CHEMICAL ENGINEERING TRANSACTIONS VOL. 70, 2018 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Timothy G. Walmsley, Petar S. Varbanov, Rongxin Su, Jiří J. Klemeš Copyright © 2018, AIDIC Servizi S.r.l. ISBN 978-88-95608-67-9; ISSN 2283-9216 Solar-Assisted Dual-Source Multifunctional Heat Pump: Field Tests Results and Thermodynamic Analysis Giorgio Besagnia,*, Lorenzo Crocia, Riccardo Nesab aRicerca sul Sistema Energetico - RSE S.p.A., Power System Development Department, via Rubattino 54, 20134 Milano, Italy bPolitecnico di Milano, Department of Energy, via Lambruschini 4a, 20156, Milano, Italy giorgio.besagni@rse-web.it The integration between heat pumps and renewable energy sources (i.e., solar energy) is a promising and recognised technology to reduce primary energy consumption for building cooling/heating. This paper contributes to the existing discussion concerning field performances of multifunctional solar-assisted heat pumps, for cooling/heating applications, and for the production of domestic hot water. The proposed system has been tested experimentally, in a detached house in Milan, to obtain the seasonal performances; in particular, based on experimental observations, energy and exergy analyses have been formulated and applied to the heat pump control volume and to the whole system control volume. The results of the energy analysis have demonstrated that the system was able to maintain high efficiency in the different seasons. The exergy efficiencies of the heat pump and of the whole system were similar in their values, thus suggesting that the heat pump control volume is the location where most of the exergy losses occur, thus pointing out where the future research activities should focus to improve the performance of the proposed system. 1. Introduction European policies aim to reduce primary energy consumption and to increase the use of renewable energy sources, to achieve a sustainable low-carbon economy in the short-term, medium-term and long-term future. In this respect, particular attention is devoted to the cooling/heating sections in buildings, which are characterized by high primary energy consumption; its reduction is very important to achieve the European targets and might be achieved by acting on the demand side (e.g. building envelope) and/or on the supply side (e.g., energy conversion technologies), as discussed by Hansen et al. (2016) and by Connolly (2017). By considering the supplies side, the integration between heat pumps and renewable energy sources is a promising technology, also when considering the broader framework of the “total heat recovery” as shown in study conducted by Liew and Walmsley (2016) and the district heating shown in the heat roadmap Europe studies (EC, 2011). An interesting coupling between renewable energy and heat pumps is represented by the solar-assisted heat pumps (SAHP); this technology was proposed in the early study of Sporn and Ambrose (1955) and, since then, it has been widely studied (Scarpa et al., 2015). Among the different technologies proposed in the literature, a growing attention is devoted to multi-functional heat pumps, which are able to produce both space heating/cooling and domestic hot water (DHW), In this complex framework, this paper contributes to the existing discussion concerning multifunctional SAHP for heating and cooling applications, as well as for the production of DHW by field study and an exergy post-processing of the data. To this end, the present study investigates, experimentally and theoretically (by energy and exergy analysis), a multifunctional SAHP system, which has been first proposed by Croci et al. (2017) and, subsequently, installed and monitored in Milan (Besagni et al., 2017). It is worth noting that, despite, exergy analysis has been applied to different heat pumps by a number of researchers (Ozgener and Hepbasli, 2007), unfortunately, exergy analysis of multifunctional SAHP are still limited. The approach proposed in this paper is interesting as it couples a seasonal field test with a thermodynamic evaluation of the system: the seasonal field study is needed to verify on field the performance of the system, rather than based on theoretical approaches. Indeed, there is still a large uncertainty concerning the energy consumptions and performance of large-scale applications, as field measurements of real DOI: 10.3303/CET1870043 Please cite this article as: Besagni G., Croci L., Nesa R., 2018, Solar-assisted dual-source multifunctional heat pump: field tests results and thermodynamic analysis , Chemical Engineering Transactions, 70, 253-258 DOI:10.3303/CET1870043 253 installations often have shown lower efficiencies compared with laboratory-scale experimentations (Caird et al., 2012). The aim of this paper is, thus, twofold: (a) assess and define the system performance relating input and output parameters, (b) provide a baseline approach to compare the proposed system performance to the other system layouts proposed by other research groups. 2. Experimental setup and methods 2.1 Experimental setup The multifunctional heat pump was installed in a detached house located in Milan, at RSE Spa headquarter (Latitude 45.47°, Longitude 9.25°). The detached house consists of a prefabricated house with a heating/cooling area equal to 60 m2, divided in four rooms (Madonna and Bazzocchi, 2013). The house is equipped with five fan coils used for space heating/cooling and has a nominal heating load equal to 4 kWth (indoor temperature equal to 20 °C and outdoor temperature equal to -5 °C). Figure 1 displays the layout of the multifunctional heat pump system; the proposed system couples hybrid photovoltaic/thermal (PVT) panels with a multifunctional and reversible heat pump. The heat pump is equipped is classified as a dual-source, as it employs both “air-source” evaporator and a “water-source” evaporator, connected in series and operated alternatively, based on the ambient conditions, system parameters and operating modes (the reader may refer to the literature survey proposed by Croci et al., (2017) and to the influence of the source describe by Besagni et al. (2017)). The PVT panels are connected with the heat pump by two storage tanks, used either to produce DHW or in “water-source” evaporator. Further details concerning the different components of the system have been discussed in our previous papers (Besagni et al., 2017). Figure 1: Solar-assisted dual-source multifunctional heat pump: the experimental layout 2.2 Experimental methods The flow rate in each circuit has been measured by an electromagnetic flowmeter meter (E&H Promag P50, ±0.2% read value). All the inlet and outlet temperatures of the main equipment, the supply and return water temperatures of the different locations, were measured by RTD Pt100 4wire 1/5DIN, inserted inside the pipes. The indoor and outdoor (near the PVT panels) temperature and humidity were measured by an Pt100 4wire hygrometer (Siap+Micros). The solar radiation intensity has been measured by a thermopile pyranometers (pyranometer Kipp&Zonen CMP11), mounted at a 45° inclined angle near the PVT panels. The power consumption of the heat pump and the circulating pumps (solar pumps, intermediate-storage tank, fan-coil pump) were measured by multifunction electric meters (Shark 100, ±0.1 %, and FRER MonoNano, ±0.5 %). Evaporating and condenser pressures were measured by pressure transducers (Keller series 21Y). All data were recorded automatically at every 6 seconds. 2.3 Operation modes The multifunctional heat pump has been operated in four different sessions. The first operation mode (mode#1) concerns heating without DHW production (from 17/01/2017 to 13/03/2017). The internal set-point temperature was set to 22 °C, with night attenuation of 4 °C. The temperature of the water produced by the heat pump and sent to the fan-coils was set by using a climatic curve (e.g., T3 = 45 °C with Tamb = 0 °C and T3 = 40 °C with Tamb 254 = 15 °C – Refer to Figure 1 for details on the subscripts). In this operation mode, valve V1 was set to deviate all thermal energy produced by the PVT panels to the “intermediate-temperature” storage tank. The “intermediate- temperature” storage tank, is used as the “water-source” for the heat pump when Tamb, is low and the resulting COP would decrease, as discussed by Besagni et al. (2017). Changes from “air-source” evaporator to “water- source” evaporator are obtained by switching on/off the pump P2 and switching on/off the outdoor “air-source” unit. The second operation mode (mode#2) concerns heating with DHW production (from 13/03/2017 to 24/05/2017). In this case, the previous operation mode was modified as follows: (a) a daily profile of DHW production was set to produce 150 l (corresponding to, approximately, 4 KWh); (b) valve V1 was set to deviate the glycol-water mixture, at the outlet of the PVT panels, depending on the temperature of the storage tanks (a) towards the “intermediate-temperature” storage tank (Tintermediate-temperature < 38 °C), (b) towards the DHW tank (Tintermediate-temperature ≥ 38 °C) or towards the “intermediate-temperature” storage tank (TDHW,tank ≥ 58 °C); (c) the DHW storage tank set-point temperature was set to TDHW,set-point = 48°C and its lower temperature has been set TDHW,mantenance = 42°C. If the temperature of the DHW storage tank fell below TDHW,mantenance, the heat pump would be used to increase the DHW tank temperature. The thirds operation mode (mode#3) concerns cooling with DHW production (from 24/05/2017 to 16/10/2017). The internal set-point temperature was set to 24 °C. The outlet water temperature was set at 7°C In addition, the operation mode mode#2 was modified as follows: (a) a daily profile of DHW production was set to produce 150 l; (b) valve V1 was set to deviate the glycol-water mixture, at the outlet of the PVT panels, depending on the temperature of the storage tanks towards the DHW storage tank (Tintermediate-temperature ≥ 36 °C) or towards the “intermediate-temperature” storage tank (TDHW,tank ≥ 57 °C). Pump P1 was activated from 19:00 until 24:00, to lower the temperature of the “intermediate-temperature” storage tank and, thus, to increase the PVT performances. The fourth operation mode (mode#4) concerns heating without DHW production (from 22/10/2017 to 14/12/2017) and was operated with the same settings as the mode#1. Please note that, owing to technical maintenance in the area of the facility, we were forced to stop the monitoring activities in the period between the 16/10/2017 and the 22/10/2017. 2.4 Energy and exergy analysis The performance of the multifunctional heat pump has been evaluated, on a daily-averaged point of view, based on the experimental data acquired in the experimental tests. In particular, both the heat pump performance (by energy and exergy approaches) and the whole system performances (by an exergy approach) have been computed. The energy performance of the heat pump has been evaluated by the COP (during heating mode, Eq. (1)) and the EER (during the cooling mode, Eq. (2)), as follows: COP = EHP→fan-coil+EHP→DHW-tank Eel | daily (1) EER = EHP←fan-coil+EHP→DHW-tank Eel | daily (2) In Eqs. (1-2) Eel is the electrical energy provided to the system, EHP→fan-coil and EHP← fan-coil refer to the thermal energy provided by the heat pump to the fan-coils (heating mode) and vice-versa (cooling mode); EHP→DHW-tank refers to the thermal energy provided by the heat pump to the DHW storage tank. The exergy performance of the heat pump (Eq. (3)) and the exergy performance (Eq. (4)) of whole system have been computed as follows: η EX,HP = EXHP-fan-coil+EXHP→DHW-tank EXel | daily = EXHP→fan-coil(1+ T0 Tin⁄ )+EXHP→DHW-tank(1+ T0 TDHW-tank⁄ ) Eel | daily (3) η EX,System = EXHP→fan-coil+EXDHW EXel + EXPVT | daily = EHP→fan-coil(1+ T0 Tin⁄ )+EDHW (1+ T0 TDHW⁄ ) Pel + EPVT(1+ T0 TPVT⁄ ) | daily (4) In Eqs. (3-4), EXel is the exergy related to the electrical energy, EXHP→fan-coil and EXHP← fan-coil refer to the exergy of the thermal energy provided by the heat pump to the fan-coils or vice-versa; EXDHW refers to the exergy of the thermal energy provided by the heat pump to the DHW tank; EXPVT refers to the exergy of the thermal energy produced by the PVT panels. In addition, T0 = 273 K is the reference temperature for the exergy analysis, Tin is the internal temperature of the detached house, TPVT is the PVT operating temperature, TDHW-tank is the DHW storage-tank temperature and TDHW is the temperature of the DHW produced. The daily-averaged performances have been considered and, in this respect, the different temperatures in Eqs. (3-4) have been computed as daily-averaged values. In the above equations, Eel considers all the auxiliaries: (a) solar circulating pump (approximately 4.1 % of the total energy consumption); (b) “intermediate-temperature” circulating pump (approximately 1.2 % of the total energy consumption); (c) Fan-coil circulating pump (approximately 3.7 % of 255 the total energy consumption), (d) DHW circulating pump and all other auxiliaries (approximately 14.2 % of the total energy consumption). Based on the experimental uncertainties the energy and exergy performances are computed within the range of ± 0.2 % of their actual value. 3. Results The results of the analysis are presented in Figure 2, Figure 3, Table 1 and Table 2. Figure 2 displays the daily- averaged energy performances of the multifunctional heat pump; Figure 3 displays the daily-averaged exergy performances of both the multifunctional heat pump and the whole system; in addition, Figure 2 and Figure 3 display the ambient temperature Tamb. Finally, Table 1 and Table 2 summarize the monthly-averaged (Table 1) and the operation mode-averaged (Table 2) energy and exergy performances. Figure 2: Daily-averaged energy performance of the multifunctional heat pump Figure 3: Daily-averaged exergy performance of the multifunctional heat pump 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 C O P - E E R [ -] Day Mode#3Mode#2 Mode#1 Mode#4 -5 0 5 10 15 20 25 30 35 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 A m b ie n t te m p e ra tu re [ °C ] C O P o r E E R [ -] Day COP [-] EER [-] Ambient temperature [°C] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 C O P - E E R [ -] Day Mode#3 Mode#2 Mode#1 -5 0 5 10 15 20 25 30 35 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 A m b ie n t te m p e ra tu re [ °C ] E xe rg y e ff ic ie n cy - S is te m [ -] Day Exergy efficiency - Heat Pump [-] Exergy efficiency - System [-] Ambient temperature [°C] Mode#4 256 Table 1: Monthly-averaged energy and exergy performances (year: 2017) Month Operation mode COP [-] EER [-] ηex,HP [-] ηex,System [-] January mode#1 3.08 - 0.21 0.21 February mode#1 3.51 - 0.24 0.23 March mode#1 3.75 - 0.25 0.21 March mode#2 2.74 - 0.27 0.19 April mode#2 2.47 - 0.24 0.15 May mode#2 2.55 - 0.20 0.14 May mode#3 - 2.99 0.32 0.29 June mode#3 - 3.45 0.41 0.38 July mode#3 - 3.12 0.38 0.36 August mode#3 - 3.02 0.37 0.35 September mode#3 - 3.06 0.33 0.30 October mode#4 3.53 - 0.21 0.17 November mode#4 2.78 - 0.19 0.18 December mode#4 3.05 - 0.23 0.23 Table 2: Operation mode-averaged performances Operation mode COP [-] EER [-] ηex,HP [-] ηex,System [-] mode#1 3.34 - 0.23 0.22 mode#2 2.58 - 0.24 0.16 mode#3 - 3.14 0.38 0.35 mode#4 2.96 - 0.22 0.21 The averaged energy performances of the multifunctional heat pump are, approximately, 3.34 in the operation mode#1, 2.58 in operation mode# 2, 3.14 in operation mode#3 and 2.96 in operation mode#4 (Figure 2, Table 1 and Table 2). Of course, the performances of the heat pump are related to the ambient temperature Tamb, owing to the variable speed compressor and the electronic expansion valve discussed (see, for example, the daily profiles presented and discussed by Besagni et al. (2017)): a variation in the boundary conditions (i.e., internal/external temperature) reflects in a variation of the operating conditions of the compressor (viz. a variation of the suction/discharge pressure and temperatures) and the electronic valve opening (to maintain the super- heat set-point value, 5° C), On the contrary, the exergy performance of the multifunctional heat pump are, approximately, 0.23 in the operation mode#1, 0.24 in the operation mode#2, 0.38 in the operation mode#3 and 0.22 in the operation mode#4. The decrease in the energy and exergy efficiencies passing from the operation mode#1 to the operation mode#2 is related to the production of heat at high temperature, to maintain the DHW storage temperature set-point (see Section 2.3). It is interesting to observe that the exergy efficiencies of the heat pump and of the whole system (Figure 3, Table 1 and Table 2) are similar in values, thus suggesting that the heat pump is the component where most of the exergy losses occurs. In agreement with our observation, Kata et al. (2008) found that the exergy efficiency of the individual components of the solar assisted heat pump (without DHW production) ranged from 10.74% to 88.87% and the higher exergy losses are located in the heat pump systems (e.g., compressor and heat exchangers, …). For this reason, the future research activities should be devoted to developing, test and validate different heat pump system layouts (i.e., an ejector-based heat pump (Besagni et al. 2016) or other integrations with the solar system). In addition, it is interesting to observe the increase in the exergy efficiencies in the cooling mode, compared with the heating modes, suggesting that the proposed system is able to produce cooling effect and DHW is a more efficient and “reversible” way, compared with the heating mode. This behavior is likely to be related to the thermal energy provided by the PVT panels, which support the production of DHW and, thus, decreases the high-temperature operation of the heat pump. This observation suggests that future works should be devoted to improving the efficiency of the system in the heating mode. Compared with the previous literature, the proposed system has shown a better performance in terms of both energy and exergy performances. On the energy point of view, Li et al. (2007) experimentally studied a solar-assisted heat pump for DHW production and found COP in the range of 1.7 – 2.9; On the exergy point of view, Ito et al. (1999) studied an heat pump system only for cooling application and observed exergy efficiency in the range of 0.1 to 0.3 (Ito et al, 1999); instead, Li et al. (2007) studied direct expansion solar- assisted heat pump water heater, which does not accounted for space cooling and heating purposes, and observed exergy efficiency in the range of 0.2 to 0.5 (Li et al., 2007). Of course, the above-mentioned comparison is qualitative, as different authors studied different systems (in laboratory facility rather than in real 257 field tests). Nevertheless, despite the comparison is qualitative only, the proposed heat pump system seems a promising technology, compared with the literature, reaching higher exergy efficiencies. 4. Conclusions This paper investigates, experimentally and theoretically (by energy and exergy analysis), a multifunctional SAHP system. The results have shown that the system has been able to maintain high energy and exergy efficiencies in the different seasons and is a promising technology for primary energy saving in domestic applications study. In particular, the averaged energy performance of the multifunctional heat pump is approximately 3, in the three different operation modes; conversely, the averaged exergy performance is approximately of 0.23 in the heating mode and 0.38 in the cooling mode. These performances have been compared with the previous literature, suggesting that the proposed system is promising technology in the framework of energy saving. As far as future studies are concerns, the exergy analysis pointed out that the future research activities should be devoted to developing, test and validate different heat pump system layouts, especially to improve the heating performances. Acknowledgments This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between RSE S.p.A. and the Ministry of Economic Development - General Directorate for Nuclear Energy, Renewable Energy and Energy Efficiency stipulated on July 29, 2009 in compliance with the Decree of March 19, 2009. The authors are also grateful to CLIVET Spa for elaborating and providing the heat pump employed in the research activities. References Besagni G., Mereu R., Inzoli F., 2016, Ejector refrigeration: a comprehensive review, Renewable and Sustainable Energy Reviews, 53, 373-407. 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