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Article

TRNSYS Simulation of a Bi-Functional Solar-Thermal-Energy-Storage-Assisted Heat Pump System

by
Mingzhen Wang
,
Eric Hu
* and
Lei Chen
School of Electrical and Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia
*
Author to whom correspondence should be addressed.
Energies 2024, 17(14), 3376; https://doi.org/10.3390/en17143376
Submission received: 28 May 2024 / Revised: 2 July 2024 / Accepted: 8 July 2024 / Published: 10 July 2024
(This article belongs to the Section A2: Solar Energy and Photovoltaic Systems)
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Abstract

:
The escalating energy demands in buildings, particularly for heating and cooling demands met by heat pumps, have placed a growing stress on energy resources. The bi-functional thermal diode tank (BTDT) is proposed as thermal energy storage to improve the heating and cooling performances of heat pumps in both summer and winter. The BTDT is an insulated water tank with a gravity heat pipe (GHP), which can harvest and store heat passively from sun radiation and the external environment during the daytime. In summer, it harvests and stores cold energy from the air and night sky during the daytime. The performance of the BTDT-assisted heat pump (BTDT-HP) system in Adelaide, Australia, during the 2021–2022 summer and winter seasons was evaluated by conducting a TRNSYS simulation. This study revealed that the BTDT-HP system outperformed the reference ASHP system, where up to 8% energy in heating and 39.75% energy in cooling could be saved. An overall reduction in the energy consumption of 18.89% was achieved. Increasing the BTDT volume and GHP panel area enabled the tank to store more thermal and cold energy across the winter and summer seasons, thereby improving the system’s performance. The maximum ESPs were found to be 31.6% and 41.2% for heating and cooling for the study case under optimal conditions. When the GHP panel area was fixed at 15 m2, the BTDT volume should be at least 28 m3 for the BTDT-HP system, boasting cooling and heating capacities of 40 kW and 43.2 kW, to achieve positive energy savings.

1. Introduction

The continuous escalation of energy demands in buildings has exerted a significant stress on global energy resources, which is particularly attributed to the vital requirements for heating and cooling purposes. Energy consumption in residential buildings constitutes around 20% of the global total, while contributing to over 20% of worldwide carbon dioxide emissions. The increasing global energy consumption is projected to cause a 50% increase in heating and cooling demands for buildings over the next three decades [1,2]. Moreover, the global energy consumption for cooling purposes will surpass that for heating by the year 2060 [3]. Therefore, addressing the challenges of energy consumption and carbon dioxide emissions from buildings has been a critical focus area in sustainability and the built environment. Recently, the thermal management of water sources has become a promising way to save energy for heating and cooling [4] and has been widely studied and applied in large-scale heating and cooling systems [5,6,7,8,9,10,11]. Considering certain drawbacks of water-sourced heat pumps, thermal energy storage (TES) is expected to offer improved performance and better serviceability, which have already played a significant role in energy conversion and conservation [12,13,14]. A thermal diode tank (TDT) is a sustainable alternative for TES that can significantly contribute to mitigating the energy crisis caused by heating and cooling systems. By integrating the TDT with a normal cooling system, the integrated system could reduce energy consumption by around 40% [15]. Furthermore, a radiation-enhanced TDT (RTDT) was previously proposed and validated to outperform a normal TDT in terms of water-cooling performance [16,17]. In summer, when the water temperature is cooler than the surrounding air, and in winter, when it is warmer, using a TDT to assist a water-sourced heat pump (WSHP) is likely to enhance its coefficient of performance (COP) compared with an air-sourced heat pump (ASHP) [2]. In order to integrate the TDT with the heating system, this study further improved this device, enabling more efficient storage and utilisation of thermal energy during both the summer and winter seasons. The proposed device harvests cold energy in summer nighttime, whilst it stores heat in winter daytime; hence, it is termed a bi-functional thermal diode tank (BTDT).
In this study, the BTDT-HP system was simulated by applying the Transient System Simulation (TRNSYS) Program, which enables simulating most energy systems that incorporate other components, including solar collectors [18,19], desiccant dehumidifiers [20,21], phase change materials (PCMs) [22,23,24,25], and TES [22,23,26]. A TRNSYS case study about a cooling system coupled with a TDT under Australian summer weather conditions was thoroughly conducted by Wang et al. [27], where up to 40% energy could be saved in comparison with a standard ASHP system. Chargui et al. [28] conducted a study on simulating a geothermal heat pump in heating mode using TRNSYS 18. Their research demonstrated that the COP of the heat pump was influenced by the inlet temperature of water and electricity consumption, with a higher COP observed when the pump was operated using water due to its higher calorific capacity compared with ambient air. The study also highlighted the potential of CO2 as a working fluid in heat pumps. Bordignon et al. [29] studied a WSHP through a TRNSYS simulation, and they found the seasonal COP for heating and cooling were about 3.8 and 5.7, respectively. However, it was predicted that the seasonal COP generally shows a slight decrease over time. Chargui and Sammouda [30] used TRNSYS to study a heat pump system with a water tank, where the COP of their proposed heat pump was up to 9. Banister et al. [19] utilised TRNSYS to simulate an integrated system of a water tank and solar-assisted heat pump. Their validated TRNSYS model demonstrated a high accuracy in comparison with the experimental results. A TRNSYS simulation of a WSHP heating system was developed by Cardemil et al. [31], where the heat source was a swimming pool. In their study, a parametric analysis on the system location and economic performance was also conducted, and it demonstrated that the most economical WSHP system was with a heating capacity of 14 kW. A ground-sourced heat pump (GSHP) system was modelled by TRNSYS and validated with experimental results by Safa et al. [32]. They determined that the system COP ranged between 2.8 and 3.4, regardless of the heating or cooling mode. In addition, their GSHP system was able to save nearly 30% energy. Ruiz-Calvo et al. [33] also developed a TRNSYS model to optimise the energy-saving performance of a GSHP system. It revealed that the seasonal performance factors of the GSHP system were above 6.5 in summer, while they were around 3 in winter. Hou and Taherian [5] proposed and simulated an integrated system of a GSHP and a liquid dry cooler using TRNSYS, where a system COP of 3.5 was found when the room temperature range was set from 20 to 23 °C. Grossi et al. [34] studied an air and ground dual-source heat pump (DSHP) system using TRNSYS. They found that the system COPs for heating and cooling both increased with increased heating and cooling capacity. Moreover, across their studied cases, the system COPs for heating and cooling were found to be over 3 and 4, respectively. Hou et al. [35] developed a GSHP system using TRNSYS to compare its performance with a conventional system. The simulation revealed that within a 10-year period, their proposed GSHP system outperformed the conventional system, with a higher COP at 3.33. Jang et al. [36] introduced a WSHP system designed for a hybrid application serving both data centres and residential zones in a region with a predominant need for heating by TRNSYS. The research illustrated that this system yielded substantial energy savings, achieving a 12.3% reduction in energy consumption for data centres and a notable 31.2% decrease for residential areas. Overall, the WSHP system could annually save up to 18.8% energy in total.
In summary, extensive and in-depth investigations into heat pump systems using TRNSYS simulations have been a focal point, and research in this domain continues to be significant and noteworthy. TRNSYS has found widespread application in HVAC systems integrating solar panels, PCMs, and water storage tanks. To further improve the heat pump performance, this paper introduces the BTDT-assisted heat pump (BTDT-HP) system. By utilising TRNSYS simulations, a comprehensive case study was carried out to assess the heating and cooling performances of the BTDT-HP system in cold intervals (from 18:00 p.m. to 23:00 p.m. in winter) and hot intervals (from 10:00 a.m. to 15:00 p.m. in summer) in Adelaide, Australia. The novelty of this research in the heat pump theme primarily lay in proposing a new integrated system that merged solar energy, thermal energy storage, and heat pipes. It also aimed to study the impact of the BTDT volume and panel area on the BTDT energy-saving performance.

2. BTDT-HP System

In general, a heat pipe transfers heat from its evaporator end to its condenser end by employing the phase change process of the working fluid. It only works when its evaporator is hotter than its condenser. Significantly, the position of the liquid working fluid determines which section of the pipe is the evaporator [37]. For a gravity heat pipe (GHP), the lower section of the pipe is deemed as its evaporator section, whereas the upper one is the condenser section. Figure 1 shows the configuration of the GHP in the proposed BTDT-HP system, where its cuboid section is partitioned into upward and downward faces. During winter, the upward face features a black thermal collector for solar heat collection, while the downward face serves as a radiative surface. Conversely, in summer, the upward face becomes the radiative surface to dissipate heat.
As illustrated in Figure 2, the BTDT is an insulated tank filled with water and utilises a GHP to dissipate heat. This innovative device operates as a bidirectional energy storage system, which passively stores heat in water by absorbing the solar radiation in winter daytime, while discharging heat from the water to the surroundings in summer nighttime [16].
For the heating mode of the BTDT-HP system in winter, as shown in Figure 2, the external section of the GHP is strategically positioned to capture solar radiation at a lower position. This setup incorporates the black thermal collector to efficiently absorb solar heat in the daytime and subsequently transfer it into the BTDT water. At night, the heated water is then directed to the heat pump outdoor coil, which serves as the evaporator of the heat pump system. It is worth noting that during periods of minimal sunlight or when the ambient air is colder than the BTDT water, the GHP is inactive, preventing heat dissipation from BTDT water into the surroundings.
In Figure 3, during the summer season, the external section of the GHP is reconfigured to function as the condenser end at an elevated position to facilitate effective heat dissipation. When the ambient temperature descends below the BTDT water temperature, especially at night, the radiative cooling panel starts to operate. The radiative surface employs a dual heat transfer mechanism, releasing heat from water into air through convection and simultaneously to the night sky through radiation. However, if the ambient temperature surpasses the water temperature, the GHP will not operate [15,16]. During the system operation, the BTDT water is then used for cooling the condenser of the heat pump system.

3. The Study Case

The study case revolved around a residential house located in Adelaide, Australia, characterised by a spacious indoor area and volume of 500 m2 and 1500 m3, respectively. The house was meticulously modelled in three dimensions, and it was set to maintain a constant indoor temperature of 24 °C for 5 h from 18:00 p.m. to 23:00 p.m. in winter, between 1 June 2022 and 31 August 2022, and 22 °C for 5 h from 10:00 a.m. to 15:00 p.m. in summer, between 1 December 2021 and 28 February 2022. The heating and cooling demands were met by a BTDT-HP system and a conventional ASHP system, respectively. TRNSYS was employed to conduct performance simulations for two systems, with the aim to demonstrate the improved heating and cooling performances of the BTDT-HP system (System 1) over the conventional ASHP system (System 2, refer to [27]).
For both heat pump systems, the rated total cooling and heating capacities were 40 kW and 43.2 kW. The total air flow rate and indoor fan power of System 1 were also the same as those of System 2. More information about Systems 1 and 2 is outlined in Table 1. For System 1, a volume of 50 m3 BTDT was applied with a GHP that offered a 15 m2 thermal collector panel in winter, and a radiative cooling panel with the same surface area was applied in summer. Table 2 provides more detailed specifications of the BTDT, and Table 3 introduces the thermal performance parameters of the building enclosures.

4. TRNSYS Simulation

Figure 4 shows the schematic layout of System 1, where the weather data were the input to all the following main components of this simulation model: building, heat pump, BTDT, thermal collector panel, and radiative cooling panel. The Type 15-2 component input the weather data to the models, while the Type 56 represented the case study house component being conditioned by the heat pump systems.
As shown in Figure 5, the system comprised a water-sourced heat pump (WSHP, Type 919) responsible for providing heating and cooling to the building component. It was directly connected to a bi-functional thermal diode tank (BTDT, Type 158) through a water pump to facilitate water circulation. The input parameters governed the operation of the WSHP, which are included and shown in Figure 4.
During the winter season, the system orchestrated an interplay of components to efficiently provide heating. The BTDT was linked to a thermal collector panel (Type 1) component, which took in direct normal radiation and total horizontal radiation as its radiation input. It operated optimally when exposed to direct sunlight. At this stage, the radiative cooling panel component was non-operational. However, a noteworthy consideration was that its efficiency could be significantly affected by adverse weather conditions. On rainy or cloudy days, the panel’s capacity to store solar heat in the BTDT was limited. Under this condition, a potential challenge may arise during the operation of System 1 when the heat uptake by the evaporator surpassed the heat supplied by the BTDT. This scenario could lead to an unintended consequence: the BTDT water temperature dropped below the ambient air temperature. To address this challenge in winter, a strategically designed backup system mode was implemented. This mode allowed System 1 to seamlessly transition to a backup system (similar to System 2, ASHP) whenever the BTDT water temperature was lowered to the ambient air temperature in order to satisfy the heating demands.
In summer, the BTDT was linked to the radiative cooling panel (another Type 1 component) rather than the thermal collector. The effective sky temperature was treated as the ambient temperature of radiative cooling panel. Meanwhile, the thermal collector panel component was non-operational. On scorching summer days, the efficiency of the radiative cooling panel could be compromised due to the extreme temperatures, resulting in limited cold energy storage within the BTDT. Operating System 1 under this circumstance could present a potential challenge: the cooling water supplied to System 1’s condenser could be insufficient, while causing the BTDT water temperature to exceed the ambient air temperature. To proactively address this challenge and ensure optimal cooling performance, this system also incorporated a backup system mode. Whenever the BTDT water temperature surpassed the ambient air temperature, System 1 transitioned to the backup system (similar to System 2, ASHP) to provide uninterrupted cooling. Therefore, System 1 was essentially a dynamic integration of a WSHP and an ASHP and was specifically designed to operate under the most demanding winter and summer conditions. However, if ambient conditions permitted efficient heat exchange between BTDT water and the surrounding environment, System 1 functioned as a standalone WSHP system. This adaptive behaviour underscored the system’s versatility and capacity to optimise its operation based on prevailing environmental factors, which ensured both efficiency and effectiveness in meeting heating and cooling demands. An air flow diverter (Type 148a) was a device that redirected the air flow from one pathway to another, allowing for greater control and flexibility in air distribution. Meanwhile, an air flow mixer (Type 148b) was designed to control and adjust the flow of air within the system, which regulated the amount of fresh air or return air that entered the conditioned space.
The details of the BTDT aligned with the settings in Table 2. A heated water circulation system was applied between the BTDT unit and thermal collector panel, whilst another cooling water circulation system operated between the BTDT unit and the radiative cooling panel. In addition, two ON/OFF differential controllers (Type 165) were incorporated, with each one connected to the thermal collector and radiative cooling panel, respectively. These controllers managed the activation and deactivation cycles of the panels. During the winter season, the controller associated with the thermal collector panel activated when the BTDT water temperature was lower than the ambient temperature, when sunlight was available, or both. This enabled the storage of solar heat within the BTDT water. Otherwise, the controller remained off. In summer, however, when the BTDT water temperature exceeded the effective sky temperature, the controller associated with the radiative cooling panel activated. This facilitated the heat dissipation through both convection and radiation processes. By offering these control conditions, the BTDT unit enabled dual functionality in storing heat in winter while discharging heat in summer.
For the TRNSYS model of System 2, its design configuration was based on the previous work conducted by Wang et al. [27]. In this study, the specifications of System 2 aligned with the details outlined in Table 1.

5. Results

The visualised data from the TRNSYS simulation offered a comprehensive overview of System 1’s performance, including the ambient conditions, power, and energy consumptions, in comparison with the numerical results obtained for System 2.

5.1. Ambient Conditions

Figure 6a provides a representation of the ambient temperature difference from the highest to the lowest. Figure 6b shows the daily global radiation in kWh/m2 and Figure 6c shows the monthly sunshine duration on average. From previous literature regarding the BTDT in cooling mode, the better performance occurs while there is a higher day and night ambient temperature gap [15,16,17]. For the heating mode in winter, the ambient temperature and sun radiation are the main impact factors of the BTDT performance, and thus, the BTDT-HP system performance.

5.2. Power and Energy Consumption

Figure 7a illustrates a visual representation of the power and accumulated energy consumptions of System 1, including the consumptions of its backup system. To provide a clearer comparison, the monthly energy consumptions of System 1 are outlined in Table 4. The data reveal that System 1 exhibited a cooling power consumption that typically fluctuated within the range of 6 to 9 kW, while the heating power consumption roughly ranged from 8 to 15 kW for System 1. Notably, the WSHP itself of System 1 displayed a power consumption range from 8 to 10 kW. In contrast, the backup system’s power consumption fell within the range of 12 to 15 kW. This distinction underscored the fact that the WSHP itself outperformed the backup system when it came to heating, although it occasionally required the support of the backup system under extremely demanding winter conditions. The data also highlighted a substantial disparity between the energy consumption for heating and cooling, where the total energy consumption for heating consistently remained almost twice as high as that for cooling. Over the summer and winter seasons, the cumulative energy consumption of System 1 totalled approximately 6900 kWh.
In comparison, the power and energy consumptions by System 2 are shown in Figure 7b and Table 4. It clearly demonstrates that under the same ambient conditions, the heating power consumption of System 2 was between 12 to 15 kW, which aligned with the heating consumption of System 1’s backup system. However, the cooling power consumption of System 2 ranged from 10 to 13 kW, which was much higher than that of System 1. Overall, the total energy consumption amounted to over 8500 kWh.

5.3. Energy Savings

Figure 8 and Table 4 both reveal that the energy consumptions for the heating and cooling of System 1 were lower than those of System 2. On the premise that both systems could provide sufficient heating and cooling to condition the room at desired levels, System 1 indeed outperformed System 2, as expected.
During the winter season, System 1 and System 2 showed different energy consumption patterns for heating. System 1 consumed a total of 4417.92 kWh, boasting an average heating COP of 3.24. In contrast, System 2 consumed 4675.15 kWh in total, but with a slightly lower heating COP of 3.11. System 1 achieved monthly energy savings of approximately 85 kWh compared with System 2, translating to an energy saving percentage (ESP) of 8%.
Across the summer season, the total energy consumptions of Systems 1 and 2 amounted to 2506.81 kWh and 3861.86 kWh, respectively. The average cooling COPs of Systems 1 and 2 were 5.1 and 2.72, respectively. It was also found that over 450 kWh of energy could be saved by System 1 every summer month compared with System 2, with a remarkable ESP of 39.75%.
In summary, System 1 consistently outperformed System 2 in terms of energy efficiency and savings, which also demonstrated an overall ESP of 18.89% across both the heating and cooling seasons. The data presented reveal a clear distinction in energy consumption and COPs between Systems 1 and 2, typically with a significant difference in summer. However, in winter, the improvement in heating performance of System 1 was relatively limited.

6. Parametric Analysis

The impacts of the BTDT volume and GHP (thermal collector/radiative cooling) panel area on the performance of BTDT-HP system were investigated, focusing on the heating and cooling ESPs in winter and summer, and the overall value across two seasons.

6.1. BTDT Volume

In this analysis, the BTDT volume alone was the variable value, while all other parameters remained constant. It should be noted that the tank configuration was fixed, that is, the ratio of the tank height (3 m) to tank radius (2.3 m) was constant at approximately 1.3. This investigation primarily centred on understanding the impact of the BTDT volume variations on the heating, cooling, and overall ESPs of the BTDT-HP system compared with the reference ASHP system.
As presented in Figure 9, there were certain thresholds for the tank volume for achieving energy savings, while falling below these volume thresholds resulted in negative energy savings. This phenomenon occurred because a smaller tank size could lead to insufficient cooling water supply in summer and inadequate heat collection in winter. Consequently, the BTDT water temperature could surpass the ambient air temperature in summer, whereas it fell below it in winter. Such conditions reduced the heating and cooling performances of the BTDT-HP system. To ensure consistently positive ESPs, it is advised to maintain a minimum BTDT volume of around 28 m3.
Figure 9 also reveals the ESP trends when increasing the BTDT volume. Initially, as the tank size grew, the cooling ESP experienced a rapid increase, while the heating ESP experienced a relatively gradual ascent. This behaviour was attributed to the fact that larger tanks could store more heat in winter and accumulate more cold energy in summer. However, if the tank size continued to increase beyond a specific value of 40 m3, the increased rate in cooling ESP became less pronounced. For heating, this specific value was found to be 70 m3. Furthermore, the highest heating and cooling ESPs were found to be 31.6% and 41.2%, respectively.
It is significant to carefully assess the BTDT volume in relation to energy savings. An undersized tank risks negative ESPs, compromising the system performance. Conversely, oversizing the tank may lead to extra costs without proportional gains in ESPs. Determining a balance between BTDT volume and cost is essential for achieving optimal energy efficiency and sustainability in the BTDT-HP system.

6.2. Panel Area

This sensitivity analysis focused on the impact of varying the GHP (thermal collector/radiative cooling) panel area on the heating, cooling, and overall energy saving percentages (ESPs) within the BTDT-HP system. Figure 10 presents specific thresholds for the GHP panel area that were essential for achieving positive ESPs. This occurred due to the inherent limitations associated with an undersized panel area. Such limitations translated into inadequate heat collection during winter and insufficient heat dissipation during summer, ultimately diminishing the BTDT-HP system’s performance and efficiency in comparison with the reference system. To consistently maintain positive ESPs, the GHP panel area should be at least 11.5 m2.
Furthermore, the analysis revealed that as the panel area increased, the ESPs initially experienced a rapid increase and finally flattened out. Increasing the panel area enabled the BTDT to collect and supply more heat in winter while storing more cold energy in summer. These advantages were especially pronounced while the panel area was small, where a subtle increase could yield a substantial improvement in ESPs.
However, an important inflection point emerged in this pattern. Beyond specific thresholds, the rate of increase in ESPs became progressively less pronounced. For cooling, this inflection point occurred at approximately 10 m2, while for heating, it was around 20 m2. Additionally, once the GHP panel area was larger than 40 m2, it ultimately resulted in a plateauing effect.
These findings highlight the significance of the evaluation of the GHP panel surface area concerning energy savings. An insufficient GHP panel area can cause negative ESPs within the BTDT-HP system. Conversely, an oversized surface area may incur unwarranted costs without commensurate gains in ESPs. Striking an optimal balance between achieving a high ESP and an increased rate is paramount. In this context, a GHP panel area of 20 m2 emerged as a potential optimal point, aligning with cost-effectiveness.

7. Conclusions

The BTDT was designed to store heat from sun radiation in summer daytime, while releasing heat and storing cold energy on winter nights. This dual functionality proved advantageous in energy conversion and storage when integrated with a conventional heat pump system. By employing the TRNSYS simulation, a comparative analysis between the cooling and heating capabilities of the BTDT-HP and the reference ASHP systems was demonstrated. Two significant parameters, the BTDT volume and the GHP panel area, were investigated to understand their impacts on the BTDT-HP system’s performance. Several findings are outlined below:
  • The BTDT-HP system exhibited superior performance over the reference ASHP system, where it achieved an 8% ESP for heating and an impressive 39.75% ESP for cooling. The cumulative ESP across both modes was found to be 18.89%.
  • During winter, the BTDT-HP system faced limitations in providing sufficient heating due to the constrained heat storage capacity of BTDT water. Several approaches were explored to address this issue, such as implementing a backup heating system, increasing the BTDT volume to enhance energy storage, and expanding the GHP panel area to improve the heat transfer.
  • Increasing the BTDT volume proved effective in storing more thermal and cold energy, thereby enhancing the BTDT-HP system performance. It was observed that there was an optimal BTDT volume, beyond which further increases in tank size yielded diminishing returns. The maximum ESPs were 31.6% for heating and 41.2% for cooling. To guarantee positive heating, cooling, and overall ESPs, it is advisable to maintain a BTDT volume of at least 28 m3.
  • Augmenting the GHP panel area also facilitated more efficient heat exchange. Similarly, there existed an optimal panel area, surpassing which brought only marginal improvements in the ESPs. The peak ESPs observed were 31.6% for heating and 41.2% for cooling as well. It is recommended that a GHP panel area of 20 m2 is the optimal value.
In summary, this study summarised the remarkable potential of the BTDT-HP system as a promising solution for cooling applications, while its heating performance was relatively limited. By carefully optimising the BTDT volume and GHP panel area, significant energy savings could be realised both for heating and cooling.
While the simulation approach provided valuable insights and was extensively validated in prior research, it is important to acknowledge that this study did not include experimental validation. The choice to focus on simulation was made to leverage the robust and flexible modelling capabilities of TRNSYS, which is a well-established tool in the field. However, to further substantiate our findings, future work will aim to incorporate experimental validation. This will enhance the reliability of the results and provide a more comprehensive understanding of the system’s performance in real-world conditions. Further study on the BTDT-HP system will also include conducting an economic analysis to investigate the cost-effectiveness of the system in practical applications. Exploring the integration of enhanced phase change materials in BTDT water could also represent a promising avenue for further improving energy storage and efficiency.

8. Patents

Eric Jing Hu has the following patent: “A HEAT TRANSFER ARRANGEMENT FOR IMPROVED ENERGY EFFICIENCY OF AN AIR CONDITIONING SYSTEM–A thermal ‘Diode Tank’” issued to AU 2014202998 Al.

Author Contributions

Conceptualisation, E.H.; methodology, M.W.; software, M.W.; formal analysis, M.W.; investigation, M.W.; resources, M.W.; data curation, M.W. and E.H.; writing—original draft preparation, M.W.; writing—review and editing, M.W., E.H., and L.C.; supervision, E.H. and L.C.; project administration, E.H. and L.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to acknowledge the support, resources, and the University of Adelaide Research Scholarship provided by the University of Adelaide, which were instrumental in conducting this study.

Conflicts of Interest

The authors declare no conflicts of interest.

Nomenclature

ASHPAir-sourced heat pump
BTDTBi-functional thermal diode tank
BTDT-HPBi-functional thermal-diode-tank-assisted heat pump system
COPCoefficient of performance
DSHPDual-source heat pump
ESPEnergy saving percentage
GHPGravity heat pipe
GSHPGround-sourced heat pump
HPHeat pump
PCMPhase change material
RTDTRadiation-enhanced thermal diode tank
TDTThermal diode tank
TESThermal energy storage
TRNSYSTransient system simulation
WSHPWater-sourced heat pump

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Figure 1. The schematics of the customised GHP in the BTDT-HP system.
Figure 1. The schematics of the customised GHP in the BTDT-HP system.
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Figure 2. Schematics of a BTDT-HP system in heating mode in winter. During winter daytime, the black coloured side of the heat pipe’s condenser section faces upwards to collect solar energy and transfer it into the water inside the BTDT. At night, the heated water is then supplied to the heat pump evaporator.
Figure 2. Schematics of a BTDT-HP system in heating mode in winter. During winter daytime, the black coloured side of the heat pipe’s condenser section faces upwards to collect solar energy and transfer it into the water inside the BTDT. At night, the heated water is then supplied to the heat pump evaporator.
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Figure 3. Schematics of a BTDT-HP system in cooling mode in summer. In this mode of operation, the radiative surface of the GHP is faced upward.
Figure 3. Schematics of a BTDT-HP system in cooling mode in summer. In this mode of operation, the radiative surface of the GHP is faced upward.
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Figure 4. Schematic layout of the TRNSYS simulation model of the BTDT-HP system.
Figure 4. Schematic layout of the TRNSYS simulation model of the BTDT-HP system.
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Figure 5. Simulation of the BTDT-HP system (System 1) by TRNSYS. The thermal collector panel only worked in heating mode for 5 h from 18:00 p.m. to 23:00 p.m. on winter days, while the radiative cooling panel only worked in cooling mode for 5 h from 10:00 a.m. to 15:00 p.m. on summer days.
Figure 5. Simulation of the BTDT-HP system (System 1) by TRNSYS. The thermal collector panel only worked in heating mode for 5 h from 18:00 p.m. to 23:00 p.m. on winter days, while the radiative cooling panel only worked in cooling mode for 5 h from 10:00 a.m. to 15:00 p.m. on summer days.
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Figure 6. (a) Daily ambient temperature maximums and minimums, (b) daily global radiation, and (c) average monthly sunshine duration.
Figure 6. (a) Daily ambient temperature maximums and minimums, (b) daily global radiation, and (c) average monthly sunshine duration.
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Figure 7. Power and energy consumptions of (a) System 1: BTDT-HP system and (b) System 2: ASHP system across summer and winter seasons.
Figure 7. Power and energy consumptions of (a) System 1: BTDT-HP system and (b) System 2: ASHP system across summer and winter seasons.
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Figure 8. Results of the case study, where blue and orange represent Systems 1 and 2.
Figure 8. Results of the case study, where blue and orange represent Systems 1 and 2.
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Figure 9. Sensitivities of heating, cooling, and overall ESPs on the variable BTDT volume while other parameters remained constant and the tank configuration was fixed.
Figure 9. Sensitivities of heating, cooling, and overall ESPs on the variable BTDT volume while other parameters remained constant and the tank configuration was fixed.
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Figure 10. Sensitivities of heating, cooling, and overall ESPs on variable GHP panel areas while other parameters remained constant.
Figure 10. Sensitivities of heating, cooling, and overall ESPs on variable GHP panel areas while other parameters remained constant.
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Table 1. Systems 1 and 2 information.
Table 1. Systems 1 and 2 information.
ParameterValue
RefrigerantR410A
Rated total cooling capacity of Systems 1 and 2 (kW)40
Rated total heating capacity of Systems 1 and 2 (kW)43.2
Total air flow rate of Systems 1 and 2 (m3/h)7000
Rated indoor fan power of Systems 1 and 2 (kW)1.5
Rated outdoor fan power of System 2 (kW)1.5
Rated water flow rate of System 1 (m3/h)9.6
Operation duration (h/day)5
Winter: 18:00 p.m. to 23:00 p.m.
Summer: 10:00 a.m. to 15:00 p.m.
Temperature setpoint (°C)Winter: 24
Summer: 22
Table 2. BTDT specifications.
Table 2. BTDT specifications.
ParameterValue
Volume (m3)50
Height (m)3
Radius (m)2.3
Overall heat loss coefficient (W/m2·K)3
Total water flow rate (m3/h)6.5
Thermal collector panel area (m2)15
Thermal collector panel emittance0.92
Thermal collector panel absorptance0.92
Thermal collector panel instantaneous efficiency intercept0.75
Radiative cooling panel area (m2)15
Radiative cooling panel emittance0.7
Radiative cooling panel absorptance0.7
Intercept efficiency0.75
Efficiency slope (W/m2·K)5.2
Efficiency curvature (kJ/h·m2·K2)0.05
Water pump power (kW)0.56
Water pump efficiency (%)0.7
Table 3. Thermal performance parameters of the building enclosures.
Table 3. Thermal performance parameters of the building enclosures.
Building Information
Thermal parameters of envelopeExternal wall heat transfer coefficient: 0.65 W/m2·K
External window heat transfer coefficient: 2.5 W/m2·K
Roof heat transfer coefficient: 0.32 W/m2·K
Floor-to-ground thermal resistance: 0.625 (m2·K)/W
Internal thermal gainPeople: 6 W/m2
Lighting: 8 W/m2
Room sizeArea: 500 m2
Volume: 1500 m3
Table 4. Monthly Energy Consumption.
Table 4. Monthly Energy Consumption.
Monthly Energy Consumption
MonthWater Pump (kWh)Energy for Cooling (kWh)Energy for Heating (kWh)Backup System Heating (kWh)Total (kWh)
System 1: BTDT-HP system
December51.38649.0000700.38
January70.28923.2700993.55
February58.30754.5900812.89
June40.630692.18672.231405.04
July30.630513.991020.211564.83
August39.230662.49746.331448.04
Total290.442326.851868.662438.776924.73
System 2: ASHP system
December01087.03001087.03
January1519.5701519.57
February1255.2601255.26
June01509.771509.77
July01634.651634.65
August01530.731530.73
Total3861.864675.158537.01
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Wang, M.; Hu, E.; Chen, L. TRNSYS Simulation of a Bi-Functional Solar-Thermal-Energy-Storage-Assisted Heat Pump System. Energies 2024, 17, 3376. https://doi.org/10.3390/en17143376

AMA Style

Wang M, Hu E, Chen L. TRNSYS Simulation of a Bi-Functional Solar-Thermal-Energy-Storage-Assisted Heat Pump System. Energies. 2024; 17(14):3376. https://doi.org/10.3390/en17143376

Chicago/Turabian Style

Wang, Mingzhen, Eric Hu, and Lei Chen. 2024. "TRNSYS Simulation of a Bi-Functional Solar-Thermal-Energy-Storage-Assisted Heat Pump System" Energies 17, no. 14: 3376. https://doi.org/10.3390/en17143376

APA Style

Wang, M., Hu, E., & Chen, L. (2024). TRNSYS Simulation of a Bi-Functional Solar-Thermal-Energy-Storage-Assisted Heat Pump System. Energies, 17(14), 3376. https://doi.org/10.3390/en17143376

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