Changes in water temperature of the collector
Set the air temperature as 5 °C and the water flow of the collector as 20 g/(m2·s) to analyze the impact of hot water temperature on SEUE. With solar irradiation as 100, 300, 500, 700, 900 and 1100 W/m2, respectively, the relationship between water temperature and SEUE is shown in Fig. 7.
As observed in Fig. 7, the SEUE of the collector showed a linear correlation of water inlet temperature. When solar irradiation was constant, the lower the water inlet temperature the greater the SEUE. When the water inlet temperature was constant, the smaller the solar irradiation, the lower the SEUE. The smaller the solar irradiation, the greater the influence of inlet water temperature on SEUE. The solar irradiation was 100, 300, 500, 700, 900, and 1100 W/m2, the inlet water temperature was 10 °C, SEUE was 38.5%, 53.1%, 56.0%, 57.2%, 57.9% and 58.4%, respectively. The higher the inlet water temperature, the greater the temperature difference between the collector and the air, resulting in greater heat loss and smaller SEUE. As illustrated in Fig. 7, when SEUE was 0, the smaller the solar irradiation, the lower the water inlet temperature. When the solar irradiation was 100, 300 and 500 W/m2, SEUE was 0%, the water inlet temperatures were 19, 46 and 74 °C, respectively. When the solar radiation intensity was greater than 700 W/m2, even if the water inlet temperature was 90 °C, the SEUE was negative. Therefore, only if the inlet water temperature is less than the temperature corresponding to 0% SEUE, the collector could provide heat for the system normally. Otherwise, the SEUE will be negative, and the collector will become the heat dissipation device of the system.
The variation relationship between inlet water temperature and heat loss of collector is shown in Fig. 8.
As observed in Fig. 8, an increase in inlet water temperature led to an increase in total heat loss. When the inlet water temperature rose from 10 to 90 °C and the solar irradiation was 300, 500, 700 and 900 W/m2, the heat loss increased from 141, 220, 299 and 379 W/m2 to 491, 571, 650 and 729 W/m2, respectively. The increase was about 350 W/m2. When the inlet water temperature was 90 °C and solar irradiation was 300 W/m2 or 500 W/m2, the total heat loss of the collector was greater than the solar radiation intensity, and the collector could not provide heat for the system.
The total heat loss is the sum of heat loss of energy absorption process and conversion process. As illustrated in Fig. 8, when solar irradiation was unchanged, the heat loss of energy absorption process changed little with the change of water inlet temperature. The heat loss of energy absorption process was mainly caused by the transmittance of the glass cover, the solar absorptivity of the surface material, and the blocking of the solar rays by the collector’s frame. For four solar radiation intensity, the heat loss of energy absorption process was about 114, 190, 266 and 342 W/m2, respectively. The heat loss of energy conversion process increased with the increase of inlet water temperature. The heat loss of energy conversion process for the four solar radiation conditions were 27, 30, 33 and 37 W/m2, respectively, when the inlet water temperature was 10 °C. The loss of conversion process was much less than that of absorption process. This is because the heat loss of energy conversion process is mainly due to the temperature difference between the collector and the air, and the temperature difference is very small. When the water inlet temperature rose to 90 °C, the heat loss of energy conversion process was 377, 381, 384 and 387 W/m2, respectively. The increase of water temperature led to a great increase of heat loss in the conversion process. Under the four solar irradiation conditions, heat loss rate in the conversion process was 4.38 W/(m2·°C). With the same inlet water temperature, the higher solar radiation intensity caused a greater heat collected by the collector, leading to a higher temperature difference between the absorber plate and the hot water. Therefore, even if the average water temperature of the collector is the same, the heat loss of energy conversion process will increase with the solar irradiation increased.
From Fig. 8, the smaller the solar irradiation, the greater the proportion of heat loss of energy conversion process in the total heat loss. When solar irradiation was 300 W/m2, the inlet water temperature was greater than 30 °C, the heat loss of energy conversion process was greater than that of energy absorption process. When solar irradiation was 500, 700 and 900 W/m2, the inlet water temperature was 46, 63 and 80 °C, respectively.
In summary, when solar irradiation was constant, reducing the inlet water temperature can improve SEUE. Therefore, the solar irradiation in winter is generally small in China’s Hot Summer and Cold Winter Zones, the use of solar collectors for heating cannot meet the demand for building heat load, needing to set up auxiliary heat sources. If a combined heating system with FPSC and other auxiliary heat sources is used, the collectors and auxiliary heat sources are connected in series with collector priority, which has a higher SEUE than parallel connections. The need for cooling in summer and heating in winter in Hot Summer and Cold Winter Zones, and the general summer cooling load value is greater than the winter heat load. The chilled water supply/return temperature for summer cooling is often 7/12 °C, and the indoor air temperature is controlled at 26 °C, so the heat exchange temperature difference between chilled water and indoor air is 16.5 °C. If the winter indoor heating temperature is controlled to 20 °C, using the same indoor air and hot water heat exchange temperature difference, the average temperature of hot water is 36.5 °C to meet the winter heating demand, and its value is lower than the existing hot water temperature for winter heating in Hot Summer and Cold Winter Zones. Therefore, lowering the hot water temperature can further increase the SEUE for the system while meeting the indoor thermal comfort conditions. Under low solar irradiation, the higher the water supply temperature of the collector, the easier it is to dissipate the heat of the system into the environment. Therefore, When the solar irradiation decreases until SEUE is 0%, the collector should stop running and prevent the hot water from entering the collector.
Changes in solar radiation intensity
Hot water flow of the collector was 20 g/(m2·s), and air temperature was 5 °C. The influence of solar radiation intensity on SEUE is shown in Fig. 9.
As depicted in Fig. 9, the SEUE increased with the increase of solar irradiation. The greater the solar irradiation, the smaller the impact of the changes in solar irradiation on SEUE. When solar irradiation was small, SEUE was negative. When the water inlet temperature was 20, 30, 40, and 50 °C, the SEUE was 0%, the corresponding solar irradiation was 113, 184, 255, and 328 W/m2, respectively. When solar irradiation was greater than the corresponding value, the SEUE was greater than 0%, to provide heat for the system. Otherwise, the collector may lose the heat of the system. The reason is that the air temperature is lower than the average temperature of the collector. If the water inlet and outlet temperature of the collector were constant, there will be the heat loss of energy conversion process between the collector and air, resulting in the heat obtained by the collector being less than heat loss. From Fig. 9, SEUE was small under the simulating conditions. When solar irradiation intensity was 300 W/m2, the SEUE to water temperatures of 20, 30, 40 and 50 °C are 3.85%, 23.8%, 9.2% and -5.4%, respectively. Even when solar irradiation was 600 W/m2, the SEUE was only 49.4%, 42.1%, 34.8% and 27.5%, respectively. With the average water temperature was different, the heat loss had a great impact on SEUE.
The relationship between the heat loss of the collector and the solar radiation intensity is displayed in Fig. 10.
As observed in Fig. 10, total heat loss of the collector increased with an increase of solar radiation intensity. When water inlet temperature of the collector was 20, 30, 40 and 50 °C, the solar irradiation rose from 50 to 1100 W/m2, the total heat loss increased from 74, 117, 161 and 205 W/m2 to 502, 546, 589 and 633 W/m2, respectively. The increase of total heat loss was about 428 W/m2. Under the four water inlet temperature conditions, when solar irradiation was weak, the total heat loss of the collector was greater than solar radiation intensity. In this situation, the collector cannot supply heat for the system. As illustrated in Fig. 10, when water temperature was constant, the heat loss of energy conversion process was caused by the changes of solar radiation intensity under that of energy absorption process. When solar irradiation was enhanced from 50 to 1100 W/m2, the heat loss of energy conversion process increased from 66, 110, 154 and 198 W/m2 to 84, 127, 171 and 215 W/m2, respectively. The increase was about 17 W/m2. The heat loss of energy absorption process increased from 8 to 418 W/m2. And the variation range of the heat loss in the energy conversion process was much smaller than that in the energy absorption process. When solar irradiation was 180, 300, 425 and 550 W/m2, under the four water temperature conditions, the heat loss of energy conversion process was equal to that of energy absorption process. And the heat loss of energy conversion process was 70, 114, 160 and 205 W/m2, respectively. If the solar radiation continued to increase, the heat loss of energy absorption process will be greater than that of energy conversion process.
Based on the above analysis, under the simulation conditions, when the water temperature of the collector is constant, an increase of SEUE was caused by an increase of solar radiation intensity. However, the heat loss of the collector also rose with the enhancement of solar radiation intensity. With strong solar irradiation, the heat loss of energy absorption process is significantly greater than that of energy conversion process. Therefore, in addition to reducing the water temperature, promoting SEUE also can improve the thermal performance of the collector. If solar irradiation is weak, it can be considered to improve the absorptivity of the collector and reduce the heat loss to promote SEUE. When solar irradiation is strong, the effect of improving the absorptivity of the collector is more significant.
Operation optimization method
The SEUE of the collector is the key to affect the normal operation of solar water heating system. From the above research, in addition to the energy loss caused by the collector's material, structure and other factors, meteorological parameters such as solar radiation intensity and air temperature, and operating parameters such as the inlet and outlet water temperature will also affect the SEUE of the collector. To improve the SEUE, the supply and return water temperature of the collector can be reasonably adjusted according to the users’ heating demand. For example, the temperature of domestic hot water in winter is generally above 50 ℃, and the heating temperature is related to the heating equipment used. The temperature of heating medium for floor heating, coil heating and radiator heating varies greatly. To improve the SEUE, you can choose appropriate user heating equipment to reduce the supply and return water temperature of the collector as much as possible.
In addition, in areas with low solar radiation intensity, the SEUE of the series connection mode of the collector and the auxiliary heat device is greater than that of the parallel connection. When supply and return water temperature of the system remains unchanged, the series connection mode is preferred, which can further reduce the average water temperature in the collector and improve SEUE. However, when the solar radiation intensity is very small or there is no solar radiation at night, the collector may become the heat dissipation equipment of the system. Under the simulated conditions, when the SEUE was 0%, the inlet water temperature of the collector was 20, 30, 40 and 50 ℃, respectively, and the corresponding solar radiation intensity was 128, 193, 263 and 329 W/m2 for the parallel connection system and 99, 160, 227 and 296 W/m2 for the series connection system. At this time, whether it is connected in parallel or series, when the solar radiation intensity is lower than the working condition with SEUE of 0%, the collector should be stopped, and the water inlet valve of the collector should be closed to prevent system heat loss through the collector.