Numerical and Experimental Study on a Solar Water Heating System in Lhasa
Abstract
:1. Introduction
2. Experimental Approach
2.1. System Description
- Charging mode: When the sunshine is sufficient and there is no heating load during the daytime, 1# valve and solar collecting pumps are switched on while 2# valve, hot water pumps and intermediate circuit pumps are switched off. Under this mode, the solar collection circuit is active while the heat usage circuit is inactive. Water is extracted from the lower part of the heat storage tank (1# outlet) and transported to the solar collectors by the force of solar collection pumps, then it absorbs the solar heat from the solar collectors and directly pours into the top of heat storage tank (1-1# inlet and 1-2# inlet); The solar heat is absorbed by circulated water and transferred to the water storage tank and stored in sensible form;
- Discharging mode: When the sunshine is not sufficient and there is heating load, the 2# valve, intermediate circuit pumps and hot water pumps are switched on while the 1# valve and solar collecting pumps are switched off. Under this mode, the heat usage circuit is active while the solar collecting circuit is inactive. Water is extracted from the upper part of the heat storage tank (2# outlet) and transported to the plate heat exchanger by the force of hot water pumps. Then it releases the heat to the water coming from the HVAC terminal (FCU) and directly flows into the lower part of heat storage tank (2# inlet); The stored heat is released by circulated water and transferred to the water from the FCU;
- Simultaneous charging and discharging mode: When the sunshine is sufficient and there is heating load the 1# valve and 2# valve, solar collection pumps, intermediate circuit pumps and hot water pumps are all switched on. Under this mode, the solar collection circuit and the heat usage circuit are both active. The charging process and discharging process both take place for the water storage tank. Part of the solar collecting heat is directly supplied to the FCU though two cycles. The redundant heat is stored in the heat storage tank in sensible form.
2.2. Experimental Procedure
2.3. Experimental Results
3. Numerical Approach
3.1. Model Setting
3.2. Validation
3.3. Numeric Analysis
4. Analysis of the System Performance
4.1. System COP
4.2. Heating Effect
4.3. Optimization Directions
- Resetting the mass flow rates of the solar collecting cycles. In this study, the mass flow rates of the 1-1# and 1-2# solar collecting cycles are 5.0 and 2.5 kg/s, respectively. From Figure 4, the 1-1# inlet temperature is much lower than 1-2# inlet temperature, with a mean temperature difference of 5 °C. At some time with high solar radiation (13:00), this difference even exceeds 10 °C. This large temperature difference results in the mixture of the hot water and cold water which addresses the thermal exergy loss and the reduction of the solar energy utilization efficiency. This is mainly attributed to the unreasonable mass flow rate setting of the solar collecting cycles. Therefore, the 1-1# solar collecting cycle should set a lower mass flow rate. On the one hand, the temperature difference between the 1-1# inlet and 1-2# inlet can be balanced. On the other hand, the energy consumption of the solar collecting pumps can be reduced and the system COP can be improved.
- Setting a reasonable control stagey for the FCU. In the initial period and the last period of the heating season in Lhasa, the heat load is relatively low. Due to the different sunshine conditions, the south-facing rooms and the north-facing rooms show different thermal environments when the FCU is switched on. As shown in Figure 11, the temperature of the south-facing room exceeds 20 °C most of the time and even reaches to 24 °C in the afternoon. It is much higher than that of the north-facing room. Therefore, the electric two-way valve can be used for the FCU to realize the variable water volume operation, which can adjust the indoor temperature, especially for the south-facing room.
5. Conclusions
- The numerical calculation method of the tank temperature distribution under the simultaneous charging and discharging operation mode offers a correlation between the output water temperatures of the tank and the input water temperatures of the tank, which can be used to optimize the solar heating system in future study.
- The solar heating system under the simultaneous charging and discharging operation mode has a relatively high system COP as well as good heating effect.
- During the initial and last period of the heating season in Lhasa, the simultaneous charging and discharging operation mode of the solar water heating system is a good attempt to maintain the indoor comfort during the daytime and worth further in-depth research in the future.
Acknowledgments
Author Contributions
Conflicts of Interest
Nomenclature
flow rate of water pump (m3/h) | heat transfer rate of plate heat exchanger (kW) | ||
head of water pump (m) | length of solar collector (m) | ||
power of water pump (kW) | width of solar collector (m) | ||
length of heat storage tank (m) | thickness of solar collector (m) | ||
width of heat storage tank (m) | efficiency factor of solar collector | ||
height of heat storage tank (m) | transmittance of solar collector | ||
air flow rate of FCU (m3/h) | absorptivity of solar collector | ||
heat transfer rate of FCU (W) | heat loss coefficient of solar collector (W/(m2·°C)) | ||
power of FCU (W) | uncertainty | ||
water mass flow rate (kg/s) |
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Name | Technical Parameters |
---|---|
Hot water pump | WILO65-28/2: Lpump = 49 m3/h, Hpump = 28 m, Npump = 4 kW |
Intermediate circuit pump | WILO50-21/2: Lpump = 40 m3/h, Hpump = 21 m, Npump = 3 kW |
Solar collection pump | WILO65-45/2: Lpump = 17 m3/h, Hpump = 45 m, Npump = 5 kW |
Plate heat exchanger | QHE = 300 kW |
Solar water storage tank | Ltank = 7 m, Wtank = 4 m, Htank = 3 m |
FCU | MFMW200C: LFCU = 340 m3/h, QFCU = 3565 W, NFCU = 35 W |
Solar collector | NS-PGT2.0: Lsc = 2 m, Wsc = 1 m, δsc = 0.08 m, FR = 0.9, τ = 0.9, α = 0.92, UL = 5 W/(m2·°C) |
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Yang, X.; Wang, Y.; Xiong, T. Numerical and Experimental Study on a Solar Water Heating System in Lhasa. Energies 2017, 10, 963. https://doi.org/10.3390/en10070963
Yang X, Wang Y, Xiong T. Numerical and Experimental Study on a Solar Water Heating System in Lhasa. Energies. 2017; 10(7):963. https://doi.org/10.3390/en10070963
Chicago/Turabian StyleYang, Xun, Yong Wang, and Teng Xiong. 2017. "Numerical and Experimental Study on a Solar Water Heating System in Lhasa" Energies 10, no. 7: 963. https://doi.org/10.3390/en10070963
APA StyleYang, X., Wang, Y., & Xiong, T. (2017). Numerical and Experimental Study on a Solar Water Heating System in Lhasa. Energies, 10(7), 963. https://doi.org/10.3390/en10070963