
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39251632
71291
10.1038/s41598-024-71291-9
Article
Performance investigation of a solar-driven cascaded phase change heat storage cross-seasonal heating system for plateau applications
Gao Tianfei 12
Han Xu hanx@vip.163.com

1
Shi Luyang 1
Geng Yichao 1
Zhang Hua 1
Song Tao 3
1 https://ror.org/05mgp8x93 grid.440614.3 0000 0001 0702 1566 College of Defense Engineering, Army Engineering University of PLA, Nanjing, 210000 Jiangsu China
2 Liaoning Police College, Dalian, 116036 Liaoning China
3 https://ror.org/00cwdhv97 grid.464234.3 0000 0004 0369 0350 Liaoshen Industries Group CO., LTD, China North Industries Group Corporation, Shenyang, 110045 Liaoning China
9 9 2024
9 9 2024
2024
14 2095717 4 2024
27 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
The mismatch between solar radiation resources and building heating demand on a seasonal scale makes cross-seasonal heat storage a crucial technology, especially for plateau areas. Utilizing phase change materials with high energy density and stable heat output effectively improves energy storage efficiency. This study integrates cascaded phase change with a cross-seasonal heat storage system aimed at achieving low-carbon heating. The simulation analyzes heat distribution and temperature changes from the heat storage system to the heating terminal. The results indicate that although the solar collectors operate for 26.3% of the total heat storage and heating period, the cumulative heat stored is 45.4% higher than the total heating load. Heat transferred by the cross-seasonal heat storage system accounts for up to 61.2% of the total heating load. Therefore, the system reduces fuel consumption by 77.6% compared to conventional fossil fuel heating systems. Moreover, radiant floor heating terminals, with a wide range of operating temperatures, match well with cascaded phase change heat storage and can reduce operation time by 19.5% and heat demand by 5.2% compared to conventional radiators. In addition to demonstrating the feasibility of applying cascaded phase change technology in cross-seasonal heat storage heating, this study reveals the lifecycle sustainability due to the shortened heat storage period. The configuration, parameters, and simulation results provide a reference basis for system application and design.

Keywords

Low carbon heating
Solar thermal energy
Cross-seasonal heating
Cascaded phase change material
Energy saving
Subject terms

Solar thermal energy
Civil engineering
http://dx.doi.org/10.13039/501100001809 National Natural Science Foundation of China 52208135 Han Xu http://dx.doi.org/10.13039/501100004608 Natural Science Foundation of Jiangsu Province BK20221056 Han Xu issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

With global economic development and improving living standards, oil, coal, natural gas, and other conventional energy sources account for 80% of global energy consumption1. Building energy consumption accounts for approximately 30% of total energy consumption2, with heating energy consumption constituting 25% of building energy use3. Conventional energy reserves are extremely limited, making energy supply and demand increasingly tense. Traditional heating methods face significant challenges. Due to the unique geographical and climatic conditions of plateau regions, the temperature difference between day and night is extreme, energy reserves are insufficient, transportation is inconvenient, and winters are long and cold. Meeting the thermal comfort requirements of indoor personnel during the heating season is challenging, resulting in a high demand for heating. Traditional heating methods rely on fossil fuels, leading to severe environmental pollution. Therefore, exploring efficient, environmentally friendly, and sustainable heating methods is crucial for the sustainable development of harsh cold plateau regions.

The plateau region is rich in solar energy resources, with average annual sunshine reaching 3100–3300 h. Therefore, solar energy has the potential to provide sufficient energy for the heating season. Solar energy, a clean and renewable energy source4, has received significant attention for its application in the heating sector. Liu et al.5 analyzed the impact of an active solar heating system on the indoor thermal environment through field measurements in a residential building in the Tibetan region. The results showed that the active solar heating system could provide heat for 75% of the heating period. Fan et al.6 conducted a comparative study on the energy-saving performance and feasibility of six solar heating systems and evaluated an all-electric heating system in Lhasa. The results indicated that the solar thermal driven hot air heating system has high solar energy utilization for nighttime heating and low maintenance cost, making it the most suitable heating system for the plateau region. Zhang et al.7 discussed auxiliary heat sources, including electric boilers, gas boilers, and air-source heat pumps, suitable for centralized solar thermal driven heating systems (CSDHS) in the Tibetan region. The results demonstrated that electric boilers are suitable for areas with high solar irradiance, where the solar factor (SF) should be greater than 85%, and air-source heat pumps are suitable for areas with average or low solar irradiance, where the SF is less than 50%. K. Lebedeva et al.8 conducted a case study of a solar active heating system in Latvia and evaluated the effects of increasing the solar share of a solar district heating system under local climatic conditions using a TRNSYS model. Operational data showed that under local climatic conditions, solar collectors (SCs) with storage systems can produce heat to meet up to 20% (10,933.2 MWh) of the heating demand. N. Arbaoui et al.9 studied the effect of solar heating systems on the microclimate of agricultural greenhouses and found that during the cold season, the temperature of greenhouses increased by 4 °C compared to those without heating systems. Wang et al.10 proposed a collaborative optimal design method (CODM) to optimize heating cost and energy consumption over the building life cycle, using a railroad passenger station as an example. The results showed that the optimized total building heating cost and total energy consumption were reduced by 1948 USD/m2 and 2292 kWh/m2, respectively, over the entire building life cycle compared to the initial design.

Existing studies have demonstrated the feasibility and economic viability of using solar energy for heating in highland areas. However, the seasonal mismatch between solar radiation resources and building heating loads is a primary issue in the actual application phase11,12. Moreover, achieving high-efficiency heat storage and release, and maintaining system stability and sustainability under severe cold conditions remain significant challenges in current research. There are two modes of solar energy storage: short-term storage and long-term storage (cross-seasonal storage)13. The abundant solar energy resources in the plateau region and the high heating demand in winter highlight the potential for adopting cross-seasonal storage. Heat storage methods for solar-driven cross-seasonal heating include tank thermal energy storage (TTES), pit thermal energy storage (PTES), borehole thermal energy storage (BTES), and aquifer thermal energy storage (ATES)14–16. As heat storage volume increases, hot water preparation costs and heat loss per unit volume decrease. Thus, developing large-scale cross-seasonal thermal storage systems is an effective solution to improve the thermal efficiency and solar energy utilization of solar heating systems. TTES, with low geological requirements, is a common form of heat storage in large-scale cross-seasonal heat storage systems. Related research mainly focuses on system design, control strategies, performance evaluation, and practical application. Xu et al.17 proposed a device that combines a building cross-seasonal energy storage system with long-term and short-term heat storage tanks. They analyzed energy transfer, conversion patterns, and dynamic operation characteristics of the subsystem using simulation methods, and proposed a synergistic control strategy and parameter optimization method. The results showed that the optimized composite system improved energy efficiency and reduced carbon dioxide emissions by 10.3% compared to the electric-assisted heating system. Li et al.18 investigated the effects of different control strategies on the performance of solar cross-seasonal energy storage heating systems, particularly in the non-heating season. They built a solar heating system in Hebei, China, combined with 3,000 cubic meters of underground water pit seasonal storage (UWPS). The experimental results showed that variable flow control improved solar energy efficiency by 10% compared to temperature difference control. Guo et al.19 proposed maximizing the use of solar energy and industrial waste heat for cross-seasonal energy storage to support the heating needs of urban district heating systems. Liu et al.20 established a multi-objective optimization model to minimize total life cycle cost and CO2 emissions of a concentrated solar central heating system. Three typical cities, Lhasa, Xining, and Xi'an, were selected for analysis. The results showed that compared to optimizing the design of solar heating stations and heating networks individually, the proposed method has lower economic costs and higher environmental performance. The minimum supply water temperatures in the three cities were 55 °C, 50 °C, and 65 °C. Buscemi et al.21 proposed a solar heat pump heating system for seasonal latent heat thermal storage. The simulation results showed that the system had an average annual collector efficiency of 10% and a seasonal thermal storage efficiency of 80%. The above study demonstrates the potential of combining solar thermal collector systems with cross-seasonal heat storage systems to achieve low-carbon heating.

The high energy density and heat storage performance of phase change materials (PCMs) make them ideal for cross-seasonal heat storage. The PCM heat storage method can store more energy in a limited space. Additionally, PCMs can maintain a constant temperature during the phase change process, ensuring that the heating system provides a stable supply water temperature under different operating conditions, thereby improving comfort and system efficiency. The latent heat storage capability of PCMs can reduce the size of the heat storage device, making them widely used in cross-seasonal heat storage. Qi et al.22 proposed a system for solar heat pump heating using PCMs as the seasonal storage medium. This system can significantly reduce the volume of the storage tank and its footprint. By maintaining the PCM in a sub-stable liquid state at temperatures lower than the phase transition temperature, the subcooling nature of inorganic PCMs can be utilized for long-term thermal storage23. Wu et al.24 proposed a novel material consisting of a composite of two or three PCMs for solar water heating systems to meet seasonal thermal storage needs. Crespo et al.25 utilized a flat plate thermal storage tank set up with phase change material as a thermal storage device to provide an inlet water temperature of 15 °C to the evaporator in a cross-seasonal energy storage system. Farid and Kanzawa26 proposed cascaded phase change as a means of heat and mass transfer enhancement in latent heat storage systems in 1989. Alessio et al.27 investigated a system configuration utilizing cascaded PCMs in place of a single PCM. The results showed that the novel system was able to achieve a liquefaction ratio consumption of 0.27 kWh/kgLA, thus improving the liquefaction performance of a single PCM by 6%. Huang et al.28 proposed a novel low melting point alloy cascade PCM heat sink. The experimental study of three single PCMs and three cascade PCMs concluded that the cascade PCM had a positive effect on thermal management. The maximum management time of the cascade PCM heat sink was approximately 8.47 min under a heat flow density of 3.0 W/cm2, which was 7.2% longer than that of the single PCM heat sink.

Currently, most studies on solar energy-driven cross-seasonal heat storage systems use phase change materials with single phase change temperatures. Cascaded PCM energy storage can increase the charging and discharging rates, improving the dynamic performance of latent heat storage systems. Multiple phase change temperatures can meet the temperature demands of different heating terminals and improve solar energy utilization. Studies combining cascaded PCMs with solar energy-driven cross-seasonal heat storage systems have not been widely conducted. Therefore, this study explores the feasibility of low-carbon heating through a solar-driven cascaded phase change heat storage cross-seasonal heating (SD-CPCH) system in a plateau region with abundant solar radiation resources and a lack of traditional energy sources. To investigate the actual performance of the SD-CPCH system, year-round operation was simulated based on the climatic conditions of the Ngari Sakü region. The simulation results were analyzed in terms of solar energy efficiency, system thermal energy patterns, and indoor environment control effects. This study demonstrated that PCMs with multiple phase change temperatures can meet the temperature demands of different heating terminals, thus improving thermal energy utilization. Year-round operation simulations provide data support for practical applications. In addition to verifying the performance of the existing system, this study proposes an improved system configuration for the heating terminal under fossil-free conditions. This configuration can further reduce the carbon footprint and demonstrate the feasibility and stability of the system under harsh environmental conditions.

Methods

This study aims to utilize solar energy and phase change thermal storage technology to achieve low carbon cross-seasonal heating. The system is modelled using the open source EnergyPlus software version 23.2 (https://energyplus.net/) and an office building located in a highland area is used as the application scenario. Along the heat flow direction of the system, the heat source system, heat storage system, and terminal system are simulated to analyze the characteristics of thermal energy and heat distribution throughout the year. By comparing indoor thermal comfort and system energy consumption with different terminal systems, an optimized system configuration that achieves low-carbon heating is proposed. Figure 1 shows the specific research flowchart.Fig. 1 Research flowchart.

System description

The solar-driven cascaded phase change heat storage cross-seasonal heating system proposed in this study focuses on remote plateau areas with abundant solar radiation resources, where energy shortages and energy supply are challenging. The system is divided into a heat source loop, heat storage loop, and heating loop, as shown in Fig. 2. The heat source loop includes solar collectors and plate heat exchangers. As a completely outdoor loop, the heat medium in the heat source circuit is non-freezing to avoid the risk of freezing in winter. When the heat source loop operates during the heat storage period, the non-freezing liquid flows through the solar collector, is heated by solar thermal energy, and then flows through the plate heat exchanger to exchange heat with hot water from the heat storage loop. In the heat storage loop, the water temperature in the heat storage tank increases during the heat storage period through heat transfer between the heat storage and heat source loops. To achieve cross-seasonal heating, as much hot water as possible should be stored during the heat storage period for use in winter. Therefore, the storage tank volume is large. Using cascaded PCM energy storage modules with different phase change temperatures can effectively reduce the storage tank volume and enable cascaded utilization of solar thermal energy. The phase change temperatures of the modules were 42, 58, and 75 °C. Additionally, to reduce heat loss through the outer skin of the storage tank, it is placed in the ground where the temperature is relatively stable throughout the year. In the heating loop, the heat storage tank serves as the main heat source, supplying hot water to the terminal radiators and maintaining the indoor temperature at the setpoint. The heat source for office building heating in the region mainly depends on fossil fuels, such as diesel, coal, and natural gas29,30. To investigate the feasibility of cross-seasonal heating using solar thermal energy and cascaded PCM, changes in water temperature and indoor air temperature were compared between the operating and non-operating conditions of a diesel boiler used as an auxiliary heat source.Fig. 2 System configuration.

System operation scheme

The operation of the solar-driven cascaded phase change heat storage cross-seasonal heating system is divided into heat storage and heating periods. During the thermal storage period, the heat source and storage loops operate to collect solar thermal energy. To avoid reverse heat transfer under low solar radiation and outdoor temperature conditions, operation reset control is adopted using the solar collector inlet and outlet water temperatures as the feedback signal. When the temperature difference between the outlet and inlet of the solar collector is less than 2 °C, the heat source and heat storage loops shut down. During the heating period, the system operates in three modes: 1. All three loops operate simultaneously, with heat storage and heat release (on sunny days); 2. The heat storage and heating loops operate, using stored heat for heating (on cloudy days); and 3. The diesel boiler operates as an auxiliary heat source to ensure the supply water temperature for the terminal radiators when the heat storage tank temperature is low. The terminal radiators operate based on an indoor temperature setpoint.

System modelling process

The main components involved in the SD-CPCH system include the solar collector, diesel boiler, heat storage tank. The entire system was modeled using EnergyPlus software. The solar thermal energy collected by the heat source loop is transferred to the terminal device through coupling calculations between different loops. The indoor thermal environment and system-related parameters are obtained through coupled calculations between the building and the system. This section introduces the mathematical modeling and calculation process of the main components to guide design parameter selection.

Solar collector modelling

In this study, a double-glazed flat-plate solar collector is used, which outperforms single-glazed flat-plate and vacuum tube solar collectors at high inlet water temperatures31. The model is based on a solar thermal process model developed by Duffie and Beckman32.The thermal efficiency η of a collector is defined as the ratio of the effective heat gain of the heating medium flowing through the collector to the total solar radiation incident on its surface, as shown in Eq. (1).1 η=(q/A)Isolar

where q is the effective heat gain, in W; A is the total area of the collector, in m2; and Isolar is the total incident solar radiation, in W/m2.

For double-glazed solar collectors, the effective heat gain is related to glass properties, absorber layer properties, and environmental conditions, as shown in Eq. (2).2 qA=Isolarτg1τg2αabs-Tabs4-Tg24Rrad-Tabs-Tg2Rconv-Tabs-TairRcond

where, τg1 is the transmittance of the first glass layer; τg2 is the transmittance of the second glass layer; αabs is the absorption rate of the absorption layer; Rrad is the radiation thermal resistance of the absorption layer to the inside of the glass; Rconv is the convection thermal resistance of the absorption layer to the inside of the glass; Rcond is the thermal resistance from the absorber layer to the outdoor air; Tabs is the temperature of the absorption layer; Tg2 is the temperature of the inside of the glass; Tair is the temperature of the outdoor air.

The above equation can be approximated using a simpler formula, as follows:3 qA=FR[Isolar(τα)-UL(Tin-Tair)]

where, FR is an empirically determined correction factor; τα is the product of all transmission and absorption terms; UL is the total heat loss coefficient combining radiation, convection and conduction terms; and Tin is the inlet temperature of the fluid.

Substituting Eq. (3) into Eq. (1):4 η=FR(τα)-FRULTin-TairIsolar

Considering FR(τα) and -FRUL as characteristic constants of the solar collector, a linear correlation can be constructed as shown in Eq. (5):5 η=c0+c1(Tin-TairIsolar)

Similarly, the quadratic correlation can be constructed using the following Eq. (6):6 η=c0+c1Tin-TairIsolar+c2(Tin-Tair)2Isolar

where the correction coefficients in the thermal efficiency equation are listed in the Table 1.Table 1 Parameter settings for the double-glazed solar collectors.

Collector number	1	2	3	4	5	6	7	
Area (m2)	67	17	53	88	20	20	49	
Tilt angle (o)	30	
Mass flow (m3/s)	0.001	
Thermal efficiency coefficient	c0=0.691, c1= − 3.396, c2= − 0.00193	
Incidence correction factor	b0=− 0.1939, b1=− 0.0055	

The transmittance of the collector glass varies with the angle of incidence of the solar radiation. Typically, the transmittance is the highest when the angle of incidence is normal to the glass surface. The test conditions of the solar collector determine the thermal efficiency at normal incidence angles. For non-normal angles, the transmittance of the glass is affected by the incidence correction factor, κτα. Incidence correction was fitted as a second-order quadratic function.7 κτα=1+b01cosθ-1+b11cosθ-12

where the coefficients are listed in the Table 1.

Incidence corrections were calculated separately for solar, sky, and ground radiations. The net incidence correction for all incident radiations is the weight of the components.8 κτα,net=Isolarκτα,solar+Iskyκτα,sky+Igroundκτα,groundIsolar+Isky+Iground

For sky and ground radiations, the angle of incidence was approximated using the following equation:9 θsky=59.68-0.1388ϕ+0.001479ϕ2

10 θground=90.0-0.5788ϕ+0.002693ϕ2

Therefore, the thermal efficiency of a solar collector can be defined as11 η=FRκτα,net(τα)n-FRULTin-TairIsolar

Accordingly, the outlet temperature of the fluid flowing through the solar collector was calculated using the following equation:12 Tout=Tin+qm˙cpA

The second-order incident angle modifier equation coefficients used in EnergyPlus are only valid for incident angles of 60 degrees or less due to the uncertainty for angles greater than 60 degrees. The model cuts off collector gains for incident angles greater than 60 degrees. In addition, if there is no flow through the collector, Tout is the stagnation temperature of the fluid.

Diesel boiler modelling

The performance of boilers in EnergyPlus is primarily based on rated heating capacity Qrated and thermal efficiency Enormal33, which are used to calculated partial load ratio (PLR) and fuel used (FU)as in Eq. (13) to Eq. (15).13 PLR=QloadQrated

14 FUtheoretical=QloadEnormal

15 FU=FUtheoreticalTEC

A thermal efficiency curve (TEC), is used to represent the performance of non-electric boilers more accurately, as shown in Eq. (16):16 TEC=a1+a2(PLR)+a3(PLR)2+a4(PLR)3

where a1, a2, a3, a4 are 0.83888652, 0.132579019, − 0.17028503 and 0.047468326 respectively.

Heat storage tank modelling

A non-stratified tank model developed in EnergyPlus was used in this study33. To calculate the water temperature, the model analytically solves the differential equation governing the energy balance of the water tank:17 mcpdTdt=qnet

where, m is the total mass of water in the tank; V is the volume of the tank; cp is the specific heat of water; T is the temperature of the tank water; t is the time; qnet is the net heat transfer rate to the tank water.

The qnet, as shown in Eq. (18), is the sum of gains and losses due to multiple heat transfer pathways.18 qnet=qheater+qoncycpara+qoffcycpara+qoncycloss+qoffcycloss+quse+qsource

where, qheater is the heat added by the heating element or burner; qoncycpara is the heat added due to on-cycle parasitic loads (zero when off); qoffcycpara is the heat added due to off-cycle parasitic loads (zero when on); qoncycloss is the heat transfer to/from the ambient environment (zero when off); qoffcycloss is the heat transfer to/from the ambient environment (zero when on); quse is the heat transfer to the heating loop; qsource is the heat transfer from the heat source loop.

quse and qsource are calculated as Eqs. (19) and (20):19 ques=εusemusecp(Tuse-T)

20 qsource=εsourcemsourcecp(Tsource-T)

where, εuse is the heat exchanger effectiveness for the heating loop; muse is the mass flow rate for the heating loop; Tuse is the inlet fluid temperature of the heating loop; εsource is the heat exchanger effectiveness for the heat source loop; msource is the mass flow rate for the heat source loop; Tsource is the inlet fluid temperature of the heat source loop.

By combining the above equations into Eq. (17) yields the differential equation shown in Eq. (21):21 dTdt=1mcpqheater+qoncyc+qoffcyc+UAoncycTamb+UAoffcycTamb+εusemusecpTuse+εsourcemsourcecpTsource+-1mcpUAoncyc+UAoffcyc+εusemusecp+εsourcemsourcecpT

The differential equation can be simplified to Eq. (22):22 dTdt=a+bT

Therefore, the solution of the differential equation can be expressed in terms of a and b:23 T(t)=(ab+Ti)ebt-ab

where, Ti is the initial temperature of the tank water at time t = 0.

Since EnergyPlus lacks a cascaded PCM module model, this study connected EnergyPlus with Python via an external interface. The solar thermal energy and tank temperature calculated by EnergyPlus were input into Python. The heat stored in the cascaded PCM module was calculated based on energy storage rates from experimental tests, and the results were entered into EnergyPlus via Python. Figure 3 illustrates the experimental setup and temperature sensor arrangement.Fig. 3 Experiment setup and the temperature sensor arrangement.

By continuously heating the water tank, the material temperature change and phase change time were tested to determine the heat storage capacity of the phase change material, as shown in Fig. 4, thus quantitatively portraying the heat storage capacity of the cascaded phase change material, as shown in Table 2. Latent heat stored was integrated into the heat storage tank as a buffer heat source in EnergyPlus. Through this co-simulation, the original heat storage tank was modified to simulate cascaded PCM heat storage.Fig. 4 Temperature change processes of PCMs.

Table 2 Heat storage capacity of cascaded PCMs obtained in the experiment.

	Cascaded phase change materials	
Stage 1	Stage 2	Stage 3	
Energy storage (kJ)	1321	1155	1192	
Energy storage rate (J/s)	167.2	144.4	150.9	

Simulation conditions

The SD-CPCH system proposed in this study is primarily aimed at the energy-scarce plateau region. The Ngari Sakü region of China, characterized by a typical plateau climate, is rich in solar energy resources, with a total solar radiation of 3184 kWh/m2 annually. In addition, Fig. 5 shows that nearly 92.6% of the daytime solar radiation in the Ngari Sakü region has a direct radiation intensity of more than 200 W/m2 on the horizontal plane, with 2511 h of radiation intensity exceeding 600 W/m2. This ensures the effective operation of solar collectors for heating during winter. The year-round temperature in the Ngari Sakü region is below 0 °C most of the time, with the lowest temperature reaching -23 °C and the highest temperature being only 25.4 °C, as shown in Fig. 6. Therefore, the heating load in this region is high, the heating period is long, and the traditional diesel boiler heating method has high energy consumption. Given the resource endowment of this region, SD-CPCH systems utilizing solar energy resources have significant application prospects. In this study, the meteorological parameters of the Ngari Sakü region were used as outdoor conditions in the simulation.Fig. 5 Direct solar radiation intensity at Ngari Sakü throughout the year.

Fig. 6 Outdoor air temperature profiles.

The object building of this simulation study was an office building, and the main functional areas on each floor were classified as thermal zones (TZs), with a total of 14 TZs and a total air-conditioned area of 884.14 m2, as shown in Fig. 7. The June 24th to September 27th is selected as the heat storage period, and heating period was from September 28th to May 23rd of the following year. During winter, the indoor temperature was maintained at 18 °C, with a design supply water temperature of 60 °C for the indoor radiator terminals. Based on load calculations, the accumulated heating load for the entire year was 335.75 GJ. The total area of the solar collectors was 314 m2, accounting for 34.7% of the overall roof space resources of the building. The maximum temperature setpoint of the cascaded PCM energy storage tank was 82 °C. Based on load calculations for the building, the tank volume was determined to be 846 m3. The heat storage system employs a cascaded phase change module consisting of composite materials. The basic energy storage materials for the stage 1, stage 2, and stage 3 phase change modules are Erythritol, Sodium Acetate Trihydrate, and Sodium Thiosulphate Pentahydrate, respectively. The phase change temperatures are 42, 58, and 75 °C, respectively, achieved through material modification. Using standardized phase change modules with different melting points, the phase change temperature of the thermal storage system can match the temperature change of the solar collector and meet the demand of different heating terminals for heat grade. Table 3 shows thermophysical parameters related to cascaded PCMs.Fig. 7 3D view of the target office building: (a) Building model; (b) SC distribution.

Table 3 Thermophysical parameters of phase change modules.

Phase	Stage 1	Stage 2	Stage 3	
Solid	Liquid	Solid	Liquid	Solid	Liquid	
Thermal conductivity W/(m·K)	0.76	0.38	0.81	0.42	1.91	0.53	
Specific heat capacity J/(g·℃)	1.46	2.39	1.97	3.22	1.78	2.11	
Density g/cm3	1.725	-	1.38	-	1.42	-	

This simulation study aims to analyze the feasibility of cross-seasonal heating and explore system optimization options. The simulation results analyze the operational characteristics of the system throughout the year in three aspects. First, the performance of the heat source loop during the heat storage and heating periods was analyzed by assessing changes in the solar energy utilization rate and solar heat gain. Second, cases with and without auxiliary diesel boilers were designed, and conventional convective radiators were used as heating terminals. The tank temperature and heat distribution of the system were investigated to demonstrate the feasibility of using solar thermal energy as the sole heat source. Finally, changes in indoor temperature are discussed, and an optimized system configuration using radiant floor heating (RFH) is proposed to achieve cross-seasonal heating with full solar thermal energy utilization. Table 4 lists the building related parameters and terminal devices related design parameters.Table 4 Building and terminal devices related design parameters.

Parameters	Detail setting	
Number of floors	3	
Number of thermal zones	12	
Total air-conditioned area	884.14 m2	
Gross window-wall ratio	13.98%	
Indoor heat source	Light: 6.9 (W/m2)	
People:17 (m2/person)	
Equipment: 5 (W/m2)	
U-value of envelope	Wall: 0.4 (W/K·oC)	
Window:2.559 (W/K·oC)	
Slab 0.91 (W/K·oC)	
Roof: 0.223 (W/K·oC)	
Radiator	Type: convective	
Supply water temperature: 60 °C	
Water flow mass rate: 15 LPM	
UA: 158 (W/K)	
Radiant floor heating	Supply water temperature: 40—45 °C	
Water flow mass rate: 8 LPM	
Tube conductivity: 0.35 (W/ m·K)	
Tube spacing: 0.15 m	
Cover layers: tiles (0.01 m), concrete (0.1 m)	

Results and discussion

Analysis of heat changes in SCs during thermal storage and heating periods

During the operation of SC in the heat storage period, to prevent reverse heat transfer, the operation reset control is performed by monitoring the inlet and outlet water temperature of the collector, and the resulting operation time distribution is shown in Fig. 8. Operating hours ranged from 9:00 a.m. to 8:00 p.m. throughout the heat storage period, with 97% of the operating hours concentrated between 11:00 a.m. and 6:00 p.m., consistent with the pattern of solar radiation intensity changes. Due to the ample solar radiation resources in the Ngari Sakü region, the total operating time of the system reached 2099 h, accounting for 26.3% of the total heat storage and heating period. Compared to SC systems in general areas, the longer operational time in the plateau region makes the SC system more promising for application. However, for buildings in highland areas, the heating load during the nighttime without solar heat gain is greater than that during the daytime with solar heat gain, resulting in a temporal mismatch between renewable energy heating and heat demand. Therefore, compared to merely increasing the heat generated by a widespread deployment of solar collectors, the heat transfer facilitated by the energy storage system is more critical.Fig. 8 Distribution of solar collector operating hours.

The stacked diagram in Fig. 9 illustrates the variation in solar collector heat gain during the week with median solar radiation intensity within the heat storage period. The intraday heat gain from the solar system mainly depends on the intensity of solar radiation. When the solar radiation intensity exceeds 800 W/m2, the total solar heat gain of all collectors can surpass 300 W/ m2. Due to the thermal inertia of the water system and the feedback control preventing reverse heat transfer, there is a lag period of 1 to 3 h between the maximum solar radiation and the maximum heat gain.Fig. 9 Heat gain per unit area and solar radiation intensity.

As shown in Fig. 10, the daily heat collection of the solar system during the heat storage and heating periods initially decreases and then increases. The overall heat gain of the SC gradually decreases from 400 W/m2 to 100 W/m2, and then rises to 350 W/m2. The decline in solar heat gain during the heat storage period is primarily due to the heat exchange between the heat source and heat storage loops. Conversely, the increase during the heating period is influenced by changes in outdoor temperature. At the beginning of the heat storage period, high-temperature nonfreezing liquid heated by the solar collector passes through the heat exchanger, exchanging heat with low-temperature water drawn from the cascaded PCM energy storage tank. This warmed hot water is then circulated back into the tanks. As the temperature of the cascaded PCM energy storage tank increases, the temperature difference between the nonfreezing liquid in the heat source loop and the hot water in the heat storage loop decreases, reducing the heat exchange quantity. Consequently, the inlet temperature of the nonfreezing liquid tends to rise, which lowers the thermal efficiency and heat gain of the solar collector, resulting in an overall decreasing trend in solar heat gain over the long term. Short-term fluctuations align with changes in outdoor temperature. Upon entering the heating period, the inlet temperature of the solar collectors decreases as the tank temperature drops. This reduces the negative impact of the inlet fluid temperature on thermal efficiency, making outdoor temperature the dominant factor influencing thermal efficiency changes. Therefore, the long-term trend of solar heat gain aligns with variations in outdoor temperature.Fig. 10 Variation in heat collected from SCs during heat storage and heating periods.

Under identical design and operating conditions, the location of the collectors resulted in differences in the average heat gain per unit area, as depicted in Fig. 11. The target building is oriented north–south, and the time and intensity of solar radiation received by the collector installed on the east side of the roof is higher than those at other locations. Consequently, the average heat gain per unit area of SC 3 was higher than that of collectors at other locations, with a maximum of 272 W/m2. The average heat gain per unit area of the solar collectors on the south side (SC 1, 3, and 4) was higher than that of the collectors on the north side (SC 2, 5, and 7). Additionally, SC 6, located on the second-floor roof, had the smallest average heat gain per unit area due to the difference in heights and the shadows cast by the third-floor exterior wall.Fig. 11 Average solar heat gain per unit area for each collector.

Figure 12 shows the thermal efficiency distribution of the solar collectors SCs. SC 1, 3, and 4 exhibit relatively higher thermal efficiency distributions, with the highest probability densities of 0.43, 0.44, and 0.46, respectively. The overall thermal efficiency distribution of each SC was within the range of 0.23 to 0.54, aligning with the actual thermal efficiency of most solar collectors. Therefore, the results obtained from the SCs used in this study are representative and can provide reference values for system design.Fig. 12 Comparison of the thermal energy efficiency distribution of each collector.

The total cumulative heat gain of the solar collectors during the heat storage and heating periods was 488.2 GJ, which is 45.4% higher than the total heat load of the building in winter, as shown in Fig. 13a. This quantitative relationship between the heat supply and demand suggests the feasibility of cross-seasonal heating using large-scale solar collectors on the roofs of buildings in the plateau region, coupled with cascaded PCM energy storage tanks. The heating period in the plateau region is long, and the solar thermal heat gain accumulated during this period was 37.3% higher than that accumulated during the heat storage period. Besides the intraday mismatch between solar heat gain and indoor heating load demonstrated in Fig. 8, the solar heat gain during the heating period exhibits a monthly mismatch with the indoor heating load, as shown in Fig. 13b. In the first heating month, owing to the relatively small heating load of the building, the solar thermal heat gain of that month can meet the heating demand. However, from the beginning of the second heating month up to the fifth heating month, the monthly heating load exceeds the monthly solar thermal heat gain. This temporal mismatch between heat supply and demand can be addressed by cross-seasonal heat storage, which allows for the transfer of heat collected during the heat storage period to the middle of the heating period, filling the heat gap during the heating period.Fig. 13 Distribution of solar energy gain and indoor heating load: (a) Total solar energy gain and indoor heating load; (b) Monthly solar energy gain and indoor heating load.

The simulation results in this section highlight the feasibility of using solar energy for low-carbon heating in plateau areas, showcasing the effectiveness of cross-seasonal thermal storage in addressing the mismatch between heat supply and demand. The next section analyses the temperature and heat changes in the cascaded PCM energy storage tank during the system operation.

Temperature and heat change process in a cascaded PCM energy storage tank

To investigate the feasibility of solar thermal energy as the sole heat source for achieving low-carbon heating in existing buildings, this section compares the changes in tank temperature and heat under two cases: with and without a diesel boiler. In cases with an auxiliary diesel boiler, if the tank temperature falls below 60 °C (the designed supply water temperature for the terminal radiators), the diesel boiler reheats the water.

Figure 14 illustrates the variation in water temperature in the tank during the heat storage and heating periods for both cases. At the end of the heat storage period, the heat gained from the solar collector can elevate the water temperature from the initial 23 °C to 82 °C, fully meeting the water temperature demand for heating in winter. This temperature increase process includes the phase change temperatures of the PCMs (42 °C, 58 °C, and 75 °C, respectively). There is no significant increase in the tank temperature during the phase change process, and the tank temperature gradually rises after the completion of the phase change process. A minor momentary decrease in the tank temperature occurs upon entering the heating period due to the large thermal inertia within the building during the initial startup of the heating system. The tank temperature then slowly decreases, with the rate of decrease gradually increasing as the outdoor temperature drops. The change in tank water temperature throughout the heat storage and heating periods demonstrates the analysis of the trend of the solar collector heat gain in Section "Analysis of heat changes in SCs during thermal storage and heating periods". For the case without a diesel boiler, the water temperature steadily drops to 41.5 °C during the heating period, and the cascaded PCM module releases its latent heat completely. This low supply water temperature affects indoor air temperature control, necessitating a discussion on the adaptability of low-temperature heating terminal devices like a RFH system. As the indoor heating load gradually decreases after March, the remaining solar thermal gain is stored in the tank, in addition to meeting the basic heating load. Consequently, the water temperature gradually rises again to 63.6 °C after completing the second stage of phase change, suggesting a potential significant reduction in the duration of the next heat storage period of the system. With a diesel boiler, the tank temperature is maintained at 60 °C. Subsequently, as the heating load decreases, the tank temperature gradually increases, and the diesel boiler shuts down. Although the diesel boiler can maintain the supply water temperature for the terminal radiators, the latent heat of the stage 1 PCM remains unused.Fig. 14 Temperature changes in the cascaded PCM energy storage tank.

Figure 15 illustrates the heat supplied by the energy storage tank and the diesel boiler. Since the heat provided by the phase-change tank is considered clean energy, it is represented as a negative value in the figure. The graph indicates a gradual increase in the heat supplied by the phase-change tank as the outdoor temperature decreases. Once the tank temperature drops to 60 °C, the diesel boiler activates, providing a maximum heating power of 21.53 kW. However, the heat supplied by the diesel boiler only accounts for 22.4% of the total heat supply during the heating period, as depicted in Fig. 16. The heat provided by the tank accounts for 51% of the total heat supply during the period of dual heat source operation, which can significantly reduce the design capacity of the diesel boiler, as shown in Fig. 16.Fig. 15 Thermal heat changes drawn from heat sources during the heating period.

Fig. 16 Comparison of the total heat supplied by heat sources during the heating period.

The analysis in this section reveals that the tank temperature can reach 82 °C through continuous operation during the heat storage period. However, without the diesel boiler, the water temperature drops below the designed supply water temperature. While a diesel boiler can meet the water temperature requirements, it struggles to utilize the latent heat potential of the stage 1 PCM. Therefore, the next section will comparatively analyze the indoor temperature changes when radiators and radiant heating systems are used respectively under the condition without an auxiliary diesel boiler. This analysis will verify the match between the low-temperature heating terminal and the SD-CPCH system.

Analysis on the indoor temperature changes with using different heating terminal

Figure 17 displays the indoor temperature changes in each thermal zone when the SD-CPCH system is combined with radiator terminals. At the beginning and end of the heating period, when the indoor heating load is low, the system maintains the indoor temperature at 18 °C. However, as the tank temperature decreases, the radiator terminal struggles to meet the indoor heating load, resulting in indoor temperatures below the design temperature from December 30th to March 17th. The lowest indoor temperature recorded is 13 °C.Fig. 17 The indoor temperature changes when radiators are used as heating terminals.

On the other hand, Fig. 18 illustrates the indoor temperature changes in each thermal zone with RFH. Without an auxiliary diesel boiler, the RFH terminal maintains indoor temperatures above 17 °C by utilizing solar thermal heat stored during the heat storage period and supplemented during the heating period. The thermal inertia of the RFH terminal allows indoor temperatures to exceed 18 °C during midday hours when solar radiation intensity is high.Fig. 18 The indoor temperature changes when RFHs are used as heating terminals.

Comparing the change in tank temperature in Fig. 19, it is observed that the decrease rate in tank temperature slows down with the use of RFH, reaching a minimum of 43.5 °C. The exothermic process of the phase change materials is prolonged due to the RFH system, despite having the same phase change material capacity. The large surface area of the radiant floor enables the heating terminal to draw more instantaneous heat from the tank compared to conventional radiators. Additionally, the heat storage capacity of the floor structure allows for a slow emission of heat to the indoor environment after meeting the temperature setpoint, resulting in RFH turning off. Consequently, the operation time of RFH is 19.5% shorter than that of radiators, leading to a 5.2% reduction in the total heat drawn from the tank during the heating period.Fig. 19 The tank temperature and thermal heat transfer changes for different heating terminals.

Conclusions

The study involved modeling a solar-driven cascaded phase change heat storage cross-seasonal heating system using EnergyPlus software. The study aimed to investigate the performance of combining solar collectors and cascaded PCM heat storage to achieve cross-seasonal heating in the plateau region, which benefits from abundant solar radiation. The study included a comparative analysis between the proposed system and a conventional fossil fuel-based heating system. The following main conclusions were drawn from the simulation study:Due to the endowment of solar energy resources in the plateau region, solar collectors can operate for more than 2000 h annually. Furthermore, the total cumulative heat collected during the heat storage and heating periods is 488.2 GJ, representing a 45.4% surplus compared to the total heating load in winter. This quantitative relationship demonstrates the feasibility of achieving low-carbon heating by deploying solar collectors across extensive areas of the building roof in the plateau region. Phase-change thermal storage technology can solve the issue of mismatch between the supply and demand of heat on a time scale. The heat collected during the heat-storage period can be transferred to fill the heat gap during the middle of the heating period.

In the case of the SD-CPCH system with the diesel boiler as the auxiliary heat source, the heat supplied by the diesel boiler during the heating period only accounts for 22.4% of the total heat supplied; therefore, the SD-CPCH system can significantly reduce fossil fuel consumption. The proportion of heat supplied by the cascaded PCM energy storage tank is 51% during the diesel boiler operation period, which indicates that the design capacity of the diesel boiler can be reduced using the SD-CPCH system.

A low-temperature RFH system should be used as the heating terminal to achieve the low carbon operation of the SD-CPCH system without additional heat sources. The system can maintain an indoor temperature above the temperature setpoint using solar thermal energy as the sole heat source. The increase in the tank temperature at the end of the heating period was beneficial for shortening the duration of the heat storage period for the following year.

The feasibility of utilizing solar thermal energy and cascaded phase change heat storage for cross-seasonal heating has been demonstrated in this study. However, as the supply water temperature decreases, conventional radiator terminals may fail to meet the indoor heating load demand, resulting in the need for a diesel boiler as an auxiliary heat source. Moreover, utilizing photovoltaic direct-drive electric boilers to prepare hot water, in addition to solar thermal energy, can mitigate the unstable heat supply resulting from fluctuations in water temperature and heat loss in the heat transfer process. Further investigation is required to address the key issue of power matching between photovoltaic power generation and electric boiler power consumption. The subsequent study will compare and analyze the proposed SD-CPCH system and the photovoltaic direct-drive electric boiler system in terms of heat supply reliability and carbon emissions. Additionally, a combined system solution will be explored.

Highland areas typically experience cold winters and warmer summers, with significant temperature fluctuations between day and night. Conventional heating methods tend to be less efficient and more costly at high altitudes. Thus, the solar-driven cascaded phase change heat storage system for cross-seasonal heating holds significant application value in highland areas. The system utilizes solar energy as the primary energy source, which is abundant in the plateau region, effectively reducing reliance on traditional fossil energy sources and mitigating carbon emissions. Through the cascade design of phase change materials, phase change materials with different melting points can store and release heat at different temperatures, maximizing the efficiency of solar energy utilization. The system can efficiently collect and store thermal energy in the summer and release it in the cold winter to meet the heating demand in the winter, balancing the difference in energy supply and demand between seasons. Areas with abundant solar energy resources should be selected during practical application, and the installation position and angle of the solar collection system should be optimized to maximize solar energy capture efficiency. To enhance thermal efficiency, suitable phase change materials should be selected based on climatic conditions and required supply water temperature, ensuring alignment between the material's melting point and actual operating temperature.

Roman symbols

A Total area of the solar collector (m2)

cp Specific heat capacity (J/kg·K)

FR Correction factor

Isolar Total incident solar radiation (W/m2)

m Mass flow (kg/s)

PLR Part load ratio

q Heat transfer (W)

Q Heating capacity (W)

Rcond Thermal resistance from the absorber layer to the outdoor air (K/W)

Rconv Convection thermal resistance of the absorption layer to the inside of the glass (K/W)

Rrad Radiation thermal resistance of the absorption layer to the inside of the glass (K/W)

T Dry-bulb temperature (oC)

UL Total heat loss coefficient (W/(m2K)

V Volume of the tank (m3)

Greek symbols

α Absorption rate

κ Incidence correction factor

η Thermal efficiency of the solar collector

θ Angle of incidence

ϕ Surface tilt of the solar collector

τ Transmittance rate

Subscripts

abs Absorption layer

air Outdoor air

b Coefficient of incidence correction

g1 First glass layer

g2 Second glass layer

in Inlet fluid

n Normal angle

out Outlet fluid

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.52208135) and Natural Science Foundation of Jiangsu Province, China (No. BK20221056).

Author contributions

F. G. and L. S. wrote the main manuscript text and Y. G. prepared all figures. X. H. supervised the research. T. S. provided critical feedback. H. Z. edited the draft of the paper. All authors helped shape the research, analysis and reviewed the manuscript.

Data availability

The datasets used and/or analyzed during the present study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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