Performance Results of a Solar Adsorption Cooling and Heating Unit

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Article Performance Results of a Solar Adsorption Cooling and Heating Unit

Tryfon C. Roumpedakis 1 , Salvatore Vasta 2,* , Alessio Sapienza 2, George Kallis 1, Sotirios Karellas 1, Ursula Wittstadt 3, Mirko Tanne 3, Niels Harborth 4 and Uwe Sonnenfeld 4 1 Laboratory of Steam Boilers and Thermal Plants, School of Mechanical Engineering, National Technical University of Athens, 15780 Athens, Greece; [email protected] (T.C.R.); [email protected] (G.K.); [email protected] (S.K.) 2 Consiglio Nazionale delle Ricerche (CNR), Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano” (ITAE), 98126 Messina, Italy; [email protected] 3 Fahrenheit GmbH, 80803 Munich, Germany; [email protected] (U.W.); [email protected] (M.T.) 4 AkoTec Produktionsgesellschaft mbH, 16278 Angermünde, Germany; [email protected] (N.H.); [email protected] (U.S.) * Correspondence: [email protected]; Tel.: +39-090-624404

Received: 23 February 2020; Accepted: 19 March 2020; Published: 2 April 2020

Abstract: The high environmental impact of conventional methods of cooling and heating increased the need for renewable energy deployment for covering thermal loads. Toward that direction, the proposed system aims at offering an efficient solar powered alternative, coupling a zeolite–water adsorption chiller with a conventional vapor compression cycle. The system is designed to operate under intermittent heat supply of low-temperature solar thermal energy (<90 ◦C) provided by evacuated tube collectors. A prototype was developed and tested in cooling mode operation. The results from the testing of separate components showed that the adsorption chiller was operating efficiently, achieving a maximum coefficient of performance (COP) of 0.65. With respect to the combined performance of the system, evaluated on a typical week of summer in Athens, the maximum reported COP was approximately 0.575, mainly due to the lower driving temperatures with a range of 75 ◦C. The corresponding mean energy efficiency ratio (EER) obtained was 5.8.

Keywords: solar cooling; adsorption; evacuated tube collectors; experimental testing

1. Introduction Renewable energy sources gained significant interest over recent years owing to the depletion of fossil fuel reserves and the growing concerns over the environmental impact of conventional cooling and heating technologies. Moreover, the European Union sub-target of increasing the renewable heating and cooling by 1.3% per year, starting in 2021, outlines the necessity for more sustainable solutions in the field [1]. Taking into consideration the concurrence between the solar availability and the peak building demands, solar energy could be one of the most reliable, commercially mature alternatives toward the minimization of the heating and cooling sector environmental footprint [2]. With respect to solar-driven heating/cooling, there are two main technologies: photovoltaic (PV)-driven reversible heat pumps and solar thermally driven sorption heat pumps [3,4]. Currently, the PV-driven cooling/heating systems dominate the market, thanks to the overall growth of the PV market, which is expected to gain a share of 16% of the total energy production by 2050, according to the International Energy Agency [5]. This expansion of the PV market, along with the respective increased use of reversible heat pumps, resulted in low specific capital costs for the PV-driven cooling/heating systems, making this technology the most competitive solar-driven cooling/heating technology [6].

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On the other hand, the majority of commercially available solar thermally driven cooling/heating set-ups implement an absorption heat pump, mainly due to the fact that absorption is the most mature thermally driven cooling/heating technology [7], as well as owing to the higher involved coefficient of performance (COP, the ratio between the cold produced and the heat spent to produce it [8]) and the lower capital costs compared to the other sorption technologies [9]. Several solar absorption applications were developed and are currently in operation across the world. For instance, a solar-driven absorption system was installed at the Center for Renewable Energy Sources and Saving (CRES) in Pikermi, Athens, Greece, and it is in operation since December 2011. The solar field consists of flat plate collectors with a total surface of 149.5 m2, while an underground energy storage system 3 with a total volume of 58 m is also installed. The LiBr–H2O absorption chiller has a nominal capacity of 35 kW, while an 18-kW conventional heat pump is installed as a backup. According to measurements conducted by Drosou et al. [10], the achieved solar fraction is around 70%. The annual cooling demands were estimated to be 19.5 MWh/a, which refers to the period of May–September, while the respective heating loads were 12.3 MWh/a for the period of October–April [11]. Adsorption technology gained attention over the past few years, thanks to its potential to exploit very low-grade heat sources, the absence of crystallization issues, and the simplicity of the involved equipment due to the absence of a solution pump and a rectifier [12,13]. Furthermore, compared to conventional electrically driven systems, the adsorption technology has advantages in the lower operating costs, the absence of moving parts, and the absence of vibrations [14]. On the other hand, a key drawback for adsorption technology is the relatively low COP [15]. Solar adsorption systems were already investigated thoroughly in terms of both their theoretical and their experimental performance [16,17]. In Reference [18], Rahman et al. introduced an innovative thermodynamic cycle based on a four-bed layout and three stages which can be powered by low-temperature heat. The mathematical simulations conducted showed that the maximum coefficient of performance of the system can be achieved for a regeneration temperature at 55 ◦C, while the optimal cycle time was dependent on the corresponding heat source temperature, thus proving the possibility to employ an adsorption chiller for solar applications. Habib et al. [19] simulated the performance of a two-stage four-bed silica gel–water adsorption system, powered by evacuated tube collectors. For the needs of the simulations, the heat source temperature was varied from 40 to 95 ◦C. In single-stage mode (driving temperature of 80 ◦C, cooling water temperature of 30 ◦C, and chilled water inlet temperature 14 ◦C), the COP was around 0.48. On the other hand, when the driving temperature was lower (50 ◦C), the system operated in two stage mode, with a COP of approximately 0.27. Lemmini and Errougani [20] tested a single-bed methanol-activated carbon (AC-35) adsorption unit powered by a flat plate collector. Several experiments were conducted, achieving a maximum solar COP between 0.05 and 0.08 and a minimum temperature of 11 C. Aristov et al. [21] evaluated simulations of their solar refrigeration − ◦ system developed based on a closed adsorption cycle. Among several chemisorbents, CaCl2 in silica gel composite sorbent was found to be the most efficient sorbent for water adsorption, resulting in a cycle COP equal to 0.6–0.8 (Te = 5 ◦C, Tc = 35 ◦C, and Tdes = 80 ◦C) evaluated experimentally in a silica gel single-bed adsorption chiller driven by 4-m2 evacuated tube collectors under the climatic conditions of Baghdad, Iraq. The nominal driving heat temperature was set at approximately 90 ◦C. Under varying experimental conditions, the optimal working point of the chiller was determined to be achieved at an evaporator temperature of 6.6 ◦C, which corresponded to a COP of 0.55. Chorowski et al. [22] investigated the performance effects of the control strategy on a three-bed, two-evaporator adsorption chiller. The authors demonstrated that changes to the control system can improve the performance of the chiller, especially in terms of COP. However, improvements were still limited; average COP was improved by 7.5% while the peak COP was improved by 11% up to 0.71. Due to the aforementioned drawbacks of adsorption technology, there are fewer applications of solar adsorption cooling. One of the earliest solar thermal systems based on a 5.5-kW adsorption chiller was developed Energies 2020, 13, 1630 3 of 18 and installed at the Institute for Solar Energy System (ISE) in Freiburg, Germany. The measurements conducted between August 2008 and July 2009 revealed an average COP of 0.43 [23]. In most theoretical and experimental investigations, the adsorption chillers used a silica gel–water working pair, mainly due to the stability and the low cost of silica gel. Lu et al. [24] analyzed the performance of a novel solar-driven adsorption air-conditioning system. The cooling capacity of the measured silica gel–water adsorption system was approximately 3.6 kW, corresponding to a COP of 0.32 (Te = 15 ◦C, Tc = 27 ◦C, and Tdes = 57 ◦C). Koronaki et al. [25] simulated a two-bed silica gel–water adsorption chiller powered by various types of solar collectors. The system was evaluated for three cities in the southern Mediterranean region, namely, Athens (Greece), Nicosia (Cyprus), and Alexandria (Egypt). With respect to the adsorption module, the optimal type of solar collector was a selective flat plate one, resulting in a cooling COP of 0.51. However, with respect to the exergy efficiency maximization, the optimal type of solar collector was the unglazed photovoltaic/thermal (PV/T) system with an overall efficiency of 11.35%. Similarly, Alahmer et al. [26] assessed a two-bed silica gel–water adsorption chiller driven by concentrated solar collectors. At the peak hour, the estimated COP was around 0.5, for a cooling output of 11 kW. In terms of economics, the proposed system resulted in a payback period of 11 years, using a solar field of 38 m2. A small number of solar adsorption installations are operating across Europe for air-conditioning applications. One of the first solar cooling installations was the set-up at the university hospital of Freiburg. The system, which was installed in 1999, is used to cover the cooling loads of a lab section with a total area of 550 m2. The adsorption chiller has a cooling capacity of 70 kWc, with a nominal COP of 0.6, while the nominal output temperature of the chilled air is 9 ◦C. The system is powered by a 167 m2 vacuum tube collector field, while backup heating is supplied by the district heating network [23]. Another set-up of solar adsorption cooling is used in a cosmetics company (Sarantis S.A.) in Greece. Using a 2700 m2 field of flat plate collectors, two 350-kWc silica gel–water adsorption chillers are operated in order to air-condition a total space of 22,000 m2 [27]. Despite the attractiveness of the aforementioned solutions, the need to cover thermal loads in the absence of solar irradiance results in the use of conventional backup systems. In order to overcome this issue, recently, several hybrid adsorption/compression solutions were proposed [28,29]. Cyklis [30] investigated a two-stage cascade adsorption–compression system, consisting of a solar-driven commercial silica gel–water adsorption chiller and a low-temperature CO2 compression cycle. Compared to a conventional double-stage CO2 R410A compression cycle, an energy saving up to 30% was estimated, while the respective percentage for total equivalent warming impact was as high as 60%. Miyazaki et al. [31] evaluated two different configurations of hybrid adsorption/compression cycles, namely, the sub cooling type and the cascading type. The results of the simulations indicated that the cascading cycle enhanced the COP by 40%, while the sub cooling configuration performed better in terms of system size and, thus, capital costs of the system. Palomba et al. [32] evaluated different configurations of cascade adsorption/compression chillers for various applications and discussed the key aspects in the proper sizing of the two sub-systems for maximization of the energy savings and overall system performance. The ZEOSOL project [33] is based on the hybridization of an adsorption chiller with a conventional vapor compression cycle. The implementation of the vapor compression cycle allows covering the peak loads; hence, the adsorption chiller operates at higher COP, while, in the absence of solar irradiance, the conventional system is able to fully cover the loads of the residential building [34]. The system was designed with particular attention paid to the environmental impact of the developed system, compared to conventional alternatives, as presented by Kallis et al. [35]. The life-cycle assessment (LCA) of the investigated system, using the ReCiPe 2016 method, outlined the significant reduction in the system’s impact on global warming and ozone depletion with respect to conventional reversible heat pumps. On the other hand, the worse performance of the system in impact factors such as the land use and the mineral resource scarcity highlighted the necessity for further optimization of the coupling toward the reduction of the excessive use of copper. Energies 2020, 13, 1630 4 of 18

In the present study, the authors investigate the preliminary experimental performance of a solar-driven hybrid cooling and heating system based on a small-scale zeolite–water adsorption chiller in adsorption only mode. The prototype system was designed to fully cover the thermal loads of a residential building with a 12.5-kW-peak cooling load.

2. System Description The proposed system focuses on the coupling of a zeolite–water adsorption chiller with solar thermal collectors. The cooling capacity of the developed sorption chiller exceeds 10 kW using SAPO-34 as the adsorbent. To reduce the chiller’s capacity and, thus, the required solar field area, while simultaneously enhancing the efficiency of part-load operation, a backup electrically driven heat pump is coupled with the adsorption chiller. The backup heat pump has a nominal cooling capacity of 10 kW and is used mainly to cover peak loads. The solar field consists of three rows of advanced evacuated tube collectors with a total surface of 40 m2. The system is also equipped with a 1-m3 heat storage tank, which absorbs the spikes in solar energy, allowing for a more stable operation of the sorption module. A “V-shaped” dry cooler is implemented as the heat rejection unit for both the adsorption chiller and the backup heat pump, retrofitted for the specific application. An overview of the prototype, including the installed measuring devices, is shown in Figures1 and2, while images of the actual set-up are also provided in Figure3. To enhance the solar collector’s performance and allow risk-free operation in low ambient temperatures, a propylene glycol solution is used as the working medium for the solar subsystem. Moreover, all secondary circuits of the adsorption chiller use pure water. The 1-m3 heat storage tank is equipped with heat coils, via which heat is transferred from the glycol solution toward the hot water, which in turn drives the adsorption chiller. All temperature measurements were obtained using Pt1000 thermal sensors, class A according to DIN/EN 60751. The flow sensors used were ultrasonic in-line flow meters with a 2% accuracy. The electrical power consumption of the dry cooler, the compressor, and the circulation pumps was measured using energy analyzers. Finally, the solar radiation and the ambient conditions were monitored via a solar weather station, equipped with a second-class (as ISO 9060) pyranometer, a Pt1000 thermal sensor, an air humidity sensor, and an anemometer. The experimental procedure focused on optimizing the coupling of the solar module with the hybrid adsorption–compression chiller. Therefore, several experiments were conducted, varying the target temperature of the storage tank (Tst,3 in Figure1) and the high- and low-temperature setpoints of the chiller. The proper selection of a target temperature in the storage tank is of key importance for the operation of the system, in order to allow for maximum operation of the solar-driven system, while retaining sufficient high-temperature (HT) inlet temperatures for maximum efficiency of the adsorption chiller. In each day/experiment, the solar circuit started operation when the solar irradiance was over 200 W/m2, charging the storage tank with the collected solar heat. When the HT setpoint temperature was reached at Tst,3 (see Figure1), the pumps of the chiller were set on and the chiller initiated its start phase. The charging of the storage tank until Tst,3 reached the HT setpoint was a selection made by the user, as it was identified that it provided sufficient heat to operate the system for a prolonged period of time. The proper selection, on the other hand, of the HT and low-temperature (LT) setpoints was of key importance, as it is expected to have a direct influence on the system’s COP and the duration of the system’s solar-driven daily operation. The system’s cooling mode consists of three modes of operation, namely, adsorption-only operation which is driven solely by solar energy (Figure2a), compression-only operation, and their combined operation (Figure2b). This paper does not discuss the compression-only operation, as it is of no scientific interest, owing to its maturity. Energies 2020, 13, x FOR PEER REVIEW 5 of 18 Energies 2020, 13, 1630 5 of 18 Energies 2020, 13, x FOR PEER REVIEW 5 of 18

Figure 1. Schematic of system prototype with all the involved measuring devices. Figure 1. Schematic of system prototype with all the involved measuring devices. Figure 1. Schematic of system prototype with all the involved measuring devices.

(a)

(b)

Figure 2. Detailed schematic of the hybrid adsorption(b) chiller/backup heat pump module: (a) in adsorptionFigure 2. Detailed only mode; schematic (b) in combined of the hybrid operation. adsorption chiller/backup heat pump module: (a) in adsorption only mode; (b) in combined operation. Figure 2. Detailed schematic of the hybrid adsorption chiller/backup heat pump module: (a) in adsorption only mode; (b) in combined operation.

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(b) (a)

(d) (c)

FigureFigure 3. 3.Overview Overview of of thethe experimentalexperimental set-up set-up components components [35]: [35 (a]:) the (a) ETCs the ETCs (Evacuated (Evacuated Tubes Tubes Collectors)Collectors) solar solar ﬁeld; field; (b )(b the) the hybrid hybrid chiller–dry chiller–dry cooler cooler set-up;set-up; ((c) the solar station station and and the the storage storage tank andtank (d) theand hydronic(d) the hydronic ducted ducted fan coil fan unit. coil unit.

3. Experimental3. Experimental Measuring Measuring of of SeparateSeparate Components Components TheThe preliminary preliminary measuring measuring ofof the proposed system system was was divided divided into intothree three parts: parts: (a) the (a) the experimentalexperimental assessment assessment of of the the solar solar collectors collectors andand thethe storage tank, tank, (b) (b) the the performance performance testing testing of of the hybridthe hybrid adsorption adsorption chiller, chiller, and (c)and the (c) drythe dry cooler cooler along along with with all all the the involved involved auxiliaryauxiliary equipment equipment (e.g., (e.g., circulations pumps). circulations pumps). The solar collectors used in the system were heat pipe evacuated tube collectors, manufactured The solar collectors used in the system were heat pipe evacuated tube collectors, manufactured by Akotec specifically for the ZEOSOL system, able to operate efficiently between 65 and 95 °C. The by Akoteccollectors specifically were tested by for a thecertified ZEOSOL institute system, according able to toISO operate 9806. The effi collectorciently efficiency, between shown 65 and in 95 ◦C. TheFigure collectors 4, is werecalculated tested as byfollows: a certified institute according to ISO 9806. The collector efficiency, shown in Figure4, is calculated as follows: 𝑇 −𝑇 (𝑇 −𝑇) 𝜂=𝜂 −𝑐 −𝑐 . (1) G G 2 T Ta (Tcol Ta) η = η c col − c − . (1) As the coefficients c₀, c₁, and c₂0 are− 1confidential,G − a2 graphicalG overview of the solar collector efficiency is presented in Figure 4 with respect to other commercial solar collectors. This representationAs the coeffi wascients createdc0, cfor1, andstandardc2 are test confidential, conditions (STC), a graphical namely, an overview ambient temperature of the solar of collector25 efficiency°C andis a presentedsolar irradiance in Figure of 10004 with W/m respect². to other commercial solar collectors. This representation was created for standard test conditions (STC), namely, an ambient temperature of 25 ◦C and a solar irradiance of 1000 W/m2.

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Figure 4. Performance curve of the developed solar collectors for the ZEOSOL system (blue line) in Figure 4. Performance curve of the developed solar collectors for the ZEOSOL system (blue line) in comparison to other commercial solar collectors. comparison to other commercial solar collectors. On the other hand, the adsorption chiller was developed and experimentally tested by the On the other hand, the adsorption chiller was developed and experimentally tested by the respective manufacturers, Fahrenheit GmbH. The performance results of the separate testing of the respective manufacturers, Fahrenheit GmbH. The performance results of the separate testing of the adsorption chiller revealed a maximum thermal COP of 0.54, corresponding to an energy efficiency adsorption chiller revealed a maximum thermal COP of 0.54, corresponding to an energy efficiency ratio (EER) as high as 45 for a driving temperature of 85 ◦C[22]. The aforementioned COP for the ratio (EER) as high as 45 for a driving temperature of 85 °C [22]. The aforementioned COP for the adsorption chiller is defined by Equation (2). adsorption chiller is defined by Equation (2). . QLT𝑄 COP𝐶𝑂𝑃= . = ,, (2)(2) QHT𝑄 wherewhere HTHT refersrefers toto the driving driving heat heat supplied supplied to to the the adsorption adsorption chiller, chiller, and and LT LTrefers refers to the to low- the low-temperaturetemperature stream stream which which provides provides the thecooling cooling effe ect.ffect. On Onthe theother other hand, hand, the the EER EER is defined is defined as asthe . 𝑄 𝑊. theratio ratio between between the the cooling cooling capacity capacity QLT andand the the total total electric electric power power consumption consumption of of the system Wel..

. 𝑄 𝐸𝐸𝑅QLT = . (3) EER = . 𝑊. (3) Wel For the determination of the total electric power consumption of the system, in addition to the For the determination of the total electric power consumption of the system, in addition to the electrical consumption of the dry cooler, .𝑊, , the following aspects also contribute: (i) the power electrical consumption of the dry cooler, Wel,dc, the following aspects also contribute: (i) the power consumption of the heat pump’s compressor, 𝑊. ,, and (ii) the electrical consumption for the six consumptionpumps of the ofsystem, the heat as shown pump’s in Figure compressor, 1. The totalWel, comelectrical, and (ii)power the consumption electrical consumption is calculated for as thefollows: six pumps of the system, as shown in Figure1. The total electrical power consumption is calculated as follows: 𝑊 , =𝑊 , +𝑊 , +𝑊 ,. (4) . . . X . Wel,tot = Wel,com + Wel,dc + Wel,pumps. (4) The equations for the power consumption of the HT, medium temperature (MT), and LT pumps of FigureThe equations 1 can be forfound the in power Reference consumption [22]. Moreover of the HT,, the medium fan coil temperaturepump is identical (MT), to and the LT LT pumps pump; ofthus, Figure the1 samecan be power found consumption in Reference profile [ 22]. Moreover, is realized the. The fan solar coil collector’s pump is identical circuit pump, to the installed LT pump; at thus,the solar the same station power of the consumption set-up (Figure proﬁle 2c) is isa Grundfos realized. The pump, solar model collector’s UPM3 circuit Solar 25-145. pump, On installed the other at thehand, solar the station storage of the pump set-up is (Figurea pump3c) from is a Grundfosthe same pump,manufacturer, model UPM3 model Solar UPS2 25-145. 15-50. On The the electric other hand,power the consumption storage pump curves, is a pump as provided from the sameby the manufacturer, manufacturer, model of the UPS2 aforementioned 15-50. The electric pumps power are consumptionshown in Figure curves, 5a. as provided by the manufacturer, of the aforementioned pumps are shown in FigureThe5a. backup heat pump is a custom-made module developed for this specific application, and a series of experiments were conducted at CNR-ITAE to evaluate its performance in coupling with the dry cooler of the system. Figures 5b, c show that the performance of the backup heat pump, operating

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The backup heat pump is a custom-made module developed for this speciﬁc application, and a series of experiments were conducted at CNR-ITAE to evaluate its performance in coupling with the dry coolerEnergies 20 of20 the, 13,system. x FOR PEER Figure REVIEW5b,c show that the performance of the backup heat pump, operating8 of 18 with R134a at a cooling water temperature of 7 ◦C, is considered satisfactory, achieving COP values as highwith as 4. R134a On the at othera cooling hand, water Figure temperature5d presents of 7 the °C, power is considered dissipated satisfactory, by the dry achieving cooler as COP a function values of theas temperature high as 4. On di ﬀ theerence other between hand, Figure the water 5d presents input and the air. power dissipated by the dry cooler as a function of the temperature difference between the water input and air.

(b) (a)

Figure 5. Experimental results of (a) the electrical consumption of the pumps, as well as the developed Figure 5. Experimental results of (a) the electrical consumption of the pumps, as well as the developed backup heat pump performance; (b) at maximum flow rate; (c) as a function of the maximum backup heat pump performance; (b) at maximum flow rate;(c) as a function of the maximum temperature lift and (d) cooling map of the dry cooler for variable flow rates at maximum fan speed. temperature lift and (d) cooling map of the dry cooler for variable flow rates at maximum fan speed. Regarding the performance of the backup heat pump, its COP is defined by Equation (5). Regarding the performance of the backup heat pump, its COP is defined by Equation (5). . QLT푄̇퐿푇 COP퐶푂푃= =. .. (5) ̇ (5) Wel푊,com푒푙,푐표푚

4. Results of Solar Adsorption Cooling Unit Performance 4. Results of Solar Adsorption Cooling Unit Performance In thisIn this section, section the, the experimental experimental results results of of the the developed developed set-up set-up operating operating solesolelyly with with the the adsorptionadsorption chiller chiller are are presented. presented. FigureFig6ure shows 6 shows the the temperature temperature trend trend and and performance performance measured on on a atypical typical su summermmer day. day. In In particular,particular Figure, Figure6a 6a presents presents an an overview overview of of the the ambient ambient conditions, conditions, withwith respectrespect to the temperature andand the solarthe solar irradiance irradiance for for the the week week of of the the measurements. measurements. As shownAs shown in Figure in Figure6a, the6a, the solar solar irradiance irradiance in in the the evaluated evaluated week week (ﬁrst (first week weekof of AugustAugust 2019)2019) is ratherrather lower lower than than the the average average peak peak values values for for the the summer summer of of a a typical typicalyear year inin Athens,Athens, not exceeding exceeding 10001000 W/m W/m².2. On On the the other other hand, hand, the the simultaneouslysimultaneously higher higher ambient ambient temperatur temperatureses that that occurred occurred in the in investigated days resulted in operating all the components at higher temperature levels, which decreased their efficiency. Figure 6b, c presents the temperature profiles for the solar collector/storage tank module and for the adsorption chiller secondary streams, respectively. The adsorption chiller

Energies 2020, 13, 1630 9 of 18 the investigated days resulted in operating all the components at higher temperature levels, which decreased their efficiency. Figure6b, c presents the temperature profiles for the solar collector /storage Energiestank module 2020, 13, andx FOR for PEER the REVIEW adsorption chiller secondary streams, respectively. The adsorption chiller9 of 18 was set to start its operation at 65 ◦C for the HT in the stream, resulting, in combination with the wasavailable set to solar start irradiance,its operation in operatingat 65 °C for only the a fewHT in hours the perstream, day resulting close to the, in solar combination noon. Despite with the the availableless optimal solar conditions, irradiance the, in system operating was only able a to few cool hours down per the day water close to 7.5to the◦C, solar which noon. was Despite the setpoint the lessfor theoptimal low-temperature conditions, the circuit, system even was though able to the cool driving down temperature the water to was7.5 °C, less which than 80was◦C the in allsetpoint cases, foralthough the low at-temperature the expense circuit, of lower even overall though performance. the driving The temperature profiles of was Figure less6 cthan outline 80 °C the in necessityall cases, althoughfor a modification at the expense of the of control lower strategy overall performance for the involved. The circulating profiles of pumps Figure such6c outline that higher the necessity driving fortemperatures a modification are of obtained, the control ensuring strategy maximum for the involved efficiency circulating of the chiller. pumps At such this that point, higher it has driving to be temperatureshighlighted that are Figure obtained6 refers, ensuring to adsorption maximum only efficiency mode; thus, of inthe the chiller. absence At of this driving point, solar it has heat, to the be highlightedsystem is off that, which Figure results 6 refers in no to operation adsorption at night.only mode; thus, in the absence of driving solar heat, the system is off, which results in no operation at night.

(a)

(b)

(c)

FigureFigure 6. 6. ((aa) )Ambient Ambient conditions conditions at at the the period period of of the the experiments experiments;; ( (bb)) experimental experimental results results with with respect respect toto the the solar solar sub sub-circuit-circuit temperatures temperatures and and ( (cc)) with with respect respect to to the the chiller’s chiller’s secondary secondary stream stream temperatures temperatures..

Figure 7 presents the performance results for the entire ZEOSOL system, in adsorption-only mode, based on the definitions of Equations (2) and (3) on a typical summer day. Figure 7a shows the cooling power production of the chiller during the investigated week and the total electrical power consumption, as defined by Equation (4). As shown in Figure 7a, the maximum obtained cooling

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Figure7 presents the performance results for the entire ZEOSOL system, in adsorption-only mode, based on the definitions of Equations (2) and (3) on a typical summer day. Figure7a shows the cooling power production of the chiller during the investigated week and the total electrical Energies 2020, 13, x FOR PEER REVIEW 10 of 18 power consumption, as defined by Equation (4). As shown in Figure7a, the maximum obtained cooling power output was around 5 kW, which was approximately 40% of the nominal chiller’s cooling power output was around 5 kW, which was approximately 40% of the nominal chiller’s cooling capacity, mainly attributed to the lower driving temperatures during the period of the measurements. capacity, mainly attributed to the lower driving temperatures during the period of the measurements. On the other hand, the electrical power consumption was significantly low, with a maximum of 900 W, On the other hand, the electrical power consumption was significantly low, with a maximum of 900 W,mainly mainly due due to theto the operation operation of the of drythe cooler,dry cooler, which which accounted account fored more for more than 60%than of 60% the of total the power total powerconsumption consumption of the of system. the system. The correspondingThe corresponding performance performance indicators indicators are are presented presented in Figurein Figu7reb. 7Theb. The maximum maximum obtained obtained thermal thermal COP COP was was approximately approximately 0.535, 0.535, forfor aa maximum reported driving driving temperaturetemperature of of 79 79 °C,◦C, while while the the corresponding corresponding maximum maximum EER EER was was as as high high as as 12, 12, with with an an average average operationoperation at at approximately approximately 5.8. 5.8. Similarly Similarly to to Fig Figureure 6,6, both the coo coolingling production and the EER of the systemsystem highlight the the necessity necessity for for further further optimization optimization of the of system’s the system’s control control and operational and operational strategy strategyto maximize to maximize cooling outputcooling at output a reduced at a reduced specific powerspecific consumption. power consumption.

Figure 7. Performance results of proposed system prototype with respect to (a) the produced cooling output and the respective electrical consumption and (b) the corresponding coefficient of performance Figure 7. Performance results of proposed system prototype with respect to (a) the produced cooling (COP) and energy efficiency ratio (EER) values. output and the respective electrical consumption and (b) the corresponding coefficient of performanceFigures8–10 (COP present,) and energy instead, efficiency the performance ratio (EER) values. results for the ZEOSOL system, during a hot summer week and on cloudy days (September). Figures 8–10 present, instead, the performance results for the ZEOSOL system, during a hot As shown in Figure8a, the solar field is capable of providing hot water at a good thermal level summer week and on cloudy days (September). when the solar radiation ranges between 800 and 1000 W/m2, while the pump of the solar loop As shown in Figure 8a, the solar field is capable of providing hot water at a good thermal level continuously switches off and on to preserve water stratification in the storage tank on cloudy days or when the solar radiation ranges between 800 and 1000 W/m², while the pump of the solar loop when the solar radiation drops below 800 W/m2 (Figure8b). continuously switches off and on to preserve water stratification in the storage tank on cloudy days As shown in Figure9, the adsorption chiller can produce cold water at 15 C and release the or when the solar radiation drops below 800 W/m² (Figure 8b). ◦ process heat at 25–30 C, both on a hot day and on a cloudy day; however, cold production is not As shown in Figure◦ 9, the adsorption chiller can produce cold water at 15 °C and release the continuous with a lower solar radiation, thus demonstrating the need for the operation of the electrical process heat at 25–30 °C, both on a hot day and on a cloudy day; however, cold production is not backup unit, not only to cover the remaining loads but also to allow for sufficient re-charging of the continuous with a lower solar radiation, thus demonstrating the need for the operation of the storage tank for more efficient operation of the adsorption module upon resumption of its operation. electrical backup unit, not only to cover the remaining loads but also to allow for sufficient re- Finally, as depicted in Figure 10, the maximum obtained cooling power output was around 6 kW charging of the storage tank for more efficient operation of the adsorption module upon resumption on a hot day, while it ranged between 2 and 5 kW when it was cloudy, due to the lower driving of its operation. temperatures. During the same measurements, the electrical power consumption was significantly Finally, as depicted in Figure 10, the maximum obtained cooling power output was around 6 low, ranging between 50 (electronic consumption) and 900 W (operation of the fans the dry cooler), kW on a hot day, while it ranged between 2 and 5 kW when it was cloudy, due to the lower driving while the maximum obtained thermal COP was approximately 0.53 for a driving temperature of 80 C, temperatures. During the same measurements, the electrical power consumption was significantly◦ with the corresponding average EER of 7. low, ranging between 50 (electronic consumption) and 900 W (operation of the fans the dry cooler), while the maximum obtained thermal COP was approximately 0.53 for a driving temperature of 80 °C, with the corresponding average EER of 7.

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(a) (a)

(b) (b) Figure 8. ZEOSOL system behavior on hot summer days (a) and on cloudy days (b). FigureFigure 8. 8.ZEOSOL ZEOSOL system system behavior behavior on on hot hot summer summer days days (a ()a and) and on on cloudy cloudy days days (b ().b).

(a) (b) (a) (b) FigureFigure 9. Temperature 9. Temperature of secondary of secondary chiller’s chiller’s streams streams on ( ona) hot (a) hotsummer summer days days and and (b) (cloudyb) cloudy days. days. Figure 9. Temperature of secondary chiller’s streams on (a) hot summer days and (b) cloudy days.

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(a)( a) (b)( b)

Figure 10. Performance results of proposed system prototype: the produced cooling output, the FigureFigurerespective 10. 10. Performance electricalPerformance consumption, results results of of proposed and proposed the corresponding system system prototype: prototype:COP and the theEER produced producedvalues ( coolinga ) cooling on hot output, summer output, the days the respectiverespectiveand (b) onelectrical electrical cloudy consumption days. consumption, and, and the the corresponding corresponding COP COP and and EER EER values values (a )( aon) on hot hot summer summer daysdays and and (b )( bon) on cloudy cloudy days. days. 5. Results of Combined Operation 5. Results of Combined Operation 5. ResultsIn theof Combined following figures, Operation a combined operation scheme was investigated. In general, the weight of theIn experiments Inthe the following following was figures, highlyfigures, a shifted combineda combined toward operation operation the adsorption-only scheme scheme was was operation,investigated. investigated. as itIn was Ingeneral, general, the main the the moduleweight weight to of ofbe the the combined experimen experimen withts ts was the wassolar highly highly field, shifted shiftedwhile toward the toward operation the the adsorption of adsorption the backup-only-only heat operation, operation, pump was as asalready it itwas was investigatedthe the main main modulemoduleand lacked to tobe be combined significant combined with scientific with the the solar interest.solar field, field, Thewhile while interest the the operation ofoperation this experiment of ofthe the backup backup was, heat therefore, heat pump pump was shifted was already already toward investigatedinvestigatedusing the backupand and lacked lacked heat significant pump significant to cool scientific scientific down interest. the interest. water The priorThe interest interest to the of startup ofthis this experiment ofexperiment the adsorption was was, therefore, therefore chiller,(in , shiftedshiftedthe morning toward toward when using using the the the HT backup backupsetpoint heat heat was pump not pump yet to reached)to cool cool down down in order the the water to water allow prior prior for more to to the the optimal startup startup conditions of of the the adsorptionadsorptionfor the operation chiller chiller (in of( inthe the the morning adsorption morning when when chiller the the and,HT HT setpoint thus, setpoint higher was was enotffi notciencies. yet yet reached) reached) in inorder order to toallow allow for for moremore optimal Asoptimal shown conditions conditions in Figure for for 11the a,the oper the operation temperatureation of ofthe the adsorption profilesadsorption in chiller the chiller storage and and, thus, tank thus, higher and, higher the efficiencies. solarefficiencies. field were muchAsAs shown more shown stable,in inFigure Figure following 11a 11a, the, thethe temperature lesstemperature sharp increasesprofiles profiles in in inthe the the storage heat storage demands tank tank and byand the the the solar adsorption solar field field werechiller. were muchmuchThe more spikes more stable, instable, the following MT following and LT, the the shown less less sharp insharp Figure increases increases 11b, arein inthe due the heat toheat thedemands demands transition by by the from the adsorption theadsorption adsorption chiller chiller part. . TheTheto spikes the spikes on in/o inffthe operationthe MT MT and and LT, of LT, the shown shown backup in inFigure heat Figure pump. 11b 11b, are, Thisare due due behavior, to tothe the transition transition however, from from would the the adsorption not adsorption affect the part part end to touser,the the on/off sinceon/off operation oscillations operation of inofthe thethe backup LTbackup circuit heat heat were pump. pump. small This This and behavior, behavior, around however, the however, LT setpoint would would of not 15 not affect◦C. affect With the the respectend end useruserto, thesince, since system’s oscillations oscillations performance in inthe the LT LT (Figure circuit circuit were12 were), the small small combined and and around around operation the the LT enhancedLT setpoint setpoint theof of15 overall 15 °C. °C. With With performance, respect respect to totheachieving the system’s system’s close performance performance to nominal (Figure cooling (Figure 12 output,12), the), the combined withcombined a maximum operation operation EER enhanced enhanced of 11.5 the in the the overall overall combined performance, performance, operation, achievingachievingcorresponding close close to totonominal annominal adsorption cooling cooling chilleroutput, output, COP with with of a approximately maximuma maximum EER EER 0.525. of of11.5 11.5 in inthe the combined combined operation, operation, correspondincorresponding tog toan an adsorption adsorption chiller chiller COP COP of ofapproximately approximately 0.525. 0.525.

(b) (a)( a) (b)

FigureFigureFigure 11. 11. 11.Overview OverviewOverview of of( ofa )( (athea)) the the solar solar solar field field ﬁeld temperatures temperatures temperatures and and and solar solar solar irradiance irradiance irradiance and and and (b )( (bbthe)) the the adsorption adsorption adsorption chillerchillerchiller for for forcombined combined combined operation operation operation.. .

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Figure 12. Overview of the cooling/electrical loads and the system’s performance for combined operation. Figure 12. Overview of the cooling/electrical loads and the system’s performance for combined 6. Economicoperation. Evaluation of Solar Adsorption Cooling Unit Using the performance results of the previous section, an economic analysis was conducted to 6. Economic Evaluation of Solar Adsorption Cooling Unit evaluate the economic viability of the ZEOSOL system for a number of countries in comparison to otherUsing commercially the performance available results technologies of the forprevious cooling section, and heating. an economic For this purpose,analysis was a 500-m conducted2 reference to evaluatemulti-family the economic building viability was considered. of the ZEOSOL As the system reference for heatinga number system of countries of thebuilding, in comparison a 30-kW to othercondensing commercially gas boiler available was considered, technologies while, for for cooling the cooling and loads, heating. single For split this air-conditioning purpose, a 500 units-m² referencewere considered multi-family with abuilding total cooling was considered. capacity of As 24 kW.the reference Apart from heating the ZEOSOL system of system, the building the other, a 30evaluated-kW condensing technologies gas were boiler as follows:was considered, while, for the cooling loads, single split air- conditioning units were considered with a total cooling capacity of 24 kW. Apart from the ZEOSOL A boiler for the heating loads of the building and a solar cooling system for the cooling loads. system,• the other evaluated technologies were as follows: A reversible heat pump powered by the grid. • • AA boiler reversible for the heat heating pump loads powered of the by building a photovoltaic and a solar panel cooling field of system 10 kWp for capacity. the cooling loads. •• A reversible heat pump powered by the grid. • AThe reversible investigated heat pump locations powered were by Berlin a photovoltaic (Germany), panel Paris field (France), of 10 kWp Vienna capacity (Austria),. Rome (Italy), Athens (Greece), Larnaca (Cyprus), Madrid (Spain), Lisbon (Portugal), and Istanbul (Turkey). TheThe estimation investigated of the locations annual heatingwere Berlin/cooling (Germany), loads was Paris conducted (France), Vienna using data (Austria), from theRome literature. (Italy), AthensMore specifically, (Greece), Larnaca the building (Cyprus), envelope’s Madrid characteristics, (Spain), Lisbon as (Portugal) well as the, and heating Istanbul system (Turkey). per country, The estimationwere derived of fromthe annual the TABULA heating/cooling webtool [ 36loads], considering was conducted in all cases using the data most from modern the literature. typology. GivenMore specifically,that the aforementioned the building database envelope does’s characteristics not provide data, as well for the as coolingthe heating system, system in all per cases, country single-split, were derivedair conditioning from the units TABULA of seasonal webtool energy [36], e fficonsideringciency ratio in (SEER) all cases equal the to most 6.95 modern were considered. typology. In Given order thatto allow the aforeme the comparisonntioned ondatabase an equal does basis, not in provide all cases, data the for total the building cooling surface system, was in all considered cases, single equal- splitto 500 air m conditioning2, as mentioned units before. of seasonal The energy energy simulations efficiency ratio for all (SEER) buildings equal were to 6.95 eventually were considered conducted. Inusing orderEnergyPlus to allow software the comparison [37], and on the an results equal are basis, summarized in all cases in, Figure the total 13a. building With respect surface to other was 2 consideredassumptions, equal required to 500 for m the, as simulations mentioned in before.EnergyPlus The , energy the air leakage simulations was forset to all eight buildings air changes were eventuallyper hour, whileconducted the natural using EnergyPlus ventilation software was set to[37] year-round, and the res threeults daysare summarized per week. Regardingin Figure 13a the. With respect to other assumptions, required for the simulations in EnergyPlus, the air leakage was set temperature set points, we set 20 ◦C for heating, while the respective value for cooling mode was equal to eight air changes per hour, while the natural ventilation was set to year-round three days per week. to 25 ◦C; the operating schedule was in both modes between 4:00 p.m. and 8:00 a.m. With respect to Regardingthe estimation the temperature of the capital set costs points per, system,we set 20 the °C following for heating, assumptions while the respective were considered: value for cooling mode was equal to 25 °C; the operating schedule was in both modes between 4:00 p.m. and 8:00 a.m. WithThe ZEOSOLrespect to system the estimation has a specific of the capitalcapital costcosts of per 2 ksystem,€/kWc, whilethe following the installation assumptions costs were were • considered:considered equal to 1 k€. For the reference system, the cost of the boiler was considered equal to 2.8 k€, and the total cost •• The ZEOSOL system has a specific capital cost of 2 k€/kWc, while the installation costs were for the single split air-conditioning units was considered equal to 7.3 k€, based on commercial considered equal to 1 k€. prices from the Greek heating, ventilation, and air-conditioning (HVAC) market. • For the reference system, the cost of the boiler was considered equal to 2.8 k€, and the total cost For the solar cooling system, a specific cost of 3 k€/kWc was considered with installation costs of • for the single split air-conditioning units was considered equal to 7.3 k€, based on commercial 2 k€. prices from the Greek heating, ventilation, and air-conditioning (HVAC) market.

Energies 2020, 13, x FOR PEER REVIEW 14 of 18 Energies 2020, 13, 1630 14 of 18 • For the solar cooling system, a specific cost of 3 k€/kWc was considered with installation costs ofThe 2 k€. reversible heat pump cost was considered to be equal to 11.2 k€, and the speciﬁc costs of the • • ThePVs reversible were considered heat pump equal cost to was 1.1 k considered€/kWp [38]. to be equal to 11.2 k€, and the specific costs of the PVs were considered equal to 1.1 k€/kWp [38]. For simpliﬁcation, all scenarios considered a maintenance cost of 400 €/year, apart from the •• For simplification, all scenarios considered a maintenance cost of 400 €/year, apart from the reference case which was estimated to have 200 €/year. reference case which was estimated to have 200 €/year. Based on the above, the capital costs per system were estimated as listed in Figure 11b. As shown, Based on the above, the capital costs per system were estimated as listed in Figure 11b. As the costs of all systems were comparable, apart from the case of the boiler with solar cooling, which shown, the costs of all systems were comparable, apart from the case of the boiler with solar cooling, had a capital cost exceeding 35 k€. which had a capital cost exceeding 35 k€. 90 40

80 ) 35 70 € 60 30 50 25 40 20 30 15 20 10 Load (MWh/y) Load 10 Capital cost(k Capital 5

Zeosol Boiler Boiler Heat PV System with with Pump with AC solar heat cooling pump Cooling Heating

(a) (b)

FigureFigure 13. 13. (a)( aCooling/) Coolingheating/heating loads loads for for the the considered considered building building in the in theinvestigated investigated regions regions and and(b) capital(b) capital costs costs for the for considered the considered systems systems.. The economic performance for the different scenarios was estimated on the basis of considering, The economic performance for the different scenarios was estimated on the basis of considering, as a profit from each alternative system, the avoided annual costs of the reference system (maintenance as a profit from each alternative system, the avoided annual costs of the reference system and operation) reduced by the respective costs of the selected per case system. The index that was (maintenance and operation) reduced by the respective costs of the selected per case system. The used for this analysis was the payback period (PbP). index that was used for this analysis was the payback period (PbP). (퐶푎푝푖푡푎푙(Capital cost푐표푠푡) +) +(installation(푖푛푠푡푎푙푙푎푡푖표푛) ) 푃푏푃 PbPsystem=,i = .. (6) (6) 푠푦푠푡푒푚,푖 (푎푛푛푢푎푙(annual 푐표푠푡 cost 표푓 o f 푟푒푓푒푟푒푛푐푒 re f erence) ) (−annual(푎푛푛푢푎푙 cost o푐표푠푡 f system 표푓 푠푦푠푡푒푚, i) , 푖) − ForFor the the estimation estimation of of the the operational operational costs, costs, the the recent recent prices prices for for electricity electricity were were considered considered based based onon data data from from Eurostat Eurostat for for the the first first semester semester of of 2019 2019 [39] [39].. The The results results of of the the analysis analysis are are presented presented in in FigFigureure 14 14. .As As shown, shown, the the ZEOSOL ZEOSOL system’s system’s performance performance was was more more favorable favorable than than the the scenarios scenarios of of a a boilerboiler with with solar solar cooling cooling and and a a grid grid-connected-connected heat heat pump. pump. In In terms terms of of the the comparison comparison with with a aPV PV-driven-driven heatheat pump, pump, the the ZE ZEOSOLOSOL system system was was favorable favorable in in countries countries with with dominant dominant heating heating loads loads and and lower lower solarsolar irradiance irradiance during during the the heating heating period. period. This This is is because because the the custom custom-made-made backup backup heat heat pump pump of of the the ZEOSOLZEOSOL system system has has a a higher higher efficiency efficiency than than commercial commercial reversible reversible heat heat pumps, pumps, an an effect effect which which is is furtherfurther enhanced enhanced by by the the boost boost of of the the sorption sorption chiller chiller when when there there is is solar solar energy energy available. available. In In fact, fact, the the lowestlowest payback payback periods periods were were reported reported for for Paris Paris and and Vienna, Vienna, with with 7.9 7.9 and and 7.2 7.2 years, years, respectively. respectively. TThehe economic economic performance performance of of ZEOSOL ZEOSOL w wasas also also considered considered quire quire competitive competitive in in regions regions with with higherhigher cooling cooling demands, demands, with with payback payback periods periods of of 11.3 11.3 years years for for Rome Rome and and 11.9 11.9 years years for for Madrid, Madrid, respectively.respectively. However, However, the the PV PV/heat/heat pump pump system system ha hadd a abetter better performance performance in in such such regions regions overall. overall. ThisThis is is mainly mainly justified justified by by the the high high COPs COPs of of the the heat heat pump pump at at the the period period of of maximum maximum production production of of thethe PVs, PVs, which which results results in in almost almost zero zero grid grid-dependent-dependent operation. operation. On On the the contrary, contrary, the the ZEOSOL ZEOSOL system system ususeses its backup backup heat heat pump pump to cover to cover peak peak loads, loads, thus resulting thus resulting in electrical in electrical consumption consumption and eventually and eventuallysmaller profits. smaller At profits. this point, At this it has point, to be it mentionedhas to be mentioned that the analysis that the did analysis not evaluate did not the evaluate use of the any usesystem of any for system domestic for hotdomestic water hot (DHW), water which (DHW could), which significantly could significant vary thely outcomes. vary the outcomes.

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50 45 40 35 30 25 20 PbP(years) 15 10 5 0 Berlin Paris Vienna Rome Athens Larnaca Madrid Lisbon Istanbul (DE) (FRA) (AUT) (IT) (GR) (CY) (ES) (POR) (TUR) Zeosol System Boiler with solar cooling Heat Pump PV with heat pump

Figure 14. Economic performance of considered technologies for the investigated countries. Figure 14. Economic performance of considered technologies for the investigated countries. 7. Conclusions 7. ConclusionsIn this study, the preliminary performance results of a solar-driven hybrid adsorption chiller coupled with a backup heat pump were presented. The experimental analysis of the solar-driven In this study, the preliminary performance results of a solar-driven hybrid adsorption chiller adsorption chiller revealed that the system, despite non-optimal conditions (smaller solar irradiance, coupled with a backup heat pump were presented. The experimental analysis of the solar-driven high ambient temperatures), operated at a satisfactory level, with a maximum COP of 0.535. adsorption chiller revealed that the system, despite non-optimal conditions (smaller solar irradiance, The developed system was proven to significantly decrease the electrical power consumption, achieving high ambient temperatures), operated at a satisfactory level, with a maximum COP of 0.535. The a maximum EER of 5.83 (with more than 60% of the total consumption coming from the system’s dry developed system was proven to significantly decrease the electrical power consumption, achieving cooler). These results are considered optimistic for the upcoming phases of the experimental evaluation a maximum EER of 5.83 (with more than 60% of the total consumption coming from the system’s dry of the system, not only for cooling mode operation but also for heating mode operation. However, as cooler). These results are considered optimistic for the upcoming phases of the experimental the experimental testing is in a preliminary phase, there cannot be objective conclusions toward the evaluation of the system, not only for cooling mode operation but also for heating mode operation. system’s performance, especially prior to the evaluation of the combined adsorption chiller/backup heat However, as the experimental testing is in a preliminary phase, there cannot be objective conclusions pump operation, which allows for even higher EER values under an optimized operational strategy. toward the system’s performance, especially prior to the evaluation of the combined adsorption Using the combined operation, the system is expected to fully cover the loads of a 12.5-kW-peak chiller/backup heat pump operation, which allows for even higher EER values under an optimized building, with an optimum solar fraction of around 60%, depending on the climatic conditions of the operational strategy. Using the combined operation, the system is expected to fully cover the loads site of installation. An alternative to further enhance the solar fraction of the system would be the of a 12.5-kW-peak building, with an optimum solar fraction of around 60%, depending on the climatic addition of photovoltaic (PV) panels; however, this option was not evaluated within the framework of conditions of the site of installation. An alternative to further enhance the solar fraction of the system this project as it would increase the capital costs and add further complexity to the system. Regarding would be the addition of photovoltaic (PV) panels; however, this option was not evaluated within the economic aspects of this technology, the investigated system has a better economic performance in the framework of this project as it would increase the capital costs and add further complexity to the regions with dominant heating loads, with payback periods as low as 7.2 years for Vienna, Austria. system. Regarding the economic aspects of this technology, the investigated system has a better On the other hand, for the countries of the Mediterranean region, the system is outperformed by economic performance in regions with dominant heating loads, with payback periods as low as 7.2 PV/heat pump systems due to the high maturity of the latter. years for Vienna, Austria. On the other hand, for the countries of the Mediterranean region, the systemAuthor Contributions:is outperformedT.C.R. by andPV/ G.K.heat developedpump systems the model due that to the was high used andmaturity contributed of the in latter. writing the original draft. T.C.R. conducted the experimental validation of the results and also contributed in the writing of the Author Contributions: T.R. and G.K. developed the model that was used and contributed in writing the original review and editing of the manuscript. S.V. and A.S. conducted the experimental characterization of the system draft.components T.R. conducted at CNR-ITAE. the experimental S.V. supervised validation the preparation of the results of and the also manuscript contributed and in contributed the writing in of the the writing,review andediting editing and reviewing of the manuscript. the text. U.W. S.V and and M.T.A.S. developedconducted thethe back-up experimental unitat characterization Fahrenheit and ofcontributed the system in componentswriting the manuscript. at CNR-ITAE. N.H. S.V and supervised U.S. conducted the preparation the experimental of the activitymanuscript on the and solar contributed collectors in at the Akotec. writing, S.K. editingsupervised and thereviewing preparation the text. of theU.W manuscript and MT developed and was thethe projectback-up coordinator. unit at Fahrenheit All authors and contributed have read and in writing agreed to the published version of the manuscript. the manuscript. N.H and U.S. conducted the experimental activity on the solar collectors at Akotec. S.K. supervisedFunding: The the analysispreparation was conductedof the manuscript as a part and of thewas ZEOSOL the project project coordinator. fundedby the European Union’s Horizon 2020 research and innovation program under grant agreement No. 760210. This scientific paper was supported by Funding:the Onassis The Foundation, analysis was Scholarship conducted ID: as a G part ZO of 025-1 the/ 2018-2019.ZEOSOL project funded by the European Union’s Horizon 2020 research and innovation program under grant agreement No. 760210. This scientific paper was supported Conflicts of Interest: The authors declare no conflicts of interest. by the Onassis Foundation, Scholarship ID: G ZO 025-1/2018-2019.

Conflicts of Interest: The authors declare no conflicts of interest.

Energies 2020, 13, 1630 16 of 18

Nomenclature

A Aperture area, m2 c Heat loss coefficient W/(m2 K) 1 · c Heat loss coefficient W/(m2 K2) 2 · G Solar radiation, W/m2 η Efficiency η0 Collector efficiency, adm PbP Payback period, years . Q Heating/cooling capacity, kW T Temperature, ◦C . W Power, kW

Subscripts a Ambient col Collector com Compression el Electrical dc Dry cooler pumps Pumps

Abbreviations

COP Coeﬃcient of performance EER Energy eﬃciency ratio HT High temperature LT Low temperature MT Medium temperature

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