Analysis of Operation Characteristics of Phase-Change Temperature Maintenance System Heating by Solar Source and Sewage Source Heat Pump

Abstract

To address the issues of high energy consumption and operating costs in the temperature maintenance and heating of floating roof oil tanks, a phase-change temperature maintenance simulation system using a solar source–sewage source heat pump was designed. Its operating characteristics and economic benefits were studied based on the TRNSYS platform. The study analyzed the effects of the solar energy guarantee rate, phase-change heat storage tank operating temperature, and sewage source heat pump operating temperature on various indicators, such as the heat storage and release efficiency of the phase-change heat storage tank, the heating capacity and energy proportion of crude oil, and the power consumption of the sewage source heat pump system. The economic benefits were also compared. The results indicate that when the solar energy guarantee rate is below 30%, the phase-change heat storage tank does not operate, while the sewage source heat pump operates at a higher efficiency, leading to increased system power consumption. However, when the solar energy guarantee rate exceeds 30%, the phase-change heat storage tank operates normally from April to December, while the sewage source heat pump ceases to function, resulting in reduced total system power consumption. Additionally, increasing the phase-change temperature from 38 °C to 54 °C boosts the heat storage and release efficiency of the phase-change heat storage tank from 87% to 94%, without affecting the heat pump’s heating capacity. Similarly, raising the temperature of the sewage source heat pump from 20 °C to 40 °C enhances the heat pump’s heating capacity and efficiency from 4.45 to 4.84, without impacting the heat storage and release efficiency of the phase-change heat storage tank.

1. Foreword

With the development of the national economy and the tense international situation, the national strategic reserve of crude oil has further increased. Therefore, ensuring the safe demand capacity of oil reserves has become an important part of the strategic reserve plan [1]. During the storage process of crude oil, when the temperature of the crude oil drops to a certain value, the crude oil will undergo gelation [2], which will block the inlet and outlet valves of the crude oil tank and have adverse effects on the storage and use of crude oil. Generally, in order to prevent crude oil from gelling, the storage temperature of crude oil should be 3–5 °C or more. For waxy crude oil, the storage temperature is generally 5–10 °C [3]. Therefore, continuously heating crude oil to maintain its storage temperature is an effective means of preventing crude oil from gelling. The conventional heating methods for crude oil [4,5] include boiler heating, air source heat pump heating, electric heating rod heating, electromagnetic heating, etc. However, these traditional heating methods suffer from serious pollution, high operating costs, and regional limitations, greatly limiting the amount of crude oil reserves and working conditions. Solar energy, as a clean and renewable energy source, is abundant in oil-producing areas such as northeast China and Xinjiang [6,7]. By using solar radiation energy to heat crude oil, it can reduce fossil energy consumption and environmental pollution.

The design concept of using solar energy as a heating source to construct a temperature maintenance system is often seen in building heating and domestic heating. For example, Ji Xiaofang [8] installed solar hot water systems in high-rise buildings, optimized water supply parameters and system structural parameters, which can provide sustainable circulating hot water for residents and reduce CO₂ emissions by 27.7%. Li Wei et al. [9] proposed an optimized design scheme for the heat source of the roof solar hot water system, designing the collector as an integrated structure of heat collection, storage, and exchange, reducing the water pump circulation system. This not only reduces the heat load on the water tank roof but also avoids the noise impact of the circulation pump, reducing operating costs and improving living comfort.

Khalid Hamid [10] reviewed the technological progress, application potential, and future challenges of integrated high-temperature heat pump systems and proposed a multi-energy complementary integrated system design that couples HTHP with renewable energy, industrial waste heat networks, or energy storage systems to improve energy utilization efficiency and stability.

In terms of solar energy utilization, the University of Wisconsin in the United States [11] proposed the concept of solar energy guarantee rate based on a large number of simulation experiments and established an F-chart correlation model for the design of solar collector area. As an important component of solar energy combination systems, the design of phase-change thermal storage water tanks and the characteristics of phase-change materials have a significant impact on the energy efficiency of solar energy. Liu Kai et al. [12] found through experimental research that the heat storage capacity of phase-change thermal storage water tanks can reach 3.7 times that of ordinary thermal storage water tanks, and the average efficiency of the system is improved by 26%. Wang Xiaoyong [13] summarized the current situation of phase-change material selection in energy storage systems. The selection of phase-change materials needs to be comprehensively considered, and targeted performance optimization and long-term energy efficiency monitoring should be carried out based on the characteristics of the building where the project is located. In terms of the design and operational characteristics analysis of solar phase-change thermal storage and temperature maintenance systems, Zhu Shangwen et al. [14] built a solar phase-change thermal storage system based on the solar radiation and environmental conditions in Daqing. The designed phase-change thermal storage tank has high energy storage and release efficiency, and compared with solar temperature maintenance systems and boiler heating systems, the annual total investment cost can be reduced by about 6.9% and 40%, respectively.

This article proposes a phase-change hybrid temperature maintenance system for solar water source heat pumps based on the structure of the solar phase-change temperature maintenance system. A system simulation model is established through the TRNSYS platform, and the evaluation indicators include the heat storage and release efficiency of the phase-change heat storage tank, the heating capacity and proportion of crude oil and equipment, and the power consumption of equipment and system. The annual operating characteristics of the phase-change temperature maintenance system for floating solar water source heat pumps are analyzed, and the investment and operating costs of the system are compared and analyzed under different solar energy guarantee rates, providing reference for the design and operation of the phase-change temperature maintenance system for solar water source heat pumps.

2. Modeling

2.1. Physical Model

The working principle of the solar heat pump hybrid heating system involves the solar collector converting the received solar radiation into thermal energy to heat the heat transfer fluid. This fluid then flows through a phase-change heat storage tank and is recirculated to the floating roof oil tank, facilitating crude oil heating and temperature maintenance. During periods of high solar radiation intensity, the phase-change heat storage tank stores the excess heat. When solar radiation is low, the tank or a sewage source heat pump supplies heat to the crude oil tank, satisfying its static heat storage load requirements.

Based on these principles, a phase-change temperature maintenance system using solar sewage source heat pump hybrid heating was designed, taking a crude oil tank in Daqing City with a diameter of 40 m and a height of 21 m as an example. As illustrated in Figure 1a, the system primarily comprises solar collectors, phase-change heat storage tanks, crude oil storage tanks, sewage source heat pumps, and other related equipment. Additionally, temperature measurement nodes are established within the system to ensure accurate and representative temperature readings. These nodes are distributed across varying depths and radial directions within the tank, as depicted in Figure 1b.

Furthermore, from an economic perspective, this study primarily employs the TRNSYS simulation system for calculations. This simulation system includes components, such as vacuum tube collectors (type 71), floating roof oil tank modules (type 156), phase-change heat storage tanks (type 2765), sewage source heat pump modules (type 659), and water pump modules (type 114), as shown in Figure 1c.

2.2. Mathematical Model

The crude oil static storage heat load [14] is calculated as follows:

$$Q_{oil}={\displaystyle \frac{(K_{wall}{A}_{wall}+K_{top}{A}_{top}+K_{bottom}{A}_{bottom})\mathrm{\Delta }T}{1000}}$$

(1)

In the formula, Q_oil represents the static heat storage load of crude oil, kW; K_wall, K_top, and K_bottom represent the heat transfer coefficients of the floating roof oil tank wall, top, and bottom, with values of 0.475 W/(m²·°C), 1.726 W/(m²·°C), and 0.11 W/(m²·°C), respectively; A_wall, A_top, and A_bottom represent the area of the wall, top, and bottom of a floating roof oil tank, m²; and ΔT represents the difference between the outdoor design temperature and the crude oil static storage design temperature, °C.

According to the specifications, the outdoor design temperature for Daqing is −8.5 °C [14]. The design temperature for crude oil static storage should exceed the pour point temperature of crude oil, which is 30.2 °C. Therefore, the design temperature for crude oil static storage is set at 40.2 °C. The calculated thermal load for crude oil static storage is 571.24 kW.

The solar collector is a vacuum tube collector, and its collection area is calculated as follows, the results are shown in

Table 1. Calculation of system-related data.

Solar Energy Guarantee Rate f (%)	Collector Heat Collection Area Asolar (m2)	Circulating Water Pump Flow Rate qHTF (m3/h)
10	1226.32	61.32
20	2452.64	122.63
30	3678.95	183.95
40	4905.27	245.26
50	6131.59	306.58

:

$$A_{solar}={\displaystyle \frac{86400Q_{oil}f}{J_T\eta \left(1-{\eta }_L\right)}}$$

(2)

In the formula, A_solar represents the heat collection area of the vacuum tube collector, m²; J_T represents the annual average daily solar radiation on the inclined surface of the collector, kJ/m²; f represents the solar energy guarantee rate, taken as 10–50%; η represents the average heat collection efficiency of the collector, taken as 45%; and η_L represents the daily average loss rate of the solar energy system, taken as 15%.

The flow rate of the circulating water pump in the solar phase-change temperature maintenance system is determined by the flow rate per unit heat collection area. The calculation formula for the flow rate of the circulating water pump is as follows, the results are shown in Table 1:

$$q_{HTF}=q_z{A}_{solar}$$

(3)

In the formula, q_HTF represents the flow rate of the circulating water pump in the solar thermal maintenance system, m³/h; and Q_z represents the flow rate of the working fluid corresponding to the unit heat collection area, m³/(h·m²), take 0.05 m³/(h·m²). The working fluid for heat transfer is a 50% ethylene glycol solution.

Phase-change thermal storage tanks mainly rely on the latent heat of material phase-change to store and release heat, where the volume of the phase-change thermal storage tank is calculated by the following formula:

$$Q_H={\displaystyle \frac{m_{PCM}{L}_{PCM}}{1000}}$$

(4)

$$V_{PCM}={\displaystyle \frac{t{Q}_{oil}}{{\rho }_{PCM}{Q}_H}}$$

(5)

In the formula, Q_H represents the latent heat of the phase-change material, kJ; t represents the working time of phase-change thermal storage tank, s; m_PCM represents the quality of the phase-change materials, kg; L_PCM represents the specific enthalpy of the phase-change materials, J/kg; V_PCM represents the volume of the phase-change thermal storage tank, m³; and ρ_PCM is the density of the phase-change material, kg/m³.

To ensure that the phase-change thermal storage equipment can meet the heat load demand of crude oil during night or rainy weather, the phase-change temperature of the phase-change material should be 5–10 °C higher than the static storage temperature of crude oil. The phase-change material is 42 # paraffin wax (whose physical properties are taken from [15]).

The heat storage and release efficiency is an important indicator for evaluating the thermal performance of phase-change thermal storage tanks, and its formula is as follows:

$${\eta }_{PCM}={\displaystyle \frac{Q_{PCM,re}}{Q_{PCM,st}}}\times 100\%$$

(6)

In the formula, η_PCM represents the heat storage and release efficiency of the phase-change thermal storage tank; Q_PCM,st represents the heat storage capacity of the phase-change materials, kJ; Q_{PCM, re} represents the heat release of the phase-change materials, kJ.

2.3. System Control Plan

To prioritize the use of solar heat collection or heat generated by sewage source heat pumps for crude oil static storage loads, the solar phase-change temperature maintenance system adopts sewage source heat pump heating control, circulating water pump control, and phase-change heat storage tank control. The sewage source heat pump heating control is turned on when the average temperature of crude oil is lower than the design temperature of crude oil and turned off at all other times. When the outlet temperature of the collector differs from the average temperature of the crude oil by more than 8 °C, the circulating water pump of the solar phase-change temperature maintenance system is turned on. When the outlet temperature of the collector differs from the average temperature of the crude oil by less than 2 °C, the circulating water pump of the solar phase-change temperature maintenance system is turned off. When the outlet temperature of the collector is higher than the phase-change temperature of the phase-change material by more than 3 °C, or the average temperature of the crude oil is higher than the design temperature by more than 3 °C, the thermal storage mode is turned on, and the sewage source heat pump is turned off. When the vacuum tube collector and sewage source are turned off, and the average temperature of the phase-change thermal storage tank is higher than the design temperature of the crude oil static storage by more than 3 °C, the heat release mode is turned on. The rest of the time, it is in bypass mode.

2.4. Boundary Conditions and Assumptions

In order to simplify the numerical calculation complexity, the following assumptions are proposed: (1) except for specific heat, the thermal properties of phase-change materials are constant; (2) neglecting the influence of radiation on heat transfer process; (3) neglecting the contact thermal resistance between the heat transfer fluid and the heat transfer fluid pipeline, as well as between the phase-change material and the heat transfer fluid pipeline; and (4) phase-change materials and heat transfer fluids are both incompressible materials and isotropic.

The calculation time is one year, from 0 h to 8760 h, with a calculation interval of 1 min. The initial external conditions are as follows:

Storage Tank	$\begin{array}{l}0\le x\le a,0\le y\le b,\\ -\lambda {\displaystyle \frac{\partial T_{PCM}}{\partial x}}=h(T_{PCM}-T_{air}),\\ -\lambda {\displaystyle \frac{\partial T_{PCM}}{\partial y}}=h(T_{PCM}-T_{air})\end{array}$	a—Length in the x-direction of the storage tank; b—Length of the storage tank in the y-direction; Tair—Ambient Temperature; λ—coefficientof conduction; h—coefficient of convection
PipeLine	$x=r,-\lambda {\displaystyle \frac{\partial T_{\mathrm{oil}}}{\partial x}}=h(T_{oil}-T_{air})$	r—Radius of pipeline

3. Results and Discussion

3.1. Model Validation

Figure 2a is a schematic diagram of the experimental setup, in which the first tank is a floating roof oil tank, the second tank is a phase-change heat storage tank, and the third tank is used for refilling. All three tanks are cylindrical with a diameter of 0.6 m and a height of 0.9 m. The initial temperature of the phase-change thermal storage tank is 58 °C, the initial temperature of the replenishment tank is 70 °C, the temperature of the floating roof oil tank is 10.2 °C, and the water pump flow rate is 1.5 m³/h. The heating process of the phase-change thermal storage tank to the crude oil storage tank in the TRNSYS simulation system was verified using the experimental equipment. Figure 2b shows the average temperature change trend of the crude oil storage tank and the phase-change thermal storage tank. The results show that the maximum temperature error of the phase-change thermal storage tank is 4.011%, and the maximum temperature error of the floating roof oil tank is 4.463%.

3.2. Analysis of Operating Characteristics

According to literature data [6], the Daqing area in Heilongjiang Province, where the system is located, is an area with abundant or utilizable solar energy resources. The lowest temperature of the year occurs in January, with temperatures as low as −26.4 °C, and the highest temperature occurs in July, with temperatures as high as 29.4 °C. The annual sunshine hours are 1400–3000 h, and the total annual radiation is 419–586 kJ/m². Under these conditions, the operating parameters and efficiency of a phase-change heat storage system with solar sewage source heat pump hybrid heating are studied.

3.2.1. Solar Energy Guarantee Rate

Figure 3 illustrates the average temperature of system devices when the solar energy guarantee rate is set at 10%, 30%, and 50%. As evident from Figure 3a, at a 10% solar energy guarantee rate, the heat collection capacity of the collector is limited, resulting in a low outlet temperature that fails to meet the operational requirements of the phase-change thermal storage tank. Consequently, the temperature of the phase-change thermal storage tank remains constant at 20 °C. When the guarantee rate increases to 30%, the heat collection capacity of the collector enhances, leading to a higher outlet temperature compared to the 10% condition. The phase-change thermal storage tank becomes operational from April and continues to operate until December, during which the ambient temperature remains low, allowing the heat stored in the tank to be fully released, resulting in a slight decrease in its temperature. As the guarantee rate further increases to 50%, the heat collection capacity of the collector increases significantly, pushing the outlet temperature of the collector above 50 °C. This allows the phase-change thermal storage tank to commence operation from March and sustain it until December.

When the solar energy guarantee rate is low, the phase-change thermal storage tank does not operate. Under conditions where the solar energy guarantee rate exceeds 30%, the monthly trend of the heat storage and release efficiency of the phase-change thermal storage tank during the system’s annual operation is depicted in Figure 4. As shown in Figure 4a, when the solar energy guarantee rate is 30%, the phase-change thermal storage tank commences operation in April. The heat storage and release capacities of the phase-change thermal storage tank are 3.03 × 10⁸ kJ and 2.18 × 10⁸ kJ, respectively, with a heat storage and release efficiency of 72%. As the system operates from May to October, the heat storage and release efficiency climbs to 90%. After November, as the ambient temperature drops, the heat release capacity of the phase-change thermal storage tank rises, and the heat storage and release efficiency climbs to 120%. As Figure 4b,c illustrates, when the solar energy guarantee rate is 40% and 50%, the phase-change thermal storage tank starts operating in March. The heat storage and release capacities of the phase-change thermal storage tank are 7.31 × 10⁷ kJ and 1.39 × 10⁸ kJ, respectively, with heat storage and release efficiencies of only 50% and 64.4%, respectively. However, as the system operates from May to December, the collector provides ample heat, maintaining the heat storage and release efficiency at approximately 90%. Furthermore, as the solar energy guarantee rate increases, the cumulative heat collection capacity of the collector grows, leading to an increase in the total heat storage and release capacity of the phase-change thermal storage tank throughout the year. Nevertheless, the heat storage and release efficiency remains stable at around 90%. This indicates that the solar energy guarantee rate has minimal impact on the overall efficiency of the phase-change thermal storage tank throughout the year, as demonstrated in Figure 4d.

Figure 5 illustrates the total heating load of crude oil, as well as the heating supply and proportion of each heating device, under the condition of a 30% solar energy guarantee rate. As shown in Figure 5a, the ambient temperature is low from January to March, resulting in a relatively high heat supply to crude oil. During this period, as the northern hemisphere is in winter, the heat provided by solar collectors ranges from 7.5 × 10⁸ kJ to 1.2 × 10⁹ kJ. The total heat supply of crude oil is approximately 2.5 × 10⁹ kJ, necessitating a heat pump to supply 50–70% of the heat. From April to October, as the ambient temperature increases, the heat supply to crude oil decreases. The sewage source heat pump ceases to contribute heat to the system, which primarily relies on the collectors to supply heat to crude oil. Simultaneously, the phase-change thermal storage system becomes operational, providing around 20% of the heat to crude oil. In November, the ambient temperature drops, and lighting conditions are less favorable than in summer and autumn. The phase-change heat storage tank releases the previously stored heat to crude oil, as depicted in Figure 5b. The heating supply from the phase-change heat storage tank peaks at 45% in November. However, by December, its heating supply significantly decreases to approximately 1%, with the system mainly relying on heat pumps to provide 75% of the heat.

Figure 6 illustrates the total heat load, the heat supply from each equipment, and their respective proportions throughout the year, assuming a solar energy guarantee rate ranging from 10% to 50%. As evident from Figure 6a, as the solar energy guarantee rate increases, the total heat collected by the solar collector rises, resulting in an increase in the total annual heat supply from 1.59 × 10¹⁰ kJ to 1.97 × 10¹⁰ kJ. Specifically, the proportion of heat supply from the collector increases from 23.6% to 68.3%. Correspondingly, the heat provided by the sewage source heat pump decreases significantly, from 1.22 × 10¹⁰ kJ to 2.3 × 10⁹ kJ, with the proportion of heat supply dropping from 78% to 11%. The phase-change thermal storage tank starts operating when the solar energy guarantee rate reaches 30%. As the solar energy guarantee rate increases, both the stored and released heat from the thermal storage tank synchronously increase, leading to an increase in the proportion of heat supply from 9.93% to 19.65%.

Figure 7 illustrates the hourly and annual average efficiency of a sewage source heat pump under solar energy guarantee rates ranging from 10% to 50%. As seen in Figure 7a, the heat pump efficiency drops from 5.6 to 4.5. Between March and December, the sewage source heat pump either does not provide any heat to the system or only delivers a partial amount, maintaining an efficiency of 4.5. As the solar energy guarantee rate rises from 10% to 50%, the total heat collected by the solar collectors increases, leading to a decrease in the heat provided by the sewage source heat pump. Consequently, the annual average thermal efficiency decreases from 4.7 to 4.57, as depicted in Figure 7b.

When the solar energy guarantee rate is less than 30%, the total heat collected by the collector is relatively small, with the sewage source heat pump providing most of the heat for the system. Except for summer, the heat pump consumes more electricity in each month, as shown in Figure 8a. The power consumption of the water pump is relatively low, with a monthly energy consumption of approximately 1 × 10⁴ kWh. When the solar energy guarantee rate reaches 30%, the sewage source heat pump consumes only about 1 × 10⁵ kWh of electricity from January to March and December, while the monthly power consumption of the water pump remains at 1.55 × 10⁴ kWh. Compared with the 10% operating condition, the monthly power consumption has significantly decreased, as shown in Figure 8b. When the solar energy guarantee rate reaches 50%, the sewage source heat pump can provide sufficient heat to the system. The heat pump only operates from January to March, and its power consumption decreases as the system operates. The power consumption of the water pump remains at approximately 2.5 × 10⁴ kWh, as shown in Figure 8c. Compared with the first two working conditions, the monthly power consumption has increased. As shown in Figure 8d, the annual total power consumption varies with the solar energy guarantee rate. It can be seen that as the solar energy guarantee rate increases, the collection area of the collector expands, requiring a higher flow rate of the working medium transported by the water pump, which increases the power consumption of the water pump. At the same time, when the collector can provide sufficient heat to the system, the sewage source heat pump will reduce heating and significantly reduce power consumption. Therefore, as the solar energy guarantee rate increases from 10% to 50%, the total power consumption of the system decreases from 8.86 × 10⁵ kWh to 5.20 × 10⁵ kWh, with a total power consumption reduction of 40%.

3.2.2. Sewage Source Heat Pump Temperature

Figure 9 shows the total annual heat supply and the heat supply and proportion of each piece of equipment when the working temperature of the sewage source heat pump is 20–40 °C under the condition of a solar energy guarantee rate of 30%. Based on Figure 9a, it can be seen that the working temperature of the sewage source heat pump has no significant effect on the operation of the collector and phase-change heat storage tank, and the heat provided by the two is approximately 8.85 × 10⁹ kJ and 1.69 × 10⁹ kJ, respectively. As the temperature of the sewage source heat pump increases, the heat provided by the heat pump increases from 6.27 × 10⁹ kJ to 6.71 × 10⁹ kJ, and the total heating capacity of the system also increases from 1.68 × 10¹⁰ kJ to 1.71 × 10 ¹⁰ kJ. At the same time, the proportion of heating provided by collectors and phase-change thermal storage tanks has decreased, while the proportion of heating provided by heat pumps has increased from 37.3% to 39%, as shown in Figure 9b.

Figure 10 shows the hourly thermal efficiency and annual average thermal efficiency of the sewage source heat pump at different operating temperatures. From the hourly thermal efficiency graph in Figure 10a, it can be observed that as the system operates from January to March, the heat pump efficiency decreases hourly. As the system operates from April to November, the efficiency remains basically unchanged. When the operating temperature decreases from 40 °C to 20 °C, the thermal efficiency during this period decreases from 4.75 to 4.3. When the system operates in December, the heat pump continues to supply heat to the system, and the thermal efficiency decreases. From the annual average thermal efficiency graph Figure 10b, it can be seen that as the operating temperature of the sewage source heat pump increases, the efficiency of the sewage source heat pump increases from 4.45 to 4.86, with an efficiency improvement of about 10%.

Figure 11 depicts the total power consumption of the system and the power consumption of the water pump and heat pump as a function of the heat pump operating temperature throughout the year. Changes in the heat pump operating temperature have no impact on the operation of the solar collector. Therefore, the annual power consumption of the water pump remains constant at 1.88 × 10⁵ kWh and does not vary with increasing temperature. Conversely, as the heat pump operating temperature increases, the power consumption of the heat pump decreases from 4.14 × 10⁵ kWh to 4.04 × 10⁵ kWh, representing a reduction of approximately 2.4%. Correspondingly, the total system power consumption also decreases by approximately 2.4%.

3.2.3. Temperature of Phase-Change Thermal Storage Tank

Figure 12 illustrates the annual heat storage and release capacity and efficiency of the phase-change thermal storage tank under the condition of a solar energy assurance rate of 30% and an operating temperature range of 38–54 °C. As can be seen from the figure, as the temperature increases, the heat storage and release capacity of the thermal storage tank increases simultaneously. At 50 °C, the heat storage and release capacities reach their maximum values of 3.19 × 10⁹ kJ and 3.03 × 10⁹ kJ, respectively, and the heat storage and release efficiency also reaches its maximum value of 94.75%. However, when the temperature is higher than 50 °C, the heat storage and release capacities slightly decrease, and the heat storage and release efficiency also drops to around 94%.

Figure 13 shows the total annual heat supply and the heat supply and proportion of each equipment when the operating temperature of the phase-change thermal storage tank increases from 38 °C to 54 °C under the condition of a solar energy guarantee rate of 30%. From the combination of Figure 13a,b, it can be seen that the operating temperature of the phase-change thermal storage tank has no significant effect on the operation of the sewage source heat pump. The total annual heat provided by the heat pump to the system remains at around 6.55 × 10⁹ kJ, accounting for approximately 38.5% of the total heating supply. As the temperature of the phase-change thermal storage tank increases, the heat provided by the thermal storage tank increases from 1.49 × 10⁹ kJ to 2.63 × 10⁹ kJ, and the proportion of heating increases from 8.74% to 15.5%. Due to the fact that a portion of the heat collected by the collector is stored in the phase-change heat storage tank, as the heat stored in the tank increases, the amount of heat supplied by the collector to the system decreases accordingly, and the proportion of heat supply decreases from 52.6% to 46.2%.

Figure 14 shows the trend of annual power consumption with the temperature of the phase-change thermal storage tank under the condition of a solar energy guarantee rate of 30%. From the power consumption of the system and equipment, the increase in temperature of the phase-change thermal storage tank has no significant impact on the operation of the collector, and the annual power consumption of the water pump remains at 1.88 × 10⁵ kWh. The power consumption of the heat pump decreases from 3000 kWh to 5000 kWh as the temperature of the phase-change thermal storage tank increases, and the total power consumption of the system synchronously decreases from 3000 kWh to 5000 kWh.

3.3. Sensitivity Analysis of Parameters

In order to describe the effects of solar energy guarantee rate, sewage source heat pump temperature, and phase-change thermal storage tank temperature on system operation, the variation in solar heating ratio, collector efficiency, and phase-change thermal storage tank heat release efficiency with parameters was selected, as shown in Figure 15. By comparing Figure 15a–c, it is found that the solar energy guarantee rate has the greatest impact on the three parameters. When the solar energy guarantee rate increases from 10% to 50%, the solar heating ratio increases from 0.2 to 0.65, an increase of about 70%, while the solar energy collection efficiency decreases from 0.5 to 0.43, a decrease of about 14%. The temperature of the sewage source heat pump and the temperature of the phase-change heat storage tank have little effect on the solar heating ratio and collection efficiency, with two values maintained at around 0.46 and 0.67; The temperature of the sewage source heat pump has little effect on the storage and heat release efficiency of the phase-change heat storage tank, but as the temperature of the phase-change heat storage tank increases, the storage and heat release efficiency of the tank increases from 87% to 94%, an increase of about 8%.

3.4. Economic Benefit Evaluation

To better demonstrate the economic prospects of solar phase-change temperature maintenance systems in crude oil heating, an economic benefit evaluation was conducted using solar temperature maintenance systems and boiler systems as references. Introducing the average annual investment cost as an evaluation indicator [16], the formula is as follows:

$$AC=C_i\times {\displaystyle \frac{{\left(i+1\right)}^{n}i}{{\left(i+1\right)}^n-1}}+A$$

(7)

In the formula, AC—average annual investment cost, CNY;

C_i—initial investment cost of the project, CNY;

The compound interest rate for investment loans is generally set at 10%;

n—loan repayment period, generally taking the service life of the heating system as 15 years;

A—annual operating expenses, CNY.

Formula (7) shows that the total annual investment cost is composed of the average annual initial investment and annual operating costs. The annual average initial investment includes equipment costs and labor costs. According to market research, the equipment cost required for this system is 4550 CNY/ton for paraffin wax, with a total usage of 300 m³. Vacuum tube collector costs are 500 CNY/m², with a collector area ranging from 1226.32 m² to 6131.59 m², the total cost ranges from 613,200 CNY to 3,065,800 CNY, as shown in

Table 2. Initial investment cost of solar collectors.

Solar Collector
Solar Energy Guarantee Rate	Unit Price of Collector/Ten Thousand CNY/m2	Area of Collector/m2	Design Cost of Collector/Ten Thousand CNY
10%	0.05	1226.32	61.32
20%	0.05	2452.64	122.63
30%	0.05	3678.95	183.95
40%	0.05	4905.27	245.26
50%	0.05	6131.59	306.58

.

The circulating water pump costs 5000 CNY, the sewage source heat pump costs 6000 CNY, the pipeline network and other related valves total 74,000 CNY, and the labor cost is 10% of the total equipment cost; the annual operating cost mainly comes from the power consumption of water pumps and sewage source heat pumps. The industrial electricity price is 0.55 CNY/kWh, and the total operating cost ranges from 132,100 CNY to 431,200 CNY. As the solar energy guarantee rate increases, the operating cost decreases, as shown in

Table 3. Operating costs of water pumps and sewage source heat pumps.

Solar Energy Guarantee Rate	Annual Power Consumption of Water Pump/kWh	Annual Power Consumption of Sewage Source Heat Pump/kWh	Prices of Electricity/CNY/kWh	Operating Cost of Water Pump/Ten Thousand CNY	Operating Cost of Heat Pump/Ten Thousand CNY
10%	102,000	784,000	0.55	5.60	43.12
20%	99,800	556,000	0.55	5.49	30.58
30%	188,000	413,000	0.55	10.33	22.72
40%	240,000	280,000	0.55	13.19	15.41
50%	291,000	240,000	0.55	16.03	13.21

.

The phase-change thermal storage tank in the system is made by retrofitting discarded small oil tanks, with a renovation cost of 500 CNY/m³. The total price of the phase-change temperature maintenance system for solar sewage source heat pump hybrid heating is 1.8994 million CNY −4.352 million CNY, as shown in

Table 4. Investment at the beginning of the year, annual operating costs, and total annual investment expenses for each system.

Plan	Phase-Change Temperature Maintenance System Heating by Solar Source and Sewage Source Heat Pump
	Original Investment			Annual Operating Cost		Annual Total Investment Cost
Solar Energy Guarantee Rate	Paraffin Cost/Ten Thousand CNY	Collector Cost/Ten Thousand CNY	Other Cost/Ten Thousand CNY	Operating Cost of Water Pump/Ten Thousand CNY	Operating Cost of Heat Pump/Ten Thousand CNY
10%	120.12	61.32	8.5	5.60	43.12	238.66
20%	120.12	122.63	8.5	5.49	30.58	287.32
30%	120.12	183.95	8.5	10.33	22.72	345.62
40%	120.12	245.26	8.5	13.19	15.41	402.48
50%	120.12	306.58	8.5	16.03	13.21	464.44

. The average annual initial investment of the phase-change temperature maintenance system for solar sewage source heat pump hybrid heating accounts for more than 80% of the total cost, while the annual operating cost is around 20%.

Overall, the solar energy guarantee rate has a significant impact on the total investment cost. This is mainly because as the solar energy guarantee rate increases, the area of the collector increases significantly, resulting in a significant increase in investment costs at the beginning of the year but a decrease in annual operating costs. Compared with the working condition with a solar energy guarantee rate of 10%, when the solar energy guarantee rate is 20%, the annual investment cost increases by 613,100 CNY, and the operating cost decreases by 126,500 CNY. The total cost of the system will basically remain the same after about 5 years of equipment operation. When the solar energy guarantee rate is 30%, the annual investment cost increases by 1.2263 million CNY, and the operating cost decreases by 156,700 CNY. The total cost of the system will basically remain stable after about 8 years of equipment operation. When the solar energy guarantee rate is 40%, the annual investment cost increases by 1.8394 million CNY, and the operating cost decreases by 20.12 million CNY. The total cost of the system will basically remain stable after about 9 years of equipment operation. When the solar energy guarantee rate is 50%, the annual investment cost increases by 2.4526 million CNY, and the operating cost decreases by 194,800 CNY. The total cost of the system will basically remain the same after about 12 years of equipment operation. Therefore, if the project cycle is short, a plan with a lower solar energy guarantee rate should be adopted; otherwise, a plan with a higher solar energy guarantee rate should be adopted to achieve better economic efficiency.

According to the investment data of Zhu Shangwen et al. [13] on solar phase-change temperature maintenance systems, solar temperature maintenance systems, and boiler heating system, as shown in

Table 5. Comparison of initial investment, annual operating costs, and annual total investment expenses for different systems at the beginning of the year.

Plan	Original Investment/Ten Thousand CNY	Annual Operating Cost/Ten Thousand CNY	Annual Total Investment Cost/Ten Thousand CNY
Phase-change temperature maintenance system heating by solar source and sewage source heat pump	435.2	29.24	464.44
Phase-change temperature maintenance system heating by solar source	380.2	39.1	419.3
Temperature maintenance system heating by solar source	357.3	59.6	416.9
Temperature maintenance system heating by boiler	96.5	131.9	228.4

, the gas boiler prices of the systems are 270,000 CNY, 360,000 CNY, and 620,000 CNY, respectively. The annual operating costs mainly come from the power consumption of the water pump and the gas consumption of the boiler, and the total annual investment cost is calculated. By comparing the investment costs of different systems, it can be seen that the boiler system has the lowest investment cost at the beginning of the year, but the annual operating cost is higher. Based on a solar energy guarantee rate of 50%, the total cost of the two systems will be basically equal after about 3 years of operation. After 3 years of operation, the boiler system cost is higher than that of the phase-change temperature maintenance system with solar sewage source heat pump mixed heating. The initial investment cost of solar phase-change temperature maintenance and solar temperature maintenance system is lower than that of the phase-change temperature maintenance system of solar sewage source heat pump hybrid heating, but the annual operating cost is higher. Based on a solar energy guarantee rate of 50%, the total cost of the three systems will basically remain the same after about 5 years of operation. After 5 years of operation, the phase-change temperature maintenance system of solar sewage source heat pump hybrid heating has the lowest cost, followed by the solar phase-change system, which has the highest cost. Therefore, compared with boiler systems, solar energy systems have better economy. Among the three types of solar energy systems, the phase-change temperature maintenance system of solar sewage source heat pump hybrid heating has better economy.

3.5. Environmental Benefit Assessment

According to the Annual Report on Carbon Emission Factors of Provincial Power Grids (2024) released by the Chinese Ministry of Ecology and Environment, the carbon emission factor in Daqing area is 0.71 kg CO₂/kWh. According to the formula for carbon emissions in the power grid, CO₂ emissions = electricity consumption (kWh) × carbon emission factor (kg CO₂/kWh), the CO₂ emissions of the system’s electricity consumption can be calculated. According to the 2025 global carbon emission accounting standards, the conversion of gas consumption to CO₂ emissions needs to be combined with fuel type, combustion efficiency, and regional emission factors. According to the formula for natural gas carbon emissions, CO₂ emissions = gas consumption × energy conversion coefficient × carbon emission factor × oxidation rate, where the energy conversion coefficient is taken as 38.1 MJ/m³, the carbon emission factor is taken as 56.1 kg CO₂/GJ, and the oxidation rate is taken as 1. Based on this, the CO₂ emissions of natural gas consumed by the system can be calculated, as shown in

Table 6. CO₂ emissions of different systems.

Plan	Annual Power Consumption/kW·h	Carbon Emission Factor/kg CO2/kWh	Annual Gas Consumption/m3	Energy Conversion Coefficient × Carbon Emission Factor/kg CO2/m3	Emissions of CO2/kg
Phase-change temperature maintenance system heating by solar source and sewage source heat pump	531,000	0.71	0	2.137	377,010
Phase-change temperature maintenance system heating by solar source	300,041.3	0.71	48,839.8	2.137	317,400.0
Temperature maintenance system heating by solar source	300,041.3	0.71	51,847.7	2.137	323,828
Temperature maintenance system heating by boiler	311,330.4	0.71	331,724.9	2.137	929,941

. From the annual carbon emissions, it can be seen that the carbon emissions of the three systems with solar energy as the main heating source are roughly the same, but compared with the boiler system, the carbon emissions can be reduced by about 60%.

4. Conclusions

Taking a large floating roof oil tank in Daqing City as an example, a simulation model of a phase-change temperature maintenance system for solar sewage source heat pump hybrid heating was built using the TRNSYS platform. The annual operating characteristics of the system were analyzed, and the average annual total investment was introduced as an economic indicator to evaluate the economic benefits of the solar phase-change temperature maintenance system, solar energy temperature maintenance system, and boiler system. The following conclusions were drawn.

(1) The proposed method and model can be used for simulating the phase-change temperature maintenance system of solar sewage source heat pump hybrid heating, with an average error of about 4% and high computational accuracy.

(2) When the solar energy guarantee rate is lower than 30%, the phase-change thermal storage tank does not work. When the solar energy guarantee rate is higher than 30%, the energy storage and release efficiency of the phase-change thermal storage tank reaches about 90%, which can provide 10–20% of the heating capacity to the system, reducing the total power consumption of the system by about 40%.

(3) When the solar energy guarantee rate is 30%, increasing the working temperature of the sewage source heat pump increases the heat provided by the heat pump to the system by 2%, increases the thermal efficiency by 10%, and reduces the total power consumption of the system by 2.4%, while the operation of the collector and phase-change heat storage tank is not affected.

(4) When the solar energy guarantee rate is 30%, increasing the working temperature of the phase-change thermal storage tank can slightly increase the heat storage and release efficiency of the tank, increase the heat release by about 7%, reduce the heating capacity of the collector by 6%, and reduce the total power consumption of the system by 3000 kWh–5000 kWh, but the heating and thermal efficiency of the heat pump are not affected.

(5) The initial investment cost of the phase-change temperature maintenance system for solar sewage source heat pump hybrid heating is relatively high, but the operating cost is low. With the increase in solar energy guarantee rate, there will be a significant increase in investment costs at the beginning of the year, while operating costs will be significantly reduced.

(6) Compared with solar phase-change temperature maintenance systems, solar energy temperature maintenance systems, and boiler systems, the phase-change temperature maintenance system of solar sewage source heat pump hybrid heating has a higher initial investment, lower annual operating costs, and relatively lower carbon emissions compared with boiler systems. In long-term operation, it has higher economic and environmental benefits.