inlet volumetric ﬂow rate and the HRT, considering an extra EnergiesEnergies 20202020,, 1133,, x 4292 FOR PEER REVIEW 4 4of of 17 16

125 Table 1. Operating conditions of the anaerobic digesters. volume of about 17.5%. Technical and geometric data of the reactor, for both operating conditions, are Conditions presentedParameter in Table 2. Description Mesophilic Thermophilic T TableTemperature 2. Characteristics (°C) of the anaerobic digesters.37 55 HRT Hydraulic retention time (days) 21 15 Qbiogas Biogas production (Nm3/h) 140 Conditions 168 Parameter Description 3 QCH4 Biomethane production (Nm /h) Mesophilic84 Thermophilic100.8 H Height from ground to substrate (m) 9 - 126 1 Table 2. Characteristics of the anaerobic digesters. H2 Height for biogas filling (m) 2 - Hburied Buried height of the digester (m) 2Conditions - ParameterD Diameter of theDescription anaerobic digester (m) 20 16.9 Mesophilic Thermophilic V Volume of the anaerobic digester (m3) 3454 2467 H1 Height from ground to substrate (m) 9 - d1 Thickness of reinforced concrete walls (m) 0.2 H2 Height for biogas filling (m) 2 - d2 Thickness of the insulation (m) 0.15 Hburied Buried height of the digester (m) 2 - d3 Thickness of the cover (m) 0.15 D Diameter of the anaerobic digester (m) 20 16.9 V Volume of the anaerobic digester (m3) 3454 2467 Severald1 biogas upgradingThickness of technologies reinforced concrete are available walls (m) on the market, each0.2 one with specific advantagesd2 and drawbacks.Thickness However, of theythe insulation are quite (m) similar in terms of consumptions0.15 and yields [3], beingd the3 high-pressure waterThickness scrubbing of the (HPWS) cover (m) the most popular on a commercial0.15 basis [12]. The expected biomethane output flows, for the mesophilic and thermophilic conditions, are 84.00 Nm3/h 127 andThe 100.80 thermal Nm3/ h,system respectively. providing heat to the reactor consists of a solar collector field, storage tank, 128 and biogasThe thermal back-up system boiler. providing Different heatcollector to the type reactors are investigated consists of a solarto produce collector hot field,water storage which tank,feds 129 theand reactor biogas-heating back-up loop. boiler. The Di collectorfferent collectortypes utilized types in are this investigated study are todepicted produce in hotFigure water 2. whichThree 130 planefeds thecollector reactor-heating designs are loop. selected The for collector performance types utilized and economic in this studycomparison are depicted analysis: in flat Figure plate2. 131 collectThreeor plane-glazed collector FKC-2W designs by Bosch are selected[13]; flat forplate performance collector FT and226- economic2V by Bosch comparison [13]; evacuated analysis: tubular flat 132 collectorplate collector-glazed VTK 120-2 CPC FKC-2W by Bosch by Bosch[13]. Bosch [13]; flat FKC plate-2W collectorflat solar FT collector 226-2V is by composed Bosch [13 ];by evacuated the pre- 133 mouldedtubular collector tank and VTK aluminium 120-2 CPC pick by- Boschup plate [13, ].absorber Bosch FKC-2W with a selective flat solar collectorinox finish is composed[13]. FT 226 by–2 theV 134 solarpre-moulded collector tank has and a copper aluminium and pick-upaluminium plate, absorber absorber with with PVDa selective coating inox and finish a double [13]. FT-twisted 226–2V 135 geometrysolar collector of the has hydraulic a copper circuit and aluminium [8]. The third absorber solar collector with PVD is coating the Bosch and VTK a double-twisted 120-2 CPC: it geometryhas very 136 highof the performance hydraulic circuit guaranteed [8]. The throughout third solar the collector year thanks is the to Boschevacuated VTK technology 120-2 CPC:, where it has verythe pipes high 137 inperformanceside the collector guaranteed are kept throughout under vacuum the year [13] thanks. Technical to evacuated data of the technology, chosen collectors where the are pipes presented inside 138 inthe Table collector 3. are kept under vacuum [13]. Technical data of the chosen collectors are presented in Table3.

(a) (b) (c)

139 FigureFigure 2. 2. TypeType of of considered considered collectors: collectors: ( (aa)) FKC FKC-2W;-2W; ( (bb)) FT FT 226 226–2V;–2V; ( (cc)) VTK VTK 120 120-2-2 CPC. CPC.

140

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Table 3. Technical data of the three types of collectors.

FT VTK 120-2 Parameter Description FKC-2W 226–2V CPC

η0 Zero loss collector efficiency (%) 77.00 79.40 66.30 b Slope of surface 45◦ 45◦ 45◦ 2 a1 Linear heat loss coefficient (W/ m K) 3.87 3.86 0.782 2 2 a2 Quadratic heat loss coefficient (W/ m K ) 0.012 0.013 0.012 IAMtra Transversal incidence angle modifier (θ = 50◦) 0.92 0.94 1.09 IAMlon Longitudinal incidence angle modifier (θ = 50◦) 0.95 A Aperture of the collector (m2) 2.25 2.55 1.22

Incidence angle modiﬁer (IAM) is a function of declination, local hour angle, latitude, longitude, and solar azimuth angle obtained from the solar radiation and weather data processor in TRNSYS software (TRNSYS 18, Solar Energy Laboratory, University of Wisconsin, Madison, WI, USA). The IAM is provided by the manufacturer, as a set of IAM factors associated with given incidence angles. The value of IAM, for the simulation purpose, can be obtained by interpolation based on data reported in Table4.

Table 4. Prediction of incidence angle modiﬁer value according to manufacturer data [14–16].

Incidence Angle 10 20 30 40 50 60 70 θ◦ FKC–2W IAMtra 1.00 0.99 0.98 0.96 0.92 0.86 0.73 FT 226–2V IAMtra 1.00 0.99 0.98 0.97 0.94 0.90 0.80 VTK 120-2 CPC IAMtra 1.01 1.00 0.96 1.03 1.09 1.18 1.36 IAMlon 1.00 0.99 0.99 0.97 0.95 0.91 0.83

The solar system is coupled with the water thermal storage, assumed to be a shell and tube arrangement, with water as the heat transfer fluid and also circulating in the inside tube. However, when the solar radiation is not enough, the reactor heat duty is supplied by a backup biogas-fired boiler, operating in parallel with the thermal storage. Biogas and solar energy can be used in combination or independently, depending on irradiation conditions and on the reactor requests. The aim is to keep the reactor temperature at the desired level 37 ◦C in the mesophilic condition and 55 ◦C in the thermophilic condition—minimising auxiliary biogas consumption. The preferred energy source is, of course, the solar radiation, but, since it cannot be altered, the control system keeps the desired inlet temperature conditions, by handling the biogas boiler. It is assumed that the biogas boiler back-up system operates only when the storage tank level is lower than 10%, and there is no (or not enough) energy from the solar collector field available at the given moment.

2.2. Thermal Modelling The simulation is performed for each hour of one year, calculating heat losses and gains, assuming the weather conditions for three reference sites in Italy. Milano (site 1), Frosinone (site 2), and Enna (site 3) are considered as reference sites, for which the hourly meteorological data (external temperature, wind speed, direct normal irradiance, global irradiance, and incidence angle) are collected. The three sites are representative of the variation of global solar radiation (G) through the Italian territory. Figure3 shows that the ﬁrst site (site 1) falls in an area with low global irradiation of about 1300 kWh/m2 as yearly totals, while for the second (site 2) and third site (site 3) the global irradiation values are about 1600 and 1900 kWh/m2 as yearly totals, respectively. The average values of meteorological data for the reference sites are given in Table5. Energies 2020, 13, 4292 6 of 16

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171

172 FigureFigure 3. 3. GlobalGlobal Irradiation Irradiation map map.. Reproduced Reproduced from from [17] [17],, Global Global Solar Solar Atlas: Atlas: 2019 2019..

173 TableTable 5. 5.Geographical Geographical position position and and average average meteorological meteorological parameters parameters for for the the three three considered considered sites. 174 sites. Parameter Site 1 Site 2 Site 3

LongitudeParameter 9◦Site130 E 1 13◦Site19012” 2 E 14◦10Site034” 3 E LongitudeLatitude 45◦9°13'280 N E 4113°◦3819'12’’E024” N 3714°◦3110'34’’E003” N Average annualLatitude temperature 14.2945°28'◦C N 41°38'24’’ 15.33 ◦C N 37° 15.9031'03’’N◦C AverageAverage annual annual wind temperature sped (Ws) 1.6914.29 m /°Cs 15.33 1.05 m °C/s 1.3815.90 m /°Cs AverageAverage annual directannual normal wind sped irradiance (Ws) (DNI) 182.361.69 W m/s/m2 191.561.05 m/s W/m 2 211.401.38 W m/s/m 2 2 2 2 AverageAverage annual annual direct global normal irradiance irradiance (G) (DNI) 290.88182.36 W W/m/m 2 191.56315.30 W/m W/m2 409.90211.40 W W/m/m 2 Average annual global irradiance (G) 290.88 W/m2 315.30 W/m2 409.90 W/m2 The simulation of the system resolves the energy balances, on hourly basis using two software 175 (TRNSYSThe simulation 18 and Matlab of the R2018b system (R2018b, resolve MathWorks,s the energy Natick,balances MA,, on USA)). hourly Random basis using availability two software of the 176 (solarTRNSYS source 18 and is assumed Matlab inR201 the8b model, (R2018b being, MathWorks a very important, Natick feature, MA, USA) when). dealingRandom with availability renewable of 177 thesources solar [18 source]. As results,is assumed hourly in load the proﬁles model, of being the reactor a very are important obtained for feature each site when under dealing study with and 178 renewablethe two operating sources conditions[18]. As results, of the hourly anaerobic load digester profiles system.of the reactor are obtained for each site under 179 study and the two operating conditions of the anaerobic digester system. Thermal load of the reactor QAD.load is the sum of the thermal energy to heat the substrate to the 180 Thermal load of the reactor QAD.load is the sum of the thermal energy to heat the substrate to the required temperature Qsub and the thermal energy required to compensate heat losses through the 181 required temperature Qsub and the thermal energy required to compensate heat losses through the external walls of the reactor to environment Qdisp (1): 182 external walls of the reactor to environment Qdisp (1):

QAD.load = Qsub + Qdisp (1) QAD.load = Qsub + Qdisp (1) Thermal energy lost from the reactor walls to the surroundings by conduction, convection, and 183 radiationThermal is represented energy lost as from the productthe reactor of heat walls transfer to the coesurroundingsfficient Ui, wallby conduction, area Ai and convection the temperature, and 184 radiationdifference is between represented the reactoras the produc internalt of temperature heat transfer T digcoefficientand the U ambienti, wall area temperature Ai and the T Dbtemperature(2). 185 difference between the reactor internal temperature Tdig and the ambient temperature TDb (2). Q = U A T T (2) disp i· i· dig − Db Qdisp = Ui ∙ Ai ∙ (Tdig − TDb) (2) where the heat transfer coefficient is given by (3): 186 where the heat transfer coefficient is given by (3): 1 Ui = (3) 1 P d 1 α + 1 + α U = 1.sub k 1.amb i 1 d 1 (3) ( ) + ∑ + ( ) α1.sub k α1.amb

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and α1.amb is the external heat transfer coeﬃcient; it is a function of the wind speed Ws in m/s, it depends on the external conditions [19] and it is given by (4):

α = 4 + 4 Ws (4) 1.amb · Conductive and internal convective heat transfer coefficients and heat exchange areas used in the calculations are presented in Table S1 in the Supplementary Materials. Tdig is the temperature of the digester 37 ◦C or 55 ◦C, while TDb is the dry-bulb temperature supplied by the meteorological stations for each hour of the year. Qsub is given by (5): . Q = m cp ∆t (5) sub · · . where m is the mass flow of the entering substrate, equal to 157 t/day; cp is the specific thermal capacity of the substrate equal to 4186 J/kg K; ∆t is the temperature difference between the initial and final temperature of the substrate entering the reactor. Table6 shows gross annual amount as energy content of biogas produced annually by the anaerobic digester, the annual consumption of biogas, without solar collectors, fed to the boiler for heating the digester, and the annual amount of biogas converted to biomethane (cases without solar integration).

Table 6. Annual amount of biogas produced, required by the boiler and available for biomethane production, at the three considered sites, for mesophilic and thermophilic conditions, without solar system integration.

Conditions Description Mesophilic Thermophilic Biogas produced by AD (MWh/year) 7154 8585 site 1 Biogas required by the boiler (MWh/year) 1492 2729 Biogas to biomethane (MWh/year) 5662 5856 site 2 Biogas required by the boiler (MWh/year) 1476 2713 Biogas to biomethane (MWh/year) 5678 5871 site 3 Biogas required by the boiler (MWh/year) 1424 2662 Biogas to biomethane (MWh/year) 5730 5923

It is noted that even in the thermophilic case, the heat duty is higher, also the biogas sent to biomethane production is higher because of the higher speciﬁc production. In general, in the thermophilic case without solar systems, about 3.3% more biogas is available for upgrading than in the mesophilic case.

2.3. Solar System Design A parametric analysis is performed varying both solar collectors ﬁeld area and thermal storage size in order to compare the energy gain and the economic feasibility of the proposed solutions. The area of the solar collector ﬁeld is calculated for the maximum thermal power demand of the 2 reactor QAD.load.max and varying design solar irradiation DNIdesign from 100 to 800 W/m with a step of 25 W/m2 (6). Q A = AD.load.max (6) DNI η X design· th· Energies 2020, 13, 4292 8 of 16

where:

X is the collector cleanliness factor, minimization criterion, it is equal to 1 in the low range • temperature (50–100 ◦C) [20]; η is the eﬃciency of the collector evaluated by Equation (7): • th ∆t η = η IAM (a + a ∆t) (7) th 0· − 1 2· · G

η is the optical efficiency; • 0 ∆t is the difference between average fluid temperature into collectors and average surroundings • temperature [21]; a and a are the linear and quadratic heat loss coefficients given in Table3. • 1 2 The IAM values are fitted with a polynomial function reported in Equation (8), based on the data provided by the manufacturer in Table4. The coe fficients of the polynomials obtained for all the considered cases are presented in Table7.

IAM = a θ2 + b θ + c (8) tra;long · · where θ is the incidence angle (θ◦), a function of the weather conditions for the three considered sites.

Table 7. Polynomial factors.

IAM IAM IAM IAM Parameter tra tra tra lon FKC-2W FT 226-2V VTK 120-2 CPC a 1.06E-04 7.62E-05 1.88E-04 6.55E-05 − − − b 4.44E-03 3.17E-03 9.55E-03 2.70E-03 − c 9.54E-01 9.66E-01 1.10E+00 9.71E-01

In the case of VTK 120-2 CPC collector, the value of IAM is estimated as the product of longitudinal and transversal IAM as in Equation (9).

IAM = IAMtra IAM (9) · lon

The optical efficiency η0 and thermal loss coefficients a1 and a2 are determined experimentally. Optical and thermal losses both have an impact on the collector efficiency, and their relative contributions depend mainly on the physical design of the collector. In Figure4, the solar collector e fficiency ηcol, calculated according to Equation (10), at different solar radiation levels is illustrated for the three types of considered collectors. Similar values of efficiency about evacuated tube collectors are presented by Atkins [22]. ∆t η = η (a + a ∆t) (10) col 0 − 1 2· · G Different capacities of thermal storage, expressed in hours, are considered, from 0 to 24 h with a 3 h step. The resulting storage capacities are in the range from 600 to 4830 kWh and 980 to 7850 kWh, for mesophilic and thermophilic conditions, respectively. The effect of the thermal energy produced by the solar section is to reduce the biogas consumption for the reactor heating. Thus, the highest the solar energy supplied to the reactor, the highest the biogas that can be sent to the upgrading section to produce biomethane, that, in turn, can be injected into the grid. This benefit is evaluated according to the amount of saved biomethane (expressed as the energy content, in MWh, of the additional biomethane annually produced with respect to the base case mesophilic/thermophilic AD, without solar system). The calculation is carried out on an hourly basis: per each hour of the year the (i) the heat available from the solar system is calculated (including heat Energies 2020, 13, 4292 9 of 16

storedEnergies or2020 retrieved, 13, x FOR from PEERthe REVIEW storage); (ii) the AD reactor heat requirement is calculated; iii) the saved9 of 17 biogas/biomethane is obtained as the diﬀerence between (ii) and (i).

(a) (b)

(c)

235 FigureFigure 4.4. EEfficienciesﬃciencies forfor FKC-2WFKC-2W collectorcollector ((aa),), FT FT 226-2V 226-2V collectorcollector ((bb)) and and VTK VTK 120-2 120-2 CPC CPC evacuated evacuated 236 collectorcollector (c(c)) at at di differentﬀerent solar solar radiation radiation levels. levels.

2.4. Economic Analysis 237 Different capacities of thermal storage, expressed in hours, are considered, from 0 to 24 hours 238 withAn a 3 increasehour step in. T thehe savedresulting biomethane storage capacit can beies obtained, are in the obviously, range from with 600 a to larger 4830 solarkWh fieldand 980 area. to 239 Of785 course,0 kWh for, for increasing mesophilic amounts and the ofrmophilic saved biomethane conditions an, re increasespectively. in the additional revenues from the 240 biomethaneThe effect selling of are the obtained. thermal On energy the other produced hand, for by increasing the solar solar section field is area, to reduce the cost the related biogas to 241 theconsumption solar section for increases the reactor as well. heating. With Thus this in, the mind, highest a preliminary the solar andenergy simplified supplied economic to the reactor analysis, the is 242 carriedhighest out, the withbiogas the that aim can of evaluatingbe sent to the the upgrading additional investmentsection to produce cost for thebiomethane solar section, that (for, in diturn,fferent can 243 areabe injected of the solar into fieldthe grid. and This for the benefit three is sites) evaluated vs. the according additional to revenues the amount from of saved saved biomethane biomethane 244 selling.(expressed The analysisas the energy is not includingcontent, in the MWh investment, of the costsadditional for the biomethane AD and upgrading annually system produced as well with as 245 itrespect is not includingto the base the case revenues mesophilic from/thermophilic the biomethane AD, producedwithout solar in the system reference). The operatingcalculation condition is carried 246 (i.e.,out mesophilicon an hourly/thermophilic basis: per each case hourwithout of the solar year integration). the (i) the Thus, heat available the economic from evaluation the solar system is based is 247 onlycalculat oned the (including additional heat costs stored/revenues. or retrieved Even though from the this storage); approach (ii) isthe rather AD reactor simplified, heat itrequirement can provide is 248 somecalculated; interesting iii) the cluessaved onbiogas/biomethane whether the addition is obtained of the as solarthe difference section mightbetween be ( sustainable,ii) and (i). without altering the AD/biomethane profitability. A whole life cycle cost analysis, including the AD plant 249 investment2.4. Economic and Analysis operation costs and overall revenues (biomethane selling and biowaste feed-in tariff), 250 couldAn provide increase a wider in the picture saved of biomethane the economic can sustainability, be obtained, butobviously it is out, with of the a scopelarger of solar this workfield area. and 251 suggestedOf course, for for future increasing analysis. amounts of saved biomethane an increase in the additional revenues from 252 the biomethaneThe total investment selling are cost obtained. for the solarOn the section other(C handinv), is for estimated increasing starting solar fromfield area the specific, the cost costs related for 253 theto thesolar solar collectors section and increases for the storage as well. system. With The this specific in mind costs, a preli of themi solarnary collectors and simplified are presented economic in 254 Tableanalysis8, according is carried to out Bosch, with catalogue the aim costsof evaluating 2016 [ 13 the]: these additional costs include investment the supply cost for of the the solar entire section solar 255 thermal(for differen kit andt area exclude of the installation. solar field The and cost for of the the three storage sites tank) vs is. assumedthe additional equal torevenues 8.20 Euro from/kWh saved [23]. 256 Tablesbiomet S21hane to S26selling. in Supplementary The analysis is Materials not including report the the investmentinvestment costs costs for for all the the AD analysed and upgrading solutions 2 257 (mesophilicsystem as well/thermophilic; as it is not storage including hours), the withrevenues resulting from solar the fieldbiomethane area between produced 0 and in 10,000 the refer m .ence 258 operating condition (i.e. mesophilic/thermophilic case without solar integration). Thus, the economic 259 evaluation is based only on the additional costs/revenues. Even though this approach is rather 260 simplified, it can provide some interesting clues on whether the addition of the solar section might 261 be sustainable, without altering the AD/biomethane profitability. A whole life cycle cost analysis, 262 including the AD plant investment and operation costs and overall revenues (biomethane selling and

Energies 2020, 13, 4292 10 of 16

Table 8. Speciﬁc costs of diﬀerent solar collector types.

Collector Type Costs (Euro/m2) FKC-2W 237.34 FT 226-2V 414.71 VTK 120-2 CPC 773.09

Maintenance cost for the solar system (Cm) is evaluated as 1.0% of initial investment cost [24]. For the assessment of the annual revenues (Byears) from the biomethane selling, the Italian Ministerial Decree dated 2 March 2018 [25] is considered. According to this decree, the injection into the grid provides a revenue equal to the natural gas price (average of the last available three months values), reduced by 5%, assumed equal to 20.41 Euro/MWh [26]. Additionally, in the specific case of SS-OFMSW, for each 5 GCal of biomethane injected into the grid, a certificate (CIC) is acknowledged, with a value of 375 Euro each, for the first 10 years (corresponding to 64.54 Euro/MWh). The Italian incentives are representative of highly advantageous subsidy, with respect to other European countries [3]. The parameter used to evaluate the investment is the simple payback time (PBT). The PBT is calculated as the ratio between the Cinv and the net annual revenues, calculated as the difference between Byears and Cm, as given by (11):

C PBT = inv (11) (Byears Cm) − 3. Results This section summarizes the main results of the analysis. The total thermal power required by the digester—made by the two contributions Qdisp and Qload—depends mainly on the internal temperature of the anaerobic digester (i.e., mesophilic/thermophilic) and on the external temperature, changing during the year, in the diﬀerent geographical locations. The average power required to maintain the mesophilic temperature conditions in the digester varied in the range from 80–100 kW in summer (July) to 190–200 kW in winter (January). While, in the thermophilic condition case, it varied in the range from 210–230 kW in summer (July) to 310–340 kW in winter (January). The trend of average monthly thermal power required during the year is reported in Figure S1 of Supplementary Materials, showing that the contribution of the heat loss through the reactor surfaces is quite small. Speciﬁcally, the results indicate that up to 96.4% (mesophilic conditions) or 97.2% (thermophilic conditions) of the heat required by the digester operation is used to raise the feedstock temperature to the operating temperature, while the remaining part is utilised to keep the process temperature constant.

3.1. Saved Biomethane This section reports the results in term of additional amount of biomethane (i.e., saved biomethane), not used for heating the digester. The saved biomethane is calculated as the difference between the actual amount of available biomethane in each mesophilic/thermophilic case and the amount of biomethane available in the base reference case, which is the mesophilic/thermophilic one without solar system (in the different sites). Figure5 reports the amount of saved biomethane by introducing in the system three di fferent types of solar collectors, with different storage systems, in the three sites in Italy. For graphical reasons, only three levels of storage, that are 6, 12, and 18 h, are presented. Complete results are reported in Tables S3–S20 of Supplementary Materials. A considerable difference exists between the results of the mesophilic case and the thermophilic one, due to the different hourly production of biomethane (84 Nm3/h and 100.8 Nm3/h, respectively). The areas of the solar collectors in the thermophilic conditions (Figure5b,d,f) are significantly larger than in the mesophilic case, because the thermophilic reactor requires a larger amount of thermal energy compared to the mesophilic case. Saved biomethane reaches 700–750 MWh/year in the mesophilic Energies 2020, 13, x FOR PEER REVIEW 11 of 17

303 amount of biomethane available in the base reference case, which is the mesophilic/thermophilic one 304 without solar system (in the different sites). 305 Figure 5 reports the amount of saved biomethane by introducing in the system three different 306 types of solar collectors, with different storage systems, in the three sites in Italy. For graphical 307 reasons, only three levels of storage, that are 6, 12, and 18 hours, are presented. Complete results are 308 reported in Tables S3-S20 of Supplementary material. 309 A considerable difference exists between the results of the mesophilic case and the thermophilic 310 one, due to the different hourly production of biomethane (84 Nm3/h and 100.8 Nm3/h, respectively). 311 The areas of the solar collectors in the thermophilic conditions (Figure 5b,d,f) are significantly larger 312 than in the mesophilic case, because the thermophilic reactor requires a larger amount of thermal 313 energy compared to the mesophilic case. Saved biomethane reaches 700-750 MWh/year in the 314 mesophilic conditions (Figure 5a,c,e) and 1300–1500 MWh/year in the thermophilic conditions 315 (Figure 5b,d,f), for the most efficient collector (VTK 120-2 CPC) and longest storage time. However, 316 to reach similar values in the different sites, the required area increases for sites with low annual solar 317 irradiation. 318 As expected, the saved biomethane fraction (ratio of annually saved biomethane to annually 319 biogas sent to boiler in the base case without solar integration) increases with the increasing of the 320 area of collectors, storage tank size, and with collector type. Considering a thermal storage of 18 h 321 and a surface of the collectors FKC-2W of about 3300 m2 in site 1, the fractions of biomethane saved 322 annually are equal to 38% (564 MWh/year) and 24% (655 MWh/year) for the mesophilic and thermophilic 323 conditions, respectively. In addition, under the same conditions (site 1, 18 h and 3300 m2), using the 324 collector VTK 120-2 CPC type, the fractions of saved biomethane increase to 46% (683 MWh/year) and 325 35% (959 MWh/year) for mesophilic and thermophilic conditions, respectively. The additional Energies 2020, 13, 4292 11 of 16 326 biomethane produced by the integrated system might be regarded as a way to store solar energy into 327 an easily deliverable biofuel. 328 Finally, we can observe that results of saved biomethane obtained using collectors FKC-2W and conditions (Figure5a,c,e) and 1300–1500 MWh /year in the thermophilic conditions (Figure5b,d,f), 329 FT 226-2V are quite similar, at the same storage capacity conditions, while the use of collector VTK for330 the most120-2 eCPCﬃcient may collectorprovide higher (VTK savings. 120-2 CPC) and longest storage time. However, to reach similar values in the diﬀerent sites, the required area increases for sites with low annual solar irradiation.

Energies 2020, 13, x FOR PEER REVIEW 12 of 17 (a) (b)

(e) (f)

331 Figure 5.FigureAnnual 5. Annual biomethane biomethane savings savings asas a a function function area area of solar ofsolar collectors collectors,, thermal thermalstorage size storage and size 332 different types of collectors: (a) Mesophilic condition site 1, (b) Thermophilic condition site 1, (c) and diﬀerent types of collectors: (a) Mesophilic condition site 1, (b) Thermophilic condition site 1, 333 Mesophilic condition site 2, (d) Thermophilic condition site 2, (e) Mesophilic condition site 3, (f) 334 (c) MesophilicThermophilic condition condition site site 2, 3. ( d) Thermophilic condition site 2, (e) Mesophilic condition site 3, (f) Thermophilic condition site 3. 335 3.2. Economic Results As expected, the saved biomethane fraction (ratio of annually saved biomethane to annually 336 Figure 6 presents the results of the economic assessment in terms of PBT of the investment. The biogas337 sentresults to boilerare presented in the assuming base case the without 10 years solar constraint integration), as it is the increases maximum with period the for increasing receiving ofthe the area of338 collectors, economic storage incentives tank for size, biomet andhane. with T collectorhe investment type. costs Considering are higher for a thermal solar collectors storage of oflarger 18 h and a surface339 ofsurfaces the collectors with more FKC-2W hours of ofthermal about storage. 3300 m On2 in the site other 1, the hand fractions, by increasing of biomethane the solar field saved area annually are340 equal and to storage 38% (564size, more MWh biomethane/year) and is saved 24% (655and additional MWh/year) economic for thebenefits mesophilic are possible and. thermophilic conditions, respectively. In addition, under the same conditions (site 1, 18 h and 3300 m2), using the collector VTK 120-2 CPC type, the fractions of saved biomethane increase to 46% (683 MWh/year) and 35% (959 MWh/year) for mesophilic and thermophilic conditions, respectively. The additional

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 12 of 17

Energies 2020, 13, 4292 12 of 16 biomethane produced by the integrated system might be regarded as a way to store solar energy into an easily deliverable biofuel. Finally, we can observe(e that) results of saved biomethane obtained(f using) collectors FKC-2W and FT331 226-2V areFigure quite 5. Annual similar, biomethane at the savings same storageas a function capacity area of solar conditions, collectors, whilethermal thestorage use size of and collector VTK 120-2332 CPC maydifferent provide types of higher collectors: savings. (a) Mesophilic condition site 1, (b) Thermophilic condition site 1, (c) 333 Mesophilic condition site 2, (d) Thermophilic condition site 2, (e) Mesophilic condition site 3, (f) 3.2.334 EconomicThermophilic Results condition site 3. 335 Figure3.2. Economic6 presents Results the results of the economic assessment in terms of PBT of the investment. The336 results areFigure presented 6 presents assuming the results theof the 10 economic years constraint, assessmentas in itterms is the of PBT maximum of the investment. period for The receiving the337 economic results are incentives presented for assuming biomethane. the 10 years The constraint investment, as it costs is the are maxim higherum period for solar for receiving collectors the of larger surfaces338 economic with more incentives hours for of thermalbiomethane. storage. The investment On the other costs hand, are higher by increasing for solar collectors the solar of ﬁeldlarger area and 339 surfaces with more hours of thermal storage. On the other hand, by increasing the solar field area storage size, more biomethane is saved and additional economic beneﬁts are possible. 340 and storage size, more biomethane is saved and additional economic benefits are possible.

Energies 2020, 13, x FOR PEER REVIEW 13 of 17 (a) (b)

(e) (f)

341 Figure 6. Payback time as a function area and types of solar collectors and thermal storage size: (a) Figure 6. Payback time as a function area and types of solar collectors and thermal storage size: 342 Mesophilic condition location 1, (b) Thermophilic condition site 1, (c) Mesophilic condition site 2, (d) (a) Mesophilic condition location 1, (b) Thermophilic condition site 1, (c) Mesophilic condition site 2, 343 Thermophilic condition site 2, (e) Mesophilic condition site 3, (f) Thermophilic condition site 3. (d) Thermophilic condition site 2, (e) Mesophilic condition site 3, (f) Thermophilic condition site 3. 344 Figure 6a,c,e for the mesophilic conditions, show that PBT is close to 10 years for some solutions, 345 in particular with the FKC-2W collector model, in the site 2 and 3, for a limited range of areas and 346 capacity of storage tanks. In this case, the best solution from the economic point of view, 347 corresponding to the minimum PBT, in Figure 6c,e, falls in the range between 800 and 1000 m2, with 348 storage tank size equal to 12–18 h. 349 In the thermophilic conditions, as shown in Figure 6b,d,f PBT is close to 10 years only in the case 350 of site 3 for FKC-2W collector model, associated with 12–18 hours of storage time. From Figure 6f, it 351 is noted that the minimum PBT falls in the range between 1900 and 2500 m2, with storage tank size 352 equal to 12–18 h. 353 It is observed that the cost of the collector with higher efficiency (VTK 120-2 CPC), which actually 354 provides higher biomethane saving, is too high to supply economic sustainability for the studied 355 systems.

356 4. Discussion 357 From these results, it is clear that the solar integration into AD producing biomethane has a 358 limited economic sustainability and it is strongly affected by the investment cost of the solar system. 359 Uncertainty on the investment cost plays a major role of the results, however only a reduction of such 360 investment cost in the future may improve the economic sustainability of the system. For this reason, 361 a sensitivity analysis is performed in order to evaluate the influence of the specific cost reduction for 362 the different solar collector types, with respect to previously assumed values. Assuming to reduce 363 the collector cost by 10 to 70%, the PBT is recalculated, as reported in Figure 7, for two selected types 364 of collectors, i.e. FKC-2W and VTK 120-2 CPC, in thermophilic condition with 12 h of thermal storage,

Energies 2020, 13, 4292 13 of 16

Figure6a,c,e for the mesophilic conditions, show that PBT is close to 10 years for some solutions, in particular with the FKC-2W collector model, in the site 2 and 3, for a limited range of areas and capacity of storage tanks. In this case, the best solution from the economic point of view, corresponding to the minimum PBT, in Figure6c,e, falls in the range between 800 and 1000 m 2, with storage tank size equal to 12–18 h. In the thermophilic conditions, as shown in Figure6b,d,f PBT is close to 10 years only in the case of site 3 for FKC-2W collector model, associated with 12–18 h of storage time. From Figure6f, it is noted that the minimum PBT falls in the range between 1900 and 2500 m2, with storage tank size equal to 12–18 h. It is observed that the cost of the collector with higher eﬃciency (VTK 120-2 CPC), which actually provides higher biomethane saving, is too high to supply economic sustainability for the studied systems.

4. Discussion From these results, it is clear that the solar integration into AD producing biomethane has a limited economic sustainability and it is strongly affected by the investment cost of the solar system. Uncertainty on the investment cost plays a major role of the results, however only a reduction of such investment cost in the future may improve the economic sustainability of the system. For this reason, a sensitivity analysis is performed in order to evaluate the influence of the specific cost reduction for the diEnergiesfferent 20 solar20, 13, collectorx FOR PEER types, REVIEW with respect to previously assumed values. Assuming to reduce14 of the 17 collector cost by 10 to 70%, the PBT is recalculated, as reported in Figure7, for two selected types of 365 collectors,for site 3. i.e.,Table FKC-2W S27 in Supplementary and VTK 120-2 CPC,material in thermophilic reports the values condition of the with reduced 12 h of specific thermal cost storage, of the 366 forsolar site collectors 3. Table. S27 in Supplementary material reports the values of the reduced specific cost of the solar collectors.

(a) (b)

367 FigureFigure 7. 7. PaybackPayback timetime asas aa functionfunctionarea areaof of solar solar collectors collectorsand and thermal thermal storage storage size, size, varying varying the the 368 speciﬁcspecific cost cost of of solar solar collectors: collectors: (a(a)) FKC-2W FKC-2W with with 1212 hh thermal thermal storage storage thermophilic thermophiliccondition condition sitesite 3,3, 369 (b(b)) VTK VTK 120-2 120-2 CPC CPC with with 12 12 h h thermal thermal storage storage thermophilic thermophilic condition condition site site 3. 3.

370 FigureFigure7 7shows shows that that reducing reducing the the specific specific cost, cost, the the PBT PBT values values obviously obviously decrease, decrease providing, providing 371 somesome acceptable acceptable economic economic solutions. solutions. In In the the FKC-2W FKC-2W collector collector case case with with 12 12 h h thermal thermal storage storage size, size, the the 372 PBTPBT becomesbecomes inin general general lowerlower thanthan 10 10 years years when when the the specific specific cost cost is is below below 189.87 189.87 Euro Euro/m/m2 2( (-20% 20% − 373 collectorcollector costs). costs). WhileWhile inin thethe casecase ofof thethe VTKVTK120-2 120-2 CPC CPC collector collector with with 12 12 h h thermal thermal storage storage size, size, the the 2 374 PBTPBT becomes becomes lower lower than than 10 10 years years only only if if the the specific specific cost cost is is 231.93 231.93 Euro Euro/m/m 2( (-70% 70% collector collector costs). costs). − 375 Finally,Finally, we we recalculate recalculate the the PBT PBT for for some some selected selected cases cases (site (site 3, thermophilic 3, thermophilic case, case, FKC-2W FKC and-2W VTK and 376 120-2VTK CPC120-2 collectors), CPC collectors), considering consider onlying the only income the income from biomethane from biomethane selling selling at the naturalat the natural gas price gas 377 (withoutprice (without the additional the additional incentives incentives specific forspecific the Italian for the situation) Italian situation equal to) 20.41 equal Euro to 20.41/MWh Eu (whichro/MWh is 378 about(which 25% is ofabout the overall25% of sellingthe overall price selling considered price in considered the base case). in the Being base that case) the. Being Italian that incentives the Italia aren 379 incentives are quite advantageous, with respect to other European countries, we expect that other 380 results, eventually calculated considering other intermediate cases of incentives, will lay within the 381 range of our results calculated with and without the Italian incentives, providing a more general 382 interpretation of the presented results. 383 Of course, the revenues obtainable by selling the biomethane at the natural gas price, without 384 incentives, are also dependant on the trend of the natural gas market, which presented average 385 annual values in the range 19–26 Euro/MWh in the last ten years in Italy, according to a decreasing 386 trend [26]. 387 Results in Figure 8 shows that the PBT calculated without incentives is always higher than 10 388 years, as expected, confirming that the incentives mechanism is fundamental to support the economic 389 sustainability of solar integration in AD producing biomethane.

Energies 2020, 13, 4292 14 of 16

quite advantageous, with respect to other European countries, we expect that other results, eventually calculated considering other intermediate cases of incentives, will lay within the range of our results calculated with and without the Italian incentives, providing a more general interpretation of the presented results. Of course, the revenues obtainable by selling the biomethane at the natural gas price, without incentives, are also dependant on the trend of the natural gas market, which presented average annual values in the range 19–26 Euro/MWh in the last ten years in Italy, according to a decreasing trend [26]. Results in Figure8 shows that the PBT calculated without incentives is always higher than 10 years, asEnergies expected, 2020, 13 conﬁrming, x FOR PEER REVIEW that the incentives mechanism is fundamental to support the economic15 of 17 sustainability of solar integration in AD producing biomethane.

(a) (b)

390 FigureFigure 8. 8.Payback Payback time time without without incentives incentives as as a a function function area area of of solar solar collectors collectors and and thermal thermal storage storage 391 size,size varying, varying the the speciﬁc specific cost cost of of solar solar collectors: collectors: ( a()a) FKC-2W FKC-2W Thermophilic Thermophilic condition condition site site 3, 3, ( b(b)) VTK VTK 392 120-2120-2 CPC CPC Thermophilic Thermophilic condition condition site site 3. 3.

393 5.5. Conclusions Conclusions 394 TheThe possibility possibility of of integrating integrating thermal thermal solar solar energy energy into into anaerobic anaerobic digestion digestion reactor, reactor with, with biogas biogas 395 upgradingupgrading to to biomethane, biomethane, is is studied, studied, considering considering mesophilic mesophilic/thermophilic/thermophilic conditions, conditions, three three di differentfferent 396 typestypes of of solar solar collectors collectors (Bosch (Bosch FKC-2W, FKC-2W, FT FT 226-2V, 226-2V, VTK VTK 120-2 120-2 CPC), CPC), and and three three di differentfferent Italian Italiansites. sites. 397 TheThe influence influence of of the the solar solar collector’s collector’s area area and and the the thermal thermal storage storage size size are are studied. studied. 398 TheThe integration integration of of solar solar source source allows allows reducing reducing the the amount amount of of biogas biogas required required for for heating heating the the 399 digester,digester, thus thus more more biogas biogas is sentis sent to theto the upgrading upgrading section, section, producing producing more more biomethane biomethane that canthat be can sent be 400 tosent the grid.to the The grid. saved The biomethane saved biomethane amount depends amount on depends the diff erenton the studied different conditions. studied In conditions. general, it is In 401 largergeneral, in thermophilic it is larger in case thermophilic with respect case to thewith mesophilic respect to one;the mesophilic it increases withone; it the increas area ofes the with collectors, the area 402 withof the the collectors, storage capacity, with the with storage decreasing capacity, latitude with ofdecreasing the site and latitude with improvedof the site collector and with technology. improved 403 collectorThe amount technolo ofgy. yearly saved biomethane lays in the range 100–700 MWh/y and 100–1500 MWh/y 404 for theThe mesophilic amount of and yearly thermophilic saved biomethane cases, respectively.lays in the range This 100 amount–700 MWh/y represents and 100 a– fraction1500 MWh/y of the for 405 requiredthe mesophilic heat for and the ther gasmophilic boiler in cases, the base respectively. case, ranging This from amount 4% torepresents 55%, depending a fraction on of the the di requiredfferent 406 consideredheat for the variables. gas boiler in the base case, ranging from 4% to 55%, depending on the different considered 407 variablesResults. of the simplified and preliminary economic analysis show that in the mesophilic conditions 408 there areResults some of economically the simplified acceptable and pre solutionsliminary witheconomic PBT close analysis to 10 s years,how that in particular in the mesophilic with the 2 409 collectorconditions FKC-2W there are type some for the economically range between acceptable 800 and solutions 1000 m , with with PBT storage close tank to 10 size years equal, in to particular 12–18 h 410 inwith the sitesthe collector 2 and 3. InFKC the-2W thermophilic type for the conditions, range between the minimum 800 and PBT 1000 is m 102, years,with storage and it can tank be size obtained equal 2 411 withto 12 collector–18 h in areathe site betweens 2 an 1900d 3. In and the 2500 thermophilic m , with storage conditions, tank the capacity minimum 12–18 PBT h, in is the 10 siteyears, 3. and it 412 can Itbe is obtained observed with that thecollector economic area acceptabilitybetween 1900 can and be 2 reached500 m2, onlywith using storage the tank less expensivecapacity 12 collector,–18 h, in 413 eventhe site if its 3. e fficiency allows lower biomethane savings. Future reduction of solar collector costs might 414 improveIt is the observed economic that feasibility. the economic However, acceptability when the payback can be reached time is calculated only using excluding the less the expensive Italian 415 collector, even if its efficiency allows lower biomethane savings. Future reduction of solar collector 416 costs might improve the economic feasibility. However, when the payback time is calculated 417 excluding the Italian incentives, its value is always higher than 10 years. Therefore, incentives 418 mechanism is a fundamental support to the economic sustainability of solar integration in these 419 systems. 420 Further analysis, based on complete life cycle costing of the integrated AD/solar system, is 421 however required to better evaluate the main economic indicators for the whole system, also 422 considering the uncertainties associated with costs and revenues and other boundary conditions as 423 climate change impact on solar energy. 424 Finally, the proposed integration can be regarded as a way to store solar energy, considering 425 that the solar energy is the most abundant energy resource with the potential to become a major 426 component of a sustainable global energy solution.

Energies 2020, 13, 4292 15 of 16 incentives, its value is always higher than 10 years. Therefore, incentives mechanism is a fundamental support to the economic sustainability of solar integration in these systems. Further analysis, based on complete life cycle costing of the integrated AD/solar system, is however required to better evaluate the main economic indicators for the whole system, also considering the uncertainties associated with costs and revenues and other boundary conditions as climate change impact on solar energy. Finally, the proposed integration can be regarded as a way to store solar energy, considering that the solar energy is the most abundant energy resource with the potential to become a major component of a sustainable global energy solution.

Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1073/13/17/4292/s1. Author Contributions: Conceptualization, L.L.; Methodology, L.L., B.M.; Formal analysis, S.F.; Investigation, L.L., B.M.; Writing- Original draft preparation, S.F.; Writing- Reviewing and Editing, L.L., B.M.; Visualization, S.F.; Supervision, L.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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