Installation and Operation of a Solar Cooling and Heating System Incorporated with Air-Source Heat Pumps

Home , Air conditioning, Chilled water, Chiller, Cooling, Cooling tower, Radiator (heating)

Article Installation and Operation of a Solar Cooling and Heating System Incorporated with Air-Source Heat Pumps

Li Huang 1,*, Rongyue Zheng 1 and Udo Piontek 2 1 Faculty of Architecture, Civil Engineering College, Ningbo University, Fenghua Street 818, Ningbo 315211, China; [email protected] 2 Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Osterfelder Strasse 3, 46047 Oberhausen, Germany; [email protected] * Correspondence: [email protected]; Tel.: +86-574-8760-9510

Received: 21 January 2019; Accepted: 11 March 2019; Published: 14 March 2019

Abstract: A solar cooling and heating system incorporated with two air-source heat pumps was installed in Ningbo City, China and has been operating since 2018. It is composed of 40 evacuated tube modules with a total aperture area of 120 m2, a single-stage and LiBr–water-based absorption chiller with a cooling capacity of 35 kW, a cooling tower, a hot water storage tank, a buffer tank, and two air-source heat pumps, each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW as the auxiliary system. This paper presents the operational results and performance evaluation of the system during the summer cooling and winter heatingperiod, as well as on a typical summer day in 2018. It was found that the collector field yield and cooling energy yield increased by more than 40% when the solar cooling and heating system is incorporated with heat pumps. The annual average collector efficiency was 44% for cooling and 42% for heating, and the average coefficient of performance (COP) of the absorption chiller ranged between 0.68 and 0.76. The annual average solar fraction reached 56.6% for cooling and 62.5% for heating respectively. The yearly electricity savings accounted for 41.1% of the total electricity consumption for building cooling and heating.

Keywords: solar cooling; solar heating; heat pump; absorption chiller; evacuated tube collector

1. Introduction In recent years, building energy consumption has been steadily increasing with the development of the economy and the improvement of living standards in China. It occupies 30% of the total energy consumption in China [1], 50–60% of which is consumed by air conditioning in public buildings in hot summer and cold winter zones [2]. Conventional air conditioning systems use electricity to drive a compressor to circulate a refrigerant which absorbs and removes heat from the space. The rising air conditioner usage not only results in a considerable increase in power consumption, but also in a power peak demand during the hottest hours in summer, which has a strong negative impact on the existent power grid and often causes power blackouts in summer. Solar cooling technology has been developed to reduce the increasing electricity consumption for air conditioning and to shift the peak load on hot summer days. This technology uses solar energy to drive a refrigerator to cool the space and reduces the electricity consumption. Many researchers have focused on solar cooling systems and different types have been investigated. As of 2014, about 1200 solar cooling systems have been installed worldwide, most of them in Europe [3]. It is considered that about 71% of the installations are solar absorption cooling systems, and 13% are adsorption systems. Some of the installed systems have been studied in the literature, both in terms of thermodynamic and economic performance. For example, Syed et al. [4] tested a ﬂat-plate collector-driven 35 kW

Energies 2019, 12, 996; doi:10.3390/en12060996 www.mdpi.com/journal/energies Energies 2019, 12, 996 2 of 17 single-effect LiBr–water absorption chiller in Madrid. The maximum instantaneous coefficient of performance (COP) (COP is the ratio of cooling energy yield and heat supplied by the generator), daily average COP and average COP during the period of 20 days of monitoring (25/07/2003 to 19/08/2003) were 0.60, 0.42, and 0.34, respectively. Pongtornkulpanich et al. [5] tested a 10-ton single effect LiBr–water absorption chiller powered by an evacuated tube solar collector in Thailand where the solar collector delivered 81% of the thermal energy for the chiller. Rosiek and Batlles [6] analyzed the behavior of the solar-assisted air-conditioning system installed in the CIESOL building to maximize the use of solar thermal energy supplying the cooling and heating system. The average values of coefficient of performance and the cooling capacity in summer months were 0.6 and 40 kW, respectively. Bermejo et al.[7] built and tested a solar/gas-driven double effect LiBr–water absorption system in Spain. The system could be driven by a direct-fired natural gas burner or by pressurized hot water delivered from a linear concentrating Fresnel collector. The average and maximum daily efficiencies of the solar collector were 0.35 and 0.4, respectively. The daily average COP of the chiller was 1.1–1.25. Lizarte et al. [8] experimentally studied a vacuum flat-plate collector-driven 4.5 kW air-cooled single-effect LiBr–water absorption chiller in Madrid. It was found that the mean COP and solar coefficient of performance (SCOP) (the ratio of cooling energy yield and the total solar energy available) were 0.53 and 0.06, respectively. Balghouthi et al. [9] presented a solar absorption cooling installation in Tunisia where it was found that the coefficient of performance (COP) ranged between 0.8 and 0.91 during the installation running, while the solar coefficient of performance (SCOP) was between 0.1 and 0.43. The drain backup night storage improved the average solar fraction (SF) of the system, which increased from 0.54 to 0.77. Hang et al. [10] built a double-effect solar absorption cooling test facility and investigated the experimentally based energy performance of the system. The results showed that the system could provide adequate cooling for a test facility between 11 a.m. and5 p.m. on both sunny and cloudy days. The daily average collector efficiency was in the range of 36% to 39%. The average coefficient of performance (COP) of the LiBr-water absorption chiller was between 0.91 and 1.02 with an average of 1.0, and the daily solar COP was approximately at 0.374. Marc et al. [11] presented a dynamic modeling of a single-effect absorption chiller working with a LiBr–water solution used in a solar cooling installation operating without any backup systems (hot or cold). The numerical model was based on the mass and energy balances of each component, equations of state, and equations of heat transfers. Experimental validation elements were proposed to validate pressures and temperatures of the chiller. Xu et al. [12] studied a variable effect LiBr–water absorption chiller based upon a real developed 50 kW prototype. The results showed that the COP increased from 0.69 to 1.08 under generation temperature from 95 ◦C to 120 ◦C. The chiller performance will be higher under high chilled water temperature, low cooling water temperature, proper frequency of generation pump, optimized high pressure absorber valve opening, and high frequency of hot water pump. Since the first solar cooling system was installed in Shenzhen in 1987, there have been over 10 demonstration projects put into service in China. The first system in Shenzhen provided cooling in summer to guestrooms with a total area of 80 m2 by applying two single-stage LiBr–water absorption chillers and a hot water boiler as the auxiliary heat source. It was found that the LiBr–water absorption chiller could not always operate at its nominal rating during periods of low solar radiation and high cooling water temperature as the temperature in the storage could not always be maintained at temperatures as high as 88–90 ◦C throughout the day even on clear sunny days [13]. Two of the most famous solar absorption cooling systems of Tianpu and Beiyuan were built in Beijing in 2001 and 2005, respectively. The Tianpu demonstration project consists of a 200 kW solar absorption cooling system assisted by a ground-source heat pump with a rated cooling capacity of 391 kW. It was reported that the average solar collecting efficiency in summer exceeded 40% [14]. So far, the largest solar cooling system in China is the demonstration project of Beiyuan. A single-effect LiBr-water absorption chiller was chosen to supply cooling to the building with an area of 3000 m2. An electric boiler was installed as an auxiliary heat source. When the ambient temperature was about 30.3 ◦C during the experimental Energies 2019, 12, x FOR PEER REVIEW 3 of 18

single-effect LiBr-water absorption chiller was chosen to supply cooling to the building with an area of 3000 m2. An electric boiler was installed as an auxiliary heat source. When the ambient Energiestemperature2019, 12 ,was 996 about 30.3 °C during the experimental period, the average solar collecting3 of 17 efficiency and the COP of the chiller was 0.42 and 0.75, respectively, whereas the average indoor air temperature was 23.8 °C [15]. period, the average solar collecting efﬁciency and the COP of the chiller was 0.42 and 0.75, respectively, Due to the intermittent nature of solar energy, a backup system is required in solar cooling and whereas the average indoor air temperature was 23.8 ◦C[15]. heating systems to meet the building load demand when solar inputs are insufficient. The backup Due to the intermittent nature of solar energy, a backup system is required in solar cooling and system can use either gas or electricity as the auxiliary energy source [16]. Most of the heating systems to meet the building load demand when solar inputs are insufﬁcient. The backup above-mentioned solar cooling and heating systems used a gas boiler as an auxiliary heat source. In system can use either gas or electricity as the auxiliary energy source [16]. Most of the above-mentioned this paper, a new system incorporated with two air-source heat pumps as the auxiliary system was solar cooling and heating systems used a gas boiler as an auxiliary heat source. In this paper, a new installed in Ningbo City, which is located in the hot summer and cold winter zone in China, and it system incorporated with two air-source heat pumps as the auxiliary system was installed in Ningbo has been operating since 2018. This paper presents the installation and performance evaluation of City, which is located in the hot summer and cold winter zone in China, and it has been operating the solar cooling and heating system during the summer cooling and winter heating period, as well since 2018. This paper presents the installation and performance evaluation of the solar cooling and as on a typical summer day, namely, 23 July 2018. heating system during the summer cooling and winter heating period, as well as on a typical summer day, namely, 23 July 2018. 2. Climate Conditions and Building Characteristics 2. Climate Conditions and Building Characteristics 2.1. Climate Conditions 2.1. ClimateFigure Conditions1 presents the monthly average outdoor temperature and global solar irradiation in NingboFigure City1 presents in 2018. theThe monthly average average outdoor outdoor temperat temperatureure during and July, global the solarhottest irradiation month, inis Ningbo27.6 °C, Citywhereas in 2018. the average The average outdoor outdoor temperature temperature during during January, July, thethe hottestcoldest month, month, is is 27.6 6 °C.◦C, Thus, whereas air theconditioning average outdoor systems temperature are necessary during for both January, cooling the in coldest summer month, and heating is 6 ◦C. in Thus, winter. airconditioning The average systemssunshine are hours necessary are 1855.6 for both h per cooling year inand summer the aver andage heating daily insunshine winter. Thehours average are 5.41 sunshine h. The hours total areannual 1855.6 solar h per radiation year and is the1280.5 average kWh/m daily2, sunshinewhich is hours78% of are the 5.41 average h. The value totalannual of 1627.8 solar kWh/m radiation2 in isChina. 1280.5 kWh/m2, which is 78% of the average value of 1627.8 kWh/m2 in China.

30 180 Global s o lar irra d iation Average outdoor tem perature

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123456789101112 Month FigureFigure 1. TheThe monthlymonthly average average outdoor outdoor temperature temperature and globaland global solar irradiation solar irradiation in Ningbo in inNingbo 2018. in 2018. Figure2 shows the hourly average outdoor temperature and global solar irradiation on 23 July ◦ 2018.Figure The strongest 2 shows solar the hourly radiation average occurred outdoor from 9:00temperature to 13:00, whileand global the temperatures solar irradiation exceeded on 23 32 JulyC between2018. The 11:00 strongest and 14:00,solar radiation which also occurred corresponded from 9: to00 theto 13:00, maximal while cooling the temperatures energy demand exceeded during 32 the°C between day. 11:00 and 14:00, which also corresponded to the maximal cooling energy demand during the day. 2.2. Building Characteristics The building, which the solar cooling system serves, is an ofﬁce building of a tourist center as shown in Figure3 and occupied for 10 h from 8:00 to 18:00 every day. The two-story building has a total area of 242.4 m2 and a total height of 6.9 m. The speciﬁc cooling and heating power demands are 2 2 120 W/m and 82 W/ m , respectively, and the total cooling and heating loads are 29 kW and 20 kW, Energies 2019, 12, x FOR PEER REVIEW 4 of 18

35.0 1.05 Global s o lar irra d iation

30.0 Average outdoor temperature 0.90 2 ℃ /

25.0 0.75 kW /m /

Energies 2019, 12, 996 on 4 of 17 20.0 0.60 ati tem perature tem perature

15.0 0.45 irradi respectively. There is no hot water demand in the building. The construction characteristics ar of this outdoor outdoor sol Energiesbuilding 2019, 12 are, x shownFOR PEER10.0 in REVIEW Table1. 0.30 4 of 18 G lobal

35.0 A verage 5.0 1.050.15 Global s o lar irra d iation

30.0 0.0 Average outdoor temperature 0.900.00 2 ℃ /

25.0 0.75 kW /m / on 20.0 0.60 Local time ati tem perature tem perature 15.0 0.45 irradi Figure 2. The hourly average outdoor temperature and global solar irradiation on ar 23 July outdoor outdoor sol 2018. 10.0 0.30 G lobal 2.2. Building Characteristics A verage 5.0 0.15

The building,0.0 which the solar cooling system serves, is an office building of0.00 a tourist center as shown in Figure 3 and occupied for 10 h from 8:00 to 18:00 every day. The two-story building has a total area of 242.4 m2 and a total height of 6.9 m. The specific cooling and heating power demands are 120 W/m2 and 82 W/ m2, respectively, and the total cooling and heating loads are 29 kW and 20 Local time kW, respectively. There is no hot water demand in the building. The construction characteristics of this buildingFigure are 2. The shown hourly in averageTable 1. outdoor temperature and global solar irradiation on 23 July 2018. Figure 2. The hourly average outdoor temperature and global solar irradiation on 23 July 2018.

2.2. Building Characteristics The building, which the solar cooling system serves, is an office building of a tourist center as shown in Figure 3 and occupied for 10 h from 8:00 to 18:00 every day. The two-story building has a total area of 242.4 m2 and a total height of 6.9 m. The specific cooling and heating power demands are 120 W/m2 and 82 W/ m2, respectively, and the total cooling and heating loads are 29 kW and 20 kW, respectively. There is no hot water demand in the building. The construction characteristics of this building are shown in Table 1.

Figure 3.3.The The tourist tourist center center and and the ofﬁcethe office building building marked marked with the with red frame, the red which frame, the solar which cooling the solarsystem cooling serves. system serves.

Table 1. The construction characteristics of the building.

Parameter Value Total U-value (W/m2·K) 0.5 North 13 West 6 Window-to-wall area ratio (%) East 25 South 25 Frame PVC-U Type of window Figure 3. The tourist center and the office buildingGlass marked with Double the clearred frame, (5/9/5) which the solar cooling system serves.U-window (W/m2·K) 3.0 g-value window * (-) 0.8 Air change (1/h) 0.3 * g-value window: solar heat gain coefﬁcient of window.

Energies 2019, 12, x FOR PEER REVIEW 5 of 18

Table 1. The construction characteristics of the building.

Parameter Value Total U-value (W/m2·K) 0.5 North 13 West 6 Window-to-wall area ratio (%) East 25 South 25 Frame PVC-U Type of window Glass Double clear (5/9/5) U-window (W/m2·K) 3.0 g-value window* (-) 0.8 Air change (1/h) 0.3 *g-value window: solar heat gain coefficient of window. Energies 2019, 12, 996 5 of 17 3. System Installation The installation is a solar collector-based thermal cooling system where a LiBr-water 3. System Installation absorption chiller is driven by hot water from the solar collectors. As shown in Figure 4, it consists of twoThe arrays installation of evacuated is a solar tube collector-based collectors, an thermalair-cooled cooling radiator system to prevent where overheating a LiBr-water of absorption the solar chillercollectors, is driven an absorption by hot water chiller, from a thecooling solar tower, collectors. a hot As water shown storage in Figure tank,4, it a consists buffer tank, of two and arrays two ofair-source evacuated heat tube pumps collectors, as an auxiliary an air-cooled system. radiator During to the prevent summer overheating cooling time, of the the solar solar collectors, collector anfield absorption transfers chiller,solar radiation a cooling into tower, thermal a hot waterenergy storage to produce tank, ahot buffer water tank, at a and temperature two air-source range heat of pumps70–110 as°C, an which auxiliary is stored system. in the During hot water the summer storage cooling tank. The time, absorption the solar collector chiller is field driven transfers by the solar hot ◦ radiationwater and into produces thermal chilled energy water to produce at a temperature hot water of at 10–15 a temperature °C, which range is stored of 70–110 in the C,buffer which tank is storedfor building in the hotcooling. water The storage cooling tank. tower The absorptionis used to chillerextract is waste driven heat by thefrom hot the water chiller. and producesThe heat ◦ chilledpumps wateradditionally at a temperature provide chilled of 10–15 waterC, whichwhen issolar stored energy in the is bufferinsufficient. tank for During building the cooling.winter Theheating cooling time, tower the isabsorption used to extract chiller waste is switched heat from off, the and chiller. the hot The water heat pumps with a additionally temperature provide range chilledbetween water 45 °C when and solar60 °C energyproduced is insufficient. by the solar During collector the field winter is heatingdirectly time,used thefor absorptionbuilding heating. chiller ◦ ◦ isThe switched heat pumps off, and additionally the hot water provide with hot a temperature water when range solar between energy is 45 insufficient.C and 60 C produced by the solar collector field is directly used for building heating. The heat pumps additionally provide hot water when solar energy is insufficient.

Figure 4. The schematic of the solar thermal cooling and heating system. Figure 4. The schematic of the solar thermal cooling and heating system. 3.1. Description of the System Components

3.1.1.3.1. Description The Solar of Collector the System Field Components

3.1.1.The The main Solar heat Collector driving Field source of the installation is the solar collector ﬁeld, which was installed on the roof of a steel-structure parking shed near the ofﬁce building shown in Figure5 (left). It comprises The main heat driving source of the installation is the solar collector field, which was installed 40 all-glass evacuated tube modules in two parallel arrays with a total aperture area of 120 m2. Each on the roof of a steel-structure parking shed near the office building shown in Figure 5 (left). It module contains 18 evacuated tubes and has an aperture area of 3.0 m2.

3.1.2. The Absorption Chiller The absorption chiller presented in Figure5 (right) is a single-stage and LiBr–water based machine associated with a cooling tower shown in Figure6 (left). This chiller had a cooling capacity of 35 kW. The hot water design inlet and outlet temperatures were 80 ◦C and 75 ◦C, respectively, and the rated flow was 8.6 m3/h; the chilled water design inlet and outlet temperatures were 15 ◦C and 10 ◦C respectively, and the rated flow was 6.0 m3/h; the cooling water design inlet and outlet temperatures were 30 ◦C and 35 ◦C, respectively, and the rated flow was 15.0 m3/h. The cooling tower had a rated water flow of 25.0 m3/h. Energies 2019, 12, x FOR PEER REVIEW 6 of 18

comprises 40 all-glass evacuated tube modules in two parallel arrays with a total aperture area of 120 mEnergies2. Each 2019 module, 12, x FOR contains PEER REVIEW 18 evacuated tubes and has an aperture area of 3.0 m². 6 of 18

3.1.2.comprises The Absorption 40 all-glass Chiller evacuated tube modules in two parallel arrays with a total aperture area of 120 m2. Each module contains 18 evacuated tubes and has an aperture area of 3.0 m². The absorption chiller presented in Figure 5 (right) is a single-stage and LiBr–water based machine associated with a cooling tower shown in Figure 6 (left). This chiller had a cooling capacity 3.1.2. The Absorption Chiller of 35 kW. The hot water design inlet and outlet temperatures were 80 °C and 75 °C, respectively, and the ratedThe flow absorption was 8.6 mchiller3/h; the presented chilled water in Figure design 5 inlet(right) and is outlet a single-stage temperatures and wereLiBr–water 15 °C and based 10 °Cmachine respectively, associated and with the arated cooling flow tower was shown 6.0 m in3/h; Fi gurethe 6cooling (left). Thiswater chiller design had ainlet cooling and capacityoutlet temperaturesof 35 kW. The were hot water 30 °C design and 35 inlet °C, andresp outletectively, temp anderatures the rated were flow 80 °C was and 15.0 75 °C, m 3respectively,/h. The cooling and towerthe rated had flowa rated was water 8.6 m flow3/h; theof 25.0 chilled m3/h. water design inlet and outlet temperatures were 15 °C and 10 °C respectively, and the rated flow was 6.0 m3/h; the cooling water design inlet and outlet Energiestemperatures2019, 12, 996 were 30 °C and 35 °C, respectively, and the rated flow was 15.0 m3/h. The 6cooling of 17 tower had a rated water flow of 25.0 m3/h.

Figure 5. Photos of the (left) solar collector field on the roof of a steel-structure parking Figureshed 5.and the (right) leftequipment room. Photos of the ( ) solar collector ﬁeld on the roof of a steel-structure parking shed and the (right) equipment room. Figure 5. Photos of the (left) solar collector field on the roof of a steel-structure parking shed and the (right) equipment room.

Figure 6. Photos of the (left) cooling tower and the (right) two heat pumps.

3.1.3. The ThermalFigure Energy 6. Photos Storage of the (left)cooling tower and the (right) two heat pumps. In order to make rational use of solar energy and to maximize the energy efﬁciency, two pressure storage3.1.3. The tanks Thermal were used Energy in the Storage installation, which were a hot water storage tank with a volume of 4 m3 Figure 6. Photos of the (left)cooling tower and the (right) two heat pumps. and a bufferIn order tank to with make a volumerational of use 1 m 3of. Thesolar hot energy water storageand to tankmaximize had built-in the energy heat exchanger efficiency, coils two andpressure the heat storage transfer tanks capacity were wasused 58 in kW. the installation, which were a hot water storage tank with a 3.1.3. The Thermal Energy Storage 3.1.4. TheIn Auxiliaryorder to make System rational use of solar energy and to maximize the energy efficiency, two pressureAn auxiliary storage system tanks iswere required used due in the to the installa instabilitytion, which of solar were energy. a hot Two water air-source storage heat tank pumps with a were applied in the installation as shown in Figure6 (right). The rated cooling capacity was 23.8 kW and the heating capacity was 32 kW.

3.2. Description of the Controlling and Monitoring System The controlling and monitoring system is very important for the safe and stable running of this installation. As presented in Figure7, the solar radiation Isolar, the temperatures T0–T17, the mass ﬂow rates F1–F4, and the electrical consumption E1 were recorded by the monitoring system. Table2 presents the instruments and their calibration range and uncertainty. Energies 2019, 12, x FOR PEER REVIEW 7 of 18 volume of 4 m3 and a buffer tank with a volume of 1 m3. The hot water storage tank had built-in heat exchanger coils and the heat transfer capacity was 58 kW.

3.1.4. The Auxiliary System An auxiliary system is required due to the instability of solar energy. Two air-source heat pumps were applied in the installation as shown in Figure 6 (right). The rated cooling capacity was 23.8 kW and the heating capacity was 32 kW.

3.2. Description of the Controlling and Monitoring System The controlling and monitoring system is very important for the safe and stable running of this installation. As presented in Figure 7, the solar radiation Isolar, the temperatures T0–T17, the mass flow rates F1–F4, and the electrical consumption E1 were recorded by the monitoring system. Table 2 Energies 2019, 12, 996 7 of 17 presents the instruments and their calibration range and uncertainty.

Figure 7. The schematic of the controlling and monitoring system: T0—the ambient temperature; Figure 7.The schematic of the controlling and monitoring system:T0—the ambient T1—the temperature of the collector array 1; T2—the temperature of the collector array 2; T3—the 1 2 outlettemperature; temperature T —the of the temperature collector field; ofT 4—thethe collector upper layer array temperature 1; T —the of thetemperature hot water storageof the tank;collectorT5—the array lower 2; layerT3—the temperature outlet temperature of the hot water of storagethe collector tank; T6 —thefield; inletT4—the temperature upper oflayer the collectortemperature field; Tof7—the the antifreezehot water temperature storage tank; of the collectorT5—the field;lowerT8 —thelayer hottemperature water inlet temperatureof the hot ofwater the absorptionstorage tank; chiller; T6T—the9—the inlet hot watertemperature outlet temperature of the collector of the absorption field; T7—the chiller; antifreezeT10—the chilledtemperature water outlet of the temperature collector offield; the absorptionT8—the hot chiller; waterT11 inlet—the temperature chilled water inletof the temperature absorption of thechiller; absorption T9—the chiller; hot Twater12—the outlet cooling temperature water outlet temperatureof the absorption of the absorption chiller;T chiller;10—theT 13chilled—the coolingwater outlet water temperature inlet temperature of the of theabsorption absorption chiller; chiller; T11T—the14—the chilled upper water layer temperatureinlet temperature of the bufferof the tank; absorptionT15—the lowerchiller; layer T12 temperature—the cooling of the water buffer outlet tank; T 16temperature—the inlet temperature of the absorption of the fan coils;chiller;T17 —theT13—the outlet cooling temperature water of inlet the fantemperature coils; Isolar—solar of the radiation;absorptionF1—the chiller; inlet T flow14—the rate upper of the collector field; F —the hot water inlet flow rate of the absorption chiller; F —the chilled water inlet flow layer temperature2 of the buffer tank; T15—the lower layer temperature3 of the buffer rate of the absorption chiller; F —the inlet flow rate of the fan coils; E —the electrical consumption of tank;T16—the inlet temperature4 of the fan coils; T17—the outlet1 temperature of the fan the heat pumps. coils; Isolar—solar radiation; F1—the inlet flow rate of the collector field; F2—the hot water inlet flow rateTable of 2.theThe absorption instruments chiller; and their F calibration3—the chilled range water and uncertainty. inlet flow rate of the absorption chiller; F4—the inlet flow rate of the fan coils; E1—the electrical consumption of Parameter Instrument Calibration Range Instrument Uncertainty the heat pumps. Resistance Temperature Detector Ambient temperature −40–60 ◦C ±0.6 ◦C RTD Fluid temperatureTable 2. The instruments Thermocouple, and Type their K calibration 0–200 range◦C and uncertainty.±2.2 ◦C Mass flow Coriolis flow and density meters 0–2.7 kg/s ±0.1–0.2% ParameterGlobal solar radiationInstrument Pyranometer Calibration range0–2000 W/m2 Instrument±1% uncertainty Three-phase multifunction energy Electrical consumption Resistance 0–999,999.99 kWh ±1% Ambient temperature Temperature Detectormeter −40–60 °C ±0.6 °C RTD Thermocouple, Type Fluid temperature 0–200 °C ±2.2 °C The installation had two operationalK modes, namely, the building cooling and heating modes. DuringMass the flow cooling period Coriolis in summer, flow and the valves V1–V60–2.7 andkg/s V9–V18 were open and ±0.1 V7–0.2% and V8 were closed. The controlling system was achieved through the following main cycles:

(1) The solar loop The pump, P1, was switched on when the difference between the outlet temperature of the ◦ collector ﬁeld and the upper layer temperature of the hot water storage tank T3 − T4 ≥ 4 C; the pump, ◦ P1, was switched off when the temperature difference T3 − T4 < 2 C. (2) The cooling loop The pump, P2, was switched on when the upper layer temperature of the hot water storage tank ◦ ◦ T4 ≥ 80 CAND the fan coils were turned on; the pump, P2, was switched off when T4 < 70 COR the fan coils were shut down. Energies 2019, 12, 996 8 of 17

(3) The heat pump loop The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer ◦ tank T15 > 18 C AND the fan coils were turned on; the heat pumps and pump P6 were switched off ◦ when T15 ≤ 15 C OR the fan coils were shut down. During the heating period in winter, the valves V1, V2, V7–V10, and V13–V18 were open and V3–V6, V11, and V12 were closed. The controlling system was achieved through the following main cycles: (1) The solar loop The pump, P1, was switched on when the difference between the outlet temperature of the ◦ collector ﬁeld and the upper layer temperature of the hot water storage tank T3 − T4 ≥ 4 C; the pump, ◦ P1, was switched off when the temperature difference T3 − T4 < 2 C. (2) The heating loop ◦ The pump, P2, was switched on when the lower layer temperature of the buffer tank T15 < 55 C AND the difference between the upper layer temperature of the hot water storage tank and the lower ◦ layer temperature of the buffer tank T4 − T15 ≥ 2 C; the pump, P2, was switched on when the lower ◦ layer temperature of the buffer tank T15 ≥ 60 C OR T4 ≤ T15. (3) The heat pump loop The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer ◦ tank T15 < 45 C AND the fan coils were turned on; the heat pumps and pump P6 were switched off ◦ when T15 ≥ 50 C OR the fan coils were shut down.

4. Operational Results and Performance Evaluation The solar cooling system was installed in December 2017 and has been operating since January 2018. The monthly operational results and performance evaluation as well as those on a typical summer day, namely, 23 July 2018, are presented below.

4.1. Building Cooling and Heating Energy Demand and Room Temperature The room cooling and heating temperatures were set to 22 ◦C and 18 ◦C, respectively, according to the Chinese standards GB 50189-2005 “Design standard for energy efﬁciency of public buildings” and JGJ 134-2010 “Design standard for energy efﬁciency of residential buildings in hot-summer and cold-winter zone.” Figure8 shows the monthly average outdoor temperature and building cooling and heating energy demand in 2018. The annual cooling and heating energy demands were 48,418 kWh and 24,288 kWh, respectively. The highest cooling demand occurred in July and August, which corresponded to the hottest months in 2018, while the monthly heating energy demands were similar from December to March. There was neither cooling nor heating demand in November and April. Figure9 shows the hourly building cooling load and average room temperature on 23 July 2018. The daily cooling energy demand was 435 kWh. The room temperature decreased to 21.4 ◦C as the solar cooling system started to work at 8:00 in the morning and kept in the range of 21.0–22.0 ◦C for 10 h. The room temperature increased rapidly to 27.0 ◦C after the system stopped working at 18:00. Energies 2019, 12, x FOR PEER REVIEW 9 of 18

30 12,000 C ooling energy dem and H eating energy dem and 25 Average outdoor tem perature 10,000 kW h /

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0 0 M ng Figure 8. The monthly average outdoor temperature and building cooling and heating

Tem perature Tem perature 10 4,000 energy demand in 2018. ng/heati i Figure 9 shows the hourly building cooling load and average room temperature on 23 July

5 2,000 Cool 2018. The daily cooling energy demand was 435 kWh. The room temperature decreased to 21.4 °C as the solar cooling system started to work at 8:00 in the morning and kept in the range of 21.0–22.0

°C for 10 h. The0 room temperature increased rapidly to 27.0 °C after the system0 stopped working at 18:00. 123456789101112 Month e / ℃ 25 10,B u i l d n g 0 e r a t ur 20 8,00c o l i n g 0 o r t e mp 15 6,00e n r gy 0 e o u td 10 4,00d e m a nd 0

5 A v e r ag 2,00/ k W 0

0 0 M Figure 8. The monthly average outdoor temperature and building cooling and heating energy demand Figurein 2018. 8. The monthly average outdoor temperature and building cooling and heating energy demand in 2018. 40 40 Building cooling load 35 35 Figure 9 shows theAverage hourly room building temperature cooling load and average room temperature on 23 July

2018. The daily cooling energy demand was 435 kWh. The room temperature decreased℃ to 21.4 °C 30 30 as the solar coolingkW system started to work at 8:00 in the morning and kept in the range of 21.0–22.0 °C for 10 h. The25 room temperature increased rapidly to 27.0 °C after the system stopped25 working at lo a d /

18:00. ng i 20 20

cool 15 15 ng di 10 10

Buil Average room temeprature / temeprature room Average 5 5 40 40

0 Building cooling load 0 35 35 Average room temperature ℃ 30 30 kW

25 25 lo a d / Local time ng i 20 20 Figure 9. The hourly building cooling load and average room temperature on 23 July 2018. cool Figure9. The15 hourly building cooling load and average room temperature on 1523 July 2018. ng

4.2. Solar Collectordi Field 10 10 Buil

The collector efficiency η is given by Equation (1): / temeprature room Average 5 5 Qsc F1·cp,w·(T3 − T6) 0 η = = ,0 (1) Qsolar Isolar·Acol where Qsolar and Qsc represent the total solar energy available and solar energy collected; Isolar is the solar irradiation; Asolar is the aperture area of the solar collectors; F1 is the inlet mass flow rate of the Local time collector filed; cp,w is the specific heat of water; T3 and T6 are the outlet and inlet temperatures of the

Figure9. The hourly building cooling load and average room temperature on 23 July 2018.

Energies 2019, 12, x FOR PEER REVIEW 10 of 18

4.2. Solar Collector Field The collector efficiency η is given by Equation (1):

Q Fc⋅⋅−() T T (1) η= sc = 1,pw 36 ⋅ , QIAsolar solar col

where Qsolar and Qsc represent the total solar energy available and solar energy collected; Isolar is the Energiessolar 2019irradiation;, 12, 996 Asolar is the aperture area of the solar collectors; F1 is the inlet mass flow rate10 of of 17the collector filed; cp,w is the specific heat of water; T3 and T6 are the outlet and inlet temperatures of the solar collection loop, respectively; andη is the collector efficiency, with an uncertainty of 3.3% due solar collection loop, respectively; and η is the collector efficiency, with an uncertainty of 3.3% due to theto measurementthe measurement errors. errors. FigureFigure 10 10 presents presents the the monthly monthly solar solar irradiation irradiation onto onto the the collector collector area area and and the the collector collector field field yield.yield. TheThe solar solar irradiation irradiation ontoonto the the collector collector area area from from May May to to October October was was 94,878 94,878 kWh kWh and and the the collectorcollector field field yield yield was was 41,849 41,849 kWh kWh during during the the summer summer cooling cooling time. time. The The annual annual average average collector collector efficiencyefficiency for for building building coolingcooling waswas 44%,44%, with a maximum maximum of of 47% 47% in in August. August. On On the the other other hand, hand, the thesolar solar irradiation irradiation onto onto the the collector collector area area from from De Decembercember to to March was 42,70942,709 kWhkWh andand the the collector collector fieldfield yield yield was was 18,221 18,221 kWh kWh during during the the winter winter heating heating time. time. The The annual annual average average collector collector efficiency efficiency for buildingfor building heating heating was 42%.was 42%. FigureFigure 11 11 shows shows the the hourly hourly solar solar irradiation irradiation onto onto the the collector collector area area and and the the collector collector field field yield yield onon 23 23 July July 2018. 2018. The The daily daily solar solar irradiation irradiation and and collector collector field field yield yield were were 627.7 627.7 kWh kWh and and 308.5 308.5 kWh, kWh, respectively.respectively. The The daily daily average average collector collector efficiency efficiency was was 49%. 49%.

20,000 Irra d iation onto collector area 18,000 Collector field yield

/ 16,000

area area 14,000 kW h / d ector ector l 12,000 yiel col d

el 10,000 fi onto onto

on 8,000 ector ector ati

Coll 6,000 Irra d i

4,000

2,000

123456789101112 Month

18,0

e a / 16,0

l e c t o r ar d / k Wh 1214,0

n t o c ol f i e l d yi 10,0

i a t o n l e c or 6,0 8

I r ad Co 4,0

0 2, Energies 2019, 12, x FORM PEER REVIEW 11 of 18 Figure 10. The monthly solar irradiation onto the collector area and the collector ﬁled yield in 2018. Figure 10. The monthly solar irradiation onto the collector area and the collector filed yield in 2018. 120 Irra d iation onto the collector area Colle c to r field yield

/ 100

area area kW

/ 80 d ector ector el l yi col d 60 fie l onto onto on on

e c to r 40 l ati col

Irradi 20

Local time

Figure 11. The hourly solar irradiation onto the collector area and the collector ﬁled yield on 23 JulyFigure 2018. 11. The hourly solar irradiation onto the collector area and the collector filed yield on 23 July 2018.

4.3. Absorption Chiller

The coefficient of performance of the absorption chiller COPchiller is defined as the ratio of cooling energy yield and heat supplied by the generator. It is determined on the basis of the equation (2):

Q Fc⋅⋅−() T T (2) COP = c = 3,pw 1110 chiller ⋅⋅− , QFcTTsc1, p w () 36

where Qsolar, Qsc and Qc represent the total solar energy available, solar energy collected, and cooling energy generated by the chiller driven by solar; Isolar is the solar irradiation; Asolar is the aperture area of the solar collectors; F1 and F3 are the mass flow rate of the hot water of the absorption chiller and that of the chilled water, respectively; cp,w is the specific heat of water; T3 and T6 are the outlet and inlet temperatures of the solar collection loop, respectively;T10 and T11 are the chilled water outlet and inlet temperatures of the absorption chiller, respectively; COPchiller is the coefficient of performance of the absorption chiller, with an uncertainty of 3.2% due to the measurement errors. Figure 12 shows the monthly cooling energy yield of the absorption chiller and heat supplied by the solar collector field in 2018. The annual cooling energy yield and the heat supplied were 27,399 kWh and 37,318 kWh respectively. The monthly average COPchiller ranged between 0.68 and 0.76. The yearly average COPchiller was 0.73.

Energies 2019, 12, 996 11 of 17

4.3. Absorption Chiller

The coefﬁcient of performance of the absorption chiller COPchiller is deﬁned as the ratio of cooling energy yield and heat supplied by the generator. It is determined on the basis of the Equation (2):

Qc F3·cp,w·(T11 − T10) COPchiller = = , (2) Qsc F1·cp,w·(T3 − T6) where Qsolar, Qsc and Qc represent the total solar energy available, solar energy collected, and cooling energy generated by the chiller driven by solar; Isolar is the solar irradiation; Asolar is the aperture area of the solar collectors; F1 and F3 are the mass flow rate of the hot water of the absorption chiller and that of the chilled water, respectively; cp,w is the specific heat of water; T3 and T6 are the outlet and inlet temperatures of the solar collection loop, respectively; T10 and T11 are the chilled water outlet and inlet temperatures of the absorption chiller, respectively; COPchiller is the coefficient of performance of the absorption chiller, with an uncertainty of 3.2% due to the measurement errors. Figure 12 shows the monthly cooling energy yield of the absorption chiller and heat supplied by the solar collector field in 2018. The annual cooling energy yield and the heat supplied were 27,399 kWh and 37,318 kWh respectively. The monthly average COPchiller ranged between 0.68 and Energies 2019, 12, x FOR PEER REVIEW 12 of 18 0.76. The yearly average COPchiller was 0.73.

9,000 C ooling energy yield 8,000 H eat supplied by solar collector field

7,000 kW h / /

d 6,000 el yi

g e n e5,000 ra to r energy energy b y

ng 4,000 ed i i Cool

suppl 3,000

heat heat 2,000

1,000

123456789101112 Month

Figure 12. The monthly cooling energy yield of the absorption chiller and heat supplied by the solar collectorFigure 12. field The in 2018.monthly cooling energy yield of the absorption chiller and heat supplied by the solar collector field in 2018. Figure 13 presents the hourly cooling energy yield of the absorption chiller and heat supplied by generatorFigure 13 on presents 23 July the 2018. hourly The cooling absorption energy chiller yield was of the able absorption to produce chiller chilled and waterheat supplied for 6 h fromby generator 10:00 to 16:00.on 23 July The 2018. daily The cooling absorption energy chi yieldller of was the able absorption to produce chiller chilled and water the heat for supplied 6 h from by10:00 the to solar 16:00. collector The daily field cooling were 207.1energy kWh yield and of 270.5the absorption kWh respectively. chiller and The the average heat supplied coefficient by the of performancesolar collectorCOP fieldchiller werewas 0.76,207.1 with kWh maximums and 270.5 of 0.78kWh at respectively. 13:00 and 14:00 The when average the inlet coefficient hot water of reachedperformance its highest COPchiller temperature. was 0.76, with It was maximums found that of the 0.78COP at chiller13:00increased and 14:00 withwhen the the rising inlet hothot waterwater inletreached temperature. its highest temperature. It was found that the COPchiller increased with the rising hot water inlet temperature. 80 C ooling energy yield 70 H eat supplied by solar colle c to r field kW

/ 60 / d

el 50 yi generator generator 40 by by nergy nergy ed ng i i 30 Cool suppl 20 heat heat 10

Local time Figure 13. The hourly cooling energy yield of the absorption chiller and heat supplied by generator on 23 July 2018.

Figure 14 presents the hourly hot water, chilled water, and cooling water inlet and outlet temperatures on 23 July 2018. The hot water inlet temperature from the solar collector field reached 106.6 °C at 12:00, after 2 h of operation and it remained constant for about 4 h before decreasing. The chilled water outlet temperature decreased until reaching a minimum value of 10 °C at 12:00. It remained constant for about 4 h and then started increasing. Similarly, the cooling water outlet

Energies 2019, 12, x FOR PEER REVIEW 12 of 18

9,000 C ooling energy yield 8,000 H eat supplied by solar collector field

7,000 kW h / /

d 6,000 el yi

g e n e5,000 ra to r energy energy b y

ng 4,000 ed i i Cool

suppl 3,000

heat heat 2,000

1,000

123456789101112 Month

Figure 12. The monthly cooling energy yield of the absorption chiller and heat supplied by the solar collector field in 2018.

Figure 13 presents the hourly cooling energy yield of the absorption chiller and heat supplied by generator on 23 July 2018. The absorption chiller was able to produce chilled water for 6 h from 10:00 to 16:00. The daily cooling energy yield of the absorption chiller and the heat supplied by the solar collector field were 207.1 kWh and 270.5 kWh respectively. The average coefficient of performance COPchiller was 0.76, with maximums of 0.78 at 13:00 and 14:00 when the inlet hot water Energies 2019, 12, 996 12 of 17 reached its highest temperature. It was found that the COPchiller increased with the rising hot water inlet temperature. 80 C ooling energy yield 70 H eat supplied by solar colle c to r field kW

/ 60 / d

el 50 yi generator generator 40 by by nergy nergy ed ng i i 30 Cool suppl 20 heat heat 10

Local time FigureFigure 13. 13.The The hourly hourly cooling cooling energy energy yield yield of the of absorption the absorption chiller chiller and heat and supplied heat supplied by generator by on 23generator July 2018. on 23 July 2018.

Figure 14 presents the hourly hot water, chilled water, and cooling water inlet and outlet Figure 14 presents the hourly hot water, chilled water, and cooling water inlet and outlet temperatures on 23 July 2018. The hot water inlet temperature from the solar collector ﬁeld reached temperatures on 23 July 2018. The hot water inlet temperature from the solar collector field reached 106.6 ◦C at 12:00, after 2 h of operation and it remained constant for about 4 h before decreasing. 106.6 °C at 12:00, after 2 h of operation and it remained constant for about 4 h before decreasing. The chilled water outlet temperature decreased until reaching a minimum value of 10 ◦C at 12:00. The chilled water outlet temperature decreased until reaching a minimum value of 10 °C at 12:00. It It remainedEnergies 2019 constant, 12, x FOR for PEER about REVIEW 4 h and then started increasing. Similarly, the cooling water13 outletof 18 remained constant for about 4 h and then started increasing. Similarly, the cooling water outlet temperature increased until reaching a maximal value of 35 ◦C at 12:00. It remained constant for about temperature increased until reaching a maximal value of 35 °C at 12:00. It remained constant for 4 h, and then started decreasing. about 4 h, and then started decreasing.

120 H o t w a te r inlet temperature 110 H o t w a te r o u tle t te m p e ra tu re 100 Chilled water inelt tem perature Chille d w a te r o u tle t te m p e ra tu re 90 C ooling water inlet temperature 80 C ooling water outlet temperature ℃ / 70

Tem perature Tem perature 40

Local time Figure 14. The hourly hot water, chilled water, and cooling water inlet and outlet temperatures on 23 Figure 14. The hourly hot water, chilled water, and cooling water inlet and outlet July 2018. temperatures on 23 July 2018. 4.4. Heat Pumps 4.4. Heat Pumps The coefﬁcient of performance of the heat pumps COPhp is deﬁned as the ratio of cooling/heating energy yieldThe andcoefficient electrical of consumption.performance Itof is determinedthe heat pumps on the COP basishp ofis the defined Equations as (3)the andratio (4): of cooling/heating energy yield and electrical consumption. It is determined on the basis of the

Equations (3) and (4): Qt − Qc F4·cp,w·(T17 − T16) − F3·cp,w·(T11 − T10) COPhp,c = = , (3) QQ− E1 Fc⋅⋅−−⋅⋅−() T T FcE1 () T T (3) ==tc 4pw , 17 16 3 pw , 11 10 COPhp, c , EE11

QQ− Fc⋅⋅−−⋅⋅−() T T Fc () T T (4) ==th 4pw , 17 16 2 pw , 4 5 COPhp, h . EE11

The solar fraction SFn is defined as the ratio of the generated cooling or heating by solar to the total generated cooling or heating energy by solar and the heat pumps which corresponds to the total cooling or heating energy used by the fan coils. It can be calculated with the Equations (5) and (6): Q F ⋅⋅−cTT() (5) SF ==c 3,pw 1110 nc, ⋅⋅−, QFcTTtpw4 ,() 17 16

Q F ⋅⋅−cTT() (6) SF ==h 2,pw 45 nh, ⋅⋅− , QFcTTtpw4,() 1716

where Qt, Qc, and Qh are the total cooling or heating energy used, cooling energy generated by the chiller driven by solar and heating energy generated by solar, respectively; F2, F3, and F4 are the hot water inlet flow rate of the absorption chiller, chilled water inlet flow rate of the absorption chiller and inlet flow rate of the fan coils, respectively; cp,w is the specific heat of water; T4 and T5 are the upper layer temperature and lower layer temperature of the hot water storage tank, respectively; T10 and T11 are the chilled water outlet and inlet temperatures of the absorption chiller, respectively;T16 and T17 are the inlet temperature and outlet temperature of the fan coils, respectively; E1 is the electrical consumption of the heat pumps; COPhp,c and COPhp,h are the

Energies 2019, 12, 996 13 of 17

Qt − Qh F4·cp,w·(T17 − T16) − F2·cp,w·(T4 − T5) COPhp,h = = . (4) E1 E1

The solar fraction SFn is deﬁned as the ratio of the generated cooling or heating by solar to the total generated cooling or heating energy by solar and the heat pumps which corresponds to the total cooling or heating energy used by the fan coils. It can be calculated with the Equations (5) and (6):

Qc F3·cp,w·(T11 − T10) SFn,c = = , (5) Qt F4·cp,w·(T17 − T16)

Qh F2·cp,w·(T4 − T5) SFn,h = = , (6) Qt F4·cp,w·(T17 − T16) where Qt, Qc, and Qh are the total cooling or heating energy used, cooling energy generated by the chiller driven by solar and heating energy generated by solar, respectively; F2, F3, and F4 are the hot water inlet flow rate of the absorption chiller, chilled water inlet flow rate of the absorption chiller and inlet flow rate of the fan coils, respectively; cp,w is the specific heat of water; T4 and T5 are the upper layer temperature and lower layer temperature of the hot water storage tank, respectively; T10 and EnergiesT11 are 2019 the, 12 chilled, x FOR waterPEER REVIEW outlet and inlet temperatures of the absorption chiller, respectively; T1416 ofand 18 T17 are the inlet temperature and outlet temperature of the fan coils, respectively; E1 is the electrical coefficientconsumption of performance of the heat pumps; of the heatCOP pumpshp,c and duriCOPnghp,h theare summer the coefficient cooling oftime performance and that during of the heatthe winterpumps heating during time, the summer respectively, cooling with time an and uncertainty that during of the 3.5% winter due heatingto the measurement time, respectively, errors; with SFn,c an anduncertainty SFn,h are ofthe 3.5% solar due fraction to the for measurement building cooling errors; andSFn,c heating,and SF respectively,n,h are the solar with fraction an uncertainty for building of 3.2%cooling due and to the heating, measurement respectively, errors. with an uncertainty of 3.2% due to the measurement errors. FigureFigure 1515 shows thethe monthlymonthly cooling/heating cooling/heating energy energy supplied, supplied, the the electricity electricity consumption, consumption, and andthe coefficientthe coefficient of performance of performance of the of heatthe heat pumps pumpsCOP COPhp inhp 2018. in 2018. The The heat heat pumps pumps produced produced a total a totalof 21,019 of 21,019 kWh kWh cooling cooling energy energy and 9104 andkWh 9,104 heating kWh heating energy inenergy 2018, in corresponding 2018, corresponding to the electrical to the electricalconsumption consumption of 7439 kWh of for7439 cooling kWh andfor 2851cooling kWh and for heating,2,851 kWh respectively. for heating, The averagerespectively. coefficient The averageof performance coefficient for of cooling performanceCOPhp,c forwas cooling 2.83, COP withhp,c minimums was 2.83, ofwith 2.74 minimums and 2.75 inof July2.74 andand August,2.75 in Julyrespectively. and August, The respectively. average coefficient The averag of performancee coefficient for of heatingperformanceCOPhp,h for washeating 3.20. COPhp,h was 3.20.

6.00 6,000 Electrical c o n s u m p tion Supplied cooling/heating energy

5.00 C o e fficient of performance CO P 5,000 / - energy energy COP/ COP/ 4.00 4,000 on ng

3.00 3,000 consum pti

ng/heatyi i performance performance cal of of cool 2.00 2,000 ent ent ed electri i ci Suppl Coeffi 1.00 1,000

0.00 0

123456789101112 Month

3.50 Su 3.0 Colin g perfomance fac COPtor 5,000el f a c t o r p l i e d 2.50 4,000e c t r i al r m a n ce c o l i ng 2.0 3,000 c o n s um n g p e r fo e n r gy 1.50 2,000p t i o n / C o li / 1.0 k W h

0.5 1,000

0. 0 M FigureFigure 15. 15. TheThe monthly monthly supplied supplied cooling/heating energy,energy, the the electricity electricity consumption, consumption, and and the coefﬁcient of performance of the heat pumps COP in 2018. the coefficient of performance of the heat pumpshp COPhp in 2018. In order to determine the inﬂuence of the heat pumps on the system performance, the heat pumps were switched off700 on 25, 27, and 28 July 2018 and the system performance was analyzed. As shown in Irradiation onto collector area Figure 16, the energy yield andCol thele c to solarr field yiel fractiond distinctly decreased without the heat pumps as the 600 C ooling energy yield kW h

/ 500 d el yi 400 energy energy 300 and and on on

ati 200 Irra d i 100

23.07 25.07 27.07 28.07

Date

Figure 16. The irradiation onto the collector area, the collector field yield, and the cooling energy yield on 23, 25, 27, and 28 July; the heat pumps were switched off on 25, 27, and 28 July2018.

In order to determine the influence of the heat pumps on the system performance, the heat pumps were switched off on 25, 27, and 28 July 2018 and the system performance was analyzed. As

Energies 2019, 12, x FOR PEER REVIEW 14 of 18 coefficient of performance of the heat pumps during the summer cooling time and that during the winter heating time, respectively, with an uncertainty of 3.5% due to the measurement errors; SFn,c and SFn,h are the solar fraction for building cooling and heating, respectively, with an uncertainty of 3.2% due to the measurement errors. Figure 15 shows the monthly cooling/heating energy supplied, the electricity consumption, and the coefficient of performance of the heat pumps COPhp in 2018. The heat pumps produced a total of 21,019 kWh cooling energy and 9,104 kWh heating energy in 2018, corresponding to the electrical consumption of 7439 kWh for cooling and 2,851 kWh for heating, respectively. The average coefficient of performance for cooling COPhp,c was 2.83, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20.

6.00 6,000 Electrical c o n s u m p tion Supplied cooling/heating energy

5.00 C o e fficient of performance CO P 5,000 / - energy energy COP/ COP/ 4.00 4,000 on ng

3.00 3,000 consum pti

ng/heatyi i performance performance cal of of cool 2.00 2,000 ent ent ed electri i ci Suppl Coeffi 1.00 1,000 Energies 2019, 12, 996 14 of 17

0.00 0 auxiliary system. The daily123456789101112 global irradiation onto the collector area was 661.6 kWh on 28 July, which Month

0.5 1,000

0. 0 was similar toM that of 627.7 kWh on 23 July. However, the collector ﬁeld yield and the cooling energy yieldFigure were 183.9 15. The kWh monthly and 133.7 supplied kWh, respectively,cooling/heating which energy, were the only electricity 60% and consumption, 50% of those values and on 23 July when incorporated with the heat pumps. the coefficient of performance of the heat pumps COPhp in 2018.

700 Irradiation onto collector area Colle c to r field yield 600 C ooling energy yield kW h

/ 500 d el yi 400 energy energy 300 and and on on

ati 200 Irra d i Energies 2019, 12, x FOR100 PEER REVIEW 15 of 18 shown in Figure 16,0 the energy yield and the solar fraction distinctly decreased without the heat pumps as the auxiliary system.23.07 The daily 25.07 global irradiation 27.07 onto the 28.07collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWhDate on 23 July. However, the collector field yield and the cooling energy yield were 183.9 kWh and 133.7 kWh, respectively, which were only 60% and 50% Figureof those 16.16. valuesThe The irradiation irradiation on 23 July onto whenonto the collectorthe incorporated collector area, thearea, with collector the the collector heat ﬁeld pumps. yield, field and yield, the cooling and the energy cooling yield onenergy 23, 25, yield 27, and on 2823, July; 25, the27, heatand pumps28 July; were the switchedheat pumps off on were 25, 27, switched and 28 July2018. off on 25, 27, and 28 July2018. 4.5.4.5. Electricity SavingsSavings andand EmissionEmission ReductionReduction

Figure 17 shows the monthly solar fraction SFn and electricity savings in 2018. The annual FigureIn order 17 toshows determine the monthly the influence solar fraction of theSF heatn and pumps electricity on the savings system in 2018.performance, The annual the mean heat solarpumpsmean fractions solar were fractions switched for cooling for off cooling andon 25, heating 27,and and heating were 28 July 56.6% were 2018 and 56.6% and 62.5%, the and system respectively. 62.5%, performance respectively. This means was This thatanalyzed. means 43.4% that As of the43.4% annual of the building annual cooling building demand cooling and demand 37.5% ofand the 37.5% annual of building the annual heating building demand heating were demand covered bywere the covered heat pumps. by the heat pumps.

0.90 1,800 Sola r fra c tion Electricity s a v ings

- 0.80 1,600 / n

SF 0.70 1,400

0.60 1,200 system system kW h / to 0.50 1,000 ngs ngs energy energy savi

ar ar 0.40 800 ty ci sol

of of 0.30 600 on on Electri

0.20 400 Fracti

0.10 200

0.00 0

123456789101112 Month

0.8 Fract ofsoin lar energy tos stem SF ,6001 S F 0.7n / 1,400 El s y t em 0.6 1,200 e c t r i it e r g y to 0.5 1,000y s a v i ng s o l a r en 0.34 60 8 s / k Wh

0.2c t i o n of 40

0.1F ra 20

0 0 Mo Figure 17. The monthly solar fraction SFn and electricity savings in 2018. Figure 17. The monthly solar fraction SFn and electricity savings in 2018.

One of the essential purposes of the solar cooling and heating systems is to reduce the use of non-solar energy sources such as electricity or fossil fuel burning, which also results in a reduction of CO2 emissions [6]. There are 536 g of CO2 emitted into the atmosphere for each kWh of electricity produced. Thus, the conversion factor to calculate the quantities of CO2 emitted is fc = 0.54 kg/kWh. Assuming that all building cooling and heating demands were covered by the two air-source heat pumps with the mean cooling COPhp,c of 2.83 and mean heating COPhp,h of 3.20, the annual total electricity consumption was 24,699 kWh. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for the building cooling and heating and corresponds to 5445 kg of CO2 emissions prevented from being released into the atmosphere.

4. Discussion and Conclusions An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system started operating in 2018 and the operational results were evaluated. Based on this study, the main conclusions are as follows:  The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44%for building cooling and 42% for building heating.

Energies 2019, 12, 996 15 of 17

5. Discussion and Conclusions An absorption solar cooling and heating system assisted by two air-source heat pumps located in Ningbo City, China was studied in this paper. The system started operating in 2018 and the operational results were evaluated. Based on this study, the main conclusions are as follows:

• The solar collector field was comprised of 40 all-glass evacuated tube modules with a total aperture area of 120 m2. The annual average collector efficiency was 44%for building cooling and 42% for building heating. • The single-stage and LiBr–water based absorption chiller had a cooling capacity of 35 kW. The monthly average coefficient of performance COPchiller ranged between 0.68 and 0.76 in 2018. The COPchiller increased with the rising hot water inlet temperature. • Two air-source heat pumps each with a rated cooling capacity of 23.8 kW and heating capacity of 33 kW were used as the auxiliary system for the solar cooling and heating installation. The average coefficient of performance COPhp,c was 2.83 in 2018, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COPhp,h was 3.20. • The energy yields distinctly decreased without the heat pumps as the auxiliary system. In comparison with the case combined with the heat pumps under similar irradiation conditions, the collector field yield and cooling energy yield decreased by more than 40%. • Two kinds of operational modes were conducted, namely, the building cooling and heating modes. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. The yearly electricity saving was 10,158.6 kWh when combined with the solar cooling and heating system, which accounted for 41.1% of the total electricity consumption for building cooling and heating and corresponds to 5445 kg of CO2 emissions prevented from being released into the atmosphere.

Author Contributions: Methodology and writing—original draft preparation, L.H.; Project administration, R.Z.; Data curation, U.P. Funding: This research was funded by a grant from the National Key Technology Support Program of the Twelfth Five-Year of the People’s Republic of China, grant number 2013BAJ10B06, project title “The integration and demonstration of construction technology on the ecosystem villages and towns and well-off residences in the southeast coastal area.” Acknowledgments: All support given for this paper is covered by the author contribution and funding sections. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2019, 12, 996 16 of 17

Nomenclature

2 Asolar Aperture area of solar collectors, m COPchiller Coefficient of performance of the absorption chiller COPhp,c Coefficient of performance of the heat pump for cooling COPhp,h Coefficient of performance of the heat pump for heating E1 Electrical consumption of the heat pumps, kWh F1 Inlet flow rate of the collector field, kg/s F2 Hot water inlet flow rate of the absorption chiller, kg/s F3 Chilled water inlet flow rate of the absorption chiller, kg/s F4 Inlet flow rate of the fan coils, kg/s 2 Isolar Solar radiation, W/m Qc Cooling energy generated by the chiller driven by solar, kWh Qh Heating energy generated by solar, kWh Qsc Solar energy collected, kWh Qsolar Total solar energy available, kWh Qt Total cooling or heating energy used, kWh SFn,c Coefficient of performance of the heat pumps during the winter heating time SFn,h Coefficient of performance of the heat pumps during the summer cooling time ◦ T0 Ambient temperature, C ◦ T1 Temperature of the collector array 1, C ◦ T2 Temperature of the collector array 2, C ◦ T3 Outlet temperature of the collector field, C ◦ T4 Upper layer temperature of the hot water storage tank, C ◦ T5 Lower layer temperature of the hot water storage tank, C ◦ T6 Inlet temperature of the collector field, C ◦ T7 Antifreeze temperature of the collector field, C ◦ T8 Hot water inlet temperature of the absorption chiller, C ◦ T9 Hot water outlet temperature of the absorption chiller, C ◦ T10 Chilled water outlet temperature of the absorption chiller, C ◦ T11 Chilled water inlet temperature of the absorption chiller, C ◦ T12 Cooling water outlet temperature of the absorption chiller, C ◦ T13 Cooling water inlet temperature of the absorption chiller, C ◦ T14 Upper layer temperature of the buffer tank, C ◦ T15 Lower layer temperature of the buffer tank, C ◦ T16 Inlet temperature of the fan coils, C ◦ T17 Outlet temperature of the fan coils, C η Collector efficiency

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