Design and Transient Analysis of a Natural Gas-Assisted Solar LCPV / T Trigeneration System

: This paper proposes a natural gas assisted solar low-concentrating photovoltaic / thermal trigeneration (NG-LCPV / T-TG) system. This novel system simultaneously provides electrical, thermal and cooling energy to the user. The design and dynamic simulation performance of the NG-LCPV / T-TG system is completed using Transient System Simulation (TRNSYS) software. The results show that the system can satisfy the requirements of the cooling and heating load. The proposed system maintains the experimental room temperature at about 25 ◦ C under the cooling mode, at about 20 ◦ C under the heating mode. The electrical and thermal energy produced by the low-concentrating photovoltaic / thermal (LCPV / T) system are 3819 kWh and 18,374 kWh. Meanwhile, the maximum coe ﬃ cient of performance (COP) of the low temperature heat pump (LHP), high temperature heat pump (HHP) and chiller are 5, 2.2 and 0.6, respectively. This proposed system realizes the coupling of natural gas and solar energy in a building. In summary, this trigeneration system is feasible and it promotes the implementation of building integrated high-e ﬃ ciency energy supply system. addition, since the selection of the three pumps is based on the data of previous experiment, the energy consumption of the pumps is too large. Future research will further optimize the energy consumption of the pumps.


Introduction
There is no doubt that China is the country with the highest energy consumption in the world. Unlike other countries with slow energy growth, China's energy growth accelerated in 2019 [1]. Within this growth, the proportion of coal consumption showed a downward trend, while the total consumption of renewable energy continued to increase [2].
However, the utilization of renewable energy was restricted by environmental and technological factors. Therefore, a complementary system of non-renewable energy and renewable energy came into being [3]. Additionally, solar energy is a clean and inexhaustible renewable energy, while natural gas is a non-renewable energy with the characteristics of high calorific value, convenient transportation, stable and continuous [4,5]. In the early and mid 21st century, the combination of solar energy and natural gas has been listed as the development goal of solar thermal utilization [6].
The concentrating solar photovoltaic/thermal (CPV/T) system uses water as a circulating cooling medium. The cooling water reduces the working temperature of photovoltaic (PV) cells and improves photovoltaic conversion efficiency [7]. The CPV/T system significantly improves the comprehensive utilization rate of solar energy and realizes high efficient power generation [8]. Due to the concentrator, the efficiency of the CPV/T system is higher than that of the conventional PV/T system. In particular, the thermal and electrical efficiency of the LCPV/T system has been shown to reach 54.48% and 14.49%, respectively [9]. Haiping Chen et al. designed a distributed compound parabolic concentrator (WD-CPC). The result showed that the light uniformity value was 0.153, which decreased the light non-uniformity about 0.13 or less compared with the flat-plate compound parabolic concentrator [10]. triple-generation system was proposed by Liu Yang et al. [27]. This system could produce hot water at 45-90 • C. Meanwhile, the electrical efficiency of the LCPV/T system was approximately 10%. The COP of the single-effect lithium bromide absorption chiller was above 0.5. To summarize, there are few studies on the dynamic characteristics of the trigeneration system that based on a CPV/T system and natural gas.
As discussed above, the trigeneration system has attracted the attention of a lot of scientists. However, research on the multi-source complementary system combining CPV/T technology with natural gas is extremely scarce. Therefore, this paper proposed a NG-LCPV/T-TG system in a TRNSYS environment. The electrical and thermal energy are produced by the LCPV/T system. This system uses different heat pumps as a primary auxiliary device and natural gas heater as a secondary auxiliary device. A novel form of energy cascade utilization is proposed in this paper. In addition, this system achieves a balance between supply and demand according to the requirements of the building. The temperature of the experimental room was controlled at 20 • C in winter and 25 • C in summer. Subsequently, the COP value of each equipment was improved. This research not only proposes a new energy utilization method, but also proves the feasibility of this novel system by one-year data. In particular, this research has a crucial effect on the promoting the comprehensive utilization of traditional energy and renewable energy.

Systems Description
A design and transient analysis of the NG-LCPV/T-TG system is carried out by TRNSYS software [28]. The proposed system is based on the previous research of the LCPV/T triple-generation system [27], adding an auxiliary natural gas heater to ensure the indoor temperature stable. In particular, the proposed model could simultaneously satisfy the demands of space cooling and heating, electricity, and domestic hot water. In order to simulate the dynamic characteristics of the proposed system throughout one year, the local weather condition needs to be determined. The location selected is Beijing (40 • 09 33" N, 116 • 31 47" E), China. The building module is established using TRNBUILD in TRNSYS software.
In particular, the proposed system is based on an LCPV/T system. The simplified layout of the NG-LCPV/T-TG system is depicted in Figure 1. Moreover, there are two water source heat pumps in the proposed model: a low-temperature water source heat pump (LHP) using for space heating and a high-temperature water source heat pump (HHP) using for space cooling. For cooling purposes, a single-effect absorption lithium bromide chiller is adopted. Additionally, the capillary net system is used as the heat dissipation terminal. The electricity produced by the LCPV/T module is stored in the accumulator, which is used by the user or connected to the grid.

Operating Principle
The operating principle of this trigeneration system is briefly described as follows. The LCPV/T modules is applied as the main heat source. In addition, a natural gas heater and heat pumps are applied for auxiliary thermal energy, and the absorption chiller is used to achieve cooling energy. The operation control is based on two parameters. The temperature of the experimental room is the first parameter, which is the basic parameter to control the starting and stopping of the trigeneration system. The second parameter is the temperature of storage tank 1. In order to maintain the temperature comfortable, the room temperature is set according to the design manual. During winter, the room temperature is maintained at 20 • C. When the room temperature is below 20 • C, the heating mode is opened. Likewise, the room temperature is maintained at 25 • C in summer. Similar to the heating mode, when the room temperature is above 25 • C, the cooling mode is opened.

Operating Principle
The operating principle of this trigeneration system is briefly described as follows. The LCPV/T modules is applied as the main heat source. In addition, a natural gas heater and heat pumps are applied for auxiliary thermal energy, and the absorption chiller is used to achieve cooling energy. The operation control is based on two parameters. The temperature of the experimental room is the first parameter, which is the basic parameter to control the starting and stopping of the trigeneration system. The second parameter is the temperature of storage tank 1. In order to maintain the temperature comfortable, the room temperature is set according to the design manual. During winter, the room temperature is maintained at 20 °C. When the room temperature is below 20 °C, the heating mode is opened. Likewise, the room temperature is maintained at 25 °C in summer. Similar to the heating mode, when the room temperature is above 25 °C, the cooling mode is opened.
The temperature of storage tank 1 determines which device to use as a heat source for heating or cooling.
1. The schematic diagram of the NG-LCPV/T-TG in heating model is demonstrated in Figure 2. Loop 1, called the direct supply mode, is the cycle of NG-LCPV/T-TG system when the temperature of storage tank 1 is greater than or equal to 40 °C. This mode directly uses LCPV/T system as the heat source for space heating. When the temperature of storage tank 1 is below 40 °C but greater than 15 °C, the loop 2 mode is adopted. In loop 2, the outlet water of LCPV/T is used as the heat source of the LHP. This operational mode of loop 2 improves the comprehensive utilization efficiency of LCPV/T system, and the COP of LHP. Finally, when the temperature of storage tank 1 is below 15 °C, a natural gas auxiliary heater (NGAH) is used as the auxiliary heat source. In loop 3, LCPV/T system and LHP are all closed, only natural gas is used for space heating. The temperature of storage tank 1 determines which device to use as a heat source for heating or cooling.
1. The schematic diagram of the NG-LCPV/T-TG in heating model is demonstrated in Figure 2. Loop 1, called the direct supply mode, is the cycle of NG-LCPV/T-TG system when the temperature of storage tank 1 is greater than or equal to 40 • C. This mode directly uses LCPV/T system as the heat source for space heating. When the temperature of storage tank 1 is below 40 • C but greater than 15 • C, the loop 2 mode is adopted. In loop 2, the outlet water of LCPV/T is used as the heat source of the LHP. This operational mode of loop 2 improves the comprehensive utilization efficiency of LCPV/T system, and the COP of LHP. Finally, when the temperature of storage tank 1 is below 15 • C, a natural gas auxiliary heater (NGAH) is used as the auxiliary heat source. In loop 3, LCPV/T system and LHP are all closed, only natural gas is used for space heating.
2. The schematic diagram of the NG-LCPV/T-TG in cooling model is demonstrated in Figure 3. When the temperature of storage tank 1 is greater than 40 • C, the loop 1 mode is adopted. In loop 1, the storage tank 1 is the heat source of the HHP. The HHP can provide hot water about 90 • C to the single-effect absorption lithium bromide chiller. When the temperature of the storage tank 1 is less than 40 • C, the natural gas auxiliary (NGAC) is used as the heat source of the lithium bromide chiller. 2. The schematic diagram of the NG-LCPV/T-TG in cooling model is demonstrated in Figure 3. When the temperature of storage tank 1 is greater than 40 °C, the loop 1 mode is adopted. In loop 1, the storage tank 1 is the heat source of the HHP. The HHP can provide hot water about 90 °C to the single-effect absorption lithium bromide chiller. When the temperature of the storage tank 1 is less than 40 °C, the natural gas auxiliary (NGAC) is used as the heat source of the lithium bromide chiller.   Figure 3. When the temperature of storage tank 1 is greater than 40 °C, the loop 1 mode is adopted. In loop 1, the storage tank 1 is the heat source of the HHP. The HHP can provide hot water about 90 °C to the single-effect absorption lithium bromide chiller. When the temperature of the storage tank 1 is less than 40 °C, the natural gas auxiliary (NGAC) is used as the heat source of the lithium bromide chiller.   3. When neither space heating nor space cooling is needed, the trigeneration system provides domestic hot water.

Simulation Model
The main components of the proposed NG-LCPV/T-TG system are listed in Table 1. Table 1 lists the corresponding modules in TRNSYS software and briefly describes the equipment. All the components are implemented in the TRNSYS environment. In particular, the design parameters in the proposed system are set according to the previous experimental work [27]. Thermal storage tank that stores the cold water circulating from the cooling /heating system, at the same time use it as the source of the LCPVT system Room temperature controller Type 108 A Module that can control the cooling or heating mode of the experimental room controller Type 2 The on/off differential controller generates that can control the on/off of devices in the proposed system

Building Model
The experimental room was built using TRNBUILD in TRNSYS software. In addition, the capillary net is laid on the wall as the heat dissipation terminal. The detailed design parameters of the experimental room are listed in Table 2. The cooling and heat load of one year are calculated by the TRNSYS software. Moreover, the start and stop of equipment in the system are controlled based on the feedback of the temperature of the experimental room. As described above, the results simulated by the building model are closer to the actual test value.

LCPV/T Model
In the proposed system, the parameters of each LCPV/T modules are listed in Table 3. Each group of LCPV/T module is composed of four photovoltaic (PV) cells. The cooling channels of the PV cells are connected in series. To ensure the supply of the heat and the flow, six groups of LCPV/T modules are connected in parallel. In particular, a dual-axis tracking mode is applied in the proposed system.

Heat Pump Model
A low-temperature water source heat pump (LHP) is applied in the heating mode and a high-temperature water source heat pump (HHP) is used as auxiliary device in the cooling mode. This combination mode is determined according to the previous experimental construction. Besides, the electrical energy produced by the LCPV/T system is used for driving heat pumps and circulating pumps.

Low Temperature Water to Water Heat Pump
The requirement of heat load is not satisfied by a single LCPV/T system when the temperature of storage tank 1 is between 15-40 • C. In this case, the LHP is turned on and the heat source comes from storage tank 1. Under the closed-loop condition, the performance of LHP is significantly improved. Meanwhile, the inlet temperature of LCPV/T system decreases and the comprehensive efficiency is enhanced. When the temperature of storage tank 1 is below 15 • C, LHP is switched off because of the low energy efficiency. At this time, auxiliary natural gas heater directly provides hot water for space heating. The detail parameters of LHP are shown in Table 4.

High Temperature Water to Water Heat Pump
The high temperature water source heat pump (HHP) is used to provide 85 • C-90 • C hot water as the heat source for the lithium bromide absorption chiller. The detailed parameters of HHP are shown in Table 5.

Lithium Bromide Absorption Chiller
For space cooling, a single-effect lithium bromide absorption chiller is employed during summer. The reason for choosing the absorption chiller is that it can effectively utilize the thermal energy generated by the LCPV/T system and improve the comprehensive utilization rate of the NG-LCPV/T-TG system. The parameters of the chiller are demonstrated in Table 6. Table 6. Description of the lithium bromide absorption chiller.

Parameter Value Unit
Rated capacity 10 kW Rated COP 0.6 -Auxiliary electrical power 0.5 kW

Cooling Tower
The function of cooling tower is to cool down the steam in condenser into water. The cooling tower uses cold water as a circulating coolant. Meanwhile, the absorbed heat is discharged to the atmosphere with steam. The parameters of the cooling tower are demonstrated in Table 7.

Natural Gas Auxiliary Heater
The output performance of the LCPV/T system is not stable due to the instability of solar energy. Additionally, natural gas is a traditional fossil energy with low pollution, low emission and high calorific value. The coupling of the two sorts of energy could reduce the irreversible loss of natural gas and enhance the utilization efficiency of solar energy. Therefore, the auxiliary natural gas heater is adopted in the proposed system to ensure the efficient and stable operation throughout one year. The auxiliary natural gas heater 1 is set under the heating mode, while the auxiliary natural gas heater 2 is set under the cooling mode. Detailed parameters of the auxiliary natural gas heater are manifested in Table 8.

Pumps
The system consists of 5 circulating pumps, which make up the cycle of the NG-LCPV/T-TG system. The function of pump 1 is to import water from LCPV/T system into storage tank 1. Pump 2 is used to support the heating cycle of the experimental room. The function of pump 3 is to realize the cycle of hot water which can drive the absorption chiller. The pump 4 is used to support the cycle of cooling tower. Pump 5 is applied to realize the coolant cycle of the experimental room. The specific parameters of the circulating pumps are listed in Appendix A- Table A1.

Water Storage Tanks
Two storage tanks are installed in the proposed system. Storage tank 1 is utilized as the heat source of the NG-LCPV/T-TG system. It stores the hot water heated by LCPV/T system. Storage tank 2 works as the inlet water tank of the LCPV/T system. The basic parameters of the tanks are demonstrated in Appendix A- Table A2.

Results and Discussion
Since this research is to investigate the full-condition performance of the NG-LCPV/T-TG system, the data of continuous operation in one year is simulated. The experimental data in reference [26] is Energies 2020, 13, 5930 10 of 24 used for the verification of the model used in this article. Take the COP value of the lithium bromide absorption chiller, as a parameter for comparison. The average deviation value of the modeling results with the experimental results is 10%, and the maximum value is 18%. These errors are mainly caused due to the start and stop of the equipment in the experiment, which causes the COP value to be lower than the simulated value. However, an acceptable range is observed among the results. Therefore, the models presented in this paper are feasible and reliable to simulate the performance of the trigeneration system.
The location simulated by the proposed system is Beijing, China. Beijing is a city with four distinct seasons throughout the year. Outdoor meteorological parameters mainly focus on solar radiation intensity, wind speed and outdoor ambient temperature. The annual variation of these three parameters are manifested in Figure 4. This meteorological parameter uses the CN-Beijing-545110.tm2 from Meteonorm in TRNSYS software. Both load of the experimental room and LCPV/T performance are affected by outdoor meteorological parameters.
The location simulated by the proposed system is Beijing, China. Beijing is a city with four distinct seasons throughout the year. Outdoor meteorological parameters mainly focus on solar radiation intensity, wind speed and outdoor ambient temperature. The annual variation of these three parameters are manifested in Figure 4. This meteorological parameter uses the CN-Beijing-545110.tm2 from Meteonorm in TRNSYS software. Both load of the experimental room and LCPV/T performance are affected by outdoor meteorological parameters.
Moreover, the average beam radiation intensity of Beijing in summer is lower than that in winter. This is because Beijing is frequently rainy in summer. The daul-axis tracking system is applied such that the beam radiation is always normal to the surface of PV cells. Because the low-concentrating technology is adopted, only beam radiation intensity has an impact on the performance of the proposed system. In Figure 4, the average value of beam radiation intensity per month is calculated according to the sum of hours that has solar radiation each day. The maximum and minimum amount of the average beam radiation intensity are 391.8 W/m 2 in February and 185.7 W/m 2 in July. In addition, the hours for calculating average value of ambient temperature and wind velocity are the sum of hours in a whole day per month. The detailed number of hours used for calculating the average value per month are listed in Appendix A- Table A3. The ambient temperature follows a seasonal law such that is high in summer and low in winter. In winter, it is maintained at around 0 °C. The height measurement for wind velocity is 10 m. The wind speed is slightly higher in March and April. In order to maintain comfortable, the temperature of the experimental room is controlled around 20 °C during winter and around 25 °C during summer. The allowable fluctuation range is 1 °C. A Moreover, the average beam radiation intensity of Beijing in summer is lower than that in winter. This is because Beijing is frequently rainy in summer. The daul-axis tracking system is applied such that the beam radiation is always normal to the surface of PV cells. Because the low-concentrating technology is adopted, only beam radiation intensity has an impact on the performance of the proposed system. In Figure 4, the average value of beam radiation intensity per month is calculated according to the sum of hours that has solar radiation each day. The maximum and minimum amount of the average beam radiation intensity are 391.8 W/m 2 in February and 185.7 W/m 2 in July. In addition, the hours for calculating average value of ambient temperature and wind velocity are the sum of hours in a whole day per month. The detailed number of hours used for calculating the average value per month are listed in Appendix A- Table A3. The ambient temperature follows a seasonal law such that is high in summer and low in winter. In winter, it is maintained at around 0 • C. The height measurement for wind velocity is 10 m. The wind speed is slightly higher in March and April.
In order to maintain comfortable, the temperature of the experimental room is controlled around 20 • C during winter and around 25 • C during summer. The allowable fluctuation range is 1 • C. A reference room that is not provided space heating and cooling is set for comparison. As demonstrated in Figure 5, the temperature of the reference room is associated with the outdoor ambient temperature. In contrast, the temperature of the experimental room is comfortable throughout the year due to the effect of the NG-LCPV/T-TG system.
Energies 2020, 13, x FOR PEER REVIEW 11 of 23 reference room that is not provided space heating and cooling is set for comparison. As demonstrated in Figure 5, the temperature of the reference room is associated with the outdoor ambient temperature. In contrast, the temperature of the experimental room is comfortable throughout the year due to the effect of the NG-LCPV/T-TG system.  Figure 6 depicts variations of the electrical and thermal energy produced by the LCPV/T system during a day. In this paper, the 240th hour to 264th hour (24 h) is randomly selected as a typical day. The maximum amount of the electrical and thermal energy are 2.4 kW and 11.1 kW, respectively. The cumulative value of the electrical energy is 16.4 kWh, and the cumulative thermal energy is 73.5 kWh. In summary, the LCPV/T system has excellent capacity for producing electrical and thermal energy simultaneously.   Figure 6 depicts variations of the electrical and thermal energy produced by the LCPV/T system during a day. In this paper, the 240th hour to 264th hour (24 h) is randomly selected as a typical day. The maximum amount of the electrical and thermal energy are 2.4 kW and 11.1 kW, respectively. The cumulative value of the electrical energy is 16.4 kWh, and the cumulative thermal energy is 73.5 kWh. In summary, the LCPV/T system has excellent capacity for producing electrical and thermal energy simultaneously.
Energies 2020, 13, x FOR PEER REVIEW 11 of 23 reference room that is not provided space heating and cooling is set for comparison. As demonstrated in Figure 5, the temperature of the reference room is associated with the outdoor ambient temperature. In contrast, the temperature of the experimental room is comfortable throughout the year due to the effect of the NG-LCPV/T-TG system. Figure 5. The temperatures of experimental, reference room, and ambient throughout one year. Figure 6 depicts variations of the electrical and thermal energy produced by the LCPV/T system during a day. In this paper, the 240th hour to 264th hour (24 h) is randomly selected as a typical day. The maximum amount of the electrical and thermal energy are 2.4 kW and 11.1 kW, respectively. The cumulative value of the electrical energy is 16.4 kWh, and the cumulative thermal energy is 73.5 kWh. In summary, the LCPV/T system has excellent capacity for producing electrical and thermal energy simultaneously.  As shown in Figures 7 and 8, the electrical and thermal energy produced by the LCPV/T system during one year is presented. At the 7883th hour, when the radiation intensity is 776 W/m 2 , the maximum amount of electrical energy is 2.53 kW. At the 7885th hour, when the radiation intensity is 805 W/m 2 , the maximum amount of thermal energy is 13.85 kW. The electrical energy and thermal energy are affected by the radiation intensity.
Energies 2020, 13, x FOR PEER REVIEW 12 of 23 As shown in Figures 7 and 8, the electrical and thermal energy produced by the LCPV/T system during one year is presented. At the 7883th hour, when the radiation intensity is 776 W/m 2 , the maximum amount of electrical energy is 2.53 kW. At the 7885th hour, when the radiation intensity is 805 W/m 2 , the maximum amount of thermal energy is 13.85 kW. The electrical energy and thermal energy are affected by the radiation intensity.   As shown in Figures 7 and 8, the electrical and thermal energy produced by the LCPV/T system during one year is presented. At the 7883th hour, when the radiation intensity is 776 W/m 2 , the maximum amount of electrical energy is 2.53 kW. At the 7885th hour, when the radiation intensity is 805 W/m 2 , the maximum amount of thermal energy is 13.85 kW. The electrical energy and thermal energy are affected by the radiation intensity.    The outlet water temperature on a day and a year of the LCPV/T system are obviously obtained in Figures 9 and 10, respectively. The supreme outlet temperature in a day is detected at nearly 45 • C. In addition, the outlet temperature gradually rises from 08:00 and increases rapidly at 10:00, then decreases until 16:00. The outlet temperature in one year is around 45 • C, which only reaches 65 • C for a few days in summer. The lowest temperature in winter is relatively low, while that in summer is about 20 • C.
Energies 2020, 13, x FOR PEER REVIEW 13 of 23 The outlet water temperature on a day and a year of the LCPV/T system are obviously obtained in Figures 9 and 10, respectively. The supreme outlet temperature in a day is detected at nearly 45 °C. In addition, the outlet temperature gradually rises from 08:00 and increases rapidly at 10:00, then decreases until 16:00. The outlet temperature in one year is around 45 °C, which only reaches 65 °C for a few days in summer. The lowest temperature in winter is relatively low, while that in summer is about 20 °C.  In particular, the temperature of the storage tank is not below zero in the non-operation stage of LCPV/T system (shown in Figure 11) due to the heat storage property. Because of the thermal inertia,  The outlet water temperature on a day and a year of the LCPV/T system are obviously obtained in Figures 9 and 10, respectively. The supreme outlet temperature in a day is detected at nearly 45 °C. In addition, the outlet temperature gradually rises from 08:00 and increases rapidly at 10:00, then decreases until 16:00. The outlet temperature in one year is around 45 °C, which only reaches 65 °C for a few days in summer. The lowest temperature in winter is relatively low, while that in summer is about 20 °C.  In particular, the temperature of the storage tank is not below zero in the non-operation stage of LCPV/T system (shown in Figure 11) due to the heat storage property. Because of the thermal inertia, In particular, the temperature of the storage tank is not below zero in the non-operation stage of LCPV/T system (shown in Figure 11) due to the heat storage property. Because of the thermal inertia, the rising time of water temperature in the storage tank is later than that the rising time of the outlet temperature of the LCPV/T system. Moreover, the temperature of storage tank 1 after 18:00 exceeds that before 08:00. As shown in Figure 12, the temperature of tank 1 is maintained between 10-45 • C throughout the year. Consequently, storage tank 1 can be treated as a stable cryogenic heat source of the NG-LCPV/T-TG system.
Energies 2020, 13, x FOR PEER REVIEW 14 of 23 the rising time of water temperature in the storage tank is later than that the rising time of the outlet temperature of the LCPV/T system. Moreover, the temperature of storage tank 1 after 18:00 exceeds that before 08:00. As shown in Figure 12, the temperature of tank 1 is maintained between 10-45 °C throughout the year. Consequently, storage tank 1 can be treated as a stable cryogenic heat source of the NG-LCPV/T-TG system.  In the heating mode, it is clearly visible that the LHP is on when the temperature of storage tank 1 is between 15 °C and 40 °C. Additionally, when the temperature of storage tank 1 is below 15 °C, the auxiliary natural gas heater is applied. As shown in Figure 13, the time nodes of heating mode  the rising time of water temperature in the storage tank is later than that the rising time of the outlet temperature of the LCPV/T system. Moreover, the temperature of storage tank 1 after 18:00 exceeds that before 08:00. As shown in Figure 12, the temperature of tank 1 is maintained between 10-45 °C throughout the year. Consequently, storage tank 1 can be treated as a stable cryogenic heat source of the NG-LCPV/T-TG system.  In the heating mode, it is clearly visible that the LHP is on when the temperature of storage tank 1 is between 15 °C and 40 °C. Additionally, when the temperature of storage tank 1 is below 15 °C, the auxiliary natural gas heater is applied. As shown in Figure 13, the time nodes of heating mode In the heating mode, it is clearly visible that the LHP is on when the temperature of storage tank 1 is between 15 • C and 40 • C. Additionally, when the temperature of storage tank 1 is below 15 • C, the auxiliary natural gas heater is applied. As shown in Figure 13, the time nodes of heating mode are the 1761th hour and 7328th hour, respectively. Additionally, the COP of the LHP is around 5, which indicates a significant improvement compared with the rated value of 3.8. Since the terminal of heat dissipation adopts capillary network system, the outlet water temperature at the load side of the LHP is maintained at about 35 • C to satisfy the temperature requirements of the experimental room. Obviously, such a combination takes sufficient advantage of the low temperature heat source, and makes a remarkable contribution to saving energy.
Energies 2020, 13, x FOR PEER REVIEW 15 of 23 are the 1761th hour and 7328th hour, respectively. Additionally, the COP of the LHP is around 5, which indicates a significant improvement compared with the rated value of 3.8. Since the terminal of heat dissipation adopts capillary network system, the outlet water temperature at the load side of the LHP is maintained at about 35 °C to satisfy the temperature requirements of the experimental room. Obviously, such a combination takes sufficient advantage of the low temperature heat source, and makes a remarkable contribution to saving energy.  Figure 14 clearly shows the proportion of the thermal energy produced by LHP and NGAH in heating mode, respectively. When the thermal energy exceeds the demand of space heating, it will preheat domestic hot water. Obviously, the produced thermal energy is higher than the heat load of the experimental room. Furthermore, the produced thermal energy of NGAH is higher than that of the LHP in January and December. The reason for this is that the outdoor environment temperature is low, and the radiation intensity is weak in January and December. Under the above environment conditions, the thermal energy produced by LCPV/T system is slighter, which leads to the low temperature of storage tank 1. In February and November, the produced thermal energy of the LHP is significantly higher than that of the NGAH, because the outdoor environment temperature and the radiation intensity increase significantly in February and November.  Figure 14 clearly shows the proportion of the thermal energy produced by LHP and NGAH in heating mode, respectively. When the thermal energy exceeds the demand of space heating, it will preheat domestic hot water. Obviously, the produced thermal energy is higher than the heat load of the experimental room. Furthermore, the produced thermal energy of NGAH is higher than that of the LHP in January and December. The reason for this is that the outdoor environment temperature is low, and the radiation intensity is weak in January and December. Under the above environment conditions, the thermal energy produced by LCPV/T system is slighter, which leads to the low temperature of storage tank 1. In February and November, the produced thermal energy of the LHP is significantly higher than that of the NGAH, because the outdoor environment temperature and the radiation intensity increase significantly in February and November.
In addition, the monthly data relevant to the accumulated thermal energy under the heating mode is listed in Table 9. In January, the heat load is the highest, with the accumulated value up to 906 kWh. The thermal energy produced by the LHP and the NGAH are 311 kWh and 664 kWh, respectively. Besides, the thermal energy to preheat domestic hot water is 573 kWh. In particular, the maximum amount of preheating domestic hot water is 928 kWh in February. Furthermore, LCPV/T system provides 131 kWh thermal energy for heating directly. The thermal energy produced by NGAH is only 101 kWh and 17 kWh in February and November. In addition, the monthly data relevant to the accumulated thermal energy under the heating mode is listed in Table 9. In January, the heat load is the highest, with the accumulated value up to 906 kWh. The thermal energy produced by the LHP and the NGAH are 311 kWh and 664 kWh, respectively. Besides, the thermal energy to preheat domestic hot water is 573 kWh. In particular, the maximum amount of preheating domestic hot water is 928 kWh in February. Furthermore, LCPV/T system provides 131 kWh thermal energy for heating directly. The thermal energy produced by NGAH is only 101 kWh and 17 kWh in February and November. Table 9. Produced thermal energy in heating mode. January  906  311  664  573  February  440  208  101  928  November  236  192  17  793  December  860  322  580  286 In cooling mode, the HHP provides 85-90 °C hot water as the heat source of the chiller. Figure  15 refers to the performance when outlet temperature of HHP maintained at about 90 °C in a stable manner. In addition, the COP of HHP can reach 2.2. Because the outlet temperature on the load side of HHP is high, the COP is lower than that of LHP.  In cooling mode, the HHP provides 85-90 • C hot water as the heat source of the chiller. Figure 15 refers to the performance when outlet temperature of HHP maintained at about 90 • C in a stable manner. In addition, the COP of HHP can reach 2.2. Because the outlet temperature on the load side of HHP is high, the COP is lower than that of LHP.  Figure 16 depicts that the single-effect absorption chiller can run continuously with the COP up to 0.7 in a year. The COP for this model is defined as the energy transferred from the chilled water divided by the sum of the electrical energy required by the chiller and the energy provided to the chiller by the inlet hot water. Since the energy consumed by an absorption chiller is mainly the heat energy, energy efficiency ratio (EER) is often used as the economic evaluation index. The EER is the  Figure 16 depicts that the single-effect absorption chiller can run continuously with the COP up to 0.7 in a year. The COP for this model is defined as the energy transferred from the chilled water divided by the sum of the electrical energy required by the chiller and the energy provided to the chiller by the inlet hot water. Since the energy consumed by an absorption chiller is mainly the heat energy, energy efficiency ratio (EER) is often used as the economic evaluation index. The EER is the ratio of the cooling capacity obtained by the absorption chiller to the heat consumed. The maximum amount of the EER is 0.69 during a year. The chiller is closed when the experimental room falls below 26 • C. Moreover, in the cooling mode, the capillary network system is adopted as well. The outlet temperature of the chiller is generally maintained between 10-20 • C, which can fulfil the requirement of space cooling.  Figure 16 depicts that the single-effect absorption chiller can run continuously with the COP up to 0.7 in a year. The COP for this model is defined as the energy transferred from the chilled water divided by the sum of the electrical energy required by the chiller and the energy provided to the chiller by the inlet hot water. Since the energy consumed by an absorption chiller is mainly the heat energy, energy efficiency ratio (EER) is often used as the economic evaluation index. The EER is the ratio of the cooling capacity obtained by the absorption chiller to the heat consumed. The maximum amount of the EER is 0.69 during a year. The chiller is closed when the experimental room falls below 26 °C. Moreover, in the cooling mode, the capillary network system is adopted as well. The outlet temperature of the chiller is generally maintained between 10-20 °C, which can fulfil the requirement of space cooling.   Figure 17 demonstrates the outlet temperature curve of the cooling tower. The cooling tower is started only when the chiller is running. Obviously, the outlet temperature of the cooling tower is higher when the demand of the cooling load is large. Cooling tower outlet water temperature is up to 30 • C, which can fulfill the demand of the chiller. Table 10 lists the cooling capacity and thermal energy produced in the cooling mode. In March, the cooling load is relatively low, so the energy demand is small. At the same time, the radiation intensity increases compared with that in winter. Therefore, the thermal energy to preheat domestic hot water is the highest in March, up to 1295 kWh. As the ambient temperature rises gradually, the cooling load increases, then the cooling capacity needed is increased. It is also worth noting that the maximum amount of cooling load is 1655 kWh in July. The cooling capacity provided by the chiller is 2389 kWh. The auxiliary natural gas heater is the main heat source of the chiller which provided 3297 kWh thermal energy. In addition, the minimum preheated thermal energy of domestic hot water is 104 kWh in July. Figure 17 demonstrates the outlet temperature curve of the cooling tower. The cooling tower is started only when the chiller is running. Obviously, the outlet temperature of the cooling tower is higher when the demand of the cooling load is large. Cooling tower outlet water temperature is up to 30 °C, which can fulfill the demand of the chiller.  Table 10 lists the cooling capacity and thermal energy produced in the cooling mode. In March, the cooling load is relatively low, so the energy demand is small. At the same time, the radiation intensity increases compared with that in winter. Therefore, the thermal energy to preheat domestic hot water is the highest in March, up to 1295 kWh. As the ambient temperature rises gradually, the cooling load increases, then the cooling capacity needed is increased. It is also worth noting that the maximum amount of cooling load is 1655 kWh in July. The cooling capacity provided by the chiller is 2389 kWh. The auxiliary natural gas heater is the main heat source of the chiller which provided 3297 kWh thermal energy. In addition, the minimum preheated thermal energy of domestic hot water is 104 kWh in July.  Figure 18 clearly draws the cooling load and the cooling capacity provided by the chiller under the cooling mode. The cooling capacity of the chiller is greater than the cooling load of the room. Due to the fluctuation of the indoor temperature and thermal loss of the water tanks, the cooling capacity provided by the chiller is greater than the cooling load. It is also apparent from Figure 18 that most of the thermal energy required during summer is supplied by the NGAC. This is because the   Figure 18 clearly draws the cooling load and the cooling capacity provided by the chiller under the cooling mode. The cooling capacity of the chiller is greater than the cooling load of the room. Due to the fluctuation of the indoor temperature and thermal loss of the water tanks, the cooling capacity provided by the chiller is greater than the cooling load. It is also apparent from Figure 18 that most of the thermal energy required during summer is supplied by the NGAC. This is because the temperature of storage tank 1 is set at 40 • C when the system determines the mode of heat source supply. In Beijing, although the ambient temperature is high, the radiation intensity is not high enough in summer. Thus, the thermal energy produced by the LCPV/T system cannot satisfy the temperature requirement of the chiller. At this time, the heat source of the chiller is mostly supported by the natural gas heater, which has more economic and energy saving benefits. temperature of storage tank 1 is set at 40 °C when the system determines the mode of heat source supply. In Beijing, although the ambient temperature is high, the radiation intensity is not high enough in summer. Thus, the thermal energy produced by the LCPV/T system cannot satisfy the temperature requirement of the chiller. At this time, the heat source of the chiller is mostly supported by the natural gas heater, which has more economic and energy saving benefits. Conspicuously, Figure 19 demonstrates the power consumption of all the electrical equipment of the proposed system and produced electrical energy of LCPV/T system in one year. In the heating mode, electrical energy produced by the LCPV/T system is sufficient to meet the power consumption of the LHP and circulating water pumps (pump 1 and pump 2) in the proposed system. Nevertheless, the electrical energy produced by the LCPV/T system can only satisfy the power consumption of the chiller and the HHP in the cooling mode. The power consumption of the circulating water pumps (pump 3, pump 4, and pump 5) are supported by external grid power. In addition, since the selection of the three pumps is based on the data of previous experiment, the energy consumption of the pumps is too large. Future research will further optimize the energy consumption of the pumps.  Conspicuously, Figure 19 demonstrates the power consumption of all the electrical equipment of the proposed system and produced electrical energy of LCPV/T system in one year. In the heating mode, electrical energy produced by the LCPV/T system is sufficient to meet the power consumption of the LHP and circulating water pumps (pump 1 and pump 2) in the proposed system. Nevertheless, the electrical energy produced by the LCPV/T system can only satisfy the power consumption of the chiller and the HHP in the cooling mode. The power consumption of the circulating water pumps (pump 3, pump 4, and pump 5) are supported by external grid power. In addition, since the selection of the three pumps is based on the data of previous experiment, the energy consumption of the pumps is too large. Future research will further optimize the energy consumption of the pumps. enough in summer. Thus, the thermal energy produced by the LCPV/T system cannot satisfy the temperature requirement of the chiller. At this time, the heat source of the chiller is mostly supported by the natural gas heater, which has more economic and energy saving benefits. Conspicuously, Figure 19 demonstrates the power consumption of all the electrical equipment of the proposed system and produced electrical energy of LCPV/T system in one year. In the heating mode, electrical energy produced by the LCPV/T system is sufficient to meet the power consumption of the LHP and circulating water pumps (pump 1 and pump 2) in the proposed system. Nevertheless, the electrical energy produced by the LCPV/T system can only satisfy the power consumption of the chiller and the HHP in the cooling mode. The power consumption of the circulating water pumps (pump 3, pump 4, and pump 5) are supported by external grid power. In addition, since the selection of the three pumps is based on the data of previous experiment, the energy consumption of the pumps is too large. Future research will further optimize the energy consumption of the pumps.  In particular, the specific data of the power consumption and electrical energy produced by the LCPV/T system are listed in Table 11. According to the data, the maximum amount of the electrical energy produced by LCPV/T is 403.6 kWh in April. In July, the maximum power consumption of the circulating pumps, chiller and HHP are 3370.5 kWh, 245.5 kWh, and 30.3 kWh, respectively. The maximum amount of the power consumption of HHP is 64.9 kWh in May. In February, March and November, the power consumption and the electrical energy produced by the LCPV/T system are balanced. Nevertheless, some power needs to be purchased from the grid in other months. The proposed system can simultaneously provide the space cooling and heating, part of the electrical energy required in the building, throughout the year.

Conclusions
In the present paper, a design and transient analysis of an NG-LCPV/T-TG system is demonstrated. The continuous operation of the proposed system is realized by TRNSYS software throughout the year. In addition, the building model is set up according to the real data of the experimental room in our previous work. The main component of this system is the LCPV/T system, which adopts the low-concentrating technology to provide electrical energy and low-temperature thermal energy at the same time. The auxiliary equipment of heating mode adopts LHP and a natural gas heater. In the cooling mode, the single-effect absorption lithium bromide absorption chiller is applied. Moreover, the HHP and the natural gas auxiliary heater are used as auxiliary heat source for the chiller. In addition, the coupling method of traditional energy natural gas and renewable energy solar energy is proposed. Based on previous work, this research obtains the operation data of various equipment throughout the year, which proved the feasibility of the trigeneration system. The main outcomes of this paper are as follows: • The proposed system maintains the experimental room at 25 • C under the cooling mode and 20 • C under the heating mode.

•
The maximum amount of the electrical and thermal energy during a day are 2.4 kW and 11.1 kW, respectively. Moreover, 16.4 kWh of electrical energy and 73.5 kWh of thermal energy are produced by the LCPV/T system for a day. In addition, the maximum amount of electrical and thermal energy are 2.53 kW and 13.85 kW in a year. The electrical and thermal energy produced by the LCPV/T system throughout the year are 3819 kWh and 18,374 kWh.
• During the heating mode, the COP of LHP reaches 5. The outlet water temperature at the load side of the LHP is about 35 • C. The produced thermal energy of the LHP and the NGAH are 311 kWh and 664 kWh in January. In particular, the highest thermal energy for preheating domestic hot water is 928 kWh in February. LCPV/T system provides 131 kWh thermal energy for heating directly. The thermal energy produced by NGAH is only 101 kWh and 17 kWh in February and November.

•
In the cooling mode, the average COP of HHP is 2.2. The maximum COP and EER of the single-effect absorption chiller are 0.7 and 0.9, respectively. It is also worth noting that the maximum amount of cooling load is 1655 kWh in July. The cooling capacity provided by the chiller is 2389 kWh. The auxiliary natural gas heater is the main heat source of the chiller which provided 3297 kWh thermal energy.

•
In this research, the maximum amount of the electrical energy produced by LCPV/T system is 403.6 kWh in April. In July, the maximum power consumption of the circulating pumps, chiller and HHP are 3370.5 kWh, 245.5 kWh, and 30.3 kWh, respectively. The maximum power consumption of HHP is 64.9 kWh in May.
In summary, the proposed system can simultaneously produce electricity, undertake space cooling and heating and produce domestic hot water. It can improve the conversion efficiency of the LCPV/T system, improve the thermal economy of a natural gas thermal system, reduce pollutant emissions, and reduce the irreversible loss of natural gas combustion. Our future work will be to discuss the optimization design of the NG-LCPV/T-TG system.

Conflicts of Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

COP
Coefficient of performance CPC Compound parabolic concentrator CPV/T Concentrating photovoltaic /thermal EER Energy efficiency ratio HHP High-temperature water source heat pump LCPV/T Low-concentrating photovoltaic /thermal LHP Low-temperature water source heat pump NGAC Natural gas assisted cooling mode NGAH Natural gas assisted heating mode NG-LCPV/T-TG Natural gas assisted solar low-concentrating photovoltaic/thermal trigeneration system PV/T Photovoltaic/thermal