Preliminary Study on Optimization of a Geothermal Heating System Coupled with Energy Storage for Office Building Heating in North China
Abstract
:1. Introduction
1.1. Background
1.2. Literature Review
2. Model Description
2.1. System Model
2.1.1. Heating Load
2.1.2. Heat Pump
2.1.3. Heat Exchanger
2.1.4. Energy Storage Tank
2.1.5. Water Pump
2.2. Operation Model
2.2.1. Operation Modes
2.2.2. Operation Strategies
2.2.3. Operation Conditions
- (1)
- The supply water temperature (T10) for conventional heating is same as the average of the upper and lower limit temperatures of the energy storage tank (i.e., to maintain the same quality of heating). The upper limit temperature of the water tank (Tmax) is a decision variable which represents the highest temperature that the water tank can reach during heat storage at night. The lower limit temperature of the water tank (Tmin) is designed to be 40 °C, corresponding to the lowest temperature that the water tank can reach during daytime heating.
- (2)
- The outlet temperature of Heat Exchanger 1 (T2) in the conventional heating mode is the same as the averaged T2 in the water tank energy storage mode.
- (3)
- The difference between the inflow and outflow water temperatures for building heating is not more than 8 °C; this is necessary to make sure that the heating water temperature is not low when it reaches the last room.
- (4)
2.3. Optimization Model
2.3.1. Decision Variables
- (1)
- The upper limit temperature of the energy storage tank (Tmax): ranging from 46 °C to 55 °C (subjects to heating temperature of the heat pump).
- (2)
- The energy storage ratio (ε): ratio of the maximum heat-stored in the energy storage tank to the average daily heating load of the design week, ranging from 0 to 1.
- (3)
- End temperature difference at low temperature side of the Heat Exchanger 1 (∆Te = T2–T5, Figure 2): ranging from: 1 °C to 7 °C (subject to the heat exchanger specification).
- (4)
- Maximum temperature difference of the heat pump system (∆Thp = T8–T7, Figure 2): ranging from: 35–40 °C (subject to the heat pump specification).
2.3.2. Objective Function
3. Results and Discussion
3.1. Optimal Values of Decision Variables in Different Scenarios
3.2. Minimum LCOH of each Scenario
3.3. Sensitivity Analysis
3.3.1. The Influence of Energy Storage Ratio on Operating Cost
3.3.2. The Influence of Electricity Price and Tank Price on εo and LCOH
4. Conclusions
- (1)
- The four operating strategies show that coupling an energy storage tank to the geothermal heating system can reduce operating costs by more than 25%. Case 2 is the best among the four operating strategies, and can reduce operating costs by 30% when the energy storage ratio is 74%.
- (2)
- When the system design is optimized in the consideration of both operating costs and investment costs, the operation strategy (Case 3) where the energy storage tank is used for supplying the peak heat load has a lower LCOH than other cases. The operation strategy (Case 4) where the energy storage tank is used to meet the basic heat load has the worst performance.
- (3)
- It is applicable to couple an energy storage tank to the heating system in scenarios 4, 8, and 10. In scenario 8, the coupled system can save up to 10% LCOH compared with the system without using an energy storage tank. It is worth noting that none of the optimal storage ratios exceed 55% for all scenarios.
- (4)
- The sensitivity analysis shows that coupling an energy storage tank to a geothermal heating system can reduce LCOH when peak-valley electricity price difference is higher than CNY 0.566/kW·h (USD 0.0847/kW·h) or the tank price is lower than CNY 900/m3 (USD 134.64/m3); otherwise, the techno-economy may not be good.
- (5)
- The results obtained in this study provide a reference for the design of an energy storage–geothermal coupled heating system. In the near future, an experimental study will be carried out to further validate the model, as well as the results obtained here.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Nomenclature
Roma symbols | |
A | Area |
Ch | Hazen–Williams coefficient |
Cmain | Annual maintenance cost |
Cp | Specific heat at constant pressure |
dj | Inside diameter |
g | Gravitational acceleration |
H | Head loss |
h | Enthalpy |
i | Interest rate |
iu | Pressure loss per unit length |
K | Correction coefficient |
k | Heat transfer coefficient |
L | Length |
M | Mass |
Mass flow rate | |
n | Service life of the system |
P | Pressure |
P1, P2...... | Pump |
Q | Heat transfer rate |
Q′ | Heat |
qg | Volume flowrate |
T | Temperature |
V | Volume |
V1, V2...... | Valve |
W | Power |
Greek symbols | |
α | Margin coefficient |
β | Proportion of annual maintenance cost |
ε | Energy storage ratio |
εo | Optimal energy storage ratio |
η | Efficiency |
λ | Ratio of mass flow rate |
ξ | Coefficient of local resistance |
ρ | Density of water |
τ | Time |
ν | Flow velocity |
Subscripts | |
al | Along |
B | Building |
com | Compressor |
con | Condenser |
cw | Cooling water |
d | Design |
e | Heat exchanger |
ele | Electricity |
eva | Evaporator |
he | Heating |
hp | Heat pump |
hw | Hot water |
i | In |
ie | Isentropic |
lo | Local |
m | Mean |
max | Maximum |
min | Minimum |
o | Out |
S | Season |
st | Storage |
t | Tank |
tot | Total |
W | Weak |
wf | Working fluid |
wp | Water pump |
Superscripts | |
ex | Heat exchanger |
hp | Heat pump |
Acronyms | |
COP | Coefficient of performance |
COST | Cost |
CPF | Capital recovery factor |
GSHP | Ground source heat pump |
HVAC | Heating, ventilation, and air conditioning |
LCOH | Levelized cost of heat |
MD-GHP | Medium and deep geothermal heat pump |
OC | Operating cost |
PR | Price |
TES | Thermal energy storage |
Appendix A. Water Pump Efficiency Model
Appendix B. Investment Cost (COST) and Operating Cost (OC)
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Period of Using Electricity in the Course of a Day | Price Names * |
---|---|
8:00 a.m.–12:00 p.m.; 4:00 p.m.–8:00 p.m. | Peak price |
6:00 a.m.–8:00 a.m.; 12:00 p.m.–4:00 p.m.; 8:00 p.m.–10:00 p.m. | Flat price |
10:00 p.m.–12:00 a.m.; 12:00 a.m.–6:00 a.m. | Valley price |
Scenario | Region | Energy Storage Tank Price (CNY/m3) | Heat Pump Price (CNY/kW) | Heat Exchanger Price (CNY/m2) | Water Pump Price (CNY/kW) |
---|---|---|---|---|---|
1 (basic scenario) | Xianxian Region | 1000 | 1000 | 800 | 2000 |
2 | - | - | - | 2000 | - |
3 | - | - | 500 | - | - |
4 | - | - | 1500 | - | - |
5 | Shanghai Region | - | - | - | - |
6 | - | - | - | - | 1000 |
7 | - | - | - | - | 3000 |
8 | - | 500 | - | - | - |
9 | - | 1500 | - | - | - |
10 | Tianjin Region | - | - | - | - |
Time | Status | Xianxian Region | Shanghai Region | Tianjin Region |
---|---|---|---|---|
8:00 a.m.–12:00 p.m.; 4:00 p.m.–8:00 p.m. | High peak | 0.9304 | 1.11 | 1.2760 |
12:00 p.m.–4:00 p.m. | Flat stage | 0.6724 | 1.11 | 0.8305 |
10:00 p.m.–12:00 a.m.; 12:00 a.m.–6:00 a.m. | Low valley | 0.4144 | 0.527 | 0.4050 |
Abscissa | −5 | −4 | −3 | −2 | −1 | 0 | 1 | 2 | 3 | 4 | 5 |
---|---|---|---|---|---|---|---|---|---|---|---|
The peak-valley electricity price difference (CNY/kW·h) | 0.266 | 0.316 | 0.366 | 0.416 | 0.466 | 0.516 | 0.566 | 0.616 | 0.666 | 0.716 | 0.766 |
Energy storage tank price (CNY/m3) | 500 | 600 | 700 | 800 | 900 | 1000 | 1100 | 1200 | 1300 | 1400 | 1500 |
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Ren, Y.; Lu, X.; Zhang, W.; Zhang, J.; Liu, J.; Ma, F.; Cui, Z.; Yu, H.; Zhu, T.; Zhang, Y. Preliminary Study on Optimization of a Geothermal Heating System Coupled with Energy Storage for Office Building Heating in North China. Energies 2022, 15, 8947. https://doi.org/10.3390/en15238947
Ren Y, Lu X, Zhang W, Zhang J, Liu J, Ma F, Cui Z, Yu H, Zhu T, Zhang Y. Preliminary Study on Optimization of a Geothermal Heating System Coupled with Energy Storage for Office Building Heating in North China. Energies. 2022; 15(23):8947. https://doi.org/10.3390/en15238947
Chicago/Turabian StyleRen, Yapeng, Xinli Lu, Wei Zhang, Jiaqi Zhang, Jiali Liu, Feng Ma, Zhiwei Cui, Hao Yu, Tianji Zhu, and Yalin Zhang. 2022. "Preliminary Study on Optimization of a Geothermal Heating System Coupled with Energy Storage for Office Building Heating in North China" Energies 15, no. 23: 8947. https://doi.org/10.3390/en15238947