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

: 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 m 2 , 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.


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 Figure 1 presents the monthly average outdoor temperature and global solar irradiation in Ningbo City in 2018. The average outdoor temperature during July, the hottest month, is 27.6 • C, whereas the average outdoor temperature during January, the coldest month, is 6 • C. Thus, air conditioning systems are necessary for both cooling in summer and heating in winter. The average sunshine hours are 1855. 6 h per year and the average daily sunshine hours are 5.41 h. The total annual solar radiation is 1280.5 kWh/m 2 , which is 78% of the average value of 1627.8 kWh/m 2 in China.
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 m 2 . An electric boiler was installed as an auxiliary heat source. When the ambient temperature was about 30.3 °C during the experimental period, the average solar collecting 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]. Due to the intermittent nature of solar energy, a backup system is required in solar cooling and heating systems to meet the building load demand when solar inputs are insufficient. The backup system can use either gas or electricity as the auxiliary energy source [16]. Most of the above-mentioned solar cooling and heating systems used a gas boiler as an auxiliary heat source. In this paper, a new system incorporated with two air-source heat pumps as the auxiliary system was installed in Ningbo City, which is located in the hot summer and cold winter zone in China, and it has been operating since 2018. This paper presents the installation and performance evaluation of the solar cooling and heating system during the summer cooling and winter heating period, as well as on a typical summer day, namely, 23 July 2018. Figure 1 presents the monthly average outdoor temperature and global solar irradiation in Ningbo City in 2018. The average outdoor temperature during July, the hottest month, is 27.6 °C, whereas the average outdoor temperature during January, the coldest month, is 6 °C. Thus, air conditioning systems are necessary for both cooling in summer and heating in winter. The average sunshine hours are 1855. 6 h per year and the average daily sunshine hours are 5.41 h. The total annual solar radiation is 1280.5 kWh/m 2 , which is 78% of the average value of 1627.8 kWh/m 2 in China.   Figure 2 shows the hourly average outdoor temperature and global solar irradiation on 23 July 2018. The strongest solar radiation occurred from 9:00 to 13:00, while the temperatures exceeded 32 °C between 11:00 and 14:00, which also corresponded to the maximal cooling energy demand during the day.  Figure 2 shows the hourly average outdoor temperature and global solar irradiation on 23 July 2018. The strongest solar radiation occurred from 9:00 to 13:00, while the temperatures exceeded 32 • C between 11:00 and 14:00, which also corresponded to the maximal cooling energy demand during the day.

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 m 2 and a total height of 6.9 m. The specific cooling and heating power demands are 120 W/m 2 and 82 W/ m 2 , 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.

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 m 2 and a total height of 6.9 m. The specific cooling and heating power demands are 120 W/m 2 and 82 W/ m 2 , 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.

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 m 2 and a total height of 6.9 m. The specific cooling and heating power demands are 120 W/m 2 and 82 W/ m 2 , 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.

System Installation
The installation is a solar collector-based thermal cooling system where a LiBr-water absorption chiller is driven by hot water from the solar collectors. As shown in Figure 4, it consists of two arrays of evacuated tube collectors, an air-cooled radiator to prevent overheating of the solar collectors, an absorption chiller, a cooling tower, a hot water storage tank, a buffer tank, and two air-source heat pumps as an auxiliary system. During the summer cooling time, the solar collector field transfers solar radiation into thermal energy to produce hot water at a temperature range of 70-110 • C, which is stored in the hot water storage tank. The absorption chiller is driven by the hot water and produces chilled water at a temperature of 10-15 • C, which is stored in the buffer tank for building cooling. The cooling tower is used to extract waste heat from the chiller. The heat pumps additionally provide chilled water when solar energy is insufficient. During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 • 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.

System Installation
The installation is a solar collector-based thermal cooling system where a LiBr-water absorption chiller is driven by hot water from the solar collectors. As shown in Figure 4, it consists of two arrays of evacuated tube collectors, an air-cooled radiator to prevent overheating of the solar collectors, an absorption chiller, a cooling tower, a hot water storage tank, a buffer tank, and two air-source heat pumps as an auxiliary system. During the summer cooling time, the solar collector field transfers solar radiation into thermal energy to produce hot water at a temperature range of 70-110 °C, which is stored in the hot water storage tank. The absorption chiller is driven by the hot water and produces chilled water at a temperature of 10-15 °C, which is stored in the buffer tank for building cooling. The cooling tower is used to extract waste heat from the chiller. The heat pumps additionally provide chilled water when solar energy is insufficient. During the winter heating time, the absorption chiller is switched off, and the hot water with a temperature range between 45 °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.

The Solar Collector Field
The main heat driving 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 Figure 5 (left). It

The Solar Collector Field
The main heat driving 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 Figure 5 (left). It comprises 40 all-glass evacuated tube modules in two parallel arrays with a total aperture area of 120 m 2 . Each module contains 18 evacuated tubes and has an aperture area of 3.0 m 2 .

The Absorption Chiller
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 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 m 3 /h; the chilled water design inlet and outlet temperatures were 15 • C and 10 • C respectively, and the rated flow was 6.0 m 3 /h; the cooling water design inlet and outlet temperatures were 30 • C and 35 • C, respectively, and the rated flow was 15.0 m 3 /h. The cooling tower had a rated water flow of 25.0 m 3 /h. machine associated with a cooling tower shown in Figure 6 (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 m 3 /h; the chilled water design inlet and outlet temperatures were 15 °C and 10 °C respectively, and the rated flow was 6.0 m 3 /h; the cooling water design inlet and outlet temperatures were 30 °C and 35 °C, respectively, and the rated flow was 15.0 m 3 /h. The cooling tower had a rated water flow of 25.0 m 3 /h.

The Thermal Energy Storage
In order to make rational use of solar energy and to maximize the energy efficiency, two pressure storage tanks were used in the installation, which were a hot water storage tank with a °C respectively, and the rated flow was 6.0 m 3 /h; the cooling water design inlet and outlet temperatures were 30 °C and 35 °C, respectively, and the rated flow was 15.0 m 3 /h. The cooling tower had a rated water flow of 25.0 m 3 /h.

The Thermal Energy Storage
In order to make rational use of solar energy and to maximize the energy efficiency, two pressure storage tanks were used in the installation, which were a hot water storage tank with a

The Thermal Energy Storage
In order to make rational use of solar energy and to maximize the energy efficiency, two pressure storage tanks were used in the installation, which were a hot water storage tank with a volume of 4 m 3 and a buffer tank with a volume of 1 m 3 . The hot water storage tank had built-in heat exchanger coils and the heat transfer capacity was 58 kW.

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.

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 I solar , the temperatures T 0 -T 17 , the mass flow rates F 1 -F 4 , and the electrical consumption E 1 were recorded by the monitoring system. Table 2 presents the instruments and their calibration range and uncertainty.

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 presents the instruments and their calibration range and uncertainty. Figure 7.The schematic of the controlling and monitoring system:T0-the ambient temperature; T1-the temperature of the collector array 1; T2-the temperature of the collector array 2; T3-the outlet temperature of the collector field; T4-the upper layer temperature of the hot water storage tank; T5-the lower layer temperature of the hot water storage tank; T6-the inlet temperature of the collector field; T7-the antifreeze temperature of the collector field; T8-the hot water inlet temperature of the absorption chiller; T9-the hot water outlet temperature of the absorption chiller;T10-the chilled water outlet temperature of the absorption chiller; T11-the chilled water inlet temperature of the absorption chiller; T12-the cooling water outlet temperature of the absorption chiller; T13-the cooling water inlet temperature of the absorption chiller; T14-the upper layer temperature of the buffer tank; T15-the lower layer temperature of the buffer tank;T16-the inlet temperature of the fan coils; T17-the outlet temperature of the fan coils; Isolar-solar radiation; F1-the inlet flow rate of the collector field; F2-the hot water inlet flow rate of the absorption chiller; F3-the chilled water inlet flow rate of the absorption chiller; F4-the inlet flow rate of the fan coils; E1-the electrical consumption of the heat pumps.  Figure 7. The schematic of the controlling and monitoring system: T 0 -the ambient temperature; T 1 -the temperature of the collector array 1; T 2 -the temperature of the collector array 2; T 3 -the outlet temperature of the collector field; T 4 -the upper layer temperature of the hot water storage tank; T 5 -the lower layer temperature of the hot water storage tank; T 6 -the inlet temperature of the collector field; T 7 -the antifreeze temperature of the collector field; T 8 -the hot water inlet temperature of the absorption chiller; T 9 -the hot water outlet temperature of the absorption chiller; T 10 -the chilled water outlet temperature of the absorption chiller; T 11 -the chilled water inlet temperature of the absorption chiller; T 12 -the cooling water outlet temperature of the absorption chiller; T 13 -the cooling water inlet temperature of the absorption chiller; T 14 -the upper layer temperature of the buffer tank; T 15 -the lower layer temperature of the buffer tank; T 16 -the inlet temperature of the fan coils; T 17 -the outlet temperature of the fan coils; I solar -solar radiation; F 1 -the inlet flow rate of the collector field; F 2 -the hot water inlet flow rate of the absorption chiller; F 3 -the chilled water inlet flow rate of the absorption chiller; F 4 -the inlet flow rate of the fan coils; E 1 -the electrical consumption of the heat pumps. The installation had two operational modes, namely, the building cooling and heating modes. During the cooling period in summer, the valves V1-V6 and V9-V18 were open and V7 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 T 3 − T 4 ≥ 4 • C; the pump, P1, was switched off when the temperature difference T 3 − T 4 < 2 • C.
(2) The cooling loop The pump, P2, was switched on when the upper layer temperature of the hot water storage tank T 4 ≥ 80 • CAND the fan coils were turned on; the pump, P2, was switched off when T 4 < 70 • COR the fan coils were shut down.

(3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T 15 > 18 • C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T 15 ≤ 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 T 3 − T 4 ≥ 4 • C; the pump, P1, was switched off when the temperature difference T 3 − T 4 < 2 • C.
(2) The heating loop The pump, P2, was switched on when the lower layer temperature of the buffer tank T 15 < 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 T 4 − T 15 ≥ 2 • C; the pump, P2, was switched on when the lower layer temperature of the buffer tank T 15 ≥ 60 • C OR T 4 ≤ T 15 . (

3) The heat pump loop
The heat pumps and pump P6 were switched on when the lower layer temperature of the buffer tank T 15 < 45 • C AND the fan coils were turned on; the heat pumps and pump P6 were switched off when T 15 ≥ 50 • C OR the fan coils were shut down.

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.

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." Figure 8 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. Figure 9 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.

Solar Collector Field
The collector efficiency η is given by Equation (1): where Q solar and Q sc represent the total solar energy available and solar energy collected; I solar is the solar irradiation; A solar is the aperture area of the solar collectors; F 1 is the inlet mass flow rate of the collector filed; c p,w is the specific heat of water; T 3 and T 6 are the outlet and inlet temperatures of the solar collection loop, respectively; and η is the collector efficiency, with an uncertainty of 3.3% due to the measurement errors. Figure 10 presents the monthly solar irradiation onto the collector area and the collector field yield. The solar irradiation onto the collector area from May to October was 94,878 kWh and the collector field yield was 41,849 kWh during the summer cooling time. The annual average collector efficiency for building cooling was 44%, with a maximum of 47% in August. On the other hand, the solar irradiation onto the collector area from December to March was 42,709 kWh and the collector field yield was 18,221 kWh during the winter heating time. The annual average collector efficiency for building heating was 42%. Figure 11 shows the hourly solar irradiation onto the collector area and the collector field yield on 23 July 2018. The daily solar irradiation and collector field yield were 627.7 kWh and 308.5 kWh, respectively. The daily average collector efficiency was 49%.
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 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 to the measurement errors. Figure 10 presents the monthly solar irradiation onto the collector area and the collector field yield. The solar irradiation onto the collector area from May to October was 94,878 kWh and the collector field yield was 41,849 kWh during the summer cooling time. The annual average collector efficiency for building cooling was 44%, with a maximum of 47% in August. On the other hand, the solar irradiation onto the collector area from December to March was 42,709 kWh and the collector field yield was 18,221 kWh during the winter heating time. The annual average collector efficiency for building heating was 42%. Figure 11 shows the hourly solar irradiation onto the collector area and the collector field yield on 23 July 2018. The daily solar irradiation and collector field yield were 627.7 kWh and 308.5 kWh, respectively. The daily average collector efficiency was 49%.

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):

Absorption Chiller
The coefficient of performance of the absorption chiller COP chiller 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): where Q solar , Q sc and Q c represent the total solar energy available, solar energy collected, and cooling energy generated by the chiller driven by solar; I solar is the solar irradiation; A solar is the aperture area of the solar collectors; F 1 and F 3 are the mass flow rate of the hot water of the absorption chiller and that of the chilled water, respectively; c p,w is the specific heat of water; T 3 and T 6 are the outlet and inlet temperatures of the solar collection loop, respectively; T 10 and T 11 are the chilled water outlet and inlet temperatures of the absorption chiller, respectively; COP chiller 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 COP chiller ranged between 0.68 and 0.76. The yearly average COP chiller was 0.73.  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 reached its highest temperature. It was found that the COPchiller increased with the rising hot water inlet temperature.   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 COP chiller was 0.76, with maximums of 0.78 at 13:00 and 14:00 when the inlet hot water reached its highest temperature. It was found that the COP chiller increased with the rising hot water inlet temperature. 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 reached its highest temperature. It was found that the COPchiller increased with the rising hot water inlet temperature.

Heat Pumps
The coefficient of performance of the heat pumps COPhp is defined as the ratio of cooling/heating energy yield and electrical consumption. It is determined on the basis of the Equations (3) and (4) (4) Figure 14. The hourly hot water, chilled water, and cooling water inlet and outlet temperatures on 23 July 2018.

Heat Pumps
The coefficient of performance of the heat pumps COP hp is defined as the ratio of cooling/heating energy yield and electrical consumption. It is determined on the basis of the Equations (3) and (4): The solar fraction SF n 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): where Q t , Q c , and Q h are the total cooling or heating energy used, cooling energy generated by the chiller driven by solar and heating energy generated by solar, respectively; F 2 , F 3 , and F 4 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; c p,w is the specific heat of water; T 4 and T 5 are the upper layer temperature and lower layer temperature of the hot water storage tank, respectively; T 10 and T 11 are the chilled water outlet and inlet temperatures of the absorption chiller, respectively; T 16 and T 17 are the inlet temperature and outlet temperature of the fan coils, respectively; E 1 is the electrical consumption of the heat pumps; COP hp,c and COP hp,h are the 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; SF n,c and SF n,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 COP hp in 2018. The heat pumps produced a total of 21,019 kWh cooling energy and 9104 kWh heating energy in 2018, corresponding to the electrical consumption of 7439 kWh for cooling and 2851 kWh for heating, respectively. The average coefficient of performance for cooling COP hp,c was 2.83, with minimums of 2.74 and 2.75 in July and August, respectively. The average coefficient of performance for heating COP hp,h was 3.20.
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.  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 shown in Figure 16, the energy yield and the solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto the collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh 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% of those values on 23 July when incorporated with the heat pumps. 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  Figure 17 shows the monthly solar fraction SF n and electricity savings in 2018. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. This means that 43.4% of the annual building cooling demand and 37.5% of the annual building heating demand were covered by the heat pumps. shown in Figure 16, the energy yield and the solar fraction distinctly decreased without the heat pumps as the auxiliary system. The daily global irradiation onto the collector area was 661.6 kWh on 28 July, which was similar to that of 627.7 kWh 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% of those values on 23 July when incorporated with the heat pumps. Figure 17 shows the monthly solar fraction SFn and electricity savings in 2018. The annual mean solar fractions for cooling and heating were 56.6% and 62.5%, respectively. This means that 43.4% of the annual building cooling demand and 37.5% of the annual building heating demand were covered by the heat pumps. 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.  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 CO 2 emissions [6]. There are 536 g of CO 2 emitted into the atmosphere for each kWh of electricity produced. Thus, the conversion factor to calculate the quantities of CO 2 emitted is f c = 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 COP hp,c of 2.83 and mean heating COP hp,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 CO 2 emissions prevented from being released into the atmosphere.

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 m 2 . 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 COP chiller ranged between 0.68 and 0.76 in 2018. The COP chiller 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 COP hp,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 COP hp,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 CO 2 emissions prevented from being released into the atmosphere.